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In Vitro Disc Pressure Profiles Below Scoliosis Fusion Constructs

Buttermann, Glenn R., MD*; Beaubien, Brian P., MS

doi: 10.1097/BRS.0b013e31817d1d7f
Biomechanics

Study Design. Biomechanical human cadaveric study comparing straight and scoliotic spines with healthy and degenerated L4/5 discs.

Objective. To describe the biomechanical environment of discs under various spinal alignments by measuring the coronal intradiscal pressure profiles.

Summary of Background Data. Abnormal loading of the lumbar discs in the concavity of scoliotic curves may accelerate disc degeneration, which may be related to pain.

Methods. Eight intact human cadaver spines (T1–S1; mean donor age 47 years old) underwent radiographs, DEXA, and MRI and were graded for disc degeneration. Each specimen was instrumented in a normal (straight coronal) spinal alignment from T4–L4. Intradiscal pressure profiles for the L4/5 disc and resultant moments were obtained under axial follower loads up to 1500 N. Testing was repeated for bilateral 3-cm decompensation. Posterior instrumentation was used to induce scoliosis (thoracic and lumbar curve average = 25°, fractional lumbosacral curve average = 5°), and testing was repeated for all load states.

Results. MRI found 4 healthy (grade I and II) and 4 degenerated (grade III to V) L4/5 discs. Scoliosis and decompensation significantly increased coronal moments (P < 0.003). Disc pressures increased linearly with greater applied loads for all specimens. Healthy L4/5 discs exhibited uniform pressure profiles with normal spinal alignment and minimal effect with simulated scoliosis or decompensation. For degenerated discs, there was a relative pressure profile depression in the nucleus relative to the anulus region; with spinal malalignment, either due to scoliotic curvature, decompensation, or both, there was disc pressure profile asymmetry. The ratio of maximum intradiscal pressure at the concavity relative to the convexity was 1.1 (range, 1.0–1.2) for healthy discs and 3.6 (range, 2.2–4.4) for degenerated discs in the scoliotic specimens (P = 0.008).

Conclusion. Disc pressure profilometry below long spinal constructs found asymmetric loading with the greatest loads at the concave inner anulus, especially in the presence of disc degeneration, scoliosis, and decompensation. For the degenerated cases, there was substantial disc pressure profile asymmetry despite only mildlysevere scoliotic curvatures. These results suggest that scoliosis surgeons should minimize end-vertebra tilt, maximize lumbar curve, and balance correction at the time of surgical intervention. These results combined with prior animal studies suggest a compounding effect of asymmetric loading and progression of disc degeneration.

Spinal malalignment produced large coronal moments. Disc pressure profilometry below long spinal constructs found asymmetric loading with the greatest loads at the concave inner anulus, especially in the presence of disc degeneration, scoliosis, and decompensation.

From the *Midwest Spine Institute, Stillwater; and †Midwest Orthopaedic Research Foundation, Orthopaedic Biomechanics Laboratory, Minneapolis, MN.

Acknowledgment date: November 16, 2007. Revision date: February 22, 2008. Acceptance date: March 28, 2008.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Corporate/Industry funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Supported by Synthes Spine, Midwest Spine Institute, and Midwest Spine Research Foundation.

Address correspondence and reprints requests to Glenn R. Buttermann, MD, 1950 Curve Crest Boulevard, Stillwater; E-mail: butte011@umn.edu

© 2008 Lippincott Williams & Wilkins, Inc.