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

Introduction: The intervertebral disc being avascular, the essential nutrient supply and waste removal are governed by the mechanisms of diffusion and convection. While diffusion is the predominant source of nutrition, convective transport occurs when the disc experiences deformation through mechanical loading. There are no reliable methods to simultaneously study these two mechanisms in humans under natural loads. For the first time, present study investigates these by strategically employing positional MRI and post-contrast studies in three physiological positions: i.e. supine, standing and post-standing recovery supine.

Methods: 100 healthy lumbar intervertebral discs from 20 volunteers were subjected to serial post-contrast MR studies after intravenously injecting Gadodiamide (0.3mmol/kg). T1 weighted MR images were obtained at 0,2,6,12, and 24 hrs. At each time interval, images were obtained in three positions: i.e Supine, Standing and Post-standing recovery supine. Each patient had 15 MRIs (Three positional MRIs at 0, 2, 6, 12, and 24 hours) in 24 hr period. A total of 300 MRI sequences constituted the study material. The signal intensity values and enhancement percentages were calculated and analysed comparing three positions.

Results: During unloaded supine position, there was slow gradual increase in enhancement reaching peak at 6 hrs. When the subjects assumed standing position, there was immediate loss of enhancement at nucleus pulposus which resulted in reciprocal increase in enhancement at Endplates. On standing, while central nucleus pulposus (NP) on average lost 26.2% of enhancement, superior peripheral NP lost 175% whereas superior endplate gained 105.3%. Similarly, inferior peripheral NP lost 96% and inferior endplate gained 90% of enhancement. Suggesting a contrast march from central nucleus pulposus towards Endplate (Wash-out phenomenon). Interestingly, when subjects assumed the post-standing recovery position, the nucleus pulposus regained the lost enhancement and Endplate showed reciprocal loss of gained enhancement (Pumping-in phenomenon). During recovery supine position, while the endplate lost an average of 901.2% of enhancement, the peripheral nucleus pulposus recovered its lost enhancement by 920%, which resulted in the 104% recovery in the central nucleus pulposus enhancement. It was interesting to note that, while the discs are gradually gaining contrast enhancement over a period of time by the process of diffusion, standing position acutely “squeezed out” an average of 26% of small molecules out of central nucleus pulposus, the immediate post-standing recovery position “sucked in” an average of 104% of small molecules into the central nucleus pulposus by the process of forced convection.

Discussion: To our knowledge it’s the first human in-vivo study which non-invasively evaluated the effect of diurnal natural physiological loads on solute transport across the disc. Standing caused rapid solute depletion from the nucleus pulposus which was promptly regained by assuming resting supine position. While during rest, solutes diffuse gradually into disc, the diurnal short loading and unloading redistributed small solutes by convection. The study helps in understanding the intricate interplay between mechanical loading and disc nutrition and opens up the opportunity for further investigations. Tailoring treatment approaches to account for the diurnal cycle of loading and resting phases may hold promise in optimizing disc health.