Fixation Screw Torque Versus Bone Integration for Swine TMJ Implants

Objectives: This study investigated the osseous integration of titanium fixation screws of temporomandibular joint (TMJ) implants in 10 swine for times up to 12 months of function.
Methods: The TMJ implants produced by Vista Engineering were characterized and implanted into adult swine before euthanization, necropsy, and analyses of fixation screws (IRB-APN 130709461). Maximum magnitudes for screw removal (N-cm) was measured for each screw before preparing samples into non decalcified and decalcified 5 micron paraffin sections for histomorphometry analyses. Bone integration of screws was measured to obtain Bone to Implant Contact (BIC) and Total New Bone Area (mm²). Data collected through histomorphometry were compared to respective torque of removal. Twenty-five of 71 samples that demonstrated measurable torque were included in this study. The twenty-five samples represented different anatomical locations (left, right, fossa, ramus) of implant fixation screws in three animals.
Results: Histomorphometry confirmed variable amounts of bone integration (0%-16.4%) for the screws. Torque magnitudes for the screws compared with BIC showed a range from 0 to 41.8 N-cm, with higher bone linear contact yielding higher torque of removal. Torque magnitudes versus % BIC for 15 samples demonstrated a linear regression fit exceeding 0.89.
Conclusions: Within the available sample, a significant result was that greater linear contact of bone along the screws correlated with higher torque magnitudes. These data provide a direct input for theoretical predictions of the size and number of fixation screws needed for biomechanical implant stability. The locations of the implant fixation screws should therefore be planned for sites with the maximum amount and quality of bone.