Biomechanical analysis of rotational motions after disc arthroplasty: implications for patients with adult deformities.
METHODS
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.


OBJECTIVE
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.


BACKGROUND
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 degrees 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.


CONCLUSIONS
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.


CONCLUSIONS
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.
McAfee
Paul C
PC
Spine and Scoliosis Center, St. Joseph's Hospital, Baltimore, MD, USA. mack8132@aol.com
Cunningham
Bryan W
BW
Hayes
Victor
V
Sidiqi
Farhan
F
Dabbah
Michael
M
Sefter
John C
JC
Hu
Nianbin
N
Beatson
Helen
H
D000328:Adult; D000367:Age Factors; D001696:Biomechanical Phenomena; D002102:Cadaver; D017586:Diskectomy / Q000009:adverse effects*; D005260:Female; D006801:Humans; D007049:Iatrogenic Disease / Q000517:prevention & control; D007403:Intervertebral Disc / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D017846:Longitudinal Ligaments / Q000033:anatomy & histology / Q000293:injuries / Q000502:physiology; D008159:Lumbar Vertebrae / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D008297:Male; D011183:Postoperative Complications / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control; D019736:Prostheses and Implants / Q000009:adverse effects; D016059:Range of Motion, Articular; D012399:Rotation / Q000009:adverse effects; D012600:Scoliosis / Q000209:etiology* / Q000503:physiopathology*; D013123:Spinal Fusion / Q000009:adverse effects*; D021801:Zygapophyseal Joint / Q000033:anatomy & histology / Q000502:physiology