Case Diagnosis: MCL tear in the residual limb of a patient with a prior BKA due to improper prosthetic engagement, diagnosed on bedside ultrasound examination.
Case Description: 76 year-old Female with a history of diabetes mellitus and peripheral vascular disease, and status-post Right transtibial below knee amputation secondary to gangrene. She received her permanent transtibial patella tendon bearing prosthesis with a K2 foot at 14 months post amputation. 1 month later she presented to our clinic with acute right knee pain and swelling following a fall at home after failing to fully engage the shuttle-lock mechanism of the prosthesis. Knee XRay revealed no fracture. She was unable to don the prosthesis due to edema. Ultrasound evaluation of the right knee revealed a partial thickness tear of the Medial Collateral Ligament and possible tear of the medial meniscus. She underwent a corticosteroid injection and was advised to wear a shrinker. Swelling resolved after 1 month, however the prosthetic alignment required multiple readjustments before she was comfortable ambulating again.
Discussions: Patients with transtibial amputations are at risk of developing structural injuries to the intact joint in the residual limb. Failure to completely engage the patella tendon bearing prosthesis puts the joint at further risk. Physical examination of the knee in these patients can be challenging, as numerous studies have demonstrated the unreliability of standard provocative maneuvers such as varus stress, valgus stress, and Lachman (posterior drawer testing is considered most reliable). Some reports have suggested stress radiographic imaging to help diagnose injury to the collateral ligaments, however application is limited. Bedside ultrasound evaluation of the residual limb, when available, is a useful imaging modality to aid diagnosis.
Improper engagement of a below knee prosthesis will put the knee joint at risk of structural injury. Bedside ultrasound is a useful diagnostic tool when evaluating injury in residual limb.