EARLY CLINICAL RESULTS OF THE BOX ANKLE PROSTHESIS

Abstract

A new design of total ankle replacement was developed. According to extensive prior research, the design features a spherical convex tibial component, a talar component with radius of curvature in the sagittal plane longer than that of the natural talus, and a meniscal component fully conforming to these two. The shapes of the tibial and talar components are compatible with a physiologic ankle mobility and with the natural role of the ligaments.

Within an eight-centre clinical trial, 114 patients were implanted in the period July 2003 – September 2006, with mean age 62.2 years (range 29 – 82). The AOFAS clinical score systems and standard radiographic assessment were used to assess patient outcome, here reported only for those 75 patients with follow-up longer than 6 months.

Intra-operatively, the components maintained complete congruence at the two articulating surfaces of the meniscal bearing over the entire motion arc, associated to a considerable anterior motion in dorsiflexion and posterior motion in plantarflexion of the meniscal-bearing, as predicted by the previous mathematical models. Mean 10.0 and 23.5 degrees respectively of dorsi- and plantar-flexion were measured immediately after implantation, for a mean additional range of motion of 19.2, which was maintained at follow-ups. Radiographs showed good alignment and no signs of evolutive radiolucency or loosening. The mean AOFAS score went from 40.8 pre-op to 66.2, 74.6 and 77.2 respectively at 3, 6 and 12 month follow-ups. One revision only was performed successfully three days after implantation because of a technical error.

In the score system utilized, Function and RoM sections scored better than any average previous total ankle result, Pain scored similarly. The satisfactory though preliminary observations from this novel design encourage continuation of the implantation, which is now extended over a few European countries. Instrumented gait and three-dimensional fluoroscopic analyses are in progress to quantify functional progresses.