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

INTRODUCTION: Intervertebral disc degeneration is strongly implicated as a cause of low back pain [1]. Although the precise pathophysiological mechanisms remain elusive, perturbations in nutrition that adversely impact the cellular microenvironment of the central nucleus pulposus (NP) may be a contributing factor [2]. Quantifying this microenvironment, including changes in nutrition as a function of degeneration, is critical for development of more effective cell-based treatments. Previous studies have predicted reduced glucose and oxygen, and increased lactate with degeneration [3]; however, experimental evidence through direct in situ measurements is lacking. Here we adapt Licox oxygen probes and microdialysis catheters, widely used for brain tissue monitoring, for in situ measurement of NP tissue oxygen, glucose and lactate in a preclinical goat model of disc degeneration [4].

METHODS: For ex vivo optimization, a 16G needle was used to place a Licox oxygen probe or microdialysis catheter in the NPs of bovine caudal discs. Oxygen (n=4 discs) was monitored for 90 minutes. Glucose and lactate were collected using microdialysis at either 0.3µL or 1µL/min (n=3 discs each) for 150 minutes, and measured using two independent assays. For in vivo assessments, with IACUC approval and under general anesthesia, a customized 16G spinal needle was used to position Licox probes and/or microdialysis catheters in the lumbar NPs of 3 goats. Oxygen was monitored for 30-45 mins (n=7 discs), and microdialysates were collected at 0.3 µL/minute for 150 mins (n=5-6 discs). Degeneration was then induced by injecting 2U chondroitinase ABC (ChABC). After 12 weeks, measurements were repeated. Glucose and lactate were assessed using single analyte kits. Following euthanasia, vertebral end plate porosity and disc degenerative condition were evaluated using microCT and histology, respectively.

RESULTS: In bovine discs (Fig 1), oxygen measurements equilibrated after ~45 mins. Glucose and lactate measured using microdialysis equilibrated after 30 mins, and both were higher in samples collected at 0.3µL/min versus 1µL/min (p<0.05 for lactate). Measurements were similar for both assay types. In vivo, in goat lumbar NPs (Fig 2), equilibrated median baseline oxygen in healthy discs was 50.0 mmHg, increasing to 108.0 mmHg in degenerate discs (p<0.05). With respect to glucose and lactate, following equilibration, median baseline levels in healthy discs were 0.063 and 0.069 mmol/L, respectively, increasing to 0.104 and 0.883 mmol/L in degenerate discs (p<0.05 versus baseline for lactate). MicroCT revealed higher vertebral end plate porosity and histology revealed moderate to severe degeneration in all ChABC-injected discs compared to healthy discs.

DISCUSSION: Here we successfully optimized techniques including probe placement, equilibration time, flow rate and detection method, for the in-situ measurement of oxygen, glucose, and lactate in a goat model of disc degeneration. For glucose and lactate, lower flow rates were necessary for accurate measurements due to the low levels of these metabolites in the NP. Interestingly, while increased lactate with degeneration was expected, increased oxygen levels were also measured. Potential explanations could be reduced disc height (reducing diffusion distances), increased cell death (reducing overall metabolite consumption) or increased end plate porosity (enabling increased diffusion) in degenerate discs.