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Operative Time Less Than 1.5 Hours, Male Sex, Dependent Functional Status, Presence of Dyspnea, and Reoperations Within 30 days Are Independent Risk Factors for Readmission After ACLR

Open AccessPublished:June 13, 2022DOI:https://doi.org/10.1016/j.asmr.2022.04.010

      Purpose

      The purposes of this study are to use a large, patient-centered database to describe the 30-day readmission rate and to identify predictive risk factors for readmission after elective isolated ACLR.

      Methods

      The National Surgical Quality Improvement Program Database was retrospectively queried for isolated ACLR procedures between 2011 and 2017. Current Procedural Terminology (CPT) codes were used to identify isolated ACLR patients. Those undergoing additional procedures such as meniscectomy or multi-ligamentous reconstruction were excluded. Readmissions were analyzed against demographic variables with bivariate analysis. Multivariate logistic regression was used to find independent risk factors for 30-day readmissions after ACLR.

      Results

      A total of 11,060 patients (37.2% female) were included with an average age of 32.2 ± 10.6 years and mean body mass index (BMI) of 27.9 ± 6.5 kg/m2 (29.2% were >30). The overall readmission rate was 0.59%. The most reported reason for readmission was infection 0.22 (24 out of 11,060). The following variables were associated with significantly higher readmission rates: male sex (P = .001), history of severe chronic obstructive pulmonary disease (COPD) (P = .025), cardiac comorbidity (P = .034), operative time >1.5 hours (P <.001), partially dependent functional health status (P = .002), high preoperative creatinine (P = .009), normal preoperative albumin (P = .020), hypertension (P = .034), and reoperations (P < .001). Operative time >1.5 hours, male sex, dependent functional status, the presence of dyspnea, and undergoing a reoperation were identified as independent risk factors for 30-day readmissions (P < .05 for all).

      Conclusions

      Isolated ACLR is associated with low 30-day readmission rates. Operative time >1.5 hours, male sex, dependent functional status, the presence of dyspnea, and 30-day reoperations are independent risk factors for readmission that should be considered in patient selection and addressed with preoperative counseling.

      Level of Evidence

      Level III, retrospective cohort study.

      Introduction

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      Therefore, physicians must be cognizant of the impact of postsurgical readmissions on hospital costs, as well as the patient experience.
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      ; however, this estimate does not consider the impact of postsurgical hospital readmissions. Therefore, this is likely to be a conservative projection of the true financial impact of ACLR procedures. Currently, several studies have investigated the risk factors in ACLR, overnight admissions after isolated ACLR, and readmission rates in other types of knee surgery.
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      ,
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      • et al.
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      Bokshan et al. performed a retrospective study of 9,146 patients undergoing ACLR and found that Hispanic ethnicity, use of epidural anesthesia, a known bleeding disorder, increased body mass index (BMI), longer operation time, and younger age were all independently associated with readmissions after surgery.
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      In another study of 14,159 isolated ACLRs focused on the effect of increased operative times, a 15-minute increase led to significantly higher rates of readmission, postoperative complications, and extended lengths of stay in the hospital.
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      More recently, Min et al. performed a similar study of 20,819 standard ACLR patients and used multivariate regressions to determine that regional anesthesia alone, increasing concurrent procedures, and obesity contributed to hospital readmissions in the immediate postoperative period.
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      They also found that the same risk factors applied to their smaller subgroup analysis of isolated ACLRs.
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      To our knowledge, only this minor subanalysis reporting on hospital readmissions after isolated ACLR exist in the current literature.
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      ,
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      The purposes of this study are to use a large, patient-centered database to describe the 30-day readmission rate and to identify predictive risk factors for readmission after elective isolated ACLR. We hypothesize that rates of readmission will be low and that obesity, surgical duration, and an increased number of comorbidities will be contributing risk factors.

      Methods

      In December 2019, the American College of Surgeons National Surgical Quality Improvement Program database (ACS-NSQIP) was queried for isolated ACLR procedures between 2011 and 2017, as this was the only interval during which readmission data were collected. The American College of Surgeons National Surgical Quality Improvement Program database ACS-NSQIP is a deidentified database with information on procedure type and outcomes data from 684 hospitals across the United States.
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      Current Procedural Terminology (CPT) codes were used to identify elective knee arthroscopies involving ACLR. The CPT code, 29888, was then used to identify isolated ACLR cases. Surgeries involving concomitant procedures such as meniscectomy or multiligamentous reconstruction were excluded. CPT codes used for patient identification are provided in a complete list in Table 1.
      Table 1Definitions of the CPT Codes for NSQIP Query
      CPT CodeDefinitionIncluded or Excluded
      29888Arthroscopically aided anterior cruciate ligament repair/augmentation or reconstructionIncluded
      27403Arthrotomy with meniscus repair, kneeExcluded
      29868Arthroscopy, knee, surgical; meniscal transplantation (includes arthrotomy for meniscal insertion)Excluded
      29880Arthroscopy, knee, surgical; with meniscectomy (medial AND lateral, including any meniscalExcluded
      29881Arthroscopy, knee, surgical; with meniscectomy (medial OR lateral, including any meniscal shaving)Excluded
      29882Arthroscopy, knee, surgical; with meniscus repair (medial OR lateral)Excluded
      29883Arthroscopy, knee, surgical; with meniscus repair (medial AND lateral)Excluded
      29889Arthroscopically aided posterior cruciate ligament repair/augmentation or reconstructionExcluded
      27557Open treatment of knee dislocation, with or without internal or external fixation; with primary ligamentous repairExcluded
      27558Open treatment of knee dislocation, with or without internal or external fixation; with primary ligamentous repair, with augmentation/reconstructionExcluded
      27412Autologous chondrocyte implantation, kneeExcluded
      27415Osteochondral allograft, knee, openExcluded
      29877Arthroscopy, knee, surgical; debridement/shaving of articular cartilage (chondroplasty)Excluded
      29885Arthroscopy, knee, surgical; drilling for osteochondritis dissecans with bone grafting, with or without internal fixation (including debridement of base of lesion)Excluded
      29886Arthroscopy, knee, surgical; drilling for intact osteochondritis dissecans lesionExcluded
      29887Arthroscopy, knee, surgical; drilling for intact osteochondritis dissecans lesion with internal fixationExcluded
      29879Arthroscopy, knee, surgical; abrasion arthroplasty (includes chondroplasty where necessary)Excluded
      29866Arthroscopy, knee, surgical; osteochondral autograft(s) (e.g., mosaicplasty) (includes harvesting of the autograft(s)Excluded
      29867Arthroscopy, knee, surgical; osteochondral allograft (e.g., mosaicplasty)Excluded
      27405Repair, primary, torn ligament and/or capsule, knee; collateralExcluded
      27409Repair, primary, torn ligament and/or capsule, knee; collateral and cruciate ligamentsExcluded
      CPT, Current Procedural Terminology.

      Variables Collected

      Demographic information retrieved from the ACS-NSQIP included age, sex, race, ethnicity, and inpatient/outpatient status. Age was categorized into five groups (16-20, 21-30, 31-40, 41-50, and >50). Patient variables were recorded, including body mass index, American Society of Anesthesiologist (ASA) class, diabetes, dyspnea, chronic obstructive pulmonary disease (COPD), hypertension, renal disease, heart disease, smoking history, alcohol intake, bleeding disorders, functional health status, steroid use, and recent weight loss. Examined laboratory values included hematocrit, creatinine, and serum albumin. Perioperative factors, including principal anesthesia technique and operative time, were recorded. Thirty-day reoperation and 30-day readmission rates were compiled from International Classification of Diseases (ICD) 9/10 codes along with frequency of readmission. Readmissions were defined as postoperative hospital admissions related to and within 30-days of the index isolated ACLR procedure.

      Statistical Analysis

      Bivariate analysis of the collected demographics, patient variables, laboratory values, and perioperative factors was conducted against the isolated ACLR cases that resulted in a readmission. Categorical comparisons between variables were then made using chi-squared tests. Finally, multivariate logistic regression was used to find independent risk factors for readmission after ACLR by calculating odds ratios (OR) and 95% confidence intervals (CI). All statistical analyses were conducted using SAS 9.2 software (SAS Institute Inc., Cary, NC), and the alpha was set at P < .05 to define significance.

      Results

      Retrospective analysis of the ACS-NSQIP database identified 11,060 patients (37.2% female) that met the eligibility criteria of undergoing an isolated ACLR between 2011 and 2017. The mean age of included patients was 32.2 ± 10.6 years and the average body mass index (BMI) was 27.9 ± 6.5 kg/m2 with 29.2% of patients having a BMI > 30. Of the total patient cohort, 65 (0.59%) were readmitted within 30 days of the index surgery, and the most frequent reason for readmission was postoperative infection (0.22%). A complete list of reported reasons for readmissions is presented in Table 2, while rates of readmission have been differentiated by year in Table 3 and reproduced graphically using a Cochran-Armitage Trend Test in Fig 1. Readmission rates were not significantly associated with year (P = .315).
      Table 2Reported Reasons for 30-Day Readmission After Isolated ACLR and Their Incidence rates.
      Readmission ReasonsFrequencyPercentage (%)
      Infection of Any Type240.22
      Deep Infection80.07
      Organ/Space Infection70.06
      Superficial Infection50.05
      Graft-Related Infection10.01
      Abscess/Furuncle/Carbuncle10.01
      Cellulitis10.01
      Unspecified Infection10.01
      Venous Thromboembolism120.11
      Vein Thrombosis requiring Therapy60.05
      DVT Requiring Therapy20.02
      Pulmonary Embolism40.04
      Structural and Stability Problems290.26
      Disruption of ACL10.01
      Hematoma10.01
      Effusion30.03
      Joint instability10.01
      Sprain of ACL10.01
      Intestinal Obstruction10.01
      Hiccough10.01
      Atelectasis10.01
      Ankylosis10.01
      Dermatitis10.01
      Residual Pain60.05
      Other110.10
      Not Readmitted1099599.41
      Total11060100.00
      Table 3A Summary of 30-Day Isolated ACLR Readmission Rates by Year From 2011 to 2017
      Operative YearReadmission Rate (%)
      20110.38
      20120.93
      20130.48
      20140.24
      20150.71
      20160.64
      20170.63
      Figure thumbnail gr1
      Fig 1Cochran-Armitage Trend Test of 30-day readmission rates from 2011-2017 indicated no significant relationship between readmission rate and year (P = .3145).
      Bivariate analysis revealed that the following patient characteristics was significantly associated with higher readmission rates: male sex (P = .001), history of severe COPD (P = .025), cardiac comorbidity (P = .034), operative time >1.5 hours (P < .001), partially dependent functional health status preoperatively (P = .002), high preoperative creatinine (P = .009), normal preoperative albumin (P = .02), history of hypertension (P = .034), and undergoing a reoperation (P < .001) (Table 4). Multivariate analysis determined that operative time >1.5 hours (P < .05), male sex (P < .05), dependent functional status (P > .005), the presence of dyspnea (P < .05) and undergoing a reoperation within 30 days of the index procedure (P < .001) were all independent risk factors for 30-day readmission after ACLR (Table 5).
      Table 4Bivariate Analysis of ACLR Patient Demographics by Readmission Status
      Readmission Bivariate Analysis
      No (n = 10995)Yes (n = 65)Total (n = 11060)P Value
      Age Group.142
      Chi-square
       16-201335 (99.8%)3 (0.2%)1338 (12.1%)
       21-304294 (99.4%)25 (0.6%)4319 (39.1%)
       31-402927 (99.2%)25 (0.8%)2952 (26.7%)
       41-501743 (99.5%)9 (0.5%)1752 (15.8%)
       >50696 (99.6%)3 (0.4%)699 (6.3%)
      Sex.001
      Chi-square
       Female4108 (99.7%)11 (0.3%)4119 (37.2%)
       Male6887 (99.2%)54 (0.8%)6941 (62.8%)
      Race.360
      Chi-square
       American Indian or Alaska Native173 (100.0%)0 (0.0%)173 (1.6%)
       Asian533 (99.8%)1 (0.2%)534 (4.8%)
       Black or African American941 (99.2%)8 (0.8%)949 (8.6%)
       Hispanic882 (99.0%)9 (1.0%)891 (8.1%)
       Native Hawaiian or Pacific Islander133 (99.3%)1 (0.7%)134 (1.2%)
       White5475 (99.4%)31 (0.6%)5506 (49.8%)
       Other/Not Reported2858 (99.5%)15 (0.5%)2873 (26.0%)
      Hispanic Ethnicity.241
      Chi-square
       Yes1030 (99.0%)10 (1.0%)1040 (9.4%)
       No6805 (99.5%)37 (0.5%)6842 (61.9%)
       Unknown2760 (99.5%)14 (0.5%)2774 (25.1%)
       NULL400 (99.0%)4 (1.0%)404 (3.7%)
      Obese.267
      Chi-square
       No7794 (99.5%)42 (0.5%)7836 (70.8%)
       Yes3201 (99.3%)23 (0.7%)3224 (29.2%)
      ASA.520
      Chi-square
       ≤310501 (99.4%)61 (0.6%)10562 (95.5%)
       >3494 (99.2%)4 (0.8%)498 (4.5%)
      Diabetes.315
      Chi-square
       No10827 (99.4%)65 (0.6%)10892 (98.5%)
       Yes168 (100.0%)0 (0.0%)168 (1.5%)
      History of severe COPD.025
      Chi-square
       No10971 (99.4%)64 (0.6%)11035 (99.8%)
       Yes24 (96.0%)1 (4.0%)25 (0.2%)
      Current smoker within one year.404
      Chi-square
       No9062 (99.4%)51 (0.6%)9113 (82.4%)
       Yes1933 (99.3%)14 (0.7%)1947 (17.6%)
      Cardiac comorbidity.034
      Chi-square
       No10334 (99.5%)57 (0.5%)10391 (94.0%)
       Yes661 (98.8%)8 (1.2%)669 (6.0%)
      Renal comorbidity.913
      Chi-square
       No10993 (99.4%)65 (0.6%)11058 (100.0%)
       Yes2 (100.0%)0 (0.0%)2 (0.0%)
      Bleeding disorders.712
      Chi-square
       No10972 (99.4%)65 (0.6%)11037 (99.8%)
       Yes23 (100.0%)0 (0.0%)23 (0.2%)
      Inpatient/outpatient.090
      Chi-square
       Inpatient640 (98.9%)7 (1.1%)647 (5.8%)
       Outpatient10355 (99.4%)58 (0.6%)10413 (94.2%)
      Operative Time<.001
      Chi-square
       ≤1.5 hours5271 (99.7%)16 (0.3%)5287 (47.8%)
       >1.5 hours5724 (99.2%)49 (0.8%)5773 (52.2%)
      Functional health status prior to surgery.002
      Chi-square
       Independent10887 (99.4%)62 (0.6%)10949 (99.0%)
       Partially Dependent16 (94.1%)1 (5.9%)17 (0.2%)
       Unknown92 (97.9%)2 (2.1%)94 (0.8%)
      Steroid use for chronic condition.079
      Chi-square
       No10961 (99.4%)64 (0.6%)11025 (99.7%)
       Yes34 (97.1%)1 (2.9%)35 (0.3%)
      >10% loss body weight in last 6 months.818
      Chi-square
       No10986 (99.4%)65 (0.6%)11051 (99.9%)
       Yes9 (100.0%)0 (0.0%)9 (0.1%)
      EtOH > 2 drinks/day in 2 weeks before admission.512
      Chi-square
       No673 (99.3%)5 (0.7%)678 (6.1%)
       Yes10 (100.0%)0 (0.0%)10 (0.1%)
       NULL3384 (99.6%)14 (0.4%)3398 (30.7%)
       Missing6928 (99.3%)46 (0.7%)6974 (63.1%)
      Hematocrit level.134
      Chi-square
       Low (<35.5%)125 (100.0%)0 (0.0%)125 (1.1%)
       Normal3083 (99.2%)25 (0.8%)3108 (28.1%)
       Null7787 (99.5%)40 (0.5%)7827 (70.8%)
      Creatinine level.009
      Chi-square
       High (>1.21)101 (97.1%)3 (2.9%)104 (0.9%)
       Normal2355 (99.4%)14 (0.6%)2369 (21.4%)
       Null8539 (99.4%)48 (0.6%)8587 (77.6%)
      Albumin level.020
      Chi-square
       Low (<3.4)18 (100.0%)0 (0.0%)18 (0.2%)
       Normal956 (98.8%)12 (1.2%)968 (8.8%)
       Null10021 (99.5%)53 (0.5%)10074 (91.1%)
      Principal anesthesia technique.960
      Chi-square
       Epidural22 (100.0%)0 (0.0%)22 (0.2%)
       General10372 (99.4%)61 (0.6%)10433 (94.3%)
       MAC/IV Sedation120 (100.0%)0 (0.0%)120 (1.1%)
       Regional145 (99.3%)1 (0.7%)146 (1.3%)
       Spinal328 (99.1%)3 (0.9%)331 (3.0%)
       Other6 (100.0%)0 (0.0%)6 (0.1%)
       Unknown2 (100.0%)0 (0.0%)2 (0.0%)
      Reoperation within 30-days of index procedure<.001
      Chi-square
       No10971 (99.7%)36 (0.3%)11007 (99.5%)
       Yes24 (45.3%)29 (54.7%)53 (0.5%)
      Hypertension requiring medication.034
      Chi-square
       No10334 (99.5%)57 (0.5%)10391 (94.0%)
       Yes661 (98.8%)8 (1.2%)669 (6.0%)
      Significantly different values are in bold.
      1 Chi-square
      Table 5Multivariate Logistic Regression for Predictive Factors of Readmission After Isolated ACLR
      OutcomeOdds Ratio (95% Confidence Interval)P Value
      Age group.1237
       16-20Reference-
       21-302.454 (0.739, 8.153).4724
       31-403.881 (1.162, 12.962).1454
       41-502.494 (0.652, 9.542).3485
       >501.899 (0.356, 10.121).4210
      Operative time >1.5 hours2.769 (1.554, 4.931).0298
      Non-White race1.080 (0.659, 1.777).1766
      BMI <300.830 (0.441, 1.595).5680
      Smoker within 1 year1.317 (0.633, 2.595).4403
      Dyspnea6.509 (1.324, 31.992).0454
      Male sex0.358 (0.185, 0.694).0284
      Partially dependent Functional status10.171 (1.203, 85.996).0049
      Steroid use for a chronic condition5.934 (0.694, 50.750).1356
      No cardiac comorbidity0.536 (0.235, 1.223).1060
      No history of COPD0.372 (0.043, 9.299).3177
      ASA >30.847 (0.220, 2.467).2244
      Low hematocrit0.551 (0.265, 1.183).1167
      High creatinine3.396 (0.736, 11.064).0685
      Low albumin0.572 (0.240, 1.403).2118
      Use of general anesthesia1.508 (0.474, 6.373).5313
      Reoperation within 30 days of index procedure478.711 (234.233, >999.999)<.0001
      Hypertension requiring medication1.483 (0.499, 3.920).4505
      Operative time >1.5 hours, male sex, dependent functional status, presence of dyspnea, and a reoperation within 30 days of the index procedure were all independent risk factors for readmission. Significant values in bold.

      Discussion

      In this study, we found that 30-day readmission rates were low at just 0.59% of patients. Within that cohort, operative time >1.5 hours, male sex, dependent functional status, the presence of dyspnea, and undergoing a reoperation within 30 days of the index procedure were identified as independent risk factors for readmission after ACLR using multivariate logistic analysis. Notably, postsurgical infection was the most common cause of readmission and bivariate analysis associated male sex, history of severe COPD, cardiac comorbidity, operative time >1.5 hours, partially dependent preoperative functional health status, high preoperative creatinine, normal preoperative serum albumin, history of hypertension, and 30-day reoperations with an increased likelihood for readmission.
      As hypothesized, surgical duration and number of comorbidities were identified as risk factors for ACLR readmission. Obesity, however, was not predictive of readmission in this analysis. This result contrasts with our hypothesis as well as the findings of a recent study by Cooper et al. in which obesity was, in fact, determined to be a predictive risk factor for ACLR readmission.
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      However, two important distinctions exist between our study methodologies. First, Cooper et al. investigated all elective ACLR cases in their readmission analysis, not just isolated ACLR. The inclusion of concomitant procedures may have impacted operative times, and therefore, complication and readmission rates, confounding the results. Second, in a multivariate analysis, Cooper et al. found a BMI 40 (class III) to be predictive of readmission. While the authors concluded that obesity is a risk factor for ACLR readmission, BMI >40 is often classified as morbid obesity.
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      By contrast, our analysis used a BMI of 30 (classes I-III) to report obesity without substratification by class. Our larger study size and inclusion of BMI 30 allowed us to capture far more obese patients (3,224 vs 46) and provides a more complete assessment of the effect of obesity on isolated ACLR readmission rates. Currently, the knee arthroplasty and arthroscopy literature is conflicted in establishing patient obesity as a predictive factor of readmissions,
      • Cvetanovich G.L.
      • Chalmers P.N.
      • Verma N.N.
      • Cole B.J.
      • Bach Jr., B.R.
      Risk factors for short-term complications of anterior cruciate ligament reconstruction in the United States.
      ,
      • Belmont Jr., P.J.
      • Goodman G.P.
      • Rodriguez M.
      • Bader J.O.
      • Waterman B.R.
      • Schoenfeld A.J.
      Predictors of hospital readmission following revision total knee arthroplasty.
      ,
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      ,
      • Schairer W.W.
      • Vail T.P.
      • Bozic K.J.
      What are the rates and causes of hospital readmission after total knee arthroplasty?.
      ,
      • Mednick R.E.
      • Alvi H.M.
      • Krishnan V.
      • Lovecchio F.
      • Manning D.W.
      Factors affecting readmission rates following primary total hip arthroplasty.
      • Sing D.C.
      • Luan T.F.
      • Feeley B.T.
      • Zhang A.L.
      Is obesity a risk factor for adverse events after knee arthroscopy?.
      • Martin C.T.
      • Pugely A.J.
      • Gao Y.
      • Wolf B.R.
      Risk factors for thirty-day morbidity and mortality following knee arthroscopy: A review of 12,271 patients from the national surgical quality improvement program database.
      but it is well known that obesity is associated with increased surgical times and postoperative complications, and so further study of this particular variable is warranted for this procedure.
      Regarding the variables that were identified as predictors of readmission—operative time >1.5 hours, male sex, dependent functional status, the presence of dyspnea, and 30-day reoperations—these findings are consistent with what has been previously reported in the orthopedic literature. Longer procedural times have been repeatedly associated with an increase in postsurgical complications and occasionally with readmissions.
      • Bokshan S.L.
      • DeFroda S.F.
      • Owens B.D.
      Risk factors for hospital admission after anterior cruciate ligament reconstruction.
      ,
      • Agarwalla A.
      • Gowd A.K.
      • Liu J.N.
      • et al.
      Effect of operative time on short-term adverse events after isolated anterior cruciate ligament reconstruction.
      ,
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      Meanwhile, the high predictive significance of 30-day reoperations indicates that the majority of such procedures required hospital admission in this cohort. Male sex has also been linked to a greater number of intra-articular pathologies in ACLR,
      • Kluczynski M.A.
      • Marzo J.M.
      • Bisson L.J.
      Factors associated with meniscal tears and chondral lesions in patients undergoing anterior cruciate ligament reconstruction: A prospective study.
      which may then translate to longer OR times. Additional systemic comorbidities and older age have been known to complicate surgeries as well and may require more involved perioperative management.
      • Min C.J.
      • Partan M.J.
      • Koutsogiannis P.
      • Iturriaga C.R.
      • Katsigiorgis G.
      • Cohn R.M.
      Risk factors for hospital admission in patients undergoing outpatient anterior cruciate ligament reconstruction: A national database study.
      ,
      • Schoenfeld A.J.
      • Carey P.A.
      • Cleveland 3rd, A.W.
      • Bader J.O.
      • Bono C.M.
      Patient factors, comorbidities, and surgical characteristics that increase mortality and complication risk after spinal arthrodesis: a prognostic study based on 5,887 patients.
      • Dombrowski M.
      • Murawski C.D.
      • Yasui Y.
      • et al.
      Medical comorbidities increase the rate of surgical site infection in primary Achilles tendon repair.
      • Kraus Schmitz J.
      • Lindgren V.
      • Janarv P.M.
      • Forssblad M.
      • Stalman A.
      Deep venous thrombosis and pulmonary embolism after anterior cruciate ligament reconstruction: Incidence, outcome, and risk factors.
      However, age was not significantly associated with ACLR readmission rates in our study. The effects of age on short-term ACLR outcomes may be better elucidated by future investigations of graft choice in varying age groups, especially since allografts tend to be favored in older ACLR patients. Although conflicting evidence exists regarding other demographic factors such as race and ethnicity, our results support previous studies that have found no significance in their predictive capacities in ACLR outcomes
      • Cvetanovich G.L.
      • Chalmers P.N.
      • Verma N.N.
      • Cole B.J.
      • Bach Jr., B.R.
      Risk factors for short-term complications of anterior cruciate ligament reconstruction in the United States.
      ,
      • Group M.K.
      • Spindler K.P.
      • Huston L.J.
      • et al.
      Ten-year outcomes and risk factors after anterior cruciate ligament reconstruction: A MOON longitudinal prospective cohort study.
      ,
      • Dodwell E.R.
      • Lamont L.E.
      • Green D.W.
      • Pan T.J.
      • Marx R.G.
      • Lyman S.
      20 years of pediatric anterior cruciate ligament reconstruction in New York State.
      • Group M.K.
      • Spindler K.P.
      • Huston L.J.
      • et al.
      Anterior cruciate ligament reconstruction in high school and college-aged athletes: Does autograft choice influence anterior cruciate ligament revision rates?.
      • Inacio M.C.
      • Chen Y.
      • Maletis G.B.
      • Ake C.F.
      • Fithian D.C.
      • Granan L.P.
      Injury pathology at the time of anterior cruciate ligament reconstruction associations with self-assessment of knee function.
      or postoperative readmissions.
      • Cooper J.D.
      • Lorenzana D.J.
      • Heckmann N.
      • et al.
      The effect of obesity on operative times and 30-day readmissions after anterior cruciate ligament reconstruction.
      Finally, readmissions for a procedure performed as commonly as ACLR are likely to be one contributor to hospital costs and may extend into the tens of millions of dollars when averaged annually across the United States. Although the average readmission rate found in this study was low (0.59%), this could amount to nearly 1,800 readmission events in the United States each year based on the estimated 300,000 ACLR procedures performed annually. Given its frequency, ACLR is a valuable target for studies aiming to identify predictive factors of postsurgical readmissions with the goal of preventing adverse events, maximizing hospital reimbursements, and improving patient outcomes. Although prior studies have investigated overnight admissions after isolated ACLR, risk factors for readmission after isolated ACLR have not been described in detail. A major strength of this study is the analysis of more than 11,000 cases using a large, national, and patient-centered database to identify pertinent patient and procedural risk factors. The present study confirms some familiar variables as risk factors including longer operative times, preoperative comorbidities, and male sex, but also better characterizes the roles of other variables debated in the literature, including obesity, age, race, and ethnicity. These results may help surgeons identify patients at risk for readmission, guide patient education, and improve perioperative management for those undergoing ACLR. Although not possible in this analysis, further investigations of ACLR readmissions by graft type and/or primary versus revision procedures, or predictors of PRO score quality in isolated ACLR would be of great interest to this topic.

      Limitations

      This study is not without limitations. A significant limitation is the inability to determine the type of graft or surgical technique used for each ACLR procedure from the NSQIP database. Graft type—hamstring autograft versus patellar bone-tendon-bone (BTB) versus quadriceps autograft versus allograft—can significantly influence operative times and has been shown to affect surgical outcomes.
      • Kaeding C.C.
      • Aros B.
      • Pedroza A.
      • et al.
      Allograft versus autograft anterior cruciate ligament reconstruction: Predictors of failure from a MOON prospective longitudinal cohort.
      In addition, the preferred use of allografts in older ACLR patients may have also affected the distribution of these findings. Second, the NSQIP database does not record information regarding the details of perioperative care. When one considers the variety of protocols and health care professionals that are involved in presurgical administration to postoperative management, it is possible that an event necessitating readmission could be initiated by circumstances extending beyond the variables included in this analysis. Finally, while the data of the NSQIP are known to be of high quality, the data reviewed for this study represent only a sample of ACLRs between 2011 and 2017. As a result, there is a gap of time between the most recently collected data and the date of our retrospective query, which may be a source of bias. Similarly, the NSQIP does not encompass all surgical settings, like independent surgical centers, and, therefore, may not be generalizable for all surgeons. This analysis has provided a focused assessment of an understudied outcome of isolated ACLR; however, a paucity of information on this topic remains and should continue to be investigated.

      Conclusions

      Isolated ACLR is associated with low 30-day readmission rates. Operative time >1.5 hours, male sex, dependent functional status, the presence of dyspnea, and 30-day reoperations are independent risk factors for readmission that should be considered in patient selection and addressed with preoperative counseling.

      Supplementary Material

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