Poster Presentation Sydney Spinal Symposium 2024

Clinical kinematics of cervical total disc replacement, fusion and hybrid procedures with a proposed biomechanical model for adjacent segment disease. (#34)

William Parr 1 , Simon Luong 2 , Jack Hill 1 , Charles Furtado 1 , Jessica Wind 1 , Chloe Amaro 1 , Mai Beathyate 1 , John Ward 1 , Adam Schuman 1 , William Walsh 2 , Ralph Mobbs 3
  1. 3DMorphic, Matraville, NSW, Australia
  2. Medicine, UNSW, Randwick, NSW, Australia
  3. Neurosurgery, Prince Of Wales Private, Randwick, NSW, Australia

Introduction

In patients indicated for multi-level interbody devices, ‘hybrid’ procedures, in which Total Disc Replacement (TDR) is performed at one level with other levels being fused, have gained popularity. Hybrid procedures have a biomechanical rationale focussed on range of motion (ROM) of the adjacent segments for preventing development of Adjacent Segment Disease (ASD).

Here we calculate superior adjacent segment range and quality of motion for patients that underwent: single level fusion, single level TDR, double level fusion, hybrid (superior level TDR, inferior level fused).

Method

The planned accrual target was 70 patients. In-vivo cervical spine motion was calculated from planar flexion and extension x-rays for 53 patients (43.5 : 56.5 Female : male, mean age 49.9yrs) receiving the Redmond fusion cage (A-Spine Taiwan) and/or the Mobi-C TDR (ZimmerBiomet, USA). Range Of Motion and Centre Of Rotation (COR), which is indicative of motion quality, for cervical spine levels C23, C34, C45, C56, C67 were calculated and compared.

Results

There was no difference in superior adjacent segment ROM between any of the groups. Variance in COR was significantly different between fusion patients (1 and 2 levels) and TDR / hybrid patients (P < 0.01). Qualitative assessment of the fusion patients showed out-of-(sagittal)-plane motion for some of the fusion patients during flexion and extension movements.

Finite Element Analyses (FEA) showed that fusion device placement asymmetry leads to abnormal strains in the adjacent segments, which could lead to the observed out-of-plane motion. Device placement asymmetry can occur during device insertion through asymmetric disc preparation or device placement, or soon after through asymmetric subsidence of the device. 

Conclusion

Reports of increased incidence of ASD in fusion patients compared to TDR patients may result from differences in quality of motion (COR) at the adjacent segment, rather than ROM.

Asymmetrical interbody fusion device placement may increase variance in COR of fusion patients. This is a biomechanical model that may contribute to the explanation of some incidence of ASD development in spinal fusion patients.