Anatomy of the knee
The knee is the largest joint in the body and is a hinge joint made up of the lower end of the thigh bone and upper end of the shin bone and the knee cap. The surfaces of the three bones are covered with a cartilage at the point of contact and it helps in the smooth movement of the joint. There are C shaped wedges located between the thigh and shin bone surfaces called menisci which act as shock absorbers and cushion the joint. Ligaments around the joint hold the bones together and provide stability, while the thigh muscles give the joint its strength.
The joint surface is covered by a thin membrane called the synovial membrane which release fluid to keep the joint well oiled and reduces friction.
All these part work together in harmony normally and if this is disrupted due to wear and tear or injury, it results in pain, muscle weakness and deformity with loss of mobility.