BOX PLATE FIXATION OF THE SYMPHYSIS PUBIS - BIOMECHANICAL EVALUATION OF A NEW TECHNIQUE

The purpose of this study was to compare common techniques of pubic sy mphyseal fixation with a new method, the ''box plate,'' for fractures of the pelvis where the bone is osteopenic. This symphyseal fixation c onstruct consists of two, two-hole, 4.5-mm narrow dynamic compression plates (DCP) oriented parallel to one another. One plate is recessed w ithin the symphysis, and the other is located on the pubic tubercles. The plates are interlocked using two 6.5-mm fully threaded screws, for ming a box-like construct. To determine the mechanical properties of t his construct, five fresh, cadaveric pelvic specimens with a mean age of 75 years were harvested. The femora of each specimen were potted in to containers and fixed to the base of a materials testing machine. Th e pelvis was constrained from rotating about the hip joints by anterio r and posterior restraints. A vertical compressive load was applied th rough the lumbar spine. Force to a magnitude of 1,000 N was applied th rough three cycles. Capping motions at the symphysis pubis (SP) and th e sacroiliac (SI) joints, and flexion-extension of the sacrum with res pect to the ilia were measured under the following conditions: (a) int act, (b) SP ligament, unilateral anterior SI ligaments, and ipsilatera l sacrospinous and sacrotuberous ligaments disrupted (anteroposterior compression type II injury), and these injuries fixed using (c) a 4.5- mm narrow two-hole DC plate placed on the superior SP held by two canc ellous bone screws, (d) the DC plate well as a single 7.0-mm cannulate d cancellous iliosacral lag screw across the injured SI joint, (e) the DC plate and a five-here 3.5-mm reconstruction plate on the anterior SP, (f) a 3.5-mm, four-hole, DC plate on the superior SP using four fu lly threaded screws, and (g) the box plate symphyseal construct descri bed above. All fixations reduced SP joint gapping compared to the disr upted joint. However, all but the box plate still allowed significantl y greater motion than the intact SP joint. No fixation significantly r educed SI joint gapping or sacral flexion compared to the injured stat e.