Biomechanics of theKnee Complex : 1DR. DIBYENDUNARAYAN BID [PT]THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY, RAMPURA, SURAT
Introduction The knee complex is one of the most often injured joints in the human body. The myriad of ligamentous attachments, along with numerous muscles crossing the joint, provide insight into the joint’s complexity. This anatomic complexity is necessary to allow for the elaborate interplay between the joint’s mobility and stability roles.
The knee joint works in conjunction with the hip joint and ankle to support the body’s weight during static erect posture. Dynamically, the knee complex is responsible for moving and supporting the body during a variety of both routine and difficult activities. The fact that the knee must fulfill major stability as well as major mobility roles is reflected in its structure and function.
The knee complex is composed of two distinct articulations located within a single joint capsule: the tibiofemoral joint and the patellofemoral joint. The tibiofemoral joint is the articulation between the distal femur and the proximal tibia. The patellofemoral joint is the articulation between the posterior patella and the femur.
Although the patella enhances the tibiofemoral mechanism, the characteristics, responses, and problems of the patellofemoral joint are distinct enough from the tibiofemoral joint to warrant separate attention. The superior tibiofibular joint is not considered to be a part of the knee complex because it is not contained within the knee joint capsule and is functionally related to the ankle joint.
Structure of the Tibiofemoral Joint The tibiofemoral, or knee, joint is a double condyloid joint with three degrees of freedom of angular (rotatory) motion. Flexion and extension occur in the sagittal plane around a coronal axis through the epicondyles of the distal femur, medial/lateral (internal/external) rotation occur in the transverse plane about a longitudinal axis through the lateral side of the medial tibial condyle, and abduction and adduction can occur in the frontal plane around an anteroposterior axis.
The double condyloid knee joint is defined by its medial and lateral articular surfaces, also referred to as the medial and lateral compartments of the knee. Careful examination of the articular surfaces and the relationship of the surfaces to each other are necessary for a full understanding of the knee joint’s movements and of both the functions and dysfunctions common to the joint.
Femur The proximal articular surface of the knee joint is composed of the large medial and lateral condyles of the distal femur. Because of the obliquity of the shaft of the femur, the femoral condyles do not lie immediately below the femoral head but are slightly medial to it (Fig. 11-1A).
As a result, the lateral condyle lies more directly in line with the shaft than does the medial condyle. The medial condyle therefore must extend further distally, so that, despite the angulation of the femur’s shaft, the distal end of the femur remains essentially horizontal.
In the sagittal plane, the condyles have a convex shape, with a smaller radius of curvature posteriorly (see Fig. 11-1B). Although the distal femur as a whole has very little curvature in the frontal plane, both the medial and lateral condyles individually exhibit a slight convexity in the frontal plane. The lateral femoral condyle is shifted anteriorly in relation to the medial femoral condyle.
In addition, the articular surface of the lateral condyle is shorter than the articular surface of the medial condyle. When the femur is examined through an inferior view (Fig. 11-2), the lateral condyle appears at first glance to be longer. However, when the patellofemoral surface is excluded, it can be seen that the lateral tibial surface ends before the medial condyle.
The two condyles are separated inferiorly by the intercondylar notch through most of their length but are joined anteriorly by an asymmetrical, shallow groove called the patellar groove or surface that engages the patella during early flexion.
Tibia The asymmetrical medial and lateral tibial condyles or plateaus constitute the distal articular surface of the knee joint (Fig. 11-3A). The medial tibial plateau is longer in the anteroposterior direction than is the lateral plateau; however, the lateral tibial articular cartilage is thicker than the articular cartilage on the medial side.
The proximal tibia is larger than the shaft and, consequently, overhangs the shaft posteriorly (see Fig. 11-3B). Accompanying this posterior overhang, the tibial plateau slopes posteriorly approximately 7° to 10°.
The medial and lateral tibial condyles are separated by a roughened area and two bony spines called the intercondylar tubercles (Fig. 11-4). These tubercles become lodged in the intercondylar notch of the femur during knee extension.
The tibial plateaus are predominantly flat, with a slight convexity at the anterior and posterior margins, which suggests that the bony architecture of the tibial plateaus does not match up well with the convexity of the femoral condyle. Because of this lack of bony stability, accessory joint structures (menisci) are necessary to improve joint congruency.
Tibiofemoral Alignment and Weight-Bearing Forces The anatomic (longitudinal) axis of the femur, as already noted, is oblique, directed inferiorly and medially from its proximal to distal end. The anatomic axis of the tibia is directed almost vertically.
Consequently, the femoral and tibial longitudinal axes normally form an angle medially at the knee joint of 180° to 185°; that is, the femur is angled up to 5° off vertical, creating a slight physiologic (normal) valgus angle at the knee (Fig. 11-5).
If the medial tibiofemoral angle is greater than 185, an abnormal condition called genu valgum (“knock knees”) exists. If the medial tibiofemoral angle is 175° or less, the resulting abnormality is called genu varum (“bow legs”). Each condition alters the compressive and tensile stresses on the medial and lateral compartments of the knee joint.
An alternative method of measuring tibiofemoral alignment is performed by drawing a line from the center of the femoral head to the center of the head of the talus (see Fig. 11-5). This line represents the mechanical axis, or weight bearing line, of the lower extremity, and in a normally aligned knee, it will pass through the center of the joint between the intercondylar tubercles.
The weight-bearing line can be used as a simplification of the ground reaction force as it travels up the lower extremity. In bilateral stance, the weight-bearing stresses on the knee joint are, therefore, equally distributed between the medial and lateral condyles (or medial and lateral compartments).
However, once unilateral stance is adopted or dynamic forces are applied to the joint, compartmental loading is altered. In the case of unilateral stance (e.g., during the stance phase of gait), the weight-bearing line must shift medially across the knee to account for the now smaller base of support below the center of mass (Fig. 11-6A).
This shift increases the compressive forces on the medial compartment (see Fig. 11-6B). Abnormal compartmental loading may be also be caused by frontal plane malalignment (genu varum or genu valgum). Genu valgum, for instance, shifts the weight-bearing line onto the lateral compartment, increasing the lateral compressive force while increasing the tensile forces on the medial structures (Fig. 11-7A).
whereas the tensile stresses are increased laterally (see Fig. 11-7B). The presence of genu valgum or genu varum creates a constant overload of the lateral or medial articular cartilage, respectively, which may result in damage to the cartilage and the development of frontal plane laxity. Genu varum, for instance, may con-tribute to the progression of medial compartment knee.
In the case of genu varum, the weight-bearing line is shifted medially, increasing the compressive force on the medial condyle, causes osteoarthritis and lead to excessive medial joint laxity as the medial capsular ligament’s attachment sites are gradually approximated through the erosion of the medial compartment’s articular cartilage.