A biomechanical comparison of screw and wire fixation with and without polymethylmethacrylate re-enforcement for acetabular osteotomy stabilization in dogs

Objective-Compare the biomechanical characteristics of screw and wire fixat ion with and without polymethylmethacrylate (PMMA) re-enforcement for aceta bular osteotomy stabilization in dogs. Animals-Pelves removed from 8 adult mixed breed dogs weighing between 25 an d 30 kg, Procedure-The pubic symphysis of each pelvis was split and a central transv erse acetabular osteotomy was performed. One hemipelvis from each dog was s tabilized with the composite fixation (interfragmentary Kirschner wire, two screws and a figure-of-eight orthopedic wire with PMMA). The contralateral hemipelves was stabilized with an interfragmentary Kirschner wire, two scr ews, and a figure-of-eight orthopedic wire without PMMA. All hemipelves wer e tested in bending by using a materials testing machine at a cross head sp eed of 5 mm/min. An extensometer was placed on the dorsomedial surface of t he hemipelves centered over acetabular osteotomy to record distraction of t he osteotomy during loading. A load/deformation curve and a load/distractio n curve was produced for each hemipelvis. The slope for the initial linear portion of the load/deformation curve and the load/distraction curve, yield load and maximum load sustained were compared between repair groups using a paired t-test with P < .05 considered significant. Results-The slope of the load/deformation curve was significantly greater ( P = .001) for hemipelves stabilized with the composite fixation (mean +/- S D: 69 +/- 18 N/mm) compared with hemipelves stabilized without PMMA (mean /- SD: 39 +/- 8 N/mm). There was no significant difference (P = .593) betwe en repair groups in the slope of the load/distraction curves as measured on the extensometer. Yield load was significantly greater (P = .0002) for hem ipelves stabilized with the composite fixation (mean +/- SD: 184 +/- 25 N) compared to hemipelves stabilized without PMMA (mean +/- SD: 74 +/- 12 N). Maximum load sustained was also significantly greater (P = .013) for hemipe lves stabilized with the composite fixation (mean +/- SD: 396 +/- 71 N) com pared to hemipelves stabilized without PMMA (mean +/- SD: 255 +/- 94 N). Fa ilure of hemipelves stabilized with the composite fixation occurred primari ly by ventrolateral bending of the cranial and caudal pelvic segments at th e osteotomy site. Failure of hemipelves stabilized without PMMA occurred by ventrolateral bending of the cranial and caudal pelvic segments at the ost eotomy site with pronounced concurrent ventrolateral rotation of the crania l pelvic segment. Conclusion-PMMA improves the mechanical characteristics of acetabular fract ure fixation, at least in part by neutralization of rotational forces. The results of this study justify use of PMMA as a component of the composite f ixation when repairing acetabular fractures (C) Copyright 1999 by The Ameri can College of Veterinary Surgeons.