Biomechanics of theKnee Complex : 2DR. DIBYENDUNARAYAN BID [PT]THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY, RAMPURA, SURAT
Menisci Tibiofemoral congruence is improved by the medial and lateral menisci, forming concavities into which the femoral condyles sit (Fig. 11-8). In addition to enhancing joint congruence, these accessory joint structures play an important role in distributing weight-bearing forces, in reducing friction between the tibia and the femur, and in serving as shock absorbers. The menisci are fibrocartilaginous disks with a semicircular shape.
The medial meniscus is C-shaped, whereas the lateral meniscus forms four fifths of a circle.8 Lying within the tibiofemoral joint, the menisci are located on top of the tibial condyles, covering one half to two thirds of the articular surface of the tibial plateau (Fig. 11-9). Both menisci are open toward the intercondylar tubercles, thick peripherally and thin centrally. The lateral meniscus covers a greater percentage of the smaller lateral tibial surface than the medial meniscus.
As a result of its larger exposed surface, the medial condyle has a greater susceptibility to the enormous compressive loads that pass through the medial condyle during routine daily activities. Although compressive forces in the knee may reach one to two times body weight during gait and stair climbing and three to four times body weight during running, the menisci assume 50% to 70% of this imposed load.
These loads, however, can be influenced by the presence of frontal plane malalignment. The greater the degree of genu varum, for instance, the greater is the compression on the medial meniscus.
Meniscal Attachments The open anterior and posterior ends of the menisci are called the anterior and posterior horns, each of which is firmly attached to the tibia below. Meniscal motion on the tibia is consequently limited by multiple attachments to surrounding structures, some common to both menisci and some unique to each.
The medial meniscus has greater ligamentous and capsular restraints, limiting translation to a greater extent than the lateral meniscus. The relative lack of mobility of the medial meniscus may contribute to its greater incidence of injury.
Anteriorly, the menisci are connected to each other by the transverse ligament. Both menisci are also attached directly or indirectly to the patella via the patellomeniscal ligaments, which are anterior capsular thickenings. At the periphery, the menisci are connected to the tibial condyle by the coronary ligaments, which are composed of fibers from the knee joint capsule. Some of these connections can be seen in Figure 11-9.
The medial meniscus has less relative motion than does the lateral meniscus, and it is more firmly attached to the joint capsule through medial thickening of the joint capsule that extends distally from the femur to the tibia. This capsular thickening, referred to as the deep portion of the medial collateral ligament (MCL), further restricts the motion of the medial meniscus.
The anterior and posterior horns of the medial meniscus are attached to the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), respectively. Through capsular connections, the semimembranosus muscle connects to the medial meniscus. Posteriorly, the lateral meniscus attaches to the PCL and the medial femoral condyle through the meniscofemoral ligaments.
Some of the ligamentous attachments are shown in Figure 11-10. In much the same way that the semimembranosus tendon is attached to the medial meniscus, the tendon of the popliteus muscle attaches to the lateral meniscus. The attachment to the popliteus tendon helps restrain or control the motion of the lateral meniscus.
Role of the Menisci The strong attachments to the menisci prevent them from being squeezed out during compression of the tibiofemoral joint, allowing for greater contact area between the menisci and the femur. If the femoral condyles sat directly on the relatively flat tibial plateau, there would be little contact between the bony surfaces.
With the addition of the menisci, the contact at the tibiofemoral joint is increased and joint stress (force per unit area) is, therefore, reduced on the joint’s articular cartilage (Fig. 11-11).
After the removal of a meniscus, the contact area in the tibiofemoral joint is decreased, which thus increases joint stress. Specifically, removal of the menisci nearly doubles the articular cartilage stress on the femur and multiplies the forces by six or seven times on the tibial plateau. The increase in joint stress may contribute to degenerative changes within the tibiofemoral joint.
For this reason, total meniscectomies are rarely performed after a meniscal tear; instead, care is taken to preserve as much of the meniscus as possible, either through débridement (removal of damaged tissue) or repair.
Meniscal Nutrition and Innervation The location of a meniscal lesion and the age of the patient influence the options available after injury because of the capacity of the meniscus to heal. During the first year of life, the meniscus contains blood vessels throughout the meniscal body. Once weight-bearing is initiated, vascularity begins to diminish until only the outer 25% to 33% is vascularized by capillaries from the joint capsule and the synovial membrane.
After 50 years of age, only the periphery of the meniscal body is vascularized. Therefore, the peripheral portion obtains its nutrition through blood vessels, but the central portion must rely on the diffusion of synovial fluid.
The process of fluid diffusion to support nutrition requires intermittent loading of the meniscus by either weight-bearing or muscular contractions. Subsequently, during prolonged periods of immobilization or conditions of non–weight-bearing, the meniscus may not receive appropriate nutrition.
The avascular nature of the central portion of the meniscus reduces the potential for healing after an injury. In adults, only the peripheral vascularized region of the meniscus is capable of inflammation, repair, and remodeling after a tearing injury.
The horns of the menisci and the peripheral vascularized portion of the meniscal bodies are well innervated with free nerve endings (nociceptors) and three different mechanoreceptors (Ruffini corpuscles, pacinian corpuscles, and Golgi tendon organs). The presence of nociceptors in the meniscus could explain some of the pain felt by patients after a meniscal tear, at least for tears located in the periphery.
Proprioceptive deficits may potentially occur after meniscal injury as a result of injury to the mechanoreceptors within the meniscus.