The mean follow-up was 22.6 months with a range of 18-28 months. There were no conversions from the minimally invasive approach to an open surgery. Bilateral pedicle screws were placed in all cases. The average operative time was 240 minutes (range 110-310 minutes). The mean estimated blood loss was 140 mL (range 50-450 mL). The average length of stay was 1.9 days (range 1-4 days). All 45 patients presenting with preoperative radiculopathy had resolution of their symptoms postoperatively. All patients with mechanical low back pain noted improvement of their pain. Typically, patients progressively increased their activity levels and resumed full activity at 3 months postoperatively. Outcomes were quantified using the VAS and the Oswestry Disability Index. From a preoperative average of 7.2 cm, the VAS score decreased to 2.2 cm at 1 year postoperatively and 2.1 cm at last follow-up. The average Oswestry score decreased from 46% preoperatively to 18% at 1 year and 14% at last follow-up. Narcotic use was discontinued, on average, between 2 and 4 weeks postoperatively. At a minimum of 18 months' follow-up, all cases appeared to have solid radiographic fusions as judged by the presence of trabecular bony bridging, <3° motion on flexion-extension views, and intact hardware.
There were a total of four complications in the series. Two patients required repositioning of misplaced pedicle screws. There were no neurologic sequelae as a result of this. Two other patients developed radiculopathies postoperatively: one due to graft dislodgement and a second to contralateral neural foraminal stenosis. These new radiculopathies resolved with reoperation.
This patient sustained a distractive flexion injury at L4-L5 in a motor vehicle accident (he was wearing a lap belt but no shoulder harness). He presented 1 week post trauma with mechanical low back pain and bilateral lower extremity pain. Figure 5A is a plain lateral radiograph of the injury. Figure 5B is a magnetic resonance image (MRI) of the same patient that reveals discoligamentous disruption at L4-L5 and retropulsion of disc material into the spinal canal. The patient underwent a minimally invasive TLIF via a 22-mm-diameter tubular retractor placed from a left-sided approach. The herniated disc material was removed, and the interbody fusion was supplemented with percutaneous pedicle screws and rods. The patient's surgery was uneventful. He was discharged on postoperative day 2 and returned to school 1 week later. Figure 5C and D are AP and lateral radiographs taken 2 years postoperatively. They demonstrate a solid fusion, intact hardware, and re-establishment of lumbar lordosis with good maintenance of disc space height. The patient's skin incisions are shown in Figure 5E. The 1-in incision on the left was the site of the METRx tubular retractor placement and subsequent placement of percutaneous pedicle screws (once the tubular retractor had been removed). The pedicle screws on the right were placed through two separate 0.5-in incisions, as the patient's spinal deformity did not reduce with simple positioning and the divergent pedicles at L4-L5 required significantly different trajectories for screw insertion.
The TLIF procedure was popularized by Harms as a method to achieve interbody lumbar fusion through a unilateral posterior approach.1,2 As with the PLIF procedure, TLIF offers a number of potential benefits over conventional posterolateral intertransverse arthrodesis, including increased fusion surface area, copious fusion blood supply via cancellous vertebral body bone, complete access for medial and lateral decompression, and restoration of intervertebral body height.6 However, PLIF has been associated with a significant rate of neurologic injury secondary to the retraction and manipulation of the neural elements that are required to access the disc space.19-21 Ray20 described a series of 236 patients who underwent posterior placement of an interbody cage and noted a 10% incidence of transient foot weakness postoperatively.
The transforaminal approach of TLIF provides access to the disc space without the need for significant retraction of the nerve roots or thecal sac. Additionally, the lateral approach makes revision surgeries less challenging, as there is less need to mobilize the nerve roots away from scar tissue. The TLIF approach also preserves the posterior longitudinal ligament complex as well as other midline supporting bony and ligamentous structures, which are frequently disrupted during the PLIF procedure. Last, TLIF is a unilateral procedure and therefore avoids the need for bilateral dissection within the epidural space.
Rationale for Minimally Invasive TLIF
Open instrumented lumbar fusion procedures are associated with significant morbidity related, in large part, to the iatrogenic soft tissue and muscle injury that occurs during routine surgical exposure. The deleterious effects of the extensive muscle stripping and retraction have been well documented in the medical literature.8-13,22 These undesired side effects of lumbar surgery occur so commonly that the term “fusion disease” has been coined to describe their occurrence. Kawaguchi et al8,9 evaluated the effects of retractor blade pressure on the paraspinous muscles during surgery. They found that elevated serum level of creatine phosphokinase MM isoenzyme, a direct marker of muscle injury, is related to the retraction duration and pressure. This was corroborated by Styf et al,13 who determined that the retractor blades may, in fact, increase intramuscular pressure to ischemic levels. This iatrogenic muscle injury can cause long-term problems that can negate the beneficial effects of the surgery itself. Rantanen et al11 concluded that patients who had poor outcomes after lumbar surgery were more likely to have persistent pathologic changes in their paraspinous muscles. Mayer et al10 evaluated trunk muscle strength in patients with previous lumbar surgery and determined that patients who had undergone fusion procedures were significantly weaker than discectomy patients.
The goal of minimally invasive spinal surgery is to achieve the same objectives as the comparable open procedure via a less traumatic approach. Although lessening the approach-related morbidity is a primary aim of minimally invasive spine surgery, this must be accomplished without compromising the efficacy of the procedure.
In the current study, the authors successfully performed the minimally invasive TLIF procedure in 49 patients using the METRx tubular retractor system. This system evolved from the MED procedure and was initially developed for microdiscectomy; customized instruments as well as larger tubular retractors have made the TLIF procedure feasible.15 Serial dilation of the paraspinous operative corridor allows the surgeon to dissect through the muscle and fascia with minimal tissue trauma. Percutaneous pedicle screws can be placed through the same incisions.17,18
The creation of a working channel between the muscle fibers permits access to the bony anatomy without the need for muscle stripping, unlike the open TLIF procedure. As a result, the estimated blood loss in our series averaged only 140 mL, including pedicle screw placement. Blood loss during conventional lumbar fusion surgery can be quite significant; in fact, patients commonly donate autologous blood preoperatively or a cell saver is used during the surgery. None of the 49 patients required a blood transfusion. They appeared to have less postoperative pain following the minimally invasive TLIF procedure as compared with similar open procedures. This likely underlies the relatively short average hospital stay in this series of <2 days. Moreover, narcotic use was completely discontinued, on average, between 2 and 4 weeks postoperatively.
Although there are many potential benefits to the minimally invasive TLIF procedure, the technique does have its drawbacks and limitations. As with any new surgical technique, there is a learning curve that must be surmounted before technical proficiency can be achieved. This is particularly true for minimally invasive procedures, where anatomic disorientation can occur because standard landmarks that are visualized during open procedures may be unexposed. Minimally invasive TLIF is more technically demanding than open TLIF because of the smaller working area and the need for longer and bayoneted surgical instruments. Furthermore, placement of percutaneous pedicle screws requires the surgeon to be able to accurately interpret AP and lateral fluoroscopic images to safely insert these devices.
There were four complications in this series, which occurred fairly early in our experience. Two were secondary to screw misplacement, and the remaining two were because of graft dislodgement and contralateral neural foraminal stenosis. Screw misplacement can be minimized by attention to anatomic detail. Use of intraoperative electromyography might also prove helpful in avoiding this potential complication. Compression of the screw-rod construct at the end of the case, in conjunction with pre-existing (although asymptomatic) contralateral lateral recess stenosis, likely led to the occurrence of contralateral radiculopathy. This could be minimized by avoiding overcompression of the screw-rod construct. Also, when neural compression is present on the side contralateral to the TLIF approach, consideration should be given to direct decompression of the neural structures on that side. This can be accomplished by inserting a tubular retractor through the contralateral incision prior to contralateral percutaneous pedicle screw placement.
Although technically demanding, minimally invasive TLIF offers a number of potential advantages over traditional open, dorsal lumbar fusion techniques. The procedure is feasible and can be performed with a relatively low complication rate. Our preliminary results appear promising. Further analysis and follow-up will be required to determine rates of fusion at 2 years and long-term outcomes.
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Keywords:© 2005 Lippincott Williams & Wilkins, Inc.
lumbar; fusion; minimally invasive; transforaminal lumbar interbody fusion