Elsevier

The Spine Journal

Volume 17, Issue 5, May 2017, Pages 681-688
The Spine Journal

Clinical Study
The impact of obesity on compensatory mechanisms in response to progressive sagittal malalignment

https://doi.org/10.1016/j.spinee.2016.11.016Get rights and content

Abstract

Background Context

Obesity's impact on standing sagittal alignment remains poorly understood, especially with respect to the role of the lower limbs. Given energetic expenditure in standing, a complete understanding of compensation in obese patients with sagittal malalignment remains relevant.

Purpose

This study compares obese and non-obese patients with progressive sagittal malalignment for differences in recruitment of pelvic and lower-limb mechanisms.

Study Design/Setting

Single-center retrospective review.

Patient Sample

A total of 554 patients (277 obese, 277 non-obese) were identified for analysis.

Outcome Measures

Upper body alignment parameters: sagittal vertical axis (SVA) and T1 spinopelvic inclination (T1SPi). Compensatory lower-limb mechanisms: pelvic translation (pelvic shift [PS]), knee (KA) and ankle (AA) flexion, hip extension (sacrofemoral angle [SFA]), and global sagittal angle (GSA).

Methods

Inclusion criteria were patients ≥18 years who underwent full-body stereographic x-rays. Included patients were categorized as non-obese (N-Ob: body mass index [BMI]<30 kg/m2) or obese (Ob: BMI≥30 kg/m2). To control for potential confounders, groups were propensity score matched by age, gender, and baseline pelvic incidence (PI), and subsequently categorized by increasing spinopelvic (pelvic incidence minus lumbar lordosis [PI−LL]) mismatch: <10°, 10°–20°, >20°. Independent t tests and linear regression models compared sagittal (SVA, T1SPi) and lower limb (PS, KA, AA, SFA, GSA) parameters between obesity cohorts.

Results

A total of 554 patients (277 Ob, 277 N-Ob) were included for analysis and were stratified to the following mismatch categories: <10°: n=367; 10°–20°: n=91; >20°: n=96. Obese patients had higher SVA, KA, PS, and GSA than N-Ob patients (p<.001 all). Low PI−LL mismatch Ob patients had greater SVA with lower SFA (142.22° vs. 156.66°, p=.032), higher KA (5.22° vs. 2.93°, p=.004), and higher PS (4.91 vs. −5.20 mm, p<.001) than N-Ob patients. With moderate PI−LL mismatch, Ob patients similarly demonstrated greater SVA, KA, and PS, combined with significantly lower PT (23.69° vs. 27.14°, p=.012). Obese patients of highest (>20°) PI−LL mismatch showed greatest forward malalignment (SVA, T1SPi) with significantly greater PS, and a concomitantly high GSA (12.86° vs. 9.67°, p=.005). Regression analysis for lower-limb compensation revealed that increasing BMI and PI−LL predicted KA (r2=0.234) and GSA (r2=0.563).

Conclusions

With progressive sagittal malalignment, obese patients differentially recruit lower extremity compensatory mechanisms, whereas non-obese patients preferentially recruit pelvic mechanisms. The ability to compensate for progressive sagittal malalignment with the pelvic retroversion is limited by obesity.

Introduction

The rising prevalence of obesity within modern industrialized society is cause for serious public health concern and has prompted deepened attention from the orthopedic surgical community. The effects of obesity on musculoskeletal systemic degeneration, including early degenerative disease and functional impairment, are well established; however, these associations are less investigated in the context of spinal deformity. Given concurrently increasing rates of obesity and adult spinal deformity (ASD) in the United States, a deeper understanding of obesity's role as a supplemental driver of sagittal deformity is of significant value [1].

Recent research involving compensatory mechanisms used in response to progressive sagittal malalignment highlights the importance of minimizing energetic expenditure to maintain an erect, upright posture [2], [3], [4]. Deviations in spinopelvic harmony from ideal and age-specific norms are believed to contribute to regional malalignment and, subsequently, the recruitment of selective compensatory mechanisms to realign the center of gravity about the feet [4], [5]. In a long-term effort to optimize operative correction, novel analyses of compensatory mechanisms involving the pelvis and lower extremities emphasize both the importance of full-body preoperative radiographic assessment and the evaluation of patient-specific factors that may impact compensation, such as age, gender, and weight [6], [7], [8], [9].

Broadly, the impact of obesity in the setting of spinal deformity has been previously described with regard to contributions to biomechanical stress, complication rates, and clinical outcomes [10], [11], [12]. Previous biomechanical studies, for example, have demonstrated that increased body mass generates elevated compressive loads on discs and shear stresses within the lumbar spine [13], [14]. Furthermore, Vismara et al. importantly identified restricted forward flexion in obese patients with chronic back pain and postural adaptation marked by anterior pelvic shift (PS) and lumbar hyperlordosis [13].

Comprehensive radiographic analyses of obese patients with spinal deformity are rare within the literature and conflicting. In a 2013 study of 200 participants with no deformity, Romero-Vargas et al. failed to identify differences in the sagittal spinopelvic parameters of normal, overweight, and obese subjects [15]. Park et al., although noting no differences in preoperative sagittal alignment among 77 morbidly obese patients with ASD undergoing minimally invasive surgery compared with non-obese controls, did observe a significant difference in postoperative pelvic incidence minus lumbar lordosis (PI−LL) mismatch and sagittal vertical axis (SVA) [16]. Additionally, body mass has been shown to influence preliminary changes in focal alignment: in a study of 30 obese patients with back pain undergoing bariatric surgery, Lidar et al. demonstrated that following significant weight loss, patients experienced a marked increase in the L4–L5 intervertebral disc height [17]. Despite some evidence that the obese spine differs from the non-obese spine, these conflicting results collectively illustrate the need for increased attention, including head-to-toe radiographic analysis of these patients.

Recent efforts to describe compensation in response to progressive sagittal malalignment using the pelvis and lower limbs have demonstrated success using full-body stereoradiographic imaging (EOS imaging). The head-to-toe evaluation provides an illustration of musculoskeletal interdependence in the weight-bearing position, and has already been used successfully to examine a variety of lower extremity compensatory parameters in adults with spinal pathology [18].

The primary goals of this study were to compare sagittal malalignment in obese and non-obese patients with spinal pathology, and to investigate weight-dependent preferential recruitment of compensatory mechanisms with progressive sagittal malalignment. This analysis is the first to use a full-body imaging system to evaluate these differences.

Section snippets

Data collection

This study was an institutional review board–approved retrospective review of patients visiting a single academic center for spine-related complaints from November 2013 to June 2015. Inclusion criteria were age >18 years, available full-body radiographs, and degenerative spinal pathologies (scoliosis, kyphosis). Demographic data including age, body mass index (BMI), and gender were collected.

Biplanar radiographic acquisition

All included patients underwent biplanar full-body stereographs (EOS Imaging, Paris, France) [19]. The

Study sample

A total of 2,391 patients meeting inclusion criteria were identified. Before propensity score matching, obese patients (N=516) in the total cohort were significantly older (60.41 vs. 49.76 years, p<.001) and contained a greater proportion of females (57.2% vs. 42.8%, p<.001) than did the non-obese group. Following propensity score matching, there were 554 total patients identified for analysis (Ob=277; N-Ob=277). The total cohort comprised 62.3% women, had a mean age of 60.29±15.38 years, and a

Discussion

Previous studies have demonstrated that several patient-specific factors influence the activation of compensatory mechanisms to restore sagittal alignment for a given deformity [6], [7], [18], [29]. Although there are many compensatory mechanisms involving the pelvis and lower extremities, the primary aim of these efforts remains to maintain Dubousset's “conus of economy,” whereby the center of gravity is centrally positioned over the feet. To achieve this end, multiple positional adaptations

Conclusion

Obese patients demonstrate a different chain of compensatory events for a given spinopelvic mismatch when compared with non-obese cases. As mismatch increases, obese patients preferentially recruit lower extremity compensatory mechanisms, including knee flexion and posterior PS, to maintain the conus of economy. In contrast, non-obese patients recruit compensatory mechanisms involving the pelvis. This study demonstrates the negative impact of obesity on the ability to compensate for sagittal

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    Author disclosures: CMJ: Nothing to disclose. BGD: Nothing to disclose. DLC: Nothing to disclose. GWP: Nothing to disclose. SV: Nothing to disclose. AJB: Nothing to disclose. RL: Nothing to disclose. SB: Royalties: Pioneer (B); Consulting: K2 Medical (B), NuVasive (none), Innovasis (none), Allosource (B); Grants: K2 Medical (D, Paid directly to institution/employer), NuVasive (D, Paid directly to institution/employer), Innovasis (F, Paid directly to institution/employer), DePuy Synthes (F, Paid directly to institution/employer), outside the submitted work. TJE: Royalties: Fastenetix (F); Consulting: K2 Medical (C); Speaking and/or Teaching Arrangements: K2 Medical (C); Trips/Travel: K2 Medical (D); Research Support (Investigator Salary, Staff/Materials): Pfizer (B, Paid directly to institution/employer); Grants: Fridolin (E, Paid directly to institution/employer), ISSGF (E, Paid directly to institution/employer); Fellowship Support: OMEGA (E, Paid directly to institution/employer), AOSpine (E, Paid directly to institution/employer), outside the submitted work. VL: Stock Ownership: Nemaris INC (none); Speaking and/or Teaching Arrangements: NuVasive (B), DePuy Spine (B), Nemaris INC (B), Medicrea (B); Board of Directors: Nemaris INC (none); Grants: SRS (D, Paid directly to institution/employer), NIH (D, Paid directly to institution/employer), DePuy Spine (H, Paid directly to institution/employer), outside the submitted work. PGP: Consulting: Medicrea (none), outside the submitted work.

    The disclosure key can be found on the Table of Contents and at www.TheSpineJournalOnline.com.

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