Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION: With 350,000 cases per year in the USA, spinal fusions are the gold standard for treating spinal disorders [1]. The anterior lumbar interbody fusion is intended for fusion of pathological segments and performed with a monobloc cage (one piece) inserted by impact forces. Recently developed three-part modular cages (two endplates and a central core) attempt to provide a new, more advantageous implantation mechanism. This is expected to reduce the impact forces during implantation, minimize the damage caused to the endplates and allow more lordosis angle correction [2]. This in vitro study aims to investigate and compare the modular and monobloc cages’ implantation procedures under real surgical condition in vitro.

METHODS: Twelve segments (L23, L45) from six fresh-frozen human spines were divided into two groups distributed among the spinal levels (n=6). The specimens were screened for signs of deformity and degeneration. The implant size was selected according to the manufacturer's instructions by choosing the desired lordosis angle and posterior height based on the CT images. After preparing and embedding, the specimens were placed in a compression device (100N) to simulate the physiological situation (Fig1A). A partial discectomy was performed followed by one of the two different cage implantation procedures using surgical tools provided by Axis Spine Technologies Inc: recently developed distraction instrument for modular and impaction with mallet for monobloc (n=6 each). Every implantation step was monitored with fluoroscopy and x-ray. Macroscopic pictures and µCT scans were taken of the dissected fused segments and processed (Avizo 2020.2) [3]. By using a fully automated radiological analysis, the lordosis angle was calculated in the intact, post-discectomy and implanted state, followed by data analysis to verify the sagittal plane correction [4]. A visual semi-quantification of the damage was performed, identifying scratches (due to the cage’s anchorage), bone fragmentation and defects (at the endplate). An exploratory and descriptive statistical analysis was conducted to determine the damage entity (R-studio).

RESULTS: The modular implantation achieved a median lordosis of 21.3° compared to 19.5° for the monobloc implantation. Considering the difference between the achieved and the intact lordosis, the modular implant had a 34% higher increase compared to the monobloc implant (p=0.132, Fig1B). The modular and monobloc group had similar impact on the damage classes because of the implantation procedure: both groups fell into the mid-damage class (29%), but the modular group gave a slightly greater contribution in the low-damage class (13%) while the monobloc one in the high-damage class (17%). Fragmentation appeared more in the monobloc group (71%), while defects appeared equally. 

DISCUSSION: This in vitro study has improved the understanding of two implantation procedures. The modular procedure seems to allow for more lordosis angle increase than the monobloc one, which does not always achieve the desired lordosis angle. Furthermore, the results suggest that modular implantation causes less damage, but the scratches generated by implantation will always present. In the monobloc group more fragmentation occurs. The limitations of the study are the small number of specimens and the semi-quantitative aspect of the damage quantification.