Update on Knee Osteonecrosis from the Johns Hopkins University

Sometimes orthopedic surgeons are faced with rare conditions they either haven’t seen before or have only treated a handful of times. Osteonecrosis of the knee is one of those problems. Osteonecrosis of the hip is a much more common problem. Developing diagnostic and treatment guidelines for rare conditions based on clinical experience isn’t always possible. That’s when physicians rely on articles like this one that provides a review of recently published literature.

Osteonecrosis is the death of bone tissue. There are three types of knee osteonecrosis: 1) spontaneous (occurs without a known cause), 2) post-arthroscopy (occurs after an arthroscopic procedure), and 3) secondary to some other condition such as lupus, use of steroids, or alcohol abuse.

Spontaneous osteonecrosis of the knee is also referred to as SPONK. It usually occurs in one compartment or section of the knee, while secondary osteonecrosis (caused by disease or medical therapy) affects more than one compartment. The bottom, round part of the femur (thighbone) called the femoral condyle is affected most of the time.

Spontaneous osteonecrosis usually occurs in patients older than 55 years, while secondary osteonecrosis can occur at any age. Women are affected by SPONK three times more often than men. The reason for this is unknown.

Osteonecrosis of the knee is rare after arthroscopy. It usually occurs when some form of heat such as laser or other thermal devices were used during the procedure. The patient starts to develop worse pain after arthroscopy than before. Knee swelling is a common feature of this problem.

MRIs are relied upon to identify and diagnose osteonecrosis of the knee. Bone scans are only reliable 56 per cent of the time. Osteonecrosis shows up on MRIs 100 per cent of the time. The main disadvantage of MRIs is the delay in findings after symptoms have started. Early on in the disease process, nothing unusual shows up on MRIs. The exact best timing for identifying this condition using MRIs remains unknown.

Once the condition has been diagnosed, then treatment begins. Everything is done to preserve the joint and prevent further breakdown of the bone. Early lesions can be treated conservatively (without surgery). The types of lesions that respond to nonoperative care have no low-density lines deep in the femoral condyles (as viewed on MRI scans) and no defects in the shape of the femoral condyles.

Patients are directed to avoid putting weight on the knee along with activity limitations. They must be patient as this protective process can take from three to eighteen months. Bone resorption may be stopped by the use of medications called bisphosphonates. Knee pain can be managed with analgesics (pain relievers). Treatment with bisphosphonates is fairly new and has not been proven effective for all patients yet. Further study of these drugs must be completed to guide the surgeon in knowing when and how to use bisphosphonates, as well as which patients would benefit the most.

Another newer drug treatment for knee osteonecrosis is tumor necrosis factor alpha (TNFA). This substance is injected right into the knee joint. Case reports show rapid (one-week later) improvement in pain and stiffness. Signs of healing are seen on MRIs after only one month.

But when the case is too far advanced or when nonoperative care doesn’t work, then surgery to repair the lesion may be needed. The type of surgery done depends on where the damage is located and how severe it is. The surgeon can drill holes in the bone, a procedure called core decompression. Debridement (scraping the damaged area) followed by bone grafting to replace the missing bone has also been tried.

Bone decompression combined with bone grafting may be one way to speed up healing and recovery. Biologic substitutes and tissue-engineered cartilage are two other proposed techniques for the surgical treatment of knee osteonecrosis. With tissue-engineered cartilage, the surgeon builds a scaffold with donated bone. The bone is cemented into interconnected pores. Then cartilage is used to repair the defect in the bone.

There haven’t been very many cases treated with these various techniques. So, which one works best and for what types of knee osteonecrosis are also unknown factors. The most information we have is on the outcomes using unicompartmental knee arthroplasty. In this procedure, the surgeon replaces just the half of the joint that’s been affected by the necrosis (rather than doing a full knee joint replacement).

Results reported from a limited number of studies report excellent results with this technique. Researchers consider the unilateral knee arthroplasty a very promising approach, but once again, more studies are needed to confirm these results and to see what happens in the long-run.

Right now, we only have limited information and understanding of what causes knee osteonecrosis and how to treat it. At the present time, research efforts are directed toward finding ways to preserve the joint, rather than replace it. Nonoperative treatments with new methods of tissue engineering may eventually provide a breakthrough in the treatment of this disease.