News Category: Knee

The Multicenter Orthopaedic Outcomes Network (MOON) was created in 2002 to begin collecting information on a large group of anterior cruciate ligament (ACL) reconstructions in order to improve research and treatment. Since then this group has included over 4,400 participants from many surgical centers, and more than 40 publications have used this data. A lot of good information has come from this group which has helped guide surgeons and patients in decisions about their surgery. This article went into detail about many of the findings, but here are a few of the more interesting.

ACL injuries are a common injury to the knee which often occurs in young active individuals and limits their ability to continue to participate in high-level competitive sports. Most people with this injury have a reconstruction of the ACL with surgery. The repair is to done to restore ability for high level sports and to decrease the risk of further wear and tear in the knee. It is estimated that between 175,000 and 200,000 ACL reconstructions are performed every year in the United States and with such a high number of injuries it is important to have high-quality research guiding the treatment decisions.

Of particular interest to this young athletic group is how will they get back to their previous level of sport. The break down for individual sports is as follows: For high school and college football the rate of return to play was in the mid to high sixties, forty-three per cent reported that they returned to the same level of play, the rest either at a lower level or did not return at all. For soccer players, the return to play was seventy two per cent, and of this more than three quarters returned to the same or higher level of play. It appears that a major factor in not returning to play is fear of re-injury.

One of the most important things to consider when having an ACL reconstruction is what to use as the graft. The choice of graft and the patients age were the most predictive variables to success. Use of an allograft (donor, someone else’s, tissue) had a four times greater risk of re-rupture than when an autograft (patients own tissue) was used. Tearing the recently repaired ACL is most likely in the age group 10 to nineteen year olds and then every 10 years the risk decreased slightly. The take home message is that for younger patents the best choice is autograft, with the least chance for re-tear.

For rehabilitation the take home information is that you don’t need a continuous passive motion machine following surgery, but that early knee range of motion was important to prevent stiffness in the knee. Immediate weight bearing is also helpful to decrease pain. Bracing right after the surgery did not offer any benefit. It works fine to rehab at home. Closed chain exercises, like squats, were found to be good in the first six weeks. High-intensity neuromuscular electrical stimulation is safe and believed to improve quadriceps strength, but is not necessary for a successful rehabilitation.

The biggest complication to ACL reconstruction is the risk of re-injury, both of the same operated knee and the other “good” knee. Wright et al, in a study of 460 patients who had to have a revision surgery, found that the most common reason for re-tear was traumatic injury (thirty-two per cent) and then technical error from the initial surgery (twenty-four per cent). This same author discovered that in the first two years after the initial ACL surgery there is a 5.8 per cent chance of tearing the other ACL and an 11.8 per cent chance of re-tearing the repaired ACL.

Another interesting finding made by Mather et al through the MOON group is that in the long term, there is a lifetime cost to society based on whether a person chooses surgery or just rehabilitation following an ACL injury. Looking at the MOON participants and comparing the costs associated with reconstruction, including the medical costs and other things such as lost wages from time off work, and even disability payments, it was shown that the cost to society for an ACL reconstruction was $38,121 compared with $88,538 for rehabilitation only and no surgery. So, it appears that it may be better to have surgery both in terms of knee health, overall cost and quality of life.

The MOON has provided a lot of data for researchers looking into important factors of ACL surgery, and is already helping to guide decisions. There are many more future plans to continue using this as well as other similar large subsets of data to continually improve outcomes from ACL surgery.

Total knee replacement (TKA) is a surgery which has been increasing in frequency in recent years. In addition, hospital stays have shortened and people are heading home sooner than in the past. This means that the role of rehabilitation has become more important for these folks, and therapists who travel for in home treatments tend to have a lot of these people on their caseload. In home physical therapy can be time consuming and expensive in rural or urban areas and it there is recent interest in seeing if tele rehabilitation (instruction via video link through the internet) is a viable and quality option. This is the first large study to try and determine if tele rehabilitation is equally effective as face-to-face home therapy after a total knee replacement.

This study included two groups, one who had in home physical therapy for the first two months following their TKA, and another who had tele rehabilitation for the first two months following TKA. There were about one hundred people in each group, and each group had approximately sixteen sessions of therapy. Progress was evaluated with two well known questionnaires, range of motion, a six minute walk test and a timed stair test based four months after discharge from the hospital.

The treatments were provided by trained physical therapists and were the same for each group. They included evaluation, supervised exercises, instruction in exercises to complete on days without a session, advice about pain control, use of walking aids and return to activities. The intensity and difficulty was based on each individual patients progress.

The results are very promising for tele rehabilitation. This study found that at the last follow up, four months after discharge from the hospital, the mean differences between the groups was basically zero for all the measures. Tele rehabilitation can improve accessibility to rehabilitation services in remote or rural communities, or even in urban areas where volume can be a challenge for in home therapists. And knowing that the outcomes are similar means that as communication technologies become more available, reliable, and affordable, this is a very good alternative for health professionals and patients trying to maximize results from TKA.

Transfusions of packed red blood cells are fairly common for patients undergoing surgery, especially for a total hip or total knee replacement, due to blood loss during the procedure. Some studies have shown rates for blood transfusion with a joint replacement to be between eighteen and sixty-eight percent. There has been a push by hospitals and orthopedic surgeons to minimize the use of blood transfusion for reasons of cost, supply concerns, inappropriate use, serious complications, prolonged hospital stay and mortality. Studies that indicate complications and overuse tend to focus on the after effects, but are not accounting for the fact that many patients requiring a blood transfusion after joint surgery are likely to have preoperative risk factors as well. To look further into this effect research needs to better understand the reasons for and the patient status prior to the transfusion in order to better understand the risks afterward. This study has been set up to try to determine, for hip and knee replacement, the preoperative factors associated with needing a blood transfusion and the onset of serious complications in the first thirty days post surgery.

The National Surgical Quality Improvement Program is a nationally validated, outcome based program which collects data about preoperative risk factors, variables during surgery, thirty-day postoperative complications and mortality rates. This database was used to identify patients undergoing elective primary hip and knee replacements in 2011. A total of 9362 hip replacements and 13,622 knee replacements were identified. Demographic characteristics such as age, sex and race were noted as well as pre surgical factors such as body mass index (BMI), diabetes mellitus, smoking, alcohol use, congestive heart failure, hypertension, bleeding disorder, and other chronic conditions. Lab values and surgical variables such as duration of the procedure and American Society of Anesthesiologists (ASA) class were noted. The thirty-day complications of interest included infection, venous thromboembolism and mortality.

For this set of patients the blood transfusion rate for hip replacement was 22.2 percent, and for total knee replacement 18.3 percent. Significant risk factors associated with needing a blood transfusion were similar for both knee and hip replacements and included (in order of importance) increasing age, preoperative hematocrit (red blood cell count), BMI 2. These results were somewhat surprising, for example the authors did not expect lower BMI to be associated with increased transfusion requirement, and they also found that smoking was actually a protective factor against needing a transfusion.

This study did not indicate that having a blood transfusion made the patient any more likely for serious complications including infection, venous thromboembolism or death. Some other studies have shown some increase in such complications with blood transfusion, but the studies mentioned here by Pedersen et al and Browne et al were smaller and limited only to total hip replacements.

In any observational study there cannot be proven causality, but the authors feel that this study has used a high quality database, large sample size and comprehensive analysis of preoperative factors, and can provide useful information. This study has shown that blood transfusion following total joint replacement is fairly common and generally safe.

It is known that knee joint laxity is a risk factor that predisposes a knee to an anterior cruciate ligament (ACL) tear and a late failure of ACL reconstruction. Knee hyperextension and a large amount of tibial outward rotation are risk factors for poor outcome after ACL reconstruction. Reseachers have noted that anteriorlaxity (hyperextension) of the uninjuried knee may be able to give an indication of how successful an ACL reconstruciton would be on that persons’ injured knee. This study went back through medical records of 163 patients who had undergone ACL repair from January 2002 to August 2009 and split the patients into groups based on how much laxity was in the non-operated knee. The purpose was to evaluate the association between postoperative outcomes of ACL reconstruction and the anterior laxity (how much the tibia slides forward in the knee joint) of the uninjuried knee. There were three groups as follows: Group 1 had 7.5mm. Each pateint had bone-patellar tendon-bone graft. Functional outcomes were assessed with a Lysholm score (an assessment used for ACL reconstruction that asks how the knee functions with daily tasks) and the International Knee Documentation Committee (IKDC) score (assess ligament reconstruction function).

The three groups did not differ significantly in age (avg. 28.6 years), male to female ratio, injured side, dominant limb involvement, time between injury and operation, the Lysholm score, or IKDC score. The difference between injured in laxity in the groups was not significant in a clinical regard but, for comparison of laxity the preoperative side had 4.1 +/-0.7mm in Group 1, 6.3mm +/- 0,7mm in Group 2, and 8.6mm +/- 0.8mm in Group 3.

The postoperative stability of the knee did differ substantially between groups. Group 1 had 2.1mm +/- 1.3mm, Group 2 had 2.2mm+/- 1.3mm and Group 3 had 2.9mm+/- 1.4mm of anterior laxity. The study focused on the anterior laxity of the uninjuried knee, to examine the innate characteristic that could be related to anterior laxity of a knee that has undergone a surgical ACL reconstruction to stop excessive anterior laxity. The primary finding of the study was that patients who had >7.5mm of anterior laxity on the unijured knee and greater postoperative anterior laxity and worse functional outcomes after ACL reconstruction then those patients who had <7.5mm of laxity. However, caution should be used interpreting the results. While there were differences in the functional outcome scores they were not so great as to be considered of minimal clinical important difference (MCID). The MCID are patient derived scores that reflect changes in a clinical intervention that are meaningful for the patient. So while the scores were worse for Group 3 that Group 1 it may not carry much impact on the function. Another caution to the study is that a persons knee laxity can differ from side to side; the anterior laxity of the injured knee was not able to be assessed PRIOR to the ACL tear, so the injured knee may have had more anterior laxity to start with.

With all that, in conclusion, there does appear to be an association between anterior laxity of the uninjured knee and stability of other knee following ACL reconstruction, how much impact it has is difficult to say. It should be noted that anterior laxity of the unijured knee may be an indicator of success following ACL repair.

An ACL tear does not necessarily mean surgery. A recent study found that surgical and nonsurgical treatment for ACL tears have about the same outcomes at a two year follow-up. The study took 143 participants with ACL tears less than three months old and followed them through their treatment over a two year period. They measured initial strength and functional capabilities, then had them participate in a five week rehabilitation program emphasizing strength. Participants were then counseled on recommended treatment–surgical or non-surgical. Of this group, 100 went ahead with the surgery and 43 opted to not have surgery. The decision to have surgery hinged on the additional tear of the medial meniscus, the desire to return to a pivoting type sport (such as soccer or basketball), as well as incidences of “giving out” of the knee. The non-surgical group had an additional two to three months of rehabilitation. Following surgery, the surgical group had six to 12 months of rehabilitation consisting of strengthening, plyometrics, and neuromuscular training.

Overall, there were no significant differences between the two groups at the two-year follow up, with the majority of both groups reporting significant improvements in their self-reported knee function and having comparable strength. Of the entire participant population one-fifth did have knee re-injury and one-third had muscle strength deficits, neither of which were correlated with either treatment option. These results must be interpreted with caution however, because the surgical group was significantly younger and more likely to be participating in pivoting type sports than the non-surgical group.

Articular Cartilage is a cushing agent in your joints and overtime it can degenerate for a host of reasons and it has limited inherent ability for repair. There are many options for surgical treatment of knee pain that is caused by cartilage degeneration. A lot of research is being done on the different surgeries available and this article will give a brief overview of what is availble in the medical community for restoration of articular (joint) cartilage.

Cartilage imaging is common and you will most like have a specific type magnetic resonance imaging (MRI) performed on your knee. This is a noninvasive method used to visualize your knee’s cartilage status and health. The mechanical alignment of your knee (how you are built) will also influence what are available options or combinations of options for you.

There are some prescription and over-the-counter (OTC) drugs available that are being studied to examine their ability to protect cartilage and prevent or delay cartilage degeneration. A couple common OTC drugs are glucosamine and chondroitin sulfate. Studies on the drugs out there have been mixed in reporting benefit, so there is not a clear benefit of taking one of these medications yet. Prescription medications and OTCs effectiveness also depend on your mechanical alignment. If there is overloading on one side of your knee it is not likely that a medication will be of significant help alone.

Bone Marrow Stimulation and Microfracture

These techniques encourage the formation of new cartilage cells by stimulating underlying bone. Very simply put, in these procedures little divots are made in the bone and they bleed stimulating new cartilage growth. A systematic review (summary) of studies with greater than 3000 patients found that knee function was improved two years after microfracture. After two years, function declined but was still above pre-operative levels. Some shortcomings of microfracture are: unpredictable repair cartilage volume and higher failure rates for cell transplantation surgery (this well be described later). At the present time procedures that stimulate bone marrow are best for a first choice if the defect is isolated to the femoral condyle and smaller than 2.5cm^2.

Cell-Based options

These types of surgeries attempt to repair cartilage in the joint by injecting stem cells or your own chondrocytes (a cell found in cartilage). Implanting chondrocytes requires two separate surgeries with a period of growing the cells in-between. This can create a high cost and a substantial amount of time to finish the procedure. One study found superior results with chondrocyte implantation vs. microfracture when surgery was performed within three years of symptom onset. A study of 100 patients with 10-year follow up reported autologous chondrocyte implantation for lesions greater than 2cm^2 had a low failure rate (17 per cent). There is limited information on success rates with stem cell implantation but clinical trials to validate the technique are underway.

Chondral and Osteochondral Grafts

Cartilage autografts (your own body tissue) and cartilage allografts (donor tissue) have demonstrated growth of new tissue at the cartilage lesions. Operative procedures are fairly straightforward and require just one operation. Osteochondral (bone/cartilage) plugs have properties of both tissues so they provide a complete unit of graft tissue. Research has shown them to be a good option for lesions less than 3cm^2.

Rehabilitation

Non-weight bearing and partial weight bearing are used for a time during rehabilitation times frames range from eight to twelve weeks. Rehabilitation programs will depend on surgery, health status, cartilage lesion size, structural joint factors and age.

Restoration of Mechanical Environment and Summary.

If there is a structural deformity in the knee a surgery may need to be performed to improve success of of cartiage repair procedure.Techniques are used to cut away bone to make an equal loading surface. Cartilage surgery must focus on restoration of joint mechanics, address the defect-specific variables (location, size, numbers, bone health). Patient characteristics are also considered such as age, body mass, symptoms, lifestyle and surgical history. These procedures are fairly new and the quality of research is limited.

ACL rupture affects an estimated 35 people per 100,000 and can increase based on gender and activity. Female athletes face a two to eight times increase in risk of ACL rupture compared to their male counterparts. With numbers like this, it is not wonder that the annual estimated health care cost for ACL repair is three billion dollars just in the United States. The most common treatment for ACL rupture is surgical repair, though non surgical rehabilitation is also an option. The decision to repair a ruptured ACL depends on many factors, including age, desired level of activity, episodes of instability and general health.

Before reviewing the surgical options for ACL reconstruction, it is important to understand the basic anatomy of the ACL, or anterior cruciate ligament. It is composed of two functional bundles of ligaments, the anteromedial and posterolateral bundle, which are so named due to their insertion sites on the tibia. Both bundles work together to stabilize the knee into flexion. The anteromedial bundle length remains constant throughout flexion and extension of the knee but is most taut at 45 to 60 degrees flexion. The posterolateral bundle is tight with extension but loosens with flexion in order to allow some rotation to take place at the knee joint.

Once the the decision to have surgery has been made, there are several factors to consider including timing of the surgery, technique used by the surgeon, and the graft site from which to build the new ACL. When making these decisions one must look at preoperative range of motion, swelling and strength as well as individual anatomy, post-operative activities and goals.

Timing of the knee surgery is one of the first factors to consider. There is evidence supporting early surgery, as it may lead to improved functional outcomes and decreased rate of future meniscal damage. However, delayed surgery can allow for potential avoidance of surgery all together if the individual is satisfied with their knee function. Allowing more time before surgery can definitely help an individual improve preoperative strength and range of motion.

Preoperative strength of at least 90 per cent of the quadriceps, is correlated with improved long term functional outcomes. Preoperative swelling and limitations in range of motion are correlated with increased arthrofibrosis after surgery. Thus, a preoperative rehabilitation program focusing on decreased swelling, improved range of motion and quadriceps strength is beneficial.

There are two main surgical techniques utilized in ACL reconstruction, single or double bundle. Though the single bundle technique is far more common, with the double bundle technique being used primarily in Europe and Asia, the rupture pattern of the ACL in that individual and their unique anatomy should be considered by the surgeon when making the ultimate decision on which technique to use. Variations in the tibial notch, arthritic changes, multiligament injuries and bone bruising are all taken into consideration by the surgeon with the aid of a detailed flow chart. Outcomes measures detect no difference in long term functional outcomes between the two techniques, with the exception of fewer reported meniscal injuries with double bundle repair. Regardless, it is important that the surgery match anatomical placement of the ligament in order to help restore optimal biomechanics.

After the technique has been selected, the graft site is the next major decision. Typical graft options include bone-patellar-bone autograft, hamstring tendon autograft, quadriceps tendon autograft, and allograft. If a double bundle repair has been selected the bone-patellar-bone graft cannot be utilized. MRI scans can be helpful in allowing the surgeon to determine which tendon may be most useful based on graft size. The long term goals of the patient are also important in selecting the graft site. For example, an athlete that relies heavily on hamstring strength will not want to use the hamstring autograft. Similarly an individual who has to do a lot of kneeling will not want to choose the bone-patellar-bone autograft.

Once the surgery has been performed, several questions arise, including when one can return to sport, what is the chance of re-injury and/or developing osteoarthritis in the future. Return to sport is dependent on many factors including the healing of the graft, the individuals anatomy and the desired sporting level. For those who do not return to high level of sport, fear of re-injury is a common reason. Graft failure rate is about 11 per cent and does not seem to be dependent of the choice of graft site. Several authors have actually reported a higher rate of ACL injury in the opposite leg compared to a re-injury of the repaired ACL. In general, those who do re-injury a repaired ACL are younger and returning to a higher level of activity. Arthritic changes and the development of osteoarthritis after ACL rupture is more common in those who have sustained some meniscal damage or lose range of motion in the knee joint.

Total knee arthroplasty or total knee as it is commonly called, is surgery that is being performed more and more frequently, with more and younger people opting for a total knee.

It is estimated that by 2030 people younger than 65 will make up one million of the total knee replacement surgeries performed. It has been established that intermediate and long-term failure rates are higher in younger patients with a total knee, however not much is known about the short-term failure rates. The purpose of this study was to determine if patients younger than 55 years old where at a higher risk of getting an infection at the total knee site or having a mechanical failure of the knee prosthetic (hardware breaking anywhere in the total knee) one year after the surgery. After a total knee failure a revision surgery is performed.

The examiners used report information from California hospitals of patients that had only one total knee surgery from 2005-2009. Patients could be any age and male or female. They found that at one year, 1.36 per cent (5301 patients and 72 revisions) of those younger than 50 had a major knee infection and 3.49 per cent (5301 patients and 185 revisions) had a mechanical failure that needed revision surgery. The revision percentage decreased as age increased and those older than 65 had .73 per cent failure rate (74,570 patients and 545 revisions) due to knee infection and .75 per cent failure rate (74,570 patients and 560 revisions) due to mechanical failure. In all age groups men had slightly higher infection rate compared to women (1.04 per cent vs. .68 per cent), but there was no difference in percentages of mechanical failure. People that had multiple (four or more) health issues such as diabetes, anemia, depression, and peripheral vascular disease had higher chances of revision then those without other health issues.

This study demonstrated younger age (less than 50 years old) is a higher risk age for getting infection around a total knee or having it break due hardware failure with one year. While long-term failure rates due to the total knee breakdown are typically attributed to the younger total knee population being more active than their older counterparts, less is known about short-term breaking of the hardware. The examiners give a possible explanation for the higher percentage of revision with the younger population is that revision may be due to the greater amount of arthritis after a major injury that younger patients have. Whereas older patients do not typically have arthritis from a traumatic event in the knee to be replaced. The study also found that hospitals that performed greater than 200 total knee arthroplasties per year had lower infection rates compared with lower volume hospitals. If you are debating getting a total knee and are under fifty, you should discuss this with your health care provider.

A recent review of the most up-to-date research found that large cartilage tears at the knee joint are best repaired with donations from cadavers. The review found that a technique called Osteochondral Allograft Transplantation,. or OCA, is versatile in terms of what kinds of repairs it can help and has the best long-term effects when compared to alternative surgical options.

Chondral is a fancy word for cartilage.  Cartilage is a protective layer of rubbery tissue that covers the ends of bones to prevent rubbing.  There are two important layers of cartilage in the knee- one layer of articular cartilage that covers the end of each leg bone and your knee meniscus, which resemble rubbery washers that sit on top of the articular cartilage.  Both of these can be damaged from trauma (like a side blow to the knee or excessive twisting forces) or they can degrade over time from normal wear and tear. Sometimes, due to abnormal forces across the knee joint, or excessive use with improper form, these pieces of tissue rub and tear earlier in life.  This often happens to athletes who perform the same repetitive movements again and again or in athletes with high impact activities. In addition, if there are any muscle imbalances the knee joint moves at the less than optimal angle speeding up the wear and tear on the cartilage.  This breakdown in the cartilage causes swelling at the knee, pain, and interferes with a person’s ability to perform their sport or typical functional tasks of life. 

Cartilage does not have a good blood supply which means that it does not heal well.  What’s more, it has no nerve endings so you do not really realize there is a problem until damage is done. Chondral degradation is graded on a scale from one to five, with five being the worst. Repair options hinge on the size and location of the tear as well as the goals of the patient.

Smaller lesions (2cm^2 to <10 cm^2) or deep tears. Bigger tears are treated by either OCA or by an autologous chondrocyte implantation (ACI). An ACI procedure involves harvesting the cartilage cells and growing them outside the body and then planting them in the effected area. It is worth noting, however, that an OCA is the only back up procedure for a failed ACI. Authors of this review found that an OCA is less invasive (only one procedure), is more versatile, and has better long-term outcomes than an ACI.

The OCA procedure has become refined with time. The cartilage donation must be collected within 24 hours of the person passing away and is taken from people with healthy knees. The tissue is screened for a host of diseases. This process takes anywhere from 14 to 28 days, during which the cartilage is kept at body temperature, its ideal environment. The cartilage is then selected based on a size and location match, as there is a very minimal risk of tissue rejection since there is little to no immune response in cartilage. If the tear is deep and a bone graft is also required then the risk of rejection is only slightly higher.

An OCA procedure includes several different techniques depending on the type of tear. The most common technique is called a plug, where the chunk of torn cartilage, and perhaps bone, is removed and the new piece of cartilage is fitted perfectly in its place with as tight of a fit as possible. If the fit is not completely snug the surgeon can fasten it in using dissolvable materials or tiny hardware that will not disturb the knee function.

Rehabilitation after the surgery is broken into three phases. The first phase is a period of rest to allow the tissue to heal, with the amount of use of the leg depending on the type of repair. Typically phase one lasts six weeks. Phase two is from week six to twelve and involves return to daily activities, strengthening, and full motion of the knee. Phase three is from three months on and involves full return to sport with the guidance of a physical therapist. From six months up to one year after surgery repetitive high impact activities should be avoided.

Long-term outcomes for OCA procedures are promising with the greatest percentage of success in a younger, active population with traumatic onset of cartilage damage less than one year prior to surgery. That being said, however, the numbers are also promising for the non-traumatic middle-aged population with tears greater than 2cm. Authors suggest that an OCA become the standard practice for larger tears of these populations.

Every five years or so the American Academy of Orthopedic Surgeons (AAOS), along with a cohort of other professions (like physicians and physical therapists) publish a guideline to treat certain conditions based on the latest and greatest evidence.  These guidelines offer a quick look into what’s proven to work, what does not work, and what still needs to be further investigated.  

Below are the items that the AAOS recommends for the most up to date treatment of knee arthritis.

People with knee arthritis should:
1. Routinely take part in a strengthening program, neuromuscular education (or using techniques to restore balance, improve coordination and fine tune awareness of where your leg is in space), perform low-impact aerobic exercises, and keep physically fit to national standards in regards to heart health and body weight.  
2. Maintain a body mass index (BMI) of less than 25.
3. Use nonsteroidal anti-inflammatory drugs (oral or topical) or tramadol to help with symptom management.

The following are NOT recommended for treatment of knee arthritis:
1. Acupuncture
2. Lateral wedge insoles are not supported in the literature.  This being said, however the recommendation is moderate and patient preference should be kept in mind.
3. The use of glucosamine and chondroitin.
4. The injection of hyaluronic acid into the knee joint.
5. Performing an arthroscopy with lavage and/or debridement in which the fluid of the knee joint is removed, the joint is washed, and any loose bodies or debris are removed.
6. The use of needle lavage where saline is injected into the joint and then removed in attempts to wash the joint and remove inflammatory factors and debris.
7. The use of free-floating (not cemented or screwed into place) interpositional devices in the inner knee compartment to alleviate pain and mimic meniscus function.  (This was a general consensus recommendation due to the lack of research available for these devices.)

Evidence is inconclusive for the following due to either lack of available evidence or inconsistencies in the studies that have occurred.  
Practitioners should be on the lookout for future evidence, but in the meantime decisions regarding their use should be influenced by their clinical judgment and patient preference.
1. The use of physical agents, such as electrical stimulation and ultrasound.  
2. Manual therapy.
3. Valgus knee brace (to unload the inner knee compartment).
4. The use of acetaminophen, opioids, or pain patches.
5. The use of injections into the knee joint of corticosteroid.
6. The use of growth factor injections and/or platelet rich plasma.
7. A valgus-producing proximal tibial osteotomy, or bone shaving that changes the direction of forces across the knee joint to relieve pressure at the inner knee.

As the evidence changes and our knowledge evolves, it is good to keep the AAOS standards in mind and to be on the look out for future recommendations.

Anterior cruciate ligament injuries are one of the more common injuries impacting young athletes. The impacts are not only physical in nature but have psychological components and add financial stress as well. As the prevalence of youth participating in sports has increased, ACL injuries have also increased. Some research estimates that return to sport after ACL rupture and repair is as low as fifty per cent in young athletes, while epidemiological studies estimate that females are four to six times more likely to suffer and ACL injury compared to their male counterparts. For these reasons, a considerable amount of attention has been paid to programs aimed to decrease ACL injury rate, particularly in the female athlete.

Research supports the effectiveness of many ACL injury prevention programs that have been developed in the past decade. These programs typically involve an altered warm up and inclusion of certain fitness drills in practice that include core work, stretches, plyometrics, strengthening and sport-specific agility drills. The end goal is to optimize muscle balance and improve the athletes biomechanics, particularly with jumping and cutting type movements that typically stress the ACL. Aside from these ACL injury prevention programs, more recently certain researchers have recommended screening programs to identify the young athletes that may be at higher risk for ACL injury. Medical screening tools ideally are designed to be sensitive enough to identify only the high risk individuals so that interventions can target this population rather than those who do not need the intervention, thus saving money. With an effective ACL injury screening tool, the athletes that are at highest risk can then participate in an injury prevention program, rather than having all of the athletes in the program, again with a goal of saving time and money.

As finances for youth and college sports can be limited and injury rates continue to rise with increased sport participation, it is important to find the most cost-effective program to implement that also provides the best results. It necessary to take into consideration the cost and accuracy of a screening tool, as the purpose of the screen is to identify those at risk. Similarly it is important to make sure that the intervention is targeting the correct population and is effective in making the modifications desired. Though research supports the sensitivity of screening tools as being effective in predicting ACL injury risk, these screening tests require extensive set-up, expensive motion analysis video equipment, and a skilled tester. Typically the athlete will run through a battery of jump and landing tests to determine the knee abduction moment. Even if a coach is educated on what to look for with a screening test and is able to use a simple camera, the time and cost required to conduct the testing exceeds the benefit.

The incidence of ACL injuries in youth is high enough that a screening tool is just not warranted. The high incidence rate equates to the fact that most athletes will benefit from an ACL injury prevention program, which are very inexpensive and highly effective as supported by research. On average, such programs decrease the incidence of ACL injury from three per cent to one per cent in a single season, saving on average $100 per player per season in expenses related to such injuries. Universal ACL injury prevention programs for young athletes, male and female, are a cost effective strategy for reducing the physical, psychological and financial burden of ACL pathology.

Physicians are changing how they manage meniscal tears, according to a recent study that reviewed treatment methods over a seven year period.  Often when new evidence shows a better way to treat a problem it takes years for surgeons to alter their practice methods.  This study concluded that surgeons have changed their treatment to reflect the most up to date practice. The authors attribute the change in treatment to new evidence and changes in physician education regarding effective treatment. This is good news for the prevention of knee arthritis.

The meniscus is a c-shaped piece of cartilage that is found on both sides of the knee joint sandwiched between the ends of your bones.  It serves as a barrier between the leg bones, helps to redistribute twisting forces and decreases the wear and tear on the underlying cartilage covering the end of the bones.

The meniscus may be torn either by degeneration or by acute trauma.  Sometimes from a blow to the knee both the meniscus and  the anterior cruciate ligament (ACL) tear.  A tear typically causes knee locking and catching, swelling, and pain.  In the past, the standard treatment for a torn meniscus was removal, or meniscectomy.  However, because recent studies have shown 60 per cent of menisectomies result in osteoarthritis (or the wearing away of the cartilage covering the end of the bones), physician education has changed to emphasize preserving the meniscus.  The attempt to preserve the meniscus is called a meniscus repair and involves suturing the structure back together as best possible.  

Review of over two billion patient records from 2005-2011 showed an increase of 11.4 per cent in the number of meniscal repairs, with young males and patients under 25 years old having the greatest increase in meniscal repair surgeries. Additionally, there was a 48.3 per cent increase in ACL reconstruction in conjunction with meniscus repairs. This data suggests that physicians are changing their method of treatment of meniscus tears to reflect their training and are repairing meniscus when able instead of simply removing the tissue.

While wisdom, self-confidence, and the ability to go easy oneself often increases with age, the physical body is not always so kind. As Dolly Parton once said, Time marches on and sooner or later you realize it is marching across your face. But time also has a way of creating degenerative changes in other places as well, such as the knee.

For example, tears of the meniscus (cartilage in the knee) are more common in late middle age. The posterior horn of the medial meniscus is especially likely to develop tears as we get older. The medial meniscus is the portion of the cartilage along the inside of the knee joint (closest to the other knee). The posterior horn is located on the back half of the meniscus.

Horizontal meniscal tears (from side to side dividing the posterior horn in two parts) are the topic of this study from the Center for Joint Disease in Korea. The goal was to see how patients with degenerative horizontal tears of the posterior horn of the medial meniscus responded to surgical treatment versus conservative care (strengthening program).

Treatment choice for this type of meniscal tear is not always easy. These are difficult tears because they can go deep into the joint capsule. Removing part or the entire meniscus can result in ongoing painful symptoms. A partial meniscectomy eventually leads to a second surgery to remove the entire meniscus (total meniscectomy). Studies also show that partial or total meniscectomy can lead to early arthritis.

All patients included (total of 102) in this study were between the ages of 43 and 62 and experiencing intense knee pain. Mechanical symptoms such as clicking and/or popping were also reported by most of the patients. Two groups were followed for two years after treatment: the surgical (meniscectomy) group and the strengthening (nonoperative) group. Patients were assigned to their group using random selection. There was a four-to-one ratio of women to men (81 women and 21 men).

The exercise group was supervised by a physical therapist as they worked on muscle strength, flexibility, and endurance. Details of the eight-week exercise program (including a home exercise portion) were provided. For the surgical group, arthroscopy was used to remove frayed tissue and smooth the joint surface (partial meniscectomy). One orthopedic surgeon performed all of the procedures. No one had a complete meniscectomy. Everyone participated in the same exercise program as the nonoperative group but without the benefit of a physical therapist’s supervision.

A variety of measurements were used to compare results including pain, knee motion, activity, and patient satisfaction. The statistical analysis showed no difference in outcomes between the two groups. Pain relief, improved function, and very satisfied patients were the final results for both groups. Only a small number of patients in both groups continued to report painful symptoms at the final check-up.

In summary, this study provides evidence that horizontal meniscal tears can be treated successfully with a nonoperative approach. The tear tends to remain stable and no further treatment is required. Previous routine management with arthroscopic partial meniscectomy may not be needed after all. The authors comment that the symptoms associated with this type of degenerative change in the meniscus may get better in time no matter how it is treated. Further study is required to compare treatment groups with non-treatment groups to know for sure.

Arthroscopic examination and surgery of the knee have become mainstays in the diagnosis and treatment of knee problems. Yet every knee is slightly different in shape and the position of vital structures (e.g., blood vessels and nerves may vary from person to person). Depending on the position of the knee and the portal (opening) used, the surgeon can be challenged by the smallest anatomic difference.

And living tissue is dynamic, not static. In other words, these important structures can move during the procedure. That factor alone can increase the risk of damage or injury caused by the arthroscopic technique. Only a thin layer of fat separates the popliteal artery along the back of the knee from the thin posterior capsule. This is another reason why the risk of damage is high during posterior arthroscopic approaches to the knee. Sometimes the surgeon must change the knee position during the procedure. Saline fluid flows through the joint during the procedure and can also push soft tissue structures away from their normal positions.

And more posterior arthroscopic procedures are being done now as the surgical techniques and tools have improved over time. For example, repair and reconstruction of the posterior cruciate ligament can be done using a posterior arthroscopic portal. Likewise, removing loose fragments of cartilage, repairs of avulsion fractures of ligaments, synovectomies, and repairing tears of the posterior horn of the menisci can be done with this posterior technique.

In this study, experienced orthopedic surgeons from France studied 17 cadaveric knees (preserved after death). Donors were both men and women between the ages of 72 and 82 at the time of death. The surgeons specifically looked at the location of nerve and artery structures in relation to standard portals (places where surgeons routinely insert the long needle-like scope).

One of the positions used most often is 90 degrees of knee flexion. The scope can be placed in a posterolateral, posteromedial, or transseptal position. Posterolateral refers to the back and outside location of the knee. Posteromedial scope placement comes in from the back and inside edge (closest to the other knee).

Transseptal arthroscopic placement describes the passing of the scope from the posteromedial portal through the posterior septum of the knee. The posterior septum is an anatomical structure in the back of the knee. It divides the posterior compartment of the knee into two parts: lateral and medial. It is in the posterior compartment where some of the blood vessels and nerves are located.

There is some thought that the risk of injury to the popliteal artery and peroneal nerve might be less when the knee is bent more than 90 degrees. Likewise, it’s possible the risk of damage could be greater the straighter the knee during the arthroscopic procedure. Let’s see what they found out in this study.

After performing each arthroscopic procedure, the knees were opened up (with the arthroscopic needles still in place). Distances from the needles to nerves and blood vessels were measured by two independent surgeons who did not perform the arthroscopic procedures. The measurements were taken with the knees in three different positions (30, 90, and 120 degrees of flexion). Exact placement of the scopes (21-gauge needle, No. 11 blade, and cannula) and details of measurements were published in this study for those who are interested.

The conclusions reached by these researchers are that inserting arthroscopic needles into the back of the knee can be done safely when the knee is bent 90 degrees. As suspected, using the straighter knee position (only 30 degrees of flexion) is not advised because of the risk of damaging the peroneal nerve in that area.

Greater flexion (120 degrees) is considered safe only for the posteromedial and transseptal approaches (not for the posterolateral approach). Transseptal portals can be used safely with the knee flexed between 90 and 120 degrees. The authors note that other studies have shown the use of MRIs before surgery can aid in preventing injuries during posterior knee arthroscopic procedures.

Orthopedic surgeons from the United Kingdom have proven for themselves that cementless unicompartmental knee replacements are as good (if not better) than cemented implants of the same type. They conducted this preliminary study using the Oxford unicompartmental knee replacement. The study was partially funded by the company (Biomet) that makes these implants.

The results were so positive for the cementless version, they say that they only use this type of implant now when unicompartmental replacement is needed. In fact, they have already studied 1,000 patients for a year who received the Oxford cementless unicompartmental knee replacement. Results of that study will be published in the near future.

What makes the cementless implant better than the cemented device? And how do we know this is true? These are the two questions the researchers answered in this article. They compared two groups of patients: 32 who received the cemented Oxford unicompartmental knee replacement and 30 others who were implanted with the cementless Oxford device. All of the procedures were done using a minimally invasive surgical technique.

Participants in the study were followed for five years using fluoroscopic (real-time) X-rays. The advantage of this type of imaging study is that the X-ray beam can be focused on the underside of the implant. This gives the surgeon a better view of the bone-to-implant interface (where the implant sits against the bone). This type of imaging study is helpful since the most common cause of revision surgery after unicompartmental knee replacement is loosening of the implant without infection (called aseptic loosening).

Other measures of outcome included functional and activity scores on three valid, reliable tools: the Oxford Knee Score, the Tegner Activity Score, and the Knee Society Scores. These questionnaires are filled out by the patient providing information about symptoms (pain, swelling, clicking, locking, knee giving out), function (ability to kneel, squat, stand up from a chair, go up and down stairs), and activities of daily living (shopping, cooking, bathing, driving).

Results showed that after five years, there was no significant difference between the two groups based on most of the functional outcome measures. The one exception was the Knee Society Functional Score, which was significantly better for the cementless group. Overall, everyone in both groups improved significantly from before surgery to after surgery.

The major difference of note was seen in the X-rays. The cementless group had better fixation with fewer cases of aseptic (without infection) loosening. The end-result was fewer revision surgeries for the cementless group. In fact, even the cementless implants that were not placed with perfect alignment were still in place and working well without symptoms or problems for the affected patients.

The authors concluded that the use of the cementless unicompartmental knee replacement has a lower failure rate compared with the cemented version of this implant. They point out the advantages of the cementless device as: 1) shorter surgical time, 2) simpler surgical procedure, 3) more forgiving when a less than optimal position of the implant is achieved, and 4) no complications caused by cement.

Problems associated with cemented implants (from the cement itself) include tightening tissues from excess cement oozing into the nearby soft tissue structures, loose fragments of cement causing pain and mechanical problems, and excess wear and failure of components requiring additional surgery. These observations and the better results for the cementless group provide the evidence needed for these surgeons to shift to the exclusive use of cementless fixation for unicompartmental knee replacements.

The posterior cruciate ligament (PCL) is one of two ligaments that criss-cross inside the knee joint to hold the two leg bones (femur and tibia) together. The second (more commonly injured) ligament is the anterior cruciate ligament (ACL). These two ligaments provide stability to the knee joint while still allowing motion.

There is interest in knowing if non-surgical treatment has a higher rate of osteoarthritis later in life. A second question is whether surgery to reconstruct the PCL yields better results (possibly even preventing osteoarthritis). Currently, surgeons are doing more surgeries to restore normal PCL stability but there is insufficient evidence to support this treatment approach. This is especially true when the PCL is the only structure involved (i.e., no other soft tissue injuries are present).

Results of nonoperative treatment of PCL injuries is the subject of a long-term study from the Shelbourne Knee Center in Indiana. This is one of only a handful of studies following patients with acute, isolated PCL ruptures. And it is unique in that the patients were followed a minimum of 10 years after the injury (up to 21 years for some patients).

Everyone in the study had been in a previous study by the same authors. This report is from ongoing follow-up with these same patients who were treated for their isolated PCL injury with a home rehabilitation (exercise) program.

X-rays were used to look for signs of osteoarthritis (e.g., swelling, joint space narrowing, bone spurs). Leg strength, grade (severity) of PCL laxity (looseness), knee range-of-motion, and function were also assessed at each point of follow-up. All measurements were compared with the patient’s other (uninjured) knee.

As it turned out, there wasn’t a higher rate of osteoarthritis (OA) following nonoperative treatment of isolated PCL tears. Patients who had any OA had the same amount from side to side. This suggests that regardless of the PCL injury, they would have developed some (mild to moderate) OA over time. In fact, some of the patients who did have surgery to reconstruct the PCL had worse OA compared with patients who had the conservative care.

The authors concluded that “if PCL reconstruction is being performed to prevent OA, this goal has not been met.” The amount of knee arthritis that develops years after an isolated PCL injury is actually very low. They recommend caution in doing surgery to reconstruct PCL injuries — even when patients are having acute flare-ups of painful symptoms.

Other studies show that whereas PCL surgery can improve knee joint stability, the PCL is not returned to “normal.” Complications of surgery for PCL injuries include knee stiffness, loss of knee motion, infection, and problems with hardware. As the patients in this study demonstrated, full knee motion, good muscle strength, and good function can be achieved with a nonoperative approach to this problem. It is also very possible to remain active (even in sports requiring jumping and pivoting) years after the initial injury without surgery.

Patients with painful arthritic knees may wrestle with the decision to have them replaced one at a time (two separate surgeries and rehab) or both at the same time. The proposed advantages of the simultaneous bilateral knee replacements are decreased costs and shorter recovery time. The results (or outcomes) are just as good between the staged (one-at-a-time) approach and the simultaneous (both done together) method.

Researchers at the University of North Carolina (Charlotte) and the OrthoCarolina Research Institute (also in Charlotte, North Carolina) took on the task of doing a cost-utility and cost-effectiveness study. They used the Nationwide Inpatient Sample data to compare the results of 24,574 cases of simultaneous knee replacements with 382,496 patients who had unilateral (one-at-a-time) procedures.

Measures used to compare the outcomes included perioperative complications (minor, major, and mortality), hospital costs, rehabilitation costs, anesthesia costs, and health utilities (visits to the surgeon, primary care physician, physical therapist). Most striking was the difference in overall costs: $43,401 for simultaneous procedures compared to $72,233 for staged procedures (almost double).

Previous studies have reported more complications with simultaneous knee replacements compared with staged procedures. Problems such as gastrointestinal complications, blood clots, heart attacks, and even death can occur. But this study found no difference in the overall rate of complications between these two approaches.

In fact, for minor, major and in-hospital mortality (death), simultaneous procedures had lover complication rates. For example, the major complication rate for the staged group was 2.36 per cent compared with 1.49 per cent among the simultaneous group. The minor complication rate was 8.98 per cent (staged group) versus 6.84 per cent (simultaneous group).

In summary, with almost six billion dollars being spent on total knee replacements each year in the United States, this cost analysis shows the economic benefit of simultaneous procedures. Replacing both knee joints at the same time is both safe and effective. Data analyzed provides quality comparative evidence that can guide physicians and patients when faced with this decision.

This information may be helpful to the five million adults who suffer from severe knee osteoarthritis who are looking for improved quality of life and function. Although there is a risk of increased complications with simultaneous procedures, exposure to only one anesthetic, decreased recovery time, shorter hospital stay, and lower costs may outweigh the risk. Older adults with serious health problems may not qualify as candidates for the simultaneous replacement of both knees.

Researchers at the University of Michigan (Ann Arbor) have performed a systematic review and meta-analysis on ways to prevent anterior cruciate ligament (ACL) injuries. This is important because so many of today’s sports injuries affect the ACL. Players can be sidelined for months (sometimes longer). In fact, it is estimated that one-quarter of a million ACL injuries occur each year in the United States.

What can be done to prevent these injuries? That is the question posed by many health care and sports professionals. Using a combined systematic review (searching the literature for all studies on the topic) and performing a meta-analysis (combining data together from many studies) is a good way to “mine the data” so-to-speak. The results of this current study provide us with evidence-based recommendations.

Before looking at the specific recommendations, it’s important to understand why these are so important. It’s not just that so many people experience ACL injuries. The fact is that many of these injuries are accompanied by damage to the surrounding tissues as well.

The result is a weak knee that is at risk for future injuries and even the development of early arthritis. Preventing ACL injuries not only protects people from pain and suffering, it can also protect the pocket book for the individual and for society.

Presently, there is a wide range of risk factors that might contribute to ACL injuries. Addressing any of these factors that can be modified (changed) may provide some protection. For example, muscular strength, alignment, and flexibility are modifiable factors. The use of high-risk positions and/or techniques during sports play and lack of aerobic conditioning are additional modifiable factors that can be analyzed and modified.

Improving any one or all of these features may reduce the anatomic risk of ACL injuries. Age, genetics, and hormonal factors are nonmodifiable and may contribute to ACL injuries but cannot be changed to reduce the risk of knee injury.

Risk factors outside of the human body but still within our control are referred to as environmental factors. These include playing surface, use of footwear and/or braces, playing outside in bad weather.

For anyone interested in a details of the data collection and analysis from this study, the authors provided summary tables and descriptions of eligible studies included. The majority of evidence supported two directions for prevention of ACL injuries: neuromuscular re-education and educational interventions. It turns out that training programs to address the anatomic risk factors do, indeed, reduce the risk of ACL injuries by as much as 50 per cent.

Since the studies available did not analyze individual parts of training programs, this systematic review/meta-analysis was unable to pinpoint exactly which training interventions work the best. It does appear that more time spent in training and following the exercise programs carefully (compliance) provide a direct protective effect.

The authors of this study suggest future studies need to be more complete, of higher quality, and make comparisons between specific groups and specific prevention interventions. Comparing one treatment against no treatment is not as helpful in determining the best prevention approach as comparing one intervention to another.

With more and more adults getting knee replacements, analysts are taking a closer look at the costs versus benefits to the individual patient and to society. Society can include employers and insurance providers (payers). You are a part of “society” in this sense if you pay taxes that finance Medicare, a principal payer of many surgical procedures in older adults.

There are three ways to evaluate the “cost” of surgical versus conservative care for knee osteoarthritis: 1) direct costs, 2) indirect costs, and 3) quality of life measures. Direct costs include any and all medical expenses for any treatment provided.

Indirect costs refer to lost wages when the patient can no longer work full-time or can’t work at all and to disability payments paid out over time. Indirect costs to the employer occur due to employee absenteeism and lost productivity. Quality of life is measured based on patients’ perception of pain, motion (loss of motion), function (loss of function), and level of disability.

This is the first study to take a closer look at all costs associated with knee replacement versus conservative care (without surgery) for patients with end-stage (severe) osteoarthritis. They studied the U.S. population (ages 40 and older) who received a total knee replacement in the year 2009.

After reviewing and analyzing all the data collected, the research showed that there was a 12 billion dollar savings to society in one year (2009) for the 600,000 total knee replacements that were done. They concluded this represents a significant amount of money attributed to extra work years (and increased income) made possible by the surgery.

This information will be very helpful if and when insurance companies and other third party payers suggest finding ways to limit who qualifies for a total knee replacement (referred to as coverage restrictions). Likewise, if higher copayments are proposed, research like this comparing costs and estimating savings to society is very important.

Other factors affecting lifetime savings associated with total knee replacements include age and work status. Younger patients have longer to work and earn money. The study showed that patients in the youngest category (40 to 44 years old) could potentially (each) earn $174,364 more over a lifetime by having the surgery compared with the over 80 age group.

Older adults also had higher total medical costs regardless of whether they had surgery or were treated conservatively. But the cost of a total knee replacement for severe, limiting osteoarthritis in the older group was also offset by fewer health problems (heart attacks, strokes) compared with patients of the same age with equal joint disease who did not have knee replacement surgery.

In a parallel analysis, work status (receiving disability, retired, or working part- or full-time) was equally important in calculating cost to society. As you might expect, those individuals who continue working and earning income generate greater savings compared with patients who receive disability checks each month.

In conclusion, total knee replacements are cost-effective when viewed from a societal perspective. This study showed a positive net benefit to society in terms of cost savings for all age groups but especially those younger than 70 years old (and especially for adults in their early 40s). Quality of life is improved with surgery, which can also translate into dollars saved. Payers and policy makers should take this information into consideration when making plans to restrict access to knee replacement surgery.

The likelihood of a patient with a hamstring autograft having a deep infection after a hamstring autograft for the reconstruction of an anterior cruciate ligament (ACL) is 8.24 times higher than for someone receiving a bone-patellar tendon-bone (BPTB) autograft.

These are the results of a very large study from well-known and respected institution: Kaiser Permanente in California. Almost 11,000 patients from the Kaiser Permanente ACLR registry were involved in the study. These patients were treated in 41 different medical centers in six different geographical regions.

By reviewing the patient records, they were able to determine how many patients who had any kind of ACL graft surgery developed an infection afterwards.

The three main types of ACL grafts included: 1) hamstring autograft, 2) bone-patellar tendon-bone (BPTB) autograft, and 3) allograft. An autograft refers to tendon material taken from the patient and used to replace the torn ligament. An allograft is tendon graft material from a donor bank.

Although the overall infection rate was low (0.48 per cent), the fact that there was an obvious and statistically important difference between the autografts is very significant. In more simpler terms: graft choice does make a difference. There was no apparent difference in the infection rates between autografts and allografts. These findings support (agree with) other previous studies but now provide the added information of how often this happens.

Some additional details from the study also included the fact that there was a 2:1 ratio of deep (deep soft tissue or joint) to superficial (skin only) infections. The superficial infections occurred early on (first month after surgery). Deep infections were more likely to develop up to one year after the procedure.

Further analysis of all the data also revealed that in most cases, hamstring tendon grafts were the source of both superficial and deep infections. Staphylococcus bacteria accounted for half of the deep and three-fourths of the superficial infections. More obese patients (higher body mass index) had a greater chance of developing a superficial infection. The reason for this is unknown but may be linked with the need to remove more soft tissue in larger patients.

What is behind this observed increase in superficial and deep infections associated with hamstring tendon grafts used for ACL reconstruction? There may be more than one possible avenue for this occurence. The authors suggest the following possibilities: