Novel Thread Design Reduces Migration of Screws in Surrogate Osteoporotic Bone under Multiaxial Loads

Abstract

Introduction: Conventional bone screws have relatively high levels of migration failure in patients with osteoporotic bone. Lateral forces are often ignored in screw design in favour of axial pull-out forces. We present and examine a novel thread design with an undercut feature that sandwiches bone tissue between the screw shaft and thread to improve resistance to lateral migration. Methods: Novel thread and typical buttress thread locking screws (n=5 per group; outer diameter 4.5 mm, core diameter 3.2 mm, length 42 mm) were implanted in osteoporotic bone substitute (10 PCF polyurethane foam per ASTM F1839). Individual screw lateral migration resistance was tested by recording lateral force during 5 mm/minute displacement control to a depth of 10 mm. Screw pair with plate cyclic craniocaudal/torsional migration resistance was tested by recording displacements during increasing cyclic force control. Results: (Experiment A) For lateral force at 1.7 mm lateral displacement: novel thread (296 ± 6.50 N, p<0.001); buttress thread (270 ± 2.77 N). At 5 mm: novel thread (323 ± 10.4 N, p=0.003); buttress thread (300 ± 6.16 N). (Experiment B) In cyclic craniocaudal loading resistance test with locking plate, for force and cycle number at 5 mm lateral displacement: novel thread (721 ± 29.6 N, p<0.001, 1087 ± 59.7 cycles, p<0.001); buttress thread (599 ± 32.8 N, 850 ± 61.8 cycles). (Experiment C) In cyclic torsional loading resistance test with locking plate, for torque and cycle number at 10° angular displacement: novel thread (2794 ± 22.7 Nm, p<0.001, 413 ± 7.28 cycles, p<0.001); buttress thread (2369 ± 31.2 Nm, 359 ± 22.7 cycles). Conclusion: The novel thread design significantly improves lateral migration resistance under both static/cyclic craniocaudal and torsional loading conditions in osteoporotic bone.