O1162 LOAD SHARING BETWEEN A NEWLY DEVELOPED IMPLANT AND BONE IN SPINAL FUSION

Abstract

Aims: The holding power of new designed implants, need to be improved by osseointegration, in particular in osteoporotic bone. The osseo-integration depends on the interface mechanical conditions during fracture healing or fusion leading either to bony incorporation or to micro-movement and encapsulation. The aim of the current study was to evaluate the load sharing conditions after the fusion process between implant and bone with respect to the fusion status of the stabilized segment. Methods: The hollow cylinder based implant was used to stabilize a corpectomy of L4 in 17 sheep (9 osteoporotic and 8 control). After a survival time of 4 month the spine specimens were tested in a six-degree-of-freedom (6DOF) device in the three principal directions (flex/ex, lat. bending, torsion). Hysteresis curves were recorded before and after removal of the stabilizing longitudinal bar. The changes in ROM and stiffness in the different planes were compared statistically (α < 0.05). The results were validated histologically. Results: The increase in range of motion in all planes was significantly higher after implant removal in animals that were classified as non-fused spines (+35±15% in non-fused vs. +12±8% in fused spines). At the same time stiffness decreased significantly more (stiffness ex/flex −31±14% in non-fused vs −7±7% in fused spines) without differnces between normal and osteoporotic spines. Histology revealed 3 capsules in the fused spines. Conclusions: If fusion takes place in the presence of a relatively stiff implant the load flow through the implant is interrupted by connective tissue on one of the implant/bone interfaces. The changes in ROM and stiffness indicate the remaining contribution of the implant to the load sharing of the implant/ bone complex in case of non-fusion and are highly sensitive therefore to predict micromovement.