Three-dimensional movement analysis after internal fixation of pelvic ringfractures - A computer simulation

Several studies exist describing the biomechanical behavior of several exte rnal or internal fixation techniques of the posterior and anterior pelvic r ing. Recently, the traditional models using isolated anatomical sections or fixed pelvic ring specimens for evaluation of linear or two-dimensional da ta have been replaced by three-dimensional measurement systems and simulati ons of muscle forces. These studies have contributed important information to the understanding of the biomechanics of the intact and injured pelvic r ing, however, a consequent movement analysis is still missing. In the present study, 3-D data acquired during several series of testing im plants for stabilization of the posterior pelvic ring (sacrum: sacral bars, sacral plates, transiliosacral lag screws; S1 joint: anterior plates, tran siliosacral lag screws), using a complete pelvic ring model with single leg stance and static abductor muscle simulation,were converted into a commerc ially available 3-D animation package. By use of simple graphical represent ation of anatomical elements of the pos terior pelvic ring, reproducible an d reliable movement patterns for different types of stabilization could be identified,which demonstrated potential "weakness" of the fixation before f ailure occurred. These movements were analyzed by "replay functions" and we re comparable to observations during the original experiment. The following movements were observed. Sacral fracture, transforaminal: (1) rotation of the transiliosacral lag screws around its axis, even with a second screw in to S1; (2) Sacral bars: shearing with compression of the cranial-posterior fracture zone; (3) Sacral plates: minimal translation in the proximal fract ure zone and distraction in the distal fracture line, effectively compensat ed by an additional plate at the S3 level. S1 joint disruption: (1) anterior plating (two plates), minimal translation in the plane of the S1 joint; (2) transiliosacral lag screws, rotational m ovement around the axis of the screws with only minimal movement at the S1 level. The provided information confirmed the observations and allowed a mo re detailed and comfortable examination of movement patterns. A better unde rstanding of potential "failure zones" might be useful to optimize the dime nsions, design, and the positioning of implants for the pelvic girdle. For further studies, more complex computer models including finite element tech nology might be useful to add accessory information and could result in a d ecreased need of living specimen testing.