6 Rationale for Minimally Invasive Spine Surgery

Minimally invasive spinal surgery (MISS) has experienced an exponential growth over the last decade. A growing number of spine surgeons now routinely utilize MISS techniques to treat a wide range of spinal pathologies that traditionally required large incisions with wide tissue dissections. With the continuing innovation and improvements in minimally invasive instrumentations and techniques, MISS is altering treatment paradigms across the whole realm of spine surgery.¹ There has been revolution in the treatment of spinal pathologies from the time before the advent of metallic fixation as compared to the present era of spinal surgery as can be seen in Fig. 6.1.

6.2 Goals and Benefits of MISS

The primary goals of MISS are to reduce approach-related tissue injury and complications in order to reduce postoperative pain, blood loss, and recovery time while achieving the same clinical objectives.^2,3 Preclinical, histological, serological, radiological, and clinical outcome data have demonstrated clear evidence of extensive iatrogenic tissue injuries associated with typical open posterior spinal approaches.⁴ Further evidence has emerged to reveal significant reductions in muscle injury, pain, and disability with MISS techniques compared with open techniques for lumbar fusion.⁵ With traditional open posterior approaches for lumbar decompression, extensive injury to the paraspinous soft tissue is common and midline ligaments are completely removed during the exposure. The result of these disruptions to the patient’s native anatomy can lead to substantial pain and muscular atrophy.^6,7 Several studies have documented negative effects of paraspinal muscle injury including weakness, disabilities, and pain. The MISS techniques are less destructive to the paraspinal muscles and have the potential to facilitate better clinical outcomes ( Fig. 6.2).

While initial applications of MISS were limited to conditions such as discectomy and decompression,^2,8 its applications have been expanded to include a wide variety of more complex pathologies including: traumatic spinal fractures, spinal column and spine cord tumors, and spinal deformity.^{9,10,11,12,13} Given the emphasis on “evidence-based medicine” and “cost-effectiveness” in the current health care environment, it is important to understand the rationale and available evidence that demonstrates advantages of MISS for various pathologies. This chapter will summarize the rationale for application of various MISS techniques for commonly treated pathologies of the cervical, thoracic, and lumbar spine. The purported benefits of MISS are summarized below.^{1,2,3,14,15,16}

Fig. 6.1 (a) Ancient historical treatment of deformity by traction-based apparatus inset treatment of deformity with multi-segmental instrumentation. (b) Current minimally invasive correction of spondylolisthesis using (c) minimally invasive techniques.

Fig. 6.2 (a) Postoperative magnetic resonance imaging (MRI) sagittal (left inset axial image) showing extensive posterior scar tissue and (right inset axial image) adjacent-level stenosis created by extensive removal of lamina and spinous processes following traditional laminectomy for stenosis. Compare (b) preoperative sagittal and axial T2-weighted MRI showing lumbar stenosis treated with minimal invasive laminectomy with (c) postoperative sagittal and axial MRI demonstrating adequate decompression with preservation of the spinous processes and minimal scar formation and tissue damage.

The anterior cervical microforaminotomy and the endoscopic-assisted anterior cervical discectomy and fusion are the two most commonly used MISS procedures in the anterior cervical spine. The anterior cervical microforaminotomy was first described in 1968 by Verbiest¹⁷ for the treatment of vertebral artery insufficiency. In essence, Verbiest’s approach is a slightly lateralized modification of the standard anterior approaches as described by Cloward, Robinson, and Smith.¹⁸ However, popularity of this approach for the treatment of spondylotic cervical radiculopathy has been limited due to the concerns for injuries to the vertebral artery and the intervertebral disc complex. Most recently, this approach has been popularized in the literature by Jho,^19,20 who applied modern microsurgical techniques and instruments to the operation with great success. However, the excellent clinical results and the already “minimally invasive” nature of the traditional anterior cervical approach,¹⁸ along with the lack of reproducibility of the results of anterior endoscopic approaches as reported by Jho,^19,20 have impeded the widespread application and adoption of minimally invasive anterior approaches of the cervical spine.

6.3.2 Posterior Approaches

A number of cervical spine pathologies can be effectively treated with posterior decompression. The posterior cervical approach avoids many complications associated with the anterior cervical approach, including esophageal injury, recurrent laryngeal nerve paralysis, dysphagia, and adjacent-level disease.²¹ While the standard open posterior cervical approach can achieve excellent decompression of the lateral recess and neural foramen, the exposure often requires extensive stripping of paraspinal musculature that can lead to significant postoperative pain, muscle spasm, and dysfunction.²² In addition, disruption of ligaments and the posterior tension band can contribute to development of kyphotic deformity postoperatively; the fear of this complication prompted many spine surgeons to fuse the spine along with decompression or to perform laminoplasty instead ( Fig. 6.3).

Minimally invasive approaches have been developed to avoid these pitfalls of open surgery. Developments in percutaneous surgical access, optical technology with high-magnification endoscopes, neuroanesthetic techniques, and noninvasive imaging modalities have brought posterior foraminotomy into a new millennium of spine surgery. In 2002, the senior author²³ reported the initial experience of posterior cervical microendoscopic foraminotomy (CMEF) in a case–control cohort study where 25 patients with cervical root compression from either foraminal stenosis or disc herniation were compared to another 26 patients treated via open cervical laminoforaminotomy. CMEF cases had less blood loss (138 vs. 246 mL per level), recovered more rapidly, had a shorter postoperative stay (20 vs. 68 hours), and needed fewer narcotics (11 vs. 40 equivalents). There were two durotomies after CMEF. Overall, the CMEF procedure yielded symptomatic improvement for approximately 87 to 92% of patients, depending on which symptom was analyzed. After CMEF, the patients with radiculopathy experienced resolution of their symptoms in 54%, improvement in 38%, and no change in 8% of cases. For open surgery, radiculopathy resolved in 48%, improved in 40%, and remained unchanged in 12%. For neck pain, the CMEF results were 40% resolved, 47% improved, and 13% unchanged. Open results for neck pain were 33% resolved, 56% improved, and 11% unchanged. Overall, the posterior CMEF technique yielded clinical results equivalent to those of the open surgical group; however, CMEF patients had less blood loss, shorter hospitalizations, and a much lower postoperative pain medication requirement.

Several other studies have reported excellent clinical results and have endorsed the clinical efficacy and advantages of minimally invasive cervical foraminotomy.^24,25,26 While a number of studies have utilized tubular retractors and microscopes instead of endoscopes as described initially, the overall technique remains the same. In addition to foraminotomy, posterior cervical endoscopic and percutaneous techniques have also been used for more extensive procedures including laminectomy.²⁷

6.4 Minimally Invasive Approaches in the Thoracic Spine

6.4.1 Thoracic Disc Herniation

The management of symptomatic thoracic disc herniation (TDH) has significantly evolved along with the technological advancement in the field of spine surgery. Better understanding of the pathology and evolution of minimal access techniques have provided newer treatment options with decreased morbidity as compared to open posterior and transthoracic approaches. Several minimally invasive posterolateral and anterior approaches have been developed in the last decade to treat various TDH pathologies with the selection of best approach dependent on mediolateral localization of the disc herniation, presence or absence of calcification, health of the patient, and experience of the surgeon.²⁸ There is absence of randomized controlled studies comparing minimally invasive approaches to open approaches for TDH, with most of the evidence being restricted to level III studies. Posterior approaches can be employed at any level, and its utility depends largely on the location of the disc herniation. The laterally located disc herniation is much more accessible via the posterior approach. The posterior approaches are much better tolerated by patients with medical comorbidities, especially in patients with existing pulmonary disease.²⁸ Chi et al²⁹ compared the minimally invasive transpedicular approach using tubular retractors with open posterolateral approach. The authors concluded that a mini-open transpedicular discectomy for TDHs results in better modified Prolo scores at early postoperative intervals and less blood loss during surgery than open posterolateral discectomy.

Fig. 6.3 (a,b) Postoperative plain X-ray images and incision with atrophy of paraspinous muscles seen after multilevel cervical laminectomy, fusion, and instrumentation. However, it appears this condition is better tolerated in the cervical spine compared to the lumbar spine, most likely due to less loading forces.

However, a centrally located, giant calcified TDH is best treated via an anterior approach to maximize visualization and to minimize spinal cord manipulation.³⁰ Bartels and Peul³¹ compared mini-thoracotomy (mini-TTA) versus thoracoscopy for the treatment of calcified thoracic herniated disc. Seven patients underwent a thoracoscopy, and 21 patients underwent mini-TTA. There was no statistically significant difference in duration of surgery, duration of the necessity of a chest drain, intraoperative blood loss, or duration of the postoperative stay on the intensive care unit (ICU) between the two groups. The authors concluded that though both the techniques had similar clinical outcomes, the mini-TTA had some theoretical advantages over a thoracoscopy allowing classic microsurgical bimanual techniques ( Fig. 6.4).

The key driver for minimally invasive anterior or lateral approaches for TDH has been the fact that morbidities associated with thoracotomy are significant, which may include postthoracotomy pain in as many as 50% of patients and continuing 4 to 5 years postoperatively in 30% of patients.³² Thoracoscopic approaches were the initial minimally invasive alternative to open thoracotomy for large calcified thoracic disc and were largely successful in providing results similar to those of open procedures but with less blood loss, less approach-related morbidity, and faster recovery time ultimately translating into reduced exposure-related morbidity. However, practical implementation of thoracoscopic techniques is challenging because of the 2D visualization of 3D pathology, the high costs of instrumentation, and the need for highly trained staff, the relative difficulty in managing intraoperative complications, and an extended learning curve.^33,34

The mini-open lateral approach represents a middle-ground alternative between endoscopic and open-approach procedures for the treatment of large anterior symptomatic TDH providing direct visualization and an adequate working field for TDH and does not require single-lung intubation.^34,35 In fact, the retropleural minimally invasive approach is our preferred approach for centrally located TDH ( Fig. 6.5). We published our clinical experience with this approach in seven patients (ages: 30–70 years) with central TDH.³⁵ All patients presented with thoracic myelopathy on physical examination. The average length of stay in the hospital was 2.6 days (range: 1–4 days). Three of the seven patients improved by one point on the Nurick scale. There were no complications related to the approach. Another study by Uribe et al³⁴ included 60 patients from five institutions who were treated using a mini-open lateral approach for 75 symptomatic thoracic herniated discs with or without calcification. The median operating time, estimated blood loss (EBL), and length of stay were 182 minutes, 290 mL, and 5.0 days, respectively. Four major complications occurred (6.7%): pneumonia in 1 patient (1.7%); extrapleural free air in 1 patient (1.7%), treated with chest tube placement; new lower-extremity weakness in 1 patient (1.7%); and wound infection in posterior instrumentation in 1 patient (1.7%). Excellent or good overall outcomes were achieved in 80% of the patients, a fair or unchanged outcome resulted in 15%, and a poor outcome occurred in 5%. Both studies endorse the efficacy of this technique as a less invasive option than conventional surgical techniques providing the rationale for minimally invasive approach for this condition.

Fig. 6.4 (a,b) Intraoperative images showing mini-thoracotomy technique and (c) bone harvested from surgical site for expandable cage reconstructions for treating a large calcified thoracic disc herniation with spinal cord compression. (d) Local autograft was collected during vertebral corpectomy using the BoneBac Press (Thompson MIS, Salem, NH) and used for fusion material.

Fig. 6.5 (a) Preoperative axial and (b) sagittal CT/myelogram showing large central calcified thoracic disc herniation. (c) Postoperative coronal (d) axial and (e) sagittal CT after removal of thoracic disc herniation and reconstruction. (f) Photo showing minimally invasive thoracotomy incision.

Thoracolumbar fractures are relatively common problems encountered by spine surgeons, with an estimated incidence of more than 160,000 cases per year in the United States.³⁶ Various authors have reported the utilization of endoscopic techniques in the management of thoracolumbar trauma with excellent clinical outcomes and minimal approach-related complications. In 2002, Khoo et al³⁷ published a large series consisting of 371 patients with thoracoscopic-assisted treatment of thoracolumbar fractures. There were five cases (1.3%) of serious complication consisting of aortic injury, splenic contusion, neurological deterioration, cerebrospinal fluid leak, and severe wound infection. Compared to a control group of 30 patients with open thoracotomy, the thoracoscopic group required 42% less narcotics for postoperative pain. Several other clinical series have demonstrated the feasibility of various endoscopic approaches for management of these fractures with very low rate of complications.^38,39,40,41 A recent review article by Koreckij et al⁴² demonstrated that the overall complication rate for standard open approaches to the spine was 11.5%, including 0.33% mortality, 0.16% paraplegia, and 9.17% postthoracotomy pain syndrome. Therefore, the minimally invasive endoscopic-assisted approach compares favorably in terms of approach-associated complications. Nevertheless, the current level of evidence for utilization of minimally invasive approaches to manage traumatic thoracolumbar burst fractures is low with lack of high-quality prospective controlled trials.⁴³

6.5 Minimally Invasive Approaches in the Lumbar Spine

6.5.1 Lumbar Microdiscectomy

Lumbar disc herniation is one of the most common conditions encountered by spine surgeons. This can be envisaged from the fact that an estimated 300,000 discectomies are performed annually in the United States alone.⁴⁴ Although open discectomy (OD) performed with or without an operating microscope remains the standard and highly effective treatment for lumbar disc herniations,^45,46 minimally invasive discectomy (MID) via tubular retractor system using either a microscope or an endoscope has been increasingly utilized given its potential benefits of less muscle damage, decreased pain, and faster recovery period after surgery.^{47,48,49,50,51} In addition, in morbidly obese patients, the minimally invasive approach using tubular dilators makes access to the disc space much quicker, the incision much smaller, and the risk for wound complications much less. Advances in muscle-sparing technology to approach the spine have eliminated the need for K-wire and serial muscle dilators ( Fig. 6.6).

A number of recent prospective randomized controlled studies have compared clinical outcomes following OD versus minimally invasive microdiscectomy. Righesso et al⁵² performed a prospective controlled randomized study consisting of 40 patients with sciatica secondary to lumbar disc herniation who failed conservative treatment and subsequently underwent OD or microendoscopic discectomy (MED) with 24-month follow-up. They found that the final clinical and neurological results were satisfactory in both the OD and the MED groups, but the MED group had smaller incisions and shorter length of hospital stay. This result should not be surprising given the goal of discectomy is to remove the disc fragment compressing the neural element, regardless of the approach. The advantage of MISS is smaller incision and less tissue trauma, therefore quicker recovery and shorter length of stay, which were all confirmed by the results of the study.

Many other published studies echoed these results.^53,54 Huang et al conducted a randomized trial and demonstrated that there was less systemic cytokine response in patients following microendoscopic versus open lumbar discectomy.⁵⁵ The difference in the systemic cytokine response supports the notion that MED is less traumatic and causes less inflammatory response. Teli et al⁵⁶ studied 240 patients aged 18 to 65 years affected by posterior lumbar disc herniation who failed 6 weeks of conservative management. These patients were randomized to microendoscopic, microdiscectomy, or OD groups. They reported higher risk of dural tears and recurrent herniation with lumbar MED.⁵⁶

A recent meta-analysis by Dasenbrock et al⁵¹ analyzed the results of six randomized trials comprising 837 patients (of whom 388 were randomized to MID and 449 were randomized to OD). They failed to demonstrate any advantage of MID as compared to OD and reported that intraoperative complications (incidental durotomies and nerve root injuries) were significantly more common in patients undergoing MID (relative risk [RR], 2.01; 95% confidence interval [CI], 1.07–3.77). There was no significant difference in relief of leg pain between the two approaches with either short- or long-term follow-up. While most studies concluded that both the MISS and open microdiscectomies lead to equivalent improvements in leg pain postoperatively, these studies were relatively small (n = 22–200 patients), and they were underpowered to detect some potentially clinically relevant differences between the two surgeries. Ultimately, the true benefits of MISS techniques are unlikely to be realized in a procedure that already has low complication rates and morbidity with rapid recovery rates, leading to heterogeneous outcomes from underpowered studies with variable applications of the techniques in variably skilled surgeons’ hands.

Discovering the true benefits for MID requires an understanding of the roles played by surgeon experience, patient-specific factors (e.g., obesity or comorbidities), and the type and magnitude of the procedure performed. Only then can appropriate studies be designed to address the most compelling clinical concerns. Another point to keep in mind is that MISS is a relatively new technique with significant learning curve; therefore, the complication rates may be slightly higher during the early part of the learning curve. McLoughlin and Fourney⁵⁷ demonstrated that the operative time and complication rate associated with MID both decreased over time with increased experience.

Fig. 6.6 (a–c) Surgical approach to the spine via a muscle-splitting fashion using the One-Step-Dilator (Thompson MIS) eliminates K-wire and serial muscle dilators.

6.5.2 Minimally Invasive Decompression/Laminectomy

Lumbar stenosis commonly occurs with aging and spinal degeneration. Fortunately, surgical treatment of symptomatic lumbar stenosis is highly effective in symptomatic relief.^58,59 Even though minimally invasive lumbar decompression was first independently described by the senior author and Palmer et al^2,60 more than a decade ago, there is yet to be a large prospective, randomized study that compares the clinical outcomes of open approaches versus MISS for lumbar decompression. Most of the data comparing outcome for open versus minimally invasive lumbar decompression are level II or III. Nevertheless, the unilateral laminotomy/internal laminectomy for bilateral decompression had become the minimally invasive surgery (MIS) procedure of choice for many spine surgeons in treatment of lumbar stenosis.

A number of prospective and retrospective studies with mid- to long-term outcomes suggest that this approach has a superior overall clinical success rate compared with open laminectomy. In 2002, Khoo et al² published a series consisting of 25 consecutive patients treated with MEDS and retrospectively compared to a historical control group of 25 consecutive patients treated with open laminectomies. There was a statistically significant decrease in intraoperative blood loss (68 vs. 193 mL), postoperative narcotic requirement (31.8 vs. 73.7 eq), and length of hospital stay (42 vs. 94 hours). At 1-year follow-up visit, 90% of the patients in the MEDS group reported improved or complete resolution of their pain symptoms.² In a recent prospective randomized clinical study by Mobbs et al,⁶¹ the authors compared minimally invasive unilateral laminectomy for bilateral decompression (ULBD) with open laminectomy. They found that microscopic ULBD was as effective as open decompression in improving function (the Oswestry Disability Index [ODI] score), while ULBD had better pain relief (visual analog scale [VAS] score), shorter postoperative recovery time, less time to mobilization, and less opioid use. These findings were also demonstrated in the study by Komp et al.⁶²

In another study, Asgarzadie and Khoo⁶³ compared 48 patients with MED to 32 patients with open laminectomies with a follow-up of 4 years. The average EBL for the MEDS group was 25 versus 193 mL for the open laminectomy group. The preoperative ODI score in the MEDS group was 46, and improved to 26 at 3 years. The average length of hospitalization for the MEDS group was 36 hours compared to 94 hours in the open laminectomy group. The rate of durotomies was 4% for the MEDS group.

Since open lumbar laminectomy is a procedure with an excellent surgical outcome, and it has been the standard of care for decades, it should not be surprising that the long-term outcome of various studies comparing open versus minimally invasive laminectomies may not be dramatically different. The immediate advantages of MISS which translate into better satisfaction in the early postoperative period cannot be ignored.⁶⁴

Postoperative spinal instability is also a concern in patients undergoing laminectomy for lumbar stenosis, especially in patients with some degree of spondylolisthesis preoperatively. The MISS approach maintains the posterior tension band and preserves the facet joints, which may minimize the risk of postoperative spinal instability. This has been demonstrated in a biomechanical study with cadaveric lumbar spine, in which the radiographic evidence of spondylolisthesis progression was present if greater than 50% of the facet joint was resected at any one level.⁶⁵

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