Experimental quantification of resistance to subsidence conferred by endplate-specific expandable implants in anterior lumbar interbody fusion.

BACKGROUND

In anterior lumbar interbody fusion, implant subsidence remains a major complication, influenced by both patient- and implant-related factors. Cage sizing has been shown to influence the subsidence load and is dependent on both the surgical approach and the patient's anatomic characteristics. Larger cage footprints have been shown to increase axial stability by allowing for a greater area of contact, but their limited sizes and footprints are not always able to accommodate the significant variance in vertebral body shape, endplate dimensions, and epiphysial rim depth. Therefore, the current study aims to quantify the incremental resistance to subsidence conferred by the use of novel endplate-specific expandable implants over conventional, static implants.

METHODS

Fifteen cadaveric spines were reconstructed via computed tomography (CT) scans of the L5 vertebra, and the endplates and cortical shell were isolated from the inner trabecular core to recreate bone surrogates. The novel implant was positioned on each bone surrogate in the unexpanded (control) or expanded (endplate-specific expandable) configuration and compressed at a rate of 5 mm/min. The subsidence load and construct stiffness were evaluated for each group.

RESULTS

The subsidence load of the expandable implants quantified in 3478 N ± 588 was 20% greater than that of the control group (p < 0.05). No difference was found in construct stiffness (p > 0.05). There was no correlation (r = 0.260, p = 0.350) between the stiffness and epiphyseal rim contact, whereas a positive correlation (r = 0.796, p < 0.001) was found between the subsidence load and increased epiphyseal rim contact in the expandable implant group only.

CONCLUSIONS

In our study, endplate-specific expandable ALIF implants have shown higher resistance to subsidence than static implants. While, these results must be corroborated by clinical data, the superiority here identified indicates the potential for these implants to be advantageous over static implants. The linear correlation between the epiphyseal rim cross-sectional area of the endplate and the subsidence load, as found in the current study, highlights the potential for estimating the subsidence load during preoperative planning.