ANATOMY & BIOMECHANICS
The knee is a diarthrodial joint with an intricate mechanical architecture.Compared with the ball-and-socket joint of the hip, knee movements are by far more complex, with greater definition by bone and ligament anatomy.
Flexion and extension of the knee create a simultaneous internal and external axial rotation of the tibio-femoral joint. When the knee flexes the tibia (shin bone) rotates internally and when the knee extends, the tibia rotates externally (scerw home mechanism). In full extension, the knee locks into a very stable position for the stance phase of walking. This external rotation with extension is produced by the complex geometry of the opposing surfaces of femur and tibia and the winding of the cruciate ligaments around each other.
MENISCUS
or
'SHOCKABSORBER CARTILAGE'
Menisci are extremely important for physiological knee function. Not only do they provide shock absorption but also increase the contact area between femur and tibia and hence distribute pressure more evenly. All this is facilitated by the soft texture and a specific cellular architecture, with longitudinal arrangement of collagen fibres, allowing the absorption of hub stresses transmitted across it during walking and load-bearing. Menisci also add to joint stability through being crescent shaped with a triangular cross-section allowing the round-shaped femur to sit snug on the flat surface of the tibia like a cup on a saucer.
Their importance was undervalued in the past, which led surgeons to liberally remove them when torn. Consequently most patients developed a degree of pre-mature osteoarthritis within a few years after Meniscectomy as was shown by Fairbank and Baratz (Fairbank 1948, Baratz et al. 1986). These authors suggest that up to 95% of patients will ahve developed OA at 20 years post meniscectomy. Since the advent of arthroscopy it is possible to preserve most of the meniscus by only removing the damaged part. Certain meniscal tears (bucket handle configuration) are amenable to repair either with sutures (very strong), or meniscal arrows (weak). Tissue healing is only promoted if a repair has got sufficient blood supply, which in meniscal tissue is limited to the outer 1/3. Repairing meniscal tears is technically difficult and above all requires protected weight bearing until healing is secured (6 - 8 weeks). The patient should in any case be made aware that the repair might fail and a partial meniscectomy be required.
Menisci are exposed to a degree of natural degeneration, which occures with age. This process usually starts within the meniscus, and can affect anyone from the age of 30 onwards, independant of the level of sporting activities. This process of degeneration causes the core of the meniscus to get liquified, just like a doghnut. Once degenerate, the meniscus is less well equiped to withstand shear forces or compressive stresses and may therefore tear very easily. In fact the vast majority of patients presenting to the clinician with a degeneartive meniscal tear do not give a history of trauma, but were doing activities such as prolonged squatting, kneeling or gardening after which symptoms occured. These symptoms are mainly described as either a dull ache after walking on uneven ground or sharp stabbing pains across the joint on twisting and turning.
HYALINE CARTILAGE
or
'SURFACE CARTILAGE'
Articulating surfaces of femur and tibia are covered with Hyaline Cartilage, to provide smooth and almost friction free movement. Cartilage consists predominately of water (70%), Collagen and Proteoglycanes (30%), whilst cartilage cells (Chondrocytes) represent only 2 - 5 %. Cartilage has no blood supply and therefore absorbs nutrients from the surrounding joint fluid (Synovial Fluid). The absence of blood within cartilage and the small amount of viable chondrocytes are responsible for the fact that cartilage once damaged has no potential of healing by itself. Damage to cartilage is defined in four grades and may either affect superficial layers (partial thickness damage = grade I-III), or a whole piece of cartilage may have been lost in which case the underlying bone will be exposed (full thickness damage = grade IV).
LIGAMENTS
Butler et al. introduced the concept of primary and secondary stabilisers in the human knee. He demonstrated that the Anterior Cruciate Liganment or ACL is a primary restraint to anterior translation of the tibia on the femur (Butler et al. 1980). In doing so the ACL provides an average restraint of 87.2% to the applied load at 30 degrees of knee flexion. After ACL transection, the remaining ligaments provided little restraint to anterior subluxation, leading Butler to describe the function of the remaining soft-tissue structures as secondary restraint to this particular motion.
diverging curvature of the femoral condyles, moving in concert on the differently shaped lateral and medial tibial plateau
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