Less rigid internal fixation plates: historical perspectives and new concepts

Area of Science:

• Orthopedic surgery
• Biomaterials science
• Mechanical engineering

Background:

• Rigid plates for diaphyseal fractures can lead to undesirable outcomes like lack of callus healing and bone overprotection (stress-shielding).
• Existing solutions include modified plate removal timing, biodegradable materials, and less rigid fixation systems.
• Oversimplified terms like "flexible plate" or "elastic fixation" lack precision in describing fixation system properties.

Purpose of the Study:

• To establish appropriate design criteria for less rigid plate-fixation systems.
• To investigate the influence of axial, bending, and torsional stiffness on bone stresses.
• To design and evaluate novel plate fixation systems for improved fracture healing and bone remodeling.

Main Methods:

• Utilized finite element modeling and simplified bench experiments to analyze stiffness parameters.
• Identified plate axial stiffness as the dominant factor influencing bone stresses.
• Designed and tested two experimental plates: a low-stiffness Ti-6Al-4V alloy plate and a tubular stainless steel plate.

Main Results:

• The Ti-6Al-4V alloy plate proved inadequate in demanding canine osteotomies.
• Both experimental plates showed success in unilateral osteotomies, with the tubular plate performing best.
• Bones beneath the tubular plate exhibited superior mechanical and structural properties after 6-9 months compared to rigid controls.

Conclusions:

• The tubular plate's success stems from its balanced stiffness: moderate bending/torsional for union, low axial for bone stress sharing and remodeling.
• This design facilitates physiological stress transfer, crucial for bone healing and long-term bone property maintenance.
• The study highlights the importance of considering specific stiffness parameters for optimizing internal fixation devices.