Degenerative pathology includes degenerative and spondylolisthesis diagnoses. NS = not statistically significant.

**Table 2.** Comparison of patient diagnosis by gender.


**Table 3.** Patient comorbidities separated by gender.


A total of 1045 patients (54.1%) underwent primary fusion (Table 1). There were no statistical differences in levels fused between the two groups. On average, males with deformity disease underwent 8.2 level fusions and females underwent 8.64 level fusions. For degenerative disease, males underwent 2.09 level fusions and females underwent 2.13 level fusions.

#### *3.2. Pain and Function*

Females reported slightly higher pain scores pre-operatively (6.54 vs. 6.14; *p* < 0.01). At 6 weeks post-op, females continued to describe marginally more pain than males (VAS 4.36 vs. 3.99; *p* < 0.01). By 3 months, there was no gender-based difference in pain scores (female VAS: 3.73 vs. male VAS: 3.76; *p* > 0.01). Furthermore, there was no significant gender difference in pain scores at 6 months (*p* > 0.01), or 1 year post-operatively (*p* > 0.01). Both male and female patients demonstrated significant clinical improvement in pain scores by 1 year follow-up (Table 4).

**Table 4.** Visual Analog Scale (VAS) for pain pre-operatively and at 6 weeks, 3 months, 6 months, and 1 year post-operatively, separated by gender. Oswestry Disability Index (ODI) score pre-operatively and post-operatively at 1 year, separated by gender.


Females reported lower pre-op dysfunction scores (ODI scores, Table 4) when compared to males (F = 49.73 vs. M = 46.52; *p* < 0.01). Functional improvements in both genders were significant at 1 year (*p* < 0.01). At one year, there was no gender difference in ODI scores noted (*p* > 0.01). Females experienced a slightly greater mean overall improvement in ODI by 1 year (20 points in females vs. 16.6 points for males; *p* < 0.01).

#### *3.3. Gender-Based Complication Rates*

Comparing post-operative complications in our study group, there were no gender differences in pseudarthrosis rates, re-operation rates, or other complications (Table 5). Death is listed as a complication for any patient who died within 2 years of surgery.

**Table 5.** Complications separated by gender.


#### *3.4. Predictors of Pain and Function*

Predictors of pain and function were estimated through a linear regression model of 1 year post-operative VAS scores (Table 7) and ODI scores (Table 6), respectively. Separate models were fit for each gender and prediction estimates were based on the parameter coefficients (β), with significant coefficients (*p* < 0.05) interpreted as the estimated change in VAS or ODI for a unit change in the respective factor. As can be seen in Table 6, number of comorbidities, type of diagnosis, presence of complications, and BMI were found to significantly contribute to female 1 year post-operative ODI scores, while number of comorbidities, level of fusion, type of diagnosis, presence of complications, and BMI were found to significantly contribute to male 1 year post-operative ODI scores. Likewise, as shown in Table 7, number of comorbidities, presence of complications, BMI, and age were found to significantly contribute to female 1 year post-operative VAS scores, while number of comorbidities, level of fusion, type of diagnosis, presence of complications, and age were found to significantly contribute to male 1 year post-operative VAS scores.


**Table 6.** Linear regression model of post-op ODI at 1 year for male and female.

**Table 7.** Linear regression model of post-op VAS at 1 year for male and female.


#### **4. Discussion**

Numerous studies sugges<sup>t</sup> certain patient characteristics and comorbidities affect outcomes after spinal fusion [7–12]. The few risk factors that have been shown to consistently result in worse outcomes include BMI, age, cardiovascular disease, smoking, and receiving worker's compensation or disability benefits [7–10]. However, the effect of patient gender on outcome after spinal arthrodesis has not been solidified.

In 2002, Gehrchen et al. conducted a retrospective review including 112 patients with degenerative disc disease (DDD) and spondylolisthesis that showed female gender to be an independent risk factor for a nonoptimal outcome after lumbar fusion [12]. In 2009, Ekman conducted a randomized control trial that included 164 patients treated with spinal fusion for spondylolisthesis that suggested females had worse PROs post-operatively [10]. In 1984, when analyzing the outcomes after treatment for cervical disc disease, Eriksen et al. found that females have more pain and dysfunction post-operatively after fusion surgery [13].

However, the results of our study align more closely to those of Triebel et al. and Pochon et al. [5,6]. Triebel et al., in a study that included 4780 Swedish patients with lumbar degenerative disc disease and chronic low back pain, found that Swedish women reported similar pain and function outcomes to men after lumbar spinal fusion [5]. Additionally, a 2016 study by Pochon et al. that included 1518 patients found that females who underwent decompression alone or decompression with fusion ± instrumentation did not experience a difference in outcomes when compared to men [6].

Our study shows that while females reported slightly more pain and worse function than males at the time of surgery, by 3 months and beyond, no further gender differences in post-operative pain or function existed. Our findings support the ultimate conclusions of gender outcome equality by Triebel et al. and Pochon et al. However, our study further expands their findings to the realm of deformity surgery [5,6].

An important aspect to acknowledge when reviewing the results of our study is the MCID for VAS back pain and ODI score. Previous studies have suggested that the MCID for VAS and ODI are 2.1 and 14.9, respectively [14,15]. Both MCIDs are significantly higher than the difference found in at any time point in our study. Therefore, the slightly increased pain (F = 6.54 vs. M = 6.14; *p* < 0.01) and disability (F = 49.73 vs. M= 46.52; *p* < 0.01) that females present with prior to undergoing spinal arthrodesis is not clinically relevant.

Similar results have been echoed in the total joint arthroplasty literature. For instance, Holtzman and Katz showed that females have more pain and dysfunction prior to undergoing total joint arthroplasty. However, they found that females do not recover as well post-operatively compared to their male counterparts [16,17]. Another finding of our study was that females were slightly older than males when they underwent spinal arthrodesis (61.7 years ± 12.8 vs. 59.7 years ± 14.1; *p* < 0.01). As far as we are aware, why females wait longer and endure more pain before undergoing spine or total joint surgery has not been well studied. Possible explanations for this phenomenon include that (1) females are more reluctant to choose surgical intervention [18], (2) females spend more time gathering information about risks and benefits [19], and (3) females are more likely than males to be prescribed anti-depressants or referred to mental health before being offered surgical intervention [2,20]. Another possible explanation for delayed spinal arthrodesis in females is that many gender comparative studies performed prior to 2010 showed inferior outcomes in females after spine surgery which may differentially impact the decision making from the surgeon's standpoint [9,10,12,13].

Given the higher comorbidity burden of females, it is surprising that they ultimately achieved similar outcomes to males. There are several potential explanations. Physical therapy use has been associated with improved outcomes after lumbar fusion [21], and current literature shows that females are much more likely to utilize physical therapy [22,23]. Females are also more likely to follow-up with their physician after lumbar surgery [24]. Interestingly, a study analyzing patient compliance after total knee arthroplasty showed that females are more like to be compliant when compared to males [25]. Female patients' propensity to attend physical therapy and comply with a physician's recommendation may explain their increased margin of post-operative improvement compared to males. Once again, it is important to note that although this margin of improvement is statistically significant, it does not reach the MCID and, therefore, is unlikely to be clinically significant.

There are several limitations to this study. First, this study was retrospective in nature. All patients underwent open posterior spinal arthrodesis and results may differ for other approaches or decompression without fusion. Furthermore, patient-reported pain and functional scores are individually subjective. Additionally, although our analyses accounted for many variables, possible confounding variables that we were unable to account for include patient expectations, the operating surgeon, physical therapy effort by the patient, psychosocial factors, living environment, and psychological background. Additionally, this study focused on general VAS scores for pain and did not distinguish between back pain and leg pain. Additionally, the findings here are limited to a single center's experience and may not be broadly applicable.
