Abstract
Background Total hip arthroplasty (THA) is an effective procedure that provides patients with long-term relief from pain and enables them to resume their normal daily activities. Preoperative instruction about the functional outcomes and optimum goal of rehabilitation is helpful for patients undergoing THA.
Objective The purposes of this study were: (1) to examine the relationships between preoperative physical functions and ambulation following THA and (2) to identify optimal cutoff values for estimating ambulatory status at 6 months after THA.
Design This was a retrospective study.
Methods The study participants were 204 patients who underwent a unilateral THA. Hip abductor and knee extensor strength were measured and the Timed “Up & Go” Test (TUG) was conducted preoperatively. The patients were divided into 2 groups according to self-reported walking ability at 6 months postoperatively: an independent ambulation group (n=118) and a cane-assisted ambulation group (n=86). Differences between the 2 groups were examined using an unpaired t test or the chi-square test. A stepwise multiple logistic regression analysis was performed with walking ability at 6 months postoperatively as a dependent variable and age, sex, contralateral hip osteoarthritis (ie, whether a participant had contralateral hip osteoarthritis or not), body mass index, hip abductor strength, knee extensor strength, and TUG score as independent variables. Receiver operating characteristic curve analysis was used to identify a cutoff point for classifying the participants into the 2 groups.
Results A stepwise multiple logistic regression analysis selected 3 factors (age, knee extensor strength, and TUG score) as significant variables affecting the midterm ambulatory ability after THA. Moreover, receiver operating characteristic curve analyses revealed that the midterm (ie, 6-month) ambulatory status after THA was more accurately predicted by the patient's TUG score (cutoff point=10 seconds, sensitivity=76.7%, specificity=93.2%, area under the curve=0.93) than by age and knee extensor strength.
Limitations The categorization of ambulatory status in this study was based solely on self-reported walking ability.
Conclusion The findings indicate that patients with a preoperative TUG score of less than 10 seconds are likely to walk without an assistive device at 6 months after THA.
Total hip arthroplasty (THA) is an orthopedic procedure performed to reduce pain and improve function in patients with osteoarthritis (OA) of the hip and to enable them to resume their normal daily activities. The greatest amount of functional improvement after THA is observed within 6 months postoperatively,1 with more gradual improvements occurring for up to 2 years.2 In a study of THA in a sample generalizable to the patients for whom this operation usually is indicated, the investigators reported that about 80% of the participants could walk without assistive devices 6 months postoperatively.3 However, as many patients need an assistive device prior to surgery and immediately after THA, they are anxious about their final level of ambulatory status during the preoperative or postoperative acute rehabilitation periods.
Previous studies have shown that preoperative counseling reduces unrealistic expectations regarding pain and functional recovery after THA.4,5 A Cochrane Review also concluded that preoperative education about anxiety can have beneficial effects.6 Therefore, preoperative instructions regarding the functional outcomes, including ambulatory status such as the use of assistive devices, are helpful for patients undergoing THA.
Preoperative muscle strength and Timed “Up & Go” Test (TUG) scores have been found to be sensitive to functional change in patients following arthroplasty.1,7–9 In addition, gait dysfunction resulting from lower-extremity weakness often is managed by instructing the patient in the use of assistive devices. In some studies, TUG scores were found to be related to the use of assistive devices in elderly people or patients after hip fracture.10,11 Hence, validated functional assessments such as muscle strength and the TUG may be utilized for predicting ambulatory status after THA. It is still unclear, however, which assessment tools are most important in indicating ambulatory status, especially for patients undergoing THA. Moreover, there is no clear cutoff score for estimating the use of assistive devices after THA. The development of a preoperative standardized and valid test that can predict the midterm ambulatory status will be useful for comprehensive rehabilitation after THA.
The aims of this study were: (1) to examine the relationships between preoperative physical function (eg, lower-extremity muscle strength, TUG scores) and ambulation following THA and (2) to identify optimal cutoff values for estimating the patient's ambulatory status at 6 months after THA.
Method
Design
This was a retrospective study.
Participants
A total of 307 patients underwent THA at the Department of Orthopedic Surgery in Kyoto University Hospital between April 2007 and March 2010. The exclusion criteria in this study were: (1) having advanced or terminal OA in the contralateral hip joint, according to the radiological criteria of the Japanese Orthopedic Association12; (2) having symptoms in the contralateral hip, knee, and ankle during walking; (3) having a history of contralateral THA; (4) having a leg length discrepancy of more than 3 cm; and (5) having a revision THA or rheumatoid arthritis. The study participants were 204 patients (173 women, 31 men) with a mean age of 60.4 years (SD=11.6, range=35–81) and a mean body mass index (BMI) of 22.5 kg/m2 (SD=3.6, range=15.7–37.2). There were 138 participants with unilateral hip OA and 66 participants with early-stage bilateral hip OA. The latter group had radiographically confirmed contralateral hip OA, with no symptoms in activities of daily living. All participants underwent primary THA using an anterolateral approach. All participants were followed up for 6 months postoperatively. The participants' age and BMI at the time of surgery also were recorded.
All participants were permitted full weight bearing on the third postoperative day and underwent a 4-week rehabilitation program. The rehabilitation program consisted of transfer training and progressive resistive exercises of the lower extremity, including hip extensor and abductor, knee extensor, and ankle exercises. Participants were encouraged and trained to move from bilateral to unilateral support, and the goal of physical therapy was to achieve ambulation with a cane by 4 weeks from the date of surgery. Prior to the study, each patient was informed in detail about the design and objectives of the study and provided written consent to participate in the study.
Assessment of Preoperative Physical Function
All participants underwent a preoperative assessment at 1 to 5 days before the surgery. Hip abductor and knee extensor strength were measured preoperatively. The hip abductor strength was measured using a handheld dynamometer (Nihon Medix Co Ltd, Matsudo, Japan) during isometric contraction for 3 seconds with manual resistance. The participants rested in a supine position with the hip and knee in neutral flexion and extension and the hip in neutral abduction and adduction. The force sensor was placed at 5 cm above the lateral epicondyle of the femur. An interrater reliability analysis using the weighted kappa statistic was performed to determine consistency among the assessors in hip abductor strength testing. The kappa values were .86 between assessor 1 and assessor 2 or 3 and .91 between assessors 2 and 3. The knee extensor strength was assessed using an IsoForce GT-330 (OG Giken Co Ltd, Okayama, Japan) during isometric contraction for 3 seconds. With the participants in a sitting position with the hip and knee at angles of 90 degrees, the force sensor was placed over the anterior part of the lower leg at 5 cm above the lateral malleolus. Torque was calculated by multiplying the strength by the lever arm (distance between the position of the force sensor and the greater trochanter for the hip abductor strength; distance between the position of the force sensor and the lateral epicondyle of the femur for the knee extensor strength) and expressed as a percentage of the body weight (N·m/kg).
The TUG13 was carried out to assess the participants' ambulatory ability before THA. The TUG measures the time (in seconds) that a patient requires to stand up from an armless chair (chair seat height=45 cm), walk a distance of 3 m, turn, walk back to the chair, and sit down. The test was performed with the participants wearing shoes and walking at their maximum speed without any assistive devices.
Classification of Ambulatory Status After THA
The participants were interviewed at 6 months postoperatively and were classified into an independent ambulation group (those who could ambulate without any assistive devices in their activities of daily living, including stair climbing and stair descending without handrails) and a cane-assisted ambulation group (those who required an assistive device such as a walker, crutches, or a cane).
Data Analysis
For assessments of preoperative physical function, the better score of 2 trials was used for analyses. All statistical analyses were performed using SPSS for Windows (version 17.0, SPSS Inc, Chicago, Illinois), and values of P<.05 were considered to indicate statistical significance. An unpaired t test or the chi-square test was used to examine differences between the independent ambulation and cane-assisted ambulation groups. A stepwise multiple logistic regression analysis was used to identify significant predictors of ambulatory ability at 6 months after THA. The stepwise multiple logistic regression analysis was performed, with ambulatory status at 6 months postoperatively as a dependent variable and age, sex, contralateral hip OA (ie, whether a participant had contralateral hip osteoarthritis or not), BMI, hip abductor strength, knee extensor strength, and TUG score as independent variables. For significant predictors of the ambulatory ability at 6 months identified in the stepwise logistic regression analysis, receiver operating characteristic (ROC) curves were constructed. From the ROCs, we chose an optimal cutoff point that jointly maximized sensitivity and specificity. The areas under the curves (AUCs) associated with the ROC curves also were evaluated. The variable with the greatest AUCs was defined as the most effective tool for classifying the patients into the 2 groups. Moreover, we calculated the positive and negative predictive values standardized to a pretest chance of ambulating well of 50%.
We used 2-fold cross-validation experiments to provide an unbiased estimate of the generalization error. The full data set was randomly divided into 2 subsets: one subset was used for training (50%), and the remaining subset was used for testing (50%). This selection process was done to maintain the same proportion of the independent ambulation group and the cane-assisted ambulation group in the training and testing sets. The process was repeated 50 times, and the mean accuracy rate was calculated.
Results
There were 118 participants (58.7%) in the independent ambulation group and 86 participants (41.3%) in the cane-assisted ambulation group. Table 1 shows the age, BMI, sex, status of contralateral hip, and preoperative physical function in the 2 groups. The difference in age between the 2 groups was significant. The differences in sex, status of contralateral hip, and BMI were not significant. Both preoperative hip abductor and knee extensor strength were significantly greater in the independent ambulation group than in the cane-assisted ambulation group. Furthermore, the preoperative TUG showed significantly better results in the independent ambulation group than in the cane-assisted ambulation group.
Characteristics and Preoperative Physical Function of the Participants in the 2 Groupsa
The stepwise multiple logistic regression analysis identified age, knee extensor strength, and TUG score as factors determining the midterm ambulatory ability at 6 months. The effects of sex, status of contralateral hip, BMI, and hip abductor strength were not significant. These variables, therefore, were eliminated from the ROC analyses.
The ROC curves were constructed to determine the optimal cutoffs for age, knee extensor strength, and TUG score for identifying the participants' ambulatory ability at 6 months. The cutoff point of 67.5 years for age yielded moderate sensitivity of 52.3% (95% confidence interval [CI]=41.3–63.2) and high specificity of 88.1% (95% CI=80.9–93.4). The cutoff point of 1.25 N·m/kg for knee extensor strength yielded moderate sensitivity of 79.7% (95% CI=71.3–86.5) and moderate specificity of 69.8% (95% CI=58.9–79.2). For the TUG, the cutoff point of 10.3 seconds provided moderate sensitivity of 76.7% (95% CI=66.4–85.2) but high specificity of 93.2% (95% CI=87.1–97.0). For age, knee extensor strength, and TUG score, the AUCs were 0.73 (95% CI=0.66–0.80), 0.80 (95% CI=0.75–0.86), and 0.93 (95% CI, 0.90–0.96), respectively. These findings suggest that the midterm ambulatory ability after THA can be predicted more accurately by the TUG score than by age and knee extensor strength. Table 2 shows the positive and negative predictive values standardized to a pretest chance of ambulating well of 50%. The accuracy obtained directly from the model was confirmed by 2-fold cross-validation (CI of the mean accuracy rate calculated after repeating the process 50 times=88.7±0.32).
Positive and Negative Predictive Values Standardized to a Pretest Chance of Ambulating Well of 50%
Discussion
An important goal of physical therapy after THA is to maximize function to enable independence.14,15 Although preoperative use of assistive devices is common for avoiding hip pain, most patients strive for unassisted ambulation during postoperative rehabilitation. However, there is little information in the rehabilitation literature regarding assessment tools that can predict the recovery of ambulatory status after THA. Accordingly, in this study, we investigated screening methods for predicting whether patients would be able to ambulate without any assistive devices in activities of daily living at 6 months postoperatively.
The most important finding of this study was that a cutoff point of 10 seconds for the preoperative TUG provides optimum sensitivity and specificity for predicting ambulation ability at 6 months after THA. The good positive and negative predictive values of this cutoff point indicate its practical usefulness in clinical settings. We believe that this is the first report to identify a specific preoperative physical assessment tool that can predict ambulation ability after THA. This finding may be able to provide patients with realistic expectations for ambulation and allow them to set appropriate goals for their rehabilitation after THA.
The TUG is easy and quick to administer and requires no special equipment or training. This test is commonly used to evaluate functional mobility and quantify locomotor performance in older people or in patients after orthopedic surgery.1,13,16 In the present study, the preoperative TUG score was significantly related to the midterm ambulatory ability after THA. Kennedy et al1 demonstrated that the preoperative TUG score was able to predict the functional recovery after THA using a hierarchical linear modeling analysis. However, those studies did not identify an optimal cutoff value for the TUG. The present study further confirms the importance of the preoperative TUG using a multivariate analysis, and we propose a predictive cutoff value of 10 seconds.
Prediction of postoperative ambulatory status can help patients understand their own goals in the activities of daily living and help physical therapists determine the rehabilitation goals preoperatively. The results of the present study suggest an appropriate goal premised on the use of assistive devices at 6 months after THA for the patients with a preoperative TUG score of greater than 10 seconds. On the other hand, some studies have shown that customized preoperative exercise programs are well tolerated by patients with end-stage hip arthritis and are effective in improving early recovery of physical function after THA.17–19 Preoperative training should be a fundamental component of physical therapy. Further research is needed to determine whether improving the TUG score (of less than 10 seconds) by preoperative training will improve ambulatory ability after THA.
In general, the TUG requires several movements such as standing up from and sitting down in a chair, walking, and turning around. Bischoff et al20 reported that the TUG reflects the performance of activities in daily living rather than impairments such as muscle weakness or decreased balance. This finding may partially explain the observed superiority of the TUG over knee extensor strength in predicting midterm ambulatory status after THA in the present study.
Singh and Lewallen21 reported that older age is associated with greater dependence on walking aids after THA. Similarly, the stepwise multiple logistic regression analysis in the present study selected age at surgery as a factor for discriminating the ambulation levels after THA. However, the predictive value of age appears to be uncertain because the AUC for age was less than those for the knee extensor strength and TUG score.
The present study showed that hip abductor strength on the affected side before surgery cannot predict ambulatory ability at 6 months postoperatively. Long et al2 reported that weakness of the abductor muscle leads to hip dislocation or joint instability. Some previous studies indicated that trunk compensation strategies for hip abductor muscle weakness after THA result in reduced walking ability.22–24 Therefore, exercises immediately after THA, especially hip abductor strengthening exercises, may be important for preventing the occurrence of complications and managing gait disorders, despite the results of the present study.
All participants in the present study were patients who underwent primary THA, and 58% were able to ambulate without any assistive devices in their activities of daily living at 6 months after THA. On the other hand, there are no published data regarding the use of assistive devices such as a cane, crutches, or a walker after revision surgery. Detailed analyses of ambulation after revision THA remain a matter for future studies.
The present study had several limitations. First, the categorization of walking and stair descending and ascending ability were based solely on the self-report of patients by direct interview. Ambulatory ability was not assessed using quantitative gait measures. Second, we confirmed only interrater reliability of hip abductor strength testing, although information on intrarater reliability also is important. Third, although this study showed that the TUG score obtained before THA is a predictor of future ambulatory ability, other possible factors remain to be evaluated, such as the use of gait aids preoperatively, comorbidity profile, and symptomatic contralateral hip OA. Further research is needed to investigate the recovery of ambulatory ability after THA more comprehensively.
Conclusion
The preoperative TUG with a cutoff score of 10 seconds is a reliable assessment tool for predicting ambulatory status at 6 months after a primary THA.
Footnotes
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Mr Nankaku, Dr Kakinoki, Ms Fujita, and Professor Matsuda provided concept/idea/research design and project management. Mr Nankaku, Dr Tsuboyama, and Professor Matsuda provided writing. Mr Nankaku, Dr Akiyama, Mr Nishimura, and Mr Yoshioka provided data collection. Mr Nankaku, Ms Fujita, and Ms Kawai provided data analysis. Dr Akiyama provided study participants. Professor Matsuda provided facilities/equipment, institutional liaisons, and consultation (including review of manuscript before submission).
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All of the procedures in this study were approved by the Ethics Committee of Kyoto University Graduate School of Medicine and Faculty of Medicine.
- Received January 20, 2012.
- Accepted September 17, 2012.
- © 2013 American Physical Therapy Association
References
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