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

Park, Choon Keun; Hwang, Jang Hoe; Ji, Chul; Kwun, Sung Oh; Sung, Jae Hoon; Choi, Seung Jin; Lee, Sang Won; Kim, Moon Kyu; Park, Sung Chan; Cho, Kyeung Suok; Park, Chun Kun; Yuan, Hansen; Kang, Joon Ki

Abstract

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.