Orthopedics

Feature Article 

Preoperative Patient Functional Status Is an Independent Predictor of Outcomes After Primary Total Hip Arthroplasty

Micheal Raad, MD; Raj M. Amin, MD; Jad M. El Abiad, MD; Varun Puvanesarajah, MD; Matthew J. Best, MD; Julius K. Oni, MD

Abstract

This study was designed to determine whether preoperative functional status of patients with osteoarthritis predicts outcomes after primary total hip arthroplasty. The American College of Surgeons National Surgical Quality Improvement Program database was queried for records of patients who underwent primary total hip arthroplasty for a principal diagnosis of osteoarthritis from 2009 to 2013 (N=43,179). Patients were categorized as dependent or independent according to their preoperative functional status. The groups were compared regarding several potential confounders using Student's t and chi-square tests. Logistic and Poisson regression models (inclusion threshold of P<.1) were used to assess the associations of functional status with outcomes. The alpha level was set at 0.05. Compared with independent patients, dependent patients were likely to be older (mean, 70 vs 66 years, P<.01) and to have more preoperative comorbidities. After controlling for potential confounders, preoperative dependent functional status was predictive of major complications (odds ratio, 2.34; 95% confidence interval, 1.67–3.28), nonroutine discharge (odds ratio, 2.80; 95% confidence interval, 2.35–3.34), and longer hospital stay (incidence risk ratio, 1.19; 95% confidence interval, 1.12–1.27). Rates of unplanned reoperation were similar between groups on multivariate analysis. Compared with preoperative independent functional status, preoperative dependent functional status was independently associated with worse outcomes after primary total hip arthroplasty for osteoarthritis. [Orthopedics. 2019; 42(3):e326–e330.]

Abstract

This study was designed to determine whether preoperative functional status of patients with osteoarthritis predicts outcomes after primary total hip arthroplasty. The American College of Surgeons National Surgical Quality Improvement Program database was queried for records of patients who underwent primary total hip arthroplasty for a principal diagnosis of osteoarthritis from 2009 to 2013 (N=43,179). Patients were categorized as dependent or independent according to their preoperative functional status. The groups were compared regarding several potential confounders using Student's t and chi-square tests. Logistic and Poisson regression models (inclusion threshold of P<.1) were used to assess the associations of functional status with outcomes. The alpha level was set at 0.05. Compared with independent patients, dependent patients were likely to be older (mean, 70 vs 66 years, P<.01) and to have more preoperative comorbidities. After controlling for potential confounders, preoperative dependent functional status was predictive of major complications (odds ratio, 2.34; 95% confidence interval, 1.67–3.28), nonroutine discharge (odds ratio, 2.80; 95% confidence interval, 2.35–3.34), and longer hospital stay (incidence risk ratio, 1.19; 95% confidence interval, 1.12–1.27). Rates of unplanned reoperation were similar between groups on multivariate analysis. Compared with preoperative independent functional status, preoperative dependent functional status was independently associated with worse outcomes after primary total hip arthroplasty for osteoarthritis. [Orthopedics. 2019; 42(3):e326–e330.]

Osteoarthritis of the hip is a chronic and debilitating condition with a profound effect on patients' lives and the health care system. It is one of the most common causes for years lived with a disability,1 and its annual direct medical costs exceed $100 billion.2 Given the projected rise in the prevalence of hip osteoarthritis3 and an increasing emphasis on cost-effectiveness in health care use, as well as quality reporting requirements,4 preoperative patient stratification is of increasing value. Ideally, a preoperative patient stratification tool is simple to use and effectively predicts postoperative outcomes.

Preoperative patient functional status has been shown to be predictive of postoperative outcomes in general surgery and cardiac surgery.5,6 More recently, Dasenbrock et al7 and De la Garza Ramos et al8 used the American College of Surgeons National Surgical Quality Improvement Program (NSQIP) database to show that patient functional status was predictive of outcomes after craniotomies and adult spinal deformity surgery, respectively.

The aim of this study was to investigate the association of preoperative patient functional status with outcomes after total hip arthroplasty (THA) for osteoarthritis. The authors hypothesized that patients who were functionally dependent before surgery would have worse postoperative outcomes compared with patients who were independent before surgery.

Materials and Methods

Database

The authors retrospectively reviewed deidentified patient information from the NSQIP database from 2009 to 2013. Institutional review board approval was not required. The NSQIP database prospectively collects information on preoperative risk factors, surgical procedures, and 30-day complications for patients undergoing surgery. The NSQIP database has been shown to have a 95% success rate in data capture and 95% inter-rater reliability in all variables.9

Rationale and Patient Selection

Only patients who underwent primary THA for a principal diagnosis of osteoarthritis were included. This created a homogeneous patient sample and minimized any variability caused by previous failed surgeries or concomitant musculoskeletal diseases. The Current Procedural Terminology10 code 27130 was used to identify patients who underwent “arthroplasty, acetabular and proximal femoral prosthetic replacement (total hip arthroplasty), with or without autograft or allograft.” Patients with a diagnosis of infection, trauma, or neoplasm were excluded. The International Classification of Diseases, Ninth Revision11 code 715.X was used to identify a diagnosis of osteoarthritis. Patients with missing data on baseline characteristics (n=221) and patients with a contaminated wound (n=408) were excluded. A total of 43,179 patients were included in the analysis.

Data Points and Functional Status

Preoperative patient factors analyzed were age, sex, body mass index, comorbidities, hematocrit, current smoking status, and functional status. Functional status, as defined by the NSQIP, focuses on the ability to perform activities of daily living (ADL) during the 30 days before surgery.12 Activities of daily living include bathing, feeding, dressing, toileting, and mobility. The 2 functional status categories are as follows:

  • Independent: The patient does not require assistance from another person for any ADL. This includes a person who is able to function independently with prosthetics or assistive equipment or devices.

  • Dependent: The patient requires some or total assistance from another person for ADL. This includes a person who uses prosthetics or assistive equipment or devices but still requires some assistance from another person for ADL.

The authors also analyzed data on operative time and type of anesthesia used.

Outcomes

The primary outcome was the occurrence of a major medical complication within 30 days after surgery, which was defined as having at least 1 of the following events: death, unplanned reintubation, pulmonary embolism, cardiac arrest, myocardial infarction, renal failure, sepsis, septic shock, deep surgical site infection, or need for mechanical ventilation for more than 48 hours. The length of hospital stay, unplanned reoperation for any reason within the same 30-day period, and nonroutine discharge (defined as discharge to a facility other than home) were also examined. Data on disposition status and reoperation were available for 37,621 and 30,074 patients, respectively, and only those were included in the analysis.

Statistical Analysis

The dependent and independent groups were compared regarding several baseline patient characteristics and surgical parameters to identify potential confounders. Most risk factors that the authors analyzed as potential confounders were determined according to a previous study that used NSQIP data to identify risk factors for poor postoperative outcomes.13 The Student's t and chi-square tests were used to compare continuous and categorical variables, respectively. Univariate analysis of outcomes was performed similarly. To control for potential confounders, the authors used multivariable logistic regression controlling for covariates with P<.1 on univariate analysis to assess the independent association of functional dependence with binary outcomes, including major complication, reoperation, and discharge disposition. Results were reported as odds ratios with 95% confidence intervals. Given the highly skewed nature of length-of-stay data, a Poisson regression model with a similar threshold for inclusion was used, and incidence risk ratios with 95% confidence intervals were used to report on outcomes. Multicollinearity between the covariates was assessed using the variance inflation factor. A mean variance inflation factor of less than 10 for each model was considered acceptable. Robust estimates of the standard error were used in all regression analyses. Statistical analyses were performed using Stata version 15 software (StataCorp LP, College Station, Texas). Significance was set at P<.05.

Results

Patient Sample

A total of 43,179 patients who underwent primary THA for osteoarthritis were included in the analysis. Of those, 1083 patients (2.5%) were functionally dependent preoperatively. The mean age for dependent and independent patients was 66±11 years. A total of 24,198 patients (56%) were women. The mean body mass index was 30±6.5 kg/m2. A total of 5200 patients (12%) were smokers. The 2 most common preoperative comorbidities were hypertension (58%, n=24,916) and diabetes mellitus (11%, n=4917). In total, 24,027 patients (56%) underwent surgery with general anesthesia. The mean operative time was 93±40 minutes. A major medical complication was noted in 561 patients (1.3%). The mean length of stay was 3±5 days. There were 695 patients (1.8%, based on 37,621 patients with available data) who underwent an unplanned reoperation within the 30-day period. In total, 10,172 patients (34%, based on 30,074 patients with available data) were discharged to a facility other than home.

Baseline Patient Characteristics and Surgical Parameters

Dependent patients were older (mean, 70±12 years) than independent patients (mean, 66±11 years) (P<.01; Table 1). The proportion of women was larger in the dependent group than in the independent group. Body mass index and smoking status were similar between the 2 groups. Compared with independent patients, dependent patients were more likely to have diabetes mellitus, congestive heart failure, chronic obstructive pulmonary disease, hypertension, and chronic steroid use. Mean preoperative hematocrit was also lower in the dependent group compared with the independent group. Operative time and the use of general anesthesia were similar between the 2 groups (P>.05).

Baseline Patient Characteristics and Surgical Factors by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

Table 1:

Baseline Patient Characteristics and Surgical Factors by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

30-Day Postoperative Outcomes

Patients in the dependent group were more likely than those in the independent group to experience a major complication within 30 days postoperatively (4.1% vs 1.2%, P<.01; Table 2). When assessing individual complications, the dependent group also had higher rates of death, unplanned reintubation, acute renal failure, sepsis, and deep surgical site infection (all, P<.01) than the independent group. Mean hospital stay was longer in the dependent vs independent group (4 vs 3 days, P<.01). Patients in the dependent group were more likely to be discharged to a facility other than home and to have an unplanned reoperation (both, P<.01) compared with patients in the independent group.

Univariate Comparison of Outcomes by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

Table 2:

Univariate Comparison of Outcomes by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

Multivariate Analysis of 30-Day Postoperative Outcomes

Risk factors that had P<.1 (and were included in the final models) were age, sex, body mass index, and the following comorbidities: chronic obstructive pulmonary disease, chronic steroid use, congestive heart failure, diabetes mellitus, hypertension, operative time, preoperative hematocrit levels, and use of general anesthesia. After controlling for possible confounders, functional dependence was associated with higher odds of experiencing a major medical complication (odds ratio, 2.34; 95% confidence interval, 1.67–3.28; Table 3). Similarly, the odds of nonroutine discharge (odds ratio, 2.80; 95% confidence interval, 2.35–3.34) and requiring a longer hospital stay (incidence risk ratio, 1.19; 95% confidence interval, 1.12–1.27) were higher for the dependent group compared with the independent group. On multivariate analysis, the odds of undergoing an unplanned reoperation were similar between groups (P>.05).

Multivariate Analysis of Outcomes After Total Hip Arthroplasty for Osteoarthritis Using Patients Who Were Functionally Independent as the Reference Group

Table 3:

Multivariate Analysis of Outcomes After Total Hip Arthroplasty for Osteoarthritis Using Patients Who Were Functionally Independent as the Reference Group

Discussion

The authors found that patients who undergo primary THA for osteoarthritis are likely to have worse outcomes during the 30-day postoperative period if they were dependent in performing ADL preoperatively compared with those who were independent. After controlling for potential confounders, dependent patients were more likely than independent patients to have a major medical complication, be discharged to a facility other than home, and have a longer hospital stay.

Although there are several aspects to patient activation, certain interventions in osteoarthritis, such as water exercises, have been associated with substantial improvements in patient functional status.14 Other interventions, such as pain coping skills and lifestyle behavioral weight management, have been shown by at least 1 high-quality study15 to produce substantial improvements in physical disability caused by osteoarthritis. Considering the results presented in the current study, there might be a role for interventions that aim to improve functional status in patients undergoing primary THA for osteoarthritis who report being dependent in performing ADL in the preoperative period.

Previous studies have shown that the modified frailty index is a predictor of postoperative outcomes in the setting of primary hip and knee arthroplasty.16,17 Frailty scores account for patients' functional status during the preoperative period. Although the proposed composite score is effective in predicting postoperative outcomes, it involves a set of risk factors (eg, previous hypertension and congestive heart failure), most of which are nonmodifiable. This study, however, isolated a single risk factor that is both readily screened for and potentially modifiable as an independent predictor of postoperative outcomes in this patient population.

Furthermore, the results of the current study indicate that the odds ratio for a major medical complication on multivariate analysis was 2.34 for patients reporting functional dependency during the preoperative period. Similarly, the functionally dependent patients were likely to have a 19% longer hospital stay (incidence risk ratio, 1.19) compared with independent patients. Although it is unclear whether the prolonged hospital stays were related to the increased incidence of major medical complications, one potential explanation may be the higher incidence of surgical delay in this subset of patients, as shown by Phruetthiphat et al.18 Other explanations may be the improved range of motion19 and pain levels20 postoperatively in patients with a superior functional level at baseline.

Gholson et al21 showed that patient functional status is a predictor of nonroutine discharge after total joint arthroplasty. The results of the current study confirm that finding for patients who underwent THA for osteoarthritis. In both studies, a substantial proportion of patients were discharged to a facility other than home (approximately 30%). Because patients with a dependent functional status had a higher risk for nonroutine discharge and a higher rate of medical complications, interaction between those 2 outcomes might be present. In at least 1 other study, nonroutine discharge was shown to be associated with increased morbidity after THA.22 This highlights the importance of patient selection and the need for further research in this area. Such findings may also improve pre-operative planning and patient counseling.

This study had several limitations. The database used in this study does not offer granular surgical or radiographic details, such as alignment, deformity, or the type of implant used. However, such factors are more likely to affect long-term mechanical outcomes rather than short-term outcomes. Other outcome measures, such as the Short Form-36 and the 6-minute walk test, may offer a more precise assessment of patient activity and functional status than the simple dichotomized definition used in this study. Although the NSQIP database reports a high rate of successful data capture, it is still likely that there is under-reporting of certain outcomes (eg, complications), and conclusions may be generalizable only with that limitation in mind. However, this study shows that patients' functional level may serve as a successful predictor of short-term postoperative outcomes and may aid in patient risk stratification and preoperative counseling.

Conclusion

Preoperative dependence in performing ADL is associated with worse outcomes after primary THA for osteoarthritis compared with independent functional status. Patients with preoperative dependent functional status were more likely than independent patients to experience a major medical complication, be discharged to a facility other than home, and have a longer hospital stay.

References

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Baseline Patient Characteristics and Surgical Factors by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

VariableIndependent (n=42,096)Dependent (n=1083)P
Age, mean±SD, y66±1170±12<.01
Female, No. (%)23,491 (56.8)707 (65.3)<.01
Body mass index, mean±SD, kg/m230±6.531±7.7.08
Comorbidity, No. (%)
  Chronic steroid use1136 (2.7)72 (6.6)<.01
  Congestive heart failure107 (0.3)15 (1.4)<.01
  Chronic obstructive pulmonary disease1545 (3.7)94 (8.7)<.01
  Current smoking5072 (12.1)128 (11.8).82
  Diabetes mellitus4741 (11.3)176 (16.3)<.01
  Hypertension24,161 (57.4)755 (69.7)<.01
Surgical factor
  General anesthesia, No. (%)23,389 (55.6)638 (58.9).03
  Operative time, mean±SD, min93±4096±47.06
  Preoperative hematocrit, mean±SD41%±4.1%39%±4.5%<.01

Univariate Comparison of Outcomes by Functional Status Before Total Hip Arthroplasty for Osteoarthritis, American College of Surgeons National Surgical Quality Improvement Program Database, 2009–2013

VariableIndependent (n=42,096)Dependent (n=1083)P
Length of hospital stay, mean±SD, d3±54±4<.01
Discharge to facility other than home, No. (%)a9713 (26.36)459 (59.23)<.01
Unplanned reoperation, No. (%)b670 (2.27)25 (4.16)<.01
Any major complication, No. (%)517 (1.23)44 (4.06)<.01
  Acute renal failure16 (0.04)3 (0.28)<.01
  Cardiac arrest32 (0.08)1 (0.09).85
  Death62 (0.15)8 (0.74)<.01
  Deep surgical site infection101 (0.24)11 (1.02)<.01
  Myocardial infarction95 (0.23)4 (0.37).33
  Pulmonary embolism102 (0.24)3 (0.28).82
  Reintubation77 (0.18)6 (0.55).01
  Sepsis112 (0.27)13 (1.20)<.01
  Septic shock21 (0.05)1 (0.09).54
  Ventilator dependence33 (0.08)1 (0.09).87

Multivariate Analysis of Outcomes After Total Hip Arthroplasty for Osteoarthritis Using Patients Who Were Functionally Independent as the Reference Group

OutcomeUnadjusted OR (95% CI)PAdjusted OR (95% CI)P
Major medical complication3.41 (2.50–4.66)<.012.34 (1.67–3.28)<.01
Unplanned reoperation1.87 (1.24–2.81)<.011.42 (0.93–2.18).12
Discharge to facility other than home4.06 (3.51–4.69)<.012.80 (2.35–3.34)<.01
Length of hospital stay1.28 (1.24–1.32)a<.011.19 (1.12–1.27)a<.01
Authors

The authors are from the Department of Orthopaedic Surgery, The Johns Hopkins University/Johns Hopkins Bayview Medical Center, Baltimore, Maryland.

Dr Raad and Dr Amin contributed equally to this work and should be considered as equal first authors.

The authors have no relevant financial relationships to disclose.

Correspondence should be addressed to: Julius K. Oni, MD, Department of Orthopaedic Surgery, The Johns Hopkins University/Johns Hopkins Bayview Medical Center, 4940 Eastern Ave, Baltimore, MD 21224 ( joni1@jhmi.edu).

Received: May 01, 2018
Accepted: September 12, 2018
Posted Online: March 27, 2019

10.3928/01477447-20190321-01

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