Degenerative lumbar scoliosis (DLS) is a common condition in older population with a reported prevalence ranging from 7.5% to 30% [1, 2]. Classifying adult scoliosis may facilitate decision-making based on the comparison of similar cases and provide guidance for treatment. Several different classifications have been described by Aebi [3], Lowe et al [4], Schwab et al [5] and provide spine surgeons with different information [6]. However, none of them focused on the impact of preoperative coronal deformity on the postoperative coronal imbalance.
Recently, Bao et al [7] proposed a novel classification of coronal deformity based on the absolute value of preoperative global coronal malalignment (GCM) and demonstrated that patients with type C [GCM greater than 30 mm and C7 plumb line (PL) shifted to the convex side of major curve) were at greater risk for postoperative coronal imbalance than type A (GCM less than 30 mm) and type B (GCM greater than 30 mm and C7 PL shifted to the concave side of major curve). More recently, Obeid et al [8] adhered to the same basic principle as Bao’s and proposed a similar but more extensive classification using 20 mm as the threshold of coronal imbalance. However, the big weakness of Obeid’s classification is that it was based on the authors’ personal experiences, not on clinical experiments. Therefore, it is unknown whether the classification is feasible or not. Moreover, it is still undetermined whether preoperative GCM or preoperative pattern based on spatial relationship between C7 PL and major curve had main effects on postoperative coronal imbalance. Since one of the aims of the classification is to help guide the treatment, once a classification is defective, following the classification might lead to disastrous complications.
The purpose of this study was to identify the main effects of preoperative pattern and GCM on the postoperative coronal imbalance in DLS patients and evaluate the rationality of classification of coronal deformity based on preoperative GCM by Obeid et al [8].
Materials and methods
Patient population
This study was approved by Ethical Committee of our hospital, and all methods were performed in accordance with the guidelines and regulations of the ethics review board. We retrospectively collected consecutive DLS patients with age greater than 45 years between January 2015 and December 2019. Inclusion criteria included: primary spinal deformity correction and instrumented fusion through posterior-only approach. Exclusion criteria included: fusion levels < 5, history of hip or knee arthroplasty, absolute discrepancy of leg length > 20 mm [9]. 112 DLS patients were enrolled in this study eventually. Informed consent was obtained from all subjects.
Surgical techniques
After exposure, bilateral pedicle screws were inserted at every level in the construct. 84 patients were fused to pelvis while 28 patients were fused to L5. To obtain better deformity correction, spinal osteotomies including Schwab grade I (facetectomy) or grade II (Ponte osteotomy) were conducted in all patients [9]. Decompression and transforaminal lumbar interbody fusion (TLIF) were performed at the caudal part of lumbar spine (L2-S1) if assistive anterior support was necessary, or spinal stenosis was present [9].
Radiographic evaluation
Whole spine standing posteroanterior and lateral X-rays were collected before surgery and at discharge from hospital or 2 weeks after surgery. Surgimap (version 2.2.15; Spine Software, New York, NY) was used to perform the coronal and sagittal measurements by two independent spine researchers and the mean values were collected for analysis. Coronal measurements included: (1) GCM, defined as the horizontal distance between C7 PL and central sacral vertical line (CSVL) [7]; (2) major Cobb angle, defined as the angle between superior endplate of the most tilted vertebra cranially and inferior endplate of the most tilted vertebra caudally. Sagittal measurements included:(1) thoracic kyphosis (TK, T5-T12); (2) pelvic tilt (PT); (3) pelvic incidence minus lumbar lordosis (PI-LL); (4) sagittal vertical axis (SVA); and (5) pelvic incidence (PI).
Because spinal osteotomies were conducted in all patients, osteotomy grades and osteotomy levels were recorded, too. Instrumented levels, distribution of uppermost or lowest instrumented vertebra (UIV or LIV), and levels of interbody fusion were recorded as well.
Preoperative coronal pattern evaluation
According to the spatial relationship between C7 PL and major coronal curve on full-spine standing posteroanterior radiographs, the preoperative patterns were classified into two patterns: Pattern 1(concave pattern), C7 PL shifted to the concave side of the major curve; Pattern 2(convex pattern), C7 PL shifted to the convex side of major curve [7]. Three experienced spine surgeons reviewed all radiographs and classified them into two patterns.
According to Obeid et al [8] proposed classification, based on absolute values of GCM preoperatively, patients were classified into 3 types:
Type 0: GCM less than 20 mm plus either Pattern 1 or Pattern 2. To facilitate factorial analysis, patients with type 0 were further subdivided into type 0–1 (GCM less than 20 mm plus Pattern 1) and type 0–2 (GCM less than 20 mm plus Pattern 2).
Type 1: GCM greater than 20 mm plus Pattern 1.
Type 2: GCM greater than 20 mm plus Pattern 2.
Therefore, there were 4 groups (type 0–1, type 0–2, type 1, type 2) involved in this study.
Postoperative coronal imbalance was defined as GCM greater than or equal to 20 mm. Postoperative imbalance/balance ratio was also recorded, which could reflect the incidence of postoperative coronal imbalance.
Statistics
Intra-pattern comparison (type 0–1/type 1 or type 0–2/type 2) of continuous variables were conducted using independent t test, continuous variables among 4 groups were compared using one-way analysis of variance (ANOVA). Categorical variables were compared using Chi-square analysis or Fisher’s exact test. To determine the main effects of preoperative pattern and preoperative GCM on the postoperative coronal imbalance, a two-factor ANOVA was used for further factorial analysis. During factorial analysis for postoperative imbalance/balance ratio, postoperative coronal balance was coded as “0”, and postoperative coronal imbalance was coded as “1”. The statistical analysis was performed using SPSS computer software (version 24; SPSS, Chicago, IL, USA). P < 0.05 was set as statistical significance.