by William R. Olson, D.P.M.
In sports the knee is involved in a wider variety and larger number of injuries than any other area of the body. Both acute and overuse injuries are common. Together they account for about 15% of all sports injuries. In clinical practice, knee problems are even more ubiquitous, accounting for nearly 50% of all sports injury related patient visits. It is therefore apparent that an understanding of the functional anatomy of the structures which comprise the knee joint, as well of those that extrinsically affect or are affected by the function of the knee joint, is critical for any practitioner who is involved in the management of lower extremity overuse injuries.
A simple and straightforward way to think of the functional anatomy of the knee is to subdivide it into four quadrants: anterior, medial, posterior, and lateral. The oft-injured anterior portion of the knee is largely comprised of the distal extension of the quadriceps, the patella, and extending from the patella, the patellar tendon. With an axis of motion in the transverse and frontal planes, extension of the knee, occurring in the sagittal plane, is controlled by the quadriceps mechanism. During the contact phase of gait, the quadriceps function eccentrically to decelerate the forces of impact. Injuries of the quadriceps mechanism have multiple potential causes. Some of the more common include imbalance between the strength of the medial and lateral quadricep muscles (VMO and VLO). Also causing quadricep mechanism injuries are rotational forces at the knee which cause the vector of flexion to be directed medially or laterally to patellar tendon, or cause the patella itself abnormally in the patellar groove, most commonly along the lateral aspect. Training errors which place excessive demands upon the quadricep mechanicsm, such as excessive hill training, and activities which place significant retrograde loads upon the patella, such as those activities which require over 75 degrees of knee flexion, also predispose to patello-femoral injuries. Specific anterior knee injuries resulting from the aforementioned mechanicsms would include patellofemoral dysfunction (with or without chondromalacia), medial and lateral peripatellar retinaculitis, insertional quadricep tendinitis, and patellar tendinitis.
The medial aspect of the knee is comprised of three tendons, the sartorius, gracilis, and semi-tendinosis which combine to insert on the anterior medial aspect of the proximal tibia to form the pes anserinus. The pes works primarily as a flexor of the knee and stabilizer of the medial knee and also functions to rotate the leg internally. Deep to the tendons are the ligamentous medial stabilizer of the knee (the tibial collateral ligament) and deep to that, the medial capsule of the knee. At the articulation between the medial condyle of the femur and tibial plateau is found the medial meniscus. The menisci are C-shaped mesenchymal structures which function to buffer weight bearing, enhance shock absorption, and facilitate knee motion and stability in the compartment of the knee in which they are located. The posterior aspect of the knee can be thought of as being bounded medially by the semi-membranosis and laterally by the biceps femoris. Crossing the posterior aspect of the knee longitudianally is the most proximal aspect of the gastrocnemius muscle, and deep to that the small, flat, transversely oriented popliteus muscle. The hamstrings primarily function as a posterior stabilizer of the knee and a primary flexor of the knee joint, with the semi-membranosis having some function in internally rotating the leg, and the biceps femoris laterally rotating the leg. The popliteus has two primary functions: As an internal rotator of the tibia with respect to the femur and as a dynamic stabilizer of the knee assisting the quadriceps and posterior cruciate ligament and preventing anterior displacement of the femur with respect to the tibia. Finally, the lateral aspect of the knee is comprised primarily of the iliotibial band, lateral collateral ligaments, lateral capsule, and lateral meniscus. The iliotibial band also has a dual function, as it serves to prevent excessive internal rotation of the leg while also stabilizing the lateral aspect of the knee in response to excessive varus forces. Finally, the internal anatomy of the knee centers around the cruciate ligaments, the anterior cruciate functioning to prevent anterior displacement of the tibia on the femur and, as previously mentioned, the posterior cruciate preventing anterior displacement of the femur on the tibia.
An understanding of the dynamic functional anatomy of the structures comprising and crossing the knee joint is of critical importance if the practitioner is to appreciate the pathomechanics of overuse injuries of the knee and their subsequent management. Many overuse injuries of the knee are caused by excessive subtalar pronation motion during athletic activities. Due to the increased force with which the foot strikes the ground, and due to the natural running limb varus which occurs in running, there is an increased pronation moment (ie. torque) placed on the subtalar joint during the first half of the support phase of running. The increased subtalar joint pronation moment causes the subtalar joint to pronate at a faster rate and with larger magnitudes than seen in walking activities. Since subtalar joint pronation acceleration, velocity and magnitude lead to increased tibial internal rotation acceleration, velocity and magnitude. This excessive tibial internal rotation moment is transmitted to the knee to cause internal rotation and adduction of the knee joint. If increased internal rotation of the knee causes excessive overstretching or twisting of the ligaments or tendons or increased shearing forces on the bursa surrounding the knee, then ligament strain, tendinitis or bursitis can occur. Common knee injuries caused by these mechanisms include medial peripatellar retinaculitis, pes anserinus bursitis, iliotibial band tendinitis (as the iliotibial band attempts to restrict excessive internal rotation of the tibia), and patellofemoral dysfunction (as excessive internal rotation of the tibia may also cause lateral mistracking of the patella in the patellar groove of the femur).
If the subtalar joint has excessive pronation moment being placed upon it throughout the support phase of running, then likewise the tibia will have a strong internal rotation moment placed upon it by its osseous and soft tissue connections to the foot. Since during the latter half of the support phase the support limb is moving posteriorly, and the contralateral limb is moving anteriorly, a strong external rotation moment on the femur of the supporting limb is produced. If the foot pronation causes an internal tibial rotation moment at the same time that the contralateral pelvic advancement produces an external rotation moment on the femur, then the knee will be placed under significant axial torsion loads. Over time, these excessive torsional loads at the knee may lead to accummulative trauma to the cartilage and capsule of the knee joint. Motion of the subtalar joint not only affects the knee with regard to rotation at the knee joint, but frontal plane loads selectively affecting the medial and lateral compartment of the knee joint are also appreciated as a result of subtalar joint mechanics. Excessive subtalar pronation typically places a relative increase in load upon the lateral compartment of the knee as compared to the medial compartment and, conversely, excessive subtalar joint supination selectively loads the medial compartment of the knee as compared to the lateral compartment.
Management of overuse injuries of the knee begins with management of the specific components which constitute the overuse periods. This may include alleviate of training errors, management of abnormal lower extremity biomechanical forces, and structure-specific strength and flexibility rehabilitation. While "resting" the injured structure through appropriate biomechanic management (such as control of excessive internal tibial rotation through control of excessive subtalar pronation in the runner with iliotibial band tendinitis) and appropriate activity modification, key to the management of overuse knee injuries is comprehensive rehabilitation. It must be noted that any period of disability or inactivity -- even a few days -- will result in weakness of the thigh musculature. With disability quadricep weakness is more profound than hamstring weakness. Most commonly, quadricep weakness/atrophy is most readily apparent in the vastus medialis portion of the muscle. Quadricep weakness is a key component and, in many cases, has been shown to be the single most common cause of patellofemoral symptoms (along with abnormal lower extremity biomechanics), recurrent effusions and functional instability. The rehabilitation program begins with a management of inflammation through medication, ice, physical therapy modalities, activity modification, and, on occasion, intra-articular use of corticosteroids. As soon as sufficient reduction of inflammation occurs, strength rehabilitation is initiated through isometric, then isotonic, and finally isokinetic strengthening programs. Typically, a form of the program described by DeLorme is followed, with emphasis placed on endurance without sacrifice of flexibility.
Through the accurate assessment of the pathologic forces contributing to the overuse knee injury, specifically the errors in training contributing to the injury, the intrinsic and extrinsic biomechanic forces placing excessive stress upon selective structures, and through the management of the weakness which invariably results from the antecedent injury, the vast majority of overuse knee injuries will respond to the conservative care of rest, the appropriate management of abnormal biomechanic forces, and rehabilitation.
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