BIOMECHANICAL EVALUATION OF TRANSLAMINAR FACET JOINT FIXATION - A COMPARATIVE-STUDY OF POLY-L-LACTIDE PINS, SCREWS, AND PEDICLE FIXATION

Study Design. Nine sheep cadaveric spines were used in this acute post operative model. Objectives. To compare the biomechanical performance of translaminar facet joint fixation technique with that of cortical s crews and bioabsorbable poly-L-lactide pins and with that of rigid ped icle screw fixation in the lumbar spine. Summary of Background Data. A mong numerous posterior spine fixation techniques, pedicle screw fixat ion has been reported to be the most rigid construct and to provide hi gh fusion rates. Translaminar facet joint screw fixation is an alterna tive to pedicle screw fixation and is the lowest profile construct tha t achieves stabilization. The authors have developed a new concept inv olving application of bioabsorbable poly-L-lactide pins to translamina r facet joint fixation. Degradation in the stiffness of the implants w ith time may be advantageous for fusion mass remodeling. Methods: A to tal of nine sheep L2-L6 cadaveric spines were used. Each intact spine was nondestructively tested in flexion-extension bending (+/-5-Mm peak bending moment with 100-N axial compression) on a modified testing ma chine. Loads were applied for 10-second periods using sinusoid wavefor ms. After testing the intact spine, bilateral fenestration was perform ed between L4 and L5 and the medial aspect of the facet capsule was re sected. The L4-L5 functional spinal unit was then stabilized by five m ethods: translaminar facet joint fixation with smooth poly-L-lactide p ins; translaminar facet joint fixation with cortical screws; pedicle s crew fixation with the Texas Scottish Rite Hospital system; and withou t instrumentation, in that order. Linear displacement of L4 inferior a nd L5 superior articular processes in the sagittal plain (delta(facet) ) and L4-L5 intervertebral rotation in the sagittal plain (-(sagittal) ) were measured by the extensometers mounted to the spine. Ranges of m otion (delta(facet) and -(sagittal)), neutral zones, linear elastic zo ne stiffness, and the total energy absorption during the toad-unload c ycle (hysteresis) were calculated. Results. By resecting the facet joi nt capsules and ligamentum flavum, delta(facet) and -(sagittal) were n or increased significantly, whereas the increase of neutral zones and hysteresis were statistically significant. Compared with the intact sp ine, delta(facet) was significantly reduced to 41% of normal with tran slaminar facet joint fixation with poly-L-lactide pins, to 9% with tra nslaminar facet joint fixation with screws, and to 11% with the Texas Scottish Rite Hospital system. Neutral zones of delta(facet) showed a similar pattern, and these differences were significant. Regarding lin ear elastic zone stiffness, translaminar facet joint fixation with scr ews provided a stiffer construct than did pedicle screw fixation in th e fIexion loading mode, whereas pedicle screw fixation yielded higher values for stiffness in extension loading. Translaminar facet joint fi xation with poly-L. lactide pins increased linear elastic zone stiffne ss in extension loading, but the increase was less than was achieved w ith the other constructs. Conclusions. The facet joint is the only tru e articulation in the lumbosacral spine. It is logical to fix this par t directly to achieve spine fixation. Translaminar facet joint fixatio n with screws show similar biomechanical performance to pedicle screw fixation. Translaminar facet joint fixation with poly-L-lactide pins i s significantly less stiff than either type of screw fixation, but it also restricts the facet joint and intervertebral motions significantl y when compared with the intact spine.