Introduction: Adjacent segment degeneration (ASD) is a multifactorial condition resulting in a subsequent surgery in a significant number of lumbar fusion patients. A possible risk factor for accelerated disc degeneration can be the deteriorated metabolic conditions. There is very limited knowledge about the association between the mechano-biological features of the endplate and the risk for ASD after fusion surgeries.

Aims: The present study aimed to evaluate whether cranial endplate injury related to the pedicle screw position can facilitate adjacent disc degeneration based on a patient cohort and finite element modeling.

Methods: Subsequent cranial fusion-extension surgeries after a lumbar index fusion surgery were analyzed in a tertiary spine center between 2010 and 2020. The study cohort (N=199) was divided into two groups based on postoperative lateral x-ray scans taken after the index surgery according to the position of the pedicle screws in the upper instrumented vertebra (UIV) in relation to the cranial endplate. Group 1Cranial Screw Position (CSP) where the pedicle screws bordered or penetrated the endplate (N=28); Group 2(Parallel Screw Position (PSP) (N=171). Finite Element Models (FEMs) of the lumbar spine with different levels of posterior fixation and pedicle screw positions (CSP, PSP) in the L2 vertebra were constructed using a validated lumbar FEM.

Results: Median of the time between the index and the cranial fusion-extension surgery was significantly shorter in Group 1 (397 vs. 974 days, p<0.005). Proximal junctional fractures were also significantly more common in Group 1 (p<0.001). The CSP resulted in larger highstressed areas and higher stress peaks at the endplate for any load case and level of fixation than the PSP. The maximum endplate stresses in flexion for one, two and three fixation levels, were 163.1, 178.9, and 165.3 % higher in CSP respectively.

Conclusion: Pedicle screws close to the endplate results in a higher biomechanical stress. This can result in higher risk of endplate injury and subsequent nutritional deterioration of the adjacent disc which leads to a higher risk for a fusion extension surgery because of ASD. Based on our biomechanics supported clinical results, avoiding of cranial screw position at the UIV is highly recommended.

Funding: OTKA FK123884, ÚNKP-21-5, Co-operative Doctoral Program C1014064.