SPINE Volume 25, Number 4, pp 425– 430
`©2000, Lippincott Williams & Wilkins, Inc.
`
`Posterior Lumbar Interbody Fusion Using
`Posterolateral Placement of a Single Cylindrical
`Threaded Cage
`
`Jie Zhao, MD,* Yong Hai, MD,† Nathaniel R. Ordway, MS, PE,‡ Choon Keun Park, MD,‡
`and Hansen A. Yuan, MD‡
`
`Study Design. An invitrobiomechanical study of pos-
`terior lumbar interbody fusion (PLIF) with threaded cages
`was performed on 18 bovine lumbar functional spinal
`units.
`Objectives. To compare the segmental stiffness
`among PLIF with a single long posterolateral cage, PLIF
`with a single long posterolateral cage and simultaneous
`facet joint fixation, and PLIF with two posterior cages.
`Summary of Background Data. In most cases, PLIF
`with threaded cage techniques needs bilateral facetec-
`tomy, extensive exposure, and retraction of the cauda
`equina. Posterior element deficiency is detrimental to
`postoperative segmental stiffness.
`Methods. All specimens were tested intact and with
`cage insertion. Group 1 (n5 12) had a long threaded cage
`(15 3 36 mm) inserted posterolaterally and oriented
`counter anterolaterally on the left side by posterior ap-
`proach with left unilateral facetectomy. Group 2 (n 5 6)
`had two regular-length cages (15 3 24 mm) inserted pos-
`teriorly with bilateral facetectomy. Six specimens from
`Group 1 were then retested after unilateral facet joint
`screw fixation in neutral (Group 3). Similarly, 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 compres-
`sion, flexion, extension, lateral bending, and torsion.
`Results. The PLIF procedure involving a single cage
`(Group 1) had a significantly higher stiffness than PLIF
`with two cages (Group 2) in left and right torsion (P ,
`0.05). Group 1 had higher stiffness values than Group 2 in
`pure compression, flexion, and left and right bending, but
`differences were not significant. Group 3 had a significant
`increase in stiffness in comparison with Group 1 for pure
`compression, extension, left bending, and right torsion
`(P , 0.05). For Group 4, the stiffness significantly in-
`creased in comparison with Group 1 for extension, flex-
`ion, 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 bend-
`ing, and torsion.
`
`From the *Department of Orthopedic Surgery, Changhai Hospital,
`Shanghai; the †Department of Orthopedic Surgery, Hospital 514, Bei-
`jing, People’s Republic of China; and the ‡Department of Orthopedic
`Surgery, State University of New York Health Science Center, Syra-
`cuse, New York.
`Supported in part by a grant from Sulzer Spine Tech, Minneapo-
`lis, Minnesota.
`Acknowledgment date: August 11, 1998.
`First revision date: December 3, 1998.
`Acceptance date: May 6, 1999.
`Device status category: 11.
`Conflict of interest category: 16.
`
`Conclusions. Posterior lumbar interbody fusion with a
`single posterolateral long threaded cage with unilateral
`facetectomy enabled sufficient decompression while
`maintaining most of the posterior elements. In combina-
`tion with a facet joint screw, adequate postoperative sta-
`bility was achieved. [Key words: biomechanics, lumbar
`interbody fusion, posterior lumbar interbody fusion,
`threaded fusion cages] Spine 2000;25:425– 430
`
`The posterior lumbar interbody fusion (PLIF) is a choice
`of treatment for spinal instability accompanied by spinal
`and foraminal stenosis. Compared with bone graft PLIF,
`threaded-cage PLIF has some distinct theoretical advan-
`tages based on its rationale. It increases initial stiffness by
`distracting the disc space and is held in place by thread-
`ing into the endplates above and below. Bone is allowed
`to grow through and around the cage. It reduces prob-
`lems associated with the harvesting of large amounts of
`corticocancellous bone graft from the iliac crest. It min-
`imizes the risk of bone graft extrusion and disc space
`collapse while restoring disc height. Lastly, it can be used
`to achieve interbody fusion without supplemental inter-
`nal fixation.2
`From a clinical point of view, however, it is very chal-
`lenging to accomplish insertion of two cages of approx-
`imate size posteriorly. Extensive laminectomy and bilat-
`eral facetectomy is usually hard to avoid. From a
`biomechanical point of view, posterior element defi-
`ciency adversely affects the stiffness of an intervertebral
`fusion segment immediately after insertion of the cages,
`because these structures provide resistance to flexion and
`torsion,6 in addition to the obvious potential risk for
`operative neurologic damage.
`The literature4,11,15 shows that insertion of two cages
`has been performed with success. Chang4 reported 25
`cases of anterior lumbar retroperitoneal fusions involv-
`ing 35 levels, in which threaded cages were used and in
`which solid fusion was achieved at 2 years in all cases. In
`a multicenter prospective 236-case program, Ray11 eval-
`uated safety, fusion rate, and clinical outcome of a
`threaded titanium fusion cage for PLIF and reported that
`96% of the 208 2-year follow-up cases had fusion. Some
`studies have shown negative results. In one study using
`calf and human spines, Tencer et al12 concluded that
`placement of two posterior inserts reduced torsional
`stiffness more than one posterior insert. This is because
`two inserts damage more facet structures than one insert
`
`425
`
`

`
`426 Spine • Volume 25 • Number 4 • 2000
`
`Figure 1. Anteroposterior (A),
`lateral (B), and oblique (C) radio-
`graphs of the bovine test speci-
`men with a single long threaded
`cage in place.
`
`does. In another in vitro study using pure moments,
`Abumi et al1 found that bilateral total facetectomy, in
`comparison with the intact spine, produced increases in
`motion of 63% in flexion and 106% in axial rotation.
`Because insertion of a single threaded cage compromises
`less anatomic structures in comparison with two cages,
`the current authors examined PLIF using a single long
`threaded cage by a posterolateral approach with unilat-
`eral facetectomy to accomplish spinal canal decompres-
`sion and posterior interbody fusion. The purpose of this
`study was to compare the intervertebral motion segmen-
`tal stiffness among PLIF with a single long posterolateral
`cage insertion, PLIF with a single posterolateral cage and
`facet joint fixation, and PLIF with insertion of two reg-
`ular posterior cages. Surgical application of this tech-
`nique is also described.
`
`Materials and Methods
`Biomechanical Experimental Test. Eighteen bovine lumbar
`functional spinal units (T13–L1, L2–L3, and L4 –L5) were used
`in this study because of their similarity, geometric and kine-
`matic, to human functional spinal units (FSUs) and their con-
`sistency in bone density between levels and specimens.7,14 After
`death, the spines were stored at 220 C until used. The spines
`were thawed at room temperature for 16 to 18 hours and then
`cleaned carefully of musculature and vessels leaving the osteo-
`ligamentous tissues intact. Single motion segments were dis-
`sected; each segment consisted of two vertebrae with intact
`disc, facets, and ligamentous connections, comprising an FSU.
`Three screws were placed in the upper and lower vertebra of
`the FSU before embedding into fixtures filled with automotive
`body filler. Location of the neutral axis of each FSU was found
`using an anteroposterior and a lateral radiograph. During the
`thawing process and the time between tests, the specimens were
`wrapped in saline-soaked gauze to keep the FSU moist.
`Nondestructive tests were conducted using a materials test-
`ing machine (MTS Systems, Eden Prairie, MN) under load con-
`trol. Tests consisted of pure compression, extension, flexion,
`left and right lateral bending, and left and right torsion. Five
`20-second cycles of preconditioning were run for each test fol-
`lowed by five data collection cycles.
`For pure compression, each FSU underwent a ramp load
`input from 0 to 300 N, and the load was applied along the
`
`neutral axis as measured by radiograph. For extension, flexion,
`left bending, and right lateral bending, a ramp load input from
`0 to 300 N was applied either 1.5 cm posteriorly or anteriorly
`so that 4.5-Nm moments were generated for each cycle. For left
`and right torsion, 50 N of compression was statically applied,
`and then 4.5-Nm ramp moments were applied under torsional
`control with the materials testing system.
`Load, displacement, torque, and angular data were col-
`lected during testing with a plotter for immediate hard copy
`and with a personal computer for later analysis. Data were
`acquired by the computer at a sampling rate of 10 Hz. After
`collection, the data were archived on a backup disk.
`Each specimen was biomechanically tested intact. The spec-
`imens were then randomized and tested with instrumentation.
`Group 1 (n 5 12) had a 15 3 36-mm long single threaded cage
`(Sulzer Spine Tech, Minneapolis, MN) inserted posterolater-
`ally from the left side and oriented counteranterolaterally
`across the disc space through a posterior approach with unilat-
`eral facetectomy and hemilaminectomy (Figure 1). Entry point
`of the instruments was on a line connecting the superior and
`inferior pedicles of adjacent vertebrae and directed anterolat-
`erally 45°. A 45° line was drawn on the top of the embedding
`material to assist in consistent placement of the cage. Using a
`custom-made elongated distraction plug, equal distraction and
`reaming of both endplates was obtained from the ipsilateral to
`the contralateral side across the disc space. The cage was in-
`serted so that the closest edge of the cage was at a depth of 2
`mm from the posterior vertebral cortex. Group 2 (n 5 6) had
`two regular length cages (15 3 24 mm) inserted posteriorly and
`oriented anteriorly after bilateral facetectomy and laminec-
`tomy (Figure 2). Six specimens in Group 1 were then retested
`after fixation in neutral position with a facet joint screw, and
`these formed Group 3 (Figure 3). Similarly, the remaining six
`specimens in Group 1 were retested after fixation in an ex-
`tended position with a facet joint screw and formed Group 4.
`Data for all tests were normalized using the intact specimen
`as a control. The data from all testing models were analyzed
`using data analysis and statistical software (LabView; National
`Instruments, Austin, TX; and StatView; Abacus Concepts,
`Berkeley, CA). The stiffness was determined between 200 and
`300 N for pure compression and between 3.5 and 4.5 Nm for
`flexion, extension, lateral bending, and torsion, using a linear
`regression algorithm. The stiffness values for the last three data
`collection cycles were averaged for each test. A ratio between
`instrumentation stiffness and intact stiffness was calculated and
`used to find statistical comparisons among groups.
`
`

`
`PLIF With a Single Posterolateral Threaded Cage • Zhao et al
`
`427
`
`Figure 2. Anteroposterior (A) and lateral (B) radiographs of the
`bovine test specimen with two threaded cages in place.
`
`Statistical testing was performed by an independent statis-
`tician using repeated-measures analysis of variance. Signifi-
`cance was established at 95%.
`
`Surgical Method. One fresh human cadaver was used to dem-
`onstrate the surgical approach for posterior single threaded
`cage insertion. The cadaver was placed in a 90° kneeling–sitting
`position on an Andrew frame to maintain lumbar lordosis and
`to avoid abdominal compression and reduce epidural venous
`pressure. The skin was incised in the midline at the level to be
`fused. One side of the paravertebral muscles were then split and
`retracted laterally to the outer edge of the facet joint, and the
`lamina and facet joint were exposed. The inferior hemilaminec-
`tomy was performed first, after the unilateral facetectomy.
`Then the entire nerve root and same-side intervertebral space
`were exposed. Adequate decompression of the stenosis could
`be accomplished simultaneously. Surgicel (Ethicon, Somerville,
`NJ) was used as a tampon above and below the disc space, and
`the dura and nerve root were displaced medially and laterally,
`respectively. A drill tube was inserted obliquely through a 2-cm
`incision that was made 10 –12 cm laterally from the midline,
`following normal procedures (Figure 4). The threaded cage
`insertion was then performed.
`
`Results
`
`The mean normalized stiffness values for the seven test
`modes are shown in Figure 5.
`
`Figure 3. Anteroposterior (A) and lateral (B) radiographs of the
`bovine test specimen with a single cage combined with facet
`screw fixation in neutral position.
`
`Figure 4. A diagram to show the placement of a single long cage.
`After unilateral facetectomy and hemilaminectomy, the dura and
`superior nerve root were displaced medially and laterally, respec-
`tively, to produce adequate space for posterolateral insertion of
`the cage. A drill tube was inserted obliquely through a 2-cm
`incision that was made 10–12 cm lateral from the midline.
`
`PosterolateralSingleLongThreadedCageVersus Two
`PosteriorRegularCages
`Posterior interbody fusion by a single long threaded cage
`inserted posterolaterally with unilateral facetectomy and
`hemilaminectomy was significantly stiffer than that
`achieved with two posterior regular threaded cages with
`bilateral facetectomy and laminectomy in right and left
`torsion (P , 0.05; Figure 5, F and G). Figure 5, F and G,
`also show that normalized spine stiffness with two
`threaded cages is approximately 0.5 for both right and
`left torsion. The normalized spine stiffness with a single
`threaded cage is approximately 1.2 for left torsion and
`0.7 for right torsion. The torsional stiffness was not sym-
`metric for the single cage, because the facet joint was
`removed only on the left side. The single-cage group
`(Group 1) also had higher stiffness in pure compression,
`flexion, and left and right bending but without statistical
`significance (Figure 5, A–E).
`PosterolateralSingleLongThreadedCageWithFacet
`JointFixationinNeutralPosition
`The single long cage group with additional facet joint
`fixation in neutral position (Group 3) had a significant
`increase in stiffness (P , 0.05) in comparison with the
`single-cage group (Group 1) for pure compression (Fig-
`ure 5A), extension (Figure 5B), left bending (Figure 5D),
`and right torsion (Figure 5E). As shown in the figures, the
`normalized stiffness for the group using a single cage
`with additional facet joint fixation in neutral position
`(Group 3) was higher than 1.0 in these loading modes.
`PosterolateralSingleLongThreadedCageWithFacet
`JointFixationinExtendedPosition
`For Group 4, single cage with facet joint fixation in ex-
`tended position, the stiffness significantly increased com-
`
`

`
`428 Spine • Volume 25 • Number 4 • 2000
`
`Figure 5. Mean segmental stiffness 6 SD (normalized to intact
`spine stiffness) in seven loading directions: A, pure compression;
`B, extension; C, flexion; D, left bending; E, right bending; F, left
`torsion; G, right torsion. Group 1: a single long posterolateral
`insertion; Group 2: posterior insertion of two regular cages; Group
`3: a single posterolateral cage with facet joint fixation in neutral
`position; Group 4: a single posterolateral cage with facet joint
`fixation in extended position. Error bar, SD. *P , 0.05.
`
`

`
`PLIF With a Single Posterolateral Threaded Cage • Zhao et al
`
`429
`
`pared with that in Group 1 for extension (Figure 5B),
`flexion (Figure 5C), and right torsion (Figure 5G; P ,
`0.05). In flexion, as shown in Figure 5C, stiffness ratios
`for all configurations except Group 4 were significantly
`less than 1.0, indicating that fixation of the facet joint in
`an extended position greatly contributed to the increase
`in flexion stiffness of the construct. Although there was
`no significant difference between Group 3 and Group 4,
`the latter had increased stiffness in extension (Figure 5B),
`flexion (Figure 5C), right bending (Figure 5E), and right
`torsion (Figure 5G). Addition of the facet joint screw
`restored the symmetry between the left and right tor-
`sional stiffness values.
`Discussion
`Since Cloward5 introduced his first planned posterior
`lumbar interbody fusion (PLIF) in 1953, this method has
`gained acceptance for the treatment of spinal instability
`and associated spinal stenosis. With the advantages of
`minimizing complications of graft resorption and disc
`space collapse and avoiding the use of supplemental in-
`strumentation, threaded-cage PLIF has been recom-
`mended. However, the cage PLIF carries obvious disad-
`vantages. First, this procedure requires extensive
`removal of the posterior elements, including large por-
`tions of the bilateral facet joints, laminas and spinal pro-
`cesses. Therefore, the result is a decrease in biomechani-
`cal stiffness immediately after insertion, especially for
`flexion and torsion.6 Second, it requires extreme retrac-
`tion of the cauda equina during insertion, which in-
`creases the risk of neurologic damage. The risks associ-
`ated with the necessary retraction of the cauda equina
`seem to be acceptable, but there is a long learning curve
`to gain the necessary experience.8 Finally, it tends to
`produce kyphosis after insertion, especially when a
`larger size cage is used. Posterolateral insertion of a sin-
`gle long threaded cage has managed to overcome the
`disadvantages of PLIF with two regular cages while
`maintaining its advantages.
`Using a single posterolateral cage for PLIF requires
`unilateral facetectomy and hemilaminectomy through a
`posterior approach. A large portion of the posterior ele-
`ment remains intact, including the spinous process,
`counterside facet joint, lamina, and the more posterior
`intervertebral disc anulus. The remaining posterior ele-
`ments produce a certain amount of stiffness in flexion
`and torsion compared with PLIF with two cages. Abumi1
`found that left unilateral facetectomy produced increases
`in motion of 51% in flexion and 50% in right axial
`rotation and no marked changes in other motions,
`whereas bilateral total facetectomy, in comparison with
`the intact spine, produced increases of 63% in flexion,
`137% in left axial rotation, and 106% in right axial
`rotation. In a study in which double BAK cages were
`inserted by laparoscopic surgery in a human cadaveric
`lumbar spine, Nibu et al9 showed that the BAK fusion
`system increased the ROM (range of motion) of the in-
`tact spine significantly in extension. The ROM increases
`
`in extension because these anteriorly placed implants ne-
`cessitate damaging the anterior anulus and anterior lon-
`gitudinal ligament. Tencer et al12 also found that poste-
`rior placement of an insert can compromise the facet and
`lamina structures by reducing torsional stiffness, which is
`further reduced with two inserts. They believed these
`data may be interpreted to indicate that using a single
`insert is a better choice than using two.
`These results agree with those shown in the current
`study. Torsional stiffness for the one-cage construct in-
`creased significantly compared with the two-cage con-
`struct when a 15 3 36-mm size cage for the single-cage
`construct and 15 3 24-mm size for the two-cage con-
`struct were used. This was also found for pure compres-
`sion, flexion, and left and right bending, but with-
`out significance.
`Some studies have produced different results. Brodke
`et al3 found that placing a threaded cage doubled the
`flexion– extension stiffness of the intact spine and that
`this construct was four times as stiff as the spine with a
`bone graft alone. Oxland et al10 found in a calf spine
`model that the flexion– extension stiffness of a motion
`segment with a threaded implant increased flexion stiff-
`ness almost five times that of the intact spine. In Wilder’s
`study using a baboon spine model in vitro, a threaded
`insert increased flexion stiffness by 213% and extension
`by 334% compared with the intact spine.13 The reason
`for these differences may be the way stiffness was defined
`and measured. In the current study, stiffness was mea-
`sured using a linear regression algorithm (best-fit line)
`well above the neutral zone region (in the linear portion
`of the loading curve).
`Fixation of the facet joint with a screw is an uncom-
`plicated procedure and has been used frequently in lum-
`bar reconstruction. In this study, it functioned to de-
`crease instability of the facet joint that resulted from
`counterside facetectomy. There were significant in-
`creases in stiffness in the single-cage PLIF with an addi-
`tional facet joint screw for extension and torsion. A dif-
`ference in stiffness between the specimen with facet
`fixation in a neutral position and in an extended position
`was noted also. Keeping the specimen in an extended
`position with use of the facet joint screw significantly
`increased the stiffness for flexion in comparison with that
`of a single-cage PLIF and intact spine. There was also
`increased stiffness for extension, right bending and right
`torsion but without significance. These differences may
`have resulted from induced bony contact between the
`superior and inferior articular surface in the right facet
`joint, the increased purchase area for the right facet joint,
`and the increased tension in the posterior spinal liga-
`ment. The supplemental facet joint fixation in an ex-
`tended position also can maintain lumbar spine lordosis.
`The surgical approach described is similar to one of
`the approaches used in the treatment of far lateral disc
`herniations. It provides direct identification and visual-
`ization of both nerve root and dura. It is possible to
`accomplish insertion of an appropriate size cage postero-
`
`

`
`430 Spine • Volume 25 • Number 4 • 2000
`
`laterally and decompression of most types of accompa-
`nying canal stenosis and unilateral foraminal stenosis,
`including degenerative spondylolisthesis. The contrain-
`dication for this surgical approach is bilateral foraminal
`stenosis, which necessitates decompression of both sides,
`and adhesive arachnoiditis and epidural adhesion from
`prior surgery. Because the insertion point of the cage is
`around the posterolateral corner of the intervertebral
`disc, retraction of the cauda equina is relatively mild.
`During insertion, the superior nerve root must be pro-
`tected and retracted cautiously, and part of the superior
`pedicle may also be removed to further relieve the related
`nerve root.
`Clinically, cage size and length have to be taken into
`consideration to achieve adequate bony fusion. A single
`longer threaded cage provides approximately one third
`less area than two regular cages. If increased area for
`bony fusion is deemed necessary, additional bone graft
`could be implanted before cage insertion in the antero-
`lateral intervertebral space without risk of bone graft
`retropulsion or collapse.
`Calf lumbar spine has been shown to have kinematic
`properties similar to those of the human lumbar spine.14
`In addition, calf spines of similar ages and bone quality
`are readily available. These features make them useful as
`a biomechanical model for evaluation of implants. How-
`ever, it is important to remember the limitations of this in
`vitro animal model. The calf spine has smaller disc height
`and immature endplates. In addition, the results repre-
`sent the stiffness in the immediate postoperative period
`with relatively few cycles of loading and are not neces-
`sarily indicative of cyclic repetitive loading in the hu-
`man spine.
`Despite these limitations, this study shows some bio-
`mechanical advantages of posterolateral insertion of a
`long threaded cage over two posteriorly placed cages.
`The biomechanical difference between a single long
`threaded cage and one regular-length threaded cage in-
`serted posterolaterally was not investigated. Clinically, a
`single long threaded cage in comparison with a single
`regular-length cage has a larger purchase area (load bear-
`ing surface) and a larger region for potential bony in-
`growth. The addition of facet joint fixation increases the
`biomechanical advantage and when placed in an ex-
`tended position initially helps maintain lumbar lordosis.
`
`Conclusion
`
`In this study, an in vitro bovine model was used to ex-
`amine the immediate postoperative stability of a single
`long threaded cage placed posterolaterally versus two
`shorter cages placed posteriorly. Posterior lumbar inter-
`
`body fusion with a single posterolateral long threaded
`cage with unilateral facetectomy enabled sufficient de-
`compression while maintaining most of the posterior el-
`ements. In combination with a facet joint screw, ade-
`quate postoperative stability was achieved.
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`
`Address reprint requests to
`
`Nathaniel R. Ordway, MS, PE
`Department of Orthopaedic Surgery
`SUNY Health Science Center
`750 East Adams Street
`Syracuse, NY 13210
`Email: ordwayn@hscsyr.edu