• Total hip arthroplasty;
  • Obesity;
  • Infection;
  • Sex difference


  1. Top of page
  2. Abstract
  8. Acknowledgements


To evaluate the effect of obesity on the incidence of main complications (infection, dislocation, and revision), functional outcome, and patient satisfaction 5 years after primary total hip arthroplasty (THA), and to determine whether results differ between obese women and men.


We conducted a hospital-based prospective cohort study including patients who underwent primary THA (2,495 hips) between 1996 and 2005. We used rates and rate ratios to compare the incidence of main complications in obese and nonobese patients, and we stratified the data for sex. Functional outcome was measured using the Harris Hip Score and Western Ontario and McMaster Universities Osteoarthritis Index.


The adjusted incidence rate ratio for infection (obese versus nonobese) was 4.4 (95% confidence interval [95% CI] 1.8, 10.8). Obesity substantially increased the infection rate in women (incidence rate ratio comparing obese with nonobese women 16.1; 95% CI 3.4, 75.7), whereas obesity appeared to have no effect in men (incidence rate ratio 1.0; 95% CI 0.2, 5.3). The adjusted incidence rate ratio for dislocation (obese versus nonobese) was 2.4 (95% CI 1.4, 4.2), with a higher rate increase in obese women. A total of 817 patients had a 5-year clinical followup visit. Functional outcome and satisfaction were slightly lower in obese women partly due to higher complication rates. No difference was seen in men.


Primary THA is a successful intervention in obese patients, but physician and patient must be aware of increased complications, particularly in women.


  1. Top of page
  2. Abstract
  8. Acknowledgements

A higher prevalence of hip osteoarthritis (1–4) and a subsequent increase in total hip arthroplasty (THA) (5–7) have been observed in obese patients. These trends are of particular concern in the context of the rising prevalence of obesity in developed countries (8, 9). Mechanical factors such as increased load, malalignment, and muscle weakness, as well as metabolic factors, have been proposed to explain the effect of obesity on osteoarthritis (10–15). However, studies evaluating the effect of obesity on hip replacement are less common. Some studies have not found any differences with regards to complications (16–18), quality of life (19), or patient satisfaction (20), whereas other studies have found a negative effect of obesity upon outcomes after THA in terms of a worse functional outcome (17, 21–23) and a higher risk of postoperative complications in general (24) as well as orthopedic complications such as infection and revision surgery (20, 23, 25–31).

Sex-based differences have been reported (5, 7, 32, 33) in the prevalence and management of osteoarthritis and in the need for total hip replacement. In addition, differences between women and men exist with regards to cell biology, tissue function, biochemical factors, and body composition (15, 34–37).

The first objective of the present study was to evaluate the effect of obesity on the incidence of main complications (dislocation, infection, and revision), functional outcome, and overall satisfaction in patients who had undergone THA. The second objective was to investigate whether the results differed between obese women and men. To the best of our knowledge, this is the first study analyzing sex differences as related to outcomes in obese THA patients.


  1. Top of page
  2. Abstract
  8. Acknowledgements

Study design and patient population.

We conducted a prospective cohort study of all patients who underwent primary THA between March 26, 1996 and July 31, 2005 at the University Orthopaedic Department of the only public hospital for the urban and surrounding rural population of Geneva, Switzerland. A mean of 260 THAs per year were performed by orthopedic surgeons of varying levels of experience. Followup extended through October 2005. Patients who underwent THA following an acute fracture of the hip were excluded. A total of 2,589 hips were eligible for the study. Of these, 94 hips (3.6%) were eliminated due to missing information about weight or height, thus leaving 2,495 hips in 2,186 patients for evaluation (Table 1).

Table 1. THA distribution: inclusion period March 26, 1996 to July 31, 2005*
  • *

    Values are the number of hips. THA = total hip arthroplasty; BMI = body mass index.

Total number of THAs2,636
THAs excluded for acute fracture47
Number of eligible THAs2,589
THAs with missing BMI94
Total number of THAs included2,495
Number of THAs with 5-year followup817
 Clinical evaluation731
 Telephone interview86

Exposure and outcome variables.

Outcomes of interest were compared between THAs implemented in obese and in nonobese individuals. We used body mass index (BMI) ≥30 kg/m2 as the definition of obesity (8). Weight and height were measured at the nurse's entrance examination. We obtained the information either from the nurse's report directly or via the anesthesia record.

The primary outcome was the incidence of main complications. These included deep infection, dislocation, and revision for any cause. Deep infection was diagnosed if at least 1 of the following criteria was present: growth of the same microorganism in ≥2 cultures of synovial fluid or periprosthetic tissue, purulence of synovial fluid or at the implant site, acute inflammation on histopathologic examination of periprosthetic tissue, or presence of a sinus tract communicating with the prosthesis (38).

Secondary outcomes were disease-specific quality of life, satisfaction, and general health 5 years after primary THA. We used the following instruments: Harris Hip Score (HHS) (39), a hip-specific instrument evaluating pain, function, and mobility (we assessed the total score and the score for pain); Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) (40), an arthritis-specific quality-of-life instrument completed by the patient; Short Form 12 Health Survey (SF-12) (41), a self-administered 12-item questionnaire comprising 2 summary measures (the physical and mental health component scores); and 2 visual analog scales (VAS) measuring patient satisfaction with the intervention, one VAS for satisfaction regarding pain relief and the other for function, with 0 representing the lowest satisfaction and 10 the highest. We report the mean of these 2 scales because the results are highly correlated (Pearson's correlation coefficient 0.817).

We included the following covariates: 1) age at operation investigated as continuous and as binary variable (<70 and ≥70 years); 2) sex; 3) American Society of Anesthesiologists (ASA) score investigated as categorical and binary variable (ASA ≥3); 4) preoperative functional status and pain level measured by the Merle d'Aubigné score (42), a hip-specific physician-assessed quality-of-life measure; 5) diagnosis of osteoarthritis (primary versus secondary) investigated as binary variable; 6) previous surgery of the hip as binary variable; and 7) year of operation as binary variable (1996–2000, 2001–2005).

Data collection.

Information about preoperative status and the surgical intervention was routinely documented by the operating surgeon on specifically designed data collection forms. Details regarding any perioperative complications were obtained from the medical chart by a trained medical secretary. Any main complication (deep infection, dislocation, or revision) occurring after the hospitalization and treated at our hospital was systematically included in the database by the trained secretary. As part of our routine protocol, all participants were contacted 5 years postoperative for a followup visit that included a clinical and radiologic examination. At the same time, the WOMAC and SF-12 questionnaires were sent to each patient. At followup patients were asked to rate their satisfaction with the intervention. Information about any orthopedic complications that had occurred since the intervention and had not been treated at our institution was obtained either during the visit or by telephone for all patients who were unable to attend an evaluation in person. All followup examinations were performed by 2 trained physicians who were not involved in the operation.

Statistical analysis.

To assess the primary outcome, we calculated person-time of followup from the date of operation until the first occurrence of any main complication, death, loss to followup, 5-year control visit, or end of study (October 31, 2005). We calculated crude incidence rates and incidence rate ratios with 95% confidence intervals (95% CIs) for each BMI category. We stratified the data for sex to examine effect-measure modification and confounding. We then further stratified for other possible confounding variables (age <70 and ≥70 years, diagnosis, ASA score, year of operation, and previous hip surgery) and calculated Mantel-Haenszel estimates of the pooled rate ratio (43). We also used Cox regression analysis and calculated adjusted hazard ratios.

To evaluate the secondary outcomes, mean scores and standard deviations were calculated for each instrument. Crude mean differences and their 95% CIs were determined. We used multiple linear regression analysis to adjust for confounding. We also calculated the proportion of patients with good to excellent results on the HHS, defined by scores ≥80 (44). On the VAS we used ≥8 as an arbitrary cutoff point for further evaluation.


  1. Top of page
  2. Abstract
  8. Acknowledgements

Of 2,186 patients (1,217 women and 969 men) who underwent 2,495 primary THAs, 589 THAs (23.6%) were performed in 508 obese patients. The percentage of patients who underwent a bilateral THA during the study period did not differ between obese and nonobese individuals. The prevalence of obesity was higher in male patients than in female patients (26.7% versus 20.5%, respectively).

The baseline characteristics of the obese and nonobese groups are compared in Table 2. Obese patients were younger with slightly lower preoperative functional status (differences were greater in women) and higher ASA scores. In the obese group, the intervention was less often due to secondary osteoarthritis than in the nonobese group. In 96% of patients, the surgical exposure was via a lateral approach. In 86% of patients, we used a Morscher press-fit uncemented acetabular component and a Müller straight-stem cobalt chromium femoral component with a 28-mm head (both from Zimmer, Müsingen, Switzerland). The rehabilitation protocol was the same for the 2 groups of patients. Overall, in choice of prosthesis, surgical approach, and rehabilitation, we could not identify any possible differences in the treatment given to obese and nonobese participants. Operation time was slightly lower in nonobese patients than in obese patients (120 minutes versus 129 minutes; 95% CI 5, 12 minutes for the difference).

Table 2. Baseline characteristics by BMI category for all primary total hip arthroplasties (n = 2,495) and stratified by sex*
CharacteristicIncluded hips, no.BMI <30 kg/m2Included hips, no.BMI ≥30 kg/m2
  • *

    Values are the number (percentage) unless otherwise indicated. BMI = body mass index; OA = osteoarthritis.

  • Used preoperatively from March 1996 to June 2003 (1,942 hips operated during that period).

  • Used preoperatively from July 2003 to July 2005 (652 hips operated during that period).

All1,906 589 
Women 1095 (57.5) 287 (48.7)
Men 811 (42.5) 302 (51.3)
Age at operation, mean ± SD years    
 All1,90669.0 ± 12.558967.2 ± 9.9
 Women1,09571.3 ± 11.828769.5 ± 9.3
 Men81165.9 ± 12.930265.0 ± 9.9
Age groups1,906 589 
 <50 150 (7.9) 28 (4.8)
 50–59 241 (12.6) 87 (14.8)
 60–69 472 (24.8) 225 (38.2)
 70–79 664 (34.8) 195 (33.1)
 ≥80 379 (19.9) 54 (9.2)
BMI, mean ± SD kg/m21,90624.8 ± 3.058933.1 ± 2.8
Diagnoses1,906 589 
  Primary OA 1573 (82.5) 514 (87.3)
  Secondary OA 333 (17.5) 75 (12.7)
 Women, secondary OA 175 (16.0) 41 (14.3)
 Men, secondary OA 158 (19.5) 34 (11.3)
Merle d'Aubigné preoperative score, mean ± SD    
 All1,29010.1 ± 2.03739.6 ± 2.0
 Women7529.9 ± 2.01709.2 ± 1.8
 Men53810.4 ± 2.120310.0 ± 2.0
Harris Hip Score, mean ± SD    
 All36748.0 ± 15.411446.2 ± 13.7
 Women19445.5 ± 14.36244.6 ± 12.6
 Men17350.9 ± 16.25248.0 ± 14.8
ASA score1,881 580 
  1–2 1447 (76.9) 381 (65.7)
  3–4 434 (23.1) 199 (34.3)
 Women, 3–4 259 (24.0) 89 (31.4)
 Men, 3–4 175 (21.8) 110 (37.0)
Previous surgery of the hip1,737 528 
 All 157 (9.0) 45 (8.5)
 Women 91 (9.1) 18 (7.0)
 Men 66 (8.9) 27 (10.0)
Hip contralateral1,703 524 
 Not affected 862 (50.6) 253 (48.3)
 Affected, nonoperated 443 (26.0) 140 (26.7)
 Affected, operated 398 (23.4) 131 (25.0)


Followup ranged from 3 to 72 months, with hips in obese and nonobese patients contributing a mean ± SD of 40.5 ± 22 person-months and 42 ± 21 person-months, respectively. There were 91 hips with main complications, including 17 deep infections, 53 dislocations, and 21 hips that underwent revision. Except for 1 dislocation reported by the patient at the 5-year followup contact, all complications were treated at our hospital.

Of the 17 infections, 7 (0.4%) occurred in hips of nonobese patients and 10 (1.7%) in hips of obese individuals. The crude incidence rate for infection was 4.7 times higher (adjusted rate ratio 4.4; 95% CI 1.8, 10.8) in the obese group (Table 3). Stratification for sex revealed that the effect of obesity on subsequent infection was limited to women, of whom the incidence rate ratio for infection comparing obese with nonobese women was 16.1 (95% CI 3.4, 75.7). Men had a higher rate of postoperative infection than women (1.8 infections per person-year versus 0.5 among nonobese patients), but obesity in men did not appear to increase the infection rate (rate ratio 1.0; 95% CI 0.2, 5.3). The number of infections among men was small, however, resulting in wide confidence intervals. None of the nonobese patients who developed an infection had diabetes, whereas 3 of the obese patients who developed an infection had diabetes.

Table 3. Incidence rates and rate ratios (RR) for infection, dislocation, and revision by BMI category and sex*
 Women (n = 1,382 hips)Men (n = 1,113 hips)Crude RR (95% CI)Adjusted RR (95% CI)Adjusted HR (95% CI)
BMI <30 kg/m2BMI ≥30 kg/m2TotalBMI <30 kg/m2BMI ≥30 kg/m2Total
  • *

    BMI = body mass index; 95% CI = 95% confidence interval; HR = hazard ratio.

  • Infection adjusted for sex and diagnosis (primary or secondary osteoarthritis); dislocation adjusted for age, sex, and diagnosis; revision (all causes) adjusted for sex and diagnosis using Mantel-Haenszel methods.

  • Infection adjusted for age, sex, and diagnosis; dislocation adjusted for age, sex, diagnosis, and year of operation; revision (all causes) adjusted for age, sex, and diagnosis using Cox regression.

  • §

    BMI <30 kg/m2 compared with BMI ≥30 kg/m2.

 Incidence rate (cases/1,000 person-years)   
 RR (95% CI)§16.1 (3.4, 75.7) 1.0 (0.2, 5.3)    
 Obese vs. nonobese      4.7 (1.8, 12.3)4.4 (1.8, 10.8)5.1 (1.9, 13.5)
 Incidence rate (cases/1,000 person-years)   
 RR (95% CI)§3.0 (1.3, 7.1) 1.8 (0.9, 3.6)    
 Obese vs. nonobese      2.3 (1.4, 4.0)2.4 (1.4, 4.2)2.3 (1.3, 4.0)
 Incidence rate (cases/1,000 person-years)   
 RR (95% CI)§2.3 (0.7, 7.7) 1.7 (0.5, 6.1)    
 Obese vs. nonobese      2.0 (0.8, 4.8)2.0 (0.9, 4.8)2.2 (0.9, 5.3)

Dislocation occurred in 53 hips, 31 (1.6%) in nonobese patients and 22 (3.7%) in obese patients. The crude incidence rate for dislocation was 2.3 times higher (adjusted rate ratio 2.4; 95% CI 1.4, 4.2) in the obese group (Table 3). Stratification for sex demonstrated approximately twice the incidence rate for dislocation in men as compared with women, but the relative rate increase due to obesity was greater in women than in men (rate ratio 3.0 versus 1.8).

A total of 21 hips underwent revision during the followup period, 13 in nonobese patients and 8 in obese patients (adjusted rate ratio 2.0; 95% CI 0.9, 4.8). Aseptic loosening was the indication for revision in 3 nonobese patients. No obese patients underwent revision for aseptic loosening (Table 4), but these patients had more reoperations for septic loosening (0.68% versus 0.16%; rate ratio 4.3; 95% CI 1, 19).

Table 4. Reasons for revision by body mass index (BMI) category and sex
BMI <30 kg/m2 (n = 1,095)BMI ≥30 kg/m2 (n = 287)BMI <30 kg/m2 (n = 811)BMI ≥30 kg/m2 (n = 302)
Aseptic loosening    
Septic loosening331
Recurrent dislocation/cup instability312
Secondary cup displacement/impingement2111

Five-year outcomes.

At the end of the inclusion period, 819 THAs in nonobese patients and 249 THAs in obese patients were scheduled for a 5-year followup visit. In the nonobese group, 85 patients (91 hips, 11.1%) died between the intervention and 5-year followup. No information was available for 46 THAs (5.6%) in patients who had moved from the area, whereas in 47 hips (5.8%) information was available regarding possible complications but not about functional status and satisfaction. Of the remaining 635 hips (77.5%) in nonobese patients, 573 were evaluated clinically at the 5-year followup whereas 62 were only evaluated by telephone interview and questionnaire. In the obese group, 26 patients (29 hips, 11.7%) died before the 5-year followup. No information was available for 15 THAs (6.0%), and partial information (complications only) was available for 23 hips (9.2%). Thus there were 182 hips (73.1%) in obese patients that were evaluated either by a clinical visit (n = 158) or by telephone interview and questionnaire only (n = 24).

Five years after the intervention, 81% of the hips in the nonobese group and 70% in the obese group had good to excellent results on the HHS (≥80 points). The poorer results associated with obesity were more obvious in women than in men (women: 77% versus 58%; men: 85% versus 81%). Satisfaction with interventions was high (satisfaction score ≥8) in 86% of nonobese patients as compared with 82% of obese patients. The slightly lower satisfaction associated with obesity was entirely due to poorer results in obese women (women: 86% versus 77%; men: 86% versus 87%).

Mean scores and unadjusted as well as adjusted mean differences for HHS and satisfaction are presented in Table 5. Obese patients (particularly women) had lower mean results on the HHS even after adjustment for preoperative functional status. In terms of clinical significance, the difference can be considered moderate. Mean satisfaction scores were slightly lower in obese women than in nonobese women. Further adjustment for the presence of complications revealed that satisfaction was lower in obese women primarily due to the higher incidence of complications (Table 5). WOMAC and SF-12 scores at 5 years postoperatively are presented in Table 6.

Table 5. Harris Hip Scores and satisfaction at 5 years for women (n = 401 hips) and men (n = 330 hips) by BMI category*
 BMI <30 kg/m2BMI ≥30 kg/m2Unadjusted mean difference (95% CI)Adjusted mean difference (95% CI)
  • *

    Values are the mean ± SD unless otherwise indicated. BMI = body mass index; 95% CI = 95% confidence interval.

  • Adjusted for age at operation, preoperative functional status, American Society of Anesthesiologists score, diagnosis, and year of operation.

  • Adjusted mean difference was 0.18 (95% CI −0.3, 0.6) after additional adjustment for complications.

 HHS87.8 ± 13.279.6 ± 16.48.2 (4.7, 11.6)7.5 (3.8, 11.2)
 HHS pain40.8 ± 6.639.2 ± 8.61.6 (−0.5, 3.7)1.6 (−0.4, 3.5)
 Satisfaction8.8 ± 1.68.5 ± 2.00.3 (−0.2, 0.8)0.3 (−0.2, 0.8)
 HHS90.5 ± 13.087.4 ± 10.83.1 (0, 6.2)2.8 (−0.2, 5.8)
 HHS pain41.1 ± 6.740.9 ± 5.50.2 (−1.4, 1.8)0.1 (−1.5, 1.8)
 Satisfaction8.8 ± 1.78.8 ± 1.50.01 (−0.4, 0.5)0.03 (−0.4, 0.4)
Table 6. WOMAC and SF-12 scores at 5 years for women (n = 404 hips) and men (n = 325 hips) by BMI category*
 BMI <30 kg/m2BMI ≥30 kg/m2
  • *

    Values are the mean ± SD. WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index; SF-12 = Short Form 12 Health Survey; BMI = body mass index; PCS = physical component score; MCS = mental component score.

 WOMAC pain72.3 ± 23.461.6 ± 25.3
 WOMAC function68.0 ± 24.160.5 ± 23.5
 SF-12 PCS40.2 ± 10.037.5 ± 9.8
 SF-12 MCS46.5 ± 10.942.5 ± 10.7
 WOMAC pain76.4 ± 22.172.6 ± 23.5
 WOMAC function73.6 ± 21.067.8 ± 24.0
 SF-12 PCS43.4 ± 9.240.2 ± 10.0
 SF-12 MCS47.2 ± 10.047.2 ± 10.7


  1. Top of page
  2. Abstract
  8. Acknowledgements

The primary objective of this study was to evaluate the effect of obesity on the incidence of main complications in patients undergoing THA. The results demonstrated that obesity was associated with a substantially higher risk for deep infection in women, led to more dislocations (increase greater in women), and resulted in more revisions for septic loosening. The second goal was to evaluate the effect of obesity on functional outcome and satisfaction. Our data revealed that obese women, but not obese men, reported moderately lower functional outcomes and less satisfaction, the latter partly due to a higher incidence of complications among these patients.

Stickles et al (20) reported a higher rate of orthopedic complications (infection, dislocation, component failure) with increasing BMI in the first year after THA. A higher risk of infection in obese THA patients in general has been stated (29, 45). Ridgeway et al (30) reported a higher infection risk in obese patients and in female patients in single variable analysis, but did not present subgroup analyses by BMI and sex. To the best of our knowledge, our study is the first to investigate sex differences in THA outcomes in association with obesity. Our findings reveal a striking effect of obesity and female sex on the risk of infection. Risk factors for infection including longer operative time, ASA score ≥3, and the presence of diabetes are known to be more frequent in obese patients (16, 30, 46), but they were not related to female sex in our study and do not explain the exclusively poorer results in women. Other reasons related to sex differences such as body fat distribution (34) and metabolic response might be involved.

Although an association between obesity and risk of dislocation has not previously been clearly demonstrated (47–50), we found a more than 2-fold rate increase in dislocation in obese patients. The increase due to obesity was more marked in women than men. Soft-tissue quality is known to influence dislocation rate, and the lower peripheral muscle strength that has been described in obese women (13) could be one possible mechanism leading to an increased dislocation rate in these patients. In addition, accurate cup positioning might be more difficult in obese patients.

Although a higher risk of prosthetic failure has been reported in obese patients (23, 25–28, 31), the results are inconsistent. It has been demonstrated (51) that the effect of obesity on polyethylene wear and subsequent revision may be attenuated by a lower activity level among obese patients. We observed revision for septic loosening but not aseptic loosening in the obese group. But the limitations here include the lack of information about the patients' activity level, the small number of revisions, and the short followup time to demonstrate aseptic loosening.

There have been a few and mostly short-term followup studies explicitly comparing functional outcomes and satisfaction after THA in obese and nonobese patients (17, 19, 20, 29), but none were stratified for sex. Some authors (19, 20, 29) found no difference in outcome between obese and nonobese patients. Moran et al (17) described slightly lower HHS scores in obese patients at 6 and 18 months after THA, similar to what we found for all obese patients. Using subgroup analyses, other studies have found negative effects of obesity on outcomes after THA in terms of lower functional results (21, 22). The lower functional results in obese women that we found in our study, even after adjusting for these patients' lower preoperative function and increased complication rate, could be due to additional factors such as a higher prevalence of polyarticular osteoarthritis (52).

The strengths of this prospective cohort study include a standardized clinical followup, large sample size, and a longer followup than has been previously reported. We calculated incidence rates to account for differences in the length of followup between obese and nonobese patients. Outcome instruments are validated and widely used, and we analyzed both patient- and physician-assessed outcome measures. The assessment was performed by 2 independent surgeons to prevent observer bias.

Our study has some limitations. First, with regard to the primary outcome, we do not know whether patients with a followup less than 5 years died or moved out of the region, and as a consequence we calculated their person-time until a complication occurred or until the end of the study. Nevertheless, mortality and lost-to-followup rates of obese and nonobese patients with complete 5-year followup were similar. Second, with regards to the secondary outcomes, the number of patients unable to attend the 5-year clinical followup visit was slightly higher in the obese group, and this difference may have caused a small underestimation of the effect of obesity on functional outcome, satisfaction, and general health. Third, due to the rareness of infections and revisions, the confidence intervals of the sex subgroup analyses were large. Lastly, we do not have preoperative scores for the WOMAC and SF-12 questionnaires.

Previously reported results on the relationship between obesity, complications, and outcomes after THA are contradictory (16–31). To the best of our interpretation of these articles, they do not appear to describe different patients (inclusion primary THA) than those in our study. However, literature comparison is hindered by the absence of sex information, the use of multiple definitions of obesity, and a limited interpretation of complications because of the small number of those events in former studies. In addition, implants used were different from our study but not different between the 2 groups, and surgical technique was generally not mentioned.

Our study revealed that obesity was associated with an increase in infection, dislocation, and subsequent revision surgery for septic loosening within 5 years after THA. Obese women were more at risk than men, particularly with regard to infection. Obese women had lower functional outcome and slightly lower satisfaction following primary THA, but obesity did not affect functional outcome and satisfaction in men. Because our study revealed increased complications among obese women, we suggest that surgeons counsel this group of patients so that they are made aware of this fact. In addition, participating in a weight-loss program prior to surgery might be beneficial for such patients.


  1. Top of page
  2. Abstract
  8. Acknowledgements

Dr. Lübbeke had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study design. Lübbeke, Hoffmeyer.

Acquisition of data. Lübbeke, Garavaglia, Zurcher, Hoffmeyer.

Analysis and interpretation of data. Lübbeke, Hoffmeyer.

Manuscript preparation. Lübbeke, Stern.

Statistical analysis. Lübbeke.


  1. Top of page
  2. Abstract
  8. Acknowledgements

The authors thank Kenneth Rothman for his valuable comments and Christophe Barea for his assistance with computer programming.


  1. Top of page
  2. Abstract
  8. Acknowledgements
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