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 is often misdiagnosed as a simple tendon injury.
Medically reviewed by Dr Chaminda Goonetilleke, 31st Dec. 2021
Biceps femoris avulsion symptoms
Symptoms of a Biceps femoris avulsion are similar to that of a tendon strain. They consist of:
- Sudden sharp pain at the outside back of the knee.
- It is particularly painful, so continuing to play is not usually an option.
- Swelling appears at the point of injury, where the tendon attaches to the bone.
- A significant loss in hamstring muscle strength.
- Attempting to bend the knee against resistance is painful.
- Pressing in (palpating) feels painful and tender.
- You may even feel the bone fragment through the skin.
Your doctor may use X-ray imaging to diagnose an avulsion fracture.
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. It consists of two separate muscle bellies, which originate from different sites; 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. Fatigue, not warming up properly, or a recent hamstring injury makes you more susceptible.
Injuries to the biceps femoris are more common than to the other hamstring muscles. One theory for this is the fact that each of the two heads is innervated by different branches of the sciatic nerve.
In states of fatigue or when the muscle is not fully warmed up, the uncoordinated firing of the nerves may cause the muscle to contract inappropriately during movement, leading to injury.
Treatment for biceps femoris avulsion
Immediate first aid is to apply the PRICE principles of protection, rest, ice, compression, and elevation.
Apply ice/cold therapy for 10 to 15 minutes every hour during the initial acute phase. Reduce frequency as your symptoms improve.
Conservative treatment involves a period of complete rest followed by progressive stretching and strengthening exercises.
Your doctor or surgeon may treat a more serious injury by immobilizing your knee at 90 degrees for a number of weeks. This allows your injury to heal.
The most severe cases require surgical repair. However, recovery after surgery can take significantly longer.