Abstract
Introduction
Pre-clinical testing of orthopaedic devices could be improved by comparing performance with established implants with known clinical histories. Corail and Summit (DePuy Synthes, Warsaw) are femoral stems with proven survivorship of 95.1% and 98.1% at 10 years [1], which makes them good candidates as benchmarks when evaluating new stem designs. Hence, the aim of this study was to establish benchmark data relating to the primary stability of Corail and Summit stems.
Methods
Finite Element (FE) simulations were run for 34 femurs (from the Melbourne femur collection) for a diverse patient cohort of joint replacement age (50 – 80 yrs). To account for the diversity in shape, the cohort included femurs with the maxima, minima and medians for 26 geometric parameters. Subject-specific FE models were generated from CT scans. An in-house developed algorithm positioned idealized versions of Corail and Summit (Figure 1) into each of the femur models so that the stem and femur shaft axes were aligned, and the vertical offset between the trunnion centre and the femoral head centre was minimised. For such a position, the algorithm selected the size that achieved maximum fill of the medullary canal without breaching the cortical bone boundaries.
Joint contact and muscle forces were calculated for level gait and stair climbing[2] and scaled to the body mass of each subject. Femurs were rigidly constrained at the condyles. Risk of failure was assessed based on (i) stem micromotion, (ii) equivalent strains (iii) percentage of the bone-prosthesis contact area experiencing micromotions < 50 μm, micromotions > 150 μm and strains > 7000 μstrains [3].
Results
Stair climb loads resulted in higher micromotion and interface strains, compared to level gait loads. For level gait, on average, Corail had 89% and Summit had 91% of the contact area experiencing less than 50 μm and less than 1% of the contact area with micromotion greater than 150 μm. For stair climbing, the average area experiencing <50 μm was about 75% for both stems. On average, Corail and Summit had less than 1% of the contact area with micromotion greater than 150 μm during stair climbing. The average percentage of the contact are with strains greater than 7000 μstrains was about 2% for both stems during level gait, and 8% (Corail), 10% (Summit) during stair climbing (Figure 2).
Discussion and Conclusion
It is desirable for the micromotion at the entire contact area to be below 50 μm. Despite the reported good survivorship of Corail and Summit [1], results of the FE simulations do not show such a distribution. Instead, results suggest that primary stability may be achieved with up to 25% of the contact area with micromotion greater than 50 μm. Hence, the 75th percentile may be a suitable metric for benchmarking femoral stems.
