CANCELLOUS BONE SCREW THREAD DESIGN AND HOLDING POWER

This study was designed to isolate and evaluate the parameters of host density, outer diameter (OD), root diameter (RD), and pitch in cancel lous bone screw design and their effect on holding power. Special emph asis was placed on screw pitch, which has been evaluated infrequently in the literature. Three groups of stainless steel V thread screws (gr oup I, OD 4.5 mm, RD 3.0 mm; group II, OD 6.4 mm, RD 3.5 mm; group III , OD 6.4 mm, RD 4.2 mm) were machined with progressive increases in pi tch from 12 to 32 threads per inch (TPI). Two densities of synthetic c ancellous bone material (Pedilen, Ottobock, Minneapolis, MN, U.S.A.), 0.15 g/ml and 0.22 g/ml, were then prepared and molded into sheets 1.9 cm thick and the screw threads completely engaged. Push-out tests wer e performed using a servohydraulic testing machine (MTS, Minneapolis, MN, U.S.A.). Fifteen trials of each screw were tested in each material . The effect on holding power of the different parameters of the custo m screws in order of importance was (a) host material density, (b) OD (c) pitch, and (d) RD. The groups with a 6.4-mm OD had a much greater holding power than did the group with a 4.5-mm OD (p < 0.001). A decre ase in screw pitch (increased threads per inch) did itself have a sign ificant improved effect on fixation for all groups in both pedilen den sities (p < 0.001). In the two 6.4-mm screw groups studied, the differ ence in the two root diameters (4.2 mm vs. 3.5 mm) showed the smaller root diameter to give a greater holding power in the less dense 0.15 g /ml pedilen (p < 0.001). In the more dense 0.22 g/ml pedilen there was no difference (p = 0.26) between the root diameters. To optimize hold ing power, cancellous screws may be designed with a decreased pitch (i ncreased TPI) over those commercially available today. Cannulated scre ws must have a larger cancellous thread root diameter to leave room fo r the central cannulation; this may decrease their holding power in le ss dense cancellous bone but not in denser bone.