**1. Introduction**

Historically, common stereotypes exist regarding the differences in how males and females perceive pain. Females have been reported to describe higher levels of pain when presented with equal amounts of thermal stimuli compared to males [1]. Females have also shown a lower threshold for thermal pain and lower pain tolerance than males [2]. Tonelli et al. reported female joint arthroplasty patients experienced more pain and dysfunction than males, even in the setting of less severe osteoarthritis [3].

However, in the setting of low back pain, much is unknown regarding gender perceived differences in pain and functional outcomes. Chenot et al. found that females had a lower pain threshold and lower functional capacity than males with chronic low back pain [4]. On the other hand, females with chronic low back pain treated with spinal fusion have been shown to experience similar pain and functional outcomes when compared to males [5]. For patients undergoing laminectomy alone or with fusion, similar ultimate clinical outcomes have been reported without gender differences [6].

**Citation:** Gulbrandsen, M.T.; Lara, N.; Beauchamp, J.A.; Chung, A.; Chang, M.; Crandall, D. Early Gender Differences in Pain and Functional Recovery Following Thoracolumbar Spinal Arthrodesis. *J. Clin. Med.* **2021**, *10*, 3654. https://doi.org/10.3390/ jcm10163654

Academic Editors: Takashi Hirai, Hiroaki Nakashima, Masayuki Miyagi, Shinji Takahashi and Masashi Uehara

Received: 14 July 2021 Accepted: 13 August 2021 Published: 18 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Specifically, gender differences in pain perception and function after spinal fusion surgery have not been studied in the setting of lumbar degenerative disease or thoracolumbar deformity. Consequently, gender-based outcome differences remain unclear in patients undergoing spinal surgery for these conditions. The purpose of this study is therefore to analyze how a patient's gender impacts self-reported pain and functional recovery after spinal arthrodesis surgery for thoracolumbar deformity and lumbar degenerative disease.

#### **2. Materials and Methods**

#### *2.1. Patient Sample*

This was a retrospective cohort study utilizing patient data from a single center's prospectively collected surgical database that received IRB exemption. Only adult patients (>18 years old) undergoing open posterior instrumented arthrodesis were included in this study. Included were both primary and revision surgeries of any length, with or without interbody fusions, for lumbar degenerative conditions and thoracolumbar deformity. Patients without a minimum 1 year of clinical and radiographic follow-up data were excluded. Trauma, tumor, and infection cases were additional grounds for exclusion. All surgeries were performed by 5 fellowship trained spine surgeons.

A similar strategy for post-operative pain managemen<sup>t</sup> and limited narcotic use was used throughout this study, with an effort to have all patients off opiate analgesics by 3 months post-op. Post-operative bracing was optional and provided at the request of individual patients. Post-operative physical therapy was typically instituted at 8–12 weeks post-op, and continued for 4 weeks. All patients were placed on a home exercise program after formal physical therapy was completed.

All patient demographic information and baseline characteristics including comorbidities, smoking status, body mass index (BMI), and indication for surgery were noted. Surgical factors were additionally collected.

#### *2.2. Outcome Measures*

Clinical outcome measurements included the Visual Analogue Scale (VAS) and the Oswestry Disability Index (ODI). These scores were collected pre-operatively, at 6 weeks, 3 months, 6 months, and 1 year after surgery. Radiographic data were additionally collected at similar time points. Radiographic evidence of fusion included no implant–bone interface lucency, apparent bridging bone either posterolaterally or through the interspace, and no motion on flexion-extension radiographs at the 1 year post-operative follow-up. All peri-operative complications were noted.

#### *2.3. Statistical Analysis*

Mann–Whitney U test, was employed to determine potential gender differences in ODI and VAS scores. Student's *t*-test was used to compare gender differences in age at the time of surgery. Spearman's rho analysis was used to determine the strength of association between either VAS or ODI scores and a patient's gender, BMI, and age. A Chi-Square test was used to determine potential gender differences in the presence of complications, type of diagnosis (degenerative vs. deformity), and number of comorbidities. Linear regression models were used to estimate and compare the differential effects of a patient's gender, diagnosis (degenerative vs. deformity), age, number of comorbidities, BMI, levels of fusion, revision status, and presence of complications on ODI and VAS scores over time. Statistical significance was set at *p* < 0.01. All statistical analyses were conducted with IBM SPSS Statistics for Windows (IBM Corp., Armonk, NY, USA).
