The knee consists of the lateral and medial compartments of the tibiofemoral joint and the patellofemoral joint (Figure 13-1). Motion at the knee occurs in two planes, allowing flexion and extension, and internal and external rotation. Functionally, however, these movements rarely occur independent of movement at other joints within the lower limb. Consider, for example, the interaction among the hip, knee, and ankle during running, climbing, or standing from a seated position. The strong functional association within the joints of the lower limb is reflected by the fact that about two thirds of the muscles that cross the knee also cross either the hip or the ankle.

FIGURE 13-1. Radiograph showing the bones and associated articulations of the knee.

The knee has important biomechanical functions, many of which are expressed during walking and running. During the swing phase of walking, the knee flexes to shorten the functional length of the lower limb; otherwise, the foot would not easily clear the ground. During the stance phase, the knee remains slightly flexed, allowing shock absorption, conservation of energy, and transmission of forces through the lower limb. Running requires that the knee move through a greater range of motion than walking, especially in the sagittal plane. In addition, rapidly changing direction during walking or running demands adequate internal and external rotation of the knee.

Stability of the knee is based primarily on its soft-tissue constraints rather than on its bony configuration. The massive femoral condyles articulate with the nearly flat proximal articular surfaces of the tibia, held in place by extensive ligaments, joint capsule and menisci, and large muscles. With the foot firmly in contact with the ground, these soft tissues are often subjected to large forces, from both muscles and external sources. Injuries to ligaments, menisci, and articular cartilage are unfortunately common consequences of the large functional demands placed on the knee. Knowledge of the anatomy and kinesiology of the knee is an essential prerequisite to the understanding of most mechanisms of injury and effective therapeutic intervention.

OSTEOLOGY

Distal Femur

At the distal end of the femur are the large lateral and medial condyles (from the Greek kondylos, knuckle) (Figure 13-2). Lateral and medial epicondyles project from each condyle, providing elevated attachment sites for the collateral ligaments. A large intercondylar notch separates the lateral and medial condyles, forming a passageway for the cruciate ligaments. A narrower than average notch may increase the likelihood of injury to the anterior cruciate ligament.²⁶⁷

FIGURE 13-2. Osteology of the right patella, articular surfaces of the distal femur, and proximal tibia.

The femoral condyles fuse anteriorly to form the intercondylar (trochlear) groove (see Figure 13-2). This groove articulates with the posterior side of the patella, forming the patellofemoral joint. The intercondylar groove is concave from side to side and slightly convex from front to back. The sloping sides of the intercondylar groove form lateral and medial facets. The more pronounced lateral facet extends more proximally and anteriorly than the medial facet. The steeper slope of the lateral facet helps to stabilize the patella within the groove during knee movement.

Lateral and medial grooves are etched faintly in the cartilage that covers much of the articular surface of the femoral condyles (see Figure 13-2). When the knee is fully extended, the anterior edge of the tibia is aligned with these grooves. The position of the grooves highlights the asymmetry in the shape of the medial and lateral articular surfaces of the distal femur. As explained later in this chapter, the asymmetry in the shape of the condyles affects the sagittal plane kinematics.

The articular capsule of the knee extends across all sides of the tibiofemoral joint and the patellofemoral joint (see dotted lines in Figure 13-3). Posteriorly, the capsule attaches just proximal to the femoral condyles, immediately distal to the popliteal surface of the femur.

FIGURE 13-3. Right distal femur, tibia, and fibula. A, Anterior view. B, Posterior view. Proximal attachments of muscles are shown in red, distal attachments in gray. The dashed lines show the attachment of the joint capsule of the knee.

Proximal Tibia and Fibula

Although the fibula has no direct function at the knee, the slender bone splints the lateral side of the tibia and helps maintain its alignment. The head of the fibula serves as an attachment for the biceps femoris and the lateral collateral ligament. The fibula is attached to the lateral side of the tibia by proximal and distal tibiofibular joints (see Figure 13-3). The structure and function of these joints are discussed in Chapter 14.

The primary function of the tibia is to transfer weight across the knee and to the ankle. The proximal end of the tibia flares into medial and lateral condyles, which form articular surfaces with the distal femur (see Figure 13-3). The superior surfaces of the condyles form a broad region, often referred to as the tibial plateau. The plateau supports two smooth articular surfaces that accept the large femoral condyles, forming medial and lateral compartments of the tibiofemoral joint. The larger, medial articular surface is slightly concave, whereas the lateral articular surface is flat to slightly convex. The articular surfaces are separated down the midline by an intercondylar eminence, formed by irregularly shaped medial and lateral tubercles (see Figure 13-2). Shallow anterior and posterior intercondylar areas flank both ends of the eminence. The cruciate ligaments and menisci attach along the intercondylar region of the tibia.

The prominent tibial tuberosity is located on the anterior surface of the proximal shaft of the tibia (see Figure 13-3, A). The tibial tuberosity serves as the distal attachment for the quadriceps femoris muscle, via the patellar tendon. On the posterior side of the proximal tibia is a roughened soleal line, coursing diagonally in a distal-to-medial direction (see Figure 13-3, B).

Patella

The patella (from the Latin, “small plate”) is a nearly triangular bone embedded within the quadriceps tendon. It is the largest sesamoid bone in the body. The patella has a curved base superiorly and a pointed apex inferiorly (Figures 13-4 and 13-5). The thick patellar tendon attaches to and between the apex of the patella and the tibial tuberosity. In a relaxed standing position, the apex of the patella lies just proximal to the knee joint line. The subcutaneous anterior surface of the patella is convex in all directions.

FIGURE 13-4. Lateral view of the right knee. Note the curved articular surface of the lateral femoral condyle. Proximal attachments of muscles and ligaments are shown in red, distal attachments in gray.

FIGURE 13-5. Anterior and posterior surfaces of the right patella. The attachment of the tendon of the quadriceps muscle is indicated in gray; the proximal attachment of the patellar tendon is indicated in red. Note the smooth articular cartilage covering the posterior articular surface of the patella.

Osteologic Features of the Patella

• Base

• Apex

• Anterior surface

• Posterior articular surface

• Vertical ridge

• Lateral, medial, and “odd” facets

The posterior articular surface of the patella is covered with articular cartilage up to 4 to 5 mm thick (see Figure 13-5).⁶⁵ Part of this surface articulates with the intercondylar groove of the femur, forming the patellofemoral joint. The thick cartilage helps to disperse the large compression forces that cross the joint. A rounded vertical ridge runs longitudinally from top to bottom across the posterior surface of the patella. On either side of this ridge is a lateral or a medial facet. The larger and slightly concave lateral facet matches the general contour of the lateral facet on the intercondylar groove of the femur (see Figure 13-2). The medial facet shows significant anatomic variation. A third “odd” facet exists along the extreme medial border of the medial facet.

ARTHROLOGY

General Anatomic and Alignment Considerations

The shaft of the femur angles slightly medially as it descends toward the knee. This oblique orientation is a result of the natural 125-degree angle of inclination of the proximal femur (Figure 13-6, A). Because the articular surface of the proximal tibia is oriented nearly horizontally, the knee forms an angle on its lateral side of about 170 to 175 degrees. This normal alignment of the knee within the frontal plane is referred to as genu valgum.

FIGURE 13-6. Frontal plane deviations of the knee. A, Normal genu valgum. The normal 125-degree angle of inclination of the proximal femur and the longitudinal axis of rotation throughout the entire lower extremity are also shown. B and C illustrate excessive frontal plane deviations.

Variation in normal frontal plane alignment at the knee is not uncommon. A lateral angle less than 170 degrees is called excessive genu valgum, or “knock-knee” (see Figure 13-6, B). In contrast, a lateral angle that exceeds about 180 degrees is called genu varum, or “bow-leg” (Figure 13-6, C).

The longitudinal or vertical axis of rotation at the hip is defined in Chapter 12 as a line connecting the femoral head with the center of the knee joint. As depicted in Figure 13-6, A, this longitudinal axis can be extended inferiorly through the knee to the ankle and foot. The axis mechanically links the horizontal plane movements of the major joints of the entire lower limb. Horizontal plane rotations that occur in the hip, for example, affect the posture of the joints throughout the lower limb as far distal as those in the foot, and vice versa.

Capsule and Reinforcing Ligaments

The fibrous capsule of the knee encloses the medial and lateral compartments of the tibiofemoral joint and the patellofemoral joint. The proximal and distal attachments of the capsule to bone are indicated by the dotted lines in Figure 13-3, A and B. The capsule of the knee receives significant reinforcement from muscles, ligaments, and fascia. Five reinforced regions of the capsule are described next and are summarized in Table 13-1.

TABLE 13-1.

Ligaments, Fascia, and Muscles That Reinforce the Capsule of the Knee

^*Often referred to as the medial patellofemoral ligament.

^†Often referred to as the posterior-medial capsule or the posterior oblique ligament.

The anterior capsule of the knee attaches to the margins of the patella and the patellar tendon, being reinforced by the quadriceps muscle and medial and lateral patellar retinacular fibers (Figure 13-7). The retinacular fibers are extensions of the connective tissue covering the vastus lateralis, vastus medialis, and iliotibial band. The extensive set of netlike fibers has connection to and among the femur, tibia, patella, quadriceps and patellar tendon, collateral ligaments, and menisci.

FIGURE 13-7. Anterior view of the right knee, highlighting many muscles and connective tissues. The pes anserinus tendons are cut to expose the medial collateral ligament and the medial patellar retinaculum.

The lateral capsule of the knee is reinforced by the lateral (fibular) collateral ligament, lateral patellar retinacular fibers, and iliotibial band (Figure 13-8).²³² Muscular stability is provided by the biceps femoris, the tendon of the popliteus, and the lateral head of the gastrocnemius.

FIGURE 13-8. Lateral view of the right knee shows many muscles and connective tissues. The iliotibial band, lateral head of the gastrocnemius, and biceps femoris are cut to better expose the lateral collateral ligament, popliteus tendon, and lateral meniscus.

The posterior capsule is reinforced by the oblique popliteal ligament and the arcuate popliteal ligament (Figure 13-9). The oblique popliteal ligament originates medially from the posterior-medial capsule and the semimembranosus tendon. Laterally and superiorly, the fibers blend with the capsule adjacent to the lateral femoral condyle. This ligament is pulled taut in full knee extension, a position that naturally includes slight external rotation of the tibia relative to the femur. The arcuate popliteal ligament originates from the fibular head and then divides into two limbs. The larger and more prominent limb arches (hence the term “arcuate”) across the tendon of the popliteus muscle to attach to the posterior intercondylar area of the tibia. An inconsistent and smaller limb attaches to the posterior side of the lateral femoral condyle, and often to a sesamoid bone (or fabella, meaning “little bean”) embedded within the lateral head of the gastrocnemius. The posterior capsule is further reinforced by the popliteus, gastrocnemius, and hamstring muscles, especially by the fibrous extensions of the semimembranosus tendon. Unlike the elbow, the knee has no bony block against hyperextension. The muscles and posterior capsule limit hyperextension.

FIGURE 13-9. Posterior view of the right knee that emphasizes the major parts of the posterior capsule: the oblique popliteal and arcuate popliteal ligaments. The lateral and medial heads of the gastrocnemius and plantaris muscles are cut to expose the posterior capsule. Note the popliteus muscle deep in the popliteal fossa, lying partially covered by the fascial extension of the semimembranosus.

The posterior-lateral capsule of the knee is reinforced by the arcuate popliteal ligament, lateral collateral ligament, and popliteus muscle and tendon. This set of tissues is often referred to as the arcuate complex.

The medial capsule of the knee extends in varying thickness from the patellar tendon to the posterior capsule.²²⁶^,²⁵⁸ Its anterior one third consists of a thin layer of fascia reinforced by the medial patellar retinacular fibers (Figure 13-10). The middle one third of the capsule is reinforced by a continuation of the medial patellar retinacular fibers and, more substantially, by the superficial and deep fibers of the medial collateral ligament (the deep fibers are not exposed in Figure 13-10). The posterior one third of the capsule is relatively thick, originating near the adductor tubercle and blending with tendinous expansions of the semimembranosis and to the adjacent posterior capsule.²²⁶ The posterior one third of the medial capsule is relatively well defined and is frequently described as a discrete structure, often under the name of the posterior-medial capsule or, less frequently, the posterior oblique ligament.²⁰³^,²²⁶ The posterior-medial capsule is reinforced by the flat conjoined tendons of the sartorius, gracilis, and semitendinosus—collectively referred to as the pes anserinus (from the Latin, “goose’s foot”) tendons. The posterior two thirds of the medial capsule and its associated structures provide an important source of stabilization to the knee.²²⁵

FIGURE 13-10. Medial view of the right knee shows many muscles and connective tissues. The tendons of the sartorius and gracilis are cut to better expose the superficial part of the medial collateral ligament and the posterior-medial capsule.

Synovial Membrane, Bursae, and Fat Pads

The internal surface of the capsule of the knee is lined with a synovial membrane. The anatomic organization of this membrane is complicated, in part, by the knee’s convoluted embryonic development.²⁵⁸

The knee has as many as 14 bursae, which form at inter-tissue junctions that encounter high friction during movement.²⁵⁸ These inter-tissue junctions involve tendon, ligament, skin, bone, capsule, and muscle (Table 13-2). Although some bursae are simply extensions of the synovial membrane, others are formed external to the capsule. Activities that involve excessive and repetitive forces at these inter-tissue junctions potentially lead to bursitis, an inflammation of the bursa.

TABLE 13-2.

Examples of Bursae at Various Inter-tissue Junctions

Fat pads are often associated with bursae around the knee. Fat and synovial fluid reduce friction between moving parts. At the knee, the most extensive fat pads are associated with the suprapatellar and deep infrapatellar bursae.

SPECIAL FOCUS 13-1 Development of Knee Plicae

During embryonic development, the knee experiences significant physical transformation. Mesenchymal tissues thicken and then reabsorb, forming primitive joint compartments, ligaments, and menisci. Incomplete resorption of mesenchymal tissue during development forms tissues known as plicae.⁴⁶^,²⁵⁸ Plicae, or synovial pleats, appear as folds in the synovial membranes. Plicae may be very small and unrecognizable, or so large that they nearly separate the knee into medial and lateral joint compartments. The literature reports a wide range in the presence of plicae within the knee, ranging from 20% and 70%.⁴⁶^,¹⁹⁹ Plicae may serve to reinforce the synovial membrane of the knee, although this is only speculation. Other synovial joints of the body besides the knee may have plicae.

The three most commonly described plicae in the knee are the (1) superior or suprapatellar plica, (2) inferior plica, first called the ligamentum mucosum by Vesalius in the sixteenth century,⁴⁶ and (3) medial plica. The most prominent medial plica is known by about 20 names, including alar ligament, synovialis patellaris, and intra-articular medial band. Plicae that are unusually large, or are thickened owing to irritation or trauma, can cause knee pain. Because this pathology occurs most often in the medial plica, pain is often reported in the anterior-medial region of the knee. If particularly large, some medial plicae are visible or palpable under the skin.²⁴⁵ Observations during arthroscopy suggest that an enlarged medial plica can cause abrasion of the facing articular cartilage of the medial femoral condyle.¹⁴⁹ Inflammation and pain of the medial plica may be easily confused with patellar tendonitis, a torn medial meniscus, or patellofemoral joint pain. Treatment includes rest, anti-inflammatory medication, physical therapy, and, in some cases, arthroscopic resection.

Tibiofemoral Joint

The tibiofemoral joint consists of the articulations between the large, convex femoral condyles and the nearly flat and smaller tibial condyles (see Figure 13-4). The large articular surface area of the femoral condyles permits extensive knee motion in the sagittal plane for activities such as running, squatting, and climbing. Joint stability is provided not by a tight bony fit, but by forces and physical containment provided by muscles, ligaments, capsule, menisci, and body weight.

MENISCI

Anatomic Considerations: The medial and lateral menisci are crescent-shaped, fibrocartilaginous structures located within the knee joint (Figure 13-11). The menisci transform the articular surfaces of the tibia into shallow seats for the larger convex femoral condyles. This transformation is most important laterally because of the flat to slightly convex shape of the tibia’s lateral articular surface.

FIGURE 13-11. A, The superior surface of the tibia shows the menisci and other cut structures: collateral, cruciate, and posterior meniscofemoral ligaments, as well as muscles and tendons. (This specimen does not have an anterior meniscofemoral ligament.) B, The superior view of the right tibia marks the attachment points of the menisci and cruciate ligaments within the intercondylar region.

The menisci are anchored to the intercondylar region of the tibia by their free ends, known as anterior and posterior horns. The external edge of each meniscus is attached to the tibia and the adjacent capsule by coronary (or meniscotibial) ligaments (see Figure 13-11, A). The coronary ligaments are relatively loose, thereby allowing the menisci, especially the lateral meniscus, to pivot freely during movement.²³² A slender transverse ligament connects the two menisci anteriorly.

Several muscles have secondary attachments into the menisci. The quadriceps and semimembranosus attach to both menisci,¹²⁴ whereas the popliteus attaches to the lateral meniscus.⁵⁴^,²³² Through these attachments, the muscles help stabilize the position of the menisci.

Blood supply to the menisci is greatest near the peripheral (external) border. Blood comes from capillaries located within the adjacent synovial membrane and capsule.²⁵⁸ The internal border of the menisci, in contrast, is essentially avascular.

The two menisci have different shapes and methods of attaching to the tibia. The medial meniscus has an oval shape, with its external border attaching to the deep surface of the medial collateral ligament and adjacent capsule; the lateral meniscus has more of a circular shape, with its external border attaching only to the lateral capsule. The tendon of the popliteus passes between the lateral collateral ligament and the external border of the lateral meniscus (Figure 13-12).

FIGURE 13-12. Posterior view of the deep structures of the right knee after all muscles and the posterior capsule have been removed. Observe the menisci, collateral ligaments, and cruciate ligaments. Note the popliteus tendon, which courses between the lateral meniscus and lateral collateral ligament.

SPECIAL FOCUS 13-2 A Closer Look at the Meniscofemoral Ligaments

The posterior horn of the lateral meniscus is usually attached to the lateral aspect of the medial condyle of the femur by anterior or posterior meniscofemoral ligaments.⁷⁷^,⁷⁸ The meniscofemoral ligaments are named for their position relative to the posterior cruciate ligament (PCL), with which they share similar femoral attachments. Only the posterior meniscofemoral ligament is present in the specimen illustrated in Figure 13-11, A.

Cadaveric studies reveal that at least one of the meniscofemoral ligaments is present in 92% of knees, and both are present in 32% of knees.⁷⁶ The posterior meniscofemoral ligament is usually the more substantial of the two structures. After arising from the posterior horn of the lateral meniscus, the posterior meniscofemoral ligament attaches to the femur just posterior and slightly medial to the PCL (see Figure 13-12). The meniscofemoral ligaments sometimes serve as the only bony attachment of the posterior horn of the lateral meniscus.²⁵⁸ The exact function of the meniscofemoral ligaments is not certain. The ligaments may help stabilize the posterior horn of the lateral meniscus during movement. In addition, these ligaments may provide secondary (and likely minor) sagittal plane dynamic stability to the knee, an assumption based on research showing that the anterior ligament becomes more taut in flexion and the posterior ligament more taut in extension.¹⁷⁵

Functional Considerations: The primary function of the menisci is to reduce the compressive stress * across the tibiofemoral joint.¹²³^,¹³⁷ Other functions of the menisci include stabilizing the joint during motion, lubricating the articular cartilage, providing proprioception,²⁸⁴ and helping to guide the knee’s arthrokinematics.

Compression forces at the knee joint routinely reach 2.5 to 3 times body weight while one is walking, and over 4 times body weight while one ascends stairs.¹⁷⁶^,²⁸³ By nearly tripling the area of joint contact, the menisci significantly reduce pressure (i.e., force per unit area) on the articular cartilage. This method of attenuating peak pressure is essential to the health and protection of the knee joint.⁵¹ A complete lateral meniscectomy has been shown to increase peak contact pressures at the knee by 230%, which increases the risk of development of stress-related arthritis.^52,163,192 Even a tear or a partial meniscectomy significantly increases local stress, which is strongly believed to cause excessive wear on the articular cartilage.¹³⁷ When possible, surgically repairing a meniscus instead of removing the damaged regions is clearly the treatment of choice.¹⁶³^,²¹⁶ In certain cases, after a complete meniscectomy a meniscal allograft transplantation may be indicated, with goals of limiting the degeneration of the articular cartilage.³⁵^,²²⁹

At every step, the menisci deform peripherally as they are compressed.¹²³^,²⁴⁸ This mechanism allows part of the compression force at the knee to be absorbed as a circumferential tension (known as hoop stress) throughout each meniscus. Studies indicate that a torn medial meniscus, most notably with an avulsion tear of its posterior horn, loses its ability to optimally resist hoop stress, thereby reducing the capacity for protecting the underlying articular cartilage and bone.¹⁶¹

Common Mechanisms of Injury: Tears of the meniscus are the most common injury of the knee, occurring relatively frequently in both the athletic and the general population.¹⁴⁵^,¹⁸⁷ According to research cited by Lohmander and colleagues, 50% of all acute injuries of the anterior cruciate ligaments are associated with a concurrent injury to a meniscus.¹⁴⁵ In general, meniscal tears are often associated with a forceful, axial rotation of the femoral condyles over a partially flexed and weight-bearing knee. The axial torsion within the compressed knee can pinch and dislodge the meniscus. A dislodged or folded flap of meniscus (often referred to as a “bucket-handle tear”) can mechanically block knee movement.

The medial meniscus is injured twice as frequently as the lateral meniscus.²⁹ The mechanism of injury for a medial meniscus tear often involves axial rotation, and also may involve an external force applied to the lateral aspect of the knee. This force—typically described as a “valgus force”—can cause an excessive valgus position of the knee and subsequent large stress on the medial collateral ligament and posterior-medial capsule. Because of the anatomic connections between the medial meniscus and these connective tissues, a significant valgus force delivered to the knee can indirectly strain and thereby injure the medial meniscus.

This risk of developing tears in the meniscus of the knee increases if the knee is malaligned or has a history of ligamentous instability, most notably in the anterior cruciate.¹⁴⁵^,¹⁶³

OSTEOKINEMATICS AT THE TIBIOFEMORAL JOINT

The tibiofemoral joint possesses two degrees of freedom: flexion and extension in the sagittal plane and, provided the knee is at least slightly flexed, internal and external rotation. These motions are shown for tibial-on-femoral and femoral-on-tibial situations in Figures 13-13 and 13-14. Frontal plane motion at the knee occurs passively only, limited to about 6 to 7 degrees.¹⁵⁹

FIGURE 13-13. Sagittal plane motion at the knee. A, Tibial-on-femoral perspective (femur is stationary). B, Femoral-on-tibial perspective (tibia is stationary).

FIGURE 13-14. Internal and external (axial) rotation of the right knee. A, Tibial-on-femoral (knee) rotation. In this case the direction of the knee rotation (internal or external) is the same as the motion of the tibia; the femur is stationary. B, Femoral-on-tibial rotation. In this case the tibia is stationary and the femur is rotating. The direction of the knee rotation (external or internal) is the opposite of the motion of the moving femur: external rotation of the knee occurs by internal rotation of the femur; internal rotation of the knee occurs by external rotation of the femur.

Flexion and Extension: Flexion and extension at the knee occur about a medial-lateral axis of rotation. Range of motion varies with age and gender, but in general the healthy knee moves from 130 to 150 degrees of flexion to about 5 to 10 degrees beyond the 0-degree (straight) position.⁸³^,²²⁴

The medial-lateral axis of rotation for flexion and extension is not fixed, but migrates within the femoral condyles.²⁵¹ The curved path of the axis is known as an “evolute” (Figure 13-15). The path of the axis is influenced by the eccentric curvature of the femoral condyles.⁹⁷^,²⁵¹

FIGURE 13-15. The flexing knee generates a migrating medial-lateral axis of rotation (shown as three small circles). This migration is described as “the evolute.”

The migrating axis of rotation has biomechanical and clinical implications. First, the migrating axis alters the length of the internal moment arm of the flexor and extensor muscles of the knee. This fact explains, in part, why maximal-effort internal torque varies across the range of motion. Second, many external devices that attach to the knee, such as a goniometer, an isokinetic testing device, or a hinged knee orthosis, rotate about a fixed axis of rotation. During knee motion, therefore, the external devices may rotate in a slightly dissimilar arc than the leg. As a consequence, a hinged orthosis, for example, may act as a piston relative to the leg, causing rubbing against and abrasion to the skin. To minimize this consequence, care must be taken to align the fixed axis of the external device as close as possible to the “average” axis of rotation of the knee, which is close to the lateral epicondyle of the femur.

Internal and External (Axial) Rotation: Internal and external rotation of the knee occurs about a vertical or longitudinal axis of rotation. This motion is also called “axial” rotation. In general, the freedom of axial rotation increases with greater knee flexion. A knee flexed to 90 degrees can perform about 40 to 45 degrees of total axial rotation.¹⁷⁸^,¹⁹⁰ External rotation range of motion generally exceeds internal rotation by a ratio of nearly 2 : 1.¹⁷⁸ Once the knee is in full extension, however, axial rotation is maximally restricted. Rotation of the knee is significantly blocked by passive tension in the stretched ligaments, parts of the capsule, and increased bony congruity within the joint.

As depicted in Figure 13-14, axial rotation of the knee occurs by either tibial-on-femoral or femoral-on-tibial rotation. (Although not depicted, axial rotation can also occur as a result of both rotational perspectives occuring simultaneously.) Axial rotation of the knee provides an important functional element of mobility to the lower extremity as a whole. The terminology used to describe axial rotation of the knee is important to understand. As a rule, the naming of axial rotation of the knee is based on the position of the tibial tuberosity relative to the anterior distal femur. External rotation of the knee, for example, occurs when the tibial tuberosity is located lateral to the distal anterior femur. This rule, however, does not stipulate whether the femur or tibia is the moving bone; it only stipulates the relative articular orientation of the rotated knee. To demonstrate, compare external rotation of the knee in Figures 13-14, A and B. Tibial-on-femoral external rotation of the knee occurs as the tibia rotates externally relative to a stationary femur. On the other hand, femoral-on-tibial external rotation of the knee occurs as the femur rotates internally relative to a stationary tibia (and foot). Both examples fit the definition of external rotation of the knee because both motions end up with a similar articular orientation: the tibial tuberosity is located lateral to the anterior distal femur. The distinction between bony rotation (tibial or femoral) and knee joint rotation must always be clear to avoid misinterpretation. This point is particularly important in describing femoral-on-tibial osteokinematics.

ARTHROKINEMATICS AT THE TIBIOFEMORAL JOINT

Extension of the Knee: Figure 13-16 depicts the arthrokinematics of the last 90 degrees of active knee extension. During tibial-on-femoral extension, the articular surface of the tibia rolls and slides anteriorly on the femoral condyles (see Figure 13-16, A). The menisci are shown pulled anteriorly by the contracting quadriceps muscle.

FIGURE 13-16. The active arthrokinematics of knee extension. A, Tibial-on-femoral perspective. B, Femoral-on-tibial perspective. In both A and B, the meniscus is pulled toward the contracting quadriceps.

During femoral-on-tibial extension, as in standing up from a deep squat position, the femoral condyles simultaneously roll anteriorly and slide posteriorly on the articular surface of the tibia (see Figure 13-16, B). These “offsetting” arthrokinematics limit the magnitude of anterior translation of the femur on the tibia. The quadriceps muscle directs the roll of the femoral condyles and stabilizes the menisci against the horizontal shear caused by the sliding femur.

“Screw-Home” Rotation of the Knee: Locking the knee in full extension requires about 10 degrees of external rotation.¹⁰⁹ The rotary locking action has historically been referred to as the “screw-home” rotation, based on the observable twisting of the knee during the last 30 or so degrees of extension. The external rotation described here is fundamentally different from the axial rotation illustrated in Figure 13-14. Screw-home (external) rotation has been described as a conjunct rotation, emphasizing the fact that it is mechanically linked (or coupled) to the flexion and extension kinematics and cannot be performed independently.²⁰⁰^,²⁵⁸ The combined external rotation and knee extension maximizes the overall contact area of the adult knee: 375 mm² in the medial tibiofemoral joint and about 275 mm² in the lateral tibiofemoral joint.²⁰⁰ This final position of extension increases joint congruence and favors stability.

To observe the screw-home rotation at the knee, have a partner sit with the knee flexed to about 90 degrees. Draw a line on the skin between the tibial tuberosity and the apex of the patella. After the partner completes full tibial-on-femoral extension, redraw this line between the same landmarks and note the change in position of the externally rotated tibia. A similar but less obvious locking mechanism also functions during femoral-on-tibial extension (compare Figure 13-16, A with B). When one rises up from a squat position, for example, the knee locks into extension as the femur internally rotates relative to the fixed tibia. Regardless of whether the thigh or leg is the moving segment, both knee extension movements depicted in Figure 13-16, A and B show a knee joint that is relatively externally rotated when fully extended.

The screw-home rotation mechanics are driven by at least three factors: the shape of the medial femoral condyle, the passive tension in the anterior cruciate ligament, and the slight lateral pull of the quadriceps muscle (Figure 13-17). The most important (or at least obvious) factor is the shape of the medial femoral condyle. As depicted in Figure 13-17, B, the articular surface of the medial femoral condyle curves about 30 degrees laterally, as it approaches the intercondylar groove. Because the articular surface of the medial condyle extends farther anteriorly than the lateral condyle, the tibia is obliged to “follow” the laterally curved path into full tibial-on-femoral extension. During femoral-on-tibial extension, the femur follows a medially curved path on the tibia. In either case, the result is external rotation of the knee at full extension.

FIGURE 13-17. The “screw-home” locking mechanism of the knee. A, During terminal tibial-on-femoral extension, three factors contribute to the locking mechanism of the knee. Each factor contributes bias to external rotation of the tibia, relative to the femur. B, The two arrows depict the path of the tibia across the femoral condyles during the last 90 degrees of extension. Note that the curved medial femoral condyle helps to direct the tibia to its externally rotated and locked position.

Flexion of the Knee: The arthrokinematics of knee flexion occur by a reverse fashion as that depicted in Figure 13-16. For a knee that is fully extended to be unlocked, the joint must first internally rotate slightly.^200,206,225 This action is driven primarily by the popliteus muscle. The muscle can rotate the femur externally to initiate femoral-on-tibial flexion or can rotate the tibia internally to initiate tibial-on-femoral flexion.

Internal and External (Axial) Rotation of the Knee: As described earlier, the knee must be flexed to maximize independent axial rotation between the tibia and femur. Once the knee is flexed, the arthrokinematics of internal and external rotation involve primarily a spin between the menisci and the articular surfaces of the tibia and femur. Axial rotation of the femur over the tibia causes the menisci to deform slightly, as they are compressed between the spinning femoral condyles. The menisci are stabilized by connections from active musculature such as the popliteus and semimembranosus.

MEDIAL AND LATERAL COLLATERAL LIGAMENTS

Anatomic Considerations: The medial (tibial) collateral ligament (MCL) is a flat, broad structure that crosses the medial side of the joint.²⁵⁸ Although different terminology exists, this chapter describes the MCL as having superficial and deep parts.²²⁵^,²²⁶ The larger superficial part consists of a relatively well-defined set of parallel running fibers about 10 cm in length (see Figure 13-10).²²⁶ After arising from the medial epicondyle of the femur, the superficial fibers course distally to blend with medial patellar retinacular fibers before attaching to the medial-proximal aspect of the tibia. The fibers attach just posterior to the distal attachments of the closely aligned tendons of the sartorius and the gracilis.

The deep part of the MCL consists of a shorter and more oblique set of fibers, lying immediately deep and slightly posterior and distal to the proximal attachment of the superficial fibers. Although not visible on Figure 13-10, the deep fibers attach distally to the posterior-medial joint capsule, medial meniscus, and tendon of the semimembranosus muscle.²²⁶^,²⁵⁸

The lateral (fibular) collateral ligament consists of a round, strong cord that runs nearly vertically between the lateral epicondyle of the femur and the head of the fibula (see Figure 13-8).¹³⁵^,²³² Distally, the lateral collateral ligament blends with the tendon of the biceps femoris muscle. Unlike its medial counterpart, the MCL, the lateral collateral ligament does not attach to the adjacent lateral meniscus (see Figure 13-12). As described later in this chapter, the tendon of the popliteus courses between these two structures.

Functional Considerations: The primary function of the collateral ligaments is to limit excessive knee motion within the frontal plane. With the knee extended, the superficial part of the MCL provides the primary resistance against a valgus (abduction) force.⁷⁴^,²²⁵ The lateral collateral ligament, in comparison, provides the primary resistance against a varus (adduction) force.⁹⁵^,²³⁷ Table 13-3 lists several other tissues that provide restraint against valgus and varus applied forces to the knee.

TABLE 13-3.

Tissues That Provide Primary and Secondary Restraint to the Knee in the Frontal Plane *

^*Assume a fully extended knee.

A secondary function of the collateral ligaments is to produce a generalized stabilizing tension at the knee throughout the sagittal plane range of motion. Although some of the fibers that constitute the collateral ligaments are taut throughout the full range of knee flexion and extension, most are positioned slightly posterior to the medial-lateral axis of rotation of the knee and therefore are pulled relatively taut in full extension.197,225,268 Other structures that become more taut in full extension are the posterior-medial capsule, the oblique popliteal ligament (representative of the posterior capsule), the knee flexor muscles, and the components of the anterior cruciate ligament.^197,203,225 Figure 13-18 demonstrates these tissues as being relatively slack in flexion (A) and more taut as the knee assumes the locked position of full femoral-on-tibial extension (B). Full extension—which includes the kinematics of the screw-home rotation—elongates the collateral ligaments roughly 20% beyond their length at full flexion.²⁷⁰ Although a valuable stabilizer in full extension, a taut MCL and posterior-medial capsule are especially vulnerable to injury from a valgus (i.e., an abduction) load delivered over a planted foot. Because the deeper fibers of the MCL are shorter than the superficial, the deeper fibers experience a greater percentage of stretch when subjected to similar valgus (abduction) strain.²²⁵ Primarily for this reason, the deeper fibers of the MCL are more frequently injured than the superficial fibers during an excessive valgus-related trauma, such as what typically occurs in the “clip” injury in American football.²⁵⁰

FIGURE 13-18. Medial view of the knee shows the relative elongation of some fibers of the medial collateral ligament, oblique popliteal ligament, posterior capsule, and components of the anterior cruciate ligament (ACL) during active femoral-on-tibial extension. A, In knee flexion the structures are shown in a relatively slackened (or least taut) state. B, The structures are pulled relatively taut as the knee actively extends by contraction of the quadriceps. Note the “screw-home” rotation of the knee during end-range extension. The combined external rotation and extension on the knee specifically elongate the posterior-medial capsule and oblique popliteal ligament (within the posterior capsule).

The collateral ligaments and adjacent capsule also provide resistance to the extremes of internal and external rotation.²³²^,²⁷⁰ Most notable in this regard are the elongation and subsequent increased passive tension in the superficial fibers of the MCL at the extremes of external rotation of the knee.⁷⁴^,²²⁵ Planting the right foot securely on the ground and vigorously rotating the superimposed femur (and body) to the left, for example, may damage the superficial fibers of the right MCL. This potential for injury increases if the externally rotating knee (i.e., internally rotating femur) is simultaneously experiencing a substantial valgus load.

Table 13-4 provides a summary of the functions and common mechanisms of injury for the major ligaments of the knee, including the posterior-medial and posterior capsule.

TABLE 13-4.

Function of Ligaments at the Knee and Common Mechanisms of Injury