#519 In Vitro Biomechanics of Human Cadaveric Cervical Spines with Mature Fusion

General Session: Cervical Motion Preservation

Presented by: N. Rodriguez-Martinez

Author(s):

N. Rodriguez-Martinez (1)
L. Perez-Orribo (1)
S. Kalb (1)
A.G.U.S. Newcomb (1)
P.M. Reyes (1)
N. Theodore (1)
N.R. Crawford (1)

(1) Barrow Neurological Institute, Spinal Biomechanics, Phoenix, AZ, USA

Abstract

Introduction: Although numerous laboratory studies have looked at biomechanics after cervical spine fusion, these studies, by their nature, address only immediate post-operative mobility. There is limited information on the biomechanics at fused and adjacent levels of human cadaveric cervical spine segments with mature fusion after bony healing has occurred.

Methods: Eight human cadaveric cervical spines (C2-T1) with mature fusion (7 anterior, 1 anterior-posterior) were studied. Donor ages were 49 to 87 years (mean 69.9±13.8, 6F/2M); age at time of fusion was 34 to 76 years (51.6±16.2) and time between fusion surgery and death was 5 to 40 years (mean 18.2±12.1). Intervertebral angular range of motion (ROM) of spine segments was determined optoelectronically at fused (0) and adjacent levels (+1: one level rostral to fusion mass, +2: two levels rostral, -1: one level caudal, - 2: two levels caudal) during application of 1.5 Nm pure moments inducing flexion-extension (FE), lateral bending (LB) and axial rotation (AR). Data were compared to that of a separate group of immediate postoperative (time-zero) cervical spine specimens (n=32) tested intact and then with one-level anterior plate/graft using the same test method. Specimens with mature fusion (including fused and adjacent levels) are also undergoing analysis using regional DEXA scans of anterior vertebral bodies and histology.

Results: The ROM for levels with mature fusion was 46% (FL-EX), 47% (LB) and 26% (AR) of the ROM at the fixation level in time-zero specimens (Fig. 1, P< 0.02, two-tailed non-paired t-tests). The ROM at +1, +2, -1 and -2 levels was greater than at the index level (0) during all directions of loading (Fig. 2, P< 0.04, one-way ANOVA/Holm-Sidak); the ROM at -1 was greater than at 0 during FE (P< 0.004), LB (P< 0.02) and AR (P=0.095). The ROM at -1 was less than at +1 and -2, but not significantly so (P>0.3). There were signs of significant wear between screws and plates in two of five cases fused with anterior plates. One case (20-year C5-7 fusion) involved excessive wear particles around an inferior screw angled into the caudal disc, which incurred increased ROM at the level of the penetrated disc.

Conclusions: Cadaveric cervical spine segments with mature fusion have less than half the ROM of screw/plate fixated levels immediately post-op. Long-term mobility changes at adjacent levels are inconsistently affected and may be slightly different caudal than rostral to the fusion. Factors such as time since fusion, number of fused levels, whether hardware is present, hardware orientation, and age must also be considered.

Fig 1. ROM for immediate postop vs. mature fusion.

Fig 2. ROM for mature fusion and adjacent levels.