THE TORSIONAL STRENGTH OF BONES WITH RESIDUAL SCREW HOLES FROM PLATESWITH UNICORTICAL AND BICORTICAL PURCHASE

Objective. To evaluate the effect of unicortical and bicortical screw holes on residual bone strength by comparing the in vitro torsional st rength of cadaveric sheep tibiae with screw holes from plates with uni cortical and bicortical purchase relative to each other and to intact bone. Design. The paired tibiae were grouped randomly and torsion test ed to failure as follows: Group I - unicortical screw holes versus int act bone; Group II - bicortical screw holes versus intact bone; and Gr oup III - bicortical versus unicortical screw holes. Background. Recen tly the point contact fixator, or PC-Fix (using screws with unicortica l purchase), was designed to minimize bone devascularization beneath t he plate compared to the conventional dynamic compression plate, or DC P (bicortical purchase), and possibly reduce refracture rates after pl ate removal. However, the effects of unicortical versus bicortical res idual screw holes on potential refracture are unknown. Methods. Screw holes were drilled and tapped through either a 7-hole bicortical DCP o r a unicortical PC-Fix in 18 paired cadaveric sheep tibiae. The bones were then tested in torsion. Results. The average decreases in the tor sional strength of unicortical screw holes versus intact bone, bicorti cal screw holes versus intact bone, and bicortical versus unicortical screw holes were 21.6, 31.4, and 26.7% respectively. Mean torque value s to failure were statistically significant within each of the three g roups: P<0.01, 0.001, and 0.001 respectively. Conclusions. Bones with unicortical screw holes are significantly weaker in torsion than intac t bones, but significantly stronger than bones with bicortical holes.