The knee contains two menisci, the medial and lateral (Tortora & Grabowski 2000). The menisci are both C shaped structures, made up of semilunar fibro-cartilage which is arranged parallel to the circumference (Evans 1986). The lateral meniscus is nearly circular and has a constant width (Peterson & Renstöm, 2001). The posterior horn merges with the posterior portion of the anterior cruciate ligament (ACL), whilst its anterior horn merges with ACL’s attachment (Segal & Jacob 1983). It has a range of movement of approximately 1cm. The medial meniscus is less circular than its lateral counterpart, but also is equal in width, and is 3-4cm in length (Peterson & Renstöm, 2001). The medial meniscus’ border is attached to the joint capsule, its anterior horn is attached to the anterior surface of the tibia and the posterior horn is attached to the posterior surface of the tibia (Gray, 2002).
As previously mentioned the menisci help to stabilise the knee statically, the lateral meniscus less so than the medial, against anteroposterior motion (Shelbourne, Rask & Hunt, 1999). These structures, described as “extensions of the tibia” by Engebretsen et al (2003, p.564), were previously thought to be non-essential, and were removed if damaged. It has now been shown that they have a protective role, transmitting between “30-70% of the load applied across the joint” (Peterson & Renstöm 2001, p.297) by elongating when the femur applies pressure on the tibia. Shelbourne, Rusk & Hunt claim this number is slightly higher, with the meniscus transmitting 50-100% of the load (1997).When the knee is flexed, to 90 , the menisci transmits approximately 85% of the load applied, it transmits at least 50 % when the knee is extended. The menisci have shock absorbing properties, and are most effective under low loading rates. They aid the articular cartilage by limiting the shock, which would have to be absorbed by it, and by compressing synovial fluid into the cartilage Segal & Jacob, 1983).
The bursae are sacs of synovial membrane, and within the knee they help to prevent frictional forces, created between knee structures, from damaging the joint (Norkin & Levangie 1992). The bursae, of the knee can be separated into two groups; those that communicate with the synovial capsule and those that do not (Segal & Jacob, 1983). The bursae, which do not communicate with the capsule, include the deep infrapatellar bursa, the subcutaneous infrapatella bursa and the prepatella bursa. Those that do communicate with the capsule are; the suprapatella bursa, the gastrocnemius bursa and the subpopliteal bursa. The deep infrapatella bursa is located between, and reduces friction between, the tibial tuberosity and the patellar ligament joint (Norkin & Levangie 1992). The subcutaneous infrapatella bursa separates the skin and the patella ligament. The prepatella ligament is situated between the anterior surface of the patella and the skin, during flexion and extension it allows smooth movement of the skin over the patella. The suprapatella bursa separates the anterior aspect of the femur and the quadriceps tendon. The gastocnemius bursa is found between the medial femoral condyle and the tendon of the medial head of the gastrocnemius (Segal & Jacob, 1983). The subpopliteal bursa is situated between the popliteus muscle tendon and the lateral femoral condyle. During flexion and extension the suprapatella bursa, the gastrocnemius bursa and the subpopliteal bursa compress, this aids the synovial fluid to move across the joint and lubricate it joint (Norkin & Levangie 1992).
The knee also contains a number of ligaments, which connect bone to bone, limit the range of movement and prevent movement in undesired planes (Bartlett, 2001). They’re very strong, white fibrous structures, made up of 10% elastic fibre and 90% collagen (dry weight); water comprises approximately 70% of wet weight (Norris, 1993). The ligaments working with the knee joint are both extra-capsular and intra-capsular ligaments. The extra-capsular ligaments include; the patella ligament (PLg), the medial collateral ligament (MCL), the lateral collateral ligament (LCL), and the oblique and arcuate popliteal ligaments (Tortora & Grabowski 2000). The intra-capsular ligaments are the posterior cruciate ligament (PCL), the anterior cruciate ligament (ACL), and the anterior intermesical ligament (AIL) (Tortora & Grabowski, 2000, Nelson & LaPrade2000).
The tendon of the quadriceps femoris continues to form the patella ligament; it extends, approximately 3 inches, to attach to the tibial tuberosity (Gray 2002). A deposit of adipose tissue separates the posterior surface from the patella. The ligament is essential for extension of the knee.
The MCL provides much of the knees medial stability and limits outward rotation. Its attachment is located on the medial femoral condyle; the ligament then runs distally, attaching to the tibia in an assortment of places (Terry & LaPrade 1996). It is made up of three distinct units; the posterior oblique ligament, the superficial MCL, and the deep MCL, which work as an integrated unit (Peterson & Renstöm, 2001). During external rotation the MCL is tight; the anterior fibres loosen during extension, whilst the posterior fibres become taut. When the knee is flexed at 25 the superficial MCL is responsible for 78.2% of medial stability, and the deep MCL accounts for 7.6%; when flexed at 5 superficial MCL accounts for 57.4% and the deep MCL is responsible for 25.2% (Grood, Noyes, Butler & Suntay 1981, in Kjær et al 2003).
The LCL is often grouped with the popliteus complex to form the posterolateral complex. It is 66±6 mm long and has femoral attachments halfway between the distal and posterior borders of the lateral femoral condyle (Meister, Michael, Moyer, Kelly & Schneck, 2000). The LCL extends distally and fans out to attach to the popliteus tendon (Laprade, Thuan, Wentorf & Engebretsen 2003).This complex limits outward rotation, but it stabilises the lateral aspect of the knee. It allows internal and external rotation by slackening during flexion (Fowler 1980).
The oblique popliteal ligament originates from the intercondylar fossa of the femur and extends to the head of the tibia (Fowler 1980). The arcuate popliteal ligament originates from the lateral condyle of the femur and extends to the styloid process of the head of the femur (Terry & LaPrade 1996). These two ligaments, along with the semimembranosus tendon strengthen the posterior aspect of the joint (Tortora & Grabowski, 2000).
Two cruciate ligaments, named posterior and anterior due to the positioning of their attachment to the tibia, are found in the intercondylar notch of the knee (Fowler 1980). The PCL and ACL both get thinner towards the middle of the structure and then flare out towards the relevant attachment. The fibres are organised, both, spirally and in a straight line between the origin and attachment. The ligaments are made up of three bands; the anteromedial (AMB), the intermediate (IB), and the posterolateral PLB. In both ligaments, the IB is constantly taut throughout the flexion and extension, the AMB is taut during flexion and slack during extension, the PLB is slack during flexion and taut during extension.
The PCL helps to avert posterior movement of the tibia in relation to the femur and during extension, the tension of the PCL, may play an important role in stabilizing the knee. It originates on the posterior aspect of the tibia and attaches to the inner aspect of the medial femoral condyle; it is, on average, 38mm long (Peterson & Renström, 2001). The ACL inhibits anterior displacement of the tibia in relation to the femur. Its origin is located on the anterior aspect of the tibia and attaches to the posterior part of the lateral femoral condyle (Armour,1983).
The anterior meniscal ligament (AML), or ligament of Humphrey, is presumed to play a significant role in the stabilisation of the anterior horn of the medial meniscus (Nelson and LaPrade 2000). It does not occur in all people, Bertlet & Fowler found the ligament present in 71% of specimens (1998). However, Nelson and LaPrade found that it appeared in 94% (2000). The ALM’s attachments vary between individuals, the most commons sites are the anterior horn of the medial meniscus to the anterior margin of the lateral meniscus (Nelson and LaPrade 2000).
To summarise, the knee joint is both large and complex, allowing motion in all degrees of freedom. It is comprised of four bones, which are stabilised statically, by the ligaments and menisci, and dynamically, by the surrounding musculature. The tension and stress placed upon the joint are absorbed by the menisci and the articular cartilage. The bursae help prevent frictional forces damaging the joint.
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