Muscle strength is the main associated factor of physical performance in older adults with knee osteoarthritis regardless of radiographic severity

Archives of Gerontology and Geriatrics 56 (2013) 377–382

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Muscle strength is the main associated factor of physical performance in older adults with knee osteoarthritis regardless of radiographic severity Se-Woong Chun a, Kyoung-Eun Kim a, Soong-Nang Jang b, Kwang-Il Kim c, Nam-Jong Paik a, Ki Woong Kim d, Hak Chul Jang c, Jae-Young Lim a,* a

Department of Rehabilitation Medicine, Seoul National University Bundang Hospital, 300, Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-707, Republic of Korea Red Cross College of Nursing, Chung-Ang University, 221, Heukseok-Dong, Dongjak-gu, Seoul 156-756, Republic of Korea Department of Internal Medicine, Seoul National University Bundang Hospital, 300, Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-707, Republic of Korea d Department of Neuropsychiatry, Seoul National University Bundang Hospital, 300, Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-707, Republic of Korea b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 August 2012 Received in revised form 22 October 2012 Accepted 26 October 2012 Available online 17 November 2012

The aim of the study is to compare factors associated with physical performance in older individuals with severe knee osteoarthritis and those with less-severe osteoarthritis. This is an ancillary cross-sectional study to a population-based cohort study focusing on Koreans age 65 years or older. The analysis included 553 subjects with information about knee pain, depressive symptoms, and comorbidities collected by self-reported questionnaire, and body weight, knee osteoarthritis severity, muscle strength, and physical measures by observer-rated tests. Stepwise logistic regression analyses were performed with physical performance as an outcome variable and the others as independent variables across radiographic knee osteoarthritis severity. In the minimal-to-moderate-severity group, muscle strength, knee pain, BMI, and age were related to poor performance (OR [CI] 0.81 [0.73–0.90], 1.12 [1.03–1.21], 0.87 [0.79–0.96], and 1.09 [1.05–1.14], respectively). In the severe group, muscle strength was the only factor significantly associated with poor performance (OR [CI] 0.72 [0.58–0.89]). Muscle strength, knee pain, and BMI were important determinants of physical performance in the older population with knee osteoarthritis. In severe knee osteoarthritis patients, muscle strength was the only significant determinant. ß 2012 Elsevier Ireland Ltd. All rights reserved.

Keywords: Muscle strength Physical performance Knee osteoarthritis

1. Introduction Population aging is occurring in all developed countries and has led to great interest in promoting health and quality of life (QoL) in the elderly. For older persons, preserving physical function is a key component to maintaining well-being. Knee osteoarthritis (OA) is a leading cause of disability and poor QoL in the communitydwelling elderly individuals (Guccione et al., 1994). The diagnosis of knee OA is based on various findings, including knee pain and radiographically proven degenerative changes. Knee pain can wax and wane, whereas radiographic findings do not improve. Consequently, we often classify knee OA severity according to the Kellgren–Lawrence grading system, which is based on radiographic findings. The prevalence of radiographically more

* Corresponding author at: Department of Rehabilitation Medicine, Seoul National University College of Medicine, Seoul National University Bundang Hospital, 300, Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-707, Republic of Korea. Tel.: +82 31 787 7732; fax: +82 31 787 4056. E-mail address: [email protected] (J.-Y. Lim). 0167-4943/$ – see front matter ß 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.archger.2012.10.013

severe knee OA increases with age. Thus understanding the physical performance of persons with severe knee OA is important in promoting optimal QoL in older adults. Knee OA may influence performance in many ways. Pain, structural damage, compensatory patterns, and limited range of motion caused by knee OA can affect physical performance (Maly, Costigan, & Olney, 2006). Anthropological properties such as body mass (Lohmander, Gerhardsson de Verdier, Rollof, Nilsson, & Engstrom, 2009), height (Hunter et al., 2005), and muscle strength (Hart, 2004) that also affect physical performance have unidirectional or bidirectional effects on knee OA. Furthermore, unmodifiable factors such as age and gender also affect both knee OA and physical performance (Samson et al., 2000). These complex interactions of various factors related to knee OA and physical performance may differ in patients with severe versus less-severe knee OA. Severe knee OA patients tend to be older and heavier, and their body mechanics are usually altered more seriously (Astephen, Deluzio, Caldwell, & Dunbar, 2008). Life-style modification including regular exercise is the firstline treatment for knee OA. Currently, this principle has not been applied properly to patients with severe knee OA because factors

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such as pain, depressive symptoms, joint deformity, and muscle weakness causing physical dysfunction may act differently in severe cases. However, few studies have addressed physical function in severe knee OA patients. Clearly, physical performance in subjects with severe knee OA is poorer than that in a comparison group (Thomas, Pagura, & Kennedy, 2003). However, knowledge of the attributes of the disability seen in severe knee OA patients is still deficient. One study identified muscle strength, knee range of motion, and BMI as significant predictors of functional task performance and proposed muscle strength as the main target of treatment for functional improvement in patients with severe knee OA (Brown et al., 2009). However, data from subjects who were waiting for surgical treatment should be applied cautiously to general severe knee OA population. Moreover, there was no comparison with normal or less-severe knee OA patients. Thus, we sought to determine the attributes of physical performance in patients with radiographically severe knee OA using baseline data from a population-based cohort study on aging. In this study, we compared factors associated with physical performance in a geriatric population with radiographically severe knee OA versus those with no or less-severe OA. We hypothesized that there would be an identifiable difference in the relationship between muscle strength, and other adjustable factors, and physical performance in severe knee OA patients compared with patients with no or less-severe OA. Understanding the relationship between the adjustable factors and performance in severe knee OA patients may provide a basis for designing a targeted treatment program for this population. 2. Methods 2.1. Study sample This study was an ancillary cross sectional study of a population-based longitudinal cohort study, the Korean Longitudinal Study on Health and Aging (KLoSHA). KLoSHA focuses on the general health, functional status and risk factors for geriatric disorders among Korean elders. The baseline study was conducted on older adults aged 65 years or older living in households in Seongnam City selected by multistage stratified probability sampling (Park et al., 2007). A total of 1000 individuals completed the baseline survey conducted by three trained nurses and three medical doctors at Seoul National University Bundang Hospital from September 2005 through September 2006. From among the 1000 participants, we recruited individuals who were able to walk independently with or without an aid and without acute exacerbation of chronic diseases or terminal illnesses in the second evaluation, which included the isokinetic muscle strength and the short physical performance battery (SPPB). In total, 690 persons participated in the second evaluation. Among these, 11 hemiplegic persons were excluded because hemiplegia directly affects muscle strength and physical performance. Another 126 persons were excluded because they had missing data for isokinetic muscle strength, SPPB, or knee radiograph. Ultimately, 553 subjects (295 men, 258 women) with a mean age of 74.2  7.7 years were included in the analysis. All subjects were fully informed regarding the study protocol, and written informed consent was obtained from the participants or their legal guardians. The study protocol was approved by the Institutional Review Board of Seoul National University Bundang Hospital. 2.2. Measurement 2.2.1. Physical performance The short physical performance battery (SPPB) is a valid measure of lower extremity mobility that is predictive of mortality

and institutionalization in elderly adults. The SPPB consists of three tests: (1) three standing-balance trials (tandem, semi-tandem, and side-by-side standing); (2) five continuous chair stands; and (3) a 4-m walk. Based on normative data, the performance times of these tasks were graded on a scale from 0 to 4. The sum of the three subscores gave the total SPPB score, ranging from 0 (worst) to 12 (best function). Elderly persons with a SPPB score 9 have a significantly higher risk of subsequent disability compared with those with a SPPB score >9 (Guralnik et al., 2000). Thus, the SPPB score was converted into a binary variable by recording scores 9 as poor performance and those >9 as good performance. 2.2.2. Radiographic knee OA severity Radiographic knee OA severity can be represented by degenerative changes that include joint space narrowing, osteophytes, and subchondral sclerosis seen in a simple anterior–posterior knee Xray (AP-knee). Where both knees were affected, the more severe one was selected for the analysis. The AP-knee was taken with the subject standing erect with feet pointing forward in a comfortable manner. This gives information on the tibiofemoral joint space in a weight-bearing posture, a more functional posture than the supine position, and on the osteophytes on the lateral and medial sides of the femoral epicondyle and tibial condyle. All AP-knee images were acquired digitally using the picture archiving and communication system (PACS; IMPAX; Agfa, Antwerp, Belgium) and assessed using PACS software. The radiographs were evaluated using the Kellgren–Lawrence (K/L) grading system by one rater blinded to all clinical information. The K/L grades are defined as follows: Grade 0, no features of OA; Grade 1, small osteophyte of doubtful importance; Grade 2, definite osteophyte, but an unimpaired joint space; Grade 3, definite osteophyte with moderate diminution of the joint space; and Grade 4, definite osteophyte with substantial joint space reduction and sclerosis of the subchondral bone. The K/L classification is one of the simplest, most reliable radiographic scoring systems for knee OA. The reliability of K/L grades has been demonstrated using intra-rater and inter-rater reliability by intraclass correlation coefficients (ranging from 0.82 to 0.95 and from 0.63 to 0.83, respectively). Generally, we define radiographic knee OA as having grade 2 or more in K/L grading (Petersson, Boega˚rd, Saxne, Silman, & Svensson, 1997). However, the ambiguity of interpretation of K/ L grade 2 and grade 3 is the major problem of K/L grading system because definite osteophytes do not always precede joint space narrowing (Altman et al., 1987) and knee pain is better predicted by osteophyte rather than joint space narrowing (Spector et al., 1993). Thus, we considered K/L grade 0/1 as ‘no-or-doubtful OA’, grade 2/3 as ‘minimal-to-moderate OA’, and grade 4 as ‘severe OA’. This grouping was used in the analysis of OA severity. 2.2.3. Demographic factors and anthropometric measurements Patients were surveyed for demographic information and specific medical history issues that might be related to knee OA. Demographic information included gender, age, height, weight, and BMI, because these factors are commonly selected as candidate determinants of physical function (McAlindon, Cooper, Kirwan, & Dieppe, 1993). 2.2.4. Muscle strength Muscle strength was represented by normalized dynamic knee extension torque. Dynamic torque was measured on an isokinetic dynamometer (Biodex Medical Systems1, System 3, Shirley, NY, USA). Unilateral strength tests were performed in the seated position in a reclining (58) chair. The rotational axis of the dynamometer was aligned with the transverse knee-joint axis and connected to the distal end of the tibia using a length-adjustable lever arm. The thighs, hips, and shoulders were stabilized with

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safety belts. Once the test was finished on one side, it was repeated on the contralateral side. To determine the maximum dynamic torque, the subjects performed five back-to-back maximal consecutive isokinetic knee extension at a velocity of 608/s after several submaximal trial contractions at the same speed followed by a 5-min rest. The highest of the five isokinetic extension torques (N m) was selected as the maximum dynamic torque. The average value of the maximum dynamic knee extension torque for both knees was divided by 1/10 of the body weight to adjust for interindividual differences in body size.

geriatric and arthritic populations. To evaluate the potential impact of comorbidity, the Charlson Comorbidity Index (CCI) was used. CCI is an index that standardizes the weight of various coexisting illnesses to consolidate individual condition into a single, predictive, numerical comorbidity score It includes a range of comorbid conditions such as heart disease, diabetes, and cancer, with a higher score indicating a greater number of and/or more severe comorbidities (Charlson, Pompei, Ales, & MacKenzie, 1987).

2.2.5. Knee pain Pain severity was measured using the Western Ontario McMaster University (WOMAC) index. WOMAC is a well-established questionnaire rating current symptoms related to osteoarthritis and can be confined to the knee. It contains three sections: pain, stiffness and disability. Each question has five response options (none, mild, moderate, severe, extreme) and the pain section consists of questions asking about pain in five different settings (walking on a flat surface, going up- or down-stairs, at night while in bed, sitting or lying, and standing upright). The pain subscale is frequently used separately as a quantitative measure of knee pain. The WOMAC is based on symptoms during the past 48 h. Thus, temporary pain without any organic cause which is inappropriate to record as knee pain caused by OA might be captured by WOMAC. To overcome this, activity-induced knee pain was also assessed based on symptoms over the past 4weeks. As the major function of the lower limbs is locomotion and walking and climbing stairs are the two major activity components of independent locomotor function (Keith, Granger, Hamilton, & Sherwin, 1987), patients who consistently had knee pain during walking and/or using the stairs were defined as having activity induced knee pain. Activity induced knee pain data were extracted from the Knee Society scoring system. The Knee Society score embraces a relatively objective clinician-based rating of pain (Insall, Dorr, Scott, & Scott, 1989).

One-way analysis of variance (ANOVA) was used to compare the means of interval- or ratio-type variables (age, BMI, muscle strength, CES-D CCI), and x2 tests to compare the ratios of nominaltype variables (knee pain, poor performance, female gender) across the levels of radiographic knee OA severity. Each comparison of multiple means was followed by Scheffe´ post hoc analysis. Subjects were divided into two groups according to muscle strength, i.e., upper and lower halves of the distribution. ANOVA and the Scheffe´ post hoc analysis were performed to compare mean SPPB scores across radiographic knee OA severities in both groups. Spearman’s correlation analyses between performance group and the other variables were performed in each knee OA severity group to select candidate variable to be entered in the subsequent regression analyses. To identify the factors predicting poor physical performance, backward stepwise logistic regressions were performed in each radiographic knee OA severity group. Model fit was measured by comparing the observed and expected frequency applying the Hosmer–Lemeshow goodness-of-fit test.

2.2.6. Depressive symptoms Depressive symptoms are associated with a decline in physical performance in community-dwelling older persons (Penninx et al., 1998). The Center for Epidemiological Studies Depression Scale (CES-D) was used to assess depressive symptoms. CES-D is a 20-item self-report of depressive symptoms that shows good agreement with clinician interview ratings and other longer self-report scales (Weissman, Sholomskas, Pottenger, Prusoff, & Locke, 1977). The Korean version of CES-D is well validated (Cho & Kim, 1993). 2.2.7. Comorbidity Comorbidity heightens the risk of disability (Verbrugge, Lepkowski, & Imanaka, 1989) and has a high prevalence in both

2.3. Statistical analyses

3. Results Means  standard deviations or ratios for each variable are displayed below according to radiographic knee OA severity (Table 1). This revealed that means or ratios differed across the levels of radiographic knee OA severity for all variables except for CES-D and CCI. The percentage of females and poor performers increased with radiographic knee OA severity. Post hoc analysis using Scheffe´’s criterion for p < 0.01 significance indicated that muscle strength differed significantly among radiographic knee OA-severity groups. The no-or-doubtful-OA group showed lower BMI and age and higher WOMAC pain subscale scores compared with the other groups. However, these were not significantly different between the minimalto-moderate-OA and the severe-OA groups. The SPPB score was lower in the severe-OA group but did not differ between the less-severe groups. One-way ANOVA was used separately in the higher and lower half muscle strength groups to test for physical performance

Table 1 Descriptive values according to knee OA severity. Descriptive value

SPPB Poor performance (number of subjects (%)) Age (year) Female (persons (%)) Muscle strength (N m/kg) BMI (kg/m2) WOMAC pain Activity induced knee pain (number of subjects (%)) CES-D CCI

Radiographic knee OA severity (n = 553)

p-Value

No-or-doubtful (n = 224)

Minimal-to-moderate (n = 258)

Severe (n = 71)

9.82  2.32a 77 (34.4) 72.7  7.3a 71 (31.7) 11.60  4.17a 23.5  2.9a 2.24  3.47a 35 (15.6)

9.27  2.32a 126 (48.8) 74.8  7.6b 129 (50.0) 9.76  3.54b 24.6  3.1b 3.56  3.91b 62 (24.0)

7.80  2.92b 49 (69.0) 76.9  8.4b 58 (81.7) 7.50  2.78c 25.1  3.3b 7.56  5.53c 37 (52.1)

31.6  8.7 0.63  0.84

31.9  8.6 0.58  0.82

33.7  9.3 0.61  0.83

Sheffe´’s post hoc test is implemented to compare ground differences. Same alphabetical letters indicates no statistical differences between two mean values.

<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.214 0.743

[(Fig._1)TD$IG]

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Fig. 1. Mean SPPB scores by muscle strength level and radiographic knee OA severity. * indicates statistical difference between groups.

differences among radiographic knee OA severities. Physical performance differed significantly among the three knee OA severity groups in those with lower muscle strength: F(2,293) = 5.228, p = 0.006. Scheffe´ post hoc comparisons of the three groups indicated that the severe-OA (M = 7.31, 95% CI 6.55– 8.06) showed significantly poorer physical performance than the no or-doubtful-OA (M = 8.52, 95% CI 8.01–9.03) or the minimal-tomoderate-OA group (M = 8.46, 95% CI 8.07–8.85). However, in those with higher muscle strength, we found no significant differences among the no-or-doubtful-OA (M = 10.67, 95% CI 10.37–10.98), minimal-to-moderate-OA (M = 10.35, 95% CI 10.03–10.68), and severe-OA (M = 10.25, 95% CI 9.20–11.30) groups (Fig. 1). Means  standard deviations or ratios for each variable are displayed according to the physical performance group (Table 2). The poor-performance group showed higher proportions of severe knee OA, of females, and of activity-induced knee pain. This group was older and had poorer muscle strength, lower BMI, more knee pain, and depressive symptoms. The comorbidity index did not differ between the two performance groups. Correlation analyses between performance group and the other variables are shown in Table 3. Most variables showed a tendency toward a correlation (p < 0.1) with physical performance. CCI was not correlated with performance group regardless of knee OAseverity group. Gender was not correlated with performance group in the minimal-to-moderate-severity group, and BMI and activityinduced knee pain showed no significant correlation in the severe knee-OA group. Variables with significant correlations (p < 0.1) with physical performance in each knee OA-severity group were entered into a

Table 2 Descriptive values according to physical performance. Physical performance

Severe radiographic knee OA (persons (%)) Age (year) Female (persons (%)) Muscle strength (N m/kg) BMI (kg/m2) WOMAC pain Activity induced knee pain (persons (%)) CES-D CCI

p-Value

Poor (N = 252)

Good (N = 301)

49 (19.4)

22 (7.3)

<0.001

77.4  8.1 145 (57.5) 8.23  3.49 23.91  3.32 5.03  5.10 86 (34.1)

71.6  6.2 113 (37.5) 11.88  3.54 24.43  2.93 2.29  3.01 48 (15.9)

<0.001 <0.001 <0.001 0.052 <0.001 <0.001

33.74  10.11 0.65  0.86

30.57  7.11 0.56  0.81

<0.001 0.207

regression analysis of the corresponding group. Possible multicollinearity between covariates using correlation analysis and collinearity statistics (tolerance and variance-inflation factor tests) was evaluated. There was no significant collinearity between any covariates. Stepwise logistic regressions with poor performance as an outcome yield were repeated in all subjects and within each OAseverity group (Table 4). In all subjects, radiographic knee OA severity was also included as an independent variable. Significant attributes of poor performance in the total sample were age (OR 1.07), knee pain (OR 1.10), BMI (OR 0.92), and muscle strength (OR 0.79). In the no-or-doubtful-OA group, they were age (OR 0.05), depressive symptoms (OR 0.76), and muscle strength (OR 0.74). In the minimal-to-moderate-OA group, age (OR 1.09), knee pain (OR 1.12), BMI (OR 0.87), and muscle strength (OR 0.81) were related to poor physical performance. In contrast, in the severe-OA group, muscle strength was the only significant factor (OR 0.72). No significant interaction effect was detected between SPPB and muscle strength. None of the models was rejected for goodness-offit due to p-values >0.05 according to the Hosmer–Lemeshow test. The frequency of poor performance predicted by the model can be considered close to the observed frequency, because the p-value in the Hosmer–Lemeshow test indicated a good fit to the model. 4. Discussions Physical performance was lower in the severe-OA group compared with the others. However, when we subdivided the population according to muscle strength, in the subgroup with greater muscle strength, the physical performance of the severeOA group did not differ from that of the less-severe-OA groups. This indicates that the patients with severe OA with good muscle strength do not show the decrease of physical performance usually seen in severe knee OA. KLoSHA data show that the determinant profile of physical performance in subjects with radiographically severe-OA differed from those of subjects with less-severe-OA. Many factors were related to physical performance; however, muscle strength was the only factor related to physical performance regardless of radiographic knee OA severity. That is, the association of muscle strength with physical function appears to be important regardless of knee OA severity, whereas the significance of the other factors varied according to knee OA severity. The physical function of the knee is determined by neuromusculoskeletal factors such as body weight (Creamer, LethbridgeCejku, & Hochberg, 2000), range of motion (ROM) (van Dijk et al., 2010), muscle strength, muscle activation and proprioception (Hurley, Scott, Rees, & Newham, 1997). The group with severe knee OA showed particularly poor physical performance (Table 1), whereas, muscle strength was different with each severity group and BMI was lower only in the no-or-doubtful OA group. It is difficult to identify which variable contributed to the decline of physical performance in the severe OA group. However, logistic regression analysis revealed that only muscle strength remained a significant attribute even after controlling for other factors. BMI did not correlate with physical performance in the no-ordoubtful- or the severe-OA group (Table 3). As higher body weight brings about larger energy consumption during the same activity, it may seem reasonable that being overweight would be associated with functional decline in the lower extremities (Woo, Leung, & Kwok, 2007). However, in old people, BMI might not effectively reflect obesity. Both muscle and fat mass contribute to body mass, and weight loss is accompanied by loss of muscle mass, particularly in older adults (Newman et al., 2005). Sarcopenia is an early characteristic of knee OA (Toda, Segal, Toda, Kato, & Toda, 2000) and is closely related to physical dysfunction (Janssen, Heymsfield, & Ross, 2002). Moreover, knee malalignment, which is related to progression of knee OA, may play a mediating role in the

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Table 3 Correlation analysis with poor physical performance group. Subgroup

Age

Gender

Muscle strength

Total rs p-Value No or doubtful rs p-Value Minimal to moderate rs p-Value Severe rs p-Vlue

0.38 <0.01 0.32 <0.01 0.40

0.2 <0.01 0.23 <0.01 0.05

0.49 <0.01 0.54 <0.01 0.38

<0.01 0.22 0.06

0.46 0.23 0.05

<0.01 0.43 <0.01

BMI

WOMAC pain

Activity induced knee pain

CES-D

CCI

Radiographic knee OA severity

0.08 0.08 0.13 0.05 0.15

0.28 <0.01 0.19 <0.01 0.19

0.21 <0.01 0.18 <0.01 0.18

0.14 <0.01 0.12 0.08 0.11

0.05 0.23 0.04 0.51 0.07

0.22 <0.01 na

0.02 0.07 0.57

<0.01 0.32 <0.01

<0.01 0.09 0.46

0.08 0.21 0.07

0.25 0.01 0.91

na

na

rs: Spearman correlation coefficient.

relationship between obesity and physical function (Sharma, Lou, Cahue, & Dunlop, 2000; Sharma et al., 2001). In short, the effect of BMI on physical performance might differ according to severity of knee OA. Knee pain was not significantly associated with physical performance in the no-or-doubtful- and the severe-OA groups. First, pain might not have direct effect on physical performance. Knee pain reduces knee extension strength (Henriksen, Rosager, Aaboe, Graven-Nielsen, & Bliddal, 2011) Thus, it is reasonable to suggest that the deterioration of physical performance in patients with pain is not a result of pain itself but of decreased muscle strength caused by pain. Another plausible explanation may be the floor or ceiling effect of pain on physical performance. In the no-ordoubtful-OA, the knee pain seems to be too mild to have an influence on physical performance. WOMAC pain and activity induced knee pain extracted from the Knee Society score (KSS) showed different results. WOMAC is based on symptoms during the last 48 h, whereas KSS is based on those during the last 4weeks. Pain assessed by KSS can thus include pain of a few weeks ago, which would have little relationship with performance at the evaluation point. Previous studies of severe knee OA patients recruited their subjects from patients waiting for knee surgery. A few focused on various factors associated with preoperative performance but did not make comparisons with less-severe OA. Barker et al. investigated the associations among radiographic findings, self-reported function, observer-rated performance, pain, and lower extremity power. Only lower extremity power was strongly correlated with performance, whereas the pain and self-reported function showed moderate correlations (Barker, Lamb, Toye, Jackson, & Barrington, Table 4 Stepwise logistic regression model on poor physical performance (SPPB 9) by radiographic knee-OA severity.

Total (74.9%) Age WOMAC pain BMI Muscle strength No-or-doubtful knee-OA (81.7%) Age CES-D Muscle strength Minimal-to-moderate knee-OA (72.1%) Age WOMAC pain BMI Muscle strength Severe knee-OA (74.6%) Muscle strength

OR

95% CI

p-Value

1.07 1.10 0.92 0.79

1.04–1.10 1.05–1.16 0.86–0.98 0.73–0.84

<0.001 <0.001 0.013 <0.001

1.05 1.04 0.74

1.00–1.11 1.00–1.08 0.66–0.83

0.032 0.051 <0.001

1.09 1.12 0.87 0.81

1.05–1.14 1.03–1.21 0.79–0.96 0.73–0.90

0.000 0.007 0.007 0.000

0.72

0.58–0.89

0.002

2004). Brown et al. studied the predictors of various functional tasks of the lower extremity. The regression equations identified strength as the most significant predictor of the performance, whereas age, BMI, and ROM were significant only in some tasks. Despite the difference in performance assessment tools, the results of previous investigators are similar to ours. However, lack of comparison with a less-severe knee OA group may fail to identify the factors that clinicians should focus on in patients with severe knee OA. Some studies conflict with our results. Kauppila et al. evaluated disability and associated factors in old patients with advanced knee OA (Kauppila et al., 2009). The linear regression model for the WOMAC score showed that knee strength was not a significant indicator, whereas pain was. This is because self-reported function is influenced more by pain than are performance-based measures in knee OA patients (Terwee et al., 2006). Additionally, previous studies have defined severe knee OA patients as those awaiting knee surgery, whereas we focused on patients with radiographically severe knee OA. Consequently, there are some limitations when comparing our study with others. Our study has some shortcomings. First, it might have failed to identify other meaningful predictors due to the small sample size of severe OA patients (n = 71). A larger sample size may have identified other meaningful variables. The small sample size also precluded us from analyzing the factors associated with physical performance in men and women. Muscle strength and the prevalence of knee OA differs between men and women, and analyses by gender would have given us a deeper insight. However, the sample was sufficient to suggest that poor muscle strength was a significant predictor of poor performance. Second, no causal relationship can be drawn because of the cross-sectional study design. The modifiable variables that truly contribute to maintaining good physical performance remain to be revealed by a longitudinal study. The second-wave evaluation of the parent cohort study, KLoSHA, is in process. The relationship between muscle strength and the change in physical performance after some time period, along with further analysis of other variables in the severe-OA group, will reveal more detailed information on the true determinants of physical performance. 5. Conclusions The factors of physical performance in the geriatric population differ according to the radiographic knee OA severity. Among the modifiable factors, muscle strength was the only factor that was related to physical performance regardless of radiographic knee OA severity. Thus, to improve the physical performance of severe OA patients, the strategy used should differ from that for patients with less-severe OA. Muscle strengthening in severe OA patients should be emphasized as a targeted treatment program.

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