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TECHNICAL PAPERS: Bone/Orthopedics

# Disc Mechanics With Trans-Endplate Partial Nucleotomy are not Fully Restored Following Cyclic Compressive Loading and Unloaded Recovery

[+] Author and Article Information
Edward J. Vresilovic

Department of Orthopaedic Surgery,  Beth Israel Deaconess Medical Center, Boston, MA 02215

McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery,  University of Pennsylvania, Philadelphia, PA 19104

Dawn M. Elliott1

McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery,  University of Pennsylvania, Philadelphia, PA 19104delliott@mail.med.upenn.edu

1

Corresponding author.

J Biomech Eng 128(6), 823-829 (Jun 14, 2006) (7 pages) doi:10.1115/1.2354210 History: Received January 03, 2006; Revised June 14, 2006

## Abstract

Mechanical function of the intervertebral disc is maintained through the interaction between the hydrated nucleus pulposus, the surrounding annulus fibrosus, and the superior and inferior endplates. In disc degeneration the normal transfer of load between disc substructures is compromised. The objective of this study was to explore the mechanical role of the nucleus pulposus in support of axial compressive loads over time. This was achieved by measuring the elastic slow ramp and viscoelastic stress-relaxation mechanical behaviors of cadaveric sheep motion segments before and after partial nucleotomy through the endplate (keeping the annulus fibrosus intact). Mechanics were evaluated at five conditions: Intact, intact after $10,000cycles$ of compression, acutely after nucleotomy, following nucleotomy and $10,000cycles$ of compression, and following unloaded recovery. Radiographs and magnetic resonance images were obtained to examine structure. Only the short time constant of the stress relaxation was altered due to nucleotomy. In contrast, cyclic loading resulted in significant and large changes to both the stiffness and stress relaxation behaviors. Moreover, the nucleotomy had little to no effect on the disc mechanics after cyclic loading, as there were no significant differences comparing mechanics after cyclic loading with or without the nucleotomy. Following unloaded recovery the mechanical changes that had occurred as a consequence of cyclic loading were restored, leaving only a sustained change in the short time constant due to the trans-endplate nucleotomy. Thus the swelling and redistribution of the remaining nucleus pulposus was not able to fully restore mechanical behaviors. This study reveals insights into the role of the nucleus pulposus in disc function, and provides new information toward the potential role of altered nucleus pulpous function in the degenerative cascade.

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## Figures

Figure 1

Overall testing sequence indicating cyclic loading and unloaded recovery periods on days 1 and 2, and the nucleotomy treatment on day 2. Assessment of elastic and viscoelastic mechanical behavior was performed at five conditions (initial, Cyclic I, acute nucleotomy, Cyclic II, and final) according to the “loading protocol” to be described in Fig. 3.

Figure 2

Schematic of the trans-endplate nucleotomy (A) A sagittal section showing the 4.0 mm diameter perforation made through the superior vertebral body into the disc. (B) An axial section with the location of nucleotomy denoted by circle, removing approximately 10% of the nucleus pulposus volume.

Figure 3

The “loading protocol” was used to assess elastic and viscoelastic mechanical behavior in axial compression (time not to scale). The loading protocol consisted of 50 preconditioning cycles at 1Hz, a 30min stress relaxation test from a 200N compression load, and a 1N∕s slow ramp test to 400N compression.

Figure 4

(a) Representative stress-relaxation response (points) and two-phase exponential model-fit (line). R2 value of 0.99. (b) Average stiffness during the stress relaxation response calculated from parameters in Table for each condition.

Figure 6

(A) Post-test MRI of a sagittal section showing the 4.0mm diameter perforation made through the superior vertebral body into the disc. (B) Post-test MRI of an axial section with the location of nucleotomy denoted by dotted circle. Note in both images that there is evidence of swelling and redistribution of the remaining nucleus pulposus tissue. This tissue exhibited a resistance to manual palpation confirming that nucleus tissue, not fluid, was present throughout the disc space.

Figure 5

Mean (standard deviation) material properties at each condition for (a) τ1, the short time constant (b) S1, the short damping coefficient, and (c) Sslow, the slow ramp stiffness. ∗ represents p<0.05 and ∗∗ represents p<0.005.

## Errata

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