Obesity and early reoperation rate after elective lumbar spine surgery: a population-based study

ABSTRACT

Study design: Population-based retrospective cohort study.

Clinical question: Are patients with a body mass index (BMI) of 35 or more who undergo elective lumbar spine surgery at increased risk of post-surgical complications, as evidenced by reoperation within a 3-month period?

Methods: The Alberta Health and Wellness Administrative database was queried to identify patients who underwent elective lumbar spine surgery over a 24-month period. This same database was used to classify subjects as obese (BMI ≥35) and non-obese (BMI <35) and to determine who underwent repeated surgical intervention. The rate of reoperation was determined for both the obese and non-obese groups; further analyses were performed to determine whether certain subjects were at increased risk of reoperation.

Results: The point estimate for relative risk for requiring reoperation was 1.73 (95% confidence interval, 1.03–2.90) for obese subjects compared with non-obese subjects. The adjusted point estimate shows that deformity correction surgery is predictive for early reoperation while obesity is not.

Conclusions: In obese subjects we observed an increased complication rate after elective lumbar spine surgery, as evidenced by reoperation rates within 3 months. When we considered other possible associations with reoperation, in adjusted analysis, deformity surgery was found to be predictive of early reoperation.

The definiton of the different classes of evidence is available here.

STUDY RATIONALE AND CONTEXT

There is conflicting evidence regarding whether obese patients have higher complication rates after spine surgery than non-obese patients [1], [2], [3], [4], [5], [6], [7], [8], [9]. This is of particular concern given that obese patients have a greater incidence of chronic back pain with associated neurological signs and symptoms, thus it is expected that a higher proportion of obese patients will undergo lumbar spine surgery [10].

METHODS

Study design: Population-based retrospective cohort study.

Primary question: Are patients with a body mass index (BMI) ≥35 undergoing elective lumbar spine surgery at increased risk of postoperative complications, as defined by reoperation within a 3-month period?

Secondary questions: Are any patient subgroups at a higher risk of reoperation? Subgroups, identified a priori, included gender, age, location of surgery (urban versus rural setting), and type of procedure performed (decompression, decompression with instrumented fusion, deformity correction, and arthroplasty).

Inclusion criteria:

• Age ≥20 years

• Elective lumbar spine surgical procedure

• Surgery between July 1, 2007, and June 30, 2009

• Surgery funded by Alberta Health and Wellness, in Alberta, Canada, the provincial medical insurance entity

Exclusion criteria:

• Trauma

• Underlying malignancy

• Infection

• Incomplete data

Patient population: We identified all patients having had a lumbar spine procedure funded by Alberta Health and Wellness during the study period. The obese group was defined as those with a BMI ≥35, corresponding to class II and III obesity as per the World Health Organization classification system. These patients were identified using a BMI billing code; we did not stratify reoperation rates according to differing BMI cut-points due to availability of data.

Clinical risk factors:

• Age

• Gender

• Procedure (decompression only, decompression with instrumented fusion, deformity correction, or arthroplasty)

Other confounding factors, such as medical comorbidities and smoking, were not available through this database.

Outcomes: The primary outcome measure was the rate of reoperation, for any reason, within 3 months of the index procedure.

Analysis: The data was verified to ensure all subjects had a complete dataset. We calculated the reoperation rates for both obese and non-obese subjects who underwent elective lumbar spine surgery. The relative risk (RR) of reoperation for obese subjects and the 95% confidence interval (CI) was determined (RR = risk of reoperation in obese group/risk of reoperation of entire cohort; 95% CI = (x−1.96σ/√n, x+1.96σ/√n). We performed subgroup analyses in the same manner. Calculations were performed using SPSS v 18 (SPSS Inc, Chicago, IL, USA).

RESULTS

• 4023 lumbar spine procedures were performed, 635 were excluded, thus leaving 3388 for analysis ([Fig 1]).

• 52.2% of the subjects were male and most patients (61.5%) were aged between 40 and 74 years. 61.8% had isolated decompression surgery while 33.4% had some form of instrumentation of fusion ([Table 1]).

• 101 subjects (3%) required reoperation in the 3 months after elective lumbar spine surgery. The obese group had a statistically significant higher reoperation rate compared with the non-obese group (4.8% versus 2.8%) ([Table 1]). This corresponds to RR of 1.73 (95% CI, 1.03–2.90) ([Fig 2]).

• Using in a logistic regression analysis, controlling for age, gender and type of procedure performed, the odds ratio for obese patients requiring reoperation was 1.57 (95% CI, 0.89–2.77). The variable predictive for early reoperation was type of procedure; where instrumented fusion had an odds ratio of 1.64 (95% CI, 1.05–2.56) and deformity correction had an odds ratio of 4.46 (95% CI, 2.34–8.53) ([Table 2]).

• The most common reason for reoperation, according to billing codes, was repeated nerve root compression followed by infection ([Table 3]). There were no differences between reason for reoperation and time to reoperation between obese and non-obese groups.

Fig 1

 Subject selection.

Table 1

 Demographic data and differences in reoperation rates between obese and non-obese subjects.

Fig 2

 Unadjusted relative risk of reoperation within 3 months of elective lumbar surgery (solid lines represent 95% confidence intervals).

Table 2

 Adjusted point estimates (odds ratios) for early reoperation.

Table 3

 Explanation for reoperation stratified by obesity status.

DISCUSSION

• While some studies have shown an increased risk of complications for obese patients after lumbar spine surgery, others have shown the contrary. The point estimate from this population-based study suggests that obese subjects have nearly double the reoperation rate.

• The adjusted estimate suggests that instrumented fusion and deformity correction surgery is predictive for early reoperation.

• We report on a complete population. The one-payer health insurance system in Alberta allowed the capture of all elective lumbar spine surgical patients in the province.

• Given the restrictions imposed by the data available, we could not account for comorbid conditions. Data quality was dependent on the accuracy submissions by the surgeons for billing purposes; incomplete or inconsistent entries were not included in the analysis to reduce bias.

• The reasons for reoperation were likely biased due to the higher fees associated with repeated decompression procedures compared with irrigation and debridement.

• Morbidly obese patients were overrepresented in this population (9.8%) compared with the Canadian national average (7.8%) [11].

SUMMARY AND CONCLUSION

Five percent of obese subjects required early reoperation after elective lumbar spine surgery. Compared with non-obese subjects, this corresponds to a point RR of 1.73. When accounting for possible known confounders, deformity correction is associated with the largest risk for early reoperation.

The authors have no conflicts of interest to declare.
This study was funded by Calgary Surgical Research Development Fund.
This study has been approved by the University of Calgary Office for Medical Bioethics.

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• REFERENCE EDITORIAL PERSPECTIVE

• 1 Kalanithi PS, Arrigo R, Boakye M. 2011; Morbid obesity increases cost and complication rates in spinal arthrodesis. Spine (Phila Pa 1976) [Epub ahead of print]
• 2 Koutsoumbelis S, Hughes AP, Girardi FP et al. 2011; Risk factors for postoperative infection following posterior lumbar instrumented arthrodesis. J Bone Joint Surg Am 93 (17) 1627-1633
• 3 Mehta AI, Babu R, Karikari IO et al. 2011; The distribution of body mass as a significant risk factor for lumbar spinal fusion postoperative infections. Spine (Phila Pa 1976). [Epub ahead of print]

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