A correlation study of objective functionality and WOMAC in total knee arthroplasty

Transcription

1 Knee Surg, Sports Traumatol, Arthrosc (2002) 10 : KNEE DOI /s E. Witvrouw J. Victor J. Bellemans B. Rock R. Van Lummel R. van der Slikke R. Verdonk A correlation study of objective functionality and WOMAC in total knee arthroplasty Received: 11 December 2001 Accepted: 25 April 2002 Published online: 22 June 2002 Springer-Verlag 2002 E. Witvrouw ( ) Department of Rehabilitation Sciences and Physiotherapy, University Hospital, 6K3, De Pintelaan 185, 9000 Ghent, Belgium erik.witvrouw@rug.ac.be, Tel.: , Fax: E. Witvrouw R. Verdonk Department of Orthopedic Surgery and Physical Medicine, Faculty of Medicine, Ghent University, Ghent, Belgium J. Victor B. Rock Department of Orthopedic Surgery, Sint Lucas Hospital, Bruges, Belgium J. Bellemans Department of Orthopedic Surgery, Catholic University of Leuven, Leuven, Belgium R. Van Lummel R. van der Slikke McRoberts, The Hague, The Netherlands Abstract Function studies after total knee arthroplasty (TKA) generally use either self-reported instruments or objective quantitative measurements of walking activities, but it is unclear how objective factors influence self-reports. This retrospective multicenter study assessed the impact of objective variables on subjective self-reported functional outcome after TKA in 128 patients at a mean of 41 months after TKA. Selfreported disability was measured by the Western Ontario and McMaster University (WOMAC) scale, and the quality of walking and representative daily activities were determined objectively by the DynaPort knee test. Objective assessment was found to be a poor predictor of WOMAC abnormality, with correlations of 0.343, 0.246, and with WOMAC function, stiffness, and pain, respectively. The results of WOMAC are thus strongly influenced by factors other than quantitative parameters of the lower extremity. These findings provide support for including an objective assessment of activities of daily living together with a subjective self-report. Keywords Total knee arthroplasty Western Ontario and McMaster University scale Objective measurement Functional outcome Dynaport knee test Introduction Total knee arthroplasty (TKA) is performed as an endstage procedure for patients with degenerative arthritis to eliminate pain and improve function [5, 8]. The surgical techniques and prosthetic engineering developments associated with these elective procedures have been highly refined over the past 20 years and have resulted in high success and survival rates [4, 23]. During the past decade the technology and design of joint implant systems have progressed considerably, and a great variety of systems are now on the market. From the patient s perspective the goals of knee joint replacement are pain relief and restoration of function and mobility [5]. The surgeon has the same objectives, and additionally aims to improve range of motion, stability, walking distance, and stair climbing. Most of these variables are addressed in the various knee scoring scales [7, 8]. Therefore most previous studies of function after TKA have used these self-report instruments to obtain an understanding of the patients functional ability. However, the results of these self-report instruments have recently been shown to reflect psychological and constitutional status as well as regional lower limb abnormalities [22]. To our knowledge, no study has examined the extent to which objective locomotive factors influence the data obtained by these self-report instruments. Therefore the goal

2 348 of this study was to assess the correlation between quantitative performance with the results of self-report instruments. Materials and methods This retrospective multicenter study examined the results of 128 total knee arthroplasties (94 men, 34 women). Only patients with a unilateral total knee prosthesis, implanted for primary osteoarthritis at least 1 year previously were included in this study. Patients with posttraumatic, postinfectious, or rheumatoid arthritis were excluded. All patients were evaluated by the same independent observer using the Western Ontario and McMaster Universities (WOMAC) function subscale OA index, and the DynaPort knee test. This WOMAC index is a well-validated scale [1] designed to reflect the problems experienced by individuals with lower limb deficiencies. It assesses pain (five items), stiffness (two items), and physical function (17 items) related to the hip or knee, with higher scores indicating greater difficulty. The DynaPort knee test is a short standardized test to assess the quality of movement of patients with knee dysfunction. It is suitable for clinical applications, and takes only min to administer. No further equipment or specialized laboratory is required. Scores reflect the level of disability in daily living; a higher score indicates a better function. The test uses sensors which are fixed to the patient s body, while the patient performs predetermined tasks (Fig. 1). The patient is asked to perform 14 selected activities, using 29 easy-to-measure test items that are often problematic for the patients. These tasks can be categorized under locomotion (walking), rising and descending (stairs, slopes, and wooden blocks), lifting and moving (carrying a tray or a bag, picking up a weight, and walking with a shopping trolley), and transfers (going to sit or lie down and then standing up again, as well as bending forwards to pick up a weight and returning to the upright position). To standardize the test a standard package with all equipment is included in the test material (such as wooden blocks, stairs of three steps, Fig. 1 The Dynaport knee test measuring device and a slope). The sensors are sensitive to both slow and fast changes in acceleration; they are small and of low weight. The sensors react to gravity and movement acceleration, thus giving information about posture and movement of the test person [21]. A portable recorder worn around the waist allows easy and stable data acquisition. The whole system is fixed to the patient with elastic straps over his daily clothing (Fig. 1). In total six sensors are fixed to the chest, the pelvis (two), the left thigh, and the left and right shanks. The raw signals are stored in a portable recorder and are analyzed with special software which extracts movement features. All signal properties are derived from the original accelerometer signals, while several movement features are calculated in terms of angle, duration (time), frequency, or a dimensionless number [6]. Before the Dynaport knee test was used in the present investigation it was deemed necessary to obtain preliminary information on its reliability and validity (responsiveness). In a test-retest study with 37 healthy subjects, measured twice at an interval of 1 week the reliability of the device was found to be high, with an overall intraclass correlation of 0.81 (95% confidence interval ) and values for the cluster ranging from 0.73 (95% CI ) to 0.84 (95% CI ). To test the validity of the Dynaport knee test its responsiveness was tested on 244 TKA patients. These patients were tested before the procedure and 3, 6, 12, and 24 months thereafter. This study [6] showed a clear trend to improvement, with an average extent of effect after 24 months (mean improvement) of 0.7 (when expressed in preoperative SD of TKA patients) or 1.5 (SD of the healthy group). Such responsiveness is satisfactory. In addition to the reliability and validity (responsiveness) of the Dynaport Knee Test, it is imperative to know whether it discriminates between injured and uninjured persons. In a previous study [6] 140 persons with knee problems (most of them diagnosed with osteoarthritis of the knee while waiting for TKA) and 32 matched persons without knee pain underwent a Dynaport knee test. The results of this study showed that the majority of item X movement feature combination discriminated significantly between the healthy subjects and TKA patients. The number of significant differences per item ranged from 10 to 24 (out of 30) and per movement feature from 1 to 29 (out of 29). The mean follow-up period of the patients in this study was 41 months (SD 22.6). All patients in our study signed an informed consent statement. All the ethics committees of the various cooperating hospitals approved this study. Data were analyzed using the SPSS software version Correlations between the different assessment scores were evaluated in bivariate analyses with Spearman s rank order correlation. To explore multivariate relationships we performed multiple regression with WOMAC function, pain, and stiffness as the dependent variables and Dynaport score as the independent variable. Statistical significance was accepted at the level of P<0.05. Results Mean and standard deviations of the anthropometric variables of the 128 patients are given in Table 1. Table 2 illustrates the results of the Dynaport knee test and the WOMAC score. The relationship between the Dynaport knee test and the WOMAC scores (Fig. 2) shows that the two are inversely: the higher the Dynaport score, the less the disability. However, the correlation between Dynaport score and WOMAC pain score was poor (r=0.359; Fig. 2). Similarly, there was a poor correlation between Dynaport score and the results of the WOMAC function and stiffness scores (r=0.342 and r=0.216, respectively). The R 2

3 349 Table 1 Anthropometric data on the patient group (n=128) Mean ±SD Range Age (years) 62.5± Weight (kg) 84.0± Height (cm) 169.3± Body mass index 29.3± Table 2 Dynaport knee test and WOMAC scores for pain, function, and stiffness (n=128) Mean ±SD Range Dynaport 53.6± WOMAC pain 7.2± WOMAC function 8.3± WOMAC stiffness 10.8± Fig. 2 Correlation between WOMAC pain score (100=max) and the result of the Dynaport knee test (100=max; n=128, r=0.359) with WOMAC pain, function, and stiffness scores as dependent variables was 0.14, 0.12, and 0.05, respectively. These data indicate that during daily living in bivariate analyses WOMAC scores are poorly associated with quantitative measures of activities. To identify the impact of the three WOMAC scores on the Dynaport score, a multiple regression analysis was performed with the Dynaport score as the independent variable and with WOMAC function, pain, and stiffness scores as the dependent variables. Statistical analysis failed to identify any of the three WOMAC scores as important predictors of the Dynaport score (P>0.05). Discussion The literature indicates that TKA is highly successful, and that it contributes significantly to improving an individual s functional status and well-being, with a considerable individual variation being shown in treatment response [13, 16, 19]. Therefore it is important to understand clearly the factors that contribute to the functional outcome after TKA [20]. This study showed that the quantitative variables measured by the Dynaport knee test are poorly correlated with self-reported disability as obtained by the WOMAC questionnaire. The correlation coefficient r measures the strength of the linear association between two variables. An interpretation of this strength is found by the R 2 statistic, which represents the proportion of the variance in one of the variables that can be explained by the values of the other. The data found in this study show that the quantitative measurement accounts for only 5 14% of the subjective assessment. This demonstrates the variability and discrepancy between these two evaluation methods. Our results are in accordance with those of Lamb et al. [9] who found that pain was associated with limited mobility (obtained by measuring ability to rise from a chair and walking ability) only when severe. Consistent with these findings, Madsen et al. [11] found no correlation between walking or stair climbing and pain level in patients with gonarthrosis. Based on their results these authors concluded that walking time and stair climbing seem to have little clinical value in predicting self-reported pain and pain disability. This may not be very striking since self-reported instruments appear to indicate more than merely regional pain [22]. These functional self-reported scales are sensitive to psychological and nondisease factors such as fatigue and depression. This can be considered as a merit of these scales since such factors contribute significantly to the patient s actual pain and dysfunction. Previous research has shown that depression is associated with and predictive of physical disability [18] and is strongly related to pain [12]. On the basis of these findings conclude that the evaluated outcome measurements in this study (WOMAC and Dynaport knee test) include numerous dimensions such as quality of life, general health, and functional impairment, which reflect the different viewpoints and relative importance to the patient, physician, and society. Regarding the results of this study, the clear discrepancy between self-report questionnaire and objective functionality suggests that both assessments should be used to provide the most complete evaluation. In addition, research has revealed the importance of using both a generic quality-of-life measuring instrument and a disease-specific instrument [2, 10, 15]. We believe that when evaluating patients with TKA both a self-report questionnaire and an objective evaluation of function are essential because, as shown in this study, they provide distinct but complementary aspects.

4 350 However, our results contrast with those results of previous research [14, 17] which found an association between quadriceps strength and disability and knee pain in patients with knee OA. Likewise, Boardman et al. [2] found a sound statistical relationship between walking ability and the functional aspects of the WOMAC in patients with total hip arthroplasty. The discrepancies between our results and these findings can be explained by the fact that a different patient population and different parameters were studied. In our study we did not measure quadriceps strength, although this has been shown by several authors [3, 11, 14, 15, 17] to be correlated with selfreported disability. This can be considered an important limitation of this study. However, the strength of this study is that all objective functional tests were performed by the same independent person who was blinded to the results of the self-report questionnaire. Conclusions An important principle in evaluating the outcome of a surgical procedure is that tools used to measure the results must be relevant to the clinical situation. The purpose of this study was to obtain a better understanding of how the subjective scores derived from the self-reported instrument (WOMAC) are correlated with objective functional outcome data measured by the Dynaport knee test. We found that the relationship between the scores of quality of movement during daily activities and the WOMAC is poor. The objective functional measurements predicted only 5 14% of the WOMAC score. The results of this study show that patient subjective assessment is poorly related to the extent of movement, and that the result is influenced (85 95%) by many other factors. We believe that our results support the continued use of a self-reported instrument (WOMAC) combined with an objective measurement of the quality of movement. Since the results suggest that the outcome measurements evaluated in this study (WOMAC and Dynaport knee test) cover different dimensions of the functional outcome, their combined use provide the clinician with a more complete evaluation of his patients and allows more appropriate decision for rehabilitation strategy. Acknowledgements The authors thank the Belgian Society of Orthopaedics and Traumatology (BVOT) for a research grant, and the members of the Belgian Knee Group for their cooperation in this study. In addition we thank A.A. Amis, N. Friederich, and P. Neyret for their important assistance in this study. References 1. Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt L (1988) Validation study of the WOMAC: a health status instrument for measuring clinically-important patient relevant outcomes following total hip- or knee arthroplasty in OA. 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