Effect of Screw Type and Pilot Hole on Screw Fixation in Osteoporotic Metaphyseal Bone

Introduction: Osteoporosis is a disease in which weakening of the bone occurs due to increased bone resorption and/or decreased bone deposition. Adequate screw fixation can be difficult to achieve in osteoporotic bone leading to hardware loosening or failure. With the steady increase in the number of geriatric fractures as well as the incidence of osteoporosis, it is important to investigate the relationship between screw type and pilot hole diameter in order to use the best combination when fixating screws in osteoporotic bone. Thus, this study aims to compare osteoporotic bone screw fixation using cortical and cancellous screws placed with two different pilot holes. Fixation success was mechanically evaluated using screw pullout strength and stiffness. Methods: Eighty osteoporotic metaphyseal sawbone model blocks (10# cellular foam 40 mm thick, laminated on 1 side with 20# solid foam 3 mm thick, finished dimension 40x40x43 mm, standard tolerances; Pacific Research Laboratories Inc., Vashon, WA) were equally divided into 2 separate groups. Forty blocks received 2.0 mm pilot holes and the other forty 2.5 mm pilot holes. Each of the two different pilot hole groups then had a total of twenty 3.5 mm cortical screws and twenty 3.5 mm cancellous screws placed by a junior resident and a staff orthopedic surgeon. This resulted in the following four, 20 sample, testing configurations: (A) 3.5 mm cortical, 2.5 mm pilot; (B) 3.5 mm cortical, 2.0 mm pilot; (C) 3.5 mm cancellous, 2.5 mm pilot; and (D) 3.5 mm cancellous, 2.0 mm pilot. Pullout strength and stiffness were studied using a material testing system (MTS 858 Mini Bionix, MTS Systems Corp., Eden Prairie, MN) following the American Society for Testing and Materials standard (ASTM F543-13). A two-way balanced ANOVA was used to compare and identify differences between the 4 different configurations. Results: Configuration (D) resulted in the highest mean pullout strength (481.4 N) and stiffness (557.8 N/mm) and was thus the best configuration for this bone type. The 2.0 mm pilot holes outperformed the 2.5 mm pilot holes for each screw type. Furthermore, the 2.0 mm pilot holes were more important in the success of fixation than the type of screw. These conclusions are evident when comparing the resulting 372.2 N and 433.4 N pullout strengths of configurations (A) and (B), respectively, and the 433.4 N and 413.2 N pullout strengths of configurations (B) and (C). Significant differences in pullout strength (p=1.17e-5) and stiffness (p=0.0087) were found between different pilot holes. Significant differences were also found in pullout strength (p=0.0018) but not stiffness (p=0.3022) between the different screw types. Conclusions: With the incidence of osteoporosis on the rise, there is a need for improved hardware fixation. The results from this study support the use of configuration (D) for this type of bone. This configuration, with the 3.5 mm cancellous screw and the 2.0 mm pilot hole, resulted in highest pullout strength and stiffness as compared to the others tested in this study. Furthermore, the results show that pilot hole size is more important in successful fixation than screw type.