Biomechanical Evaluation of Recurrent Dissociation of Modular Humeral Prostheses

Area of Science:

• Biomechanical Engineering
• Orthopedic Surgery
• Materials Science

Background:

• Modular total shoulder replacements are susceptible to humeral head dissociation.
• Potential causes include impingement, taper contamination, and inadequate compressive forces.
• Understanding dissociation forces is crucial for improving implant design and patient outcomes.

Purpose of the Study:

• To quantify the force and torque required to dissociate modular humeral heads from different total shoulder systems.
• To investigate whether bodily fluids reduce the load and torque needed for dissociation.
• To compare experimental findings with existing literature on humeral head dissociation.

Main Methods:

• Biomechanical testing of unimplanted modular total shoulder replacement systems under dry and wet (bovine serum, water) conditions.
• Measurement of load and torque to dissociation using clinically relevant methods.
• Statistical analysis of R-squared correlation coefficients and p-values to assess the impact of fluid presence.

Main Results:

• Mean load to dissociation (1513 N ± 508 N) was significantly higher than forces from activities of daily living (578 N).
• Mean torque to dissociation was 49.77 N·m ± 19.07 N·m.
• Presence of bovine serum or water did not statistically reduce dissociation force or alter stiffness.

Conclusions:

• The dissociation force of modular humeral heads exceeds typical daily living forces.
• Bodily fluids do not significantly decrease dissociation force, indicating other factors are critical.
• Torque and insufficient rotator cuff compressive forces may contribute to dissociation at sub-daily activity levels, warranting further investigation.