The CT gap sign, a case of failure to seat the polythene component in total ankle replacement (infinity)

We present a case of incorrect seating of the polyethylene liner in an INFINITY total ankle replacement causing discomfort and eventually leading to a revision ankle replacement. This malposition of the implant was observed on a computed tomography (CT) scan as a 'gap' between the liner and the body of the replacement. The patient complained of discomfort on walking postoperatively, which limited activity. All investigations were normal including X-Ray, inflammatory markers, ultrasound, and it was only the subtle CT changes along with history that confirmed the diagnosis. This case demonstrates a complication of the design of the INFINITY prosthesis and the approach taken to the ankle when inserting. The patient fortunately made a full recovery after revision surgery, returning to normal activities with minimal pain.