In vitro measurement of bone-implant micromotion of tibial implants after total ankle replacement
Background: Total Ankle Replacement (TAR) o_ers the advantage of preserving ankle joint motion compared with ankle arthrodesis. However, compared to hip or knee arthroplasty, TAR is a less successful procedure when considering the relatively low implants survival rate at 10 years, the high revision rate and also numerous complications. The main complications after TAR are periprosthetic cysts (up to 77% of cases), fractures of the malleolus and bone loosening. However, the mechanism that leads to these complications remains unclear and the high incidence of periprosthetic cysts raises worries for long-term survivorship of the implants. Therefore, there is a need for better understanding of shortcomings associated with TAR, and for improvement in implant design and surgical technique. Goal: To determine, in vitro, the in_uence of mobile-bearing position on bone strain and primary stability after Total Ankle Replacement by measuring bone surface strain and bone-implant relative movement. The study was focused on tibial components. Methods: Prostheses from the company Tornier were implanted in 8 cadaveric tibiae by an experienced surgeon. A load of 2000 [N] was applied on the prosthesis at three di_erent positions: posterior, neutral and anterior. The displacement was measured on the anterior surface of the tibia using 3D Digital Image Correlation. The bone axial strain was then evaluated from the displacement. Results: Bone strain was higher when the mobile-bearing was placed in the anterior position compared with the neutral position (p < 0.00001) and smaller when placed in the posterior position (p < 0.00001). The order of magnitude of bone strain varied from +500 microstrain up to -5600 microstrain. Most of the time, the maximum compressive strain was found around the keel of the implant. The bone-implant relative movements showed a mean value (SD) of 142 microns (40 microns) for the neutral position. Conclusions: The axial strain measured on the anterior bone surface was for most of the tibiae in the physiological zone, between 200 and 2000 microstrain according to Frost (1987). However, some values mainly measured around the keel of the implant reached the pathological zone where the risk for bone fracture is higher (> 4000 microstrain). The bone-implant relative movements were in the same order of magnitude of in vivo measurements reported in the literature. In conclusion, we confirmed the importance of mobile-bearing position in TAR, which could be optimized during surgery.