THE EFFECTS OF DEPTH OF PENETRATION, SCREW ORIENTATION, AND BONE-DENSITY ON SACRAL SCREW FIXATION

Obtaining adequate sacral fixation is an important and challenging cli nical problem. Bicortical screw fixation has been used to provide impr oved mechanical strength. The goal of this study was to evaluate the e ffects of depth of penetration (unicortical versus bicortical), orient ation (medial versus lateral), and bone density on the strength of the bone-screw interface. Seven-millimeter Steffee screws (Acromed, Cleve land, Ohio) were placed in human cadaveric sacra. The screws were plac ed either medially into the centrum of the Sl body or laterally into t he sacral ala. The fixation systems used, either bicortical or unicort ical, were tested on a Universal testing machine. The specimens were l oaded to failure and biomechanical parameters were measured. Medially oriented screws sustained the highest load to failure. In an older pop ulation, unicortical fixation sustained load to failure and initial co mpliance findings were similar to those for bicortical fixation. The r egional bone density in the centrum of the first sacral body was dense r than that in the lateral alar region, and also provided more rigid b one screw fixation.