Original ArticleA radiographic analysis of cage positioning in lateral transpsoas lumbar interbody fusion
Introduction
The lateral transpsoas approach is a novel minimally invasive technique for accessing the anterior vertebral column of the lumbar spine. Characterised by a unique lateral line of attack, it allows placement of cages spanning the entire width of the vertebrae without disrupting the normal stabilising ligaments of the spine. This affords a unique capability for enabling the interbody cages to engage the dense cortical bone at the apophyseal rings, thus reducing the risks of graft subsidence and maintaining indirect decompression unparalleled by any traditional anterior or posterior approaches.
Nevertheless, one major caveat in exploiting the advantages of this approach is the proximity of the lumbar plexus. As the nerves of the plexus descend within the psoas muscle, they migrate progressively anteriorly towards the centre of the intervertebral disc.1, 2, 3 As a result, the available operating window becomes increasingly narrow and shifted towards the anterior quadrants of the disc space as one approaches the lower lumbar segments. This in turn restricts the anteroposterior (AP) width of the cages implantable and potentially precludes a safe working zone particularly at L4-5.
To overcome this obstacle, specially designed surgical platforms incorporating tailor-made retractor and neuromonitoring system have been introduced in recent years to maximise access and promote safety. Conceivably systems that would allow positioning the working channel as close to the lumbar plexus as possible without risking neural injury would be most desirable in maximising the AP width of the operating window and the choice of larger cages. Currently there are two main surgical platforms known as eXtreme Lateral Interbody Fusion (XLIF, NuVasive Inc., San Diego, CA)4 and Direct Lateral Interbody Fusion (DLIF, Medtronic, Memphis, TN)5 for achieving this. While both systems similarly embrace the core techniques of triggered electromyographic (EMG) monitoring and the use of expandable tubular retractors to establish minimally invasive access through the psoas muscle, they differ significantly in their initial approach to the disc space and the establishment of the operating window.
In DLIF, the design is based on a pair of traditional two-blade expandable tubular retractors—the retractors were split into left and right halves allowing craniocaudal expansion (Fig. 1). The initial transpsoas approach targets at anchoring the muscle dilators to the centre of the disc space as guided by lateral fluoroscopy and evoked EMG monitoring. The retractor system (22 mm diameter) was then placed over the dilators and secured in place with a table-mounted arm assembly. In XLIF, the design is based on a three-blade system—the tubular retractors are split into left, right and centre blades. The initial approach aims to anchor the muscle dilators posterior to the centre of the disc space. The retractor system (12 mm diameter) was then inserted and fixed in position by anchoring the centre blade to the vertebral interspace with an intradiscal shim and also to a table mounted arm assembly. The surgical field can then be expanded by retracting the muscle with the right and left blades craniocaudally and anteroposteriorly.
Whilst these two systems are similarly based on the principles of tubular surgery, the difference in the design of the retractors implies an important distinction in their strategies for establishing the initial transpsoas entry. In DLIF, because of the concentric design, the centre of the operative window coincides with the initial docking position of the muscle dilators. Thus when searching for an initial safe passage through the psoas, good clearance of the lumbar plexus is mandatory as subsequent advancement of muscle dilators and retractors may unduly displace and stretch the lumbar plexus located posteriorly. In contrast, for XLIF, the initial entry is designed to be a posterior fixation point. The operative window is established by winding the left and right retractor blades anteriorly and craniocaudally. The final expanse of the operative window is anterior to the initial entry point.
Thus, collectively, when applying the DLIF retractors there may exist a tendency to place the initial dilator more anteriorly to safely avoid the lumbar plexus than when applying the XLIF retractors as the initial dilator marks the posterior fixation point in the latter and the lumbar plexus will be protected behind it. Currently no study has been performed to compare which system would favour wider operative window and cages. We therefore performed a radiographic study to evaluate quantitatively cage positioning in a cohort of patients who had undergone either a DLIF or XLIF procedure and conducted a multiple regression analysis to determine whether a difference exists between these two systems.
Section snippets
Methods
We performed a retrospective chart review to identify patients who had undergone either a DLIF or XLIF procedure from October 2012 to January 2015. During this period these two systems were used non-selectively by a single surgeon (TS) at our institution. The indications for surgery included spondylolisthesis, degenerative scoliosis, degenerative disc disease, canal and lateral recess stenosis, and adjacent segment disease. Patient parameters including treatment level, the presence of
Results
We identified 57 lumbar segments in 43 patients who had a lateral transpsoas interbody fusion performed through either DLIF or XLIF. The baseline characteristics are summarised in Table 1. Univariate analysis revealed no difference between DLIF and XLIF regarding patient demographics (sex and age), the spinal level treated, and the presence of spondylolisthesis. However, for DLIF, the normalised mean cage position at the L4-5 level was found to be 0.65 (0.60–0.69, 95% CI) compared with a mean
Discussion
The development of lateral transpsoas approach has been advanced by specially designed surgical platforms with tailored neuromonitoring console, muscle splitting retractor system, and long tapered instruments to allow a minimally invasive access to the anterior column of the lumbar spine. Our study has demonstrated for the first time that the choice of surgical platforms can influence radiographic outcome when performing lateral transpsoas interbody fusion to the L4-5 segment. Previous cadaver
Funding
No funding was received for this research.
Ethical approval
This study was conducted in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
Conflicts of interest
The authors have none to declare.
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