The knee joint is one of the major weight bearing joints, it has to cope with walking, running, bending, jumping and lifting objects. It also works in conjunction with the hip & ankle joints, assisting in static erect posture (standing). So not only does the knee joint need to offer stability & weight support, but it must also offer considerable mobility. It is no surprise then that it is one of the most commonly injured joints in the human body.
The joint is comprised of four main bones, the femur - the large bone in the thigh, attaches by ligaments to the tibia, the fibula which runs parallel to the tibia, and the patella (commonly known as the knee cap) which "rides" on the joint as the knee bends.
The joint itself has three main compartments, the main joint being the attachment of the femur & the tibia, this has an inner (medial) & an outer (lateral) section, the third compartment is the joining of the patella to the femur, namely the patellofemoral joint. The Patellofemoral joint is unique in that it protects the human body's other joints by acting as a "shock absorber".
The knee joint is, in good function, equipped with a large range of movement, strong ligaments and powerful muscles. The knee, unlike any other joint in the body, depends almost completely on its surrounding ligaments for stability. The two most important sets of ligaments are the cruciate ligaments located in front and back of the knee, and the collateral ligaments located on the sides of the knee. The ligaments strap the inside and outside of the joint (collateral ligaments) as well as crossing within the joint (cruciate ligaments).
The muscles which go across the knee joint are the quadriceps and the hamstrings. The quadriceps are the big muscle group making up the front of the thigh. The muscle starts from the thigh bone, narrows down towards the knee to the kneecap and attaches to the "bump" on the shin bone just below the knee called the tibial tuberosity. The quadriceps are a very powerful muscle group and straightens the knee in such activities as standing up, going up stairs or running.
The hamstrings make up the back of the thigh, coming from the pelvis, running down the back of the thigh to attach to the back of the fibula and tibia just below the knee. This muscle group bends the knee and straightens the hip. The hamstrings are essential to the activities of sprinting and pushing against something or someone.
These are the two major muscle groups which control knee movement and are vital to the stability of the joint. There are other muscle groups which affect knee movement and stability, namely the calf muscles, the hip abductors located on the outer thigh, and the hip adductors located on the inner thigh..The iliotibial band also affects knee stability as do the glutes (buttocks).
The knee joint also has a structure made of cartilage, which is called the meniscus or meniscal cartilage. The meniscus is a C-shaped piece of tissue which fits into the joint between the tibia and the femur. It helps to protect the joint and allows the bones to slide freely on each other, as well as absorbing some of the load of the joint. There is also a bursa around the knee joint. A bursa is a little fluid sac that helps the muscles and tendons slide freely as the knee moves.
Below the kneecap, there is a large tendon, the patellar tendon which attaches to the front of the tibia.
A knee that is perfectly aligned has its load-bearing axis on a line that runs down the middle of the leg -- through the hip, knee and ankle. When the knee is not perfectly aligned (also referred to as malaligned), it is known as either varus (bow legged) or valgus alignment (knock-kneed).
Varus alignment causes the load-bearing axis to shift to the inside, causing more stress and force on the medial (inner) compartment of the knee. Individuals with varus alignment are highly susceptible to arthritis in the knee. Individuals who are either bow-legged or knock-kneed, are at higher risk for osteoarthritis, meaning they may be prone to knee pain and function problems later in life.
Imbalanced use of muscles is the major cause of joint dysfunction. The dysfunction may manifest as pain or limitations in movement, or both. If these symptoms are ignored, the dysfunction can result in deterioration of the cartilage in the joint. Joint dysfunction can be further exacerbated in the hips and knees due to the consistent weight-bearing on the legs while standing or walking. Without intervention, the cartilage will eventually become so worn away that the result will be "bone-on-bone" where there is virtually no cartilage left in the joint. At this point, joint movement is usually severely restricted.
The patella, the small bone in the front of the knee is embedded in the quadriceps (thigh muscle) tendon and acts to increase the biomechanical leverage of the quadriceps. The patella slides in a groove on the femur as the knee flexes and extends. Because the patella 'floats' within the substance of the quadriceps, proper tracking of this bone in the femoral groove is dependent on correct muscle balance to maintain a central position. Congenital anatomic factors such as the shape of the patella also influence this tracking. Because of the location of the patella, it is subject to higher stresses than other joint surfaces. So, despite having a thicker cartilage lining than any other bone, it often begins to wear out before other parts of the knee. Patella malalignment is an abnormality of the position or tracking of the patella, and has the potential to cause pain and/or instability.
The normal patella should track straight down the middle of the femoral groove. There are varying degrees of abnormal tracking, or patella malalignment. In mild cases of malalignment the patella is simply tilted in the groove, leading to increased pressure on the downward tilted side of the patella. In more severe cases, the patella will actually sublux, or slide partially out of the groove. In the most severe cases of malalignment, the patella can actually completely dislocate.
Proper tracking of the patella is influenced by many factors. Proper muscle balance is important and is one of the few factors that we can control. Usually the patella wants to sublux toward the outside of the knee (lateral). Strengthening the inside thigh muscle, the vastus medialis oblique can act to counter this tendency.
Tracking is also influenced by the anatomical shape of your patella, femoral groove, the angle your knee makes with your hip (knock knees) and even the position of your foot (pronation). The hip knee angle is important because the patella is embedded in the quadriceps tendon which originates at the hip and attaches at the knee. The more knock kneed someone is, the more of an angular pull occurs on the patella every time the quadriceps contracts.
Increased pronation of the foot (flat feet) can influence the tracking of the patella. This occurs because the rotation of the rest of the leg is affected by the way the foot contacts with the ground. Pronation of the feet can be caused by a number of factors including an imbalance in strength or tightness between the muscles in the calf (lateral gatrocnemius & the perroneals) and a comparative weakness in the glutes and the anterior tibialis & posterior tibialis.
Another common imbalance within the quadriceps muscle group in the front of the thigh, is between the outer quadriceps muscle (vastus lateralis) and the inner quadriceps muscle (vastus medialis), can also cause kneecap problems. These two muscles run down either side of the front of the thigh and attach to the kneecap. Part of their role is to stabilize the kneecap. When one side is stronger than the other, the kneecap can be pulled to one side. Runners frequently have comparatively stronger, tighter outer quadriceps muscles than inner quadriceps muscles, the kneecap can be pulled to the outer side. This mechanism is a common cause of patellofemoral pain syndrome, a common complaint of runners.
Another factor that can pull the knee out of alignment is tightness in the tensor fascia latae and more specifically the iliotibial band (a thick tendon-like portion of the tensor fasciae latae). This band passes down the outside of the thigh and inserts just below the knee. Tightness in this area can cause the tendon to pull the knee joint out of alignment and rub against the outside of the knee, which results in inflammation and pain. Such tightness is known "iliotibial band syndrome".
There are two main causes of knee pain associated with iliotibial band syndrome. The first is "overload" and the second is "biomechanical errors."
Overload is common with sports that require a lot of running or weight bearing activity. This is why ITBS is commonly a runner's injury. When the tensor fasciae latae muscle and iliotibial band become fatigued and overloaded, they lose their ability to adequately stabilize the entire leg. This in-turn places stress on the knee joint, which results in pain and damage to the structures that make up the knee joint. Biomechanical errors can be from muscle imbalance, compensatory or postural dysfunction, hip torsion, pronation of the feet or leg length difference.
During certain weight bearing exercises the knees may fall in towards the centre of the body (adduct). This may not be due to any problem with the knee, but rather a relative imbalance between the tightness the adductors & the ITB (iliotibial band), and the weakness or inhibition of the glutes. Conversely, where the knees fall outwards (abduct) this may be due to a comparative tightness of the biceps femoris, the iliopsoas & the piriformis in relation to the gluteal group.
Another area which is prone to injury is the anterior cruciate ligament (ACL) a vital stabilizing ligament in the knee. It is located deep inside the knee joint and provides nearly all of the stability to forward force on the joint. Injuries to this ligament are very common in aggressive sports and usually occur with a sudden hyperextension or rotational force to the join ie twisting fast etc.
With a torn ACL, there is increased play in the joint allowing shearing forces across the cartilage surface, and leading to progressive tearing of the cartilage discs (menisci) and breakdown of the joint surface. Over time, this breakdown leads to degenerative arthritis.
Another crucial factor that can cause dysfunction in the knee is a restriction in movement in the hip or ankle areas. If you lack movement at a joint which has a high degree of movement capacity (hips, or ankle), then another joint which has a lower degree of movement capacity, in this instance, the knees, is forced to compensate.
Indeed, such is the nature of the relationship between the hip and the knee, knee pain is frequently simply a manifestation of poor motor control or range in the hip, whether it be flexion, extension or rotation. Strengthening the hip stabilizers is a sound way to avoid common knee injuries.
Restriction in the movement of the hip may cause pain in the knee. The hip's normal range of internal and external rotation is 35-50 degrees internally and 50 degrees externally in a healthy hip. The knee can only perform this internal and external rotation minimally. When rotating the entire leg, most of the motion should come from the hip so as not to place too much torsion on the knee joint. If the hips are tight and range of motion is restricted, excess movement may be required of the knee and could account for pain felt in the knee joint.
Likewise, restriction of movement in the knee can result in pain in the hip. The knee's normal range of motion during flexion is 150 degrees and 180 degrees during extension. Although the hip can flex up to 135 degrees, it can only extend 30 degrees in a normal hip. So, compromised movement in the knee can require the hip to extend beyond its normal range of motion and reveal itself as hip pain.
Balanced movement in the hip and knee as well as between the hip and knee is the best prevention against deterioration and pain in these joints. Furthermore, mild to moderate deterioration may also be helped by restoring range of motion and balancing the actions of the muscles around the joint.
Meniscal tears occur when excessive motion of the knee places stress on these cushions between the femur and tibia. This can be from forced extension, flexion, side-to-side, or rotational motions. The tearing may be minor and have no mechanical effect on the normal gliding of the knee, or it may be greater and cause catching, popping, and even locking of the knee so that it will not extend completely.
Inappropriate ranges of motion within the knee in flexion and/or extension are common dysfunctions of the knee, and can create significant problems in the kinetic chain. Inadequate knee flexion reduces the limbs "shock absorption" qualities. This can affect an individual's gait. Insufficient knee flexion may actually be a secondary symptom of insufficient hip flexion. These dysfunctions can affect toe drag.
Weak quadriceps are a common cause of inadequate knee flexion or excessive knee extension. Excessive ankle plantar flexion is the most common cause of knee hyperextension. Excessive knee flexion and inadequate knee extension can be caused by a number of factors, including soleus & gastrocnemius weakness, or quadriceps weakness.
Some people stand and move while "locking" the knee out straight, even pressing it backward. This posture is sometimes called "splay-legs," and makes the leg look more crescent-shaped than straight. It puts body weight onto the joint while pressing the joint slightly out of place, putting damaging forces on the cartilage. Many people push their knees into hyperextension when standing and walking. Others "bang" the joint into straight position during exercise.
Any of these factors can cause varying degrees of pain, and short to long term damage, it is therefore important to look at the knee not simply as a separate part of the anatomy, but also as an integral part of the kinetic chain.
