Doctors refer to pain at the back of the knee as 'popliteal pain'. Popliteal pain may arise from a number of causes -
One of the common causes of popliteal pain is a Baker's cyst (or popliteal cyst). This is a fluid-filled lump at the back of the knee caused by fluid which has built up under pressure within the knee cavity in response to an inflammatory problem within the knee and not directly related to the lump itself.
The space within the knee joint is lined by cells that secrete lubricating fluid (synovial fluid). If the knee is stressed for any reason excessive fluid can be secreted. In the popliteal region of the knee there is an anatomical connection between the main joint cavity and a lubricating 'pocket' at the back of the knee (the gastrocnemio-semimembranous bursa). When the fluid builds up under pressure, synovial fluid can leak into the popliteal bursa to form a cystic swelling known as a Baker's cyst (or popliteal cyst). The cyst may be obvious to the eye or it may be palpable as a tense 'balloon-like' swelling on the inner (medial) aspect of the back of the knee.
Another condition that can cause popliteal pain is 'popliteus tendinitis'. This is an inflammation of the tendon of the popliteal muscle. The popliteus tendon is very unusual. If you look at the second illustration, where the knee capsule has been sketched in, you can see that the popliteus tendon actually passes through the capsule of the knee and into the knee joint itself, where it attaches to the outer side of the femur.In doing so, it runs behind the lateral meniscus and in close proximity to it.
The popliteus tendon is sensitive to overuse activity and may become inflamed (tendinitis) causing pain at the back of the knee, aggravated during deep squats. The patient may also have trouble fully straightening the knee. Popliteus tendinitis pain can often be elicited by bending the knee and resting the ankle onto the shin of the other leg in a figure-of-4 position. Sitting on a table with the lower legs dangling, pain may also be elicited when an examiner rotates the foot and tibia outwards.
Injury to the posterior horn of the meniscus
Another cause of popliteal pain is a disruption at the back of the lateral meniscus, in the region known as the 'posterior horn'. The illustration on the left shows a bird's-eye view of the menisci sitting on the top of the tibia. You can see that the lateral meniscus is a different shape from the medial meniscus in being more 'O'-shaped rather than 'C'-shaped, and in having a recess at the point where the popliteal tendon passes up to the femur bone. The presence of the popliteus means that the lateral meniscus is not tethered at its outer rim as well as the medial meniscus, and it is consequently more mobile.
If we consider just the bit at the back of the knee - the posterior horn - of the lateral meniscus, there may be a tear of the meniscus itself, or there may be a disruption in the fibres that frequently attach the rim of the posterior horn to the popliteus tendon as it sweeps along the back of the meniscus. Either of these possibilities may result in pain at the back of the knee. The pain may be accompanied by feelings of knee instability and giving way.
Hamstrings tendinitis (biceps femoris tendinitis)
Inflammation of the biceps femoris tendon as it sweeps along the back of the knee may give rise to pain in this area. The biceps femoris tendon is one of the hamstrings tendons. It may become damaged via overuse during a repetitive activity such as running or cycling, where the tendon abrades over the bone of the femur at the back of the knee. The pain is experienced at the outer side of the knee at the back.
The other tendon inflammation that may cause pain at the back of the knee is gastrocnemius tendinitis. Pain may be experienced on the inner or outer aspects of the back of the knee.
There are less common but nonethless important conditions that can cause pain at the back of the knee -
Popliteal artery aneurysm
A popliteal aneurysm is a defect of the popliteal artery where the wall of the artery loses its elasticity and the artery bulges out into a spindle shape. This condition usually occurs in older people and the cause is usually age-related loss of elasticity and hardening of the artery wall.
The bulging of the aneurysm can cause local compression and pain, but more important symptoms may be the result of clots forming on the walls and shooting down to the lower leg, obstructing the blood supply initally causing claudication (pain in the calf and foot on walking) and later more advanced arterial obstruction and a cold white foot (a medical emergency).
Posterolateral corner injury
The posterolateral corner is the region on the outer aspect of the back of the knee where several structures contribute to stability in the knee. There is always a story of a significant injury followed by feelings of knee instability accompanying the pain.