Chapter 35 Minimally Invasive Percutaneous Lumbar Fusion Technique

John H. Shim, MD, MBA, FACS, Jason S. Mazza, MSc, OPA-C, Daniel H. Kim, MD, FACS

The development of minimally invasive techniques to address disease pathologies among a number of surgical specialties in place of traditional open surgery has become a noteworthy trend. Examples include the laparoscopic approach to gallbladder disease as well as knee and shoulder arthroscopy as a successor to open surgery for meniscal and rotator cuff disorders, respectively.

This shift toward minimally invasive or minimal-access technologies has also taken place over the last decade in the specialty of spinal surgery. The goal of the procedure remains the same: to achieve outcomes equivalent to or better than that of traditional open surgery while trying to reduce overall postoperative pain, intraoperative blood loss, hospital stay, and surgical scarring.

It is estimated that more than 200,000 lumbar procedures are performed yearly for underlying discogenic disease and instability [1]. The degenerative process, often a result of normal aging, leads to degenerative disc disease, spondylolisthesis, and segmental instability. The traditional open posterior lumbar fusion yields an acceptable efficacy with a high fusion rate [2]. However, the exposure that is needed to achieve optimal visualization of the transverse processes, lamina, and facet joints often requires a midline incision of more than 5 to 7 cm, depending on the number of levels involved. Disruption and retraction of the deep paraspinal musculature has been shown to cause localized denervation, which may lead to continued back pain and spasm after the procedure [3]. Multiple studies in this area have shown deleterious histologic effects, including regional ischemia from increased intramuscular pressure secondary to the use of retractor blades [4,5].

Less invasive techniques and instrumentation have been and continue to be developed to address the effects of extensive soft tissue disruption. In 1995, Mathews and Long [6] published their experience with the use of percutaneous pedicle screws. Unfortunately, they noted a high nonunion rate. Lowery and Kulkarni [7] published a review of eight cases in which they utilize a similar technique but with the use of rods instead of longitudinal connectors; they reported a 96% fusion rate for a mini–open anterior approach supported with a minimally invasive posterior fusion. Advances to the procedure described by Harms and Rollinger [8] have produced a method of achieving a solid arthrodesis through a posterior interbody approach.

Subsequently, the transforaminal lumbar interbody fusion (TLIF) approach was developed as a means of addressing the pathology in the disc space while minimizing the risk of iatrogenic injury to nerve roots and soft tissue. Foley and colleagues [11] described a minimally invasive method of performing TLIF that has become popular with trained surgeons [9–11]. The TLIF procedure continues to evolve as a minimal access procedure that is ideal for a patient who has clinically defined symptoms that correlate with results of diagnostic studies related to radiculopathy pain secondary to degenerative disc disease, recurrent herniated nucleus pulposus, or segmental instability. The approach allows for adequate access to the affected facet joint and lamina to perform a laminotomy and discectomy. Furthermore, through a mini–open and percutaneous approach, restoration of disc height, sagittal balance, and stabilization of the vertebral segments can be effectively performed. A study by Schwender and colleagues [12] reported clinically significant improvements in visual analog scale and Oswestry Disability Index scores along with a 100% fusion rate in a cohort of patients who underwent a minimally invasive TLIF procedure [12].

At the time of this writing, several minimally invasive systems are available for this purpose. Each of the systems features a top-loading polyaxial screw, with a unique rod delivery system that has traditionally been a challenge for minimally invasive systems. Most systems allow for compression and distraction. The Pathfinder Minimally Invasive Spine Instrumentation System (Abbott Spine, Austin, TX), PIVOT (Globus Medical, Audubon, PA), and VIPER (Depuy Spine, Raynham, MA) allow for a true percutaneous approach with the use of cannulated pedicle screws that are delivered over a guidewire. These systems allows for performance of a posterolateral fusion. TLIF can also be performed through a mini–open approach. The Atavi Atraumatic Spine Surgery System by Endius (Zimmer Spine, Minneapolis, MN) utilizes an illuminated expandable retractor system that allows for direct visualization and offers the surgeon flexibility in performing single or multiple procedures including interbody and posterolateral fusions. The SEXTANT System (Medtronic Sofamor Danek, Memphis, TN) is a minimally invasive system that requires three incisions to complete a fusion. The pedicle screws are placed through separate incisions as in the previously described systems. The rod is delivered through the third incision by means of a patented arc delivery system. TLIF can be performed with a mini-open approach using retractor tubes. In early 2007, the U.S. Food and Drug Administration (FDA) approved the Silverbolt MIS [minimally invasive screw] System by VertiFlex (VertiFlex, Inc., San Clemente, CA) for marketing and sales in the US. This system, designed by spine surgeons in conjunction with engineers, is a platform for percutaneous and mini–open approaches. The Silverbolt was built to address the difficult challenges of minimally invasive spine surgery including fusion at L5-S1.

Presurgical treatments

In the absence of cauda equina syndrome, progressive loss of motor function, infection, unstable fracture, and neoplasm, conservative management of back pain with or without radiculopathy should first be instituted. Nonsurgical interventions include a period of activity modification, physical therapy, exercise, nonsteroidal anti-inflammatory medications, muscle relaxers, and therapeutic corticosteroid injections. Referral to a pain management specialist to address nociceptive and neuropathic pain may be necessary for patients in whom basic conservative measures fail. Additional referrals to address psychological, social, and vocational issues also may be necessary as a part of a comprehensive workup and treatment plan.

No clearly defined time frames exist for the consideration of nonemergency spinal fusion surgery. The natural history of acute low back pain with or without radiculopathy resulting from a disc herniation is favorable. According to the literature and our experience, more than 90% of patients recover within 3 months [13,14]. The same holds true for those who suffer from lumbar spinal stenosis, a large percentage of whom can be managed with conservative care and no need for spinal surgery. Patients in whom nonsurgical treatment fails, who have documented clinical and diagnostic findings consistent with their symptoms, or who have neurologic deficits, surgical management may be warranted.

Preoperative preparations

Assessment

The presurgical evaluation should include a thorough history and physical examination. A detailed patient history, including the onset of complaints, aggravating and mitigating factors, and response to any treatments tried, should be entered into the medical record. This step allows the surgeon or practitioner to better target the physical examination. A comprehensive review of the patient’s medical co-morbidities is necessary to assess and identify potential contraindications to spinal surgery. Symptoms of spinal disorders must be differentiated from other causes, including infection, fracture, and neoplasms. A physical examination to include assessment of gait and range of motion, as well as motor, reflex, and sensory testing is necessary to establish a baseline for the patient’s neurologic status. Provocative testing should also be conducted to determine the source of pain. Nonorganic or psychological signs as described by Waddell and colleagues [15] should be noted during the examination. Combined, the history and physical examination provide a basis on which a diagnosis can be made and are fundamental to making decisions about further testing and treatment.

Diagnostic Testing

Radiography should be the initial imaging study and can performed during the presurgical evaluation if radiographs have not already been obtained. We advocate the use of standing radiographs that include an anteroposterior (AP) view and lateral flexion and extension views. The rationale is that most other diagnostic studies are performed in a supine position, which may not detect changes that occur in the spine in an erect position. Although no standard definition of radiographic instability exists, it is generally accepted that 3 to 4 mm of translation or greater than 11 degrees of angular motion can suggest segmental instability. Flexion and extension radiographs allow for identification and quantification of abnormal motion. Another important component to identify is the presence of a transitional vertebra. In a normal spine, the 25th vertebral body below the occiput is S1. A review of the literature shows that transitional vertebrae occur in as much a 4% to 21% of the population [16]. Sacralization of the last lumbar vertebra results in four non–rib-bearing vertebrae, whereas lumbarization results in six rib-bearing segments. Additionally, congenital anomalies are also commonly seen. It is imperative that prior to surgery, the levels are properly identified and are consistent with further diagnostic studies that may be ordered. Evaluation of the disc space for decreased disc height and sclerosis surrounding the end plates is often indicative of advancing degenerative disc disease. Assessment of the sagittal curve for irregularity should be noted. The examiner should also scrutinize the films for lytic and blastic lesions.

Diagnostic testing can be used to help confirm a disease process and its location. Each imaging test has its pros and cons, but it is the surgeon who must ultimately determine whether a particular finding has clinical meaning. Magnetic resonance imaging (MRI) has become the most commonly used modality for evaluating the disc, neural elements, surrounding soft tissue, and marrow. High-field MRI system allow for excellent visualization of soft tissue detail in multiple planes. Computed tomography (CT) can be considered when MRI is contraindicated. A CT scan allows for better delineation of osseous anatomy, which offers an advantage in cases of spondylolysis and facet osteoarthropathy. Three-dimensional reconstructions are also possible with CT scanning.

CT in conjunction with myelography can be used as an adjunct study to standard MRI and CT. Weight-bearing views can provide more definitive information on neural compression that may otherwise be missed. A CT myelogram may also be advantageous in cases of lumbar spinal stenosis. Because it is invasive, most surgeons order and utilize the results of this study only for surgical planning purposes.

Provocative discography remains controversial in its value and approach. The role of discography is that it may help differentiate painful discs from other causes of back pain [17]. It is understood that as a part of the natural aging process, the disc can develop fissures with a loss of fluid content. For many people, this process does not cause pain, but for others such a disc can be a pain generator. Inflammatory mediators are believed to be involved in causing irritation of the nerve endings embedded within the anulus. Provocative discography represents a method of assessing whether a degenerated disc is a potential pain generator. A post-discography CT scan is used to identify the contrast dye pattern from the injection. An injection into another disc used as a control is necessary to confirm a concordant pain response. When the procedure is performed by a well-trained discographer, the findings can help identify both the disc integrity and the physiologic response to the defect. Like myelography, discography is used to delineate a disease process for surgical intervention.

Electromyography (EMG) combined with nerve conduction studies can be used to establish a cause for radicular complaints and determine whether an identified lesion is acute or chronic.

Diagnostic injections commonly performed by interventional radiologists and pain management physicians are used to target a suspected pain generator. Diagnostic spinal injections include epidural steroid injections, facet and medial branch blocks, and selective nerve root blocks.

Dual energy x-ray absorptiometry (DEXA) is an enhanced form of x-ray technology used to measure bone density. Although the reliability of results may be questionable, many surgeons have begun to include an osteoporosis evaluation as a part of the presurgical workup, especially in postmenopausal women, as a guide for preoperative planning when fusion surgery with instrumentation is being considered.

Indications and contraindications

Decompressive surgery with fusion is commonly indicated for recurrent disc herniations, spondylolisthesis, or segmental instability causing radiculopathy in the lower extremity. Symptomatic degenerative disc disease that is unresponsive to conservative care with diagnostic evidence of the disease process and pseudoarthrosis are also accepted indications for fusion surgery. Conversely, fusion surgery is not indicated for nonspecific low back pain and findings that do not correlate with results of diagnostic studies. Patients with medical co-morbidities that preclude surgery or that would put them at high risk should not be considered for elective spinal surgery. Patients who present with unrealistic expectations of the outcome and those with psychosocial issues may not be good candidates for surgical intervention. Lastly, fusion surgery utilizing instrumentation is not advised in patients with advanced osteoporosis.

Patient selection is paramount, in that poor selection may result in less than optimal results. The surgeon should weigh the results of the presurgical workup and diagnostic studies, assess the medical risks, and consider the psychological status of the patient before considering surgery.

Informed consent and preoperative planning

Once the decision to pursue surgery has been made, several patient-related issues should be considered. A visit between the patient and the surgeon for a discussion of the risks and benefits of as well as alternatives to the operation as well as informed consent should be performed. The use of anatomical models, diagnostic study films, printed educational materials, and web-based portals are all appropriate patient educational tools that should be employed to help the patient and family members fully understand the proposed surgery. A review of the technical aspects of the procedure along with expected hospital stay and short- and long-term outcomes should be included in the discussion. Patient and surgeon expectations, limitations, and goals are also defined.

A thorough medical evaluation may be necessary for older patients and those with medical co-morbidities and risk factors. This includes a full system evaluation, appropriate preoperative laboratory studies, electrocardiogram, and chest radiograph when indicated. The primary care physician may recommend further evaluation by specialists in the fields of cardiology and pulmonology prior to clearing a patient for surgery. Not only should the patient’s risk level be defined from a medical standpoint, but the patient should also be optimized preoperatively to mitigate overall risks. This includes normalizing blood glucose values in diabetic patients, adding or changing medications in patients with cardiac disease, and appropriate management of any metabolic abnormalities.

Coordination of surgical equipment, implants, and hospital facilities should be considered during the immediate presurgical stage. A review of the patient’s risk profile should be made to determine postoperative needs. These may include in-hospital consultations by other specialists for postoperative management and arranging for a bed in the intensive care unit or telemetry unit. Ensuring the presence of the appropriate surgical table, fluoroscopy unit, specialized instrumentation, and implants in the operating room should be carried out far enough in advance to avoid conflicts. A preoperative evaluation by the anesthesiologist also is recommended to determine and discuss any specials needs in relation to preoperative medications, intubation, and any previous adverse reactions to anesthesia.

Related anatomy and physiology

A thorough understanding of the patient’s specific anatomy, including orientation of the pedicles, is crucial to the consideration of minimally invasive spinal fusion. Coronal angulation of the pedicles typically increases from the top to the bottom of the lumbar spine, but anatomic variants of this pattern exist. The transverse pedicle width increases from L1 to the sacrum [18,19]. Pedicle width is more important than pedicle height in the determination of screw placement. Accurate and safe placement of pedicle screws is highly dependent on the surgeon’s understanding of this concept. As previously discussed, the presence of sacrilization or lumbarization should be clearly defined preoperatively on standard radiographs, which should compared with other diagnostic studies to ensure that surgery is being performed on the appropriate levels. The lordosis of the sagittal curve in a normal lumbar spine averages approximately 50 degrees. A large percentage of lumbar lordosis results from wedging of the intervertebral discs. Disease processes, including advanced degenerative disc disease and segmental instability, can negatively affect the overall alignment in the sagittal plane. Inclusion of the restoration of sagittal balance in the goals of the procedure improves the chances of a successful surgical outcome [20]. Other anatomically and physiologically related considerations are the presence of scar from previous surgery, bone quality, preoperative assessment of adjacent levels, and morphology of the pelvis for operations at L5-S1, as well as the presence of conjoined nerve roots [21].

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