Fracture line stability as a function of the internal fixation system: an in vitro comparison using a mandibular angle fracture model.

PURPOSE This study was conducted to determine and compare the initial mechanical stability and functional capability of six contemporary internal fixation systems used to fix mandibular angle fractures. MATERIALS AND METHODS An iterative analog of a mandibular angle fracture was developed to ensure replicability of material properties and fracture configuration across the test constructs. Each of six sets of mandible analog (1 set = 3 mandibles) was reduced according to prescribed technique by a variety of compressive and adaptive fixation systems. The compressive systems included the 1) eccentric dynamic compression plate, 2) Würzburg plate, 3) Luhr plate, and 4) solitary lag screw technique. The Champy miniplate and the Mennen clamp plate represented the adaptive fixation systems. The reduced analogs were placed in a straining frame, and simulated masticatory loads were applied to predetermined occlusal sites. Fracture line displacements were acquired and registered by displacement transducers attached to a computer-based data acquisition program. A coordinate transformation procedure was used to convert the generalized displacements at the fracture line into the individual rotations of the segments. An "instability factor" computed from the force-displacement data recorded at various loading conditions for each test construct was used to characterize a particular system's ability to restrain relative motion at the fracture surfaces. RESULTS There were minimal variations in the stability profiles of the individual compressive fixation systems. However, the fixation stability provided by the compressive and adaptive systems differed significantly (P < or = .0001). A large initial setting and a susceptibility to variations in loading patterns characterized the functional stability provided by the adaptive systems. Even at low masticatory loads (2 DaN), the adaptive systems had an instability that was two to three times as much as that of the compressive systems. Post-hoc comparisons between pairs of devices showed that angle fractures fixed by compressive systems provided significantly greater stability (P < or = .05) than those fixed by the Champy and Mennen systems. Between the adaptive systems tested, fracture fixation with Mennen plates was more stable than reduction by Champy miniplates (P < or = .05) when averaged over loads. CONCLUSIONS Compressive fixation systems are biomechanically superior to adaptive systems and provide good immediate functional stability to reduced mandibular angle fractures. The Champy and Mennen systems permit significantly higher motion at the fracture site, even at the attenuated masticatory forces encountered in the early postoperative period. Because the functional stability afforded by these adaptive systems is influenced by variations in the biting patterns, the risk of infection and complicated healing is correspondingly increased. Also, the low displacement resistance of the adaptive systems may not protect the alignment of the mandibular segments through the healing period and may manifest as occlusal discrepancies in the dentate patient. The biomechanical test system developed for this study allows an equitable comparison of fixation stability and appears to be a promising tool for investigating a variety of fixation systems and optimizing device design on a rational basis.