P3. Total implant contact area versus contact surface topology: Which one has a greater role on subsidence resistance of an interbody device, comparison of two 3D-printed titanium anterior lumbar spine fixation devices

Abstract

<h3>BACKGROUND CONTEXT</h3> Anatomical loads in combination with poor bone quality may lead to interbody fixation device subsidence. Severe subsidence can often cause nerve compression and pain that may require revision surgery. It is often believed that the total contact area or footprint size of the implant has a direct impact on subsidence resistance of the interbody device. Traditionally, it was believed that cages with larger contact area have low incidence and severity of subsidence versus cages with smaller total contact area at bone-implant interface. With the advancement in additive manufacturing, we are able to create implants with advanced structural designs and surface topology that can improve subsidence resistance of the interbody device. These new designs focus on architecture and load distribution at the bone implant interface rather than overall dimensional design specifications. <h3>PURPOSE</h3> This study is aimed to evaluate subsidence resistance of two 3D-printed titanium ALIF interbody devices of the same size, but with different contact surface topology. <h3>STUDY DESIGN/SETTING</h3> Mechanical testing using foam blocks. <h3>OUTCOME MEASURES</h3> Subsidence rate of different interbody constructs. <h3>METHODS</h3> Two ALIF 3D-printed titanium interbody fixation devices were used. The first device had a truss design that spans throughout the entire construct and contains a snowshoe style bone interface and the other had two cylindrical holes through the center with micro lattice truss structure filling the rest of the volume. Both interbody devices were the same dimensions (36mm x 25mm, width x depth and 18mm tall). The truss device had the total graft volume of 7cc whereas the predicate device had the overall graft volume of 1cc. The total metal-to-bone contact area at the endplate was 401mm2 for the truss device versus 620mm2 for the predicate device. To mimic mechanical properties similar to human bone density (osteoporotic through normal), 5, 10, 15 and 20 PCF SawBone® foam blocks were utilized for testing. To mimic clinical subsidence, the interbody device was placed over the foam block and compressive displacement was applied to push the device into the foam substrate at the rate of 5mm/min. The test was repeated 6 times for each block/implant combination. The subsidence load at 1 and 2mm displacement was collected for each test combination. To calculate the modulus/stiffness, the ratio of load (normalized to contact area) to axial compression rate of the cage-only construct was calculated. Two-way ANOVA with Sidak-Bonferroni Posthoc was used for multiple comparisons and analysis of statistical significance. <h3>RESULTS</h3> The truss interbody device had subsidence resistance significantly greater than the generic titanium device at both 1mm and 2mm subsidence rates (p<0.05) across all density blocks. In foam representing osteoporotic bone (5pcf), the truss cage had subsidence resistance load of 238.3N±25.6N vs 172.2N ±13N in the predicate cage at 1mm displacement respectively. The corresponding numbers at in rigid bone (20PCF) were 1774N±174N vs 1428N ±66N. <h3>CONCLUSIONS</h3> The truss device with snowshoe design had metal-to-bone contact area of 60% of the predicate device and had total graft volume of 7 times of the predicate device; nevertheless, the truss device demonstrated significantly better resistance to subsidence for all foam densities including those that mimic osteoporotic bone compared to the same size predicate titanium device. The subsidence resistance of the truss cage, despite its smaller contact area for the same footprint size, may be due to the a more efficient distribution of load across the bone/implant interface where the load is uniformly distributed across more of the interbody footprint. The data suggests that the implant's surface contact topology at the endplate plays a greater role than the overall contact area for resisting subsidence. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.