The CT gap sign, a case of failure to seat the polythene com
Although total ankle replacement (TAR) is an increasingly common procedure in specialist foot and ankle units, the number of ankle replacements remains fewer than hip and knee. Current TARs are either two- or three-component designs. INFINITY, which is a fixed bearing implant, has rapidly become the most common prosthesis

A 56-year-old man underwent TAR due to post-traumatic osteoarthritis using an ‘INFINITY’ TAR (Wright medical implant, size 4 tibial tray, size 3 talar component and 9 mm poly component).

His initial postoperative period was uneventful. Postoperative X-rays showed good alignment and implant position. Over the following months, he complained of persistent medial and posterior ankle pain. On examination, he had minimal swelling over the medial side of the ankle. His range of movement was 20 degrees of dorsi-flexion and 30 degrees of plantar-flexion, which was painless. He had no rest pain but described something ‘pressing inside’ limiting his walking distance.
All inflammatory markers were within normal limits. Upon suspicion that the talar component might be irritating the tibialis posterior tendon, he underwent an ultrasound which was normal. His computed tomography (CT) was reported as normal by the radiologist. Symptoms suggested possible impingement of a slightly oversized talar component on medial malleolus. On close scrutiny, it was apparent that the CT postoperatively showed the poly to be seated anteriorly with a failure to engage posteriorly, leaving a void in the socket. We describe this as a ‘CT GAP sign’, and he was offered a revision ankle replacement.

Revision ankle surgery was undertaken by anterior approach through the previous incision. Intraoperative samples were taken which ruled out infection. The tibial component was well fixed to bone and showed no evidence of loosening, subsidence or rotational malalignment. Macroscopic examination of the retrieved insert showed grooves on its side. The poly was not fully reduced in its groove and had dislodged from the groove subsequently on the posterior aspect of the implant . He had a revision of the talar component to size 2 and change of polythene with good clearance of his medial gutter

In conclusion, incorrect seating of poly component is a rare complication, which can be a cause of persistent pain in ankle replacements. This can be easily missed by the untrained eye even on CT scans. Prompt revision is needed to prevent late consequences.