THE KNEE

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6 THE KNEE

Hans J. Kreder, Gillian A. Hawker

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.

FIGURE 6-1 THE BONES OF THE KNEE.

JOINT STABILITY

The lack of congruity between the round femoral condyles and the flat tibial plateau requires that the tibiofemoral articulation be stabilized by soft-tissue structures to prevent joint dissociation. Various knee ligaments, the knee joint capsule, and the menisci act as static stabilizers that control knee motion, assisted by the surrounding muscles, which act as dynamic stabilizers. The capsule is reinforced by bands from the fascia lata, iliotibial tract, and tendons. Posteriorly the capsule is strengthened by the oblique popliteal and arcuate popliteal ligaments. The matching geometry of the femoral trochlea and the corresponding triangular undersurface of the patella contribute to patellofemoral joint stability. Patellar motion during knee flexion and extension is controlled by dynamic muscle-stabilizing forces, especially from the vastus medialis.

The exposed position of the knee joint and the lack of protective layers of fat and muscle make the knee highly susceptible to traumatic injuries. The location of the joint between two long bones also predisposes the articulation to maximum stresses.

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.)

FIGURE 6-3 LOWER-LIMB ALIGNMENT.

Straight medial border of the lower extremity.

FIGURE 6-4 MEASUREMENT OF THE Q ANGLE.

The undersurface of the patella is divided into a larger lateral and a smaller medial side by a vertical ridge. There are six paired facets—two each of a superior, middle, and inferior—and a seventh facet along the medial border. By holding the patellar tendon away from the axis of movement, at the femoral epicondyles, the patella provides leverage for the quadriceps to improve the efficiency of the last 30° of extension. During knee flexion, the patellar contact forces move upward on the undersurface of the patella, from the inferior to the superior facets.

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.

FIGURE 6-5 POSTERIOR ASPECT OF THE KNEE.

Capsular, ligamentous, and muscular attachments.

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 6-6 LIGAMENTS OF THE KNEE.

A, Anterior aspect. B, Posterior aspect.

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.

FIGURE 6-8 THE KNEE: MEDIAL ASPECT.

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).

FIGURE 6-9 THE KNEE: LATERAL ASPECT.

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.

FIGURE 6-10 THE KNEE: CRUCIATE LIGAMENTS AND THEIR TIBIAL ATTACHMENTS.

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.

During the open-packed position (knee flexion), the collateral and cruciate ligaments are lax, and flexion is not checked until the leg contacts the thigh. During the close-packed position (full extension), the cruciate ligaments become taut, preventing further extension. The collateral ligaments also prevent hyperextension as a result of their posterior attachment to the tibia and fibula.

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.

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):

1. Prepatellar bursa—overlies the lower half of the patella and the upper half of the patellar ligament

2. Superficial infrapatellar bursa—overlies the ligamentum patellae

3. Deep infrapatellar bursa—lies beneath the ligamentum patellae

4. Anserine bursa—located between the pes anserinus and the superficial part of the MCL over the anteromedial surface of the proximal tibia

5. Medial gastrocnemius-semimembranosus bursa

A popliteal cyst, also known as Baker cyst, is most commonly caused by a fluid-distended medial gastrocnemius-semimembranosus bursa, which may communicate with the knee through a posteromedial capsular defect in the medial joint compartment. The size of the cyst often varies over time and with knee position. A popliteal cyst can rarely be caused by a fluid-distended, communicating popliteus bursa (subpopliteal recess) through a defect in the posterior capsule of the lateral knee compartment.

Blood and Nerve Supply

The knee derives its blood supply from a number of genicular branches from the femoral, profunda femoris, popliteal, and anterior tibial arteries. Its nerve supply is from the femoral, obturator, tibial, and common peroneal nerves.

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

If an injury is involved, it may be helpful to know the direction of force applied at impact and whether the knee was in the open- or close-packed position at the time. Knee pain is often diffuse and difficult to localize, but if the patient is able to delineate the maximum area of pain, this can be helpful in determining which underlying structure might be injured. Knee pain may be referred from hip pathology. Occasionally, a patient is able to describe certain activities or movements that aggravate or alleviate the pain. For example, posteromedial knee-joint pain with squatting may be caused by a tear of the posterior horn of the medial meniscus. Tendinitis pain is often improved as a workout continues, whereas most other causes of pain usually increase with activity. Finally, the pain character may provide some clues as to the underlying pathology. Muscle pain is often felt as a deep, dull ache that is difficult to localize, whereas meniscus pain may be sharp, localized, and intermittent.

Clicking, Snapping, and Clunking Noises and Sensations

Normal knees often make painless, high-pitched clicking sounds during squatting. Patients with arthritis or an inflamed synovium may note a crackling sound or sensation with knee movement. Low-pitched clunks may be caused by a loose body, a meniscus, or a cartilage fragment that is getting caught in the joint mechanism during movement. A patient may experience an audible snap or pop during rupture of a ligament or capsular structure or as a bone fractures.

Swelling

Diffuse knee-joint swelling is a nonspecific symptom that suggests knee synovitis and joint effusion. Knee swelling that occurs immediately after an injury is often caused by bleeding into the joint from a ligament or capsular rupture or by an intraarticular fracture. Such bleeding is usually accompanied by bruising. Swelling that arises hours or days after an injury is more likely to be caused by inflammation rather than bleeding (e.g., acute synovitis). Swelling of the knee that occurs without injury suggests an inflammatory arthritis with synovitis and effusion.

Heat and Redness

Swelling, heat, and redness are all signs of inflammation in the knee joint or periarticular tissues.

Stiffness

Stiffness may accompany knee swelling, or it may be an independent symptom. With a large effusion, the knee is more comfortable in slight flexion, and patients have difficulty with both full extension and full flexion. They describe the knee as feeling “tight.” Arthritis and periarticular contractures can cause decreased range of motion that is not due to fluid in the knee joint. A sense of stiffness in the morning is common with both rheumatoid arthritis and osteoarthritis.

Deformity

Acute deformity after trauma suggests a major muscular, ligamentous, or bony injury that is accompanied by loss of integrity of the knee joint or the surrounding musculature sufficient to cause the deformity. For example, the patient may report noticing a lump at the lateral aspect of the knee that jumped back into place later on, suggesting a patellar dislocation that spontaneously reduced. The insidious onset of deformity often accompanies knee arthritis. Varus deformity of the knee is particularly common in medial compartment osteoarthritis.

Giving Way and Instability

The knee collapses when the patient bears weight if the extensor mechanism is disrupted or inhibited by pain or muscle weakness. Patellofemoral problems often cause a sense of the knee’s giving way, especially when descending stairs, because the quadriceps muscle is contracting eccentrically with controlled lengthening of the muscle. Disruption of the ACL can cause collapse of the knee with certain maneuvers that result in femoral translation into an abnormal position on the planted tibia.

Pivoting

An ACL disruption allows the tibia to slide forward on the femur, especially on the lateral side of the knee. The patient may notice that this happens when the body is swung around to the outside of a planted leg during cutting sports. This movement rotates the femur around the tibia, and, because of the incompetent ACL, the femur rides back excessively on the lateral side. Patients often describe what happens by using their fists to indicate how the knee pivots. Pivoting is often accompanied by painful collapse as the knee joint gives way.

Locking

True locking of the knee refers to sudden or recurrent inability to flex or to extend the knee. It can occur because of a mechanical block to knee-joint motion, such as a loose body, meniscus flap, cruciate ligament stump, cartilage flap, or scar tissue that interferes with knee flexion or extension by its interposition between the joint surfaces. True locking must be distinguished from pseudolocking, which is caused by knee-joint stiffness or lack of movement due to pain inhibition, as opposed to a mechanical block. Asking the patient whether the knee “jams up” or “gets stuck” because something seems to be caught in it may be helpful in differentiating true locking from pseudolocking.

INFLAMMATORY ARTHRITIS AND OSTEOARTHRITIS

Symptoms of arthritis include pain, swelling, stiffness, crepitus, and, in the late stages, deformity and instability. Physical findings include disuse atrophy of the vastus medialis, joint tenderness, joint effusion, joint instability, and decreased range of motion. There may be similar changes found in other joints, the distribution of which may provide clues for the diagnosis of inflammatory arthritis. For example, rheumatoid arthritis generally produces symmetrical swelling of small and large joints.

LIGAMENTOUS INJURY

Ligamentous injury should be ruled out in the assessment of any patient who presents with knee pain after an acute injury. The direction of impact should be sought if possible, considering that the ligaments opposite to the impact may have been ruptured. Whereas complete ligament disruption is associated with gross instability, a grade 1 tear may be painful in the absence of instability on history or physical examination.

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.

TABLE 6-1 ACCURACY OF DIAGNOSTIC TESTS USED IN KNEE EXAMINATION