- Email: [email protected]
Is Obesity a Risk Factor for Adverse Events After Knee Arthroscopy?
Is Obesity a Risk Factor for Adverse Events After Knee Arthroscopy? David C. Sing, B.S., Tammy F. Luan, Brian T. Feeley, M.D., and Alan L. Zhang, M.D.
Purpose: To evaluate how body mass index (BMI) affects rates of 30-day complication, hospital readmissions, and mortality in patients undergoing knee arthroscopy. Methods: Patients undergoing knee arthroscopy procedures between 2006 and 2013 were identified in the American College of Surgeons National Surgical Quality Improvement Program database. Patient demographics and preoperative risk factors including BMI were analyzed for postoperative complications within 30 days. Cochran-Armitage testing was performed to detect differences in complication rates across BMI categories according to World Health Organization classification. The independent risk of BMI was assessed using multivariate regression analysis. Results: Of 41,919 patients with mean age 48 years undergoing knee arthroscopy, 20% were classified as normal weight (BMI 18.5 to 24), 35% overweight (BMI 25 to 29), 24% obese class I (BMI 30 to 34), 12% class II (BMI 35 to 40), and 9% class III (BMI 40). Risk of complication increased significantly with increasing BMI (normal: 1.5%, overweight: 1.6%, obese class I: 1.7%, obese class II: 1.8%, obese class III: 1.9%, P ¼ .043). On multivariate analysis, there was no increased risk of postoperative complication directly attributed to patient BMI. Independent risk factors for medical and surgical complications after knee arthroscopy included American Society of Anesthesiologists (ASA) rating (class 4 v class 1 odds ratio [OR]: 5.39 [95% confidence interval: 3.11-9.33], P < .001), functional status for activities of daily living (dependent v independent OR: 2.13 [1.42, 3.31], P < .001), history of renal comorbidity (presence v absence OR: 5.10 [2.30, 11.29], P < .001), and previously experienced history of wound infection prior to current surgery (presence v absence OR: 4.91 [2.88, 8.39], P < .001). Conclusions: More than 40% of knee arthroscopy patients qualify as obese. Although univariate analysis suggests that obesity is associated with increased postoperative complications within 30 days of surgery, BMI alone does not predict complications. Independent predictors of complications include patients with high ASA classification, dependent functional status, renal comorbidities, and a recent history of wound infection. Level of Evidence: Level IV, prognostic case series.
O
besity is a rapidly growing epidemic in the United States.1,2 Data from the Center for Disease Control and Prevention estimate that 78 million US adults, or 35.7% of the population, are currently obese with a body mass index (BMI) greater than or equal to 30.3 As the obesity epidemic continues to increase worldwide, obese patients will form a larger portion of patients undergoing elective orthopedic procedures.4 Therefore, understanding the effect of modifiable risk factors such as obesity on early outcomes after surgery may improve
From the Department of Orthopaedic Surgery, University of California San Francisco, San Francisco, California, U.S.A. The authors report the following potential conflict of interest or source of funding: B.T.F. received support from Zimmer. Received May 3, 2015; accepted January 8, 2016. Address correspondence to Alan L. Zhang, M.D., Department of Orthopaedic Surgery, University of California San Francisco, 1500 Owens Street, Box 3004, San Francisco, CA 94158, U.S.A. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15383/$36.00 http://dx.doi.org/10.1016/j.arthro.2016.01.017
preoperative risk stratification and medical optimization. Several studies have reported the effect of obesity on outcomes of major reconstructive orthopaedic surgeries. Haverkamp et al.5 found that obese patients undergoing total hip arthroplasty had a 3-fold higher wound infection and complication rate. Similarly, obesity negatively affects outcomes and complication rates for patients receiving total knee arthroplasty (TKA) with higher rates of reoperation and lower patient function.6 Buerba et al.7 also reported that obesity was a direct risk factor for wound, urinary, and pulmonary postoperative complication rates after lumbar surgery. Few studies have evaluated complication rates in obese patients after less invasive orthopaedic procedures such as arthroscopic surgery. Martin et al.8 previously analyzed 12,271 patients undergoing knee arthroscopy and found no significant differences on univariate analysis in patients with BMI <35 compared with BMI 35. Warrender et al.9 found that
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obesity negatively impacts operative time, length of stay, and functional outcomes for patients undergoing arthroscopic rotator cuff repairs. Harrison et al.10 found that obese women reported inferior quality of life and satisfaction with surgery compared with normal weight women after arthroscopic debridement of the knee. In addition, obese patients have lower functional scores after isolated arthroscopic partial meniscectomy than nonobese patients.11 Knee arthroscopy is consistently among the most commonly performed procedures by orthopedic surgeons12 as 4 million operations are performed worldwide each year.13 With the rising incidence of knee arthroscopies and increasing obesity rates, it becomes increasingly important to understand the effects of obesity on complications. Cross-sectional analyses of large national quality databases are useful for informing surgeons about the relative importance of modifiable preoperative risk factors, including obesity or other medical comorbidities. The purpose of this study was to evaluate how BMI affects rates of 30-day complications, hospital readmissions, and mortality in patients undergoing knee arthroscopy. We hypothesized that obesity is associated with an increased incidence of 30-day complications, hospital readmissions, and mortality after knee arthroscopy.
Methods Study Population The American College of Surgeons (ACS) National Surgical Quality Improvement Program (NSQIP) database is a nationally validated prospective outcomesbased program designed with the purpose to measure and improve the quality of surgical care. Data collection and performance evaluation of this database has been previously described.14 The ACS-NSQIP now includes 462 US and 34 international hospitals.15 Data is collected across 135 Health Insurance Portability and Accountability Act (HIPAA)ecompliant variables for adult patients undergoing both inpatient and ambulatory surgery. A representative sample of operative cases from each participating center is collected and submitted by certified surgical clinical reviewers (SCRs) using standardized data definitions and protocols. Regular systematic audits ensure interrater reliability.16 Definitions for all clinical variables including comorbidities and complications can be found from publicly available ACS-NSQIP sources.17 The ACS-NSQIP participant-use data files from 2006 to 2013 were used for the current analysis. Inclusion criteria were all patients identified by Current Procedural Terminology coding for arthroscopy of the knee (Appendix Table 1, available at www.
arthroscopyjournal.org). Patients presenting underweight were excluded from analysis to focus analysis on presence or absence of obesity. Patients considered moribund on the American Society of Anesthesiologists (ASA) classification score were also excluded. Patients were stratified by BMI, categorized into groups according to World Health Organization guidelines: normal (18.5 to 24.9), overweight (25 to 25.9), obese class I (30 to 34.9), obese class II (35 to 39.9), and obese class III (40). Demographic variables included age and gender. Age was stratified into 5 groups: <25, 25 to 44, 45 to 64, and >65. Other clinical variables included inpatient versus outpatient setting, preoperative functional status, presence of comorbidities, and ASA score. Comorbidities were grouped by system for cardiac (angina, hypertension, congestive heart failure, myocardial infarction, previous percutaneous coronary intervention or cardiac surgery, ischemic rest pain, peripheral vascular disease), respiratory (chronic obstructive pulmonary disease, dyspnea, pneumonia, smoking), renal (on dialysis, renal failure), or cerebrovascular (stroke, hemiplegia, quadriplegic, paralysis, transient ischemic attack). Outcome Measures The primary outcome variable was computed to indicate the presence of any medical or surgical complication. Complications were coded to include events occurring within 30 postoperative days, and were grouped into minor or major categories. Minor complications included superficial surgical site infection, pneumonia, urinary tract infection, deep vein thrombosis, bleeding transfusions, peripheral nerve injury, and renal insufficiency. Major complications included organ space infection, sepsis, septic shock, wound dehiscence, deep surgical site infection, pulmonary embolism, requiring ventilator for greater than 48 hours, unplanned intubation, acute renal failure, cardiac arrest requiring CPR, myocardial infarction, stroke, return to operating room, readmission within 30 days, and death. Readmission data were collected starting in 2011. Thus readmission rates were analyzed for 34,834 patients who underwent knee arthroscopy in the years 2011 to 2013. Statistical Analysis Differences between BMI classifications in baseline characteristics, comorbidities, and complications were assessed with Pearson c2 test. The Cochran-Armitage c2 trend test was performed to evaluate differences in consideration of the ordinal nature of increasing BMI categories. BMI, comorbidities, ASA rating, procedure type, demographics, and facility type were entered as covariates into a multivariable logistic regression model, with outcome variable assigned as presence of any complication. Forward stepwise regression was also
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performed to analyze the strongest contributing risk factors for predicting any complication. The order of entry into the model reflects each covariate’s strength of association, and the first 10 covariates entered were analyzed. Significance was assessed as P < .05. All analysis was performed using Python 2.7 (Python Software Foundation, www.python.org) and R 2.13 (R Foundation, www.r-project.org).
Results
Fig 1. Distribution of body mass index by patient age.
Demographics A total of 41,919 arthroscopic knee procedures were included in the analysis (Table 1). The mean age of the study cohort was 48.6 (standard deviation: 15.4). Overall, 8,563 patients were normal weight (20.4%), 14,525 patients were overweight (34.7%), 9,910 patients were obese class I (23.6%), 5,008 patients were obese class II (11.9%), and 3,913 patients were obese class III (9.3%). By age group, patients <25 years old were less likely to be obese, with 21.7% having BMI 30. In comparison, patients >80 had the next lowest percentage of obesity of 25%, followed by 42.5%, 44.9%, and 52.2% obesity in age groups 25 to 44, 65 to 79, and 45 to 64, respectively (Fig 1). Prevalence of obesity was significantly different between age groups (P < .001). Table 1. Characteristics of Study Population Characteristic Total Sex Female Male Age group <25 25-44 45-64 >65 ASA class 1 ¼ no disturbance 2 ¼ mild disturbance 3 ¼ severe disturbance 4 ¼ life threatening BMI Normal (18.5-24) Overweight (25-29) Obese class I (30-34) Obese class II (35-40) Obese class III (>40) Functional status Independent Partially/completely dependent Surgical setting Inpatient Outpatient
No. of Patients (%) 41,919 20,083 (47.9) 21,803 (52.0) 4,367 12,612 19,580 5,331
(10.4) (30.1) (46.7) (12.7)
11,205 23,146 7,335 233
(26.7) (55.2) (17.5) (0.6)
8,563 14,525 9,910 5,008 3,913
(20.4) (34.7) (23.6) (11.9) (9.3)
Women comprised 61.0% of obese class III patients, 56.9% of patients with normal BMI, 53.1% of obese class II patients, 45.0% of obese class I patients, and 39.3 of overweight patients (P < .001). ASA ratings of 3 or 4 comprised 54.1% of obese class III patients, 31.2% of obese class II patients, 18.3% of obese class I patients, 10.3% of overweight patients, and 6.8% of normal weight patients (P < .001) (Fig 2). Patient functional status was equivalent regardless of BMI, with 98.5% of patients functionally independent. Obesity Associated With Baseline Comorbidities Comorbidities increased with increasing BMI (Table 2). Notably, the percentage of patients with a history of cardiac comorbidity increased with each BMI stratification (normal weight: 12.8% v obese class III: 51.3%, P < .001). This was also observed in diabetic patients (normal weight: 2.1% v obese class III: 21.4%, P < .001). These trends were found to be significant for both cardiac comorbidity (P ¼ .048) and diabetes (P ¼ .047). Obesity was not associated with increased likelihood to be dependent or partially dependent for daily activities compared with normal weight patients (2% v 1.4%, P ¼ .071). Pulmonary, renal, and cerebrovascular comorbidities were all significantly different across BMI groups, although a significant trend was not observed.
41,307 (98.5) 612 (1.5) 1,373 (3.3) 40,546 (96.7)
NOTE. Percentage of patients in group is shown in parentheses. ASA, American Society of Anesthesiologists; BMI, body mass index.
Fig 2. Distribution of body mass index by American Society of Anesthesiologists rating.
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Table 2. Comorbidities by Body Mass Index Comorbidity Total, n Functional status, n (%) Independent Dependent/partially dependent History of cardiac comorbidity, n (%) History of pulmonary comorbidity, n (%) History of renal comorbidity, n (%) History of cerebrovascular comorbidity, n (%) History of diabetes mellitus, n (%) History of wound infection, n (%)
Normal (18.5-24) 8,563 8,443 120 1,100 1,499 6 48 176 26
Overweight (25-29) 14,525
(98.6) (1.4) (12.8) (17.5) (0.1) (0.6) (2.1) (0.3)
14,330 195 3,513 2,386 17 122 716 47
(98.7) (1.3) (24.2) (16.4) (0.1) (0.8) (4.9) (0.3)
Obese Class I (30-34) 9,910 9,764 146 3,717 1,765 10 103 1,055 33
(98.5) (1.5) (37.5) (17.8) (0.1) (1) (10.6) (0.3)
Obese Class II (35-40) 5,008 4,934 74 2,314 946 5 67 790 22
(98.5) (1.5) (46.2) (18.9) (0.1) (1.3) (15.8) (0.4)
Obese Class III (>40) 3,913 3,836 77 2,008 786 10 37 839 23
(98) (2) (51.3) (20.1) (0.3) (0.9) (21.4) (0.6)
P (Trend)
.022 < .001 < .001 .036 < .001 < .001 .013
NOTE. Percentage of patients with presence of comorbidity in each body mass index classification is shown in parentheses.
Complication Rates by Procedure The overall complication rate in all knee arthroscopies was 2.3% (Table 3). The most common procedures were partial meniscectomy (n ¼ 26,623, 63.5% of procedures; complication rate: 1.4%) and anterior cruciate ligament repair (n ¼ 6,310, 15.1%; complication rate: 1.5%). Arthroscopic drilling for osteochondritis dissecans lesions (n ¼ 93, 0.22%) had the highest complication rate among knee arthroscopies at 4.3%, whereas meniscal transplantation (n ¼ 122, 0.29%) had no reported complications. Univariate Analysis of Complications In consideration of the ordering of BMI categories, significant trends were observed with association of Table 3. Complication Rate by Knee Arthroscopy Procedure Procedure Total Drilling for osteochondritis dissecans lesion Synovectomy Lysis of adhesions, with or without manipulation Lateral release Debridement/shaving of articular cartilage (chondroplasty) Posterior cruciate ligament repair/ augmentation or reconstruction Abrasion arthroplasty (includes chondroplasty where necessary) Meniscus repair Anterior cruciate ligament repair/ augmentation/reconstruction Meniscectomy Osteochondral transplant Removal of loose body or foreign body (e.g., osteochondritis dissecans fragmentation, chondral fragmentation) Meniscal transplantation
n 41,919 93
% Complication 2.3 4.3
1,957 230
3.4 3.0
586 2,841
2.9 2.5
148
2.0
1,242
2.0
1,247 6,310
1.6 1.5
26,623 82 438
1.4 1.2 1.1
122
0.0
any complication (P ¼ .043), major complication (P ¼ .021), return to operating room (P ¼ .046), and superficial surgical site infection (P ¼ .002) (Table 4). No other differences among BMI groups were observed for any other complications, including readmissions (overall risk: 1.0%) and mortality (overall risk: 0.03%). Multivariate Analysis of Complications On multivariate logistic regression, obesity was not a significant predictor of having at least 1 complication in knee arthroscopy (obese class III v normal weight odds ratio [OR]: 0.89 [95% confidence interval: 0.64-1.22], P ¼ .465) (Table 5). Independent risk factors for complication included ASA rating (ASA class 4 v ASA class 1 OR: 5.39 [3.11-9.33], P < .001), functional status (dependent v independent OR: 2.13 [1.42-3.21], P < .001), history of renal comorbidity (presence v absence OR: 5.1 [2.3-11.29], P < .001), history of wound infection (presence v absence OR: 4.91 [2.88-8.39], P < .001), and procedure type in reference to meniscectomy (anterior cruciate ligament reconstruction OR: 1.46 [1.13-1.89], P ¼ .004; abrasion arthroplasty OR 1.52 [1.02-2.3], P ¼ .045; articular debridement/chondroplasty OR: 1.86 [1.43-2.42], P < .001; drilling for osteochondritis dissecans lesion OR: 3.95 [1.43-10.86], P ¼ .008; synovectomy OR: 2.3 [1.75-3.04], P < .001; lateral release OR: 2.27 [1.37-3.77], P ¼ .002; lysis of adhesions OR: 2.2 [1.03-4.73], P ¼ .043). Evaluating the same covariates in a forward stepwise regression, the ranking of the 10 most important covariates was as follows: presence of renal complication, ASA rating of 4, history of recent wound infection, ASA rating of 3, synovectomy procedure, dependent functional status, articular chondroplasty procedure, lateral release procedure, osteochondritis dissecans drilling procedure, and history of diabetes. BMI grouping was the 14th covariate selected in the forward stepwise model.
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OBESITY AND KNEE ARTHROSCOPY Table 4. Complications by Body Mass Index Complication Total, n Any complication, n (%) Major complications, n (%) Organ space infection Sepsis Septic shock Deep SSI Wound dehiscence Pulmonary embolism Ventilator >48 hours Unplanned intubation Acute renal failure Cardiac arrest with CPR Myocardial infarction Stroke Return to operating room Minor complications, n (%) Superficial SSI Pneumonia Urinary tract infection DVT or thrombophlebitis Bleeding transfusions Peripheral nerve injury Renal insufficiency Death, n (%) Readmission (out of 22,854), n (%)
Normal (18.5-24) 8,563 127 (1.5) 62 (0.7) 10 (0.1) 4 (0) 0 (0) 6 (0.1) 1 (0) 6 (0.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (0) 61 (0.5) 71 (0.8) 16 (0.2) 6 (0.1) 10 (0.1) 26 (0.3) 11 (0.1) 2 (0) 0 (0) 3 (0) 51 (1.1)
Overweight (25-29) 14,525 229 (1.6) 114 (0.8) 20 (0.1) 14 (0.1) 2 (0) 8 (0.1) 3 (0) 13 (0.1) 2 (0) 3 (0) 1 (0) 2 (0) 5 (0) 0 (0) 109 (0.5) 131 (0.9) 29 (0.2) 6 (0) 18 (0.1) 66 (0.5) 13 (0.1) 0 (0) 1 (0) 3 (0) 71 (0.9)
Obese Class I (30-34) 9,910 165 (1.7) 84 (0.8) 15 (0.2) 6 (0.1) 2 (0) 7 (0.1) 2 (0) 14 (0.1) 1 (0) 1 (0) 2 (0) 0 (0) 1 (0) 1 (0) 69 (0.5) 97 (1) 22 (0.2) 8 (0.1) 12 (0.1) 42 (0.4) 10 (0.1) 1 (0) 4 (0) 5 (0.1) 56 (1)
Obese Class II (35-40) 5,008 89 (1.8) 52 (1) 9 (0.2) 5 (0.1) 0 (0) 5 (0.1) 1 (0) 8 (0.2) 1 (0) 0 (0) 0 (0) 1 (0) 2 (0) 1 (0) 50 (0.7) 46 (0.9) 15 (0.3) 0 (0) 7 (0.1) 23 (0.5) 3 (0.1) 0 (0) 0 (0) 1 (0) 29 (1)
Obese Class III (>40) 3,913 75 (1.9) 40 (1) 6 (0.2) 4 (0.1) 0 (0) 5 (0.1) 1 (0) 2 (0.1) 1 (0) 2 (0.1) 0 (0) 1 (0) 2 (0.1) 0 (0) 36 (0.7) 39 (1) 19 (0.5) 1 (0) 5 (0.1) 13 (0.3) 1 (0) 0 (0) 0 (0) 0 (0) 28 (1.3)
P (Trend) .043 .021 .396 .401 .933 .184 .641 .425 .229 .211 .868 .247 .150 .868 .046 .321 .002 .234 .788 .641 .082 .204 .645 .532 .507
NOTE. Percentage of patients with presence of complication in each body mass index classification is shown in parentheses. CPR, cardiac pulmonary resuscitation; DVT, deep vein thrombosis; SSI, surgical site infection.
Discussion Increasing BMI was significantly associated with higher rates of complication after knee arthroscopy in univariate analysis; however, after adjusting for confounders through multivariate analysis, BMI alone was not a significant predictor of complication. In ranking the importance of risk factors through stepwise regression analysis, presence of renal comorbidities, ASA class, functional status, recent history of wound infection, and procedure type all were superior predictors of complication than BMI grouping. Thus, although increasing BMI results in greater chances of complication, it is likely attributable to the presence of comorbidities rather than obesity alone. Our hypothesis that obesity would increase the risk of postoperative complication after knee arthroscopy procedures as an independent variable was found to be incorrect. No previous studies analyzing the role of BMI on complications after knee arthroscopy using a large national database have shown this using a multivariate analysis and adjusting for other relevant clinical variables. In contrast, previous studies evaluating obesity in orthopedic procedures have demonstrated increased incidence of wound complications using similar methodology. An analysis using the NSQIP database showed that obesity significantly increased postoperative complications and readmissions after THA.18 Obesity has
also been described to increase tension and risk of avulsion of the medial collateral ligament during knee flexion in TKA.19 Surgical exposure is often more difficult in obese patients requiring longer patient setup, more soft tissue releases, and prolonged operative time. Because arthroscopic procedures for the knee are performed through small portal incisions, difficulty with exposure is less likely to be problematic. This may also decrease the likelihood of wound complications and prolonged operative times that commonly accompany open procedures in patients with high BMI. Postoperative recovery after knee arthroscopy typically allows patients to resume normal daily activities, which may also decrease the risk of complications and readmissions associated with immobilization. Overall complication rates after arthroscopy in this study were similar to previous large retrospective analyses, including a National Health Service (England) study of 301,701 knee arthroscopies showing <1% 30day complication rates.20 Both major and minor complication rates in knee arthroscopies were reported to be <1%, with approximately a 1% incidence of readmission in a previous NSQIP analysis by Martin et al.8 analyzing 21,681 combined knee and shoulder arthroscopy cases, roughly one-third of our study population size. This study also did not find a BMI cutoff of 35 to be associated with significantly increased
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Table 5. Multivariate Regression Predicting 30-Day Complication After Knee Arthroscopy Variable Body mass index (ref.: normal) Overweight (25-29) Obese class I (30-34) Obese class II (35-40) Obese class III (>40) Sex: male v female Age (ref.: 45-64) <25 25-44 >65 ASA class (ref.: 1 ¼ no disturbance) 2 ¼ mild disturbance 3 ¼ severe disturbance 4 ¼ life threatening Functional status: dependent v independent History of cardiac comorbidity History of pulmonary comorbidity History of renal comorbidity History of cerebrovascular comorbidity History of diabetes mellitus History of wound infection Procedure (ref.: meniscectomy) Anterior cruciate ligament repair/ augmentation or reconstruction Abrasion arthroplasty (includes chondroplasty where necessary) Debridement/shaving of articular cartilage (chondroplasty) Drilling for osteochondritis dissecans lesion Synovectomy Lateral release Lysis of adhesions Meniscus repair
Adjusted OR (95% CI) 1.04 1.00 0.95 0.89 1.01
P
(0.83-1.3) (0.78-1.27) (0.71-1.28) (0.64-1.22) (0.87-1.18)
.763 .952 .749 .465 .88
0.83 (0.6-1.15) 0.91 (0.74-1.11) 1.07 (0.85-1.35)
.262 .344 .566
1.13 1.81 5.39 2.13
(0.9-1.41) (1.34-2.44)* (3.11-9.33)* (1.42-3.21)*
.304 <.001 <.001 <.001
1.07 1.1 5.1 1.16 1.2 4.91
(0.88-1.31) (0.91-1.34) (2.3-11.29)* (0.64-2.12) (0.94-1.54) (2.88-8.39)*
.486 .321 <.001 .624 .142 <.001
1.46 (1.13-1.89)*
.004
1.52 (1.01-2.3)*
.045
1.86 (1.43-2.42)* 3.95 (1.43-10.86)* 2.3 2.48 2.2 1.31
(1.75-3.04)* (1.5-4.11)* (1.03-4.73)* (0.83-2.07)
<.001 .008 <.001 <.001 .043 .244
NOTE. Less frequently performed procedures found to be statistically insignificant were omitted. ASA, American Society of Anesthesiologists; CI, confidence interval; OR, odds ratio; ref., referent. *Statistically significant.
complication rates. Although our findings of low early postoperative complication rates in knee arthroscopy are reassuring, it is important to distinguish that not all arthroscopy procedures are equivalent in risk as shown in Tables 3 and 4, with certain more invasive procedures that require longer operative time associated with higher complication rates on both univariate and multivariate analysis. Although obesity was not an independent risk factor for complication, several other strong predictors were identified. With 18% of patients classified as ASA 3 or 4, patients in ASA class 4 had an odds ratio greater than 5 for a complication compared with ASA class 1 patients (P < .001). Used globally as a standard preoperative measurement, ASA classification has previously been described to predict longer length of stay in surgical
treatment of isolated orthopaedic fractures and total knee arthroplasties.21,22 Santhiyakumar et al.23 found that ASA was the strongest predictor of 30-day readmission for selected orthopaedic trauma procedures. Higher ASA score was correlated with greater risk of surgical complications, functional status, mortality, and prosthesis failure after both total shoulder and knee arthroplasty.24,25 Previously, Martin et al. described ASA score as a risk factor for complication in knee and shoulder arthroscopies in univariate analysis using NSQIP data from 2005 to 2010.13 Using a multivariate model, our results further substantiated these findings, indicating that patients undergoing knee arthroscopies with ASA scores of 3 or 4 are at higher risk of complication and should be counseled appropriately. Renal comorbidities, including chronic kidney disease and need for dialysis, and presence of open wound/ wound infection preoperatively were significant predictors of complications after knee arthroscopy. Renal failure has been cited as a significant cause of postoperative complications after orthopaedic knee and hip arthroplasty.26,27 Chronic kidney disease combines increased inflammatory response and enhanced coagulability, all of which may contribute to increased rates of complication.28 In addition, patients with a history of wound infection may be expected to be more likely to have a subsequent wound complication owing to various metabolic and socioeconomic factors. Therefore, even with smaller portal incisions from knee arthroscopy, it is possible that these patients are at increased risk of postoperative complications as shown by the multivariate analysis in this study. Finally, with respect to surgical procedures, a recent literature review reported inconclusive evidence over association of BMI with complications in arthroscopy in meniscectomy, debridement, or anterior cruciate ligament reconstruction.29 In our study, the procedures that showed higher risk of complication than arthroscopic meniscectomy included ones that could lead to greater postoperative hemarthrosis such as lateral release, synovectomy, and drilling for osteochondral dissecans lesion. In these procedures, it may be assumed that inadequate hemostasis intraoperatively led to higher rates of complications. Limitations Several limitations inherent to large database analyses should be noted. Clinical data outside the 30-day postoperative period is not collected, limiting the knowledge of important parts of patient presentation and diagnoses, as well as complications that may have occurred after 30 days. Important measures including pain, stiffness, range of motion, or diagnostic criteria for infection are not captured. Patients with missing data were excluded from the multivariate model, and this may introduce a potential source of bias. Because of the large number of patients in the database, some
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significant differences identified may not be relevant on a clinical level. Other modifiable risk factors specific to surgeon preference or surgical technique that may contribute to complication rates are also not available. Because arthroscopic procedures may be performed at unaffiliated smaller ambulatory surgery centers that do not participate in ACS-NSQIP registry data collection, there may be bias in only observing higher-volume, larger hospitaleaffiliated centers in the database. Because surgical facility is not a specific descriptor of the patient or the procedure, and more so reflective of a clinical treatment plan based on physician judgment, geographical constraints, and overall health status, we chose to omit surgical facility as a predictor in our model. Finally, for certain arthroscopic procedures that are less frequently performed such as meniscal transplantation, the sample size from this database is underpowered for subanalysis.
Conclusions More than 40% of knee arthroscopy patients qualify as obese. Although univariate analysis suggests that obesity is associated with increased postoperative complications within 30 days of surgery, BMI alone does not predict complications. Independent predictors of complications include patients with high ASA classification, dependent functional status, renal comorbidities, and a recent history of wound infection.
References 1. Ells LJ, Lang R, Shield JP, et al. Obesity and disability: A short review. Obes Rev 2006;7:341-345. 2. Ogden CL, Carroll MD, Curtin LR, McDowell MA, Tabak CJ, Flegal KM. Prevalence of overweight and obesity in the United States, 1999-2004. JAMA 2006;295: 1549-1555. 3. Centers for Disease Control and Prevention. Overweight and obesity: Adult obesity facts. Available at http://www. cdc.gov/obesity/data/adult.html. Published September 21, 2015. Accessed November 29, 2014. 4. Bostman OM. Prevalence of obesity among patients admitted for elective orthopaedic surgery. Int J Obes Relat Metab Disord 1994;18:709-713. 5. Haverkamp D, Klinkenbijl MN, Somford MP, Albers GH, van der Vis HM. Obesity in total hip arthroplasty: Does it really matter? A meta-analysis. Acta Orthop 2011;82: 417-422. 6. Amin AK, Clayton RA, Patton JT, Gaston M, Cook Re, Brenkel IJ. Total knee replacement in morbidly obese patients. Results of a prospective, matched study. J Bone Joint Surg Br 2006;88:1321-1326. 7. Buerba RA, Fu MC, Gruskay JA, Long WD 3rd, Grauer JN. Obese class III patients at significantly greater risk of multiple complications after lumbar surgery: An analysis of 10,387 patients in the ACS NSQIP database. Spine J 2014;14:2008-2018.
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8. Martin CT, Pugley AJ, Gao Y, Wolf BR. Risk factors for thirty-day morbidity and mortality following knee arthroscopy. J Bone Joint Surg Am 2013;98:1-10. 9. Warrender WJ, Brown OL, Abboud JA. Outcomes of arthroscopic rotator cuff repairs in obese patients. J Shoulder Elbow Surg 2011;20:961-967. 10. Harrison M, Morrell J, Hopman W. Influence of obesity on outcome after knee arthroscopy. Arthroscopy 2004;20: 691-695. 11. Erdil M, Bisel K, Sungur M, et al. Does obesity negatively affect the functional results of arthroscopic partial meniscectomy? A retrospective cohort study. Arthroscopy 2013;29:232-237. 12. Garrett WE Jr, Swiontkowski MF, Weinstein JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: Part-II, certification examination mix. J Bone Joint Surg Am 2003;88:680-687. 13. American Academy of Orthopaedic Surgeons. Knee arthroscopy. Available at: http://orthoinfo.aaos.org/topic. cfm?topic¼a00299. Published March 2010. Accessed February 21, 2015. 14. Khuri SF, Henderson WG, Daley J, et al. Principal Site Investigators of the Patient Safety in Surgery Study. The Patient Safety in Surgery Study: Background, study design, and patient populations. J Am Coll Surg 2007;204: 1089-1102. 15. Fink AS, Campbell DA Jr, Mentzer RM Jr, et al. The National Surgical Quality Improvement Program in nonVeterans Administration hospitals: initial demonstration of feasibility. Ann Surg 2002;236:344-353. 16. Hall BL, Hamilton BH, Richards K, Bilimoria KY, Cohen ME, Ko CY. Does surgical quality improve in the American College of Surgeons National Surgical Quality Improvement Program: An evaluation of all participating hospitals. Ann Surg 2009;250:363-376. 17. ACS NSQIP. American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP). Available at http://site.acsnsqip.org. Accessed November 20, 2014. 18. Pugley AJ, Callaghan JJ, Martin CT, Cram P, Gao Y. Incidence of and risk factors for 30-day readmission following elective primary total joint arthroplasty: Analysis from the ACS-NSQIP. J Arthroplasty 2013;28:1499-1504. 19. Winiarsky B, Barth P, Lotke P. Total knee arthroplasty in morbidly obese patients. J Bone Joint Surg Am 1998;80: 1770-1774. 20. Jameson SS, Dowen D, James P, Serrano-Pedraza I, Reed MR, Deehan DJ. The burden of arthroscopy of the knee: A contemporary analysis of data from the English NHS. J Bone Joint Surg Br 2011;93:1327-1333. 21. Kay HF, Sathiyakumar V, Yoneda ZT, et al. The effects of American Society of Anesthesiologists physical status on length of stay and inpatient cost in the surgical treatment of isolated orthopaedic fractures. J Orthop Trauma 2014;28:e153-e159. 22. Van den Belt L, van Essen P, Heesterbeek PJ, Defoort KC. Predictive factors of length of hospital stay after primary total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2015;23:1856-1862. 23. Sathiyakumar V, Molina CS, Thakore RV, Obremskey WT, Sethi MK. ASA scores as a predictor of
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30-day perioperative readmission in patients with orthopaedic trauma injuries: A NSQIP analysis. J Orthop Trauma 2015;29:e127-e132. 24. Hooper G, Rothwell A, Hooper N, Frampton C. The relationship between the American Society of Anesthesiologists physical rating and outcome following total hip and knee arthroplasty. J Bone Joint Surg Am 2012;94:1065-1070. 25. Johnson CC, Sodha S, Garzon-Muvdi J, Petersen SA, McFarland EG. Does preoperative American Society of Anesthesiologists score relate to complications after total shoulder arthroplasty? Clin Orthop Relat Res 2014;472: 1589-1596. 26. Ackland GL, Moran N, Cone S, Grocott MPW, Mythen MG. Chronic kidney disease and postoperative
morbidity after elective orthopedic surgery. Anesth Analg 2011;112:1375-1381. 27. Augustin ID, Yeoh TY, Sprung J, Berry DJ, Shcroeder DR, Weingarten TN. Association between chronic kidney disease and blood transfusions for knee and hip arthroplasty. J Arthroplasty 2013;28:928-931. 28. Wannamethee SG, Shaper AG, Lowe GD, Lennon L, Rumley A, Whincup PH. Renal function and cardiovascular mortality in elderly men: The role of inflammatory, procoagulant, and endothelial biomarkers. Eur Heart J 2006;27:2975-2981. 29. Kluczynski MA, Bisson LJ, Marzo JM. Does body mass index affect outcomes of ambulatory knee and shoulder surgery? Arthroscopy 2014;30:856-865.
OBESITY AND KNEE ARTHROSCOPY Appendix Table 1. Current Procedural Terminology (CPT) Codes for Knee Arthroscopy Procedure Osteochondral transplant Meniscal transplantation Diagnostic arthroscopy Lateral release Removal of loose body or foreign body Synovectomy Debridement of articular cartilage/ chondroplasty Abrasion arthroplasty Meniscectomy Meniscus repair Lysis of adhesions Drilling for osteochondritis dissecans Anterior cruciate ligament repair Posterior cruciate ligament repair
CPT Codes 29866, 29867 29868 29870 29873 29874 29875, 29876 29877 29879 29880, 29881 29882, 29883 29884 29885, 29886, 29887 29888 29889
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Purpose: To evaluate how body mass index (BMI) affects rates of 30-day complication, hospital readmissions, and mortality in patients undergoing knee arthroscopy. Methods: Patients undergoing knee arthroscopy procedures between 2006 and 2013 were identified in the American College of Surgeons National Surgical Quality Improvement Program database. Patient demographics and preoperative risk factors including BMI were analyzed for postoperative complications within 30 days. Cochran-Armitage testing was performed to detect differences in complication rates across BMI categories according to World Health Organization classification. The independent risk of BMI was assessed using multivariate regression analysis. Results: Of 41,919 patients with mean age 48 years undergoing knee arthroscopy, 20% were classified as normal weight (BMI 18.5 to 24), 35% overweight (BMI 25 to 29), 24% obese class I (BMI 30 to 34), 12% class II (BMI 35 to 40), and 9% class III (BMI 40). Risk of complication increased significantly with increasing BMI (normal: 1.5%, overweight: 1.6%, obese class I: 1.7%, obese class II: 1.8%, obese class III: 1.9%, P ¼ .043). On multivariate analysis, there was no increased risk of postoperative complication directly attributed to patient BMI. Independent risk factors for medical and surgical complications after knee arthroscopy included American Society of Anesthesiologists (ASA) rating (class 4 v class 1 odds ratio [OR]: 5.39 [95% confidence interval: 3.11-9.33], P < .001), functional status for activities of daily living (dependent v independent OR: 2.13 [1.42, 3.31], P < .001), history of renal comorbidity (presence v absence OR: 5.10 [2.30, 11.29], P < .001), and previously experienced history of wound infection prior to current surgery (presence v absence OR: 4.91 [2.88, 8.39], P < .001). Conclusions: More than 40% of knee arthroscopy patients qualify as obese. Although univariate analysis suggests that obesity is associated with increased postoperative complications within 30 days of surgery, BMI alone does not predict complications. Independent predictors of complications include patients with high ASA classification, dependent functional status, renal comorbidities, and a recent history of wound infection. Level of Evidence: Level IV, prognostic case series.
O
besity is a rapidly growing epidemic in the United States.1,2 Data from the Center for Disease Control and Prevention estimate that 78 million US adults, or 35.7% of the population, are currently obese with a body mass index (BMI) greater than or equal to 30.3 As the obesity epidemic continues to increase worldwide, obese patients will form a larger portion of patients undergoing elective orthopedic procedures.4 Therefore, understanding the effect of modifiable risk factors such as obesity on early outcomes after surgery may improve
From the Department of Orthopaedic Surgery, University of California San Francisco, San Francisco, California, U.S.A. The authors report the following potential conflict of interest or source of funding: B.T.F. received support from Zimmer. Received May 3, 2015; accepted January 8, 2016. Address correspondence to Alan L. Zhang, M.D., Department of Orthopaedic Surgery, University of California San Francisco, 1500 Owens Street, Box 3004, San Francisco, CA 94158, U.S.A. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15383/$36.00 http://dx.doi.org/10.1016/j.arthro.2016.01.017
preoperative risk stratification and medical optimization. Several studies have reported the effect of obesity on outcomes of major reconstructive orthopaedic surgeries. Haverkamp et al.5 found that obese patients undergoing total hip arthroplasty had a 3-fold higher wound infection and complication rate. Similarly, obesity negatively affects outcomes and complication rates for patients receiving total knee arthroplasty (TKA) with higher rates of reoperation and lower patient function.6 Buerba et al.7 also reported that obesity was a direct risk factor for wound, urinary, and pulmonary postoperative complication rates after lumbar surgery. Few studies have evaluated complication rates in obese patients after less invasive orthopaedic procedures such as arthroscopic surgery. Martin et al.8 previously analyzed 12,271 patients undergoing knee arthroscopy and found no significant differences on univariate analysis in patients with BMI <35 compared with BMI 35. Warrender et al.9 found that
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obesity negatively impacts operative time, length of stay, and functional outcomes for patients undergoing arthroscopic rotator cuff repairs. Harrison et al.10 found that obese women reported inferior quality of life and satisfaction with surgery compared with normal weight women after arthroscopic debridement of the knee. In addition, obese patients have lower functional scores after isolated arthroscopic partial meniscectomy than nonobese patients.11 Knee arthroscopy is consistently among the most commonly performed procedures by orthopedic surgeons12 as 4 million operations are performed worldwide each year.13 With the rising incidence of knee arthroscopies and increasing obesity rates, it becomes increasingly important to understand the effects of obesity on complications. Cross-sectional analyses of large national quality databases are useful for informing surgeons about the relative importance of modifiable preoperative risk factors, including obesity or other medical comorbidities. The purpose of this study was to evaluate how BMI affects rates of 30-day complications, hospital readmissions, and mortality in patients undergoing knee arthroscopy. We hypothesized that obesity is associated with an increased incidence of 30-day complications, hospital readmissions, and mortality after knee arthroscopy.
Methods Study Population The American College of Surgeons (ACS) National Surgical Quality Improvement Program (NSQIP) database is a nationally validated prospective outcomesbased program designed with the purpose to measure and improve the quality of surgical care. Data collection and performance evaluation of this database has been previously described.14 The ACS-NSQIP now includes 462 US and 34 international hospitals.15 Data is collected across 135 Health Insurance Portability and Accountability Act (HIPAA)ecompliant variables for adult patients undergoing both inpatient and ambulatory surgery. A representative sample of operative cases from each participating center is collected and submitted by certified surgical clinical reviewers (SCRs) using standardized data definitions and protocols. Regular systematic audits ensure interrater reliability.16 Definitions for all clinical variables including comorbidities and complications can be found from publicly available ACS-NSQIP sources.17 The ACS-NSQIP participant-use data files from 2006 to 2013 were used for the current analysis. Inclusion criteria were all patients identified by Current Procedural Terminology coding for arthroscopy of the knee (Appendix Table 1, available at www.
arthroscopyjournal.org). Patients presenting underweight were excluded from analysis to focus analysis on presence or absence of obesity. Patients considered moribund on the American Society of Anesthesiologists (ASA) classification score were also excluded. Patients were stratified by BMI, categorized into groups according to World Health Organization guidelines: normal (18.5 to 24.9), overweight (25 to 25.9), obese class I (30 to 34.9), obese class II (35 to 39.9), and obese class III (40). Demographic variables included age and gender. Age was stratified into 5 groups: <25, 25 to 44, 45 to 64, and >65. Other clinical variables included inpatient versus outpatient setting, preoperative functional status, presence of comorbidities, and ASA score. Comorbidities were grouped by system for cardiac (angina, hypertension, congestive heart failure, myocardial infarction, previous percutaneous coronary intervention or cardiac surgery, ischemic rest pain, peripheral vascular disease), respiratory (chronic obstructive pulmonary disease, dyspnea, pneumonia, smoking), renal (on dialysis, renal failure), or cerebrovascular (stroke, hemiplegia, quadriplegic, paralysis, transient ischemic attack). Outcome Measures The primary outcome variable was computed to indicate the presence of any medical or surgical complication. Complications were coded to include events occurring within 30 postoperative days, and were grouped into minor or major categories. Minor complications included superficial surgical site infection, pneumonia, urinary tract infection, deep vein thrombosis, bleeding transfusions, peripheral nerve injury, and renal insufficiency. Major complications included organ space infection, sepsis, septic shock, wound dehiscence, deep surgical site infection, pulmonary embolism, requiring ventilator for greater than 48 hours, unplanned intubation, acute renal failure, cardiac arrest requiring CPR, myocardial infarction, stroke, return to operating room, readmission within 30 days, and death. Readmission data were collected starting in 2011. Thus readmission rates were analyzed for 34,834 patients who underwent knee arthroscopy in the years 2011 to 2013. Statistical Analysis Differences between BMI classifications in baseline characteristics, comorbidities, and complications were assessed with Pearson c2 test. The Cochran-Armitage c2 trend test was performed to evaluate differences in consideration of the ordinal nature of increasing BMI categories. BMI, comorbidities, ASA rating, procedure type, demographics, and facility type were entered as covariates into a multivariable logistic regression model, with outcome variable assigned as presence of any complication. Forward stepwise regression was also
OBESITY AND KNEE ARTHROSCOPY
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performed to analyze the strongest contributing risk factors for predicting any complication. The order of entry into the model reflects each covariate’s strength of association, and the first 10 covariates entered were analyzed. Significance was assessed as P < .05. All analysis was performed using Python 2.7 (Python Software Foundation, www.python.org) and R 2.13 (R Foundation, www.r-project.org).
Results
Fig 1. Distribution of body mass index by patient age.
Demographics A total of 41,919 arthroscopic knee procedures were included in the analysis (Table 1). The mean age of the study cohort was 48.6 (standard deviation: 15.4). Overall, 8,563 patients were normal weight (20.4%), 14,525 patients were overweight (34.7%), 9,910 patients were obese class I (23.6%), 5,008 patients were obese class II (11.9%), and 3,913 patients were obese class III (9.3%). By age group, patients <25 years old were less likely to be obese, with 21.7% having BMI 30. In comparison, patients >80 had the next lowest percentage of obesity of 25%, followed by 42.5%, 44.9%, and 52.2% obesity in age groups 25 to 44, 65 to 79, and 45 to 64, respectively (Fig 1). Prevalence of obesity was significantly different between age groups (P < .001). Table 1. Characteristics of Study Population Characteristic Total Sex Female Male Age group <25 25-44 45-64 >65 ASA class 1 ¼ no disturbance 2 ¼ mild disturbance 3 ¼ severe disturbance 4 ¼ life threatening BMI Normal (18.5-24) Overweight (25-29) Obese class I (30-34) Obese class II (35-40) Obese class III (>40) Functional status Independent Partially/completely dependent Surgical setting Inpatient Outpatient
No. of Patients (%) 41,919 20,083 (47.9) 21,803 (52.0) 4,367 12,612 19,580 5,331
(10.4) (30.1) (46.7) (12.7)
11,205 23,146 7,335 233
(26.7) (55.2) (17.5) (0.6)
8,563 14,525 9,910 5,008 3,913
(20.4) (34.7) (23.6) (11.9) (9.3)
Women comprised 61.0% of obese class III patients, 56.9% of patients with normal BMI, 53.1% of obese class II patients, 45.0% of obese class I patients, and 39.3 of overweight patients (P < .001). ASA ratings of 3 or 4 comprised 54.1% of obese class III patients, 31.2% of obese class II patients, 18.3% of obese class I patients, 10.3% of overweight patients, and 6.8% of normal weight patients (P < .001) (Fig 2). Patient functional status was equivalent regardless of BMI, with 98.5% of patients functionally independent. Obesity Associated With Baseline Comorbidities Comorbidities increased with increasing BMI (Table 2). Notably, the percentage of patients with a history of cardiac comorbidity increased with each BMI stratification (normal weight: 12.8% v obese class III: 51.3%, P < .001). This was also observed in diabetic patients (normal weight: 2.1% v obese class III: 21.4%, P < .001). These trends were found to be significant for both cardiac comorbidity (P ¼ .048) and diabetes (P ¼ .047). Obesity was not associated with increased likelihood to be dependent or partially dependent for daily activities compared with normal weight patients (2% v 1.4%, P ¼ .071). Pulmonary, renal, and cerebrovascular comorbidities were all significantly different across BMI groups, although a significant trend was not observed.
41,307 (98.5) 612 (1.5) 1,373 (3.3) 40,546 (96.7)
NOTE. Percentage of patients in group is shown in parentheses. ASA, American Society of Anesthesiologists; BMI, body mass index.
Fig 2. Distribution of body mass index by American Society of Anesthesiologists rating.
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Table 2. Comorbidities by Body Mass Index Comorbidity Total, n Functional status, n (%) Independent Dependent/partially dependent History of cardiac comorbidity, n (%) History of pulmonary comorbidity, n (%) History of renal comorbidity, n (%) History of cerebrovascular comorbidity, n (%) History of diabetes mellitus, n (%) History of wound infection, n (%)
Normal (18.5-24) 8,563 8,443 120 1,100 1,499 6 48 176 26
Overweight (25-29) 14,525
(98.6) (1.4) (12.8) (17.5) (0.1) (0.6) (2.1) (0.3)
14,330 195 3,513 2,386 17 122 716 47
(98.7) (1.3) (24.2) (16.4) (0.1) (0.8) (4.9) (0.3)
Obese Class I (30-34) 9,910 9,764 146 3,717 1,765 10 103 1,055 33
(98.5) (1.5) (37.5) (17.8) (0.1) (1) (10.6) (0.3)
Obese Class II (35-40) 5,008 4,934 74 2,314 946 5 67 790 22
(98.5) (1.5) (46.2) (18.9) (0.1) (1.3) (15.8) (0.4)
Obese Class III (>40) 3,913 3,836 77 2,008 786 10 37 839 23
(98) (2) (51.3) (20.1) (0.3) (0.9) (21.4) (0.6)
P (Trend)
.022 < .001 < .001 .036 < .001 < .001 .013
NOTE. Percentage of patients with presence of comorbidity in each body mass index classification is shown in parentheses.
Complication Rates by Procedure The overall complication rate in all knee arthroscopies was 2.3% (Table 3). The most common procedures were partial meniscectomy (n ¼ 26,623, 63.5% of procedures; complication rate: 1.4%) and anterior cruciate ligament repair (n ¼ 6,310, 15.1%; complication rate: 1.5%). Arthroscopic drilling for osteochondritis dissecans lesions (n ¼ 93, 0.22%) had the highest complication rate among knee arthroscopies at 4.3%, whereas meniscal transplantation (n ¼ 122, 0.29%) had no reported complications. Univariate Analysis of Complications In consideration of the ordering of BMI categories, significant trends were observed with association of Table 3. Complication Rate by Knee Arthroscopy Procedure Procedure Total Drilling for osteochondritis dissecans lesion Synovectomy Lysis of adhesions, with or without manipulation Lateral release Debridement/shaving of articular cartilage (chondroplasty) Posterior cruciate ligament repair/ augmentation or reconstruction Abrasion arthroplasty (includes chondroplasty where necessary) Meniscus repair Anterior cruciate ligament repair/ augmentation/reconstruction Meniscectomy Osteochondral transplant Removal of loose body or foreign body (e.g., osteochondritis dissecans fragmentation, chondral fragmentation) Meniscal transplantation
n 41,919 93
% Complication 2.3 4.3
1,957 230
3.4 3.0
586 2,841
2.9 2.5
148
2.0
1,242
2.0
1,247 6,310
1.6 1.5
26,623 82 438
1.4 1.2 1.1
122
0.0
any complication (P ¼ .043), major complication (P ¼ .021), return to operating room (P ¼ .046), and superficial surgical site infection (P ¼ .002) (Table 4). No other differences among BMI groups were observed for any other complications, including readmissions (overall risk: 1.0%) and mortality (overall risk: 0.03%). Multivariate Analysis of Complications On multivariate logistic regression, obesity was not a significant predictor of having at least 1 complication in knee arthroscopy (obese class III v normal weight odds ratio [OR]: 0.89 [95% confidence interval: 0.64-1.22], P ¼ .465) (Table 5). Independent risk factors for complication included ASA rating (ASA class 4 v ASA class 1 OR: 5.39 [3.11-9.33], P < .001), functional status (dependent v independent OR: 2.13 [1.42-3.21], P < .001), history of renal comorbidity (presence v absence OR: 5.1 [2.3-11.29], P < .001), history of wound infection (presence v absence OR: 4.91 [2.88-8.39], P < .001), and procedure type in reference to meniscectomy (anterior cruciate ligament reconstruction OR: 1.46 [1.13-1.89], P ¼ .004; abrasion arthroplasty OR 1.52 [1.02-2.3], P ¼ .045; articular debridement/chondroplasty OR: 1.86 [1.43-2.42], P < .001; drilling for osteochondritis dissecans lesion OR: 3.95 [1.43-10.86], P ¼ .008; synovectomy OR: 2.3 [1.75-3.04], P < .001; lateral release OR: 2.27 [1.37-3.77], P ¼ .002; lysis of adhesions OR: 2.2 [1.03-4.73], P ¼ .043). Evaluating the same covariates in a forward stepwise regression, the ranking of the 10 most important covariates was as follows: presence of renal complication, ASA rating of 4, history of recent wound infection, ASA rating of 3, synovectomy procedure, dependent functional status, articular chondroplasty procedure, lateral release procedure, osteochondritis dissecans drilling procedure, and history of diabetes. BMI grouping was the 14th covariate selected in the forward stepwise model.
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OBESITY AND KNEE ARTHROSCOPY Table 4. Complications by Body Mass Index Complication Total, n Any complication, n (%) Major complications, n (%) Organ space infection Sepsis Septic shock Deep SSI Wound dehiscence Pulmonary embolism Ventilator >48 hours Unplanned intubation Acute renal failure Cardiac arrest with CPR Myocardial infarction Stroke Return to operating room Minor complications, n (%) Superficial SSI Pneumonia Urinary tract infection DVT or thrombophlebitis Bleeding transfusions Peripheral nerve injury Renal insufficiency Death, n (%) Readmission (out of 22,854), n (%)
Normal (18.5-24) 8,563 127 (1.5) 62 (0.7) 10 (0.1) 4 (0) 0 (0) 6 (0.1) 1 (0) 6 (0.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (0) 61 (0.5) 71 (0.8) 16 (0.2) 6 (0.1) 10 (0.1) 26 (0.3) 11 (0.1) 2 (0) 0 (0) 3 (0) 51 (1.1)
Overweight (25-29) 14,525 229 (1.6) 114 (0.8) 20 (0.1) 14 (0.1) 2 (0) 8 (0.1) 3 (0) 13 (0.1) 2 (0) 3 (0) 1 (0) 2 (0) 5 (0) 0 (0) 109 (0.5) 131 (0.9) 29 (0.2) 6 (0) 18 (0.1) 66 (0.5) 13 (0.1) 0 (0) 1 (0) 3 (0) 71 (0.9)
Obese Class I (30-34) 9,910 165 (1.7) 84 (0.8) 15 (0.2) 6 (0.1) 2 (0) 7 (0.1) 2 (0) 14 (0.1) 1 (0) 1 (0) 2 (0) 0 (0) 1 (0) 1 (0) 69 (0.5) 97 (1) 22 (0.2) 8 (0.1) 12 (0.1) 42 (0.4) 10 (0.1) 1 (0) 4 (0) 5 (0.1) 56 (1)
Obese Class II (35-40) 5,008 89 (1.8) 52 (1) 9 (0.2) 5 (0.1) 0 (0) 5 (0.1) 1 (0) 8 (0.2) 1 (0) 0 (0) 0 (0) 1 (0) 2 (0) 1 (0) 50 (0.7) 46 (0.9) 15 (0.3) 0 (0) 7 (0.1) 23 (0.5) 3 (0.1) 0 (0) 0 (0) 1 (0) 29 (1)
Obese Class III (>40) 3,913 75 (1.9) 40 (1) 6 (0.2) 4 (0.1) 0 (0) 5 (0.1) 1 (0) 2 (0.1) 1 (0) 2 (0.1) 0 (0) 1 (0) 2 (0.1) 0 (0) 36 (0.7) 39 (1) 19 (0.5) 1 (0) 5 (0.1) 13 (0.3) 1 (0) 0 (0) 0 (0) 0 (0) 28 (1.3)
P (Trend) .043 .021 .396 .401 .933 .184 .641 .425 .229 .211 .868 .247 .150 .868 .046 .321 .002 .234 .788 .641 .082 .204 .645 .532 .507
NOTE. Percentage of patients with presence of complication in each body mass index classification is shown in parentheses. CPR, cardiac pulmonary resuscitation; DVT, deep vein thrombosis; SSI, surgical site infection.
Discussion Increasing BMI was significantly associated with higher rates of complication after knee arthroscopy in univariate analysis; however, after adjusting for confounders through multivariate analysis, BMI alone was not a significant predictor of complication. In ranking the importance of risk factors through stepwise regression analysis, presence of renal comorbidities, ASA class, functional status, recent history of wound infection, and procedure type all were superior predictors of complication than BMI grouping. Thus, although increasing BMI results in greater chances of complication, it is likely attributable to the presence of comorbidities rather than obesity alone. Our hypothesis that obesity would increase the risk of postoperative complication after knee arthroscopy procedures as an independent variable was found to be incorrect. No previous studies analyzing the role of BMI on complications after knee arthroscopy using a large national database have shown this using a multivariate analysis and adjusting for other relevant clinical variables. In contrast, previous studies evaluating obesity in orthopedic procedures have demonstrated increased incidence of wound complications using similar methodology. An analysis using the NSQIP database showed that obesity significantly increased postoperative complications and readmissions after THA.18 Obesity has
also been described to increase tension and risk of avulsion of the medial collateral ligament during knee flexion in TKA.19 Surgical exposure is often more difficult in obese patients requiring longer patient setup, more soft tissue releases, and prolonged operative time. Because arthroscopic procedures for the knee are performed through small portal incisions, difficulty with exposure is less likely to be problematic. This may also decrease the likelihood of wound complications and prolonged operative times that commonly accompany open procedures in patients with high BMI. Postoperative recovery after knee arthroscopy typically allows patients to resume normal daily activities, which may also decrease the risk of complications and readmissions associated with immobilization. Overall complication rates after arthroscopy in this study were similar to previous large retrospective analyses, including a National Health Service (England) study of 301,701 knee arthroscopies showing <1% 30day complication rates.20 Both major and minor complication rates in knee arthroscopies were reported to be <1%, with approximately a 1% incidence of readmission in a previous NSQIP analysis by Martin et al.8 analyzing 21,681 combined knee and shoulder arthroscopy cases, roughly one-third of our study population size. This study also did not find a BMI cutoff of 35 to be associated with significantly increased
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Table 5. Multivariate Regression Predicting 30-Day Complication After Knee Arthroscopy Variable Body mass index (ref.: normal) Overweight (25-29) Obese class I (30-34) Obese class II (35-40) Obese class III (>40) Sex: male v female Age (ref.: 45-64) <25 25-44 >65 ASA class (ref.: 1 ¼ no disturbance) 2 ¼ mild disturbance 3 ¼ severe disturbance 4 ¼ life threatening Functional status: dependent v independent History of cardiac comorbidity History of pulmonary comorbidity History of renal comorbidity History of cerebrovascular comorbidity History of diabetes mellitus History of wound infection Procedure (ref.: meniscectomy) Anterior cruciate ligament repair/ augmentation or reconstruction Abrasion arthroplasty (includes chondroplasty where necessary) Debridement/shaving of articular cartilage (chondroplasty) Drilling for osteochondritis dissecans lesion Synovectomy Lateral release Lysis of adhesions Meniscus repair
Adjusted OR (95% CI) 1.04 1.00 0.95 0.89 1.01
P
(0.83-1.3) (0.78-1.27) (0.71-1.28) (0.64-1.22) (0.87-1.18)
.763 .952 .749 .465 .88
0.83 (0.6-1.15) 0.91 (0.74-1.11) 1.07 (0.85-1.35)
.262 .344 .566
1.13 1.81 5.39 2.13
(0.9-1.41) (1.34-2.44)* (3.11-9.33)* (1.42-3.21)*
.304 <.001 <.001 <.001
1.07 1.1 5.1 1.16 1.2 4.91
(0.88-1.31) (0.91-1.34) (2.3-11.29)* (0.64-2.12) (0.94-1.54) (2.88-8.39)*
.486 .321 <.001 .624 .142 <.001
1.46 (1.13-1.89)*
.004
1.52 (1.01-2.3)*
.045
1.86 (1.43-2.42)* 3.95 (1.43-10.86)* 2.3 2.48 2.2 1.31
(1.75-3.04)* (1.5-4.11)* (1.03-4.73)* (0.83-2.07)
<.001 .008 <.001 <.001 .043 .244
NOTE. Less frequently performed procedures found to be statistically insignificant were omitted. ASA, American Society of Anesthesiologists; CI, confidence interval; OR, odds ratio; ref., referent. *Statistically significant.
complication rates. Although our findings of low early postoperative complication rates in knee arthroscopy are reassuring, it is important to distinguish that not all arthroscopy procedures are equivalent in risk as shown in Tables 3 and 4, with certain more invasive procedures that require longer operative time associated with higher complication rates on both univariate and multivariate analysis. Although obesity was not an independent risk factor for complication, several other strong predictors were identified. With 18% of patients classified as ASA 3 or 4, patients in ASA class 4 had an odds ratio greater than 5 for a complication compared with ASA class 1 patients (P < .001). Used globally as a standard preoperative measurement, ASA classification has previously been described to predict longer length of stay in surgical
treatment of isolated orthopaedic fractures and total knee arthroplasties.21,22 Santhiyakumar et al.23 found that ASA was the strongest predictor of 30-day readmission for selected orthopaedic trauma procedures. Higher ASA score was correlated with greater risk of surgical complications, functional status, mortality, and prosthesis failure after both total shoulder and knee arthroplasty.24,25 Previously, Martin et al. described ASA score as a risk factor for complication in knee and shoulder arthroscopies in univariate analysis using NSQIP data from 2005 to 2010.13 Using a multivariate model, our results further substantiated these findings, indicating that patients undergoing knee arthroscopies with ASA scores of 3 or 4 are at higher risk of complication and should be counseled appropriately. Renal comorbidities, including chronic kidney disease and need for dialysis, and presence of open wound/ wound infection preoperatively were significant predictors of complications after knee arthroscopy. Renal failure has been cited as a significant cause of postoperative complications after orthopaedic knee and hip arthroplasty.26,27 Chronic kidney disease combines increased inflammatory response and enhanced coagulability, all of which may contribute to increased rates of complication.28 In addition, patients with a history of wound infection may be expected to be more likely to have a subsequent wound complication owing to various metabolic and socioeconomic factors. Therefore, even with smaller portal incisions from knee arthroscopy, it is possible that these patients are at increased risk of postoperative complications as shown by the multivariate analysis in this study. Finally, with respect to surgical procedures, a recent literature review reported inconclusive evidence over association of BMI with complications in arthroscopy in meniscectomy, debridement, or anterior cruciate ligament reconstruction.29 In our study, the procedures that showed higher risk of complication than arthroscopic meniscectomy included ones that could lead to greater postoperative hemarthrosis such as lateral release, synovectomy, and drilling for osteochondral dissecans lesion. In these procedures, it may be assumed that inadequate hemostasis intraoperatively led to higher rates of complications. Limitations Several limitations inherent to large database analyses should be noted. Clinical data outside the 30-day postoperative period is not collected, limiting the knowledge of important parts of patient presentation and diagnoses, as well as complications that may have occurred after 30 days. Important measures including pain, stiffness, range of motion, or diagnostic criteria for infection are not captured. Patients with missing data were excluded from the multivariate model, and this may introduce a potential source of bias. Because of the large number of patients in the database, some
OBESITY AND KNEE ARTHROSCOPY
significant differences identified may not be relevant on a clinical level. Other modifiable risk factors specific to surgeon preference or surgical technique that may contribute to complication rates are also not available. Because arthroscopic procedures may be performed at unaffiliated smaller ambulatory surgery centers that do not participate in ACS-NSQIP registry data collection, there may be bias in only observing higher-volume, larger hospitaleaffiliated centers in the database. Because surgical facility is not a specific descriptor of the patient or the procedure, and more so reflective of a clinical treatment plan based on physician judgment, geographical constraints, and overall health status, we chose to omit surgical facility as a predictor in our model. Finally, for certain arthroscopic procedures that are less frequently performed such as meniscal transplantation, the sample size from this database is underpowered for subanalysis.
Conclusions More than 40% of knee arthroscopy patients qualify as obese. Although univariate analysis suggests that obesity is associated with increased postoperative complications within 30 days of surgery, BMI alone does not predict complications. Independent predictors of complications include patients with high ASA classification, dependent functional status, renal comorbidities, and a recent history of wound infection.
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OBESITY AND KNEE ARTHROSCOPY Appendix Table 1. Current Procedural Terminology (CPT) Codes for Knee Arthroscopy Procedure Osteochondral transplant Meniscal transplantation Diagnostic arthroscopy Lateral release Removal of loose body or foreign body Synovectomy Debridement of articular cartilage/ chondroplasty Abrasion arthroplasty Meniscectomy Meniscus repair Lysis of adhesions Drilling for osteochondritis dissecans Anterior cruciate ligament repair Posterior cruciate ligament repair
CPT Codes 29866, 29867 29868 29870 29873 29874 29875, 29876 29877 29879 29880, 29881 29882, 29883 29884 29885, 29886, 29887 29888 29889
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