1 Introduction

Despite decades of awareness, the obesity epidemic remains a nationwide challenge. 39.8% of adults were considered obese with a BMI greater than 30 in 2015–2016.¹ Using current trends, it is now predicted that approximately 50% of Americans will be overweight by the year 2030.² In orthopaedics, obesity significantly increases mechanical loading through lower extremity joints, confers greater joint reactive forces, and may complicate postoperative care. Specifically, the tibiotalar joint is subject to joint reaction forces greater than four times body weight during walking and greater than ten times with running.³ Additionally, obesity poses greater challenges with rehabilitation and disrupts normal musculoskeletal healing.^4–7

Ankle fractures are one of the most common orthopaedic injuries with approximately 260,000 ankle fractures per year in the United States.⁸ Depending on the fracture pattern, some fractures are amenable to conservative management. However, most ankle fractures achieve favorable results with operative management.⁹ Several studies have examined the relationship between obesity, as measured by body mass index (BMI), and ankle fracture complexity with differing conclusions regarding their association.^10–13 Furthermore, there is a paucity of information on the relationship between BMI and the need for open reduction and internal fixation of syndesmotic injuries. However, ankle injuries with associated syndesmotic failures tend to be more severe and syndesmotic injury itself is a predictor for worse outcomes.^14–16

Therefore, the purpose of this study was to evaluate the relationship between BMI and operative ankle fracture pattern as well as the likelihood of requiring open syndesmotic reduction and fixation. We hypothesized that increasing BMI would be associated with higher ankle fracture complexity and an increased likelihood of requiring open reduction and internal fixation of the syndesmosis.

2 Methods

Data Set: This study was granted exemption by the Institutional Review Board at our institution because the deidentified data was acquired through the American College of Surgeon's National Surgical Quality Improvement Program (ACS-NSQIP) database. This database is nationally validated and peer-controlled. Over 600 hospitals nationwide supply perioperative data which is then deidentified and shared with participating institutions. This includes patient demographics and comorbidities, surgical details, and any postoperative complications within 30 days from the date of surgery. This data is obtained by trained Surgical Clinical Reviewers and frequent internal audits ensure a high quality of standardized reporting.¹⁷

Study Population and Outcomes: Patients treated with open reduction and internal fixation of ankle fractures between the years of 2013–2017 were included in this study. Patients were identified by the Current Procedural Terminology (CPT) codes 27766 (open treatment of medial malleolus fracture), 27769 (open treatment of posterior malleolus fracture), 27792 (open treatment of lateral malleolus fracture), 27814 (open treatment of bimalleolar ankle fracture), 27822 (open treatment of trimalleolar ankle fracture without fixation of posterior lip), and 27829 (open treatment of distal tibiofibular joint, syndesmotic fixation). Open fractures, fractures involving the tibial plafond (i.e. pilon fractures), and polytrauma patients were all excluded from the study.

Patient-specific data was collected and included demographic information, comorbidities, and surgery-specific variables. Demographic and comorbidity information included age, race, sex, and height, American Society of Anesthesiologists (ASA) class, smoking status, diabetes, chronic obstructive pulmonary disease, hypertension, and independent functional status prior to surgery. Body mass index (BMI) was then calculated from height and weight data ([weight in kilograms]/[height in meters²]).

Statistical analyses: Patients were stratified by fracture pattern and syndesmotic injury into 6 groups according to the appropriate CPT codes: unimalleolar fracture with or without syndesmotic reduction, bimalleolar fracture with or without syndesmotic reduction, and trimalleolar fracture with or without syndesmotic reduction. The frequencies of baseline characteristics and demographics were calculated and compared between groups using the chi-square test for categorical variables and one-way ANOVA with Tukey's post hoc test for continuous variables.

Univariate ordinal regression was used to determine the unadjusted risk that increasing BMI had on malleolar fracture complexity with and without syndesmotic injury. Three univariate logistic regression analyses were utilized to assess the unadjusted risk that increased BMI has on a syndesmotic injury accompanying a malleolar fracture for unimalleolar, bimalleolar, and trimalleolar fractures, respectively. A multivariate ordinal regression model assessing the risk increasing BMI had on malleolar fracture pattern with adjustments for demographic and comorbid variables. Multivariate logistic regression models were then created to assess the risk of syndesmotic injury accompanying a malleolar fracture due to increased BMI for unimalleolar, bimalleolar, and trimalleolar fractures. All models used an alpha of 0.05 and reported 95% confidence intervals (CI). Statistical analysis was performed using IBM SPSS Statistics for Windows, Version 25.0 (IBM Corp., Armonk, NY, USA).

3 Results

Baseline Characteristics: A total of 15,841 patients met inclusion criteria for this study. The mean age of the patients was 48.9 years. The majority of patients were white, female, and non-smokers. When stratifying patients by fracture pattern and syndesmotic injury, there were significant differences in age, sex, race, BMI, and ASA classification. Patients with trimalleolar fractures were more likely to be older, female, white, and obese than patients with unimalleolar fractures (Table 1). Additionally, patients with a syndesmotic injury were more likely to be younger, male, black, obese, and have lower ASA classifications compared to patients without syndesmotic injuries (Table 2).

Syndesmotic Injury and BMI: Univariate logistic regression models were created to analyze the effect of demographic factors and comorbidities on the need for open syndesmotic reduction and fixation accompanying malleolar fractures. Substantial predictors (p < 0.1) were then included as variables in the adjusted, multivariate logistic regression models for unimalleolar fractures, bimalleolar fractures, and trimalleolar fractures (Table 3).

Elevated BMI was found to be associated with an increased need for open syndesmotic reduction and fixation. Specifically, the risk for syndesmotic fixation increased by 4.2% per increase in BMI point in unimalleolar fractures, 3.0% in bimalleolar fractures, and 3.4% in trimalleolar fractures (Table 3). Additionally, all fracture patterns were at a substantially greater risk for open reduction and fixation of the syndesmosis when patient BMI was greater than 40 compared to normal weight individuals (BMI 18.5–24.9, Fig. 1). Specifically, unimalleolar ankle fractures had almost three times the risk (OR 2.876, CI 2.238–3.696, p < 0.001) and the risk was similarly increased for bimalleolar (OR 2.325, CI 1.665–3.246, p < 0.001) and trimalleolar (OR 2.249, CI 1.503–3.365, p < 0.001) ankle fractures.

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Fig. 1 Risk of syndesmotic fixation by BMI category. The odds ratios for the risk of needing syndesmotic fixation for patient with unimalleolar, bimalleolar, and trimalleolar fractures were calculated using a multivariate logistic regression model for each fracture type.

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In addition to BMI, age and sex were also significant predictors for syndesmotic reduction and fixation for all three malleolar fracture patterns. Specifically, the risk for syndesmotic fixation decreased with increasing age for all three malleolar fracture patterns (Table 3). Women were also at a lower risk of experiencing a syndesmotic injury with all three fracture patterns, and black race was only a predictor for syndesmotic fixation in the setting of unimalleolar injuries.

Fracture Pattern and BMI: Univariate ordinal regression analysis of BMI and malleolar fracture complexity demonstrated that BMI had a significant effect on the likelihood of more complex malleolar fracture patterns. Specifically, the risk of fracture complexity increased by 0.5% for every increase in one BMI point (OR 1.005, 95% CI 1.001–1.009, p = 0.028, Table 4). Additionally, females, diabetics, older patients, white patients, and patients with COPD, congestive heart failure, hypertension, and non-independent functional status were also at risk for more complex fracture patterns (Table 4, Model 1).

Therefore, all of these factors were included in an adjusted, multivariate ordinal regression model, and only age, race, and sex remained significant predictors in this model (Table 4, Model 2). BMI was no longer a significant predictor of malleolar fracture complexity (p = 0.146). Females had more than twice the risk of experiencing a more complex malleolar fracture than males (OR 2.049, 95% CI 1.914–2.194) and every one-year increase in age resulted in a higher risk for more complex fractures (OR 1.013, 95% CI 1.011–1.016, p < 0.001).

4 Discussion

This study found that elevated BMI correlated to increased need for open reduction of the syndesmosis in operatively managed malleolar ankle fractures, yet BMI was not a significant predictor of ankle fracture complexity based on malleolar involvement. Additionally, age was found to be a positive predictor of increasing fracture complexity but decreasing need for open reduction of the syndesmosis.

There remains much debate in the literature as to the relationship between obesity and ankle fractures. For example, Spaine et al. found that BMI was significantly greater in patients with displaced fractures as compared to patients who sustained non-displaced ankle fractures.¹⁰ However, Greenfield et al. studied 103 women aged 50–80 years compared to a population-based cohort of 375 women and found no significant differences in the prevalence of osteoporosis. The only statistical difference was that the ankle fracture cohort had a higher body mass index than the control group.¹⁸ Interestingly, Acosta-Olivo et al. studied 188 patients with acute ankle injuries treated in the emergency department and concluded that obesity was not the main factor that influenced the complexity of ankle fractures. The authors suggested that age was the primary determinant of ankle injury severity and noted that patients over the age of 30 were 20% more like to sustain an ankle fracture.¹³ In our multivariate model, this data found that obesity was not a significant predictor of increasing ankle complexity based on malleolar involvement. Rather, age was identified as the most significant predictor of fracture complexity.

Identification and appropriate treatment of syndesmotic injuries is critical for successful outcomes, yet accurate diagnosis of these injuries remains challenging. While Weber C fibula fractures are most commonly associated with syndesmotic disruption, prior studies suggest that roughly 40% of Weber B bimalleolar ankle fractures have syndesmotic instability.¹⁹ However, there is a paucity of literature investigating the relationship between syndesmotic injury and obesity. King et al. reviewed the radiographs of 280 patients and discovered that obese patients (defined by BMI >30) had a 1.78 times greater odds of a Weber C ankle fracture compared to a Weber A or Weber B ankle fracture but did not comment specifically on incidence of syndesmotic injury.¹¹ Our study revealed that BMI was a significant predictor for requiring open reduction of the syndesmosis in patients with concomitant malleolar ankle fractures. In addition to this, age and sex were also found to be significant predictors of needing syndesmotic fixation for all three malleolar fracture patterns.

There are limitations to this study that merit discussion. The NSQIP database has limited information regarding clinical outcomes and complications beyond 30 days. These studies work under the assumption that all data was recorded accurately, and appropriate CPT coding was documented. The authors acknowledge the possibility of inappropriate coding when surgeons are treating neuropathic ankle fractures as the current recommendation is to augment with quadricortical fixation.²⁰ Due to the retrospective nature of the study and data collection strictly based off of CPT coding, the authors were not able to obtain information regarding the mechanism of injury or bone mineral density and how they may impact ankle fracture complexity and need for syndesmotic fixation. The authors also acknowledge that CPT codes are an inexact method of data collection and use the CPT codes as a surrogate for fracture complexity.

5 Conclusion

We can conclude from this study using nationwide data, that there is a positive association between obesity and the need for open reduction of the syndesmosis in operatively treated malleolar ankle fractures. However, obesity does not appear to be linked to ankle fracture complexity based on degree of malleolar involvement. This data adds to the existing literature regarding the impact of obesity as well as other modifiable and nonmodifiable risk factors on the degree of ankle fracture complexity. Understanding how obesity can impact operative management of syndesmotic injuries and ankle fractures is paramount to setting expectations for patients, and also provides a framework for surgeons and hospitals to identify patients at risk for more complex ankle fractures.

Funding

This research did not receive any funding from agencies in the public, commercial, or not-for-profit sectors. All authors had full access to the data in the study and approved the manuscript for publication.