Deformity and MIS: Limitation or Opportunity?

4 Deformity and MIS: Limitation or Opportunity?

Catherine Miller, Ibrahim Hussain, Juan S. Uribe , Kai-Ming Fu, and Praveen Mummaneni

Summary

Adult spinal deformity comprises a complex and wide spectrum of spinal disease. Sagittal balance and spinopelvic parameters have been shown to correlate with disability and patient reported outcomes. Adult deformity correction has traditionally been treated with open surgical correction but is associated with high complication rates. Minimally invasive surgery (MIS) techniques were developed as alternative corrective strategies in an effort to decrease the perioperative complications and morbidity associated with open deformity correction surgery. Proper patient selection is critical to ensure adequate deformity correction using MIS techniques and depends on the patients, their symptoms, the deformity, and the spinopelvic parameters.

Keywords: minimally invasive adult spinal deformity spinopelvic parameters sagittal balance pelvic incidence lumbar lordosis MISDEF2 algorithm

4.1 Introduction

Adult spinal deformity comprises a complex and wide spectrum of spinal disease ranging from progression of adolescent idiopathic scoliosis to degenerative scoliosis from asymmetric degeneration of the spine.1 Depending on the type and severity of the spinal deformity, patients often have significant disability. They can present with obvious abnormalities in posture, deformity of the body, and pain and neurologic dysfunction as a result of structural changes and/or impingement of neural elements.

Adult spinal deformity continues to be a challenging disease faced by spine surgeons. In 2016, elderly patients (age >65 years) made up 14.5% of the United States population.² The prevalence of scoliosis in this age group has been reported to be as high as 68%.³ With the increasing elderly population, there is a rising number of patients being evaluated for possible deformity correction. Significant investigation is underway to determine which of these patients will benefit from correction of their deformity and through which surgical approach (traditional open surgery versus minimally invasive techniques).

4.2 Spinopelvic Parameters

The extent of spinal deformity is determined through radiographic evaluation of the spine. Sagittal balance is influenced by the relationship between the pelvis and the spine. Ideal balance maintains the head over the center of the pelvis. If sagittal imbalance is present, a person can use compensatory measures (hip extension, pelvic retroversion) to bring the spine toward normal alignment, but this compensation has limits. Spinopelvic parameters are used in the assessment of deformity to determine the degree of sagittal imbalance and its relationship to the pelvis (Fig. 4.1). These parameters have been shown to correlate with health-related quality of life scores and disability, both prior to and after deformity correction.

Fig. 4.1 Spinopelvic parameters: Lumbar lordosis, pelvic incidence, pelvic tilt, and sacral slope.

Sagittal balance is measured by the sagittal vertebral axis (SVA) from 36 inch standing X-rays. SVA is the distance from a plumb line drawn down from the midpoint of the C7 vertebral body to the dorsal corner of the end plate of S1. The SVA is positive if the plumb line falls ventral to the dorsal corner of the end plate of S1 and negative if the plumb lines fall dorsal to this point. Normal SVA is 0 to 5 cm.

Pelvic incidence (PI) is the angle between a line drawn perpendicular to the midpoint of the sacral end plate and a line from this point to the center of the femoral heads. PI is a constant value that is not affected by body posture and is the sum of the sacral slope (SS) and pelvic tilt (PT). SS is the angle between the horizontal and a line along the sacral end plate. Pelvic compensation is measured by the PT. PT is the angle between a vertical line drawn upwards from the center of the femoral heads and a line drawn from the center of the femoral heads to the midpoint of the sacral end plate.

Lumbar lordosis (LL) is the angle between the lines drawn parallel to the superior end plate of L1 and the sacral end plate. It has been shown that there is a relationship between LL and PI with the ideal difference less than 10 degrees (PI-LL ≤ 10 degrees).

Glassman et al reviewed 752 patients with adult spinal deformity from a multicenter prospective database and found that increasing SVA was associated with a linear worsening of health-reported quality of life scores (Scoliosis Research Society patient questionnaire [SRS-29], Oswestry Disability Index [ODI], and Medical Outcomes Study Short Form [SF-12]).⁴ Schwab et al evaluated disability in 492 adult spinal deformity patients from a multicenter database. They found that SVA, PT, and PI-LL mismatch were key factors to impact disability in deformity patients.⁵ In a study reviewing 125 adult patients with flatback deformity (PI-LL mismatch >10 degrees), with and without normal SVA, Smith et al found that a PI-LL mismatch was associated with significant disability.⁶

4.3 Opportunity for MIS Deformity Correction

Degenerative adult scoliosis is a progressive disease which worsens significantly over time. Curves tend to increase 1 to 2 degrees per year, with lateral listhesis being a documented risk factor for progression. Patients with lumbar stenosis who have concomitant scoliosis often demonstrate a significant increase in deformity 1 year after decompression surgery.⁷^,⁸^,⁹ The operative treatment of patients with deformity takes this into account by relieving compression of neural structures, realigning the spine to achieve global spinal balance, and promoting arthrodesis. This has traditionally been accomplished with open surgical techniques, which require extensive spinal exposure with muscle retraction, long operative times, and significant blood loss. Deformity correction surgery has been associated with high perioperative morbidity, with historical reports citing complications rates ranging from 20 to 80%.¹⁰^,¹¹^,¹² The International Spine Study Group (ISSG) performed a multicenter study in which they reviewed 953 adult spinal deformity patients who underwent surgical correction. They reported a 7.6% rate of major complications, with the most common complications being excessive blood loss, pulmonary embolism, and wound infection requiring reoperation.¹³ Another study performed a retrospective review of 206 patients who underwent deformity correction from a multicenter database. They found major complication rates varied from 6 to 29%, with complications occurring more frequently in older patients.¹⁴ In a study examining 407 deformity corrective procedures, Pateder et al found a 30-day mortality rate of 2.4%.¹⁵

In an effort to decrease the perioperative complications and morbidity associated with open deformity correction surgery, minimally invasive surgery (MIS) techniques were developed as alternative corrective strategies. These techniques allow for less disruption to the paraspinal musculature through the utilization of various MIS retractor systems while achieving the same goals as open deformity correction surgery. Several studies have shown that MIS for deformity can result in less blood loss, shorter hospital stays, and earlier postoperative recovery for appropriately selected patients with moderate deformities.16^,¹⁷^,¹⁸^,¹⁹^,²⁰

MIS techniques were developed as alternative corrective strategies in an effort to decrease the perioperative complications and morbidity associated with open deformity correction surgery. Proper patient selection is critical to ensure adequate deformity correction using MIS techniques.

4.4 Limitations of MIS

As MIS techniques gained popularity for deformity correction, additional research has been published which identify some of the limitations of MIS. Early studies of MIS techniques showed improved coronal balance; however some patients did not achieve optimal sagittal correction.²¹^,²² Dakwar et al published a series of 25 adult patients who underwent MIS for deformity correction using the minimally invasive lateral transpsoas approach. Perioperative complications occurred in 12% of the patients. Although they had improvement in clinical outcomes and coronal balance, one-third of the patients in this early study did not have restoration of their sagittal balance.²³

Because of limited bony exposure for a fusion surface, pseudarthrosis has been reported in MIS for deformity, especially at levels where an interbody fusion is not used. Wang and Mummaneni reviewed 23 patients who underwent MIS for deformity correction. There was improvement in both clinical and radiographic results; however, 28.6% of patients who underwent posterolateral (without interbody) fusion had pseudarthrosis.²⁴

Over time, MIS techniques have advanced with the use of hyperlordotic grafts and anterior column reconstructions with practices involving anterolateral stand-alone instrumentation, circumferential MIS (cMIS), and hybrid surgery (MIS + open). The ISSG studied 85 consecutive patents undergoing deformity correction from a multicenter database and compared the results of stand-alone fusion, cMIS, and hybrid surgeries. They found similar clinical outcomes between the three groups; however the hybrid group achieved greater coronal and sagittal correction, albeit with high complication rates. For coronal curve correction, they found a ceiling effect of typical lordosis corrections of 23, 34, and 55 degrees for stand-alone, cMIS, and hybrid, respectively. Only the hybrid approach allowed for statistically significant improvements in sagittal plane correction and also markedly greater improvement in LL.25

4.5 Patient Selection

Despite the advancements in MIS for deformity correction and its reported advantages, not every patient is a suitable candidate to undergo deformity correction through a minimally invasive approach. Literature has shown that several factors can help determine this, such as flexibility of the curve, spinopelvic alignment, and the degree of the sagittal deformity. Mundis et al investigated 63 deformity patients who underwent cMIS or posterior MIS correction and divided the patients into groups based on their baseline SRS-Schwab global alignment modifier. They found that those with less sagittal imbalance had greater improvement in ODI and numeric rating scale (NRS) in back and leg scores. The group with severe sagittal imbalance was not adequately treated with an MIS approach and similar deformities may be better suited for traditional open correction.²⁶

Eastlack et al and the ISSG evaluated baseline characteristics of 350 patients who underwent either open or MIS for deformity correction from two multicenter databases. The approach was determined by surgeon’s preference. The MIS group was slightly older with worse preoperative ODI scores. They had smaller coronal deformities but similar sagittal malalignment. MIS was more often used in patients with SVA < 6 cm and PI-LL mismatch <30 degrees.²⁷

4.6 MISDEF Algorithm

Given the complexity of adult spinal deformity and the numerous available surgical techniques, the Minimally Invasive Spinal Deformity (MISDEF) algorithm was developed to help guide surgeons in selecting the optimal surgical approach for their deformity patients who had failed nonoperative management.²⁸ This algorithm was established using the Delphi approach to ensure agreement from experts in the field. The initial iteration of the algorithm divided patients into three classes based on various radiographic parameters. As MIS techniques and instrumentation evolved, an updated MISDEF2 algorithm was developed. The MISDEF2 algorithm included a new class in which newer circumferential MIS and hybrid strategies could be utilized resulting in four total classes (Fig. 4.2).²⁸

Fig. 4.2 Minimally Invasive Spinal Deformity (MISDEF2) algorithm is a guide to assist with decision-making on which adult spinal deformity patients are candidates for minimally invasive surgical approaches. (Reprinted with permission from Mummaneni PV, Park P, Shaffrey CI, et al. The minimally invasive interbody selection algorithm for spinal deformity. J Neurosurg Spine 2021; 34(5): 741–748.)

Only gold members can continue reading. Log In or Register to continue