Abstract
INTRODUCTION
Resurfacing prostheses are implanted by impaction onto the prepared femoral head. Ceramic resurfacings can be proposed as an alternative to metal implants, combining bone conservation with mitigation of sensitivity reaction risks. With low wall-thickness required for bone conservation, their strength must be verified. This study aimed to assess a ceramic resurfacing prosthesis' strength under surgical loads using a computational model, tuned and verified with physical tests.
METHODS
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Tests were conducted to obtain baseline impact data (Fig1 left). Ø58mm DeltaSurf prostheses (Finsbury Development Ltd., UK), made from BIOLOX Delta (CeramTec AG, Germany) ceramic were cemented onto 40pcf polyurethane foam stubs (Sawbone AG, Sweden) attached to a load cell (Instron 8874, Instron Corp., USA). Ten repeatable 2ms−1 slide hammer impacts were applied with a 745g mass. The reaction force at the bone stub base was recorded, and the cumulative impulse was calculated by integrating reaction force over time.
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A half-plane symmetry model was developed using LS-DYNA (ANSYS Inc., USA) explicit dynamic FE analysis software (Fig1, right). The bone stub was constrained, and the mallet was given an initial velocity of 2.0m/s. Outputs were the impact reaction force at the bone stub base, the impact duration and the peak tensile prosthesis stress.
First, the model was solved representing the experimental setup, to fit damping parameters. Then the damped model was used to predict the peak prosthesis stresses under more clinically representative loads from a 990g mallet. The smallest (Ø40mm) and largest (Ø58mm) prosthesis heads in the size range were analysed, with two impact directions: along the prosthesis axis, and with the impactor inclined at 10°.
RESULTS AND DISCUSSION
The experimental tests gave a mean peak impact force of 4.70kN (S.D.0.11kN), an impact duration of 1.1ms (S.D.0.06ms) and a total impulse of 2.88Ns (S.D. 0.017Ns). The damped basic FE model gave a peak impact force of 7.0kN, an impact duration of 0.70ms and a total impulse of 2.88Ns. The model overestimated the measured peak impact force by 49% and underestimated the impact duration by 36% (Fig. 2), but was in close agreement with the measured cumulative impulse (Fig3).
The peak force and impulse results were consistent with surgical and cadaveric test measurements [1,2]. Comparison with a similar computational analysis [3] suggest that this study's stiff polyurethane foam stub represents a worse case than bone, with a similar overall impulse but a higher peak force and lower impact duration. The model therefore represents a conservative case, which is beneficial in pre-clinical analysis.
With the surgical impact model, peak prosthesis stresses of 42.5MPa and 68.7MPa were predicted for the Ø40mm head with axial and inclined impaction respectively. For the Ø58mm head, the peak stresses were 20.0MPa (axial) and 27.9MPa (inclined). Stresses were highest with inclined loading, which stressed the prosthesis stem root. The maximum stress predicted was 6% of the 1150MPa material strength [4], indicating that the prosthesis strength should be sufficient to sustain surgical impaction loads.
