The Knee Joint
By Dr Kenneth Backhouse OBE
In some joints, such as the hip, the bony structure gives a basic stability but the knee has no such help. The lower end of the femur divides into two barrel shaped condyles that sit on the two almost flat surfaces of the upper end of the tibia. The stability and strength of the knee joint, therefore, depends on the controlling ligaments to some extent but otherwise entirely upon the controlling muscles. Without good muscle control the knee is an unstable joint, heavily loaded by the body above and vulnerable to injury.
Although at first sight the legs appear to be in parallel throughout, this is largely due to the soft tissues. In fact the upper ends of the femora are attached to the sides of the pelvis, a structure itself being much wider in females than males, due to the needs of a birth canal through the pelvis. From the head of the femur in the hip joint, a neck extends further outwards so that the shaft of the bone runs from this wide upper end to join the tibia at the knee joint at an angle. From there the two tibiae run downward in parallel. As a result, instead of the femur sitting directly over the tibia, there is a lateral force from above that tends to drive the knee medially i.e. to produce a so called knocked knee situation, to add to the problems of stability in the joint. Because of the wider adult female pelvis the angle from straight is greater in women so that 'knock-knees' is a common problem in females.
In addition to the two main bones, the knee also has the accessory patella (knee-cap). It is really a large sesamoid bone in the central tendon of the quadriceps muscles: running over the distal end of the femur it increases the mechanical efficiency of the quadriceps in extension of the knee.
Movements of the knee joint
The knee joint appears to be a simple hinge joint, moving from straight into full flexion. In its straight position it should be in compacted state to hold the weight of the body. Its flexor activity provides the powerful movements needed in this direction. The concept of a pure hinge fails in flexion, as a certain degree of rotation is possible; as much as 60� at about 60� of flexion. As the knee is extended this is lost, so that in full firm extension, the joint should lock, the locking being linked with a slight lateral twist of the tibia on the femur. In extension no active rotation should be possible. The final locking is a function of the ligaments of the joint. In experiments on cadaver legs, with all muscles removed, the extended legs were subjected to twist on test beds and even with high power being exerted no more than about 5� was possible until the ligaments eventually gave way. Under normal circumstances such stresses do not take place in life. Furthermore the rotation possible in flexion offers a degree of vulnerability to the joint unless taken under full active muscle control.
As would be expected of a hinge joint there are taut ligaments to each side. However, due to the medially directed loading on the joint, resulting from the angle of approach of the femur, the medial ligament is much broader and more powerful than the lateral. As the joint is not designed to go beyond a straight there is a heavy check ligament posteriorly. In some people this is rather more lax, allowing a swayback knee. From the space between the two condyles of the femur to the area between tables of the tibia, two powerful ligaments run obliquely, closely crossing each other; the anterior and posterior cruciate ligaments. These ligaments prevent forward and backward sliding of the femur on the tibia. Their absence, if ruptured due to injury, is particularly felt when walking on a slope. Also, being taut and having some twist as they cross, they are responsible for producing the locking twist in full extension.
As in all joints, the load bearing surfaces over the condyles of the femur and the tables of the tibia are covered by hyaline cartilage, as also is the deep surface of the patella. In addition each table of the tibia has a semi-lunar fibro-cartilage running around the periphery; thick to the outside and thinning centrally. These are what are usually called the semi-lunar cartilages of the knee, or the menisci. They act to increase the very shallow hollow of the tibial tables but should not be considered as load bearing. Under normal circumstances the load from the tibia should be on the central hyaline cartilage of the tibial tables. The major function of these accessory cartilages is to assist in the movement of synovial fluid in lubrication of the heavily load bearing joint. They are particularly vulnerable if trapped during twisting of the joint under load, when splitting may occur; the so-called cartilage injury.
The quadriceps group of muscles act as the main powerful extensors at the knee as well as giving a major controlling influence over the joint itself. So important are these muscles that they have been dealt with separately in an earlier article, Quadriceps Control of the Knee Joint. Their tendinous expansion also replaces the ligaments over the whole anterior part of the joint, i.e. anterior to the medial and lateral ligaments.
Two powerful muscle groups act as the flexors of the joint. From above, the hamstrings (semitendinosus, semimembranosus and the two headed biceps femoris), which, with the exception of the short head of the biceps also act as extensors at the hip joint. They divide as they approach the knee, the tendons of the first two going to the medial side and that of biceps to the lateral side of the joint. This allows a wider control of flexion over the knee but also permits the muscles to induce rotation in flexion. The separation also produces the hollow behind the knee.
Running down from the two condyles of the femur are the gastrocnemius muscles. These then join the soleus, arising from the tibia, to run on to form the thick tendo-calcanium (Achillis) at the heel. As is the case of the hamstrings, they also act over two joints; the knee and the ankle. By running from the two sides of the knee they also act, with the hamstrings to control rotation in flexion.
It is important always to remember that ligaments are not designed to take up stress except in an emergency and, in fact this is when pulled or even ruptured ligaments can occur. Muscles always act in support of ligaments and should always take the strain. One such muscle assisting in flexion while covering the posterior ligament is popliteus which, also by running obliquely unlocks the straightened joint. The lateral head of gastrocnemius and biceps femoris also act as supporters of the lateral ligament.
The medial side of the joint is the one needing particular support due to the potential knock-knee stresses. It is a control so often forgotten and certainly rarely exercised even in those people with a tendency to knock-knee. In fact a thick mass of three muscles run down the medial side of the thigh, their tendons crossing the knee joint just behind the prominence of the condyle to run to the tibia. These with the vastus medialis (of the quadriceps) should give the powerful support to the medial side of the joint; the muscles so important in pulling up in dance. We can simply group these muscles as those to the inner side of the thigh or, taking them more seriously as sartorius, gracilis and the inner hamstring, semitendinosus. Sartorius is the strap-like muscle that runs superficially obliquely across the front of the thigh from the anterior superior spine of the hipbone, while gracilis runs down directly from the pubis. One important consideration is that all three tendons turn forwards below the knee to run into the tibia. In flexion therefore they act as medial rotators of the knee. Hence in pulling up on the inside of the knee it also protects the knee from twisting outwards. It is important to remember this. So often in bad ballet training, the ligaments at the knee are gradually stretched, so as to achieve a 'flat turn-out', whereas turn-out should be entirely a function at the hip joint. This not only creates ligamentous instability at the knee, with all the dangers so produced but also, in order to allow the lateral twist to occur, the medial pull up muscles must relax throwing extra strain towards knock-knee.
As in all synovial joints, delicate synovial membranes line the non-load bearing surfaces within the joint. The membranes run from the edges of the load bearing hyaline cartilages of the bones, to line the inner aspects of the joint capsule, as well as such aspects of the bones within the joint, not subject to loading. Below the level of the patella the area of membrane is not very great. Above however, in view of the considerable amount of movement of the patella around the lower end of the femur, the synovial membrane needs to be quite extensive to move freely with the patella. In full flexion the membrane runs from the anterior edge of to hyaline cartilage of the femur to the upper border of the patella, covering and adding lubricant to the lower surface of the femur. In extension however this sheet of synovial membrane must be pulled up for safety. It now forms a pouch under the lower part of the quadriceps muscles, pulled up by a tiny muscle slip for as much as three fingers breadths above the patella - the deep supra-patellar pouch.
One function of the synovial membrane is to provide small amounts of synovial fluid, a large molecular weight lubricant for the joint. Another is to act as a general cleaner of the joint cavity. However being a delicate membrane it is easily subject to injury. In addition to synovial membranes within the joint, synovial sleeves are provided for tendons where friction may occur and, particularly present in the region of the knee pockets of synovial membrane, bursae, exist, likewise to reduce friction but also subject to injury on occasions.