6 THE KNEE
Applied Anatomy
The knee is the largest synovial joint in the body. The knee joint is inherently unstable, because it is not constrained by the shape of its articulating bones. It consists of two tibiofemoral and one patellofemoral compartment. The tibiofemoral articulation is a condylar joint, whereas the patellofemoral articulation is a gliding joint (Figure 6-1). The proximal tibiofibular joint is a plain synovial articulation between the lateral tibial condyle and the fibular head. The tibiofibular joint capsule is much thicker anteriorly and is reinforced by the anterior and posterior tibiofibular ligaments. Slight movements occur at the joint with lower-limb rotation and with activities involving the ankle. The capsules and synovia of the knee and the proximal tibiofibular joints intercommunicate in about 10% of adults.
LOWER LIMB ALIGNMENT, PATELLOFEMORAL ARTICULATION, AND PATELLAR TRACKING
Normally, the center of the femoral head, the center of the knee joint, and the center of the ankle joint are aligned in the coronal plane (Figure 6-2). The adductor muscle mass normally produces the appearance of a relatively straight medial border from top to bottom in the lower extremity (Figure 6-3). Because the femoral neck offsets the femoral shaft away from the hip joint, the femoral shaft must meet the tibia at an angle (see Figure 6-2). This relationship has significant implications for the biomechanical functioning of the patellofemoral articulation. As the force of the quadriceps muscle contraction is transmitted through the tibial tubercle, at an angle to the quadriceps muscle pull, the patella experiences a laterally directed force. This force is resisted dynamically by the vastus medialis muscle (Figure 6-4), which is attached more distally to the patella than the vastus lateralis. The lateral femoral condyle projects more anteriorly than the medial condyle does, and this also helps to counteract lateral dislocation of the patella when the quadriceps contracts (see Figure 6-6A). The angle formed between the quadriceps muscle pull and the tibial shaft, known as the quadriceps angle or Q angle in a supine patient (see Figure 6-4), is normally between 8° and 14° in males and somewhat higher in females, although measurement error may be up to 5°, and there is disagreement regarding the upper limits of normal. The Q angle is measured between a line from the anterior superior iliac spine (ASIS) to the patellar midpoint and a line from the tibial tubercle through the patellar midpoint (see Figure 6-4). Weakness of the vastus medialis or a large Q angle is often associated with patellofemoral symptoms.
FIGURE 6-2 LOWER-LIMB ALIGNMENT.
(From Gross J, Fetto J, and Rosen E., eds.: Musculoskeletal Examination, 2nd ed. Malden, MA: Blackwell Publishing Company, 2002, page 293.)
When a person is standing erect, the knee is normally locked in extension, and no sustained quadriceps muscle contraction is required. Moreover, in full knee extension, the tibia rotates externally with respect to the femur, the so-called screw-home mechanism (Figure 6-5). Overextension and overrotation of the knee are prevented by the anterior cruciate, collateral, and oblique popliteal ligaments; an unexpected blow to the back of the knee causes the knee to buckle.
The knee is considered to be in the close-packed position during full extension, when the capsule and ligaments are maximally taut and the articular surfaces are compressed and maximally congruent. The open-packed position occurs when the knee is flexed. The three lower-extremity joints—hip, knee, and ankle—can be considered a kinetic chain. Open-chain movements occur when the femur is relatively stable and the tibia moves freely, whereas closed-chain movements involve femoral movement over a fixed tibia. Open- and closed-chain movements can result in different types of sports injuries.
KNEE LIGAMENTS AND SUPPORTING STRUCTURES
Although the main ligamentous structures about the knee (Figure 6-6) may be injured in isolation, knee-joint injuries often involve multiple ligaments, the joint capsule, and muscle insertions that act as static and dynamic knee-stabilizing structures (see Figure 6-5). In particular, the collateral and cruciate ligaments, posteromedial and posterolateral capsule, posterior oblique ligament, arcuate popliteus muscle complex, pes anserinus tendons, and iliotibial band represent the main static knee stabilizers (Figure 6-7). The hamstrings and quadriceps muscles serve as dynamic knee stabilizers by resisting anterior and posterior translation of the tibia on the femur, respectively. The fused tendons of the rectus femoris and vastis femoris (quadriceps tendon) insert into the upper patella, but some superficial fibers extend distally over the anterior patella to join the ligamentum patellae. Thinner bands from the sides of the patella attach to the anterior border of the tibial condyles to form the medial and lateral patellar retinacula. The gastrocnemius muscles make a more minor contribution to joint stability. The fabella, a sesamoid bone within the tendon of the lateral head of the gastrocnemius muscle, is present in approximately 10% to 20% of normal individuals. The fabella articulates on its anterior aspect with the posterior aspect of the lateral femoral condyle.
FIGURE 6-7 STABILIZING LIGAMENTS AND MUSCLES OF THE KNEE.
(From Gross J, Fetto J, and Rosen E, eds. Musculoskeletal Examination, 2nd ed. Malden, MA: Blackwell Publishing Company, 2002, page 369.)
Medial Collateral Ligament (MCL)
The medial (tibial) collateral ligament (MCL) consists of a deep and a superficial band. It attaches proximally to the medial femoral epicondyle immediately below the adductor tubercle and inserts distally into the medial tibial condyle (deep band) and into the medial surface of the tibia (superficial band or long band). The long, superficial band attaches to the tibia as a large fascial extension 7 to 10 cm below the joint line, deep to the pes anserinus tendons (Figure 6-8). The deep ligament band has attachments to the peripheral margin of the medial meniscus.
Lateral Collateral Ligament (LCL)
The relatively small-diameter lateral (fibular) collateral ligament extends from the lateral femoral epicondyle proximally to attach onto the fibular head distally (Figure 6-9).
Anterior Cruciate Ligament (ACL)
The anterior and posterior cruciate ligaments, so named for the position of their attachment to the tibia, are situated centrally between the two tibiofemoral articulations (Figure 6-10). The cruciates provide a strong mechanical tie between the femur and the tibia, providing the main resistance to sagittal displacement; they also assist the collateral ligaments in resisting lateral bending of the joint. The anterior cruciate ligament (ACL) provides strong resistance to anterior displacement and excessive internal rotation of the tibia on the femur. The ACL attaches distally on the tibia in a relatively large expanse just in front of and lateral to the tibial spine (intercondylar eminence). It spirals upward and laterally to attach onto the posteromedial corner of the lateral femoral condyle, posterior to the longitudinal axis of the femur. The ACL twists around the posterior cruciate ligament (PCL) with internal rotation of the tibia on the femur, and it may be injured either with excessive anterior translation of the tibia on the femur or with excessive internal tibial rotation. The ACL has been described as consisting of three distinct bundles; although this is a somewhat oversimplified representation of the ligament in vivo, it is nonetheless useful when dealing with partial ACL tears. The anteromedial fibers are taut in flexion, whereas the larger, posterolateral fibers are tight in extension. The intermediate fibers remain relatively taut throughout knee range of motion.
Posterior Cruciate Ligament (PCL)
The tibial attachment of the PCL is extraarticular, extending down the back of the tibial plateau over 1 or 1.5 cm distal to the joint line (see Figure 6-6) and blending with the posterior horn of the lateral meniscus. On the femoral side, the ligament attaches onto the anterolateral aspect of the medial femoral condyle in the intercondylar notch on the opposite side of, and anterior to, the ACL (see Figure 6-10). The anterior fibers of the PCL are taut in flexion, whereas the posterior fibers are taut in extension.
THE MENISCI
The menisci are semilunar structures, with a triangular cross-sectional geometry, that are situated around the periphery of the medial and lateral knee joint compartments (see Figure 6-10). They are composed of fibrocartilage and are attached to the edge of the medial and lateral tibial plateau beneath the femoral condyles. The peripheral border of the medial meniscus is firmly attached to the medial capsule in the deep portion of the MCL, whereas the free surface is invested by synovial membrane. The menisci cover about two thirds of the articular surface of the tibia. The menisci allow controlled rotatory movements during knee flexion and extension, and they attenuate forces during axial loading by increasing the contact surface area between the femur and the tibia (shock absorption). By deepening and improving joint congruity, the menisci also help to stabilize the knee. The menisci may have a role in joint nutrition by helping to distribute synovial fluid evenly to the surrounding articular cartilage of the femoral condyles.
Medial Meniscus
The medial meniscus is C-shaped and has a larger radius than the lateral meniscus. The anterior horn of the medial meniscus is firmly attached to the tibia, just anterior to the ACL attachment. The posterior horn attaches adjacent to the PCL.
KNEE BURSAE
There are several bursae around the knee joint. These usually are not palpable unless they are inflamed (bursitis). The important ones are the following (see Figures 6-8 and 6-9):
Knee Movements
The knee is not a true hinge joint, because the axis of movement is not a fixed one. Instead, the axis shifts forward during extension and backward during flexion. Also, the commencement of flexion and the end of extension are accompanied by rotatory movements. Therefore, movements of the knee from full flexion to full extension consist of three components: 1) a simple rolling movement of the tibia on the femur; 2) a gliding movement of the tibia on the femur superimposed on rolling, in which the axis of movement through the medial and lateral femoral condyle gradually shifts forward during extension (opposite to what occurs during flexion); and 3) a rotatory movement at the end of extension, consisting of external rotation of the tibia on the femur through contraction of the biceps femoris and tensor fascia lata. This rotary movement is referred to as the locking movement of the joint or the screw-home movement. At the commencement of knee flexion, the converse occurs: the tibia internally rotates on the femur through contraction of the popliteus, semitendinosus, sartorius, gracilis, and semimembranosus, thereby “unlocking” the joint.
The screw-home position on full extension contributes significantly to knee stability, particularly when standing erect. It allows the patient to maintain knee extension over prolonged periods of standing without relying on continuous quadriceps contraction; therefore, it is an energy-conserving mechanism. The presence of a knee flexion deformity abrogates this stabilizing mechanism, causing quadriceps muscle fatigue.
Common Knee Disorders and Clinical Evaluation
KNEE PAIN
Heat and Redness
Swelling, heat, and redness are all signs of inflammation in the knee joint or periarticular tissues.
LIGAMENTOUS INJURY
In the acutely injured patient, the Lachman test is particularly useful, because it has both a high positive and a high negative predictive value for ACL injury diagnosis (Table 6-1). The pivot shift test is very useful, if it is positive; but injuries can be missed, especially in the acute situation, when a patient is apprehensive and in muscle spasm. The anterior drawer test is less accurate than the Lachman test. For the diagnosis of PCL injuries, the posterior sag, posterior drawer, and quadriceps active tests are all useful, especially for diagnosis of chronic injuries (see Table 6-1). Isolated ligament ruptures are relatively rare, and combined injuries with capsular tears, tibial plateau fractures, or meniscal injuries are more common.
MENISCUS INJURY
A history of an injury with subsequent locking, clunking, and localized pain to the joint line is a classic for a meniscus tear. The accuracy of physical examination maneuvers in correctly diagnosing meniscus pathology is low according to the published literature (see Table 6-1). Unquestionably, the evaluation of meniscus pathology requires attention to detail and skill that can be acquired only with experience. Careful application of the meniscus tests described earlier, in conjunction with a detailed history, should allow the examiner to limit the use of magnetic resonance imaging (MRI) studies to those cases in which significant uncertainty remains after the clinical evaluation. An acutely injured knee may be exceedingly difficult to examine for a meniscus injury. For example, it is impossible to perform a McMurray test unless the knee can be flexed to at least 90°. A repeat examination 1 or 2 weeks after the acute injury is often very helpful in establishing the correct diagnosis.
CONSIDERATIONS IN PATIENTS AFTER TOTAL KNEE REPLACEMENT
Neurovascular Pain and Dysfunction
The common peroneal nerve is particularly at risk for injury when a preoperative fixed valgus deformity is corrected at the time of knee replacement, resulting in lengthening of the lateral side of the knee and stretching of the peroneal nerve. Complaints of pain and paresthesia should be evaluated with a complete neurological examination. The most obvious motor abnormality with complete loss of common peroneal nerve function is a dense foot drop.