Our joints are marvellously designed to allow us to do the very large number of activities and attain the very large number of postures that we wish to in doing a variety of functional deeds. Each joint is designed with a particular function in mind for the position it is placed in the body and for the forces it has to exert and resist. However, our joints are often injured in sport, leisure activities or just in everyday activities and such injuries need good management to settle down completely.
Joints may suffer a direct injury to their surface, the articular cartilage which coats the joint surfaces to a depth of several millimetres. This cartilage is slippery and dense, will indent with pressure but spring back slowly once the pressure is removed, and is fed by the synovial fluid secreted by the joint. One type of joint injury to get a direct blow across the joint so one joint surfaces impacts heavily on the other. This causes a local trauma which can vary from a bruised cartilage area to a region of the cartilage being detached by the blow and then becoming a loose body within the joint.
This kind of injury will often go together with a ligamentous injury and will be serious enough to cause a quick swelling of the joint, most commonly the knee due to its vulnerable position in the centre of the chain of the leg. The person will keep the knee bent to around 30 degrees which is the position where most of the knee structures are most lax, minimising the forces through the joint. Cartilage injuries are slow to heal and if a piece of it has been bounced off this will not heal back, leaving a bare area which may scar to some degree but not be like the previous tissue.
Another type of injury occurs when the joint is strongly stressed on one side, pushing it out of alignment and straining the ligamentous structures which hold the joint stable. Ligament injuries can vary from minor strains to complete ruptures but many will need expert evaluation, often after the acute pain has settled, to ensure that a significant injury has not occurred. Muscle injuries are less common than ligamentous ones but often involve the muscle contracting against a stronger force or when the joint is moving the other way.