COMPARISON OF FIXATION STRENGTHS OF LOCKING HEAD AND CONVENTIONAL SCREWS, IN FRACTURE AND RECONSTRUCTION MODELS

Purpose: Claimed clinical advantages of the locking-head mandibular re construction plating system include the ability to achieve stability w ith fewer numbers of screws per bony segment as compared with conventi onal screws. The purpose of this study was to test the hypothesis that increased resistance to displacement will be obtained when using lock ing-head as compared with the same number of conventional screws per s egment in both fracture and reconstruction models. Materials and Metho ds: Eight groups were tested based on the screw number (two or four), screw type (locking-head or conventional), and fracture (bony appositi on) or reconstruction model (1-cm defect). Two-dimensional beam mechan ics using adult bovine ribs and the Instron machine were used to devel op a load-displacement curve up to 150 N for each specimen. An osteoto my was then created and the segments were reduced, with preload (fract ure model) or with a 1-cm defect (reconstruction model), and plated us ing the Synthes locking-head plate with either two or four bicortical locking-head (4.0-mm) or conventional (2.7-mm) screws per segment, The fixed ribs were loaded to 150 N, and the displacement was recorded. R esults: Locking-head screws provided superior resistance when using tw o screws per segment in the reconstruction model as compared with conv entional screws. Minimal difference was seen between other screw types within a model. The fracture model offered significantly greater (3.1 to 3.7 X) resistance to displacement than did the reconstruction mode l. Conclusions: Locking-head screws provided significantly increased r esistance to displacement when only two screws per segment were used i n the reconstruction model. When four screws per segment were used, th ere was no significant difference between locking-head and conventiona l screw types in either model. The effect of bony buttressing is signi ficant and may explain why miniplates often fail in the atrophic mandi ble but are successful in the fully dentate patient.