Rigidity of unilateral external fixators--a biomechanical study.

INTRODUCTION: External fixation is the primary choice of temporary fracture stabilisation for specific polytrauma patients. Adequate initial fracture healing requires sufficient stability at the fracture site. The purpose of this study was to compare the rigidity of the Dynafix DFS(®) Standard Fixator (4 joints) with the Orthofix ProCallus Fixator(®) (2 joints), which differ in possibilities for adapting the configuration for clinical needs. MATERIALS AND METHODS: Both devices were tested 10 times in a standardised model. In steps of 10N, loading was increased to a maximum of 160N in parallel, transversal and axial direction (distraction and compression). Translation resultant and rotation resultant were calculated. RESULTS: With a force of 100N in parallel direction the mean translation resultant (Tr(mean)) of the Dynafix DFS(®) Standard Fixator (6.65±1.43mm) was significantly higher than the ProCallus Fixator(®) (3.29±0.83mm, p<0.001; Student's t-test). With a maximum load of 60N in transverse direction the Tr(mean) of the Dynafix DFS(®) Standard Fixator was significantly lower (8.14±1.20mm versus 9.83±0.63mm, p<0.005). Translation was significantly higher with the Dynafix DFS(®) Standard Fixator, for both distraction (2.13±0.32mm versus 1.69±0.44mm, p<0.05) and compression (1.55±1.08mm versus 0.15±0.33mm, p<0.005). The mean rotation resultant (Rr(mean)) at 160N distraction was lower for the Dynafix DFS(®) Standard Fixator (0.70±0.17° versus 0.97±0.21°, p<0.005). CONCLUSIONS: Both fixators were most sensitive to transverse forces. The Dynafix DFS(®) Standard Fixator was less rigid with parallel and axial forces, whereas transverse forces and rotation at distraction forces favoured the Dynafix DFS(®) Standard Fixator. Repeated heavy loading did not influence the rigidity of both devices.

Comparison of three different pelvic circumferential compression devices: a biomechanical cadaver study.

BACKGROUND: Pelvic circumferential compression devices are designed to stabilize the pelvic ring and reduce the volume of the pelvis following trauma. It is uncertain whether pelvic circumferential compression devices can be safely applied for all types of pelvic fractures because the effects of the devices on the reduction of fracture fragments are unknown. The aim of this study was to compare the effects of circumferential compression devices on the dynamic realignment and final reduction of the pelvic fractures as a measure of the quality of reduction. METHODS: Three circumferential compression devices were evaluated: the Pelvic Binder, the SAM Sling, and the T-POD. In sixteen cadavers, four fracture types were generated according to the Tile classification system. Infrared retroreflective markers were fixed in the different fracture fragments of each pelvis. The circumferential compression device was applied sequentially in a randomized order with gradually increasing forces applied. Fracture fragment movement was studied with use of a three-dimensional infrared video system. Dynamic realignment and final reduction of the fracture fragments during closure of the circumferential compression devices were determined. A factorial repeated-measures analysis of variance with pairwise post hoc comparisons was performed to analyze the differences in pulling force between the circumferential compression devices. RESULTS: In the partially stable and unstable (Tile type-B and C) pelvic fractures, all circumferential compression devices accomplished closure of the pelvic ring and consequently reduced the pelvic volume. No adverse fracture displacement (>5 mm) was observed in these fracture types. The required pulling force to attain complete reduction at the symphysis pubis varied substantially among the three different circumferential compression devices, with a mean (and standard error of the mean) of 43 ± 7 N for the T-POD, 60 ± 9 N for the Pelvic Binder, and 112 ± 10 N for the SAM Sling. CONCLUSIONS: The Pelvic Binder, SAM Sling, and T-POD provided sufficient reduction in partially stable and unstable (Tile type-B1 and C) pelvic fractures. No undesirable overreduction was noted. The pulling force that was needed to attain complete reduction of the fracture parts varied significantly among the three devices, with the T-POD requiring the lowest pulling force for fracture reduction.