Delayed Recognition of an Isolated Tear of the Posteromedial Bundle Following Anterior Cruciate Ligament Reconstruction: A Case Report

Isolated posteromedial bundle (PMB) injuries of the posterior cruciate ligament (PCL) are rare, often subtle, and easily missed on standard clinical tests and imaging. This case report highlights a delayed diagnosis of PMB injury in a 24-year-old male athlete presenting with persistent knee instability eight months following anterior cruciate ligament (ACL) reconstruction. Despite resolution of anterior laxity, the patient exhibited increased hyperextension, a pseudo-Lachman sign, and a tibiofemoral posterior step-off at 10-20° flexion, while 90° posterior drawer testing and MRI were unremarkable. These findings raised suspicion for isolated PMB injury, which was confirmed via diagnostic arthroscopy. The patient underwent nonanatomic PMB augmentation using a semitendinosus autograft with internal bracing. A structured rehabilitation protocol was followed, incorporating PCL-specific bracing, protected range of motion exercises, delayed hamstring activation, and a staged return to activity. At six months, the patient demonstrated full resolution of symptoms, with return to competitive sports by nine months. Validated functional outcome scores confirmed clinical improvement, including an International Knee Documentation Committee subjective score of 92.0 and a Tegner activity scale improvement from 4 (preoperative) to 7 (postoperative), reflecting return to high-level athletic activity. This case emphasizes the importance of recognizing underappreciated clinical signs of isolated PMB injury, particularly following hyperextension trauma. It underscores the limited sensitivity of MRI in chronic or partial PMB tears and advocates for diagnostic arthroscopy when clinical suspicion remains high. Diagnostic arthroscopy should be considered when clinical findings suggest subtle posterior instability, despite normal imaging results. Targeted PMB augmentation can restore extension stability and deliver excellent functional outcomes, even in delayed presentations.