Multiple sclerosis is a physically debilitating disease that is still not fully understood by doctors. The most likely cause is the accidental destruction of nerves in the brain and spinal cord by the patient's own immune system. This damage to the nerves results in the destruction of the outer myelin coating of nerves, which normally insulates the nerve cells and allows them to transmit signal between the brain and the body. Multiple sclerosis may cause many different symptoms, including symptoms affecting the knees.
The motor neurons, nerves which control muscles, are frequently affected in patients with multiple sclerosis. Damaged motor neurons can result in several different symptoms. One symptom that doctors frequently encounter is an increase in the deep tendon reflexes, reports the Merck Manuals Online Medical Library. This is often diagnosed with the knee-jerk test, in which doctors test tendon reflexes by gently tapping the knee with a soft hammer. In patients with multiple sclerosis, this test is often results in a stronger jerk of the knee than normal.
Another result of the damage to the motor neurons is increased spasticity of muscles, meaning that muscles tend to spontaneously contract. In some cases, a patient may not be able to consciously relax the contracted muscle. The knees are affect when spasticity occurs in the quadriceps, which are the large muscles on the front of the legs, or the adductor muscles, which are smaller muscles on the inside up the upper legs. When these muscles contract and do not release, the knees are forced to straighten and remain locked in that position, explains the National Multiple Sclerosis Society.
Patients with multiple sclerosis may alternatively experience the opposite problem of the knees becoming locked in the bent position. This locking of the knees is caused by contraction of the hamstrings, which are the long muscles located on the rear of the upper leg, and the hip flexors, which are the muscles located on the top of the upper thigh, the National Multiple Sclerosis Society explains.
Severe cases of multiple sclerosis may result in paralysis of large muscle groups, forcing a patient to spend the majority of their time in a bed or wheelchair. This immobility can cause sores to develop on the skin, particularly in pressure points where the skin presses against the bed of chair. The knees are one spot on the body that should frequently be checked for pressure sores in multiple sclerosis patients who are confined to a wheelchair, Medline Plus reports.