BIOMECHANICAL PROPERTIES OF SMALL BONE SCREWS

Purpose: To evaluate systematically the biomechanical properties of 13 popular screw designs, ranging from 0.8 to 2.0 mm in diameter. Method s: Screws were characterized in terms of external, core, and drill dia meter; cutting flute and head design; material; pitch, thread depth; a nd height of shank (unthreaded portion) and shank with plate. They wer e tested in standardized bone specimens (2 x 2.5-cm slabs of fresh bov ine femur) 1, 2, 3, and 4-mm thick. For each screw-bone thickness comb ination, 10 trials were conducted to determine push-out force (POF) an d another 10 trials to determine insertion (IT) and maximum torque (MT ) yielding a total of 1,040 tests. Results: Among the 13 different scr ews, in 1-mm thick bone, both the lowest (108.5 N) and highest (294.9 N) POF were created by 2-mm screws (P < .001); that with the lowest PO F had a long unthreaded shank and pitch, that with highest POF had a s hort unthreaded shank and pitch. Screws with 0.8- to 1.5-mm diameters showed no differences in POF. The 2-mm screw with the lowest POF also had the lowest MT in 1-mm thick bone compared with the other 2-mm scre ws (P < .001). In thicker bones (> 2 mm), two 2-mm screws showed 30% t o 50% lower MT than the other same size screws (P < .001) because thei r head slots stripped easily. When all screws were considered together for a particular bone thickness, torque was strongly predicted by scr ew diameter (MT: r = .94, P < .001; IT: r = 0.92, P < .001). Screws wi th the same diameters varied significantly in IT because of difference s in self-tapping cutting flute design. Conclusion: External diameter, unthreaded shank height, head slot, and self-tapping cutting flute de sign had the greatest impact on screw strength and efficiency in thin cortical bone. Thread depth, core diameter, and metal type did not aff ect performance significantly. Under these highly standardized in vitr o conditions, the ideal 2-mm screw has an unthreaded shank that is as short as possible, and the pitch is about 0.8 mm. Additional aspects o f a clinical situation beside holding strength must, however, be consi dered when choosing a screw.