Preliminary Investigations on Intradiscal Pressures during Daily Activities: An In Vivo Study Using the Merino Sheep

Abstract

PurposeCurrently, no studies exist, which attest the suitability of the ovine intervertebral disc as a biomechanical in vivo model for preclinical tests of new therapeutic strategies of the human disc. By measuring the intradiscal pressure in vivo, the current study attempts to characterize an essential biomechanical parameter to provide a more comprehensive physiological understanding of the ovine intervertebral disc.MethodsIntradiscal pressure (IDP) was measured for 24 hours within the discs L2-L3 and L4-L5 via a piezo-resistive pressure sensor in one merino sheep. The data were divided into an activity and a recovery phase and the corresponding average pressures for both phases were determined. Additionally, IDPs for different static and dynamic activities were analyzed and juxtaposed to human data published previously. After sacrificing the sheep, the forces corresponding to the measured IDPs were examined ex vivo in an axial compression test.ResultsThe temporal patterns of IDP where pressure decreased during activity and increased during rest were comparable between humans and sheep. However, large differences were observed for different dynamic activities such as standing up or walking. Here, IDPs averaged 3.73 MPa and 1.60 MPa respectively, approximately two to four times higher in the ovine disc compared to human. These IDPs correspond to lower ex vivo derived axial compressive forces for the ovine disc in comparison to the human disc. For activity and rest, average ovine forces were 130 N and 58 N, compared to human forces of 400-600 N and 100 N, respectively.Conclusions In vivo IDPs were found to be higher in the ovine than in the human disc. In contrast, axial forces derived ex vivo were markedly lower in comparison to humans. Both should be considered in future preclinical tests of intradiscal therapies using the sheep. The techniques used in the current study may serve as a protocol for measuring IDP in a variety of large animal models.

A new dynamic six degrees of freedom disc-loading simulator allows to provoke disc damage and herniation

“…For preconditioning of the specimen, a compressive load of 130 N was applied for 15 min, simulating a typical spinal load of a standing sheep [50]. Testing was performed using the following protocol: 0°-12°flexion, 0°-9°right lateral bending, and 0°-4°right axial rotation combined with constant axial compression of 800 N typically occurring in vivo during dynamic activities [50].…”

“…Testing was performed using the following protocol: 0°-12°flexion, 0°-9°right lateral bending, and 0°-4°right axial rotation combined with constant axial compression of 800 N typically occurring in vivo during dynamic activities [50]. The motions were applied cyclically with a frequency of 0.5 Hz.…”

Limitations of current in vitro test protocols for investigation of instrumented adjacent segment biomechanics: critical analysis of the literature

“…In a rabbit model, changes in the structure of the anulus fibrosus at adjacent segments were noted already 3 months after surgical immobilization [8], whilst in a goat model, no signs of degeneration were found after 6 months [7]. These conflicting results highlight the limited transferability of these data to human, due to deviations in anatomy [9], biology [10] and loading condition [11] in animal models.…”

Noninvasive cumulative axial load may induce intervertebral disc degeneration-A potential rabbit model

“…A previous investigation found great variations in IDP in sheep during their various daily activities. The highest IDPs (3.73–4.78 MPa) were recorded when the sheep stood up from a lying position, and were 5- to 6-fold greater than the IDPs recorded while the sheep were standing (0.70–0.73 MPa) (15). This indicates that an animal's position greatly impacts its IDP, which is considered to be a significant factor influencing disc degeneration (13,16).…”