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

Introduction: One of the primary mechanisms of low back pain can be attributed to the degeneration of the intervertebral disc [1]. In advanced cases of intervertebral disc degeneration, a spinal fusion may be a viable treatment plan. However, any immobilization of a given motion segment or segments will change the biomechanics of the spine leading to the potential for adjacent segment disease (ASD) [2]. Despite many studies on ASD, it is largely unknown how spinal fusions effect the mechanical properties of the adjacent disc.

Methods: Fifty-two porcine cervical spines containing C3-C5 were assigned to one of four conditions: 1) control (no alterations made to the C4/5 disc); 2) injured (18.5-gauge needle inserted into the nucleus of C4/5); 3) fusion (18-gauge steel wire wrapped around the transverse and spinous processes of C4/5); and 4) injured+fusion (Figure 1). Following injury and/or fusion, each specimen was subjected to 15 minutes of 300 N axial compression followed by cyclic compression which consisted of a 0.5 Hz sinusoidal waveform ranging from 300-1200 N for two hours (a total of 3600 cycles) (MTS, Eden Prairie, MN) [3]. Post-compression, three annular samples were dissected from the C3/4 IVD (adjacent to the injured/fused level). Two of the three samples were single layers of the annulus (one from the anterior region and one from the posterior region) which were subjected to a tensile test in order to measure the strength of the intRAlamellar matrix. The third sample contained multiple layers of the annulus and was obtained from the posterolateral region. This sample was subjected to a peel test in order to measure the adhesion strength of the intERlamellar matrix [4,5]. To complement the porcine data, the same loading protocol and annular testing was conducted in eight soft fixed human cadaveric lumbar spines (mean age 78.3 years (± 13.4); 5 male, 3 female). Due to the small sample size for the human cadavers, only the control and injured+fusion conditions were replicated.

Results: Both fusion and injury impacted the mechanical properties of the adjacent disc. Specifically, a reduction in single annular layer strength was observed including reduced toe region stress (p<0.001), reduced initial failure stress (p=0.03), and reduced ultimate stress (p=0.004) (Figure 4). In contrast, neither fusion nor injury impacted the interlamellar adhesive strength in the adjacent disc. Similar findings were observed in the human cadaver samples with reduced toe region stress in the fused+injured samples compared to the control (p=0.049).

Discussion: The current study observed empirical evidence of decreased lamellar strength in the disc adjacent to a fused and/or injured level following prolonged cyclic axial loading. Reduced lamellar matrix strength is associated with disc pathologies, thereby providing a mechanistic link between fusion and the development ASD. While these findings should not play a role in the decision for a surgeon to perform spine fusion, appropriate clinical therapy and education following fusion should be provided to the patient.