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Minimally Invasive Approaches to Spinal Deformity
Summary of Key Points
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The minimally invasive spinal deformity 2 algorithm guides patient selection for a minimally invasive surgery (MIS) procedure or an open deformity correction operation.
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Meticulous preoperative assessment of the patient’s spinopelvic parameters and the amount of correction needed to address the patient’s pathology is required to select the appropriate MIS operation for a given patient.
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Three-dimensional image navigation and robotics are useful adjuncts for MIS surgery and reduce the radiation exposure of the surgeon and surgical team.
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Complications of MIS surgery are different than those of open surgery, and understanding expected complications is important in choosing an operative approach for spinal deformity.
There have been significant advancements in minimally invasive surgery (MIS) techniques over the last decade. These advances have made MIS approaches safer and more efficient and have expanded the catalog of spinal deformities that may be effectively approached. Development of MIS techniques has had particularly profound implications for aging patients with greater comorbidities, who may not be able to tolerate a large deformity correction without an extended postoperative hospital stay or admission to an intensive care unit. MIS techniques are comparable to conventional surgery in terms of perioperative pain but may have quicker recovery (i.e., return to work, discharge from the hospital, etc.), lower infection rate, reduced intraoperative and postoperative blood loss, less soft tissue damage (i.e., less dissection of the multifidus muscle from the posterior spinal elements), and less surgical scarring. The purpose of this chapter is to review MIS techniques for the management of spinal deformity, including presentation of an algorithm to guide appropriate surgical strategies.
MIS techniques are a powerful component of a spine surgeon’s armamentarium to achieve symptom relief and improve outcomes. MIS techniques currently have limited corrective potential, particularly in patients with a significant fixed spinal deformity, but they are a powerful approach in patients with flexible deformity and preserved disc spaces. MIS offers a potential alternative to the open surgical approach to address spinal deformity, and the MIS technique may have significant advantages regarding risk profile, infection rate, and recovery time.
Goals of Minimally Invasive Techniques for Spinal Deformity
Realignment of the spinal column is the major goal of spinal deformity surgery. The alignment goals in MIS surgery and open surgery are the same.
Correct Sagittal Imbalance (Sagittal Vertical Axis >5 cm)
Standing sagittal alignment is measured the distance from a C7 plumb line drawn from the middle of the C7 vertebral body on a standing long cassette radiograph to the posterior edge of the S1 end plate. This sagittal alignment is defined as a sagittal vertical axis (SVA) and should typically be less than 6 cm. , Thus, MIS deformity correction seeks to improve sagittal imbalance and reduce the SVA to less than 6 cm for most patients aged 20 to 75 years, and less than 8 cm for those over 75 years old.
Correct Coronal Curvature
In patients with low back pain and leg pain, it is essential to evaluate for potential foraminal stenosis, especially in patients who predominantly experience radicular symptoms in the legs. Lumbar foraminal stenosis can result from progressive spinal degeneration resulting in a disc herniation, loss of disc height, or osteophyte formation resulting in compression of the nerve root as it exits through the foramen. Additionally, compression of the nerve root within the foramen can occur on the concavity of a coronal curve. Patients with symptoms from foraminal stenosis can experience an exacerbation of their pain, particularly with lumbar extension, which is known as Kemp’s sign.
As the major curve of a scoliotic deformity progresses, the spine develops additional minor curves as the body attempts to maintain coronal balance. For a thoracolumbar scoliotic deformity, if a compensatory “minor” curve is located at the lumbosacral junction (L4 to S1), it is called the fractional curve. The fractional curve can result in severe foraminal stenosis and compression of the L5 nerve root on the concavity of the curve. Therefore, these patients typically experience significant radicular pain. Fractional curves are one of the more difficult areas for a spinal deformity surgeon to address with MIS techniques. Nevertheless, an MIS deformity correction should try to address any coronal curve that is causing symptomatic foraminal stenosis.
Restore Pelvic Incidence and Lumbar Lordosis Harmony
Normal lumbar lordosis (LL) typically ranges anywhere from 20 to 80 degrees, with the apex of the curvature located at the L3-4 disc space or superior end plate of the L4 vertebral body. , Furthermore, approximately two-thirds of the lordosis within the lumbar spine arises from the L4–L5 and L5–S1 levels. Thus, surgeons planning major corrections with the intent of adding greater than 20 degrees of LL should concentrate the largest degree of their correction lower down in the lumbar spine.
Spinopelvic harmony is based on the principle of a “cone of economy,” which states that there is an optimal relationship between the pelvis and the spine that results in the least amount of effort and energy expenditure to maintain balance and walk upright without the use of a cane or walker. Pelvic incidence (PI) is defined as the angle drawn between the center of the femoral head and the midpoint of the sacral end plate and a line drawn downward and perpendicular to the center of the sacral end plate. In general, spinopelvic harmony is defined as the PI being within 10 degrees of the LL. Thus, any corrective surgery within the thoracolumbar spine should endeavor to bring these parameters within 10 degrees of each other. As patients age, their LL decreases. Thus, any correction intending to improve sagittal balance in advanced-age patients should be diligent not to “overcorrect the patient.” ,
Minimally Invasive Surgery Deformity Algorithm
Treatment of spinal deformity using open techniques offers a robust correction and greater exposure of bony surfaces for posterior and posterolateral arthrodesis. However, these open techniques can result in significant morbidity because the complication rates are relatively high owing to the extensive spine reconstruction performed. This consideration is particularly important, as the population of elderly patients requiring deformity operations increases as patients are living longer with a greater expectation to lead highly active lives.
The minimally invasive surgery deformity (MISDEF) 2 algorithm assists the surgeon in identifying which spinal deformities are amenable to an MIS approach, open approach, or hybrid (MIS and open) approach. As shown in Fig. 149.1 , the MISDEF 2 algorithm separates various surgical spine pathologies into four classes in order of the increasing invasiveness of surgery needed to effectively treat the patient’s pathologies.
Class I includes patients who typically present with a significant component of radicular pain without significant axial back pain. As shown in Fig. 149.2 , patients in class I have minimal or no sagittal plane deformity, with a LL/PI mismatch less than 10 degrees and pelvic tilt less than 25 degrees. Additionally, these patients have minimal coronal deformity, with a Cobb angle less than 20 degrees. These patients’ complaints primarily stem from compression of their neural elements. Therefore, the surgical treatment to address the patient’s complaints is predicated on adequate decompression of the neural elements with or without a fusion.
Class II, although similar to Class I, encompasses patients with a more significant coronal Cobb angle and/or more significant sagittal plane imbalance. More specifically, the patients within class II have a LL/PI mismatch between 10 and 30 degrees, a thoracic kyphosis under 60 degrees, and/or a thoracolumbar junctional kyphosis under 10 degrees. As shown in Fig. 149.3 , a Class II patient can be treated with a multilevel MIS approach for fusion, correction, and decompression.
Patients within Class III have significant sagittal deformity, with a LL/PI greater than 30 degrees, thoracic kyphosis over 60 degrees, and/or a thoracolumbar junction kyphosis over 10 degrees. Class III operations include a circumferential MIS fusion with anterior column release, mini-open pedicle subtraction osteotomy, expandable cage technology, and/or hybrid MIS-open approaches. As shown in Fig. 149.4 , class III operations are most amenable to patients with no preexisting hardware that requires revision and who have not had more than five levels of prior fusion that included L5-S1. Additionally, these patients may also have a fixed deformity, which allows for an adequate correction utilizing an MIS approach.
Class IV patients have greater than five levels of fusion that include L5–S1 or greater than 10 segments needing treatment or preexisting multilevel instrumentation. A class IV operation involves an open surgery with osteotomies with or without extension of the fusion to the thoracic spine. As shown in Fig. 149.5 , patients may require a posterior three-column osteotomy for correction because of a fixed/rigid deformity from prior fusion.
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