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

Introduction

Cadaveric mechanical testing is frequently used to study disc segments, with outcomes interpreted in the context of in vivo spine. However, the in vivo spine and excised motion segment experience different loading conditions. To mitigate these effects, axial preloads are often applied; however, preloads are highly variable across studies and not based on experimentally determined differences between the in vivo and cadaveric segment conditions. This limits our ability to interpret cadaveric study outcomes in the in vivo context. The purpose of this work was to quantify the progressive changes in disc geometry, opening pressure, and T₂ time from an in vivo, physiologic state to excised and loaded segments in a porcine model.

Methods

Lumbar spines of Yucatan minipigs underwent serial 3T MRI and opening pressure assessments with dissection and imposed loading (Table 1). Our previous work confirmed that Live, anesthetized animal disc geometry, T2 time, and opening pressure (OP) were not different from the fresh cadaver Torsos [2], therefore the Torso represents the physiological reference in this study. The lumbar Spine was dissected out of the Torso and then further dissected into motion Segments. The segments were Potted in bone cement and placed in a PBS bath with an axial load for the Load condition. The load was removed, and segments were allowed to free Swell in the bath. Finally, the loading was repeated for the Load 2 condition.

From the MRI, disc height was calculated as the mid-sagittal disc area/width and T₂ relaxation time, which is correlated with hydration, was evaluated in the nucleus pulposus (NP) [1-2]. Following the Potted condition, a Load (axial stress) was applied to each segment (between 0-0.7 MPa) to determine the relationship between applied stress and OP. The opening pressure OP was assessed by injecting contrast until it entered the NP [3].

Results and Discussion

The disc height increased with specimen dissection and preparation, but was recoverable with an imposed axial stress (Fig 1A). The Load and Load 2 conditions were not different from each other and importantly, an applied Load recovered disc height to the in vivo range. The T2 relaxation time was reduced with specimen preparation and, unlike disc height, remained low throughout subsequent loading and swelling conditions (Fig 1B).

The OP decreased with specimen dissection and preparation, as expected, due to loss of restraining structures (Fig 1C). The intrinsic OP (0 MPa axial stress) was 0.11+/-0.06 MPa. The axial stress (0-0.74 MPa) and OP exhibited a linear relationship in agreement with previous studies (Fig 2).

In conclusion, the disc geometry from MRI and the disc opening pressure were optimally recovered to the in vivo value with an axial stress of 0.2-0.3 MPa; however, despite hydration in a PBS bath, the T2 time was not recoverable to the in vivo range, it reduced at the Load condition and remained low for subsequent steps.