Localized trabecular damage adjacent to interbody fusion devices.

STUDY DESIGN An in vitro cadaveric study of interbody fusion devices (IFDs) with differing interface geometries. OBJECTIVE To quantify the trabecular damage and mechanical property degradation resulting from mechanical testing of bone-implant constructs and evaluate the dependence on the interface geometry. SUMMARY OF BACKGROUND DATA The success of an orthopedic implant depends, in part, on the interaction between the implant and the bone at its interface. IFDs must safely and effectively transfer load between the device and the neighboring vertebral cancellous bone on which the device is supported. METHODS Twenty-four vertebral pairs implanted with bilateral cylindrical or hexahedral IFDs were subjected to either 1% or 2.5% compressive strain. The change in structural stiffness and the presence of residual deformation caused by damage processes were measured. Histologic evidence of trabecular damage was quantified using polychromatic labeling and an image processing scheme. Statistical comparisons between groups within a strain level were made using the Mann-Whitney U test. RESULTS.: Permanent deformation and/or decreased structural stiffness was found in all specimens. The overall changes in mechanical stiffness properties were inconsistent with regard to the device interface geometry. Histologic damage was found in all specimens located in a region immediately adjacent to the implant. The distribution of the damage relative to the center of the device cross-section was significantly different for the 2 interface geometries. At the higher strain, damage and the mechanical effect of damage were greater for the cylindrical devices. CONCLUSION Histologic damage was found in close proximity to the bone-implant interface and in all specimens, including those which did not demonstrate mechanical damage. Patterns of histologic damage corresponded directly to the bone-implant interface geometry. Gross structural measurements do not reliably detect changes caused by damage at the bone-implant interface.