Mechanical Performance of Metallic Bone Screws Evaluated Using Bone Models

To evaluate mechanical performance properties of various types of cortical bone screw, cancellous bone screw, and locking bolt, we conducted torsional breaking and durability tests, screw driving torque tests into bone models, and screw pullout tests (crosshead speed: 10 mm/min) after driving torque tests. The 2° proof and rupture torques of a screw, which were estimated from torque versus rotational angle curves, increased with increasing core diameter of the screw. The durability limit of metallic screws obtained by four-point bending durability tests increased with increasing core diameter. The compressive, tensile, and shear strengths of the bone models used for the mechanical testing of orthopedic devices increased with increasing density of the bone model. The strength and modulus obtained for solid rigid polyurethane foam (SRPF) and cellular rigid polyurethane foam (CRPF) lay on the same straight line. Among the three strengths, the rate of increase in compressive strength with the increase in density was the highest. The maximum torque obtained by screw driving torque tests for up to 8.3 rotations (3000°) into the bone models tended to increase with increasing core diameter. In particular, the maximum torque increased linearly with increasing effective surface area of the screw, as newly defined in this work. The maximum pullout load increased linearly with increasing number of rotations and mechanical strength of the bone model. Screws with low driving torque and high pullout load were considered to have excellent fixation and are a target for development.

Effects of pilot hole diameter and depth on screw driving torques in plywood

Factors affecting screw driving torques in plywood were investigated in this work. The factors were number of layers (7 and 9), pilot hole diameter (3.0 and 3.5 mm), pilot hole depth (60 and 80% of the thickness of specimen), and thickness of the metal plate (7.5 and 10 mm). Screw driving torques were studied in oriented strandboard, medium-density fiberboard, particleboard, and some wood-plastic composites. There is no such information about screw driving torques in plywood (PW). Therefore, this study focused on the plywood made of aspen (Populus tremula L.). The mean seating torque (SET) values ranged from 0.31 to 0.69 N∙m, whereas mean stripping torque (STT) values ranged from 0.50 to 4.7 N∙m. The ratios of STT/SET were between 2 and 5 in PW with seven layers, whereas the ratios were between 4 and 7 in PW with nine layers. The results indicated that the four main effects of SET and STT were statistically significant with p-values of ˂ 0.0001.

Quantitative Estimation of Forearm Strength and Associated Muscle Fatigue on the Screw Driving Task

Background: Upper extremity musculoskeletal disorders are highly prevalent work-related injuries. The problem is commonly related to tasks that involve forceful exertion and repetitive motion. This study investigated forearm muscular strength and fatigue when performing a screw driving task using the screw driving model.Methods: Ten male and two female adults participated in this study. The pre- and post-fatigue maximum handgrip, driving torque, push force, insertion rate of the screws and corresponding electromyographic responses were measured to assess the muscle strength loss and fatigue of the forearm when driving screws. Results: After screwing, the maximal grip force, maximal driving torque, and maximal push force losses were approximately 32%, 24% and 27%, respectively. The percentage force loss of grip force and driving torque in the brachioradialis and extensor carpi ulnaris was greater than those of the biceps brachii. The percentage of maximum driving torque and push force decreased significantly on the eighth screw compared with the first screw. The insertion rate decreased linearly with the number of inserted screws; however, a significant decrease in the insertion rate of the fourth screw was observed. Conclusion: Muscle fatigue may occur in subjects who are inserting more than four screws. More muscle force loss and a higher risk of fatigue occurred in the brachioradialis and extensor carpi ulnaris. The results of this study can be used to assess the risk of forearm injury and potential for muscle fatigue due to exposure to repetitive driving tasks. Keywords: muscle fatigue, maximum isometric forces, driving torque