An increasing number of lumbar fusion procedures are being performed with minimally invasive exposures, despite a perception that minimally invasive lumbar interbody fusion procedures are unable to affect segmental and regional lumbar lordosis (LL).1,2 Although many studies report segmental and regional alignment outcomes in the treatment of degenerative (nondeformity) lumbar pathology, consensus on the issue has not yet been achieved. In a field with a growing focus on alignment as a predictor of outcome and long-term surgical durability in degenerative spinal surgery,3,4 a broader understanding of the potential for modern and evolving surgical techniques to preserve or correct alignment is needed. The purpose of this study was twofold: (1) to review lumbar segmental and regional alignment changes following treatment with a variety of minimally invasive surgery (MIS) interbody fusion procedures for degenerative conditions and (2) to evaluate whether baseline characteristics (e.g., preoperative lordosis) may be able to predict magnitude of alignment changes.
MATERIALS AND METHODS
A search of peer-reviewed, human-subject journals through MEDLINE and Google Scholar databases from 1980 to November 11, 2015, was conducted using search terms, including “lordosis,” “spin*,” minimally invasive,” “interbody,” and “fusion,” without “[citation],” or “cervical.” A complete list of search criteria is included in Table 1. Any full-length article that reported original data on radiographic characteristics, at least regional or segmental LL, using MIS interbody fusion surgery for short-segment (<four levels) degenerative (nondeformity) pathology was collected and included in the current study. Each article from the primary search was assessed individually across title and abstract for inclusion within the previously described parameters. A second review, which included full article review, was undertaken to further discriminate the primary findings. For each study included in the analysis, study and patient (per reported cohort) characteristics were tabulated along with all available preoperative and postoperative alignment measures [LL, segmental lordosis (SL), pelvic incidence (PI), pelvic tilt (PT), and sagittal vertical axis (SVA)]. Preoperative to postoperative change in alignment was calculated for each study cohort with paired (preoperative to postoperative) data to determine raw (° or mm) and percentage mean (study-level) differences following intervention. Weighted averages were calculated using each paired alignment set for all radiographic parameters with more than one data point available.
Statistical analyses included paired-samples t-tests for comparing preoperative to postoperative alignment changes and single-linear regressions to compare degree of alignment changes [raw value (°) and percent (%) change from preoperative] based on preoperative LL and SL. All analyses were performed using JMP version 12 (SAS Institute, Cary, NC) and significance was accepted for P < 0.05.
A total of 2218 articles were identified through the primary search. Following primary review, 113 articles underwent full-article review. Following secondary review, 23 articles were accepted for inclusion with a total of 28 unique study cohorts reported and analyzed as part of this review (Figure 1).1,2,5–25 There were two study cohorts reported for mini-ALIF approaches, 16 for lateral transpsoas interbody fusion (XLIF), and 10 for MIS posterior interbody approaches. Cohort-level study and alignment data are detailed in Table 2. The results were limited to analysis of regional lordosis (lumbar) and SL, as PI was not reported in any paper examined, while PT was reported in two and SVA in only one.
Across 19 study cohorts and 720 patients, weighted average of LL preoperatively for all procedures was 43.5° (range 28.4°–52.5°) and increased 3.4° (9%) (range −2° to 7.4°) postoperatively (P < 0.001). However, two of the studies did report postoperative decreases in LL. SL increased, on average, by 4° from a weighted average of 8.3° preoperatively (range −0.8° to 15.8°) to 11.2° at postoperative time points (range −0.2 to 22.8°) (P < 0.001) in 1182 patient from 24 study cohorts (Table 3). All studies reported increases in postoperative SL. Simple linear regression revealed a significant relationship between preoperative LL and change in LL (r2 = 0.413; P = 0.003), wherein lower preoperative LL predicted a greater increase in postoperative LL. A similar relationship between preoperative LL and percentage (from preoperative) in postoperative LL change was observed (r2 = 0.627; P < 0.001) (Figure 2). SL did not show the same inverse relationship between preoperative values and postoperative change when evaluating raw values (r2 = 0.001; P = 0.840), though a relationship was seen when percentage change from preoperative was assessed, in the same inverse correlation observed in the LL regression analysis (r2 = 0.473; P < 0.001) (Figure 3). When comparing the raw (°) and percentage magnitude change from preoperative to postoperative of LL by SL, no significant trends were observed [(r2 = 0.091; P = 0.257) and (r2 = 0.001; P = 0.896), respectively] (Figure 4).
A comparison between the MIS anterior approach interbody fusion cohorts (ALIF and XLIF) and MIS posterior approaches revealed significant increases in regional lordosis and SL from baseline within each group, with similar magnitude of increases observed between the comparative groups. Weighted average results between anterior and posterior MIS approaches are included in Table 4.
Despite the continually increasing adoption of modern minimally invasive interbody fusion techniques for the treatment of lumbar degenerative pathology, an anecdotal perception persists that these techniques are unable to affect lumbar alignment parameters, particularly LL and SL. This perception has been perpetuated by a few reports that show unchanged segmental or global LL following MIS techniques without considering whether or not lordosis increases were a goal of surgery.1,2 In 2011, Acosta et al.1 reviewed the radiographic outcomes of 23 patients who underwent lateral transpsoas interbody fusion for a variety of degenerative surgical indications. The authors concluded that although coronal measures were significantly improved, the procedure “does not improve regional lumbar lordosis or global sagittal alignment.” This is despite the fact that mean SL increased from 5.3° to 8.2° (P < 0.001) and LL improved from 42.1° to 46.2° (P > 0.05), and it was not stated whether or not the study was powered to detect a negative statistical finding. Meanwhile, other studies have found the same procedure significantly improving LL and SL and concluding the opposite findings.5,7,13 The current study sought to address this uncertainty through a thorough literature review and to provide a more objective examination of the radiographic outcomes of MIS interbody fusion techniques in treating short-segment, degenerative spinal conditions.
The primary findings of the current study were that, in aggregate, MIS techniques for lumbar interbody fusion increase both SL and LL. Also, the amount of correction that is achieved, especially for LL, is directly related to the preoperative lordosis of the patient, where relative preoperative hypolordosis predicts a greater increase in postoperative lordosis. Phillips et al.26 suggested this phenomenon in a study from 2013 analyzing clinical and radiographic characteristics in 107 patients who underwent XLIF for adult degenerative scoliosis. In this study, the authors found that patients with hypolordosis (<40°) preoperatively had a significant increase in LL postoperatively [from 27.7° to 33.6° (22%), P < 0.001], while patients with normal preoperative lordosis experienced less LL gain. On the basis of the findings of the current study, it appears that when a patient has normal lordosis and is properly aligned preoperatively, the goal of surgery should be for alignment preservation. If alignment changes, are required through surgical intervention, particularly lordosis increases, then the restoration of alignment is the goal of surgery and MIS procedures are able to significantly increase segmental and regional lordosis. The distinction between these planning goals (preservation vs. restoration) has only been reported in a few instances and not at all in the literature reviewed in the current study. Thus, a question of “did lordosis increase postoperatively?” for a specific procedure is irrelevant without first knowing the preoperative lordosis of the patient or patient sample and the goals of surgery.
There are also other factors, outside of preoperative lordosis measurements, that can impact postoperative lordosis. Some of these factors include the levels and number of levels treated (for LL), the cage position in the intervertebral space, the type of cage used (lordotic vs. nonlordotic), the type of supplemental internal fixation applied, and the use of compressive techniques with posterior instrumentation to increase lordosis. Although the current study was unable to compare these elements as part of the literature review due to the nonsystematic nature of how patients were treated (e.g., heterogeneous samples) and how data were reported across studies (e.g., many studies reported no details on these factors), several individual studies reported specific findings across these parameters. Kepler et al.7 in 2012 analyzed a series of 29 patients with 67 levels of XLIF to determine which factors influenced SL changes. Although the authors found, overall, 11% and 90% increases in lumbar and SL, respectively, the authors found that cage type (lordotic vs. parallel) or dimension (height) did not impact lordosis, though a more anterior placement of the cage in the intervertebral space significantly increased lordosis (+7.4° per level) compared with those placed posteriorly (−1.2° per level). Sembrano et al.17 studied the role of lordotic (10°) cages compared with parallel (0°) cages and found that lordotic cages significantly increased postoperative SL (+2.8°) compared with parallel cages (+0.6°). Although the authors did not find statistically significant increases in LL, the cohorts were not necessarily powered to detect such differences (20 patients in each group with 31 and 30 levels treated in each). In an examination of the role of fixation on alignment, Dahdaleh et al.6 compared alignment following unilateral or bilateral pedicle screw and rod fixation after MIS TLIF. The authors examined 16 unilateral and 20 bilateral pedicle screw and rod fixation patients and found that SL increased slightly more in the bilateral pedicle screw and rod fixation cohort than in unilateral patients (0.4° vs. 2.1°). Several studies, however, reviewed SL and LL changes after uninstrumented lateral interbody fusion, with increases in both postoperatively. Marchi et al.14 in 2012 examined 22 patients who underwent standalone lateral interbody fusion for isolated discogenic back pain and found that SL increased from an average of 12.2° preoperatively to 16.7° [4.5° (37%) mean change] postoperatively. Similarly, Marchi et al.13 in the same year reported standalone lateral interbody fusion results in 52 patients treated for Grade I or II spondylolisthesis and found that SL increased an average of 6° (9.7°–15.7°) and LL increased an average of 3.7° from 42.8° to 46.5°. The level treated also may play a role in the amount of lordosis achieved. In 2015, Pawar et al.16 examined 18 levels of L3 to L4 and 30 levels of L4 to L5 lateral interbody fusion. The authors found that, overall, LL increased an average of 8% (44.1–47.5), though SL only increased 1.5° (11.2°–12.7°) in the L3 to L4 levels compared with 4° at L4 to L5 (15.6°–19.6°). Kotwal et al.10 similarly reported individual-level lordosis measurements from T12-L1 to L4 to L5 following XLIF and found generally similar lordosis improvements at all levels, with the exception of T12-L1. All of these individual findings suggest that postoperative lordosis and alignment correction are multifactorial and cannot be predicted solely on the basis of the type of interbody fusion procedure performed.
In addition, and potentially more importantly, in emerging research, the preoperative and postoperative malaligned spine in short-segment, degenerative (nondeformity) disease and surgery has been associated with increased rates of adjacent level spondylolisthesis, depressed rates of health-related quality of life, and elevated rates of adjacent segment degeneration and construct failure.3 Lumbosacral alignment measurement requires visualization of, at least, the full lumbar spine and both femoral heads, in order to accurately measure PT, PI, and sacral slope. Spinal balance requires visualization of the lower cervical spine and the sacrum in a continuous image (either stitched or through long films). Current practice does not generally allow for collection of these images on non-deformity patients, which is reflected in the findings of the current study. Of the studies examined in this review, one reported preoperative and postoperative SVA, two reported preoperative PT (thought only one reported change in PT), and PI was not measured in any study.
Limitations of the current study include the inability to do more nuanced subgroup analyses to examine the role of fixation, procedure type, levels treated, and so on in preoperative and postoperative lordosis and alignment. Also, in general, heterogeneous individual study cohorts limit the quality of the data being reviewed as well as the strength of conclusions able to be made, as most studies were level III or IV study designs.
Significant gains in both LL and SL were seen following MIS interbody fusion in weighted average analyses across studies, with two of 19 LL and none of the SL cohorts decreasing in lordosis postoperatively. These results suggest that MIS approaches are able to impact regional and local segmental alignment and that preoperative patient factors (baseline lordosis) can impact the extent of correction gained (preserving vs. restoring alignment). However, lordosis change postoperatively is multifactorial and the interbody procedure type is one of several variables that influence radiographic outcome. Finally, lumbosacral alignment is increasingly being recognized as important for long-term surgical success, even in short-segment lumbar degenerative disease, and there is a clear lack of data or understanding in the current literature.
- A MEDLINE and Google Scholar search was performed and a total of 23 articles and 28 study cohorts were identified that reported alignment in minimally invasive lumbar interbody fusion for degenerative lumbar conditions.
- From preoperative to postoperative time points, mean regional and segmental LL increased significantly [3.4° (9%) and 4° (47%)] in a weighted average analysis of 19 and 24 study cohorts, covering 720 and 1182 patients, respectively.
- A significant negative correlation was observed between preoperative LL and change in LL postoperatively, indicating that study cohorts with lower baseline LL experienced a greater postoperative increase in lordosis.
- This study demonstrated that modern minimally invasive interbody fusion procedures are able to affect regional lordosis and SL.
The authors thank Kyle Malone, MS, at NuVasive, Inc. for his editorial and statistical help in this work.
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