Biomechanics of theKnee Complex : 2DR. DIBYENDUNARAYAN BID [PT]THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY, RAMPURA, SURAT
Joint Capsule Given the incongruence of the knee joint, even with the improvements provided by the menisci, joint stability is heavily dependent on the surrounding joint structures. The delicate balance between stability and mobility varies as the knee is flexed from full extension toward increased flexion. Bony congruence and overall ligament tautness are maximal in full extension, representing the close- packed position of the knee joint.
In knee flexion, the periarticular passive structures tend to be lax, and the relative bony incongruence of the joint permits greater anterior and posterior translations, as well as rotation of the tibia beneath the femur. The joint capsule that encloses the tibiofemoral and patellofemoral joints is large and lax. It is grossly composed of an exterior or superficial fibrous layer and a thinner internal synovial membrane that is even more complex than the already complex fibrous portion.
In general, the outer or fibrous portion of the capsule is firmly attached to the inferior aspect of the femur and the superior portion of the tibia. Posteriorly, the capsule is attached proximally to the posterior margins of the femoral condyles and intercondylar notch and distally to the posterior tibial condyle.
The patella, the tendon of the quadriceps muscles superiorly, and the patellar tendon inferiorly complete the anterior portion of the joint capsule. The anteromedial and anterolateral portions of the capsule, as we shall see, are often separately identified as the medial and lateral patellar retinaculae or together as the extensor retinaculum. The joint capsule is reinforced medially, laterally, and posteriorly by capsular ligaments.
The knee joint capsule and its associated ligaments are critical in restricting excessive joint motions to maintain joint integrity and normal function. Although muscles clearly play a dominant role in stabilization, it is difficult to stabilize the knee with active muscular forces alone in the presence of substantial disruption of passive restraining mechanisms of the capsule and ligaments.
The joint capsule plays a role beyond that of a simple passive structure, however. The joint capsule is strongly innervated by both nociceptors as well as pacinian and Ruffini corpuscles. These mechanoreceptors may contribute to muscular stabilization of the knee joint by initiating reflex- mediated muscular responses. In addition, the joint capsule is responsible for providing a tight seal for keeping the lubricating synovial fluid within the joint space.
Synovial Layer of the Joint Capsule The synovial membrane forms the inner lining in much of the knee joint capsule. The roles of the synovial tissue are to secrete and absorb synovial fluid into the joint for lubrication and to provide nutrition to avascular structures, such as the menisci. The synovial lining of the joint capsule is quite complex and is among the most extensive and involved in the body (Fig. 11-12).
Posteriorly, the synovium breaks away from the inner wall of the fibrous joint capsule and invaginates anteriorly between the femoral condyles. The invaginated synovium adheres to the anterior aspect and sides of the ACL and the PCL. Therefore, both the ACL and the PCL are contained within the fibrous capsule (intracapsular) but lie outside of the synovial sheath (extrasynovial).
Posterolaterally, the synovial lining delves between the popliteus muscle and lateral femoral condyle, whereas posteromedially it may invaginate between the semimembranosus tendon, the medial head of the gastrocnemius muscle, and the medial femoral condyle.
The intricate folds of the synovium exclude several fat pads that lie within the fibrous capsule, making them intracapsular but extrasynovial, like the cruciate ligaments. The anterior and posterior supra-patellar fat pads lie posterior to the quadriceps tendon and anterior to the distal femoral epiphysis, respectively. The infra-patellar (Hoffa’s) fat pad lies deep to the patellar tendon (see Fig. 11-9).
Patellar Plicae Formation of the knee joint’s synovial membrane occurs in early embryonic development. Initially, the synovial membrane may separate the medial and lateral articular surfaces into separate joint cavities. By 12 weeks of gestation, the synovial septae are resorbed to some degree, which results in a single joint cavity but with retention of the posterior invagination of the synovium that forms some separation of the condyles.
The failure of the synovial membrane to become fully resorbed results in persistent folds in specific regions of the membrane. These folds are called patellar plicae.
There are four potential locations where patellar plicae may be found. Because size, shape, and frequency of these plicae vary among individuals, descriptions also vary among authors.
The most frequent locations for the plicae, in descending order of incidence, are: inferior (infrapatellar plica), superior (suprapatellar plica), and medial (mediopatellar plica) (Fig. 11-13). There is also the potential for a lateral plica, although finding this lateral plica is relatively rare.
Synovial plicae, when they exist, are generally composed of loose, pliant, and elastic fibrous connective tissue that easily passes back and forth over the femoral condyles as the knee flexes and ex- tends. On occasion, a plica may become irritated and inflamed, which leads to pain, effusion, and changes in joint structure and function, called plica syndrome.
Fibrous Layer of the Joint Capsule Superficial to the synovial lining of the knee joint lies the fibrous joint capsule, which provides passive sup- port for the joint. The fibrous joint capsule itself is composed of two or three layers, depending on location. Additional structural support to the incongruent knee joint is provided by several capsular thickenings (or capsular ligaments), as well as both intracapsular and extracapsular ligaments.
The anterior portion of the knee joint capsule is called the extensor retinaculum. A fascial layer covers the distal quadriceps muscles and extends inferiorly. Deep to this layer, the medial and lateral retinacula are composed of a series of transverse and longitudinal fibrous bands connecting the patella to the surrounding structures (Fig. 11-14).
Medially, the thickest and clinically most important band within the medial retinaculum is the medial patellofemoral ligament (MPFL).
Its fibers, oriented in a transverse manner, course anteriorly from the adductor tubercle of the femur to blend with the distal fibers of the vastus medialis and eventually insert onto the superomedial border of the patella. The transversely oriented fibers within the lateral retinaculum, called the lateral patellofemoral ligament, travel from the iliotibial (IT) band to the lateral border of the patella.
The remainder of the retinacular bands include the obliquely oriented medial patellomeniscal ligament and the longitudinally positioned medial and lateral patellotibial ligaments (see Fig. 11-14). The medial portion of the joint capsule is com-posed of the deep and superficial portions of the MCL. The most superficial layer of the joint capsule on the medial side of the knee joint is a fascial layer that covers the vastus medialis muscle anteriorly and the sartorius muscle posteriorly.
Laterally, the joint capsule is composed superficially of the IT band and its thick fascia lata. The capsule is reinforced posterolaterally by the arcuate ligament and posteromedially by the posterior oblique ligament (POL).