Posterior Lumbar Interbody Fusion Using Posterolateral Placement of A Single Cylindrical Threaded Cage and Two Regular Cages: A Biomechanical Study

OBJECTIVES: An in vitro biomechanical study of posterior lumbar interbody fusion(PLIF) with threaded cage using two different approaches was performed on eighteen functional spinal units of bovine lumbar spines. The purpose of this study was to compare the segmental stiffnesses among PLIF with one long posterolateral cage, PLIF with one long posterolateral cage and simultaneous facet joint fixation, and PLIF with two posterior cages. METHODS: Eighteen bovine lumbar functional spinal units were divided into three groups. All specimens were tested intact and with cage insertion. Group 1(n=12) had a long threaded cage(15x36mm) inserted posterolaterally and oriented counter anterolaterally on the left side by posterior approach with left unilateral facetectomy. Group 2(n=6) had two regular length cages(15x24mm) inserted posteriorly with bilateral facetectomy. Six specimens from group 1 were then retested after unilateral facet joint screw fixation in neutral(group 3). Likewise, the other six specimens from group 1 were retested after fixation with a facet joint screw in an extended position(group 4). Nondestructive tests were performed in pure compression, flexion, extension, lateral bending, and torsion. RESULTS: PLIF with a single cage, group 1, had a significantly higher stiffnesses than PLIF with two cages, group 2, in left and right torsion(p<0.05). Group 1 showed higher stiffness values than group 2 in pure compression, flexion, left and right bending but were not significantly different. Group 3 showed a significant increase in stiffness in comparison to group 1 for pure compression, extension, left bending and right torsion(p<0.05). For group 4, the stiffness significantly increased in comparison to group 1 for extension, flexion and right torsion(p<0.05). Although there was no significant difference between groups 3 and 4, group 4 had increased stiffness in extension, flexion, right bending and torsion. CONCLUSION: Posterior lumbar interbody fusion with a single long threaded cage inserted posterolaterally with unilateral facetectomy enables sufficient decompression while maintaining a majority of the posterior elements. In combination with a facet joint screw fixation, adequate postoperative stability can be achieved. We suggest that posterolateral insertion of a long threaded cage is biomechanically an ideal alternative to PLIF.

Barrier Techniques for Spinal Cord Protection from Thermal Injury in Polymethylmethacrylate Reconstruction of Vertebral Body:Experimental and Theoretical Analyses

OBJECTIVES: Polymethylmethacrylate(PMMA) is often used to reconstruct the spine after total corpectomy, but the exothermic curing of liquid PMMA poses a risk of thermal injury to the spinal cord. The purposes of this study are to analyze the heat blocking effect of pre-polymerized PMMA sheet in the corpectomy model and to establish the minimal thickness of PMMA sheet to protect the spinal cord from the thermal injury during PMMA cementation of vertebral body. MATERIALS AND METHODS: An experimental fixture was fabricated with dimensions similar to those of a T12 corpectomy defect. Sixty milliliters of liquid PMMA were poured into the fixture, and temperature recordings were obtained at the center of the curing PMMA mass and on the undersurface(representing the spinal cord surface) of a pre-polymerized PMMA sheet of variable thickness(group 1:0mm, group 2:5mm, or group 3:8mm). Six replicates were tested for each barrier thickness group. RESULTS: Consistent temperatures(106.8+/-3.9degreesC) at center of the curing PMMA mass in eighteen experiments confirmed the reproducibility of the experimental fixture. Peak temperatures on the spinal cord surface were 47.3degreesC in group 2, and 43.3degreesC in group 3, compared with 60.0degreesC in group 1(p<0.00005). So pre-polymerized PMMA provided statistically significant protection from heat transfer. The difference of peak temperature between theoretical and experimental value was less than 1%, while the predicted time was within 35% of experimental values. The data from the theoretical model indicate that a 10mm barrier of PMMA should protect the spinal cord from temperatures greater than 39degreesC(the threshold for thermal injury in the spinal cord). CONCLUSION: These results suggest that pre-polymerized PMMA sheet of 10mm thickness may protect the spinal cord from the thermal injury during PMMA reconstruction of vertebral body.