The knee consists of the tibiofemoral joint and the patellofemoral joint. As illustrated in Figure 10-1, the tibiofemoral joint is formed between the large condyles of the distal femur and the relatively flat proximal tibia. The patellofemoral joint is formed between the patella and the distal femur. Both joints are considered anatomic components of the knee.

Figure 10-1 Radiograph showing the bones and associated articulations of the knee. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 13-1.)

Motion at the knee occurs in two planes: Flexion and extension in the sagittal plane and internal and external rotation in the horizontal plane. Most activities, however, require that the knee move simultaneously in both planes such as while running and quickly changing directions. Also, because the knee functions as the middle joint of the lower extremity, most everyday motions such as standing up from a seated position require simultaneous movement at the hip and ankle. This kinematic interdependence is evident by the many multi-articular muscles of the lower extremity such as the hamstrings, rectus femoris, and gastrocnemius. The anatomic and kinesiologic relationships among the knee, hip, and ankle provide the foundation for many treatment strategies used in rehabilitation of the knee.

Unlike the hip joint, the tibiofemoral joints of the knee lack a deep concave socket. From the perspective of bony fit, therefore, the knee is relatively unstable. Many strong ligaments and muscles are therefore required for stabilization, making these soft tissues vulnerable to injury. Many principles of rehabilitation and assessment of the knee require a firm working knowledge of the anatomy of these soft tissues. The anatomy and function of these tissues is an important theme of this chapter.

Osteology

Distal Femur

The medial and lateral condyles (from the Greek kondylos, meaning “knuckle”) are the large rounded projections of the distal femur that articulate with the medial and lateral condyles of the tibia. The intercondylar groove is the smooth rounded area between the femoral condyles that articulates with the posterior surface of the patella (Figure 10-2). The intercondylar notch is located on the posterior-inferior aspect of the distal femur, separating the medial and lateral condyles. This notch forms a passageway for the anterior and posterior cruciate ligaments. The medial and lateral epicondyles (Figure 10-3) are palpable bony projections on the medial and lateral femoral condyles, respectively; these projections serve as attachments for the medial and lateral collateral ligaments of the knee.

Proximal Tibia

The medial and lateral condyles of the tibia are smooth and shallow for articulation with the condyles of the femur (Figure 10-4). The flattened superior surfaces of the condyles are often called the tibial plateau. The intercondylar eminence is a double-pointed projection of bone separating the medial and lateral condyles of the tibia. This structure serves as an attachment for the anterior and posterior cruciate ligaments, as well as for the medial and lateral meniscus. The tibial tuberosity is a protrusion of bone located on the anterior aspect of the proximal tibia, which serves as the distal attachment for the quadriceps muscle.

Figure 10-4 Anterior view of the tibia showing the tibial condyles, intercondylar eminence, and tibial tuberosity.

Consider this…

Osgood-Schlatter Disease

Activities such as running and jumping require the quadriceps to generate large knee extension forces. The force generated from these muscles pulls strongly on the tibial tuberosity via the patellar tendon. During adolescence, excessively large and frequent quadriceps contractions may overwhelm the structural integrity of the immature bone of the tibia. As a result, fragments of immature bone are pulled from the tibial tuberosity. This condition, known as Osgood-Schlatter disease, is often accompanied by pain and enlargement of the tibial tuberosity.

Proximal Fibula

The fibula (see Figure 10-4) is a long, slender bone that courses along the lateral shaft of the tibia. The fibular head is the rounded superior portion of the fibula that articulates with the superior-lateral aspect of the tibia. This articulation forms the firm proximal tibiofibular joint. The head of the fibula is the distal attachment of the lateral collateral ligament and the biceps femoris muscle.

Patella

The patella, or knee cap, is a small, plate-like bone embedded within the quadriceps tendon. Because the patella exists within the quadriceps tendon, it is highly mobile and is at risk for abnormal gliding or subluxation. A portion of the stability provided to the patellofemoral joint comes from the fit of the posterior patella within the intercondylar groove of the femur (Figure 10-5, A). The base, or superior pole, of the patella accepts the quadriceps tendon; the apex, or inferior pole, accepts the proximal side of the patellar tendon (Figure 10-5, B). The posterior articular surface of the patella articulates with the intercondylar groove of the femur through medial and lateral facets. The lateral facet is steeper than the medial facet, matching the general shape of the intercondylar groove of the femur (Figure 10-5, C).

Arthrology

General Features

The tibiofemoral and patellofemoral joints both provide a unique contribution to the overall kinesiology of the knee. Consider walking, for example, in which the motion of the tibiofemoral joint is essential to the natural forward progression of the leg. Connective tissues surrounding the tibiofemoral joint not only guide these movements but also stabilize the articulation, as well as absorb and transmit forces. The surrounding musculature adds another critical element of stability and shock absorption across the knee.

The patellofemoral joint protects the delicate structures within the knee and improves the moment arm for the quadriceps, thereby improving the extensor torque-producing potential of this muscle group. Strong activation of the quadriceps produces proportionately large compression forces between the patella and the femur. These large compressive forces are important to consider when designing quadriceps strengthening exercises or when giving advice to patients on how to protect painful or arthritic knees from damaging forces.

Consider this…

Patellar Motion and Stability

The patella plays an integral part in the normal kinematics of the knee by:

• Acting as a transmitter of quadriceps force across the knee

• Enhancing the leverage (internal moment arm) of the quadriceps muscle

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.

Figure 10-12 Large valgus-producing force at the knee resulting from a tackle against the lateral aspect of the extended knee.

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.

Figure 10-13 Superior view of the tibia showing the shapes of the medial and lateral meniscus. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 13-11, B.)

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):

Figure 10-14 Blood supply to the meniscus. The outer regions **(A)** have good blood supply, the middle portion **(B)** has moderate blood supply, and the inner portion **(C)** has little to no blood supply. (Modified from Shankman G: Fundamental orthopedic management for the physical therapist assistant, ed 3, St Louis, 2011, Mosby, Figure 18-20, A.)

• 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.

Figure 10-16 Sagittal plane motion of the knee. A, Tibial-on-femoral perspective. B, Femoral-on-tibial perspective. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 13-13.)

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)

Closed-chain (femoral-on-tibial) axial rotation of the knee is an important but often over-looked motion. Consider, for example, a sharp 90-degree cutting motion while running. With the foot and attached tibia fixed to the ground, the rotating femur creates a critical horizontal plane pivot point for the entire trunk and upper body. Large force demands are placed on the muscles and ligaments of the knee as the femur accelerates or decelerates relative to the fixed tibia. This large demand partially explains the relatively high incidence of injury to the knee resulting from cutting motions associated with high-speed sporting events.

Consider this…

Common Mechanisms of Injury to the ACL

The ACL is the most frequently totally ruptured ligament of the knee. Approximately half of all ACL injuries involve individuals between 15 and 25 years of age, and most often occur during a sporting activity such as soccer, basketball, football, or skiing. It is interesting to note, however, that nearly 70% of all sport-related ACL injuries occur during non-contact or minimal contact situations. Many of these non-contact injuries occur when an individual is landing from a jump or quickly decelerates during a change of direction such as when “cutting” to the left or the right over a planted lower extremity. Research strongly suggests that three biomechanical factors that, when combined, put the ACL at high risk for injury: (1) Strong activation of the quadriceps muscle over a slightly flexed or fully extended knee, (2) a marked “valgus collapse” of the knee, and (3) excessive external rotation of the knee (often this occurs as excessive internal rotation of the femur relative to a fixed tibia).

Figure 10-18 shows an image of a young healthy woman landing from a jump in a manner that puts the ACL at high risk for injury. Notice, in particular, the extreme valgus position of the knee combined with internal rotation of the femur relative to the tibia (external rotation of the knee). Evidence indicates that weakness (and equally important—poor control) of the hip abductors and external rotators contributes to “valgus collapse” of the knee. Therefore, clinicians who work on ACL prevention programs with athletes often incorporate strengthening and neuromuscular re-education techniques for the abductors and external rotators of the hip to help dynamically control the position of the knee.