Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability

Area of Science:

• Orthopaedic surgery
• Biomaterials engineering
• Implant biomechanics

Background:

• Modular junctions are critical in modern hip arthroplasty.
• Large diameter metal-on-metal (MoM) hip failures are linked to head-trunnion junction issues, leading to significant litigation.
• Factors influencing modular connection strength are known, but head size impact was unstudied.

Purpose of the Study:

• To investigate if varying head sizes influence the initial strength of the trunnion-head connection in hip arthroplasty.
• To compare the pull-off forces between 28mm and 36mm cobalt-chromium heads assembled onto titanium alloy trunnions.

Main Methods:

• Sixty Ti-6Al-4V trunnions and 60 cobalt-chromium heads (28mm and 36mm) were tested.
• Assembly forces of 4 kN, 5 kN, and 6 kN were applied.
• Pull-off force was measured to evaluate connection strength; statistical analysis and finite element modeling were used.

Main Results:

• 36mm heads exhibited significantly lower pull-off forces than 28mm heads at 4 kN and 5 kN assembly forces (p < 0.001).
• Mean pull-off forces were approximately 20% higher for 28mm heads compared to 36mm heads at these forces.
• Finite element and analytical models confirmed that differences in structural rigidity and interface pressures explain the observed strength variations.

Conclusions:

• This study is the first to demonstrate that 36mm heads have up to 20% lower pull-off strength than 28mm heads at equivalent assembly forces.
• The reduced connection strength of larger heads may contribute to the high failure rates observed in large diameter MoM hip implants.
• Greater assembly forces may be required for larger diameter modular heads to achieve adequate initial stability in total hip arthroplasty.