Lateral migration resistance of screw is essential in evaluating bone screw stability of plate fixation

Biomechanical Investigation of Bone Screw Head Design for Extracting Stripped Screw Heads: Integration of Mechanical Tests and Finite Element Analyses

Enhancing the design of bone screw head sockets to prevent stripping and improve the torque required for smooth unscrewing is a significant challenge in orthopedic applications. This research aims to establish a quantitative methodology by integrating mechanical testing with finite element (FE) simulations to determine a safe limitation depth for the screwdriver when engaging with the hexagonal socket, thus avoiding stripped screw heads. A FE model was developed to investigate the biomechanical responses of the screw head design. Five custom-made hexagonal sockets were manufactured, and single load torsional tests were conducted to assess the mechanical performance of the screws and drivers. The results from the mechanical tests were compared with the FE simulations, demonstrating a close agreement and confirming the model's validity. Furthermore, additional FE models were created to study the impact of manufacturing tolerances on the socket width and screwdriver width. The findings revealed that the maximum torque to failure for the four designs was lower than the margins specified in ISO 6475. Additionally, increasing the depth of the screwdriver led to higher maximum torque values. This research suggests that the technique of screw insertion, specifically the depth of the driver tool within the screw socket, holds greater importance in preventing stripped screw heads than the design and manufacturing width of the bone screw's hexagonal socket and screwdriver. This confirms the importance of screwdriver engagement inside the bone screw socket to prevent stripped screw heads and sheds light on the added value of maximum torque prediction for future design modifications.

Development and initial validation of a novel thread design for nonlocking cancellous screws

High failure rates have been associated with nonlocking cancellous screws with a typical buttress thread in patients with osteoporotic bone. This study aimed to develop a novel thread design and compare its fixation stability with that of a typical buttress thread. Nonlocking cancellous screws with a novel thread design (proximal flank angle of 120 degrees, a flat crest feature, a tip-facing undercut feature) and nonlocking cancellous screws with a typical buttress thread were manufactured using stainless steel. Fixation stabilities were evaluated individually by the axial pullout and lateral migration tests, and they were evaluated in pairs together with a dynamic compression plate in an osteoporotic bone substitute (10 PCF polyurethane foam per ASTM F1839) under cyclic craniocaudal and torsional loadings. Pullout strength and lateral migration resistance for the individual screw test and the force, torque, and number of cycles required to achieve specific displacement and torsion for the multi-screw test were comparatively analyzed between both screw types. A finite element analysis model was constructed to analyze the stress distributions in the bone tissue adjacent to the threads. The biomechanical test revealed the novel undercut thread had superior axial pullout strength, lateral migration resistance, and superior fixation stability when applied to a dynamic compression plate under cyclic craniocaudal loading and torsional loading than those in the typical buttress thread. The finite element analysis simulation revealed that the novel thread can distribute stress more evenly without high-stress concentration at the adjacent bone tissue when compared to that of a typical buttress thread.

Fixation stability comparison of bone screws based on thread design: buttress thread, triangle thread, and square thread

Background

The influence of thread profile on the fixation stability of bone screws remains unclear. This study aimed to compare the fixation stability of screws with different thread profiles under several loading conditions.

Methods

Bone screws that differed in thread profile (buttress, triangle, and square thread) only were made of stainless steel. Their fixation stabilities were evaluated individually by the axial pullout test and lateral migration test, besides, they were also evaluated in pairs together with a dynamic compression plate and a locking plate in polyurethane foam blocks under cyclic craniocaudal and torsional loadings.

Results

The triangle-threaded and square-threaded screws had the highest pullout forces and lateral migration resistance. When being applied to a dynamic compression plate, higher forces and more cycles were required for both triangle- and square-threaded screws to reach the same displacement under cyclic craniocaudal loading. On the other hand, the triangle-threaded screws required a higher torque and more cycles to reach the same angular displacement under cyclic torsional loading. When being applied to a locking plate, the square-threaded screws needed higher load, torque, and more cycles to reach the same displacement under both cyclic craniocaudal and torsion loadings.

Conclusions

The triangle-threaded screws had superior pullout strength, while square-threaded screws demonstrated the highest lateral migration resistance. Moreover, dynamic compression plate fixation with triangle- and square-threaded screws achieved more favorable fixation stability under craniocaudal loading, while triangle-threaded screws demonstrated superior fixation stability under torsional loading. Locking plate fixation with a square-threaded screw achieved better fixation stability under both loading types.

Development and initial validation of a novel undercut thread design for locking screws

BACKGROUND

Locking screws with a typical buttress thread have high levels of failure in patients with osteoporotic bones. This study aims to develop a novel thread design for the locking screw and compare its fixation stability with the typical buttress thread.

METHODS

Locking screws with a novel thread design that possess an undercut feature and locking screws with a typical buttress thread were manufactured from stainless steel. Their fixation stabilities were then evaluated individually under a lateral migration test and evaluated in pairs together with a locking plate (LP) in an osteoporotic bone substitute under cyclic craniocaudal and torsional loadings. A finite element analysis (FEA) model was constructed to analyze the stress distributions present in the bone tissue adjacent to the novel thread versus the buttress thread.

RESULTS

The biomechanical test revealed that the novel thread had a significantly higher lateral migration strength than the buttress thread. When applied to a LP, the locking screw with the novel thread requires more cycles and higher forces or torque to resist migration up to 5 mm or 10° than the buttress thread. The FEA simulation showed that the novel thread can make the stress distribute more evenly at the adjacent bone tissue when compared with the buttress thread.

CONCLUSIONS

The locking screw with the novel undercut thread had superior lateral migration resistance during both initial and continued migration and superior fixation stability when applied to a LP under both cyclic craniocaudal loading and torsional loading than the locking screw with a typical buttress thread.

Screw stripping and its prevention in the hexagonal socket of 3.5-mm titanium locking screws

There have been several reports about the difficulties in removing 3.5-mm titanium locking screws from plates due to the stripping or rounding of the hexagonal screw socket. We investigated whether stripping the locking screw sockets can be prevented by using different screwdrivers or interposing materials into the socket during removal. We overtightened 120 3.5-mm titanium locking screws (Depuy Synthes, Paoli, PA) equally into locking plates on sawbone tibia models, applying a uniform torque of 4.5 Nm, exceeding the recommended torque of 1.5 Nm. Twenty screws each were removed using a straight-handle 2.5-mm screwdriver, T-handle screwdriver, hex key wrench, and straight-handle screwdriver with a non-dominant hand. In addition, 20 screws were removed using foil from a suture packet inserted into the screw socket or using parts of a latex glove inserted into the screw socket. The incidence rates of screw stripping using the straight-handle screwdriver, T-handle screwdriver, hex key wrench, non-dominant hand, foil interposition, and latex glove interposition were 75%, 40%, 35%, 90%, 60%, and 70%, respectively. When a T-handle screwdriver or hex key wrench was used, the probability of screw stripping was 4.50 times (odds ratio = 4.50, 95% confidence interval = 1.17 to 17.37, p = 0.03) and 5.57 times (odds ratio = 5.57, 95% confidence interval = 1.42 to 21.56, p = 0.01) lower than that with the straight-handle screwdriver, respectively. Foil or latex glove interpositions did not prevent screw stripping. Thus, in the current experimental study, T-handle screwdriver or hex key wrench usage decreased the incidence rate of screw stripping during removal compared to straight-handle screwdriver use.