Spine. September 2006 - Volume 31 - Issue 19S - pp S152-S160

Biomechanical Analysis of Rotational Motions After Disc Arthroplasty: Implications for Patients With Adult Deformities [Surgical Decision-Making]

McAfee, Paul C. MD; Cunningham, Bryan W. MMech, Eng; Hayes, Victor MD; Sidiqi, Farhan MD; Dabbah, Michael MD; Sefter, John C. DO; Hu, Nianbin MD; Beatson, Helen BS

Study Design. An anatomic and biomechanical bench-top basic scientific comparative analysis to determine the appropriateness of total disc replacement (TDR) in a lumbar spine with scoliotic tendencies.

Objectives. Only limited data are currently available studying the application of disc replacement adjacent to scoliosis fusions. Theoretically, motion preservation should help delay the continuum of lumbar degeneration adjacent to scoliosis fusions and rotationally unstable lumbar segments.

Summary of Background Data. As a tertiary referral center for failed TDR, we noticed an alarming number of lumbar spinal rotational iatrogenic instability patterns but none occurring in the cervical spine. It is appropriate to analyze the bench-top rotational stability of disc replacement to predict whether this new technology is feasible for a larger prospective clinical study in the treatment of degenerative scoliosis.

Methods. Measurements were taken from 60 human specimens from the Hamann-Todd Osteological Collection: 1) to determine the rotational arc of influence (AOI) = the angle formed from the center of axial rotation to the outermost extent of the facet joints; and 2) to determine the relative anatomic size discrepancy between the left and right facets proportionately with the cross-sectional area of the intervertebral disc = facet/endplate ratio (FER). Biomechanical testing was performed using fresh frozen human cadaveric spines with the following conditions to determine the rotational stability: 1) intact; 2) resection of ALL, anulus, disc, and PLL simulating the preparation for a TDR; 3) a more radical anular resection; 4) entire 360° anular resection; and 4) insertion of the respective unconstrained-type disc replacement. Using a 6 degrees of freedom spine simulator, unconstrained pure moments of ±8.0 Nm (lumbar) and ±3.0 Nm (cervical) were used for axial rotation with quantification of the operative level range of motion and neutral zone, with data normalized to the intact spine condition.

Results. There were anatomic limitations in the lumbar spine that make it less desirable to apply uncon-strained disc replacements; indeed, the spine was at risk for iatrogenic lumbar scoliosis. The anulus fibrosis, anterior longitudinal ligament, and the posterior longitudinal ligament are critical structures in preventing iatrogenic scoliosis. The lumbar facet joints are more posteriorly located and are smaller relative to the intervertebral disc, compared with this association in the cervical spine. Because the facet capsular ligaments are mechanically less effective with lower tensile strength in the lumbar spine, multiple-level arthroplasty tends to accentuate scoliotic tendencies; this is independent of prosthetic design and surgical technique.

Discussion. Implantation of the lumbar TDR never restored the motion segment back to the rotational stability of the intact segment achieving a range of 120% to 140% rotational range of motion compared with the intact condition. This rotational instability proved to be additive as a two-level lumbar TDR resulted in between 240% and 260% increase in rotational instability compared with the intact condition.

Conclusion. The neutral zone of the intact cervical spine was restored even using an unconstrained cervical TDR. The greater inherent rotational constraints of the cervical spine make it more amenable to stable multilevel arthroplasty compared with the lumbar spine.

1. There are four main rotational stabilizing considerations in the functional spinal unit, and all four favor cervical versus lumbar rotational integrity.
2. Compared with the intact nonoperated motion segment, the preparation for both cervical and lumbar disc replacements compromises the axial rotational stability of both the cervical and lumbar spine: resection of the ALL, stretching or “release” of the PLL, increasing the disc space height which “unlocks” the posterior facet joints, and removal of portions of the uncovertebral joints in the cervical spine.
3. With an unconstrained prosthesis, the uncovertebral joint can be resected and the rotational stability of the intact cervical segment can be restored, but not with bilateral uncovertebral joint resection.
4. Implantation of an unconstrained lumbar disc replacement fails to restore the acute rotational stability of the lumbar spine. However, with an unconstrained prosthesis in the cervical spine, the rotational stability is restored within the NZ of the intact condition. With multilevel or successive lumbar artificial disc replacement, the instability is additive: what is lost in resecting the anulus, ALL, and PLL is not compensated for totally in the acute postoperative condition.

Key Points
* Total disc replacement accentuates scoliotic tendencies in the lumbar spine.
* There are four main rotational stabilizing considerations in the functional spinal unit, and all four favor cervical versus the lumbar rotational integrity.
* There is a larger arc of influence for the cervical facets (153.6°) compared with the lumbar facet joints (98.0°).
* The combined joint surface area of the cervical facets relative to the cervical disc cross-sectional area (0.519) is almost twice the magnitude of the same association in the lumbar spine (0.293).

© 2006 Lippincott Williams & Wilkins, Inc.