Chapter 35 Minimally Invasive Percutaneous Lumbar Fusion Technique
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].
Presurgical treatments
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.
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.
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]
