CHAPTER 12 Structure and Function of the Knee Posterior view Clinical Connection 12.1


Although ACL injuries can occur at any time across the life span, ACL ruptures most frequently occur between the ages of 15 and 25 years. The injury is costly in terms of health care and rehabilitation services and in lost fi nancial income. A high per- centage of individuals with ACL injuries are unable to return to the level of sports activity they par- ticipated in before their injury. ACL injuries have also been identifi ed as a major risk factor for the development of knee osteoarthritis. Typically, the injury involves a noncontact mech- anism that occurs when landing a jump, decelerat- ing, cutting, or pivoting over a single leg when the foot is planted on the ground. The knee is usually near full extension and, with ground reaction forces lateral to the knee, most often sustains a marked “valgus collapse.” There may also be excessive lateral rotation at the knee measured by a femur that is medially rotating on a relatively fi xed tibia. There is a correlation between female subjects who display excessive knee valgus angles during a verti- cal jump drop test and ACL injuries. Injury prevention programs should be matched to the needs and abilities of the individual and may include core, strength, plyometric, and balance/ proprioception training. Studies show that after reconstruction surgery following a ruptured ACL, the individual should undergo an intensive reha- bilitation program that includes core strength and stability exercises; balance training; and exercises that focus on function, agility, control, and power to prevent reinjury and maximize optimal return to function.


Medial condyle Lateral condyle


Lateral collateral ligament


Medial collateral ligament


Medial meniscus


Medial tibial condyle


Anterior cruciate ligament


Lateral meniscus


Posterior cruciate ligament


Figure 12.17 Posterior view of the deep structures of the knee.


fi xed but migrates with the femoral condyles, as shown in Figure 12.19. The migrating axis changes the internal moment arm of the muscles. Average range of motion is from approximately 130° of fl exion to approximately 5° beyond the 0° where the knee is in the straight posi- tion (Fig. 12.20). Hyperextension of the knee beyond 10° from the neutral position is called genu recurvatum (Fig. 12.21). This malalignment can be caused by weak quadriceps muscles or by a tight gastrocnemius muscle and can lead to wearing of the knee articular cartilage. Knee fl exion and extension occurs in a femur-on-tibia closed chain motion, such as when moving from a sitting to a standing and then back to a sitting position. These motions can also occur with the tibia moving on the femur in an open chain motion. Kicking a ball is an example of the tibia extending on the more stable femur.


The amount of knee range of motion required to perform various functional activities is shown in Table 12.3. As the tibiofemoral joint fl exes and extends, there is a sliding motion between the patella and the intercondylar groove, as the patella functions like a cable on a pulley system (Fig. 12.22A). During open chain fl exion, the patella moves in the same direction as the tibia and slides inferiorly in the groove. During extension, the patella slides superiorly and slightly laterally and posteriorly. In closed chain knee fl exion and extension, the femoral groove is sliding in relation to the fi xed patella. Between 90° and 60° of fl exion, the patella is in maximal contact with the intercondylar groove (Fig. 12.22C). In this posi- tion, there can be large patellofemoral joint compression forces. During the last 20° to 30° of extension, the infe- rior pole of the patella is the only point of contact and in full extension is proximal to the groove (Fig. 12.22D). At greater knee fl exion ranges, the patella rests below the intercondylar groove.


Patellar Tracking


Optimal tracking of the patella in the intercondylar groove is necessary for pain-free functional knee motion. Several factors infl uence this tracking, and when tracking is suboptimal, the patellofemoral joint can be exposed to higher contact stresses. Such stresses can lead to patel- lofemoral syndrome or degenerative osteoarthritis. A shortened tight IT band or lateral patellar retinacu-


lar fi bers can contribute to the lateral pull of the patella. Lax medial retinacular tissues and a lax MCL can also be contributing factors. Typically, the height of the lateral facet of the intercondylar groove resists lateral patellar


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