Body Mass Index as a Predictor of Complications After Operative Treatment of Acetabular Fractures

Karunakar, Madhav A. MD; Shah, Steven N. MD; Jerabek, Seth BS

Journal of Bone & Joint Surgery - American Volume: July 2005 - Volume 87 - Issue 7 - p 1498–1502
doi: 10.2106/JBJS.D.02258
Scientific Articles

Background: Obesity, a growing public health concern, is often thought to be an important risk factor for postoperative complications. We hypothesized that body mass index is predictive of complications after operative treatment of acetabular fractures.

Methods: A retrospective chart review identified 169 consecutive patients in whom an acetabular fracture had been treated with open reduction and internal fixation at a level-1 trauma center. The patients were stratified into four classes according to their body mass index: normal (<25), overweight (≥25 but <30), obese (≥30 but <40), and morbidly ≥40). The perioperative outcomes that were evaluated included estimated blood loss, wound infection, nerve palsy, deep venous thrombosis, pulmonary embolism, and heterotopic ossification. Multivariate general linear models were used to test for the relationship between body mass index and perioperative outcomes while controlling for potential intervening variables (including surgical approach, fracture type, and surgeon experience). Odds ratios were calculated as well.

Results: When body mass index was measured as a continuous variable, it was found to have a significant relationship with estimated blood loss (p = 0.003), prevalence of wound infection (p = 0.002), and prevalence of deep venous thrombosis (p = 0.03). Odds ratio analysis revealed that obese subjects (body mass index of ≥30) were 2.1 times more likely than patients of normal weight (body mass index of <25) to have an estimated blood loss of >750 mL and 2.6 times more likely to have a deep venous thrombosis. Morbidly obese patients (body mass index of ≥40) were five times more likely to have a wound infection.

Conclusions: Body mass index is predictive of complications after operative treatment of acetabular fractures.

Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.

1 Department of Orthopaedic Surgery, University of Michigan Hospital, Taubman Center 2912G, Ann Arbor, MI 48109-0328

Article Outline

The prevalence of obesity is increasing in all industrialized countries and constitutes a major public health challenge1. One in five Americans is obese, and this figure is projected to reach 40% by the year 20252,3. Obesity has been linked to the development of numerous health problems including diabetes, hypertension, coronary artery disease, and cancer4. Because of the high rate of comorbidities associated with obesity, this condition is often thought to be an important risk factor for postoperative complications5.

Displaced acetabular fractures are among the most complex of all orthopaedic injuries6. Open reduction and internal fixation of such fractures requires an extensive surgical approach and is often associated with a long operative time, high blood loss, and a substantial risk of postoperative complications. The purpose of the present study was to test the hypothesis that body mass index is predictive of complications after operative treatment of acetabular fractures.

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Materials and Methods

A retrospective chart review, approved by our institutional review board, identified 169 consecutive patients in whom an acetabular fracture had been treated with open reduction and internal fixation at a tertiary care level-1 trauma center between July 1999 and June 2003. All surgical procedures were performed by one of three fellowship trained orthopaedic traumatologists with a mean of six years (range, one to thirteen years) of postfellowship practice experience at the start of this study. Demographic data including age and gender were identified. The fracture type (elementary or associated)7, surgical approach (ilioinguinal, Kocher-Langenbeck, or combined ilioinguinal and Kocher-Langenbeck), and injury severity score (ISS)8 were also recorded. The body mass index was calculated by dividing the patient's weight in kilograms by the height in meters squared. The patients were then stratified into four body-mass-index classes: normal (<25), overweight (≥25 but <30), obese (≥30 but <40), and morbidly obese (≥40), as defined by the National Heart, Lung, and Blood Institute and the World Health Organization9,10. Outcomes measured included estimated blood loss, wound infection, nerve palsy, deep venous thrombosis, pulmonary embolism, and heterotopic ossification. Estimated blood loss, which was recorded directly from the operative record, was determined according to an institutional protocol that includes (1) calculating “in and out” measurements of irrigation fluids and suction drainage and (2) weighing laparotomy sponges, with one gram equaling one milliliter of blood loss. Wound infection and nerve palsy were diagnosed on clinical examination in the immediate postoperative period as well as at subsequent clinic visits. Infection was further defined according to two criteria: (1) postoperative drainage and/or purulence prompting a secondary surgical procedure and (2) gross intraoperative signs of infection and/or positive cultures of intraoperative specimens that resulted in treatment with long-term intravenous antibiotics. An infection was defined as superficial when the deep fascia was intact and as deep when the fascia was not intact at the time of the secondary procedure.

Deep venous thrombosis was diagnosed with duplex Doppler scanning in patients with suspicious clinical signs and symptoms (e.g., unilateral swelling of the leg or calf pain). Routine duplex Doppler screening was not performed. All patients received prophylaxis against deep venous thrombosis—either subcutaneous heparin or low-molecular-weight heparin, according to the surgeon's preference. The diagnosis of pulmonary embolism was made on the basis of clinical suspicion and was confirmed by additional imaging studies, including ventilation-perfusion scanning, spiral computed tomography, and pulmonary angiography, when appropriate. Anteroposterior and Judet radiographs were made at two weeks, six weeks, three months, six months, and one year. Radiographs made at a minimum of three months postoperatively were used to look for heterotopic ossification. Indomethacin was used as prophylaxis for approximately one-half of the patients in this study.

The study group included 126 male and forty-three female patients. The average age was forty-one years (range, seventeen to sixty years). There were fifty-nine elementary and 110 associated fractures. Forty-nine operations were performed with an ilioinguinal approach; 118, with a Kocher-Langenbeck approach; and two, with combined ilioinguinal and Kocher-Langenbeck approaches. The mean preoperative ISS was 15 points (range, 9 to 59 points).

Multivariate general linear models were used to test for the relationship between body mass index and perioperative outcomes while controlling for potential intervening variables, including age, gender, fracture type, ISS, method of prophylaxis against deep venous thrombosis, surgical approach, and surgeon experience. To calculate the odds ratios, continuous measures (estimated blood loss) were recategorized as dichotomous measures by using the median value in the normal-weight subgroup (body mass index of <25) as a cut-off point. Pearson chi-square tests with exact significance were used to compare the prevalence of perioperative outcomes with the body-mass-index category; p < 0.05 was considered to be significant for all analyses. An independent biostatistician who was not directly involved with the study performed the statistical analyses.

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Results

The average body mass index was 30 (range, 17 to 60), and the median body mass index was 29. The normal, overweight, obese, and morbidly obese groups comprised forty-seven, forty-seven, fifty-five, and twenty patients, respectively. The mean, median, and minimal duration of clinical follow-up were 14.5 months, twelve months, and one month, respectively. All patients were seen for at least one month postoperatively, ten (5.9%) were seen for less than three months, and eighty (47%) were seen for less than one year. The study population included a continuous series of patients, none of whom were excluded from the data analyses. The six perioperative outcomes of interest are stratified by body-mass-index group in Table I.

There was a total of eight wound infections (4.7%). Six infections were defined as deep, and two were defined as superficial. There were no infections in the normal-weight or overweight group, five (9.1%) in the obese group, and three (15%) in the morbidly obese group. Of the five infections in the obese group, four were deep and one was superficial. Of the three in the morbidly obese group, two were deep and one was superficial.

Heterotopic ossification developed in thirty-seven (23%) of the 159 patients who were followed for a minimum of three months. Table I lists the prevalence of heterotopic ossification according to body-mass-index subgroup. Indomethacin was used as prophylaxis in seventy-seven (48%) of the 159 patients, with an even distribution among the body-mass-index subgroups. Of the thirty-seven patients in whom heterotopic ossification developed, twenty-one (57%) had received indomethacin for prophylaxis.

When body mass index was measured as a continuous variable, it was found to have a significant relationship with estimated blood loss (p = 0.003), the prevalence of wound infection (p = 0.002), and the prevalence of deep venous thrombosis (p = 0.03). The prevalences of heterotopic ossification (p = 0.095), nerve palsy (p = 0.404), and pulmonary embolism (p = 0.139) were not significantly related to body mass index.

The odds ratio analysis revealed that, compared with the normal-weight patients, the obese patients (body mass index of 30) were 2.1 times more likely to have an estimated blood loss of >750 mL (odds ratio = 2.1, 95% confidence interval = 1.1 to 4.0, p = 0.03) and 2.6 times more likely to have a deep venous thrombosis (odds ratio = 2.6, 95% confidence interval = 1.1 to 5.8, p = 0.03). Morbidly obese subjects were five times more likely to have a wound infection (odds ratio = 5, 95% confidence interval = 1.2 to 23.2, p = 0.05) than were patients of normal weight. Obesity did not increase the risk of heterotopic ossification, pulmonary embolism, or nerve palsy.

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Discussion

A widely held assumption among surgeons is that obese patients have a higher risk of surgical complications than do patients of normal weight5,11. However, a recent prospective study of 6336 elective general surgery procedures demonstrated no difference in postoperative complications other than an increased risk of infection in obese patients5. The literature contains numerous conflicting reports regarding the impact of obesity on perioperative complications5,12-18. These discrepancies are partly related to low statistical power due to small sample sizes in some studies as well as to heterogeneity with respect to study design, type of surgery, and specific outcome measures.

The relationship between obesity and perioperative complications has been investigated in a number of small studies in the orthopaedic and general trauma literature. Choban et al. found that severely overweight patients who had sustained blunt trauma had higher mortality and complication rates than did overweight and average-weight patients19. McKee and Waddell reported difficulties with positioning, increased blood loss, long operative times, and a high rate of perioperative complications in seven morbidly obese patients in whom a femoral shaft fracture was managed with reamed intramedullary nailing20. Other studies have demonstrated complications such as compartment syndrome and nerve palsy as a result of prolonged ischemic pressure due to excessive body weight as well as a relationship between body mass index and the development of thromboembolic complications21,22. In addition, a recent report showed that intraoperative blood loss is a strong predictor of postoperative morbidity and mortality23.

Wound infection rates after operative treatment of acetabular fractures have been reported to range from 3.8% to 5.6%7,24-28. In the present study, the overall prevalence of postoperative wound infection was 4.7%. Russell et al. reported seven wound problems (including five infections), with four occurring in morbidly obese patients, after operative management of 131 acetabular fractures24. In our study, the prevalence of wound infection after acetabular fracture surgery was significantly higher in obese patients than in patients of normal weight. Various methods for minimizing postoperative wound infection in obese patients, including continuous fascial closure, closure of the subcutaneous fat, and optimizing the suture length-to-wound length ratio, have been proposed in the surgical literature29-31.

The reported rates of deep venous thrombosis and pulmonary embolism have ranged from 2.3% to 27.7% and from 0.6% to 2.2%, respectively, after operative treatment of acetabular fractures25,27,32,33. We diagnosed deep venous thrombosis and pulmonary embolism in 17.8% and 3.6% of our patients, all of whom were symptomatic. Reports in the orthopaedic literature that discuss the relationship between obesity and thromboembolic complications have focused largely on total joint arthroplasty34-39, and the results of those studies are conflicting. However, a recent, large, prospective study demonstrated that each 5 kg/m2 increase in body mass index was associated with a 1.5 times greater risk of thromboembolic complications after primary total hip or knee arthroplasty22.

In the present study, we found that obese patients (body mass index of ≥30) had a 2.6 times greater risk for the development of symptomatic deep venous thrombosis compared with patients of normal weight (body mass index of <25) after surgery for an acetabular fracture. This increased risk was probably underestimated, as ultrasound is less reliable for the detection of deep venous thrombosis in obese patients than it is in patients of normal weight40,41. The fact that duplex Doppler scanning was performed only in patients with suspicious clinical signs and symptoms, and was not used as a routine screening method, probably led to an underestimation of the overall prevalence of deep venous thrombosis as well. Obesity was not shown to increase the risk of pulmonary embolism. The low prevalence of pulmonary embolism (<2%) reported in most series25,27,32,33 suggests that a much larger sample size would be necessary to provide sufficient power to identify a statistical difference in the prevalence of pulmonary embolism in obese patients. However, the 17.8% rate of deep venous thrombosis in this study underscores the importance of aggressive prophylaxis against thromboembolic complications in obese patients.

We found a 7.7% prevalence of iatrogenic nerve palsy following operative management of acetabular fractures. Postoperative nerve palsy has been reported in 1% to 18% of cases in other reports27,42-50. Iatrogenic injury to the sciatic nerve (most commonly the peroneal branch) has been attributed to excessive retraction of the nerve, malposition of retractors, penetration by drill bits, and continuous extension of the ipsilateral knee44. Russell et al. reported an iatrogenic sciatic nerve palsy in an obese patient resulting from compression by a poorly placed retractor24. An inability to visualize the end of the retractor and difficulty in maintaining the position of the retractor secondary to the massive soft tissues were mentioned as reasons for this complication. The present study did not demonstrate a significant relationship between body mass index and iatrogenic nerve palsy. The low prevalence of this complication makes it likely that a larger sample size would be necessary to identify a significant increase in iatrogenic nerve injuries in obese patients.

Heterotopic ossification is a well-recognized complication of operative management of acetabular fractures, with published rates of the complication ranging from 10% to 86%7,25,27,28,32,33. We noted a 23% prevalence of heterotopic ossification and did not find body mass index to be an independent predictor of that complication. Since this study was not designed to enable us to make recommendations about prophylaxis against heterotopic ossification, surgeons should use individual patient factors, personal experience, and the extensive literature on the subject to formulate a decision about whether to use postoperative prophylaxis against heterotopic ossification.

The results of this investigation should be viewed within the context of its limitations, one of which is its retrospective cohort design. The quality of the data depends on the accuracy and completeness of the medical record. The odds ratios that we reported are only estimates of the true relative risk (risk ratio) that could be calculated with a prospective cohort design.

In summary, the present study demonstrated that incremental increases in body mass index are associated with an increased risk of complications after open reduction and internal fixation of acetabular fractures. The specific outcomes that were significantly associated with body mass index in this study were estimated blood loss, prevalence of wound infection, and prevalence of deep venous thrombosis. Heterotopic ossification, iatrogenic nerve palsy, and pulmonary embolism were not significantly related to obesity. This information should be used to inform obese patients of their additional risks when they are to undergo operative stabilization of an acetabular fracture. Additional prospective studies with larger sample sizes should be performed to corroborate these findings and to identify methods of reducing the risk of complications in obese trauma patients. ▪

Investigation performed at the Department of Orthopaedic Surgery, University of Michigan Hospital, Ann Arbor, Michigan

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

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