During normal patellofemoral joint motion, the patella glides distally as the knee is flexed and proximally as the knee is extended (Figure 10-6, A through C). As the patella glides proximally and distally, it must remain stable within the intercondylar groove of the femur. The anatomic design of the patella is normally well suited for this function. The jigsaw fit of the convex posterior patella fits snugly against the matching concavity of the intercondylar groove of the femur (Figure 10-6, D). Strong activation of the quadriceps naturally produces a net lateral pull on the patella. The steep slope of the lateral facet of the femur—and matching patellar surface—normally helps prevent lateral subluxation (dislocation) of the patella. Persons with a flattened lateral intercondylar groove are more likely to experience lateral subluxation of their patellofemoral joint.

Figure 10-6 Kinematics of the patellofemoral joint as the knee is extending. Note the superior migration of the patella as the knee moves from 135 degrees of flexion **(A)** to 90 degrees of flexion **(B)** to 20 degrees short of full extension **(C). D** shows a front view of the path and contact area of the patella as the knee moves from 135 degrees of flexion to 20 degrees short of full extension. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 13-23, A through D.)

Normal Alignment

As illustrated in Figure 10-7, A (and studied in Chapter 9), the 125-degree angle of inclination of the proximal femur directs the shaft of this bone toward the midline, for eventual articulation with the tibia at the knee. Because the tibia is oriented essentially vertically while standing, the articulation between the femur and the tibia does not typically form a straight line. As shown in Figure 10-7, A, the femur usually meets the tibia to form a lateral angle of 170 to 175 degrees (the femur projects 15 to 20 degrees laterally—relative to the tibia). This alignment is referred to as normal genu valgum. Variations of this angle are not uncommon because the knee must adjust to malalignment at either the hip or the ankle. A lateral angle of less than 170 degrees is considered excessive genu valgum, or knock-kneed (Figure 10-7, B). A lateral angle greater than 180 degrees is called genu varum, giving a bow-legged appearance (Figure 10-7, C).

Figure 10-7 Frontal plane deviations of the knee. A, Normal genu valgum. B, Excessive genu valgum. C, Genu varum. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 13-6.)

Supporting Structures

The knee joint must remain stable even while subjected to large internal and external forces. In addition to muscle, stability of the knee is provided by the anterior and posterior cruciate ligaments, medial and lateral collateral ligaments, posterior capsule, and the menisci. The following sections will describe the structure and basic function of these important connective tissues. Table 10-1 summarizes the primary functions of these structures and lists common mechanisms of their injury.

Table 10-1

Functions and Mechanisms of Injury of the Supporting Structures of the Knee

Structure	Function	Most Common Mechanisms of Injury
Anterior cruciate ligament	1. Resists anterior translation of the tibia relative to a fixed femur or posterior translation of the femur relative to a fixed tibia 2. Resists the extremes of knee extension 3. Resists valgus and varus deformations and excessive horizontal plane rotations	1. Hyperextension 2. Large valgus- or varus-producing forces with foot planted 3. Either of the above combined with large torsional (rotational) force at the knee
Posterior cruciate ligament	1. Resists posterior translation of the tibia relative to a fixed femur or anterior translation of the femur relative to a fixed tibia 2. Resists the extremes of knee flexion 3. Resists valgus and varus deformations and excessive horizontal plane rotations	1. Hyperflexion of the knee 2. “Dashboard injuries” (i.e., the tibia being forcefully driven posteriorly relative to the femur) 3. Severe hyperextension (with a large gapping of the posterior side of the joint) 4. Large valgus- or varus-producing forces with foot planted 5. Any of the above combined with large torsional force at the knee
Medial collateral ligament	1. Resists valgus deformation of the knee 2. Resists excessive knee extension	1. Valgus-producing force with foot planted 2. Severe hyperextension of the knee
Lateral collateral ligament	1. Resists varus deformation of the knee 2. Resists excessive knee extension	1. Varus-producing force with foot planted 2. Severe hyperextension of the knee
Posterior capsule	1. Resists excessive knee extension	1. Severe hyperextension
Medial and lateral menisci	1. Improve the overall congruency (fit) of the tibiofemoral joint 2. Dissipate compression and shear forces across the knee	1. Extreme valgus- or varus-producing forces at the knee 2. Extreme rotations of the knee, especially under large compression forces

Anterior and Posterior Cruciate Ligaments

Cruciate, literally meaning “cross,” describes the X shape of the anterior and posterior cruciate ligaments as they interconnect the tibia with the femur (Figure 10-8). The predominant anterior-posterior direction of the cruciate ligaments stabilizes the knee against the large anterior-posterior shear forces that occur while walking and running. The anterior and posterior cruciate ligaments, therefore, are the most important stabilizers of the knee in the sagittal plane.

The anterior cruciate ligament (ACL) is frequently injured during sporting events such as soccer, football, or skiing—activities that generate a combination of large rotational, side-to-side, and hyperextension forces through the knee. The posterior cruciate ligament (PCL) is injured less frequently but may be ruptured along with the ACL. Surgery is generally required to repair a ruptured cruciate ligament by replacing the torn ligament with a tendon from another muscle (autograft) or with one harvested from a cadaver (allograft). Regardless of the type of surgical reconstruction, knowledge of the anatomy and function of the ACL and PCL is required for proper post-surgical rehabilitation and for protection of the reconstructed ligaments.

Table 10-1 summarizes the primary functions of the ACL and PCL, which are illustrated in Figures 10-9 and 10-10, respectively.

Figure 10-9 Primary sagittal plane functions of the anterior cruciate ligament. A, Resisting anterior translation of the tibia relative to the femur. B, Resisting posterior translation of the femur relative to the tibia.

Clinical insight

Why Full Active Knee Extension Is Often Avoided After Anterior Cruciate Ligament (ACL) Reconstruction

Many post-surgical ACL rehabilitation protocols limit active or forceful knee extension from 40 degrees of flexion through full extension. Activation of the quadriceps at these ranges produces anterior shear forces that pull the tibia anterior relative to the femur. One of the primary functions of the ACL is to resist the anterior shear of the tibia relative to the femur. As a measure of protection, therefore, motions that maximally challenge the integrity of the new graft, such as heavily resisted, end-range (open-chain) knee extension exercises, are often avoided in the early stages of post-ACL reconstruction rehabilitation. During this time, therapists often design knee extension exercises that promote co-contraction of the muscles about the knee, such as supervised, relatively shallow, short-arc squats. Assuming that the patient is progressing well, the later stage of rehabilitation may incorporate a safe balance between open- and closed-chain knee extension exercises. Therapists must always monitor the patient for complications after ACL reconstruction, such as increased pain, swelling, and instability, as well as prolonged atrophy within the quadriceps muscle.

Primary Functions of the Anterior and Posterior Cruciate Ligaments

Anterior Cruciate Ligament

• Resists anterior translation of the tibia relative to a fixed femur—open-chain perspective (see Figure 10-9, A)

• Resists posterior translation of the femur relative to a fixed tibia—closed-chain perspective (see Figure 10-9, B)

Posterior Cruciate Ligament

• Resists posterior translation of the tibia relative to a fixed femur—open-chain perspective (see Figure 10-10, A)

• Resists anterior translation of the femur over a relatively fixed tibia—closed-chain perspective (see Figure 10-10, B)

Medial and Lateral Collateral Ligaments

The medial and lateral collateral ligaments strengthen the medial and lateral sides of the capsule of the knee (Figure 10-11, A). These ligaments are the primary frontal plane stabilizers of the knee, protecting against forces that produce excessive genu valgus.

The wide, flat medial collateral ligament (MCL) spans the medial side of the knee between the medial epicondyle of the femur and the proximal medial tibia. The primary function of the MCL is to resist valgus-producing forces (Figure 10-11, B). Some fibers of the MCL attach to the medial meniscus of the knee; therefore, injury to the MCL may involve injury to the medial meniscus as well.

The lateral collateral ligament (LCL) is a round, cord-like ligament that crosses the lateral side of the knee, attaching to the lateral epicondyle of the femur and the head of the fibula. The primary function of the LCL is to protect the knee from varus-producing forces (Figure 10-11, C).

Although the knee is stressed only minimally in the frontal plane while walking or running, rapid changes in direction or a large impact of external forces frequently injures the collateral ligaments, especially the MCL. Consider, for example, a football player who is tackled from the side (Figure 10-12). With the foot in firm contact with the ground, the medially directed force on the lateral side of the knee creates a forceful genu valgus, often tearing the MCL.

In addition to providing most of the medial-lateral stability to the knee, the collateral ligaments become taut at full extension. This increased tension in the stretched ligaments is useful for locking the extended knee while standing—a mechanism that allows a person to periodically rest the quadriceps. However, increased tension in the ligaments does increase the likeliness of injury. The pre-stretched MCL of a fully extended knee places it much closer to its rupture point at the time of impact.

Primary Functions of the Collateral Ligaments

• Medial collateral ligament: Resists valgus-producing forces at the knee

• Lateral collateral ligament: Resists varus-producing forces at the knee

• Both ligaments are taut in full extension; assist with locking the knee

Consider this…

The “Terrible Triad”

The “terrible triad” describes the simultaneous injury of the anterior cruciate ligament, medial collateral ligament, and medial meniscus. Most often this injury occurs as a result of large rotational and valgus-producing forces to the nearly or fully extended knee, when the foot is firmly planted on the ground. It is interesting to note that there is a triad of forces (rotation, valgus, and extension) that produce injury to the “terrible triad” of knee structures.

Medial and Lateral Menisci

The medial and lateral menisci are crescent-shaped fibrocartilaginous discs located at the top of the medial and lateral condyles of the tibia (Figure 10-13). These structures play an important role in absorbing compressive forces across the knee caused by muscular contraction and body weight. While walking, compressive forces at the knee routinely reach two to three times body weight. By nearly tripling the area of joint contact and expanding outward on weight bearing, the menisci significantly reduce the pressure across the knee. Also, the cup-shaped menisci “deepen” the articular surface of the knee, facilitating the arthrokinematics and further stabilizing the joint.

Part of the medial meniscus attaches to the MCL. For this reason, excessive stress or deformation of the MCL may also damage the medial meniscus.

Primary Functions of the Menisci

• Act as shock absorbers for the knee; reduce friction and dissipate compressive forces

• Increase surface area of joint contact, thereby reducing joint pressure

• Improve joint congruency

• Facilitate normal joint arthrokinematics

Consider this…

Injury and Healing of the Menisci

The primary role of the menisci is to absorb and disperse the large compressive forces transferred through the knee joint. These fibrocartilaginous structures, however, are susceptible to injury from torsion or “grinding” of the femoral condyles against the tibia. Once injured, the menisci may not heal well. This is especially true with the inner one-third of the structure because of its poor blood supply (Figure 10-14):

• Inner one-third: Essentially avascular

• Middle one-third: Poor blood supply

• Outer one-third: Good blood supply

Injury to the outer one-third of the meniscus may heal without surgery because of its relatively good blood supply.

Posterior Capsule

The primary role of the posterior capsule is to prevent hyperextension of the knee. Two major ligaments or thickenings of the posterior capsule exist: The arcuate popliteal ligament and the oblique popliteal ligament (Figure 10-15).

A variety of musculoskeletal disorders can create marked hyperextension at the knee as the result of an imbalance of forces. Unlike the elbow, the knee joint has no bony block to resist full extension. As a result of prolonged hyperextension forces, the posterior capsule may become over-stretched. A knee that demonstrates marked hyperextension is referred to as genu recurvatum—a condition that strains the posterior capsule and many other structures of the knee.

Kinematics

Osteokinematics of the Tibiofemoral Joint

The tibiofemoral (knee) joint allows 2 degrees of freedom, flexion and extension, and internal and external rotation.

Flexion and extension occur in the sagittal plane about a medial-lateral axis of rotation. Motion occurs from about 5 degrees of knee hyperextension to about 130 to 140 degrees of flexion. As illustrated in Figure 10-16, the range of motion at the knee is the same whether viewed from an open-chain (tibial-on-femoral) or closed-chain (femoral-on-tibial) perspective. The only differences involve which bone is fixed and which is moving.

Internal and external rotation of the knee occurs within the horizontal plane about a vertical or longitudinal axis of rotation. This motion, also called axial rotation, refers to the rotation between the tibia and the femur (Figure 10-17). With the knee flexed, the knee joint permits 40 to 50 degrees of total rotation (Table 10-2); however, with the knee fully extended, essentially no rotation occurs between the two bones.

Table 10-2

Osteokinematics of the Knee

Motion	Axis of Rotation	Plane of Motion	Normal Range of Motion
Flexion Extension	Medial-lateral through the femoral condyles	Sagittal	0-140 degrees 0-5 degrees of hyperextension
Internal rotation External rotation	Vertical (longitudinal)	Horizontal	0-15 degrees (with knee flexed) 0-30 degrees (with knee flexed)