The influence of over-distraction on biomechanical response of cervical spine post anterior interbody fusion: a comprehensive finite element study

Abstract

Introduction: Anterior cervical discectomy and fusion (ACDF) has been considered as the gold standard surgical treatment for cervical degenerative pathologies. Some surgeons tend to use larger-sized interbody cages during ACDF to restore the index intervertebral disc height, hence, this study evaluated the effect of larger-sized interbody cages on the cervical spine with ACDF under both static and cyclic loading.Method: Twenty pre-operative personalized poro-hyperelastic finite element (FE) models were developed. ACDF post-operative models were then constructed and four clinical scenarios (i.e., 1) No-distraction; 2) 1 mm distraction; 3) 2 mm distraction; and 4) 3 mm distraction) were predicted for each patient. The biomechanical responses at adjacent spinal levels were studied subject to static and cyclic loading. Non-parametric Friedman statistical comparative tests were performed and the p values less than 0.05 were reflected as significant.Results: The calculated intersegmental range of motion (ROM) and intradiscal pressure (IDP) from 20 pre-operative FE models were within the overall ranges compared to the available data from literature. Under static loading, greater ROM, IDP, facet joint force (FJF) values were detected post ACDF, as compared with pre-op. Over-distraction induced significantly higher IDP and FJF in both upper and lower adjacent levels in extension. Higher annulus fibrosus stress and strain values, and increased disc height and fluid loss at the adjacent levels were observed in ACDF group which significantly increased for over-distraction groups.Discussion: it was concluded that using larger-sized interbody cages (the height of ≥2 mm of the index disc height) can result in remarkable variations in biomechanical responses of adjacent levels, which may indicate as risk factor for adjacent segment disease. The results of this comprehensive FE investigation using personalized modeling technique highlight the importance of selecting the appropriate height of interbody cage in ACDF surgery.

The impact of over-distraction on adjacent segment pathology and cage subsidence in anterior cervical discectomy and fusion

“…However, You et al 32 reported that patients with ASP after ACDF had longer fusion length than patients without ASP. Recent study showed that using larger-sized interbody cages can result in remarkable variations in biomechanical responses of adjacent levels, which may indicate as risk factor for adjacent segment disease 19 . In our study, the Over-distraction group had a significantly higher incidence of RASP progression than the No-distraction group (any progression: 48% vs. 15%, p < 0.001; progress ≥ 2 grades: 25% vs. 7%, p = 0.001).…”

“…Previous study showed interbody height increases 2.2 mm in high torque group and 1.9 mm in low torque group 18 . Recent biomechanical study had reported that larger-sized interbody cages (the height of ≥ 2 mm of the index disc height) could result in remarkable variations in biomechanical responses of adjacent levels 19 . To evaluate the effect of over-distraction, the patients were divided into two groups: (1) Over-distraction group (postoperative index disc height–preoperative index disc height ≥ 2 mm), and (2) No-distraction group (postoperative index disc height–preoperative index disc height < 2 mm).…”