P5. In vitro comparison of novel adjustable-lordotic expandable and static hyperlordotic lateral interbody spacers on endplate loading and lordosis correction

Abstract

BACKGROUND CONTEXT For restoration of sagittal alignment, hyperlordotic lateral lumbar interbody fusion (LLIF) spacers are available as expandable or static spacers with many sizes, and possess up to 30° of implant lordosis to maximize lumbar lordosis when used in combination with anterior longitudinal ligament (ALL) resection. However, angular mismatch between the implant and vertebral endplates may play a role in subsidence due to asymmetric loading of the anterior implant edge. The relationship between implant size and their effect on segmental lordosis correction and anterior-posterior endplate loading is poorly understood. PURPOSE The aim of this study is to compare adjustable-lordosis expandable and static LLIF spacers in regards to lordosis correction and endplate force distribution. STUDY DESIGN/SETTING In vitro human cadaveric study. PATIENT SAMPLE A total of 14 cadaveric specimens. OUTCOME MEASURES Lordosis and anterior-posterior endplate forces. METHODS Fourteen L4–L5 cadaveric segments were used. A 55 lb axial load was applied to L4 to simulate intraoperative loads. Segments were instrumented with bilateral pedicle screws and a sequence of expandable spacers with anterior fixation (n=7) or static spacers (n=7) at varied heights and implant lordosis matched between spacers (nine constructs per spacer), including (A three height and lordosis combinations with intact ALL; and B) six height and lordosis combinations following ALL resection. Segmental lordosis and endplate force-maps were collected and compared between the two spacers. An equivalent loading ratio (ELR=anterior load/total load*100%) was calculated; ELR=80% refers to 80% anterior and 20% posterior edge loading. An ELR of 50% is equivalent anterior-posterior loading. RESULTS No significant differences in lordosis was observed between spacers except at between the 20° expandable spacer (17.4mm) and the 13mm 20° static spacer (22.5°±4.9° and 16.7°±2.3°, respectively, p=0.020). Equivalent anterior-posterior loading ratios were also quantified. Prior to ALL resection, the 9mm 10° static spacer had a significantly larger ELR than the 20° expandable spacer (11.5mm, p=0.016), indicating more anterior loading with the static spacer (91.5% ± 7.6 versus 70.5% ± 18.8%). Following ALL resection, the 11mm 10°, 11mm 20°, and 13mm 20° static spacers (85.1%, 99.2%, and 94.8%, respectively) all had significantly larger ELRs than the 20° expandable at 11.5mm, 14.7mm, and 17.4mm heights (52.0%, 70.0%, and 76.2%), respectively (p CONCLUSIONS This study suggests that expandable and static spacers have comparable lordosis correction; however, greater anterior endplate loading was observed with static spacers. Increased anterior endplate loading may lead to subsidence. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.