IN BRIEF: The knee joint meniscus cartilage is especially susceptible to injury in cutting and impact sports, particularly skiing. Evolution in the understanding and treatment of this critical structure has permitted athletes to make an early return to athletics after injury, permitted surgeons to repair and save the meniscal cartilage, and permitted development of exercise regimens that can protect the knee. This article describes the mechanisms of injury to this structure and the means available to protect it.

"Doc, I fell and twisted my knee. I heard a pop. It hurt a bit, but I was able to get up and ski down. The knee swelled that night, and it doesn't quite feel right now." This most common skier complaint describes tearing of the knee joint meniscal cartilage and, with the cruciate ligament, is the most common cause for immediate swelling of the knee after a twisting fall while skiing. The injury is serious, often preventable, and fortunately treatable.

The injury, tearing of the strong fibrous tissue (meniscus cartilage) that forms the pads between the thigh bone (the femur) and the shin bone, destroys the perfect fit that the knee joint requires to be both stable and smooth. Since the end of the femur is curved and the top of the tibia is somewhat flat (see diagram), the empty space must be filled by a tissue that can absorb the tremendous forces (up to ten times body weight when a skier lands from a jump) seen by the joint surface, and yet be durable enough to tolerate over two million steps the athlete takes in a year. When the tissue is torn, some bleeding occurs into the joint, usually, but not always causing swelling and pain. If the injury is ignored, the blood is slowly absorbed. Clicking, popping, locking of the knee joint and soreness with activities are the usual symptoms the athlete notices in the months after a meniscal cartilage injury.

Once thought of as a needless remnant of tissue in the knee joint, the torn meniscus was frequently removed by surgery. Over the past few years, it has become clear that the meniscus plays a crucial role in joint stability, lubrication, and force transmission. Under a weight bearing load, the meniscus maintains the balanced position of the femur on the tibia and distributes the compressive forces by increasing the surface contact area, thereby decreasing the average stress two to three times. The surface stress becomes smaller, the load bearing area wider, the compliance higher, and the stiffness of the joint lower with the menisci in place. Additionally, the menisci interact with the joint fluid to produce a coefficient of friction that is five times as slick as ice on ice.

Loss of the protective stabilizing meniscus leads to increased joint laxity or abnormal motions that lead to joint instability. The excessive motion and narrowed contact area promotes early arthritic changes. The changes noted by X-ray include ridge formation, narrowing of the joint space, and flattening of the curved femoral condyle. At the cellular level, there is initially a loss of cells from the superficial layer of the articular cartilage followed by cartilage splitting, subsequent thinning and erosion occurs, and finally protrusion of the underlying raw bone. The earliest arthritic changes have been noted three weeks after loss of the entire meniscus.

Since loss of the meniscus leads to early arthritic changes, great efforts are now applied toward saving as much as possible of a torn meniscus and, whenever possible, repairing the torn menisci with sutures.

The torn meniscus represents a mechanical block to normal joint motion. If the meniscus is torn in the peripheral thirty percent, the blood supply is adequate to permit healing if the tear is repaired in a stable fashion. If the tear is in the inner zone, with less blood supply, the likelihood of a repair healing is diminished but not eliminated. Newer techniques to provide stable repairs and increased nutrition to this tissue have provided a high rate of healing. Due to the supreme importance of the meniscus in knee joint function as described above, every effort to repair torn menisci is now made at the time of arthroscopic surgery..

Replacement of the meniscus is also possible using an allograft donor meniscus. (see url: http://www.stoneclinic.com/menre.htm).

After meniscus surgery a conditioning program is commenced on the day after surgery. (see url: http://www.stoneclinic.com/meniscectomy_rehab.htm).

Protection against meniscal injury and from the arthritic changes after loss or injury to the meniscus can be best achieved by decreasing the impact loading the knee joint is exposed to. Joint forces are decreased by increasing the muscular conditioning of the leg; the more force that the muscles absorb, and the more they are able to protect the joint from abnormal rotations, the less damage the internal surfaces of the joint are exposed to. Specifically, for the skier, the muscular conditioning must be directed toward increasing the lower leg's ability to accept an eccentric load. Eccentric contractions are defined as force produced by the muscle as it elongates, as the quadriceps (thighs) muscle does with knee flexion (as opposed to shortens, as the quadriceps does with knee extension). The ability to accept an eccentric load is the first skill lost after a knee injury or after a period of immobilization or deconditioning, and is the-last to be regained in a general conditioning program. This combination of the most difficult muscle system to condition, and the most easily de-conditioned, explains, in part, why so many knee injuries persist, despite tremendous advances in boot and binding technology. Adaptation of the human body to the advancing technology still requires intelligent sport specific conditioning and plain old sweat.
Specific eccentric exercises and side-to-side positioning exercises are extremely effective at preparing the knee for the loading conditions seen while skiing. A preseason program should include flexibility, agility and strength workouts. Specifically for the knee a combination of core strengthening and concentric and eccentric exercises to include a daily five minute single stand one-third knee bend, a linebacker double stance five minute one-third knee bend, and a five minute side-to-side hop, with emphasis on slowly absorbing the force of the body weight as the knee bends. These simple focused exercises can be increased in intensity by adding an elastic resistance cord and can dramatically strengthen the lower limb musculature.
Protection from impact loading is also gained by using Sorbothane type inserts in running shoes and ski boots, by training on soft surfaces, and by biasing the conditioning program toward non-impact regimens, of which bicycling is by far the kindest to the knee.  
The conditioning program must be continued throughout the season, as there is overwhelming evidence that athletes who perform but do not train are quite susceptible to career ending injuries.

The menisci are complex structures playing a vital role in the knee joint. When injured, early surgical intervention can save the torn structures by suture repair. When lost a meniscus transplantation using a donor allograft can be performed.  Conditioning the knee to accept eccentric loads can prevent their injury and diminish the chances of arthritic changes after their loss. Understanding the role of the meniscus in joint stability, lubrication and force transmission should inspire the athlete to prepare and to prevent knee injuries by year round training.

REFERENCES

1. Favenesi, J.A., Shaffer, J.C., and Mow, V.C.: Biphasic mechanical properties of knee meniscus. Trans. Orthop. Res. Soc., 8:57, 1983.
2. Arnoczky, S.P. The meniscus. Injury and Repair of the Musculoskeletal Soft Tissues. Ed. Savio L-Y. Woo. American Academy of Orthopaedic Surgeons, June 1987.
3. Fairbank, T.J.: Knee joint changes after meniscectomy. J. Bone and Jt. Surg., 3OB: 664-670, 1948.
4. Cox, J.S., Nye, C.E., Schaeffer, W.W., and Woodstein, I.J.: The Degenerative Effect of Partial and Total Resection of the Medial Meniscus in Dogs. Clin. Orthop. 109:178, 1975.
5. Scott, G.A., jolly, B.L., Henning, C.E. Combined posterior incision and arthroscopic intra-articular repair of the meniscus. JBJS 68-A, 6, July 1986.
6. Stone, K.R., Walgenbach, A.W. Meniscal Allografting: The Three-Tunnel Technique. Arthroscopy: The Journal of Arthroscopic and Related Surgery, April 2003
7. Stone, K.R., Walgenbach, A.W., Turek, T., Freyer, A., Hill, M. Meniscus Allograft Survival in Patients with Moderate to Severe Unicompartmental Arthritis: A 2- to 7-Year Follow-up. The Journal of Arthroscopic and Related Surgery, March 2006