Minimally invasive surgery (MIS) techniques are well established in spine surgery and are now widely adopted. Once considered outside the mainstream, MIS techniques are an important part of the surgical armamentarium for spine surgeons today. Whether by consumer-directed or industry-based marketing, minimally invasive spine surgery is now highly sought after by many patients to address their spine pathology because of the commonly held perception that smaller incisions lead to better surgery. Today’s patients expect successful surgery to include resolution of pain and symptoms with less postoperative pain, short or no hospital stay, a quick recovery, and rehabilitation that is the least disruptive to their lives.¹ The reality of MIS techniques is that it can be technically difficult with a steep learning curve from the limited surgical exposure. This chapter will review the MIS surgical techniques as applied to the thoracolumbar spine.

A wide spectrum of MIS strategies is used with spine surgery, from mini-open, tubular, expandable, and percutaneous techniques (Figure 1). The main strategy is to limit the damage caused by surgical dissection by taking advantage of narrow surgical corridors to accomplish the goals of surgery. Because the surgical exposure is limited, the surgeon must use visual cues from radiographic imaging to successfully perform surgery, thereby increasing the radiation exposure to the surgeon, OR staff, and the patient. This requires a mastery of the three-dimensional anatomy of the spine and being able to translate the radiographic information to the spine to avoid neurological injury and complications. Not only is radiation exposure an issue, but also increased surgical times has been associated with MIS techniques, especially during the learning curve of the procedure. The various MIS techniques used in spine surgery will be reviewed in this chapter.

Pedicle screw fixation is the mainstay for fixation in the thoracolumbar spine because of the pedicle screw providing biomechanical fixation and stability in all three vertebral columns.² The open technique of pedicle screw placement requires full exposure of the bony landmarks, including the transverse process, facet joint, and the pars interarticularis to identify the start point for pedicle screw placement. The open exposure strips the paraspinal musculature off the spine, leading to denervation, devascularization, and ultimately muscle atrophy that may lead to late-onset pain and dysfunction despite a successful fusion.^3,4 In addition,
prolonged paraspinal muscle retraction greater than 60 minutes can produce muscle necrosis and neuromuscular degeneration, which can lead to increased pain and disability.⁵

Percutaneous techniques in pedicle screw placement use a muscle splitting approach to prevent detachment of the paraspinal muscles off the spine and reduce the muscle injury from retraction. Radiographic guidance with fluoroscopy allows the surgeon to determine the starting point for the pedicle. 1 to 1.5 cm Wiltse type incisions are made on each side for each level and a muscle-splitting approach is used. Cannulated Jamshidi needles are used to create a path through the pedicle and into the vertebral body (Figure 2). Guidewires are placed through the cannulated Jamshidi needles, and tubular retractors are placed using the guidewires to retract the paraspinal muscles directly at the site of the pedicle screw insertion. This technique preserves the motor nerve to the multifidus muscle 80% of the time in cadaveric studies, as compared with transection of the nerve 84% of the time with the open technique.³ The largest study to date evaluating the placement of over 2000 percutaneous pedicle screws demonstrated a pedicle breach rate of 9.4% with the majority of these measuring <2 mm.⁶ The percutaneous pedicle screw technique is accurate and versatile and can be used as a strategy for three-column vertebral fixation in many clinical scenarios including degenerative, spinal deformity, trauma, and tumor cases.

More than 60% of spine trauma cases are thoracolumbar fractures. In recent years, MIS has been used increasingly and frequently in this setting, especially for cases without neurological deficits. The primary aim of MIS in this setting is to restore segmental stability and spinal alignment while reducing the “approach-related” morbidities associated with open procedures and the long-term sequelae of fusions.

The first long-term (5-year) follow-up study of percutaneous pedicle screw fixation (PPSF) retrospectively compared the outcomes of PPSF (10 patients) and open surgery (11 patients) in thoracolumbar fracture cases.⁷ At follow-up, although patient functional scores such as the SF-36 did not significantly differ between groups, nor the loss of correction over time with implant removal, the average blood loss in the MIS group was significantly lower than that in the open group (155.6 vs 194.4 mL). A prospective RCT examined the efficacy of PPSF compared with the paraspinal approach—an open albeit relatively less-invasive procedure—in 61 unstable thoracolumbar burst fracture patients without neurological deficit but with posterior ligamentous complex injury.⁸ Again, although the two groups had similar back pain VAS and Oswestry Disability Index (ODI) scores at 3-year follow-up, and much less blood loss (79.0 vs 149.0 mL), PPSF did not correct the local kyphotic angle postoperatively as compared with the open paraspinal approach (0.39° vs 9.25°) and was also associated with a greater average degree of correction loss (3.35° vs 2.33°) at final follow-up. The authors, therefore, suggested using PPSF cautiously with ligamentous disruption, or when a postural reduction of fracture is not achievable during the operation. Another group compared PPSF (11 patients) and open surgery (10 patients) in single-level thoracolumbar burst fractures (Figure 3) and found that, although the ultimate degree of kyphosis, as well as postoperative improvement, did not significantly differ between the two techniques, patients treated with PPSF had significantly lower ODI scores than those treated with open surgeries (4 points vs 14 points), both before implant removal and at final follow-up.⁹

What seems certain is that, clinically, PPSF is almost consistently associated with less blood loss, lower infection rates, earlier mobilization, and in many cases, a faster return to work.¹⁰ Patient-reported functional scores and radiographic analysis, however, vary, and overall, do not appear dramatically different between open and MIS approaches. The risk and complications of PPSF appear to be more related to the instrumentation per se—screw misplacement with an incidence between 6.6% and 9.7% in a variety of spinal pathologies treated with MIS— than the traumatic nature of spine fractures.^11,12

In practice, it is now generally recommended that in thoracolumbar fracture patients without neurological deficits even with mild posterior ligamentous injury, PPSF without fusion can be considered if local instability is determined or nonsurgical treatments are not practically feasible.¹¹ In patients with significant posterior ligamentous disruption and/or significant deformity, however, a more invasive surgical strategy, either open surgery or PPSF combined with another approach, often with fusion, may be needed. Either way, the fundamental goals of treatment—to reestablish segmental stability and restore spinal alignment—remain the same and the surgical strategy selected is dependent upon the surgeon’s experience and preference.

Cement augmentation procedures such as kyphoplasty and vertebroplasty are commonly used to treat painful osteoporotic vertebral compression fractures (VCFs) and metastatic spine lesions. Most osteoporotic VCFs improve over 6 to 8 weeks after injury. However, some patients are afflicted with severe pain and disability that limits early mobilization. In these cases, polymethylmethacrylate (PMMA) cement can be percutaneously introduced into the vertebral body through minimally invasive techniques that are very
similar to the percutaneous pedicle screw technique.¹³ 2 to 3 mm incisions are made using fluoroscopic guidance, and Jamshidi needles are percutaneously impacted into the pedicles and posterior aspect of the vertebral body. Kyphoplasty involves introducing a balloon that is inflated with radiopaque contrast to create a void within the vertebral body and to help reduce the fracture, thereby improving the fracture morphology and alignment. PMMA cement is then injected into the bony void under low pressure. In contrast, vertebroplasty involves injecting PMMA cement into the vertebral body under higher pressure to fill the bony interstices. Complication rates are reported to be less than 1% and most commonly involve cement extravasation and infection.¹³ Neurological injury from cement leakage into the spinal canal is rare, as are cement embolism into the
lungs. A recent meta-analysis demonstrated no difference in adjacent vertebral fractures with cement augmentation as compared with conservative treatment.¹⁴

The scientific evidence on vertebroplasty has been mixed and highly controversial, starting with two double-blinded randomized clinical trials by Buchbinder et al and Kallmes et al that were published by the New England Journal of Medicine with much publicity.^15,16 These RCTs compared vertebroplasty to sham procedure and demonstrated no difference in pain or disability. These studies included a heterogenous patient population with subacute and chronic VCFs, which biases the results because of the favorable natural history of VCFs. In contrast, RCTs that compared vertebroplasty with standard medical management demonstrated greater pain reduction with vertebroplasty that was sustained through the short term.^17,18 These studies were not blinded, however, leading to questions of a possible placebo effect confounding the results. A recent Cochrane review concluded that the use of vertebroplasty in osteoporotic VCFs was not supported based on the available high to moderate quality evidence with no clinical benefit seen when compared with sham procedure or when performed in the acute period <6 weeks from onset.¹⁹ In addition, sensitivity analyses demonstrated that the trials comparing vertebroplasty with medical management were likely overestimating the actual clinical benefit. The evidence for kyphoplasty is even less robust than vertebroplasty with only one large multicenter RCT demonstrating significant pain reduction and disability with kyphoplasty when compared with medical management that was sustained up to 2 years follow-up.²⁰ A recent meta-analysis found that both kyphoplasty and vertebroplasty were efficacious in pain relief and functional improvement and kyphoplasty was superior in pain relief in the short term with better improvement in kyphotic angles.²¹ Taken together, the scientific evidence demonstrates that cement augmentation procedures are efficacious and safe for a select population of patients with osteoporotic VCFs afflicted with severe pain and disability during the acute period.

With the advent of the lateral lumbar interbody fusion (LLIF) technique, spine surgeons have the opportunity to access the anterior column of the spine from T12 to L5, which previously required an anterior approach with a vascular access surgeon.^22,23 A wide array of pathology can be addressed with the LLIF technique, including degenerative spondylolisthesis, trauma, and adult spinal deformity. Limitations of the technique are due to the position and relationship with the iliac crest to the disk space, morphology and location of the iliopsoas musculature, and the position of the femoral nerve and lumbar plexus relative to the disk space, specifically at L4-5. Triggered EMG neuromonitoring can reduce the approach-related morbidity such as femoral nerve and lumbar plexus injury. Specialized curved instruments and equipment can aid in performing the diskectomy and end plate preparation to accomplish successful fusion.

However, the L5-S1 disk space is not accessible with the lateral approach and generally requires an anterior approach because of the anatomic relationship of the iliac vasculature and the iliac crests. In addition, extensive retroperitoneal scarring from prior surgery and transitional anatomy in which anterior psoas and lumbar plexus drift may preclude the LLIF technique to access the disk space safely. Neurological injury to the lumbar plexus and complications related to the psoas dissection can occur, especially at the L4-5 level where the femoral nerve and lumbar plexus is typically adjacent to the midpoint of the lateral disk space. Thigh paresthesias, dysethesias, numbness, and hip flexor weakness can occur up to 23.7% to 30% of cases, which are typically temporary, with up to 90% of cases resolving within 1 year postoperatively.^24,25,26