Evaluation of unilateral cage-instrumented fixation for lumbar spine

Chronic discogenic back pain caused by degenerative disc disease is a common ailment in the general population [1]. The degenerative often results from arthritic changes in the intervertebral discs, facet joints, and ligaments surrounding the vertebral canal [2]. In clinical practice, lateral recess stenosis and foraminal stenosis may induce nerve root compression which can cause unilateral symptoms. Unilateral PLIF may be satisfactory in patients with unilateral symptom. This study evaluated unilateral fusion in patients with unilateral symptoms.

Posterior lumbar interbody fusion (PLIF) has proven successful for relieving motion-induced discogenic pain and was once considered standard treatment for degenerative disc disease [3‐5]. A successful PLIF can restore disc height, decompress the dural sac and nerve roots, immobilize the unstable intervertebral disc, and maintain load-bearing to anterior structure [6]. Bilateral PLIF is associated with increased complication rates [7‐9]. Elias and coworkers [7] reported a 15% incidence of dural tear and postoperative radiculopathy. It typically caused by excessive epidural bleeding and prolong or excessive retraction. In 1982, Harm et al. [10] popularized the surgical technique of transforaminal lumbar interbody fusion (TLIF). Its advantages are posterior epidural approach with interbody support and bilateral posterior segmental pedicle screw fixation. Biomechanically, preserving longitudinal ligaments is though to preserve stability, to prevent implant dislocation, and to place a compressive force on adequately sized intervetebral implants [11‐13]. This technique also avoids retraction of the ligamentum flavum as well as scarring of the spinal canal. Nevertheless, its long learning curve, prolong radiation exposure and high cost make it unsuitable for health insurance coverage in Taiwan.

Performing unilateral PLIF using a single interbody cage has several advantages. Inserting a single interbody cage through a unilateral approach compromises fewer anatomic structures than two cages inserted through a bilateral approach. However, it is unknown that unilateral PLIF can achieve biomechanical stability as the bilateral PLIF. Suke et al. [14] found that unilateral pedicle screw fixation was as effective as bilateral pedicle fixation in lumbar spinal fusion independent of one or two levels. Their conclusions cannot extend to the cage-instrumented PLIF due to the different boundary of decompression (especially facetectomy). Tencer et al. [15] demonstrated that two PLIF structural devices produced a greater reduction in torsional stiffness than single PLIF device. Chen et al. [16] demonstrated that unilateral fixation with two cage insertion is a feasible alternative in spinal surgery. Wang et al. [17] showed that an oblique insertion of a single BAK cage in instrumented PLIF might reduce exposure and enable precise implantation. These articles could not provide enough evidence to support that unilateral fusion can supply similar stability as the bilateral fusion. That is why unilateral PLIF did not become routine procedures in the treatment of degenerative lumbar spine disease. Goel [18] demonstrated that the unilateral plate system causes coupled motions due to its inherent asymmetry and was unlikely to provide sufficient rigidity in fresh cadaveric human spines. Rotational deformity of lumbar spine might develop if inherent asymmetry persisted. They considered complete excision of the disc was required. Unilateral cage-instrumented PLIF might overcome inherent asymmetry. From the previous review, this study was conducted with two aims. One was to know the biomechanical stability between unilateral and bilateral cage-instrumented PLIF. The other was to determine the unpleasant effect of couple motion resulting from inherent asymmetry was present or not in the unilateral group.

The difference in ROM of right lateral bending (6.45 ± 2.40 vs 6.29 ± 2.17) and axial rotation (0.23 ± 0.21 vs 0.28 ± 0.31) did not significantly differ after transpedicle screw as compared with the left side.

The availability of human vertebral column segments is often limited due to the ethical, religious reasons as well as increasing legal restrictions [24]. Therefore, readily available animal vertebral columns were used for testing. Calf, pig and sheep are the three popular species used models to test spinal implants [25, 26]. The calf is the species mainly used in vitro test for the pedicle screw system. Similar to the calf, the pig is also often used in vitro test to measure intradiscal pressure. The sheep and pig is mainly used in vivo test for biomechanical experiments [26]. Sheep spines have also been used in vitro to study the initial stabilizing effect of spinal implants in the lumbar [27]. The cross-section of six to seven lumbar vertebrae in sheep resembles that in humans [28, 29]. The sheep lumbar spine and the human spine also have similar intervertebral disc morphology and spinal musculature anatomy [28]. Sheep pedicles have a smaller diameter, pedicle screw for example need to be shortened to fit the sheep vertebral dimension [25]. Considerable progress in research is apparently needed to achieve satisfactory understanding of the biomechanics of the human lumbar spine before clinical application under the result of animal experiment [25].

In vitro testing is an essential for studying spinal mechanics [30, 31]. Several parameters may be obtained through biomechanical tests of flexibility to quantify mechanical properties [30]. Such parameters include range of motion (ROM) and neutral zone (NZ). In biomechanical study, the ROM represents the stability of the specimen before and after additional procedures (including destructive and stabilizing procedures). The NZ indicates the laxity around the neutral position of a motion segment as well as residual deformation after removing a defined pure moment load from a motion segment. Mimura et al. [31] revealed that, in flexion-extension and lateral bending, ROM decreases and NZ increases during disc degeneration. In the early stage of disc degeneration, ROM increases while in the late stage of disc degeneration, ROM decreases. If only ROM is used as the measurement parameter, misinterpretations are likely. Previous in vitro studies indicated that NZ typically increases after experimentally induced injuries [32, 33], and that it decreases with the addition of muscle forces and spinal instrumentation [34]. In the ROM and NE, bilateral cage-instrumented group's transpedicle screw insertion procedure did not demonstrate significantly difference in comparison with the unilateral group in flexion-extension, lateral bending and axial rotation direction except the ROM of the rotation. Fortunately, rotation movement occupied little portion of the daily activity. The major portion of daily activity is flexion-extension and lateral bending. Unilateral cage-instrumented PLIF have similar biomechanical stability to bilateral cage-instrumented PLIF in this sheep spine model. Studies have demonstrated increased complications associated with bilateral PLIF [7‐9]. Bilateral instrumental fusion technique requires wide dissection, which jeopardizes the function of the paraspinal muscle. The unilateral approach can substantially reduce exposure requirements. This technique is easier than routine bilateral cage-instrumented PLIF. When treating unilateral sciatica patients, the cage can be placed from the symptomatic side so as to avoid retraction of the nerve root and dura sac of the asymptomatic side.

Goel [18] demonstrated the difference in rigidity between unilateral and bilateral instrumentation in fresh cadaveric human spines. They reported that the unilateral plate system causes coupled motions due to its inherent asymmetry and was unlikely to provide sufficient rigidity for decompressive procedures and would therefore require complete excision of the disc. Unilateral instrumentation is unadvised due to the effect of inherent asymmetry. In the current study, no significant differences were shown between right and left side in the lateral bending and axial rotation in the fusion procedures. This method may achieve sufficient stability. The reason is that the inserted cage can be held by the screw-rod system, and the cage can maintain adequate disc space, which is particularly effective for correcting instability associated with degenerative disc disorder.

There are some limitations in this study. First, the paraspinal muscles were remved; however, these muscles had an in vivo stabilizing effect therefore, our results probably overestimated the destabilizing effect of the segmental bone and other soft tissue alternation. Second, in this in vitro test, the results only reflect acute postoperative stability with relatively few loading cycles and are not necessarily indicative of repetitive loading cycles in the human spine. The biological effects of the potential healing process are unpredictable. Third, subsidence is a complication of cage sinking with loss of normal intervertebral height. The mean subsidence was greater in the one cage model because of the less contact area than the two cage model. An in vivo study may be necessary to show whether unilateral group had higher incidence of subsidence or not.

In conclusion, based on the results of this study, unilateral cage-instrumented PLIF and bilateral cage-instrumented PLIF have similar stability in the sheep spine model. The unilateral approach can substantially reduce exposure requirements. It also offers the biomechanics advantage of construction using anterior column support combined with pedicle screws just as the bilateral cage-instrumented PLIF. The unpleasant effect of couple motion resulting from inherent asymmetry was absent in the unilateral group.