A biceps femoris avulsion strain or fracture occurs when the tendon comes away from the bone, taking a small fragment of bone with it causing pain and swelling at the outside back of the knee. It may often be misdiagnosed as a simple tendon injury and may require a much longer period of recovery.
Biceps femoris avulsion symptoms
Symptoms consist of a sudden sharp pain at the back of the knee. This injury is known to be particularly painful so continuing to play on is not usually an option. Swelling will appear at the point of injury where the tendon attaches to the bone at the outside back of the knee.
There athlete will feel a significant loss in hamstring muscle strength and attempting to bend the knee against resistance will be painful. It will be tender when pressing in (pallpating) and the bone fragment may even be felt through the skin.
Causes and anatomy
An avulsion of the biceps femoris tendon, is where the tendon comes away from the bone taking a piece of bone with it. This most commonly occurs where the long head attaches to the ischial tuberosity in the buttocks but can also occur at the back of the knee.
The biceps femoris muscle is one of the three hamstring muscles and is made up of two separate muscle bellies which originate from different sites, namely a long head and a short head. The long head originates from a region at the back of the hip bone called the ischial tuberosity. The short head originates from the back of the femur or thigh bone. Both muscle bellies join together, cross the knee joint and insert into the top outer surface of the shin bones. As with all the hamstrings it has a dual function, both to bend the knee and extend the hip.
Biceps femoris avulsions normally occur during a rapid or violent kicking movement or explosive bending of the knee for example when sprinting at top speed. If the athlete is fatigued or has not warmed up properly or suffered a recent hamstring injury they may be more susceptible.
Injuries to biceps femoris are more common than to the other hamstrings. One theory for this is the fact that each of the two heads are innervated by different branches of the sciatic nerve. In states of fatigue or when the muscle is not fully warmed up, uncoordinated firing of the nerves may cause the muscle to contract inappropriately during movement, leading to injury.
Treatment for biceps femoris avulsion
Rest and apply cold therapy and compression immediately. Ice can be applied for 10 to 15 minutes every hour initially during the acute stage. It may be that the injury is mistaken for a simple tendon rupture initially and only after an X-ray might an avulsion fracture be diagnosed.
For less serious cases conservative treatment may be advised which may include a period of rest followed by stretching and strengthening exercises. More serious injuries may be treated by keeping the knee bent at 90 degrees for a number of weeks while the injury heals. More serious cases still should be surgically repaired by a surgeon however recovery following surgery can take significantly longer than conservative methods.