The knee owes its great range of motion to the shallow socket of the upper tibia which meets with the ball–like end of the femur. The stability of the knee is provided by its restraining ligaments, most notably the medial and lateral collateral ligaments which prevent angulation of the joint medially and laterally, and the anterior and posterior cruciate ligaments, which prevent anterior and posterior displacement, respectively, of the tibia on the femur. Since the femur and tibia are not closely matched in shape, the knee cartilages, or menisci, are important in providing good contact between the bones. Any of the ligaments or cartilages can be injured, leading to instability, pain, and, eventually, arthritis.

Anterior Cruciate Ligament Injury
The anterior cruciate ligament is often injured in athletic and other types of trauma. When the ligament is ruptured, the knee is often unstable, with the anterior displacement of the tibia on the femur causing buckling episodes and can lead to meniscal injury and post–traumatic arthritis. If the knee is not satisfactorily stabilized by muscular conditioning and bracing, substitution of the ligament with tendon grafts taken from the patient's knee is undertaken. Ligament reconstruction done arthroscopically shortens the recovery period, but a return to strenuous or athletic activity must await several months of healing and rehabilitation to minimize the chance or re–injury.

Meniscus Injury
The menisci of the knee are rubbery, fibro–cartilaginous cushions which provide improved stability and gliding of the knee joint by virtue of their unique shape. Injuries of the knee can cause tearing of these cartilages. Only tears near the outer attachments to the tibia can heal if repaired. Tears in the other areas of the menisci, if they produce symptoms of pain, swelling, catching, or buckling, require arthroscopic removal of the unstable portion of the menisci. Often, the repairable meniscal injuries are found together with anterior cruciate ligament tears, which will also need to be reconstructed for the menisci to heal and avoid re–injury.

Posterior Tibial Rupture
The posterior tibial tendon runs behind the inside bump on the ankle (the medial malleolus), across the instep, and into the bottom of the foot. The tendon is important in supporting the arch of the foot and helps turn the foot inward during walking.

Causes of Posterior Tibial Rupture
Problems with the posterior tibial tendon seem to occur in stages. Initially, irritation of the outer covering of the tendon called the paratendon causes a paratendonitis. This simply indicates that there is inflammation around the tendon as it runs through the tunnel behind the medial malleolus.

As we age, a tendon is subjected to degeneration within the substance of the tendon. The term degeneration means that wear and tear occurs in the tendon over time and leads to a situation where the tendon is weaker than normal. Degeneration in a tendon usually shows up as a loss of the normal arrangement of the fibers of the tendon. Tendons are made up of strands of a material called collagen (think of a tendon as similar to a nylon rope and the strands of collagen as the nylon strands). Some individual strands of the tendon become jumbled due to the degeneration, other fibers break, and the tendon loses strength. The healing process in the tendon causes the tendon to become thickened as scar tissue tries to repair the tendon. This process can continue to the extent that a nodule forms within the tendon. This condition is called tendonosis. The area of tendonosis in the tendon is weaker than normal tendon. The weakened, degenerative tendon sets the stage for the possibility of actual rupture of the tendon.

Symptoms of Posterior Tibial Rupture
The symptoms of tendonitis of the posterior tibial tendon include pain in the instep area of the foot and swelling along the course of the tendon. In some cases, the tendon may actually rupture, due to weakening of the tendon by the inflammatory process. Rupture of the tendon leads to a fairly pronounced flatfoot deformity that is easily recognizable.