Graft impingement after anterior cruciate ligament reconstruction. Presentation as an active extension "thunk".

Twelve of 215 patients with an anterior cruciate ligament reconstruction developed a "thunk," a low-pitched sound, on active extension of the operative knee at an average of 5 months after surgery. In 4 of the patients the thunk resolved without surgical treatment at an average of 4 months after onset (range, 2 to 6). In 3 of the patients an increase in anterior knee laxity was associated with resolution of the thunk. Seven of the patients were treated with arthroscopic examination. Six of the patients had graft impingement on the lateral wall or the roof of the notch with knee extension. The thunk was eliminated with an adequate notchplasty that corrected the impingement. In the other patient the thunk was secondary to fibrosis of the anterior fat pad. One patient with a persistent thunk declined surgery and was lost to followup.