The effect of impaction energy and bone quality on cementless femoral implant fixation and femoral periprosthetic fracture risk.

Abstract

Aims: Cementless fixation is the predominant method used in hip arthroplasty worldwide; however, seating implants via impaction remains a balancing act - too much energy may cause fracture, and too little may not seat the implant adequately. Currently, the optimal energy level required for femoral implant fixation is unknown, which adversely affects the variability of long-term implant stability. This study investigated how varying impaction energies affect strain behaviour, fracture risk, and implant fixation in synthetic bone models using cementless femoral implants.

Methods: A standardized surgical technique was used to prepare a total of 20 synthetic bone models (Sawbones) - 12 healthy and eight osteoporotic - for femoral impaction. The implants were impacted at three different energy levels (low, medium, and high) using a validated in vitro rig simulator for soft-tissue compliance, with four specimens tested at each velocity and bone type. Strain behaviour and fracture risk, measured as peak strain, were analyzed using digital image correlation with two high-speed cameras. Implant stability was assessed via pull-out fixation tests.

Results: Higher impaction energy increased fracture risk but did not significantly increase implant stability. In healthy bone models, mean tensile peak strain was 0.31, 0.69, and 0.97 for low, medium, and high energy strikes, respectively. Osteoporotic models showed the greatest difference between the low and medium energies during the initial two strikes, with peak strains of 0.42 to 0.46 versus 0.79 to 0.84, with subsequent strikes resulting in minimal variation between the two velocities. Implant pull-out forces were similar among the different impaction energy levels within each group (healthy p = 0.105, osteoporotic p = 0.686), and consistently lower in osteoporotic bones across all energy levels.

Conclusion: These biomechanical findings on synthetic bones confirm that multiple lower-energy strikes lead to similar implant fixation outcomes with lower risk of periprosthetic fracture (lower peak strain).