20 Minimally Invasive Surgery in the Aging Spine
The population of the United States and other developed countries is growing progressively older. Consequently, physicians of all specialties will see a greater incidence of age-related conditions in the coming decades, including degenerative disease of the spine. Spine surgeons should therefore be prepared to treat elderly patients, who may require specialized considerations unnecessary in younger patients. Minimally-invasive surgery is one major component in the successful treatment of these patients, reducing tissue damage, blood loss, and complication risk. In this chapter, we will discuss the indications for surgery, operative techniques, risks and benefits, and potential complications associated with minimally invasive surgery of the spine in elderly patients. At the conclusion of this chapter, the reader should recognize that minimally invasive surgery is a safe and effective approach to correct pathology in the aging spine.
The goal of minimally invasive surgery (MIS) is to provide comparable results to open surgery, while at the same time preserving as much normal human anatomy and physiology as possible.
The appropriate indications for surgery and procedure selection are more important in the aging population because of the increased prevalence of medical comorbidities, higher incidence of osteoporosis, diminished functional mobility and increased fall risk, and reduced healing ability
The surgeon’s armamentarium for MIS includes the posterior cervical foraminotomy (MIS-PF), transforaminal lumbar interbody fusion (MIS-TLIF), mini-open anterior lumbar interbody fusion (ALIF), and percutaneous pedicle screw fixation.
20.1 Indications and Contraindications
Almost every surgical field is undergoing a transition from conventional, open surgery to more minimally invasive approaches. The goal of minimally invasive surgery (MIS) is to provide comparable results to open surgery, while at the same time preserving as much normal human anatomy and physiology as possible. This leads to decreased soft tissue trauma and intraoperative blood loss, as well as reduced postoperative pain and shorter hospital length of stay. Most importantly, this should accelerate a patient’s return to their preoperative functional status.
MIS of the spine has gained particular interest amongst neurosurgeons and orthopedic surgeons. One reason for this is that spine surgeries, particularly fusion procedures with instrumentation, are typically regarded as cases with higher morbidity and longer recovery times. These procedures are often indicated in the treatment of degenerative processes that primarily afflict the aging patient, who themselves have a greater number of medical comorbidities and risk factors that must be taken into account.
Methods by which to minimize surgical morbidity while maximizing effectiveness should be considered. Life expectancy in the year 2000 was 66 years of age. Compare this to 45.5 years of age in 1950 and a projected 76 years in 2045. In addition, while 606 million people throughout the world were aged 60 or older in 2000, this number is projected to approach 2 billion by 2050. 1 As the global health burden shifts from acute illness to chronic and degenerative disease, the number of spine procedures performed will also grow.
The prevalence of spinal pathology increases with age. Consequently, the demand for spine surgery to relieve pain and functional impairment in our aging population has also increased. The appropriate indications for surgery and procedure selection are even more important in this subset of the population, given an increased prevalence of medical comorbidities, higher incidence of osteoporosis, diminished functional mobility and increased fall risk, and reduced healing ability.
As with other surgical fields, conventional techniques are being condensed into attractive MIS approaches that strive to accomplish the same goals with reduced morbidity. Therefore, the indications for surgery are generally the same, with some exceptions. Typical pathologies for which surgery is indicated include cervical spondylotic myelopathy, cervical radiculopathy, cervical deformity, lumbar stenosis with neurogenic claudication, lumbar spondylolisthesis, lumbar radiculopathy, and degenerative lumbar scoliosis. The number of MIS techniques is increasing, and the specific indications for selected techniques are discussed in the next section.
For cases such as short-segment cervical fusions, lumbar laminectomy, and simple lumbar fusion, the indications for surgery are generally straightforward and independent of age. For these smaller surgeries, the risk of postoperative complications may not be significantly different when comparing younger to older patients, but this is not likely the case with more extensive surgery. This is illustrated by a large retrospective cohort study reviewing Medicare claims from 2007, which found a significant increase in the incidence of life-threatening complications in complex fusion procedures compared to decompression alone. 2 For this reason, MIS has been proposed as a means to correct spinal deformity while minimizing surgical morbidity.
Clinical research related to the effectiveness of MIS in the treatment of spinal deformity has increased over the past several years. Current studies suggest that in properly selected patients, MIS is effective in both radiographic deformity correction and complication reduction, as compared to conventional open surgery. 3 Even so, dedicated, high-quality studies on the subject of MIS deformity correction in the aging population are lacking. It does stand to reason, however, that with proper patient selection, MIS may provide beneficial outcomes with a reduced complication profile compared to open surgery, particularly in aging patients with increased comorbidities. A recent retrospective review suggests the best clinical results from MIS deformity correction are obtained when correcting the pelvic incidence-lumbar lordosis (PI-LL) mismatch to within 10° and sagittal vertical axis (SVA) to less than 5 cm. 4 Conversely, it appears that patients with a fixed SVA > 6 cm, pelvic tilt > 25°, PI-LL > 30°, and/or thoracic kyphosis > 60° are poor candidates for MIS deformity correction. 5 Reoperation rates seem to be similar between MIS and open surgery for deformity correction, albeit for different reasons. Postoperative neurologic deficit appears to be a leading source of reoperation for open and hybrid MIS-open surgeries, whereas pseudoarthrosis is a leading cause in MIS. 6
The surgeon’s armamentarium for MIS includes the posterior foraminotomy (MIS-PF), transforaminal lumbar interbody fusion (MIS-TLIF), mini-open anterior lumbar interbody fusion (ALIF), and percutaneous pedicle screw fixation. While further high-quality studies with specific focus on patients aged 65 and older taking into account medical comorbidities are necessary, MIS remains an attractive option to avoid complex, open procedures in an aging population.
20.2 Technique Descriptions
20.2.1 Cervical Spine
In the cervical region the most commonly performed procedures for cervical stenosis and associated myelopathy and radiculopathy include the anterior cervical discectomy and fusion (ACDF) and posterior cervical laminectomy with or without instrumented fusion. While an MIS endoscopic alternative to the ACDF has been described, 7 the classic ACDF typically involves minimal soft tissue trauma and blood loss, and remains a surgical staple. The ability to perform MIS foraminotomies however, provides an excellent MIS alternative to the treatment of simple cervical radiculopathy that also allows for motion-preservation.
MIS posterior foraminotomies (MIS-PF) are indicated to treat cervical radiculopathy refractory to conservative management. 8 This is accomplished most effectively when the nerve root compression can be localized to the lateral aspect of or within the neural foramen, often as a result of an osteophyte or lateral disc herniation. A radicular distribution of symptoms should be identified and be supported by radiographic findings. This procedure may be performed bilaterally if indicated. Foraminotomies are not indicated for isolated neck pain. The main contraindications include concurrent cervical myelopathy, the presence of a central disc herniation, and significant instability. In these cases, the patient would likely benefit more from an anterior or posterior approach for decompression and stabilization.
Level 1 evidence suggests that while ACDF, cervical disc replacement (CDR) and MIS-PF are all effective in treating cervical radiculopathy, MIS-PF is associated with the lowest incidence of adverse effects. 9 MIS-PF has been shown to be effective in approximately 90% of cases and is associated with less blood loss, shorter operative time, and shorter hospital stay than open posterior foraminotomies. 10
The procedure is performed prone, with the head slightly flexed. After confirming the appropriate level with fluoroscopy, a paramedian incision is made, the fascia is opened sharply, and a blunt dilator is advanced and confirmed to be over the lateral mass of interest. Sequential dilators are then placed. Using the microscope and high-speed drill, followed by Kerrison rongeurs, a hemilaminotomy and medial facetectomy is performed. The ligamentum flavum is carefully removed, exposing the nerve root. If a soft lateral disc is the suspected culprit, careful discectomy is performed at this point. Decompression may be confirmed by palpating around the nerve root with a blunt nerve hook. Fig. 20‑1 demonstrates a completed cervical laminoforaminotomy with the nerve root decompressed.
20.2.2 Lumbar Spine
MIS applications exist for most pathologies of the lumbar spine. MIS is typically performed through a tubular retractor system, either under loupe or microscopic magnification or via endoscopic assistance. In addition, a variety of “mini-open” procedures also exist, with the mini-open ALIF being the most common. This section will review a select number of MIS procedures, including tubular microdiscectomy, MIS-TLIF, and the awake endoscopic MIS-TLIF.
The microdiscectomy remains the gold standard for unilateral lumbar radiculopathy with an identifiable radiographic disc herniation. Multiple variations exist, including the tubular microdiscectomy and endoscopic-assisted microdiscectomy. The classic indication for microdiscectomy is leg pain greater than back pain in a radicular distribution that is supported by MRI findings of a herniated disc with nerve root compression at the appropriate level, usually after a 6–8 week trial of conservative management. Acute indications include acute progressive neurologic deficit, as a result of the above, and cauda equina syndrome. Contraindications to microdiscectomy include symptoms that are not explained by imaging, a primary complaint of back pain, and cauda equina syndrome in the setting of a large central disc herniation not amenable to adequate decompression by an MIS technique alone.
Level 1 evidence supports equivalent outcomes between the standard microdiscectomy and the tubular MIS approach. 11 This procedure is commonly performed on an outpatient basis and typically under general anesthesia.
The tubular microdiscectomy (Fig. 20‑2) is performed on the Wilson frame to minimize lumbar lordosis and open the disc space. A line is drawn 1.5 cm parallel to midline, and a spinal needle is inserted to localize the disc space of interest. Centered on this point, a small incision is made, and a blunt dilator is passed through the incision, resulting in a “muscle-splitting” approach, until bone is contacted. Sequential dilators are inserted, and fluoroscopy confirms placement of the tubular system over the appropriate facet complex, in line with the disc space. Using microscopic magnification, the soft tissue is dissected off the bone, and a hemilaminotomy and medial facetectomy is performed. With the traversing nerve root retracted medially, the discectomy is performed.
Degenerative lumbar stenosis is a leading cause for spine surgery in patients over age 65. The most common indication for lumbar decompression is a moderate to severe lumbar stenosis at one or more levels with signs and symptoms of neurogenic claudication that is refractory to conservative therapy. Laminectomy is the gold standard for surgical treatment; however, advances in endoscopy have paved the way for MIS interventions such as the microendoscopic decompressive laminotomy first described by Fessler et al in 2002. 12 Other variations of the same procedure exist, including a microscope-assisted approach.
Preliminary evidence reveals significant improvements in both Visual Analog Scale (VAS) scores for back and leg pain as well as Oswestry Disability Index (ODI) scores for elderly patients receiving MIS lumbar decompression. 13 Because of the favorable complication profile, this procedure is particularly attractive for the treatment of aging patients with multiple medical comorbidities who might otherwise not be candidates for open surgery. Another benefit of this procedure is that segmental stability should be adequately maintained.
The rate of lumbar fusion procedures has increased at a dramatic rate over the past 20 years. Data from the Healthcare Cost and Utilization Project Nationwide Inpatient Sample demonstrates a 137% increase in the number of spinal fusion hospital discharges from 1998 to 2008. Not unexpectedly, a significant increase in the mean age of surgery was also seen (48.8 to 54.2 years). 14 Lumbar fusion procedures typically represent the spinal surgeries with the greatest morbidity, intraoperative blood loss, length of hospital stay, and time until return to preoperative functional ability. Indications are widespread, from back pain to deformity correction. That being said, it is no wonder that the majority of MIS has targeted the lumbar spine.
MinimallyInvasive Transforaminal Lumbar Interbody Fusion
By combining the concept of the tubular retractor and the development of percutaneous pedicle screw instrumentation, the MIS-TLIF and other variations were born. The indications are the same as for open TLIF and include degenerative disc disease resulting in lower back pain with or without radiculopathy, often secondary to disc space collapse and foraminal stenosis, and segmental instability involving grade I or II spondylolisthesis. The MIS-TLIF may be performed in up to two levels reliably, and it is generally not indicated for deformity correction.
As first described by Foley et al, 15 the procedure is performed by first making a 2.5 cm skin incision centered over the disc space of interest, 4 to 5 cm lateral to midline on the side of the patient’s worse symptomatology. Sequential dilator tubes (22 or 26 mm) are used to provide a surgical corridor centered over the facet joint. Using microscopic magnification, a complete facetectomy is performed, followed by discectomy (Fig. 20‑3). The disc space is then distracted either by placement of an interlaminar spreader or by placing contralateral percutaneous screws and rods. At this point the disc space is prepared with curettes and scrapers, and the interbody is placed. Percutaneous screws and rods are then placed ipsilaterally, followed by contralaterally, if not done so previously.
Awake Endoscopic Transforaminal Lumbar Interbody Fusion
General indications for the awake endoscopic TLIF include lumbar degenerative disc disease and low-grade spondylolisthesis causing back and/or leg pain. Contraindications include grade III or IV spondylolisthesis, more than two indicated levels, and > 90% central canal stenosis. Cases with a high risk of subsidence should also be excluded, as long-lasting indirect decompression is integral to the success of the procedure. 16
In order to avoid general anesthesia, the authors have adopted an anesthetic technique that includes a continuous infusion of propofol and ketamine to achieve light to moderate sedation. Using fluoroscopy and a spinal needle, Kambin’s triangle on the side of the primary pathology is localized, and sequential dilators leading to an 8 mm working channel are placed (Fig. 20‑4). The endoscope is inserted, and the traversing and exiting nerve roots at the level of interested are localized. Discectomy and thereby direct decompression is performed with a variety of specialized rongeurs, curettes, high-speed drill, and micro-osteotomies (Fig. 20‑5). Final disc space preparation is achieved with specialized stainless steel-brushes. At this point, 2.1 mg of recombinant human bone morphogenic protein is placed into the anterior portion of the disc space and is followed by an allograft-filled meshed containment device. The interbody provides indirect decompression of the exiting nerve roots at that level. In order to achieve fixation, percutaneous pedicle screws and rods are then placed bilaterally, with the four-screw tracts having been injected with a total of 20 mL of bupivacaine to further assist with analgesia and recovery (Fig. 20‑6).
20.3 Benefit and Risks
The question of risk and benefit in spine surgery for aging patients is in many ways more complicated than in the general population. Older patients are constitutionally frailer than their younger counterparts and often carry medical comorbidities of greater severity and number. Further, a lower productive life expectancy can negatively impact the cost-effectiveness of surgical intervention (especially involving instrumentation), potentially limiting patients’ access to otherwise beneficial procedures. However, the incidence of pathological alterations in spinal anatomy increases with age, and the progression of these changes often results in symptomatology for which surgery is warranted. A thorough examination of the risks and benefits specific to the aging population is then paramount to the sound practice of spine surgery in older patients.
The benefit of a given intervention in the aging cohort should be demonstrated with a high degree of confidence before offering surgery to these patients. Specifically, ideal procedures should minimize physiologic stress, shorten operative time, reduce exposure to potent anesthetic drugs, and optimize financial cost.
Surgical procedures by definition involve a controlled trauma to the patient’s body, including tissue damage, blood loss, and metabolic derangements. A growing body of literature supports the assumption that MIS approaches involve less injury to surrounding tissues when compared to conventional open surgery. 17 , 18 Significant reductions in blood loss have also been reported. This reduces the need for intraoperative transfusion, in turn reducing demand on hospitals’ limited blood bank supply. 19 Reduced transfusion rates may be especially useful in elderly patients who may be immunocompromised, decreasing the risk of transfusion-related hypersensitivity reactions or infection.
In addition to direct surgical risks, the potential harm of anesthesia exposure should be addressed when considering older patients for surgery. 20 Extra care should be placed on protecting the airway, managing fluid balance, and maintaining ideal perfusion during surgery. Drug metabolism may be altered or impaired in patients with comorbid liver disease or chronic kidney disease, affecting medication choice and intraoperative dosing. In addition to these physiologic dangers, anesthesia exposure has been associated with cognitive decline in some elderly patients. 21 MIS techniques may allow for reduced drug administration, while still achieving operatively viable anesthesia. 22 Indeed, as we describe in this chapter, the endoscopic MIS approach has made awake surgery under conscious sedation a possibility in the lumbar spine. 16
Spine surgery, especially involving instrumentation, can frequently require revision. Indications for reoperation include progressive degeneration, hardware failure, pseudoarthrosis, and adjacent segment disease. Older adults, therefore, not only carry the baseline risks for revision after a primary spine procedure, but also may require reoperation for surgeries performed years or decades earlier in life. In the case of primary surgeries performed in elderly patients, cases should be optimized to reduce the risk of reoperation in order to avoid subjecting frail patients to anesthesia and surgical stress a second time. In cases where revision is necessary, however, MIS approaches represent an effective means to accomplish the revision safely. The trajectories afforded by novel MIS approaches may be of particular benefit when revising a prior open surgery, as the native anatomy is typically distorted or replaced by scar tissue. Conversely, an MIS primary surgery should have less associated tissue destruction, and so may result in an easier approach if a subsequent revision procedure is required.
Efficient resource utilization allows more patients to be treated at lower cost. Therefore, determining the optimal course for patients and institutions requires a frank consideration of the price of a given surgical procedure. Cost can be especially relevant to elderly patients, whose care already may include treatments for a variety of age-related comorbidities, and the associated financial burden. As we discuss above, the frequency of spine surgery in elderly patients is projected to increase in the coming decades. This demographic trend should be seen as a call for not only more effective, but also more affordable techniques. MIS procedures answer that need. Independent financial analyses in Europe and America have demonstrated lower costs associated with MIS spine surgery as compared to open procedures. 23 , 24 These savings have been attributed primarily to a reduced postoperative hospital stay, including less pharmacy utilization and laboratory cost. MIS procedures have also been associated with fewer surgical site infections, a major driver of extended postoperative length of stay and increased cost. 25
To justify the profound investment of time and resources— as well as the associated risks of injury or death— surgeons and patients should reasonably expect a good and clinically significant postoperative outcome. This is a nontrivial consideration in older patients, whose frailty may limit the extent of intervention that can be safely achieved, ultimately affecting outcomes. Further, certain pathologies resulting from chronic progressive degeneration can be multifactorial, producing complex symptomatology that may not fully resolve even after a feasible procedure. It is then necessary to understand the realistic extent of improvement or clinical stabilization we can reach through surgery in aging patients, and to what extent MIS techniques can impact the degree of improvement.
It should first be understood that spine surgery in general, whether conventional or MIS, can be safe and effective in the aging population if proper attention is paid to patient selection. 26 This involves not only choosing patients for whom intervention is truly indicated, but also optimizing those patients (and their comorbidities) for the surgical stress and trauma necessary to achieve clinical improvement. Following these principles, spine surgery can be performed in these patients safely and beneficially. In recent years, however, a preponderance of literature has largely favored the utilization of MIS techniques in this population. 27
MIS approaches to lumbar discectomy have found favor in patients of various ages and clinical conditions. 28 , 29 , 30 These studies demonstrate comparable safety and efficacy as compared to conventional techniques, with the added benefits of the MIS approach discussed previously (reduced tissue damage, blood loss, etc.). In particular, a large retrospective study of over 25,000 cases found a significantly lower complication rate with MIS compared to conventional open lumbar discectomy. 31 MIS techniques to treat adult spine deformity have also been investigated specifically in the aging population, with demonstrable safety and efficacy as compared to conventional deformity procedures. 32 , 33 Finally, clinical and radiological outcomes in aging patients undergoing MIS lumbar interbody fusion have been shown to parallel those achievable in younger patients, with significant qualitative improvements and good fusion rates. 34 , 35
But in judging outcomes of MIS surgery for older patients, we must consider not only the direct clinical results of a single procedure, but the associated perioperative costs and benefits to patient and hospital. These chiefly include postoperative infection, analgesia consumption, and length of hospital stay.
As we discuss above, MIS techniques are associated with a lower risk of surgical site infection, reducing harm to patients and hospital costs. 36 This effect may be attributed to smaller incisions with less disruption of fascia and other subcutaneous tissue, which better maintains the body’s physical barriers to infection. As we describe our awake endoscopic MIS-TLIF, our reduced administration of anesthesia includes an avoidance of potent opioid analgesics both during and after surgery, instead relying on long-acting local anesthesia, acetaminophen, and gabapentin. 16
Previous authors have demonstrated a reduced postoperative length of stay (LOS) associated with MIS procedures. 37 However, this effect has not been reproduced in all studies. 38 This discrepancy likely reflects the multiple and disparate nature of the variables that together determine LOS, only one of which is surgical technique. In the aging population, spine surgeons should seek to optimize the remaining perioperative variables that determine LOS in order to expedite recovery and remove these susceptible patients from exposure to potential hospital-acquired infection or other complication during the postoperative period.
Enhanced Recovery After Surgery (ERAS) is one avenue to achieve this end. ERAS is a multidisciplinary approach to the surgical process pioneered by European anesthesiologists and colorectal surgeons. 39 The ultimate goal of ERAS programs is to reduce postoperative LOS by accelerating the recovery process. This is accomplished by a number of specific perioperative interventions, including the coordination of all teams caring for a given surgical patient from admission to discharge toward the ultimate goal of early discharge. Preoperative interventions include patient optimization and nutritional planning. Intraoperative interventions include the strong support of MIS techniques, as discussed here, and reduced administration of general anesthetics. Postoperative interventions include early mobilization, early oral feeding, and reduced consumption of opioids analgesics.
Recently, work has begun to develop ERAS protocols for major spine surgery. 40 In hand with the authors’ development of the awake endoscopic TLIF, the authors’ have published their experience with an ERAS pilot protocol for lumbar spine fusion. 41 Since establishing this program at our institution, we have seen excellent clinical results in our enrolled patients, with associated shorter LOS and reduced institutional costs. 42 We attribute these benefits not only to the limited soft tissue damage afforded by the MIS techniques employed, but the systematic interventions the ERAS program provides our patients. Programs such as these would seem ideal in aging patients, whose frailty and comorbid conditions often require special perioperative considerations.