FACTORS AFFECTING THE PULLOUT STRENGTH OF CANCELLOUS BONE SCREWS

Screws placed into cancellous bone in orthopedic surgical applications , such as fixation of fractures of the femoral neck or the lumbar spin e, can be subjected to high loads. Screw pullout is a possibility, esp ecially if low density osteoporotic bone is encountered. The overall g oal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase streng th. Twelve types of commercially available cannulated and noncannulate d cancellous bone screws were tested for pullout strength in rigid uni cellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentall y derived pullout strength was compared to a predicted shear failure f orce of the internal threads formed in the polyurethane foam. Screws e mbedded in porous materials pullout by shearing the internal threads i n the porous material. Experimental pullout force was highly correlate d to the predicted shear failure force (slope = 1.05, R(2) = 0.947) de monstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the mate rial into which the screw is embedded and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for c annulated screws was 17 percent lower than that of noncannulated cance llous screws, and the pullout force was correspondingly less. Increasi ng the TSF, a result of decreasing thread pitch or increasing thread d epth, increases screw purchase strength in porous materials. Tapping w as found to reduce pullout force by an average of 8 percent compared w ith nontapped holes (p = 0.0001). Tapping in porous materials decrease s screw pullout strength because the removal of material by the tap en larges hole volume by an average of 27 percent, in effect decreasing t he depth and shear area of the internal threads in the porous material .