Abstract
A linkage-based mathematical model was used to design a ligament-compatible prosthesis to keep certain ligament fibres isometric during passive motion. The sagittal plane talar component radius is about 50% longer than that of the normal talus, the tibial component is spherically convex. A fully conforming meniscal bearing is interposed between them. Experiments in cadaver specimens confirmed the mathematical prediction that the bearing moves forwards on both metal components during dorsi-flexion and backwards during plantar flexion.
Between July 2003 and July 2008, the prosthesis was implanted into 250 patients at nine hospitals in Northern Italy. By November 2007, 158 in 156 patients were seen at least six months post-operatively. Mean age was 60.5 years. The diagnosis was post-traumatic osteoarthritis in 127, primary osteoarthritis in 17, rheumatoid arthritis in 10.
The mean follow-up was 32.5 months. The pre-operative AOFAS score of 36.2 rose to 75.9, 79.3, 77.9, and 79.0 respectively at 12, 24, 36, 48 months. Dorsi-flexion increased from 0.1° to 9.7°, plantarflexion from 15.1° to 24.6°. In 30 patients at one hospital, the range of postoperative motion, 14° – 53°, was significantly correlated to the range of bearing movement on the tibial component, 2mm–11mm, measured radiologically, (r2 = 0.37, p < 0.0005).
By December 2007, 2 revision operations had been performed at 24 months, one for unexplained pain not relieved by a successful arthrodesis, one in a patient with Charcot-Marie-Tooth disease. There were no device-related revisions (loosening, fracture, dislocation). The Kaplan-Meier survival rate (component-removal as end-point) at 4 years was 96% (Confidence interval 90–100%).
Early clinical results have demonstrated safety and efficacy. The survival rate at four years compares well with multi-centre 5-year rates published by the Swedish (531 cases, survival 78%), Norwegian (257, 89%) and New Zealand (202, 86%) registries.
Correspondence should be addressed to Editorial Secretary Mr ML Costa or Assistant Editorial Secretary Mr B.J. Ollivere at BOA, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England; Email: mattcosta@hotmail.com or ben@ollivere.co.uk
