The Effect of an Abnormal BMI on Orthopaedic Trauma Patients: A Systematic Review and Meta-Analysis

Aims: The aim of this systemic review is to identify the complications that arise in operating on orthopaedic trauma patients with an abnormal body mass index (BMI). Materials and Methods: Systematic literature search using a combination of MESH subject headings and free text searching of Medline, Embase, SCOPUS and Cochrane databases in August 2019. Any orthopaedic injury requiring surgery was included. Papers were reviewed and quality assessed by two independent reviewers to select for inclusion. Where sufficiently homogenous, meta-analysis was performed. Results: A total of 26 articles (379,333 patients) were selected for inclusion. All complications were more common in those with a high BMI (>30). The odds ratio (OR) for high BMI patients sustaining post-operative complication of any type was 2.32 with a 23% overall complication rate in the BMI > 30 group, vs. 14% in the normal BMI group (p < 0.05). The OR for mortality was 3.5. The OR for infection was 2.28. The OR for non-union in tibial fractures was 2.57. Thrombotic events were also more likely in the obese group. Low BMI (<18.5) was associated with a higher risk of cardiac complications than either those with a normal or high BMI (OR 1.56). Conclusion: Almost all complications are more common in trauma patients with a raised BMI. This should be made clear during the consent process, and strategies developed to reduce these risks where possible. Unlike in elective surgery, BMI is a non-modifiable risk factor in the trauma context, but an awareness of the complications should inform clinicians and patients alike. Underweight patients have a higher risk of developing cardiac complications than either high or normal BMI patient groups, but as few studies exist, further research into this group is recommended.


Introduction
The impact of obesity and increased body mass index (BMI) upon health services is a growing concern both nationally and internationally [1]. Elective orthopaedic surgery affords the opportunity for the balance of risk and adequate preparation of the patient; however, this is rarely the case in trauma surgery.
Obese patients were shown to have longer hospital stays, on average an additional 1.3 days in 9916 obese patients [28,29,32,38]. Data were not found for underweight patients.
Revision surgery rates were recorded in 723 patients [21] (564 normal and 159 obese), the obese had a hazard ratio of 3.08 and 3.10 for metal work failure. Rates of metal work failure and non-union were recorded in 13,818 patients (12,727 normal, 1091 obese) [26], and of these an average of 7.7% of normal BMI patients had metal work failure compared to 8.2% obese. Non-union was reported in 11.1% of normal and 16.2% of obese ( Figure 5). The OR for non-union in 50 obese patients [37] with tibial shaft fractures was 2.57.
In respect to cardiac and deep vein thrombosis (DVT), the OR of cardiac complications was reported for 2360 obese patients, and the average OR was calculated at 1.94 [21,25,26]. Furthermore, the proportion of patients experiencing cardiac complications was considered in 379 normal patients and 530 obese patients; on average, 2.25% of patients with a normal BMI experienced cardiac complications (including arrhythmias), compared with 3.65% of obese patients [36,38]. The risk of any event is higher in the underweight OR (1.56) when compared with normal BMI and obese.
Incidence of DVT was assessed in 1763 patients (1202 normal BMI, 561 obese) [20,23,29,38,40,41]. In total, 1.28% of normal BMI patients reported DVT, compared with an average of 3.18% of obese. purpose of this analysis categorised OR were averaged. This included 164 obese patients who suffered superficial (1.79) and deep infections (2.09) [39]. A total of 2841 underwent open reduction and internal fixation (ORIF) (1.9 (27), 3.2 (46)), with 967 undergoing intramedullary nailing (IMN) (1.4 [27], 3.4 [45]), 821 undergoing hemiarthroplasty for proximal humeral shaft fractures (4 [45]), and 256 undergoing total shoulder arthroplasty for proximal humeral shaft fractures (2.3 [45]). Data were not found for underweight patients. Obese patients were shown to have longer hospital stays, on average an additional 1.3 days in 9916 obese patients [28,29,32,38]. Data were not found for underweight patients. Revision surgery rates were recorded in 723 patients [21] (564 normal and 159 obese), the obese had a hazard ratio of 3.08 and 3.10 for metal work failure. Rates of metal work failure and nonunion were recorded in 13,818 patients (12,727 normal, 1091 obese) [26], and of these an average of 7.7% of normal BMI patients had metal work failure compared to 8.2% obese. Non-union was reported in 11.1% of normal and 16.2% of obese ( Figure 5). The OR for non-union in 50 obese patients [37] with tibial shaft fractures was 2.57. In respect to cardiac and deep vein thrombosis (DVT), the OR of cardiac complications was reported for 2360 obese patients, and the average OR was calculated at 1.94. [21,25,26] Furthermore, the proportion of patients experiencing cardiac complications was considered in 379 normal patients and 530 obese patients; on average, 2.25% of patients with a normal BMI experienced cardiac complications (including arrhythmias), compared with 3.65% of obese patients. [36,38] The risk of any event is higher in the underweight OR (1.56) when compared with normal BMI and obese.

Discussion
As the global epidemic of obesity and the incidence of eating disorders continues to rise, the number of patients with abnormal BMI and significant orthopaedic trauma will also rise.
Identification of specific risk is important, and should guide individual surgeons and units to develop strategies to mitigate these enhanced risks.
An increase in BMI brings about pathophysiological changes in almost all organ systems, including cardio-vascular disease, diabetes mellitus and cancer, largely as a display of 'metabolic syndrome' [46]. Positive energy-balance, with more calories consumed than expended and consideration of adipose distribution, with particular concern given to intrabdominal accumulation,

Discussion
As the global epidemic of obesity and the incidence of eating disorders continues to rise, the number of patients with abnormal BMI and significant orthopaedic trauma will also rise.
Identification of specific risk is important, and should guide individual surgeons and units to develop strategies to mitigate these enhanced risks.
An increase in BMI brings about pathophysiological changes in almost all organ systems, including cardio-vascular disease, diabetes mellitus and cancer, largely as a display of 'metabolic syndrome' [46]. Positive energy-balance, with more calories consumed than expended and consideration of adipose distribution, with particular concern given to intrabdominal accumulation, can explain such change. Excess accumulation around structures can result in compression leading to complications such as hypertension with renal compression and sleep apnea as a result of pharyngeal compression, both of which hold potential for anaesthetic complications [47].
Production of proinflammatory adipokines by excess adipocytes can result in persistent systemic inflammation in some obese patients which may have implications on wound healing and infection [49].
Excess weight puts increased pressure on joints and is itself a risk factor for osteoarthritis, particularly of the knee joint. OA has impacts on mobility and rehabilitation, both key considerations following orthopaedic trauma [48]. Alongside physical changes, it is vital we consider and recognise the psychiatric implications of obesity and the pathophysiological consequences resulting in increased prevalence of depression and other mental health conditions. The exact physiology behind this is unknown but suggestions such as increased social isolation due to societal stigma is proposed [48,50].
Patients with an abnormal BMI present many challenges for orthopaedic surgeons at all stages of their care. Pre-operatively, the challenge of stabilisation, access to adequate imaging and sourcing of specialist implants and operative equipment, e.g., bariatric operating tables, can delay surgery. Furthermore, excess skin can make access difficult and can create difficulties surrounding sterility if large areas are exposed. During operation, as discussed previously, anaesthesia and airway management can be challenging both from an intubation/bag-mask ventilation, extubation and drug perspective, due to 'poor respiratory mechanics' and less oxygen reserve and altered dosing requirements. Alongside increased levels of intraoperative blood loss, patients generally have longer operative times, which can lead to increased risk of nerve palsy [51].
Post-operatively, as the review indicates, patients with an abnormal BMI face higher rates of complications, from infection to poor mobilisation [52].
This review indicates that compared to patients with a normal BMI, obese orthopaedic trauma patients face greater risk of overall complications, including mortality, wound problems, cardiac events and thrombotic events. They are also more likely to need reintervention for metal work failure or non-union. Metal work failure is higher in patients who undergo IMN compared with ORIF, whilst non-union is seen more frequently in ORIF. Table 2 indicates the prevalence of these complications, with non-union and infection/wound problems being highest, and DVT being the lowest. Furthermore, obese patients have worse functional outcomes and are less likely to achieve complete bone union. This is generally reflective of the study by Chesser et al. in 2010. There is no direct reflection of the mentioned 'obesity-paradox' when compared to a normal BMI. An increased complication rate has led to an increase in the average length of stay, and so an increased cost of care per episode. With a mean cost per patient/day of GBP 143.20 on a standard orthopaedic ward and staffing costs of GBP 155.46, ward-based care alone could add GBP 388.26 to an inpatient stay. This cost is, without factoring in the much higher cost of high dependant unit (HDU)/critical care unit(CCU) care, GBP 559.42/day, which is likely to be required by patients with major complications [53][54][55][56].
The Forrest plot indicates that underweight trauma patients also face worse outcomes, with increased overall complications and a greater mortality incidence compared to normal patients. This is comparable with Whiting et al. [9]. Surprisingly, underweight patients have increased rates of cardiac complications compared to all groups, despite it being thought that obese patients are more likely to present with cardiovascular disease [10,23].

Conclusion
As a high BMI increases the risk of complications related to post orthopaedic trauma surgery, consideration of this should be at the forefront when faced with obese trauma patients and, where possible, strategies should be created to mitigate the additional risks and costs. In this patient group, the index of suspicion of infection/non-union should be higher, with the threshold to be lower for starting antibiotics and other interventions where appropriate. Post-operatively, nutritional advice should be sought to address deficiencies in macro and micro dietary requirements, along with lifestyle support. There is an urgent need for further research into strategic interventions in this patient group to improve outcomes and reduce complications.
Finally, a low BMI appears to increase the risk of mortality and cardiac complications but limited data for this population restrict any firm conclusions. Further research into this group is recommended.

Conflicts of Interest:
The co-authors declared that there are no competing interests in the production or content of this paper.