BACKGROUND Meticulous arthrotomy closure during total knee arthroplasty (TKA) is essential to preserve quadriceps strength and maintain proper patellar tracking. We recently encountered a subset of patients referred with anterior knee pain, a palpable retinaculum defect, and quadriceps weakness. Each patient was found to have an arthrotomy dehiscence. This study highlights arthrotomy dehiscence as a failure mechanism after TKA. METHODS A retrospective case series of patients who underwent primary TKA followed by subsequent capsular dehiscence was reviewed. We investigated whether the dehiscence was related to trauma, the presence of a palpable arthrotomy defect, patellar tracking, quadriceps strength, and the type of closure technique. RESULTS All patients had an atraumatic disruption and presented with anterior knee pain and quadriceps weakness. Fourteen of 18 patients (78%) had a palpable defect along the medial retinaculum. Nine of 18 patients (50%) had patellar subluxation or tilting. Barbed sutures were used in 13 of 18 (72%) patients. A simple arthrotomy repair (44%) or a vastus medialis obliquus advancement (56%) was used successfully in all patients. CONCLUSIONS Atraumatic arthrotomy dehiscence as a cause of TKA failure historically has been a rare event. Recently, we have noticed a disturbing number of patients presenting with this failure mechanism. With this study, we call attention to this unique atraumatic failure mechanism in patients presenting with anterior knee pain after TKA. Surgeons should be aware of this risk when using barbed sutures alone and should consider routinely reinforcing the arthrotomy with interrupted sutures. METHODS Level IV, Retrospective Case Series.