Influence of an auxiliary facet system on lumbar spine biomechanics.
METHODS
In vitro biomechanical study investigating L4-L5 kinematics and intradiscal pressure (IDP) with a facet replacement system.


OBJECTIVE
To assess the influence of the Auxiliary Facet System (AFS).


BACKGROUND
Posterior dynamic systems are used in the treatment of low back pain to avoid adjacent segment degeneration. Facet replacement systems are supposed to stabilize a lumbar segment after facetectomy and neural decompression, and to provide an intersegmental range of motion (ROM).


METHODS
The AFS is fixed by 4 pedicle screws, linked by 2 angulated rods, a polyaxial connector, and a crosslink. Flexibility tests were conducted on 6 human cadaver specimens (L3-S1) using a load testing device and the Polaris system. The specimens were loaded by steps of 1 Nm to 10 Nm in flexion/extension, lateral bending, and axial rotation. The following configurations were investigated: intact segment, instrumented, instrumented plus medial facetectomy, and facetectomy alone. The sagittal mean center of rotation (MCR) was calculated, and IDPs were measured in flexion/extension.


RESULTS
The ROM of the intact segment was 10.9° (9.4°-15.5°) in flexion/extension, 9.5° (6.8°-12.1°) in lateral bending, and 4.7° (3.4°-6.0°) degrees in axial rotation. Medial facetectomy and instrumentation led to -6% of ROM in flexion/extension and +1% lateral bending. Medial facetectomy without implant led to +106% of axial rotation (P = 0.028). The instrumentation reduced axial rotation to -38% (P = 0.028). This decrease was because of the presence of the cross-link. The MCR was located around the middle of the superior L5 endplate in intact and instrumented specimens. It moved cranial after facetectomy without instrumentation. The implant decreased the maximal IDP during flexion/extension to -17% (P = 0.028).


CONCLUSIONS
The AFS had a minor influence on flexion/extension and lateral bending, and the MCR kept physiologic. Bilateral facetectomy yielded an increase in axial rotation, which was stabilized by the implant. The AFS seemed to reduce IDPs.
Charles
Yann Philippe
YP
Laboratoire de Biomécanique, Arts et Métiers ParisTech, Paris, France. yann.philippe.charles@chru-strasbourg.fr
Persohn
Sylvain
S
Steib
Jean-Paul
JP
Mazel
Christian
C
Skalli
Wafa
W
D019643:Arthroplasty, Replacement / Q000295:instrumentation / Q000379:methods*; D001696:Biomechanical Phenomena; D001858:Bone Nails; D001863:Bone Screws; D002102:Cadaver; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000502:physiology* / Q000601:surgery*; D018583:Pliability; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation