Description of Snapping Hamstrings

Snapping hamstrings are a rare condition, but they can cause significant disability in patients who have this pathology.

Lateral, or biceps, snapping hamstrings are usually due to a presumed genetic or injured state where the direct arms of the long and short heads of the biceps no longer attach to the posterolateral aspect of the fibular styloid. In these patients, when they perform a deep squat, the biceps femoris tendon can roll over the fibular head when one squats down. Upon arising, the tendon will often “snap” back into position. This can be a quite debilitating problem in patients who present with these symptoms. In addition, these patients commonly have irritation of the common peroneal nerve because it is adjacent nature to the snapping biceps tendon. Patients often have paresthesias and “zingers” going down into the lateral aspect of their leg and the dorsum of the foot due to the irritation of the common peroneal nerve.

Symptoms of snapping hamstring:

  • Deep pain within knee when squatting down
  • Audible or visible “clunk” when arising after squatting
  • Numbness or tingling over the outside of the knee with activities

While the causes differ between medial and lateral snapping hamstrings, the presenting symptoms are often similar.

Snapping medial hamstrings often present with similar findings. The “clunk” that one endures with going down into a deep squat and arising can be quite dramatic and it can sometimes appear like the joint is subluxing. Just like with lateral biceps tendon snapping, the occurrence of this varies between patients, with some having a dramatic snap in all circumstances and others having it only occasionally. We have noted that about half of our patients have had this onset with no particular injury, while the other half often have it after a hamstring-based ACL reconstruction, medial meniscus repair, or other type of surgery over the medial aspect of the knee which can cause scar tissue. The snapping is believed to be due to catching of the semitendinosus and/or gracilis tendons when they cross the semimembranosus tendon. However, dynamic ultrasound studies so far have not been able to elucidate which of these tendons it may be, or if both tendons cause the snap, between patients.

On physical exam, these patients will often have pain on palpation of the biceps tendon attachment on the fibular styloid. In addition, performing a deep squat will often reproduce their symptoms. We have found that in the most severe cases, the biceps femoris will snap in all circumstances, while in others it will only occasionally snap and catch.

The physical exam almost always involves having the patient perform a deep squat to replicate their symptoms. In some circumstances, a patient can replicate their medial knee snapping symptoms by putting full weight on their involved side and trying to hyperextend their knee. In this instance, the snapping may occur directly over the posteromedial knee. The examiner must place one’s fingers directly over the hamstring tendons to verify that this is a source of where the snapping occurs. In almost all circumstances, one can localize the snapping to the hamstrings within 5 to 6 cm of the posteromedial joint line.

We recommend that a thin slice MRI, to include the posterolateral knee structures, be obtained in these patients to verify that the biceps femoris attachment on the posterolateral aspect of the fibular styloid is not normal. X-rays should also be obtained to make sure there is not an osteochondroma or other pathology that could be confused with this condition.

Plain x-rays may be useful to determine if there is an osteochondroma or a bone spur (osteophyte) causing the snapping hamstrings. We have not found the use of MRIs to be particularly useful in diagnosing medial snapping hamstrings, but they remain useful to verify that there is no other pathology in this area of the knee which could be confused with this diagnosis (posterior horn meniscal tears, large Baker’s cysts, and other causes).