13/06/2025
Uncommon injuries: medial knee pain from the pes anserinus
Knee injuries are prevalent in athletes due to the loads transmitted through the knee joint during sport. When medial knee pain presents, there are several possible causes, including an injury to the medial ligament, medial cartilage/meniscus, or medial tibial stress fracture. However, in athletes with vigorous and repetitive hamstring use, there is another possibility – pes anserinus injury. Because these injuries are comparatively rare, missed diagnosis is common and can result in unnecessary knee surgery.
The pes anserinus – also known as pes anserine or the ‘goose’s foot’ – refers to a conjoined insertion of the sartorius, gracilis and semitendinosus muscles along the proximal medial aspect of the tibia. Each of these three muscles is innervated by a different nerve: the femoral, obturator and tibial nerves respectively. Visually, these conjoined tendons form a structure reminiscent of a goose’s webbed foot and were named from the Latin roots, pes for foot and anserinus for goose. Under the conjoined tendons lies the pes anserine bursa, a sac which provides smooth movement of the conjoined hamstring tendons over the medial collateral ligament. The pes anserinus aids knee stability by helping the medial ligament resist valgus forces.
Although not especially relevant in a clinical context, it is worth appreciating that the structure of the pes anserinus differs among individuals. For example, a recent study examined pes anserinus structure in 102 cadaveric limbs. The researchers found that in all cases, the pes anserinus consisted of the sartorius, gracilis, and semitendinosus tendons. However, six distinct types of pes anserinus were distinguished based on the presence of accessory bands. Additionally, three types of insertion were noted (short, band-shaped, and fan-shaped).
Pes anserine pathology and risk factors
Pain as a result of pes anserinus pathology can arise as a result of
1) tenosynovitis of the pes anserinus tendon,
2) inflammation of the pes anserinus bursa, or
3) a combination of the above.
Clinically, it is difficult to distinguish these two pathologies. Fortunately, the treatment is the same for both conditions. Having said that, evidence suggests that a pes anserinus bursitis occurs more frequently and responds more quickly to treatment than tenosynovitis
The underlying factors increasing the risk of pes anserinus injury are often multifactorial, but typically involve high hamstring loading combined with sub-optimal biomechanics. Movements that can precipitate a pes anserinus injury are those involving valgus (eg as a result of excessive pronation in running gait ) or rotatory stresses at the knee. Because gait patterns most significantly tax the medial knee, it is unsurprising that PA tendonitis and bursitis affect long-distance runners more than other athletes. Mexican researchers evaluated 22 patients with pes anserinus bursitis and found valgus knee deformity, alone or in association with medial collateral instability, to be a risk factor for pes anserinus injury.
Some researchers have proposed that pes anserinus tendon pathology is much more likely in those who are older and with osteoarthritic co-morbidities. A Turkish study performed an ultrasonographic assessment of the pes anserinus tendon and bursa in patients with knee osteoarthritis. They found that the mean thickness of pes anserinus in knees with osteoarthritis was significantly greater than the controls and that higher pes anserinus thicknesses were associated with higher osteoarthritis scores on a visual analogue scale (VAS). However, the researchers in the Mexican study did not find such an association. Indeed, they concluded there was no association between pes anserinus injury and previously reported predisposing factors such as diabetes, knee osteoarthritis, and obesity. On the face of it, these findings seem conflicting. However, one interpretation is that in older patients with pre-existing osteoarthritis, some pathology affecting the pes anserinus is to be expected – but is distinct from a pes anserinus injury caused by repetitive overload involving the hamstring muscles.
Patients with pes anserine bursitis typically present with tenderness and swelling along the proximal medial tibia. However, symptoms may also include vague medial knee pain, which may mimic medial meniscal or tibial collateral ligament injury. Indeed, some research suggests that a significant proportion of patients may not present with proximal tibial swelling, but instead present with either posteromedial joint pain or medial joint line pain, raising a (false) suspicion of a medial meniscal tear. Other pain characteristics and functional symptoms include:
Pain experienced approximately 2-3 inches beneath the medial aspect of the anterior knee joint (which may also extend to the front of the knee and down the lower leg).
Gradual onset of pain over a prolonged period.
Exacerbation of pain on ascending or descending stairs, walking steep gradients, or sitting down/rising from chairs.
A lack of pain when walking on level surfaces.
Pain experienced when contracting the hamstrings against resistance.
Pain upon stretching the hamstring muscles.
In more severe cases, pain at night, waking the patient when he/she bends the knees resulting in sleep disturbance.
A successful diagnosis of pes anserinus requires a thorough physical examination of the patient, along with a detailed history of the symptoms and characteristics of the pain presentation (see above). In particular, the exact site of pain should be identified by both superficial and deep palpation. The pain-producing movement(s) should be performed during a physical examination in order to support the diagnosis.
Due to the proximity of pes anserinus to the skin surface, ultrasound imaging is an effective method for detecting the morphology of pes anserinus and its peripheral structures; and as such, a useful tool to help diagnose pes anserinus bursitis, cysts, tendonitis. However, the gold standard for pes anserinus imaging is magnetic resonance imaging (MRI). In particular, MRI is more sensitive to the appearance of fluid beneath the pes anserine tendon near the joint line than ultrasonography. Regardless of imaging modality, however, the patient’s history, examination, and pain presentation characteristics remain the cornerstone of diagnosis. Imaging studies are perhaps more critical for the exclusion of other pathologies that may present with similar symptoms. These include:
Tibial stress fracture
Osteoarthritis
Popliteal cysts
Bursal Infection
Malignant tumour (rare)
As mentioned earlier, a pes anserinus injury in athletes typically involves bursitis or tenosynovitis. However, other (rarer) pes anserinus pathologies are possible. In children, a type of pes anserinus syndrome may occur as a result of proximal tibial exostoses. These can take the form of an osteochondroma, producing a painful lump, or a rose-thorn shaped bone spur, which leads to a snapping/locking sensation of the PA tendons. In the former, removal resolves the issue. In the latter, symptoms may resolve without surgery, although some cases require excision.
Another (very rare) pathology is a rupture of the pes anserinus bursa. Ruptures are more likely to occur in older patients with underlying medical/metabolic conditions such as diabetes, obesity, and osteoarthritis. If the anserine bursa retains synovial fluid due to chronic inflammation from knee osteoarthritis, mechanical stress from walking or other loading activities may cause a rupture. Evidence of a rupture includes sudden and isolated swelling of the associated calf muscles. In patients at risk, weight reduction and quadriceps strengthening are recommended as preventative strategies.
Another rare pes anserinus pathology is ‘snapping pes anserinus syndrome,’ which causes medial knee snapping. This snapping sensation results from a translation of the pes anserinus tendons (usually gracilis or semitendinosus) across the posteromedial aspect of the medial femoral condyle and tibia during knee movement. Unlike the other rare conditions above, snapping pes anserinus syndrome has been reported in athletes as a result of overuse and trauma. Ultrasonography is often the diagnostic imaging test of choice in cases of mechanical snapping. However, while static imaging forms part of a diagnosis, dynamic imaging (ie during the snapping movement) is also essential to confirm a diagnosis.