Minimally Invasive Posterior Lumbar Interbody Fusion

The traditional approach for posterior lumbar interbody fusion (PLIF) requires a large dorsal midline exposure with substantial manipulation of overlying ligamentous and muscular processes. When compared to other approaches to the lumbar spine, PLIF results in the highest complication rate, possibly owing to the broad exposure that is traditionally used.¹ This rate makes the possibility of successful minimally invasive approaches particularly attractive. Few studies have examined MIS PLIF, and none have directly compared traditional PLIF to MIS PLIF.

One of the first and most illustrative reports of the use of MIS to limit collateral soft tissue damage during PLIF was presented in 2002 by Khoo et al.² Briefly, small incisions were made between 2 and 4 cm from the midline bilaterally at the level of interest (Fig. 60-1). Under fluoroscopic guidance, sequentially larger dilators were then placed over a K-wire to ultimately place a tubular retractor that provided a working corridor 15 to 20 mm in diameter.^3,4 Decompression, hemilaminotomy, and discectomy were then performed endoscopically. During interbody graft placement, the authors used endoscopic visualization to ensure neural element retraction while working within the tubular system.

FIGURE 60-1 Posterior lumbar interbody fusion (PLIF) dilation/tubular retraction. METRx Tubular Retraction System for PLIF.

(Courtesy of Medtronic, Minneapolis, MN.)

Percutaneous pedicle screw-rod systems were used in concert to complete the instrumented arthrodesis. Generally (and as reported in all patients in the cited study), cannulation of the pedicles can be performed by using the same incisions that are made for the purpose of decompression after removal of the tubular retractor and endoscope.² Fluoroscopy was utilized while a Jamshidi needle was advanced to the pedicle and subsequently swapped for a K-wire using the Seldinger technique. In lieu of traditional fluoroscopy, a fluoroscopic navigation system was used to track the needle in the sagittal, coronal, and axial planes. A tap was passed over the K-wire, and the multiaxial pedicle screws (Sextant, Medtronic Sofamor Danek) were attached to screw extender sleeves and passed over the K-wire into the screw pathway.² The remainder of the screw-rod assembly involved mating of the ipsilateral pedicle pairs using Sextant precontoured rods.

Clinical data from this series of three patients undergoing L4-5 MIS PLIF for treatment of bilateral L5 radiculopathies clearly demonstrate the steep learning curve necessary for the MIS procedure. The first operative case took 6 hours and 15 minutes to complete, but time spent in the final case had been whittled down to 4 hours and 30 minutes. Furthermore, blood loss decreased from 208 to 110 mL, as did the length of hospital stay for each patient (from 5.4 to 3 days). The authors cited 100% fusion at 8-month follow-up and an observational decrease in the amount of morphine equivalence units with the MIS approach when compared to previous unpublished open PLIF data.

Another clinical series in 2002 reported positive outcomes in 10 patients who underwent MIS PLIF with pedicle screw fixation.⁵ Again, operative time was found to be greatest during the familiarization period and declined with time. Half of the patients were discharged on postoperative day 1 or 2, the remainder being discharged on the third postoperative day. Solid fusions were documented in all patients at 13.8 months of mean follow-up time. In 2007, Park and Ha compared 32 patients who had been treated with traditional one-level PLIF with 29 who had undergone MIS PLIF.⁶ Consistent with the aforementioned comparison, there was no statistical difference in fusion rates at 1-year minimum follow-up. The MIS group did show statistically significant differences in postoperative and intraoperative metrics. These included decreased blood loss, postoperative pain, recovery time, and hospital stay.

Interestingly, a recent study has examined the MIS placement of percutaneous pedicle screws using a miniature robotic system.⁷ In this procedure, 31 patients received a PLIF with decompression, discectomy, and polyetheretherketone (PEEK) cage implantation along with percutaneous pedicle screw fixation. The SpineAssist (MAZOR Surgical Technologies Ltd., Israel) system was utilized, in which an image-correlated framing system provided an edifice for spinous process clamping and screw guidance. The authors used a modified frame to support pedicle screw fixation and computer-based trajectories. In 29 of 31 cases, seamless percutaneous pedicle screw placement was achieved, with deviations from surgical trajectories of less than 2 mm in the vast majority of cases. While this study was an interesting proof of concept and demonstrated precision in percutaneous screw placement, it utilized a modified MIS approach and did not provide outcomes for the patient population. Further work with this or similar guided systems may be of interest in the future to expand MIS PLIF approaches.

Transforaminal Lumbar Interbody Fusion

As a variation of dorsal lumbar fusion, transforaminal lumbar interbody fusion (TLIF) utilizes a far lateral, or transforaminal, approach that can be applied throughout the lumbar spine. Additionally, complications related to neural element retraction are minimized in comparison to PLIF¹ as TLIF does not necessitate retraction of the traversing nerve root during the discectomy and cage/graft placement. Furthermore, TLIF is not limited to levels L3-4 and below, as is the case with PLIF. When compared to MIS PLIF, MIS TLIF uses a similar but more lateral approach, giving way to an eventual facetectomy, along with complete resection of the inferior and superior articulating facets of the inferior and superior vertebrae, respectively. Discectomy, preparation of end plates, pedicle/facet screw placement, and interbody graft placement are then performed in the usual manner.

Schwender et al.⁸ first reported the results of 49 patients who had undergone TLIF with percutaneous pedicle screw placement. Fusion was reported in all patients at a minimum follow-up of 18 months, and narcotic use was discontinued on average between 2 and 4 weeks postoperatively. Other clinical indicators were similarly favorable, with an average blood loss of 140 mL and an average Oswestry Disability Index (ODI) score that dropped from 46 preoperatively to 14 at last follow-up. Jang and Lee also presented a cohort of 23 patients undergoing MIS TLIF with both ipsilateral pedicle screw and contralateral facet screw fixation.⁹ In this series, 21 of 24 patients achieved fusion at follow-up (mean of 19 months), with an average blood loss of 310 mL and ODI scores dropping from 33.1 to 7.6 after surgery.

In 2009, Peng et al. presented one of the most comprehensive comparisons of MIS versus open spine surgeries, comparing 29 MIS TLIF procedures to 29 open TLIF procedures.¹⁰ The benefits of MIS TLIF were most pronounced in the immediate postoperative period and included less blood loss, a shorter hospitalization period, diminished postoperative pain, and subsequent lower analgesic use (P < .05 for all parameters). However, the MIS procedure also required longer operative and fluoroscopic exposure time. Importantly, follow-up at 6 months and follow-up at 2 years were statistically equivalent when quality-of-life metrics and fusion rates were compared. The authors concluded that MIS TLIF had “retained” positive long-term outcomes associated with TLIF but with fewer immediate postoperative complications (13.8% vs. 6.9%, open vs. MIS). It should be noted that three quarters of complications in the open group likely arose from immobility (two urinary tract infections and one case of atelectasis) and were not directly attributable to the surgical procedure.

Lateral Interbody Fusion

In 2004, Bergey et al. described an endoscopic lateral transpsoas approach to the lumbar spine in 21 patients undergoing discectomy and fusion.¹¹ A modification of McAffee’s endoscopic approach,¹¹ the true lateral approach provides a retroperitoneal approach to the lumbar spine without trespassing into the peritoneum or dissecting the great vessels.¹² The exposure typically requires the patient to be in the right lateral decubitus position (Fig. 60-2). An incision is made along the lateral border of the lumbar paraspinal muscle, and a finger dissection is performed through the retroperitoneal space to the psoas.³ With the use of fluoroscopy to determine the proper starting point along the flank, a counterincision is made directly lateral to the disc space(s) of interest. The dilators are guided to the psoas using the initial incision, and a corridor is developed through the psoas to the disc space of interest. Continuous electromyogram monitoring is utilized to avoid injuring the exiting lumbar nerve roots, which also pass through the psoas muscle. A final expandable retractor is then placed over the final dilator, providing adequate access to the disc space. The discectomy and placement of the interbody graft are then followed by placement of a side plate and/or dorsal fixation of some kind (i.e., pedicle screws, translaminar or facet screws).

FIGURE 60-2 Extreme lateral interbody fusion procedure. A, Incision over the psoas. B, Sequential dilation through the retroperitoneal space. C, Insertion of the implant.

(From NuVasive, San Diego, CA, with permission.)

While a lateral approach decreases the likelihood of complications arising from anterior lumbar interbody fusion, it presents a unique set of challenges. First, it is essential to note the location of the lumbosacral plexus, as it migrates ventrally from L1 to L5¹³ and is at significant risk when approaching the L4-5 interspace.¹⁴