Knee injury: when pain serves to deceive

Injuries affecting the knee are extremely common in athletes, not least because of the wide variety of injury mechanisms and the loads transmitted through the knee joint during sport. When medial knee pain (pain affecting the inner side of the knee joint) occurs, there are a number of possible causes, including an injury to the ligaments, cartilage/meniscus of the inner knee, or even a tibial stress fracture of the lower leg. However, in athletes whose sports include vigorous and repetitive hamstring use (in short, anyone who has to run a lot or fast!) there is another possibility – a pes anserinus injury. Although comparatively rare in athletes, it is important that a pes anserinus injury is diagnosed correctly – not least because missing such a diagnosis could result in unnecessary surgery.

What is the pes anserinus?

The pes anserinus (which we will refer to simply as ‘PA’ from this point) - also known as pes anserine or the ‘goose’s foot’ – refers to a combined tendon structure of the insertion of three hamstring muscles (the sartorius, gracilis and semitendinosus) along the upper, inside portion of the tibia bone of the lower leg (see figure 1). The three combined tendons all attach together to the tibia on the inner part of the lower knee. Visually, these ‘conjoined’ tendons form a structure reminiscent of a goose’s webbed foot and are named by anatomists from the Latin pes for foot and anserinus for goose. Underneath these three conjoined tendons lies the ‘pes anserine bursa’. This is a fluid-filled sac dedicated to providing smooth movement of the three conjoined hamstring tendons and the medial collateral ligament, which runs across the inner knee.

Figure 1: Structure of the pes anserinus