268


PART IV THE LOWER EXTREMITY


achieve optimal congruency (Fig. 12.26B). This rotation is referred to as the screw-home mechanism. Tension in the ACL and the lateral pull of the quadriceps also infl uence this screw-home mechanism that occurs during knee extension (Fig. 12.26C). As the knee is unlocked from extension, in a tibia-on-femur open chain motion, the tibia slightly rotates in a medial direction. This rota- tion is mechanically linked to the osteokinematics of knee fl exion and extension and cannot be performed indepen- dent of these movements.


A Lateral Intercondylar groove Full extension Lateral epicondyle


Medial epicondyle Screw-home rotation


30° flexion


60° flexion 90° flexion


B


1. Shape of medial femoral condyle


Medial


During closed chain knee extension, where the femur is moving on the tibia, the knee locks as the femur rotates medially on the fi xed tibia. As this motion occurs, the tibia is laterally located relative to the medially rotated femur; it is just that the knee locks as a result of move- ment of the femur rather than the tibia.


Flexion


During knee fl exion, the arthrokinematics occur in the reverse direction from knee extension. To fl ex the knee with the tibia moving on the femur, the tibia must fi rst rotate medially. The popliteus muscle facilitates this action. (This function is discussed further in the section on muscles.) The muscle also is responsible for the lateral rotation of the femur to initiate the femur moving on the tibia to fl ex the knee in a closed chain motion.


Medial and Lateral (Axial) Rotation


2. Tension in anterior cruciate ligament


3. Lateral pull of quadriceps


MUSCLES


External rotation


C Extension


Figure 12.26 (A) Anterior view. The medial femoral condyle is larger than the lateral condyle. (B) Superior view. The arrows depict the path of the tibia as it moves across the femur during open chain knee extension. Because of the difference in size of the femoral condyles and the curved orientation of the medial femoral condyle, the tibia rotates laterally to its laterally rotated locked position. (C) The shape of the medial femoral condyle, the tension in the ACL, and the lateral pull of the quadriceps muscle are all factors that contribute to the screw-home rotation of the tibia.


The muscles of the knee fl ex, rotate, and extend the knee. The attachment sites of these muscles and their motor and sensory innervations are outlined in Appendix C, which is after chapter 14.


KNEE EXTENSORS


The quadriceps femoris muscle group crosses the knee anteriorly and is the primary mover of knee extension. The quadriceps femoris is a massive and powerful muscle group consisting of the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The vastus media- lis and vastus lateralis arise from the anterolateral femoral shaft and feature oblique fi bers that approach the patellar tendon at an angle to the quadriceps tendon. The vastus medialis has two distinct fi ber directions: longitudinal


The medial and lateral axial rotation at the knee is not the accessory rotation that occurs during the terminal locking and unlocking of the knee joint. Axial rotation can occur only with the knee in fl exion. During rotation, there is an arthrokinematic spin between the menisci and the femoral and tibial articular surfaces.


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