Adult spinal deformity (ASD) encompasses a spectrum of conditions with abnormal spinal alignment that can lead to pain, neurological dysfunction, and gross deformity of the body. ASD is most commonly defined in patients ≥18 years of age who have a coronal Cobb angle greater than 20°, sagittal vertebral axis (SVA) greater than 5 cm, and/or pelvic tilt (PT) greater than 20°. The goals of spinal deformity surgery, regardless of approach, include the decompression of neural elements and restoration of global spinal balance while promoting arthrodesis. Minimally invasive (MIS) techniques have garnered increased attention as alternative and/or adjunct modalities in ASD surgery with a goal of reduced surgical access morbidity and perioperative complications. The general principles of MIS techniques for ASD are the same as traditional open techniques with respect to global alignment targets. We review the literature regarding the use of MIS techniques in ASD surgery, with special attention to spinopelvic parameters and spinal balance.1–9
MIS Techniques and MIS ASD Surgery Categories
MIS techniques for adult spinal deformity surgery include lateral lumbar interbody fusion (LLIF) for anterior column reconstruction (ACR), MIS transforaminal lumbar interbody fusion (MIS-TLIF), mini-open anterior lumbar interbody fusion (ALIF), percutaneous segmental fixation, and MIS decompression utilizing muscle splitting retractors.
These MIS techniques can either be employed alone, or in combination, through three broad categories of MIS approaches for adult deformity surgery. Each category is defined by the invasiveness of the operation and degree to which MIS techniques are utilized: (A) MIS decompression, (B) circumferential MIS (cMIS) surgery, and (C) MIS + open (hybrid) surgery.9–11 The first category involves minimally invasive decompression with or without single-level/short-segment fusion in patients with mild spinal deformity and symptoms primarily of neural element compression; this category includes stand-alone lateral lumbar interbody fusion. The second category, cMIS, involves 360° deformity correction with anterior column support (interbody graft placement) and posterior segmental instrumentation through an entirely MIS approach. The third category (hybrid) involves the incorporation of the aforementioned MIS techniques with traditional open posterior surgery that include segmental osteotomies and instrumentaion. Hybrid surgery most frequently refers to MIS LLIF in combination with open posterior instrumented fusion. The cMIS approach is distinguished from hybrid surgery in that cMIS avoids the paraspinal muscle dissection and preserves the posterior ligamentous tension band.
Impact of MIS Techniques on Spinal Parameters
Several studies have demonstrated the effectiveness of MIS and hybrid surgical approaches for optimizing coronal and sagittal balance as well as spinopelvic alignment.1–9,12–15Table 1 summarizes the radiographic and clinical outcomes of various studies regarding MIS techniques in ASD. Anand et al reported statistically significant improvements in coronal Cobb angle with cMIS surgery involving multilevel LLIF combined with posterior percutaneous instrumentation; however, they did not evaluate the effect on sagittal balance or spinopelvic parameters. Wang and Mummaneni and Acosta et al demonstrated that LLIF is an effective means of improving segmental and regional lumbar lordosis (LL)5,16; however, the effect on sagittal vertical axis (SVA) and global sagittal balance was less clear. The current literature suggests that LLIF is an effective means of improving LL and SVA, particularly with ALL release and hyperlordotic cages, in patients with mild-to-moderate malalignment (LL mismatch less than 10° and SVA less than 5 cm, respectively).14,17 Not surprisingly though, the effect of LLIF on sagittal balance and spinopelvic parameters is significantly enhanced when combined with a posterior approach.
Similar to LLIF, MIS TLIF is often used as an adjunct to open or MIS posterior approaches for ASD, and is especially useful at the lumbosacral junction. Wang18 reported his experience using MIS TLIF with cMIS surgery for ASD and demonstrated statistically significant improvements in all radiographic parameters. Another option to maximize anterior column support is the ALIF. ALIF may result in greater segmental lordosis restoration in the lower lumbar spine when compared with TLIF with a gain of 5.6° versus −1.7° (P < 0.0001) at L4-L5, and a gain of 2.5° versus −1.4° (P = 0.022) at L4-L5. However, TLIF may result in greater reduction in coronal Cobb angle than ALIF (reduction of 22.4° vs. 9.9° (P < 0.0001).19 Both of these techniques have demonstrated similar results regarding restoration of global sagittal alignment.19
ACR involves the combination of lateral lumbar discectomy/ostephytectomy, anterior longitudinal ligament (ALL) release, and placement of hyperlordotic grafts.13,20 With the use of hyperlordotic grafts, ACR significantly increases the lordosis that can be induced at a treated level, often approaching changes seen with posterior three-column osteotomies. Studies by Akbarnia et al and Berjano et al have demonstrated that ACR is effective in correcting sagittal plane deformity. Their results suggest that ACR is capable of producing nearly 30° of lordosis restoration and 10° of reduction in PT.21,22 In their series of 34 patients, Turner et al23 yielded similar results, establishing that MIS ACR is capable of producing a significant reduction in SVA and PT and restoration of regional and segmental LL. Based on these studies, supplementing ACR with posterior two-column osteotomies significantly enhances the corrective power of the procedure and may be superior to either of these maneuvers alone.
In patients with adult spinal deformity, percutaneous pedicle screw instrumentation is an effective means of providing posterior support and optimizing correction of spinopelvic parameters and sagittal as well as coronal imbalance through a minimally invasive corridor.24 This is an important distinguishing factor between cMIS surgery and hybrid surgery. For constructs extending caudal to the pelvis, iliac screws or S2-alar-iliac screws may additionally be inserted in a percutaneous fashion.25
Potential Benefits and Complications of MIS Techniques
Surgical correction for ASD has high perioperative morbidity, in large part due to the need for prolonged anesthesia, significant surgical blood loss, and patient medical comorbidities. In patients undergoing open surgical correction for ASD, the 30-day mortality rate has been reported as high as 2.4% with 40% to 71% of patients experiencing a major or minor complication.9,26,27 MIS techniques have been shown to reduce blood loss and permit earlier mobilization compared with open surgery, which produces fewer perioperative complications and potentially reduced length of stay.1,3–7,14,28,29
The complication profile of MIS surgery in ASD overlaps with traditional approaches. Major complications reported for MIS surgery in adult spinal deformity include pulmonary embolus, myocardial infarction, new neurologic deficit, and excessive blood loss. Complication rates have trended higher in patients undergoing hybrid surgery compared with cMIS.9 Patients undergoing hybrid surgery often require more extensive surgery with more blood loss and a frequent need to stage the operation under multiple sessions of anesthesia. Complications specific to the LLIF approach include transient anterior thigh paresthesias/dysesthesias and hip flexor weakness, which occur in 15% to 40% of patients undergoing LLIF.30–34
Recent literature has compared cMIS surgery and hybrid surgery for adult spinal deformity. A consecutive case series reported by the International Spine Study Group (ISSG) demonstrated that the approaches are similarly effective for correcting sagittal plane deformity and improving quality of life.9 However, the hybrid group demonstrated greater absolute improvement in radiographic parameters at the expense of a higher complication rate compared with the cMIS group.9 The complication rate (major or minor) in the hybrid group was 55% compared with 33% for the cMIS group.9 This finding correlates with Wang et al14 who demonstrated a major complication rate of 40% in patients undergoing hybrid surgery compared with 14% in the cMIS group. Further, approximately twice as many patients undergoing hybrid surgery required an additional operation compared with the cMIS group. Additional data suggest that correction of severe sagittal plane deformity with cMIS surgery is more challenging, with resultant fixed sagittal deformity and worse clinical outcomes.10 Of note, this data is prior to the anterior column reconstruction era.
Proximal junctional kyphosis (PJK) at the upper instrumented vertebra is a common complication of ASD surgery and may be related, at least in part, to disruption of normal anatomic structures including the posterior ligamentous complex, the lamina, the facets, and the intervertebral disc. Additionally, patients with ASD who undergo a limited operation (i.e., single-level fusion or decompression) are at risk of deformity progression.28,29 There is evidence that MIS techniques, specifically percutaneous instrumentation, may reduce the risk of PJK for patients undergoing corrective surgery (radiographic PJK developed in 31.3% of cMIS patients vs. 52.9% of hybrid patients, P = 0.01) and limit deformity progression in ASD patients undergoing a limited operation to address a specific symptom.28,29,35,36 However, there may be a higher risk of pseudoarthrosis for patients with long-constructs performed in an entirely MIS fashion compared with hybrid (fusion to the sacrum 46.8% in cMIS patients vs. 71.6% in hybrid patients, P = 0.001) unless adequate anterior column support is achieved.36
Limitations of MIS Techniques
Recent publications have examined the capacity of MIS approaches for adult deformity to alter spinal parameters known to be associated with improved clinical outcomes following surgery.14 In a consecutive case series, the ISSG reported similar clinical outcomes for stand-alone LLIF, cMIS, and hybrid surgery; however, the degree of sagittal and coronal plane correction was greater in the hybrid group at the expense of a higher complication rate. Choice of MIS approach used was determined by surgeon preference. They reported a ceiling effect of 23°, 34°, and 55° for the stand-alone, cMIS, and hybrid groups on coronal Cobb angle respectively. With respect to sagittal plane correction, only the hybrid group achieved statistically significant improvement in SVA. Further, the hybrid group achieved dramatically greater improvement in lumbar lordosis compared with the groups undergoing exclusive minimally invasive methods (16.6° of improvement for hybrid vs. 5° for stand-alone MIS lateral procedures and 5.7° for cMIS, P < 0.001).14
Current literature suggests that the primary factors determining whether patients are suitable for cMIS surgery are the degree of sagittal plane deformity, spinopelvic malalignment, and curve flexibility.8,9,37–39 Use of hyperlordotic grafts and anterior column reconstruction through the LLIF approach allows for correction of sagittal plane deformity without subjecting the patient to extensive subtraction osteotomies.10,12,15–18,20 As anterior column reconstruction techniques increase in use, more patient profiles may become amenable to MIS approaches for ASD. However, the effectiveness of stand-alone LLIF or LLIF in conjunction with cMIS surgery for severe and fixed sagittal deformity remains less clear. Another recent study examined cMIS surgery in patients stratified by the SRS-Schwab global alignment modifier.10,37,38 This classification system describes spinal deformity on the basis of four coronal curve types with three sagittal modifiers: mismatch between pelvic incidence and lumbar lordosis (PI-LL), sagittal vertical axis (SVA), and PT. Each sagittal modifier is graded as normal, moderate, or marked. Marked sagittal deformity is determined by PI-LL > 20°, an SVA > 9.5 cm, or a PT > 30°.10,37,38 The study demonstrated that cMIS was effective at improving radiographic parameters and quality of life in patients with up to moderate sagittal deformity. However, in patients with marked sagittal plane deformity, cMIS was less effective at improving quality of life and left patients with a fixed sagittal deformity.10 The group concluded that patients with marked sagittal deformity may be better suited for a traditional open posterior procedure.10 Correction of severe fixed sagittal imbalance is technically challenging and often requires extensive two and three column osteotomies through an open approach.
Patient Selection Considerations
To date, the application of MIS approaches for adult spinal deformity surgery has been borne of surgeon preference. The choice of MIS decompression, cMIS surgery, or hybrid surgery for patients with ASD is a reflection of surgeon bias when treating each individual patient. As stated previously, the potential benefits of MIS surgery include lower blood loss, less anatomic disruption, and a reduced complication rate compared with traditional open approaches. However, there is concern that MIS approaches, especially cMIS approaches, may increase the risk of pseudoarthrosis and limit the correction of sagittal plane deformity and spinopelvic malalignment.2,4,6 Although there is no definitive evidence to prove that any single approach (cMIS, hybrid, or traditional open surgery) is superior, recent studies have sought to examine the benefits and potential consequences of each to guide surgical decision making for individual patients.
Although MIS techniques have emerged as powerful tools in the armamentarium of deformity surgeons for certain groups of patients, their limitations are well described. Although the complication profile is lower, MIS techniques are, at present, not as versatile for correction of severe deformity. These observations have generated a need to develop a systematic algorithm for determining which patients are candidates for cMIS surgery. The minimally invasive spinal deformity surgery algorithm was created by the ISSG in 2014.11 This algorithm is based on SVA, PI-LL mismatch, PT, thoracic kyphosis, coronal Cobb angle, curve flexibility, and presence of lateral listhesis.11 Based on these values, patients are categorized into one of three minimally invasive spinal deformity surgery classes depending upon the severity of the combined variables. Class I patients (Figure 1) typically present with symptoms of neural element compression in the presence of mild flexible deformity. In these patients, the treatment goal is neural element decompression; therefore, they may be suitable candidates for MIS decompression only or single-level fusion if there is a component of instability.11 Class II patients (Figure 2) present similarly but with a greater element of back pain associated with more pronounced deformity that may be flexible or fixed. These patients may be suitable for cMIS surgery with decompression and interbody fusion along the coronal curve (either LLIF or MIS-TLIF); posterior instrumentation can be placed with percutaneous or mini-open pedicle screw fixation. Class III patients (Figure 3) present with severe multiplanar fixed deformity (i.e., iatrogenic flat back) with significant osteophyte disease associated with back and often leg pain. These patients typically require open surgery with multilevel two or three column osteotomies and extension of the construct to the thoracic spine. The Minimally Invasive Surgery Section of the International Spine Study Group determined through a survey of experienced spinal deformity surgeons and inter-rater reliability analysis that major preoperative factors that increase the risk of treatment failure (as determined by each surgeon) in patients undergoing cMIS deformity correction include an SVA > 6 cm, PT > 25°, PI-LL > 30, and a coronal Cobb angle >20°. Less invasive anterior column release continues to show increasing promise as an aide to sagittal curve correction, but its limited adoption and added potential morbidity restrict an extensive evaluation to date.
Adult spinal deformity is increasing in prevalence among older adults with comorbidities that increase the risks of surgery. MIS surgical techniques have been increasingly incorporated into the operative treatment of ASD, with a goal of less morbidity and tissue injury. Recent evidence demonstrates that circumferential MIS approaches are effective at correcting coronal plane deformity. cMIS techniques are capable of maintaining or restoring sagittal balance and decompressing neural elements for patients with mild-to-moderate sagittal plane deformity, but less effective for severe sagittal plane deformities. cMIS techniques have been postulated to decrease the risk of proximal junctional kyphosis but increase the risk of pseudoarthrosis. Adult spinal deformity patients with severe fixed sagittal imbalance and spinopelvic malalignment are poor candidates for MIS surgery alone due to the high risk of residual postoperative deformity and fixed sagittal imbalance.8–11,14,37–39 These patients often require open surgery or a hybrid MIS approach (i.e., LLIF combined with anterior column reconstruction) and/or open posterior fixation to perform osteotomies and attain satisfactory correction and arthrodesis.
- Minimally invasive techniques, such as LLIF and percutaneous instrumentation, have emerged as useful tools in the treatment of ASD.
- MIS techniques may reduce the complication profile in patients undergoing corrective surgery for ASD compared with traditional open surgery.
- Open surgery for ASD is capable of achieving greater absolute correction of sagittal and coronal plane deformity.
- In patients with severe and/or fixed sagittal and/or coronal plane deformity, circumferential (cMIS) may be less effective at achieving desired radiographic correction.
- Selecting patients with ASD for stand-alone MIS versus cMIS versus hybMIS or open corrective surgery is based on several radiographic parameters and clinical presentation.
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