Investigation of the appropriate thread depth for bioabsorbable screws

Area of Science:

• Biomaterials Engineering
• Orthopedic Devices
• Surgical Implants

Background:

• Bioabsorbable screws offer advantages over traditional metallic implants.
• The clinical application of bioabsorbable screws is limited by their relatively low mechanical strength.
• Optimizing screw design is essential for enhancing their clinical efficacy and safety.

Purpose of the Study:

• To investigate the relationship between thread depth and pull-out strength in u-HA/PLLA screws.
• To determine the correlation between minor diameter and the shearing/bending strengths of these screws.
• To identify optimal design parameters for u-HA/PLLA bioabsorbable screws.

Main Methods:

• Manufactured seven types of u-HA/PLLA screws with varying thread depths (0.1-0.7 mm) and a 4.5 mm major diameter.
• Evaluated screw pull-out strength using simulated bone.
• Assessed screw physical strength through shearing and three-point bending tests.

Main Results:

• A positive biphasic linear correlation was observed between thread depth and pull-out strength, with a critical boundary at 0.4 mm.
• Positive linear correlations were found between minor diameter and both shearing and bending strengths.
• A 0.4 mm thread depth demonstrated optimal performance for the tested u-HA/PLLA screws.

Conclusions:

• A thread depth of 0.4 mm is identified as a key parameter for achieving adequate pull-out strength and mechanical integrity in 4.5 mm major diameter u-HA/PLLA screws.
• The findings provide valuable insights for the design and clinical application of advanced bioabsorbable orthopedic implants.
• Further research can explore the long-term in vivo performance and degradation characteristics of screws with optimized designs.