Pes Anserinus: Anatomy and Causes of Medial Knee Pain

The pes anserinus is a fan-shaped group of three tendons that attach together on the inner side of your shinbone, just below the knee. The name is Latin for “goose’s foot,” because the splayed arrangement of the tendons roughly resembles one. Most people encounter the term after being diagnosed with pes anserine bursitis or tendinitis, but the structure also matters in knee surgery, imaging, and understanding why inner-knee pain is so often misdiagnosed.

The Three Tendons and Where They Attach

Three muscles from the thigh each send a tendon down to converge on the upper inner surface of the tibia. From front to back, they are the sartorius, the gracilis, and the semitendinosus. The sartorius is the long, strap-like muscle that runs diagonally across the front of your thigh. The gracilis is a thin muscle on the inner thigh. The semitendinosus is one of the hamstrings, running down the back of the thigh. Each comes from a different direction, which is why they fan out like the toes of a webbed foot at their shared attachment point.

Together, these tendons help stabilize the knee against rotational forces. When you twist your lower leg inward or resist the knee from collapsing inward during activities like running or cutting, the pes anserinus tendons are doing much of that work. They also assist with bending the knee. Because they cross both the hip and the knee joint, they link hip movement to knee stability in a way that single-joint muscles cannot.

Anatomical Variations Are the Norm, Not the Exception

Textbooks typically describe the pes anserinus as a neat trio of three tendons, but this “classic” configuration is actually present in only about 62% of knees, according to a systematic review pooling data from over 700 knees across 14 studies.¹ The remaining knees show some kind of variation, most often extra tendon slips branching off the semitendinosus.

A cadaveric study found that accessory semitendinosus bands were present in over 93% of examined limbs, with nearly half of those limbs having two or more extra bands.² Accessory gracilis bands showed up in roughly a quarter of limbs, while extra sartorius bands were uncommon. A separate classification study identified six distinct types of pes anserinus based on the presence or absence of these accessory bands, with the simple three-tendon arrangement appearing in about 53% of their sample.³

An especially striking finding came from a study of a Nigerian population, where the classic three-tendon configuration appeared in only 5% of specimens. The most common pattern there, seen in 65% of cases, included an accessory semitendinosus tendon alongside the usual three.⁴ That study also found that in 85% of specimens, the tendons converged below the inner tibial bump rather than above it, and in 10% the tendons never merged at all, instead inserting side by side. These population-level differences underscore that what surgeons and radiologists encounter in the operating room or on an MRI can look quite different from the textbook diagram.

Why These Variations Matter in Surgery

The pes anserinus tendons are not just passive anatomy. They are frequently harvested as graft material for anterior cruciate ligament (ACL) reconstruction. Surgeons typically strip the semitendinosus and gracilis tendons from their attachments, bundle them, and use them to replace the torn ACL. Knowing the local anatomy in advance matters because accessory bands can catch on the harvesting tool, causing it to slip or cut the graft short.⁵ A practical guide to hamstring harvest technique emphasizes careful identification of the tendon anatomy before stripping to avoid complications.⁶

After these tendons are removed, the body does attempt to regenerate them, though the results are inconsistent. A systematic review found that the semitendinosus tendon regenerated in roughly 79% of cases more than a year after harvest, while the gracilis tendon regenerated in about 72%.⁷ However, a second review reported lower gracilis regeneration, around 43%, and noted that even when tissue does regrow, the tendons sometimes fuse together abnormally, the muscle-tendon junction retracts upward, and measurable strength deficits can persist.⁸ Whether those strength deficits translate into noticeable functional problems for the patient remains unclear. Many people return to full activity after ACL reconstruction with hamstring grafts, but some do report lingering weakness in deep knee flexion or internal rotation.

The Nerve Next Door

Running just alongside the pes anserinus is the infrapatellar branch of the saphenous nerve, a sensory nerve that supplies feeling to the skin on the inner and front part of the knee. An anatomical study measured the distance between this nerve branch and the pes anserinus at less than a millimeter on average.⁹ That proximity matters clinically in two scenarios. First, injections aimed at the pes anserine bursa can inadvertently irritate this nerve. Second, during hamstring tendon harvesting for ACL surgery, the nerve is at risk of being stretched or cut, which can leave a patch of numbness or tingling on the inner knee that sometimes persists for months. An anatomical study tracing the nerve’s branches found that it crosses the knee in several places, with most branches running through the middle third of the area between the kneecap and the semitendinosus tendon.¹⁰

Pes Anserine Bursitis and Tendinitis

Beneath the pes anserinus tendons sits a small fluid-filled sac called the pes anserine bursa. Its job is to reduce friction between the tendons and the bone. When this bursa becomes inflamed, the result is pes anserine bursitis. When the tendons themselves are irritated, it is called pes anserine tendinitis. In practice, the two conditions overlap so often that clinicians frequently lump them together as pes anserine syndrome or pes anserine tendino-bursitis.

The bursa’s exact position varies from person to person. An ultrasound study found that in roughly two-thirds of men, the bursa sat between the pes anserine tendons and the tibia, while in about a fifth it lay between the medial collateral ligament and the tendons. The distribution was similar in women.¹¹ This variability partly explains why the pain location can differ from patient to patient, and why a one-size-fits-all injection approach sometimes misses the target.

Symptoms and How It Gets Misdiagnosed

The hallmark of pes anserine syndrome is pain along the inner side of the knee, usually a few centimeters below the joint line. You may notice it most when climbing or descending stairs, getting up from a chair, or during activities that load the knee in a bent position. There is often tenderness when you press on the spot, and sometimes mild swelling. But the picture is not always that clean. Many patients report pain further back on the knee or right along the joint line, without any swelling at all, which makes it easy to confuse with a meniscal tear.¹²

This mimicry is common enough to be well documented. A study examining patients who had MRIs for knee symptoms found that pes anserine bursitis was frequently the actual diagnosis in people whose clinical presentation looked like a medial meniscal tear.¹³ The concern here is real: patients who are misdiagnosed with a meniscal tear may end up undergoing arthroscopic surgery that was never going to fix their problem, because the pain was coming from outside the joint rather than inside it. MRI can help distinguish the two, showing a characteristic pattern of fluid pooling beneath the pes anserinus tendons against the inner tibia.¹⁴

Risk Factors Are Less Straightforward Than Expected

You will often read that pes anserine bursitis is associated with obesity, knee osteoarthritis, and diabetes. These associations are plausible and widely repeated, but the evidence is thinner than you might expect. A case-control study that specifically tested these assumed risk factors found no significant difference between people with pes anserine syndrome and matched controls in terms of diabetes, obesity, knee osteoarthritis, knee instability, or foot malalignment.¹⁵ That does not mean these factors play no role, but it does suggest that the condition is not as tightly linked to any single identifiable cause as clinical teaching sometimes implies. Overuse, biomechanical quirks, and individual anatomical variation all likely contribute, making it one of those conditions where the answer to “why did I get this?” is often unsatisfying.

Treatment Options and What the Evidence Says

First-line treatment is typically conservative: rest, ice, stretching of the inner thigh and hamstring muscles, and anti-inflammatory medications. Physical therapy focused on strengthening the hip abductors and quadriceps, along with flexibility work, addresses the biomechanical loads that irritate the area. For many people, this is enough.

When it is not, the most studied intervention is a corticosteroid injection into the bursa. A trial comparing corticosteroid injections to a structured physical therapy program found that both produced significant improvements in pain and function, with no meaningful difference between the two approaches.¹⁶ Corticosteroid injections tend to work fast, but the relief does not always last. A multicenter trial comparing corticosteroid injections to oxygen-ozone injections and prolotherapy found that corticosteroids and oxygen-ozone gave the best results at one week, but by eight weeks, oxygen-ozone and prolotherapy held their gains better than corticosteroids.¹⁷

Platelet-rich plasma (PRP) injections have attracted interest as an alternative. One study reported that about 85% of patients treated with PRP achieved total or near-total pain relief within six months.¹⁸ PRP also appears safe in either single or double doses.¹⁹ However, a head-to-head comparison of PRP, corticosteroid injection, and extracorporeal shockwave therapy found that corticosteroids outperformed PRP at both one week and eight weeks for pain and function scores.²⁰ The PRP evidence, in other words, is promising but not yet consistent enough to call it clearly superior. If you are weighing options, the practical difference often comes down to cost and availability: corticosteroid injections are cheap and widely accessible, while PRP is more expensive and not always covered by insurance.

When Imaging Helps and When It Doesn’t

For a straightforward case of pes anserine tendino-bursitis, imaging is not always necessary. A clinician who can reproduce your pain by pressing on the right spot, below the joint line on the inner knee, can often make the diagnosis without any scan. Imaging becomes useful when the clinical picture is murky, especially when the pain location overlaps with where a meniscal tear would hurt. As mentioned earlier, MRI shows pes anserine bursitis as a pocket of fluid in a specific location beneath the tendons, which helps avoid unnecessary arthroscopy.²¹ Ultrasound is another reasonable option: it is cheaper, faster, and can both visualize the bursa and guide a needle for injection at the same time. The tradeoff is that ultrasound is more operator-dependent, meaning the quality of the information depends heavily on the skill of the person holding the probe.

The Medial Collateral Ligament Connection

The pes anserinus sits in tight quarters with the medial collateral ligament (MCL), the main ligament preventing the knee from buckling inward. An MRI and cadaveric study identified a bursa located between the superficial and deep layers of the MCL itself, separate from the pes anserine bursa.²² In some people, the MCL even contributes directly to the pes anserinus formation. The Nigerian cadaveric study found that the tibial collateral ligament participated in the pes anserinus in 5% of specimens.²³ This anatomical overlap explains why inner-knee pain can be so difficult to pin down: the pes anserinus, the MCL, the medial meniscus, and at least two different bursae all share a few square centimeters of real estate. A careful physical exam, sometimes supplemented with imaging, is the only way to sort out which structure is actually generating the pain.

Living With Pes Anserine Pain Long Term

Most cases of pes anserine syndrome resolve with conservative care over several weeks. For a minority of people, it becomes a chronic, recurring issue, flaring with increased activity or seasonal changes in exercise patterns. If you are dealing with recurrent episodes, a few strategies tend to help. Strengthening the muscles around the hip, particularly the gluteus medius, reduces the inward collapse of the knee during walking and running that loads the pes anserinus tendons. Avoiding prolonged sitting with the knees tightly bent can reduce irritation. If you are a runner, gradual increases in mileage and attention to running on flat surfaces (rather than banked roads, which tilt one knee inward) can prevent flares. For those whose pes anserine pain coexists with knee osteoarthritis, managing the arthritis itself through weight management, activity modification, and appropriate exercise often improves the bursitis as a side benefit, even if the two conditions are not as tightly linked statistically as once assumed.

Surgery for pes anserine bursitis is rare and generally reserved for cases that fail all conservative measures over many months. When it is performed, it typically involves removing the inflamed bursa. The outcomes data for surgical treatment are thin, which reflects how infrequently it is needed rather than any particular concern about the procedure. For the vast majority of people, the combination of targeted exercise, occasional anti-inflammatory use, and one or two well-placed injections if needed is enough to get the problem under control.