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 postoperative model.

OBJECTIVES: To compare the biomechanical performance of translaminar facet joint fixation techique with that of cortical screws and bioabsorbable poly-L-lactide pins and with that of rigid pedicle screw fixation in the lumbar spine.

SUMMARY OF BACKGROUND DATA: Among numerous posterior spine fixation techniques, pedicle screw fixation has been reported to be the most rigid construct and to provide high fusion rate. Translaminar facet joint screw fixation is an alternative to pedicle screw fixation and is the lowest profile construct that achieves stabilization. The authors have developed a new concept involving application of bioabsorbable poly-L-lactide pins to translaminar facet joint fixation. Degradation in the stiffness of the implants with time may be advantageous for fusion mass remodeling.

METHODS: A total of nine sheep L2-L6 cadaveric spines were used. Each intact spine was nondestructively tested in flexion-extension bending (+/- 5-Nm peak bending moment with 100-N axial compression) on a modified testing machine. Loads were applied for 10-second periods using sinusoid waveforms. After testing the intact spine, bilateral fenestration was performed between L4 and L5 and the medial aspect of the facet capsule was resected. The L4-L5 functional spinal unit was than stabilized by five methods: translaminar facet joint fixation with smooth poly-L-lactide pins; translaminar facet joint fixation with cortical screws; pedicle screw fixation with the Texas Scottish Rite Hospital system; and without instrumentation, in that order. Linear displacement of L4 inferior and L5 superior articular processes in the sagittal plain (delta facet) and L4-L5 intervertebral rotation in the sagittal plain (theta sagittal) were measured by the extensometers mounted to the spine. Ranges of motion (delta facet and theta sagittal), neutral zones, linear elastic zone stiffness, and the total energy absorption during the load-unload cycle (hysteresis) were calculated.

RESULTS: By resecting the facet joint capsules and ligamentum flavum, delta facet and theta sagittal were not increased significantly, whereas the increase of neutral zones and hysteresis were statistically significant. Compared with the intact spine, delta facet was significantly reduced to 41% of normal with translaminar facet joint fixation with poly-L-lactide pins, to 9% with translaminar 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 linear elastic zone stiffness, translaminar facet joint fixation with screws provided a stiffer construct than did pedicle screw fixation in the flexion loading mode, whereas pedicle screw fixation yielded higher values for stiffness in extension loading. Translaminar facet joint fixation with poly-L-lactide pins increased linear elastic zone stiffness in extension loading, but the increase was less than was achieved with the other constructs.

CONCLUSIONS: The facet joint is the only true articulation in the lumbosacral spine. It is logical to fix this part directly to achieve spine fixation. Translaminar facet joint fixation with screws show similar biomechanical performance to pedicle screw fixation. Translaminar facet joint fixation with poly-L-lactide pins is significantly less stiff than either type of screw fixation, but it also restricts the facet joint and intervertebral motions significantly when compared with the intact spine.