February 2013

The influence of obesity on ankle fracture risk

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Obese patients are more likely than nonobese individuals to sustain an ankle fracture, particularly a severe ankle fracture. Contributing factors may include increased torque on the ankle or low bone mineral density relative to body weight.

By Christy King, DPM, AACFAS 

Obesity can increase the risk and severity of musculoskeletal injury. Along with the chronic illnesses associated with obesity—such as coronary artery disease, type 2 diabetes, various types of cancer (endometrial, breast, and colon), hypertension, stroke, venous thromboembolism, sleep apnea, dyslipidemia, liver disease, gallbladder disease, abnormal menses, infertility, and osteoarthritis—obesity also is associated with an increased risk of musculoskeletal injury.1-4 While excess body adipose tissue is protective of certain fractures, it can also put an individual at a higher risk for more distal fractures and increase the force associated with the injury.5 As the battle against obesity moves into the spotlight, it is important for practitioners in many facets of medicine to understand the potential risks associated with obesity to treat this population more effectively.

Obesity is a formidable epidemic, presenting significant challenges in terms of both healthcare and economics. Researchers have predicted that by 2030 that 86.3% of the US population will be overweight or obese.6 Obesity is currently the seventh leading cause of death in the US, and as the population grows more obese it will continue to cause serious mortality and morbidity.1 In 2008, medical spending related to obesity totaled approximately $147 billion.7 By 2030, it is estimated that obesity will account for 16% to 18% of all direct and indirect healthcare costs.6

The body mass index (BMI) is a simple measure to predict body composition determined by dividing an individual’s weight in kilograms by the square of his or her height in meters (kg/m2). Obesity is defined as a BMI of more than 30 kg/m2 while individuals with a BMI between 25 and 29.9 kg/m2 are considered overweight.1 Morbid obesity is defined at 40 kg/m2.1 As obesity rates rise, the focus on its consequences on the musculoskeletal system has increased.

Obesity and injury risk

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Although some believe that lower levels of activity in obese patients may make them less susceptible to injury than normal-weight individuals, studies have shown that strains, sprains, and dislocations are actually more common in this vulnerable population.3,4  The greater risk of injury may be related to limited mobility, muscle weakness, or elevated stress or load on the musculoskeletal system.8 The effects of obesity on the lower extremity have been explored in the total joint arena as well as trauma.

Degenerative joint disease is more prevalent in obese patients than those with normal weight, and this is thought to be due to greater joint loading and varus deformity.9,10 According to the Canadian Joint Replacement Registry, patients with a BMI between 30 and 34.9 kg/m2 are 8.5 times more likely to need a total knee arthroplasty (TKA) than those with a BMI less than 25 kg/m2, while those with a BMI greater than 35 kg/m2 are 18.7 times more likely.11 Once the BMI exceeds 40 kg/m2, patients are 32.7 times more likely than individuals with a healthy BMI to need a TKA; in addition, they seek treatment about 10 years earlier than those with lower weights.11,12

Whether obesity contributes to an increased prevalence of complications following total joint replacement, including infection, revision, and mechanical loosening, is controversial.13-16 A large multi- center trial by Andrews et al of 1421 patients undergoing total hip replacement noted no difference in complications between obese and nonobese populations, while a review by Sadr Adozi et al evaluated 3309 total hip replacements and found the risk of systemic infection was 58% in obese individuals.15,16  Malinzak et al showed that with a BMI greater than 50 kg/m2 there is a 21.3 times increased risk for a deep infection after total joint arthroplasty (hip or knee) compared with nonobese patients.14

Obesity does not only affect elective procedures but may also influence the incidence and severity of fractures. In motor vehicle accidents, obese individuals are less likely than normal-weight individuals to suffer abdominal and pelvic injuries.17,18 With lower extremity trauma, obese patients are more likely to sustain a more severe distal femur, ankle, or calcaneus fracture.19-21

Knee dislocations after low-energy trauma have been reported as more common in obese patients than in normal-weight patients.22 Gnudi et al evaluated BMI and fragility fractures in 2235 postmenopausal women and found that higher BMI was associated with risk of humerus fractures while lower BMI was associated with risk of a hip fracture.5  The lower risk of hip fractures is thought to be from increased bone mineral density or increased fat mass allowing for more cushioning during falls.23,24 However, this possible protective effect may be offset by an increased risk of falling or stumbling reported in obese patients, along with more comorbidities, altered gait and balance, and greater medication use compared to their normal-weight counterparts.23

Obesity and injury severity

Obesity may contribute to injury severity. Böstman evaluated 4012 patients with elbow and ankle fractures requiring surgical stabilization and determined that the injured patients tended to have a higher BMI than a general population sample.25 An increased BMI has also been associated with a higher complication rate following operative treatment of pelvic, acetabular, and calcaneal fractures.20,26,27 As injury location becomes more distal along the appendicular skeleton, it appears that the protection afforded by fat cushioning is lost and can possibly make the injury more severe secondary to increased force from a patient’s weight.24

DJO Global Bone Healing - CMFIn the distal lower extremity, there are a variety of ways to evaluate the pattern and classify the severity of an ankle fracture. Classification systems like the Lauge-Hansen or Weber system are frequently used.28,29 The two classification systems have some overlap. The Lauge-Hansen system is descriptive and based on the position of the foot at time of injury and rotation of the injury, which create certain typical patterns.28 The Weber classification is a very simple system based on the level of the fibular fracture and the relationship to the ankle joint and syndesmosis.29 It can then be used to extrapolate the stability of the injury and assist in treatment decisions.29-31 In addition to the classification systems, an increasing number of involved malleoli and the presence of dislocation or of an open fracture can contribute to injury stability and severity.32,33

While both the syndesmosis and the deltoid ligament contribute to ankle joint stability, the Weber classification system focuses on the syndesmosis complex as the primary contributor to ankle stability.30 Type A fractures occur below the level of the ankle joint and are usually stable.29 Type B fractures occur at the level of the ankle joint and can be associated with syndesmotic instability in about 40% of cases.29,34 Type C fibular fractures occur above the level of the ankle joint and are classically associated with syndesmotic disruption.29 Instability of the syndesmotic complex is associated with an unstable ankle mortise, which typically requires surgical stabilization and can lead to higher rates of complications and poorer patient prognosis.30,32,35,36

Ankle fracture incidence has been associated with elevated BMI. Hasselman et al evaluated a cohort of 9704 elderly white women with foot and ankle fractures over a 10-year period and found that those who sustained an ankle fracture were slightly younger and had a significantly higher BMI than those who did not sustain ankle fractures.31 By contrast, BMI was not significantly associated with risk of foot fracture. In a study by Margolis et al, those with a lower BMI had a 40% lower incidence of ankle fractures than those with a BMI greater than 30 kg/m2.37 Extrapolating from the previously described findings for other injuries of the lower extremity,23 it is possible that the increased incidence of ankle fractures in the obese population is a result of a higher frequency of falls, altered gait and balance, limited mobility, muscle weakness, increases in force or load, or a combination of these factors.

Taking it a step further, displacement and loss of reduction can increase in the obese population. Spaine and Bollen examined the BMI of their patient population with ankle fractures and reported a significantly higher BMI in those with displaced ankle fractures than those with nondisplaced ankle fractures.38 Böstman examined the relationship between BMI and the loss of reduction after closed reduction or open reduction and internal fixation (ORIF) in 3061 patients with ankle fractures, and found that obese patients had a 3.04 relative risk of loss of reduction compared with nonobese patients.39 He attributed this to a range of weightbearing issues, from sporadic accidental moments of early full weightbearing to general noncompliance with prescribed weightbearing instructions, and suggested a need for stronger internal fixation techniques and prolonged nonweightbearing periods.

Obesity and Weber classification

Few studies have evaluated the association between body weight or BMI and the Weber classification. A team at the Kaiser San Francisco Bay Area Foot and Ankle Residency Program recently completed a study in which they examined the association between BMI and ankle fracture severity determined by the Weber classification.40 The study was a retrospective review of 280 patients who sustained closed ankle fractures with a distal fibular component. Radiographs were initially reviewed to identify the Weber classification, and then an analysis was performed to identify associations between the Weber classification and BMI, gender, age, type 2 diabetes, current tobacco use, and osteoporosis or osteopenia.

The incidence of each type of Weber fracture in this study was similar to previously reported percentages. In this study, 21.07% of the fractures were Weber A (literature reports 5.5% to 27%), 58.93% were Weber B (literature reports 41% to 66.5%), and 20% were Weber C (literature reports 9% to 23%).30,33,41 After controlling for covariates, obesity (BMI > 30 kg/m2) was associated with greater odds (OR = 1.78) of having a Weber C fracture (compared with Weber A and B). Male gender (OR = 1.74) and age 25 years or younger (OR = 3.97) were also associated with greater odds of having a Weber C fracture (compared with Weber A and B).

The odds ratios for male gender and younger age are likely associated with the greater activity level and propensity for riskier behavior in those patient populations.41 There was no significant association between Weber C ankle fracture and type 2 diabetes, osteoporosis or osteopenia, or current tobacco use.

While fracture pattern is usually associated with the amount of force and mechanism, it is interesting to postulate why the obese population would be at a higher risk for a Weber C fracture. The ankle joint can withstand 1.25 to 5.5 times normal body weight depending on the level of activity, and with increased weight, there is an increase in the potential load on the joint.42 Although the bone mineral density was found to be higher in obese individuals than nonobese individuals, the smaller percentage of lean body mass means that bone mineral density is actually low relative to body weight.24,43

It is also possible that, with increased weight, there is increased force on the bones and joints. Given that ankle fractures are typically associated with rotation, the increased weight associated with obesity may lead to an increase in torque on the ankle joint.31 It is well-recognized that the mechanism of injury is a major contributor to the pattern and instability of an ankle fracture. A Weber C injury is correlated with a Lauge-Hansen pronation external rotation (PER) ankle fracture.44 Obese patients have a higher prevalence of pes planus and a pronated foot type, leading to a pronatory force on the foot.45-49 An increase in force due to excess weight and increased pronatory directed force could contribute to increased occurrence of Weber C type fractures.

Even after an ankle fracture has been stabilized surgically, obesity can still potentially interfere with the postoperative outcome, but very little research has evaluated obese patients’ status following ankle ORIF. Strauss et al retrospectively reviewed 279 charts for severity of injury, postoperative complications, and functional outcome after surgical fixation of an unstable ankle fracture.21 They found that obese patients had more comorbidities and a higher incidence of Weber type B and C ankle fractures than nonobese patients.21 However, there was no difference in the risk of open fractures, mortise displacement, time to union, or postoperative complications.

Obesity presents a significant risk for venous thromboembolism, and practitioners are encouraged to treat patients postoperatively with both mechanical and chemical prophylaxis.50 Given the previously described findings linking obesity to increased postoperative complications related to total joint arthroplasty and other types of trauma, further study into the postoperative consequences of foot and ankle trauma in obese patients is warranted.

Summary

As obesity rates grow exponentially in the US, it will be important to appreciate the effects of obesity on the musculoskeletal system. The medical literature provides evidence of a relationship between obesity and increased risk of musculoskeletal injury, a greater risk of degenerative joint disease, a need for TKR at earlier age, and a higher occurrence of certain distal fractures. Moving distally, not only are ankle fractures more common in obese patients, but the risks of dislocation and loss of reduction are also increased. Our recent study suggests that obesity presents a greater risk for more severe ankle fractures as defined by the Weber classification. With the growing incidence of obesity in our patients, it is essential that we acknowledge additional potential risks of musculoskeletal injury so that we can treat our obese patients properly before, during, and after injury.

Christy King, DPM, AACFAS, is an attending foot and ankle surgeon in the Kaiser San Francisco Bay Area Foot and Ankle Residency Program, Kaiser Permanente, in Oakland, CA.

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