Although most joints of the body can suffer from haemarthrosis (bleeding within a joint) the term is usually associated with the knee and the diagnosis generally implies a significant injury to the joint.
Site of injury
Any internal structure that carries a blood supply can be traumatised and bleed but the most common causes of haemarthrosis of the knee in descending order are:
• Tear of the anterior cruciate ligament (ACL) 70-80% • Intra articular (within the joint) fracture • Peripheral meniscal (cartilage) tear • Deep fibres of medial collateral ligament/capsule
The hyaline cartilage (lining the joint) is avascular and therefore does not usually bleed but may when the subchondral bone is involved (osteochondral fracture).
Occasionally the cause of the bleeding is never identified but more commonly there are injuries to two or more structures in the joint.
If a fracture is ruled out on X-ray (and an X-ray is mandatory) haemarthrosis of the knee is caused by injury to the ACL until proven otherwise.
The anterior cruciate ligament
The ACL together with the posterior cruciate ligament (PCL) which is much less commonly injured control the front to back stability of the knee and act as a check rein to rotation (see Fig.1). As a result, twisting actions in weight-bearing sports give the highest proportion of ACL damage. However other movements including hyperflexion, hyperextension or acceleration or deceleration in a straight line can all be responsible for injury.
The type of injury also varies, for example being hit from the side by a 20 stone rugby forward tends to give a mid-substance tear of the ACL, while a low speed gentle turn in skiing may result in an avulsion (separation from the attachment) of the ACL from the femur. The avulsion injury, however, has considerable chance of settling without major instability as the femoral attachment can heal to the adjacent PCL. Children are particularly prone to avulsion of the bony attachment to the tibia due to the immaturity of their bones – this can be seen clearly on x-ray.
Because of generalised increased joint laxity females have a 8-10 times greater chance of ACL injury over any given participation period.
Presentation Figure 1: Posterior view of the knee joint 30 SportEX
The person is usually immediately aware that something significant has happened and even years later can remember vividly the game in which the ACL was injured. The injury often occurs towards the end of the game or session when fatigue sets in and 30-40% feel a pop or rip in the knee. Weight- bearing is difficult and the player is often stretchered off or sledded to the base of the slope.