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.