Venous Thromboembolism After Degenerative Spine Surgery: A Nationwide Readmissions Database Analysis

Postoperative Ketamine May be Associated with Venous Thromboembolism in Chronic Opioid Users After Spine Surgery

OBJECTIVE

Venous thromboembolism (VTE) is a serious postoperative adverse event after spine surgery. In patients with pain refractory to typical multimodal analgesia regimens after spine surgery, whom are often chronic opioid users, perioperative ketamine is an alternative analgesic that has grown in popularity. The aim of this study is to assess the risk of VTE in chronic opioid users undergoing spine surgery.

METHODS

We conducted a structured query language search to identify patients with a diagnosis of VTE after spine surgery from 2017 to 2021. structured query language identified patient demographic information and chart review was performed to confirm procedure details, including levels fused and decompressed. Opioid use was identified using the Prescription Drug Monitoring Program for each patient. Chronic opioid users were defined as having >180 days or >9 opioid prescriptions prescribed in the 12 months prior to surgery.

RESULTS

A total of 322 patients were identified with complete Prescription Drug Monitoring Program and postoperative ketamine data, including 29 patients with confirmed VTE after spine surgery. Length of stay (P < 0.001) and Elixhauser comorbidity index (P = 0.008) were significantly greater in the VTE group compared to the no VTE group. Analysis of only chronic opioid users also found a significant association between postoperative ketamine and VTE (P = 0.034).

CONCLUSIONS

Our study found no association between chronic opioid use and VTE occurrence; however, we identified a significant association between ketamine infusion and VTE after spine surgery. Further studies should be conducted to identify the risks associated with ketamine and other newly implemented alternative analgesics.

Psoriatic arthritis increases the risk of venous thromboembolism following degenerative lumbar spine surgery: An analysis of U.S. Nationwide Inpatient Sample 2005-2018

Background

This retrospective study aimed to determine the risk of venous thromboembolism (VTE) in patients with PsA after surgery for lumbar degenerative disease (LDD).

Methods

The study data of adults aged ≥20 years admitted to U.S. hospitals with diagnoses of LDD and undergoing spinal decompression or fusion between 2005 and 2018 were extracted from the National Inpatient Sample (NIS) database. Patients were further divided into two groups based on a diagnosis of PsA or not via codes ICD-9: 696.0 and ICD-10: L40.50. Patients with missing information were excluded. Propensity score matching (PSM) was employed to enhance comparability between groups. Logistic regression was used to determine associations between PsA and various outcomes, including complications, unfavorable discharge, and prolonged length of stay (LOS).

Results

Data on 471,283 patients with LDD was extracted from the NIS database.from 2005 to 2018. Before propensity score matching, patients with PsA had higher proportions of overall morbidity (8.8 % vs. 6.9 %), VTE (1.4 % vs. 0.7 %), and unfavorable discharge (20.8 % vs. 16.9 %). After matching, patients with PsA still had higher VTE incidence and unfavorable discharge proportions. After adjustments, multivariable regression analysis indicated that patients with PsA had a higher risk of unfavorable discharge (aOR: 1.26, 95 % CI: 1.03-1.55) and VTE (aOR: 1.99, 95 % CI: 1.05-3.75).

Conclusions

Among patients undergoing surgery for LDD, pre-existing PsA may be associated with increased risks of unfavorable discharge and VTE occurrence. The findings may benefit preoperative risk stratifications before LDD surgeries.

Using Machine Learning (ML) Models to Predict Risk of Venous Thromboembolism (VTE) Following Spine Surgery

STUDY DESIGN

A retrospective cohort study.

OBJECTIVES

Venous thromboembolism (VTE) is a potentially high-risk complication for patients undergoing spine surgery. Although guidelines for assessing VTE risk in this population have been established, development of new techniques that target different aspects of the medical history may prove to be of further utility. The goal of this study was to develop a predictive machine learning (ML) model to identify nontraditional risk factors for predicting VTE in spine surgery patients.

SUMMARY OF BACKGROUND DATA

A cohort of 63 patients was identified who had undergone spine surgery at a single center from 2015 to 2021. Thirty-one patients had a confirmed VTE, while 32 had no VTE. A total of 113 attributes were defined and collected via chart review. Attribute categories included demographics, medications, labs, past medical history, operative history, and VTE diagnosis.

METHODS

The Waikato Environment for Knowledge Analysis (WEKA) software was used in creating and evaluating the ML models. Six classifier models were tested with 10-fold cross-validation and statistically evaluated using t tests.

RESULTS

Comparing the predictive ML models to the control model (ZeroR), all predictive models were significantly better than the control model at predicting VTE risk, based on the 113 attributes ( P <0.001). The Random Forest model had the highest accuracy of 88.89% with a positive predictive value of 93.75%. The Simple Logistic algorithm had an accuracy of 84.13% and defined risk attributes to include calcium and phosphate laboratory values, history of cardiac comorbidity, history of previous VTE, anesthesia time, selective serotonin reuptake inhibitor use, antibiotic use, and antihistamine use. The J48 model had an accuracy of 80.95% and it defined hemoglobin laboratory values, anesthesia time, beta-blocker use, dopamine agonist use, history of cancer, and Medicare use as potential VTE risk factors.

CONCLUSION

Further development of these tools may provide high diagnostic value and may guide chemoprophylaxis treatment in this setting of high-risk patients.

The Risk of Venous Thromboembolism after Thoracolumbar Spine Surgery: A Population-Based Cohort Study

Background: Although venous thromboembolism (VTE) is rare, including deep vein thrombosis (DVT) and pulmonary embolism (PE), it is a catastrophic complication after spinal surgery. This study was aimed to investigate the risk factors and incidence of VTE after thoracolumbar spine surgery (TLSS). Methods: We retrieved the data of 8697 patients >20 years old who underwent TLSS between 2000 and 2013 from Taiwan’s Longitudinal Health Insurance Database 2000. Each patient was randomly frequency-matched with four individuals who did not undergo TLSS by age, sex, and index year (the control group). Results: The incidence rates of VTE in the TLSS and control groups were 1.84 and 0.69 per 1000 person-years, respectively. The TLSS group had a higher VTE risk (adjusted HR (aHR): 2.13, 95% confidence interval [95%CI]: 1.41−3.21), DVT (aHR: 2.20, 95%CI: 1.40−3.46), and PE (aHR: 1.60, 95%CI: 0.68−3.78) than the control group. The correlated risk factors of VTE included older age (50−64 years: aHR: 2.16, 95%CI: 1.14−4.09; ≥65 years: aHR: 3.18, 95%CI: 1.65−6.13), a history of cancer (aHR: 2.96, 95%CI: 1.58−5.54), heart failure (aHR: 2.19, 95%CI: 1.27−3.78), and chronic kidney disease (aHR: 1.83, 95%CI: 1.18−2.83). Conclusions: The overall VTE risk following TLSS was less than 2% but correlated with certain risk factors. This information could help the spine surgeon help the patient prevent this fatal complication.

Impact of Prolonged Operative Duration on Postoperative Symptomatic Venous Thromboembolic Events After Thoracolumbar Spine Surgery

OBJECTIVE

To determine the effect of operative duration on the rate of postoperative symptomatic venous thromboembolic (VTE) events in patients undergoing thoracolumbar spine fusion.

METHODS

We identified all thoracolumbar spine fusion patients between 2012 and 2021. Operative duration was defined as time from skin incision to skin closure. A 1:1 propensity match was conducted incorporating patient and surgical characteristics. Logistic regression was performed to assess predictors of postoperative symptomatic VTE events. A receiver operating characteristic curve was created to determine a cutoff time for increased likelihood of VTE.

RESULTS

We identified 101 patients with VTE and 1108 patients without VTE. Seventy-five patients with VTE were matched to 75 patients without VTE. Operative duration (339 vs. 262 minutes, P = 0.010) and length of stay (5.00 vs. 3.54 days, P = 0.008) were significantly longer in patients with a VTE event. Operative duration was an independent predictor of VTE on multivariate regression (odds ratio: 1.003, 95% confidence interval: 1.001-1.01, P = 0.021). For each additional hour of operative duration, the risk of VTE increased by 18%. A cutoff time of 218 minutes was identified (area under the curve [95% confidence interval] = 0.622 [0.533-0.712]) as an optimal predictor of increased risk for a VTE event.

CONCLUSIONS

Operative duration significantly predicted symptomatic VTE, especially after surgical time cutoff of 218 minutes. Each additional hour of operative duration was found to increase VTE risk by 18%. We also identify the impact of VTE on 90-day readmission rates, suggesting significantly higher costs and opportunity for hospital acquired conditions, in line with prior literature.

Tobacco Use Is Associated With Increased 90-Day Readmission Among Patients Undergoing Surgery for Degenerative Spine Disease

STUDY DESIGN

Retrospective database study.

OBJECTIVE

Tobacco use is associated with complications after surgical procedures, including poor wound healing, surgical site infections, and cardiovascular events. We used the Nationwide Readmissions Database (NRD) to determine if tobacco use is associated with increased 30- and 90-day readmission among patients undergoing surgery for degenerative spine disorders.

METHODS

Patients who underwent elective spine surgery were identified in the NRD from 2010 to 2014. The study population included patients with degenerative spine disorders treated with discectomy, fusion, or decompression. Descriptive and multivariate logistic regression analyses were performed to identify patient and hospital factors associated with 30- and 90-day readmission, with significance set at P value <.001.

RESULTS

Within 30 days, 4.8% of patients were readmitted at a median time of 9 days. The most common reasons for 30-day readmission were postoperative infection (12.5%), septicemia (3.5%), and postoperative pain (3.0%). Within 90 days, 7.3% were readmitted at a median time of 18 days. The most common reasons for 90-day readmission were postoperative infection (9.6%), septicemia (3.5%), and pneumonia (2.3%). After adjustment for patient and hospital characteristics, tobacco use was independently associated with readmission at 90 days (odds ratio 1.05, 95% confidence interval 1.03-1.07, P < .0001) but not 30 days (odds ratio 1.02, 95% confidence interval 1.00-1.05, P = .045).

CONCLUSIONS

Tobacco use is associated with readmission within 90 days after cervical and thoracolumbar spine surgery for degenerative disease. Tobacco use is a known risk factor for adverse health events and therefore should be considered when selecting patients for spine surgery.

Influence of tranexamic acid use on venous thromboembolism risk in patients undergoing surgery for spine tumors

OBJECTIVE

Patients with spine tumors are at increased risk for both hemorrhage and venous thromboembolism (VTE). Tranexamic acid (TXA) has been advanced as a potential intervention to reduce intraoperative blood loss in this surgical population, but many fear it is associated with increased VTE risk due to the hypercoagulability noted in malignancy. In this study, the authors aimed to 1) develop a clinical calculator for postoperative VTE risk in the population with spine tumors, and 2) investigate the association of intraoperative TXA use and postoperative VTE.

METHODS

A retrospective data set from a comprehensive cancer center was reviewed for adult patients treated for vertebral column tumors. Data were collected on surgery performed, patient demographics and medical comorbidities, VTE prophylaxis measures, and TXA use. TXA use was classified as high-dose (≥ 20 mg/kg) or low-dose (< 20 mg/kg). The primary study outcome was VTE occurrence prior to discharge. Secondary outcomes were deep venous thrombosis (DVT) or pulmonary embolism (PE). Multivariable logistic regression was used to identify independent risk factors for VTE and the resultant model was deployed as a web-based calculator.

RESULTS

Three hundred fifty patients were included. The mean patient age was 57 years, 53% of patients were male, and 67% of surgeries were performed for spinal metastases. TXA use was not associated with increased VTE (14.3% vs 10.1%, p = 0.37). After multivariable analysis, VTE was independently predicted by lower serum albumin (odds ratio [OR] 0.42 per g/dl, 95% confidence interval [CI] 0.23-0.79, p = 0.007), larger mean corpuscular volume (OR 0.91 per fl, 95% CI 0.84-0.99, p = 0.035), and history of prior VTE (OR 2.60, 95% CI 1.53-4.40, p < 0.001). Longer surgery duration approached significance and was included in the final model. Although TXA was not independently associated with the primary outcome of VTE, high-dose TXA use was associated with increased odds of both DVT and PE. The VTE model showed a fair fit of the data with an area under the curve of 0.77.

CONCLUSIONS

In the present cohort of patients treated for vertebral column tumors, TXA was not associated with increased VTE risk, although high-dose TXA (≥ 20 mg/kg) was associated with increased odds of DVT or PE. Additionally, the web-based clinical calculator of VTE risk presented here may prove useful in counseling patients preoperatively about their individualized VTE risk.

Congress of Neurological Surgeons Systematic Review and Evidence-Based Guidelines for Perioperative Spine: Preoperative Pulmonary Evaluation and Optimization

BACKGROUND

There are no current recommendations for preoperative pulmonary evaluation and management of patients undergoing elective spine surgery.

OBJECTIVE

The aim of this guideline is to determine preoperative risk factors for perioperative and postoperative pulmonary adverse events and to determine the optimal preoperative evaluation and management of at-risk patients.

METHODS

A systematic review of the literature was performed using the National Library of Medicine PubMed database and the Cochrane Library for studies relevant to postoperative pulmonary adverse events in patients undergoing spine surgery. Clinical studies evaluating preoperative patient risk factors and preoperative diagnostic and treatment interventions were selected for review.

RESULTS

The literature search yielded 152 abstracts relevant to the PICO (patient/population, intervention, comparison, and outcomes) questions included in this chapter. The task force selected 65 articles for full-text review, and 24 were selected for inclusion in this systematic review. Twenty-three articles addressed preoperative patient risk factors. One article addressed preoperative diagnostic studies of pulmonary function. There were no studies meeting the inclusion criteria for preoperative pulmonary treatment.

CONCLUSION

There is substantial evidence for multiple preoperative patient factors that predict an increased risk of a postoperative pulmonary adverse event. Individuals with these risk factors (functional dependence, advanced age [≥65 yr], chronic obstructive pulmonary disease, congestive heart failure, weight loss, and obstructive sleep apnea) who are undergoing spine surgery should be counseled regarding the potential increased risk of a perioperative and postoperative pulmonary adverse events. There is insufficient evidence to support any specific preoperative diagnostic test for predicting the risk of postoperative pulmonary adverse events or any treatment intervention that reduces risk. It is suggested, however, to consider appropriate preoperative pulmonary diagnostic testing and treatment to address active pulmonary symptoms of existing or suspected disease.The full guidelines can be accessed at https://www.cns.org/guidelines/browse-guidelines-detail/5-preoperative-pulmonary-evaluation-optimization.

Venous Thromboembolism Prophylaxis in Elective Spine Surgery

STUDY DESIGN

Review.

OBJECTIVE

Venothromboembolic (VTE) complications, composed of deep vein thrombosis and pulmonary embolism are commonly observed in the perioperative setting. There are approximately 500 000 postoperative VTE cases annually in the United States and orthopedic procedures contribute significantly to this incidence. Data on the use of VTE prophylaxis in elective spinal surgery is sparse. This review aims to provide an updated consensus within the literature defining the risk factors, diagnosis, and the safety profile of routine use of pharmacological prophylaxis for VTE in elective spine surgery patients.

METHODS

A comprehensive review of the literature and compilation of findings relating to current identified risk factors for VTE, diagnostic methods, and prophylactic intervention and safety in elective spine surgery.

RESULTS

VTE prophylaxis use is still widely contested in elective spine surgery patients. The outlined benefits of mechanical prophylaxis compared with chemical prophylaxis varies among practitioners.

CONCLUSION

The benefits of any form of VTE prophylaxis continues to remain a controversial topic in the elective spine surgery setting. A specific set of guidelines for implementing prophylaxis is yet to be determined. As more risk factors for thromboembolic events are identified, the complexity surrounding intervention selection increases. The benefits of prophylaxis must also continue to be balanced against the increased risk of bleeding events and neurologic injury.

Systemic Corticosteroids and the Risk of Venous Thromboembolism in Patients with Severe COPD: A Nationwide Study of 30,473 Outpatients

Due to frequent exacerbations, many patients with chronic obstructive pulmonary disease (COPD) are exposed to oral corticosteroids (OCS), which may be thrombogenic. We evaluated the risk of hospitalisation with venous thromboembolism (VTE) and death in patients with acute exacerbation of COPD (AECOPD) treated with long and short OCS regimens. In this nationwide cohort study of 30,473 COPD outpatients treated for AECOPD, we compared the risk of VTE hospitalisation and all-cause mortality within 6 months in OCS dose of >250 mg vs. ≤250 mg. A multivariable Cox proportional hazard regression was used to estimate the risk. The incidence of VTE hospitalisations was 0.23%. A long OCS treatment course was associated with an increased risk of VTE compared to a short course (hazard ratio (HR) 1.69, [95% confidence interval (CI) 1.05 to 2.72], p < 0.031). A higher risk of all-cause mortality was seen in the group of COPD patients treated with a long OCS course (HR 1.71, [95% CI 1.63 to 1.79], p < 0.0001). The risk of reported VTE hospitalisation was higher among AECOPD patients treated with long courses of OCS, but the absolute risk was low, suggesting under-reporting of the condition.

Pulmonary Thromboembolism Following Spine Surgery: Clinical Suspicion is the Key

Pulmonary thromboembolism following spine surgery, although rare, could end into devastating outcome. Gold standard for it diagnosis is pulmonary CT angiography but in operating theatre, clinical suspicion is the key to diagnose. Here we report a case of pulmonary embolism with classic clinical findings which approved using pulmonary CT angiography and echocardiography.

A predictive model of preoperative risk factors for venous thromboembolism after fusion for degenerative conditions of the thoracolumbar spine

BACKGROUND

The purpose of this study was to identify preoperative risk factors for VTE and develop a predictive model that can be integrated into electronic health records (EHRs).

METHODS

Retrospective review of patients undergoing thoracolumbar fusion from 2016 to 2018 in the National Surgical Quality Improvement Program database was performed.

RESULTS

Increasing age, BMI, and several other factors were significant predictors of increased risk for postoperative VTE. The predictive model had an area under the curve of 0.683, indicating acceptable discrimination.

DISCUSSION

Our findings build upon previous studies by presenting a model equation that can be rapidly integrated into EHRs for validation.

Complication rates following Chiari malformation surgical management for Arnold-Chiari type I based on surgical variables: A national perspective

Introduction:

This study aimed to identify complication trends of Chiari Malformation Type 1 patients (CM-1) for certain procedures and concomitant diagnoses on a national level.

Materials

The Kids’ Inpatient Database was queried for diagnoses of operative CM-1 by International Classification of Disease-9 codes (348.4). Differences in preoperative demographics and perioperative complication rates between patient cohorts were assessed using Pearson's Chi-squared test and t-test when necessary. Binary logistic regression was utilized to find significant factors associated with complication rate. Certain surgical procedures were analyzed for their relationship with postoperative outcomes.

Results:

Thirteen thousand eight hundred and twelve CM-1 patients were identified with 8.2% suffering from a complication. From 2003 to 2012, the rate of complications for CM-1 pts decreased significantly (9.6%–5.1%) along with surgical rate (33.3%–28.6%), despite the increase in CM-1 diagnosis (36.3%–42.3%; all P < 0.05). CM-1 pts who had a complication were younger and had a lower invasiveness score; however, they had a larger Charlson Comorbidity Index than those who did not have a complication (all P < 0.05). CM-1 pts who experienced complications had a concurrent diagnosis of syringomyelia (7.1%), and also scoliosis (3.2%; all P < 0.05). CM-1 pts who did not have a complication had a greater rate of operation than those that had a complication (76.4% vs. 23.6% P < 0.05). The most common complications were nervous system related (2.8%), anemia (2.4%), and acute respiratory distress (2.1%). CM-1 pts that underwent an instrumented fusion (3.4% vs. 2.1%) had a greater complication rate as well as compared to those who underwent a craniotomy (23.2% vs. 19.1%; all P < 0.05). However, CM-1 pts that underwent a decompression had lower postoperative complications (21.3% vs. 28.9%; all P < 0.05).

Conclusions:

Chiari patients undergoing craniectomies as well as instrumented fusions are at a higher risk of postoperative complications especially when the instrumented fusions were performed on >4 levels.

Prevalence and Predictors for Preoperative Deep Vein Thrombosis in Patients with Thoracolumbar Fractures Caused by High-Energy Injuries

OBJECTIVE

To determine the prevalence of preoperative deep vein thrombosis (DVT) and to identify the predictors for preoperative DVT in patients with thoracolumbar fractures caused by high-energy injuries.

METHODS

A total of 429 patients with thoracolumbar fractures caused by high-energy injuries from October 2016 to November 2019 were included in this retrospective study. The patients underwent ultrasonography before operation and were divided into the DVT group and non-DVT group. Demographic data and laboratory results were collected, and mechanical and chemical prophylaxis were performed to prevent DVT. The incidence of DVT was evaluated and the predictors of preoperative DVT were identified.

RESULTS

The overall incidence of preoperative DVT was 14.45% (62/429). Of all patients, 1 (0.23%) had proximal DVT and 61 (14.22%) had distal DVT. The incidence of preoperative DVT was 24.59% in patients with thoracic fractures and 11.04% in those with lumbar fractures. Multivariate logistic regression identified 3 predictors, including lower extremity motor (P < 0.001), time from injury to operation (P < 0.001), and D-dimer (P = 0.001). Lower extremity motor score <3, time from injury to operation >3 days, and D-dimer >1.81 mg/L showed satisfied accuracy for predicting preoperative DVT. The receiver operating characteristic curve analysis indicated the diagnostic value of D-dimer was the highest when the area under the receiver operating characteristic curve curve was 0.769.

CONCLUSIONS

Despite mechanical and chemical prophylaxis, the incidence of preoperative DVT is still very high, especially in thoracic fractures. Lower extremity motor score <3, time from injury to operation >3 days, and D-dimer >1.81 mg/L are predictors for preoperative DVT. More attention is warranted to prevent preoperative DVT when ≥2 predictors are present.

Lower Activity and Function Scores Are Associated with a Higher Risk of Preoperative Deep Venous Thrombosis in Patients Undergoing Total Hip Arthroplasty

This study was performed to investigate the relationship between patients’ activity and function levels and the incidence of preoperative deep venous thrombosis (DVT) prior to total hip arthroplasty (THA). We retrospectively reviewed 500 patients admitted for primary or revision THA from July 2014 to October 2018. The diagnosis of DVT was confirmed using Doppler ultrasonography 1 month before THA. The patients’ activity and hip function were evaluated using several clinical scores: the Harris Hip Score (HHS), Oxford Hip Score (OHS), University of California Los Angeles (UCLA) activity score, and visual analog scale (VAS) score. Those scores and the medical history were examined for correlations with preoperative DVT using univariate and multivariate models. Univariate regression analysis showed that older age, current steroid use, anticoagulant use, a history of DVT, collagen disease, a lower UCLA activity score, and a lower OHS were associated with an elevated risk of preoperative DVT. The multivariate analyses showed that a higher UCLA activity score (odds ratio (OR): 0.0049–0.012) and higher OHS (OR: 0.0012–0.0088) were associated with a lower risk of preoperative DVT in each model. Age (OR: 1.07 in both models), current steroid use (OR: 9.32–10.45), and a history of DVT (OR: 27.15–74.98) were associated with a higher risk of preoperative DVT in both models. Older age, current steroid use, a history of DVT, a lower UCLA activity score, and a lower OHS were risk factors for preoperative DVT before THA, even when controlling for potential confounders. Patients exhibiting low activity and low function levels were more likely to have DVT, even before surgery.

Time-to-event analyses of lower-limb venous thromboembolism in aged patients undergoing lumbar spine surgery: a retrospective study of 1620 patients

After spine surgery, venous thromboembolism (VTE) is not uncommon in aged patients. This study investigates time-to-event risk factors of postoperative VTE based on medical records of aged patients (age≥60 yr) between January 2013 and December 2018. All participants had undergone lower extremity ultrasonography pre- and postoperatively at the first, second, fourth, eighth, and twelfth weeks. Mann-Whitney U tests and chi-square tests were used for univariate analyses, and Cox regression was utilized for multivariate analyses. A total of 1620 cases were recruited, VTE group (N=382, 23.6%) and non-VTE group (N=1238, 76.4%), aged 67 (IQR 11) years and 65 (IQR 6) years, respectively. The univariate analyses indicated significant differences between the VTE and non-VTE groups regarding advanced age, VTE history, hypertension, fusion, hospital stay, FIB, HDL, D-dimer, and TC (all P<0.05). The Cox regression showed that advanced age (OR=1.108; 95% CI, 1.091–1.126), VTE history (OR=4.962; 95% CI, 3.849–6.397), and hypertension (OR=1.344; 95% CI, 1.084–1.667) were the risk factors for postoperative VTE (P<0.05). The time-to-event analyses indicated that the hazard of VTE formation was highest in the first postoperative week. In conclusion, advanced age, VTE history, and hypertension are main risks for VTE formation, particularly in the first postoperative week.