Quadriceps Weakness and Osteoarthritis of the Knee

Osteoarthritis of the knee is the most common cause of chronic disability among older persons in the United States [1]. In persons with symptomatic osteoarthritis of the knee, quadriceps muscle weakness is common and is widely believed to result from disuse atrophy secondary to joint pain. Although exercises to strengthen the quadriceps may relieve joint pain in persons with osteoarthritis of the knee [2-6], the role of periarticular muscle weakness in the pathogenesis of joint pain and disability in these persons is poorly understood. The basis for the beneficial effect of strengthening exercises is unclear, and the duration of the improvement has not been studied. Furthermore, the possibility that muscle weakness is an etiologic factor underlying the pathologic changes of osteoarthritis has seldom been considered. Elucidation of the role of muscle weakness in osteoarthritis is particularly important given our growing understanding of safe and effective methods for increasing strength in elderly persons [7, 8]. A substantial proportion of persons who have radiographic evidence of osteoarthritis of the knee have no joint pain [9]. Because asymptomatic persons with radiographic changes seldom seek medical attention for osteoarthritis, muscle weakness has not been studied previously in this group. Thus, it is not known whether quadriceps weakness precedes or follows joint pain or (if it follows joint pain) whether it is mediated by disuse atrophy or by physiologic mechanisms that may inhibit muscle contraction [10]. To address this issue, we studied the relation among lower-extremity muscle strength, lower-extremity lean tissue mass, and osteoarthritis of the knee in men and women 65 years of age and older. Methods Study Group To obtain a sample of elderly persons living in the community, we conducted brief telephone interviews with residents of households in central Indiana. Potential participants were selected through modified random-digit dialing to increase the sampled proportion of persons 65 years of age and older. Persons were eligible if they met the minimal criteria for participation: They were willing and able to provide informed consent and to undergo the necessary strength assessments and other evaluations. Persons were excluded if they had had amputations of both lower extremities, had undergone total knee arthroplasty, or had recently had a cerebrovascular accident or myocardial infarction. A total of 462 persons (approximately 55% of all who were eligible) agreed to participate and completed the following evaluations. Evaluations Radiography of the Knee Standing anteroposterior and lateral radiographs of both knees of each study participant were obtained, and the severity of osteoarthritis in the tibiofemoral compartment was graded by a musculoskeletal radiologist according to the criteria of Kellgren and Lawrence. Similar criteria, based on the presence of osteophytes and joint space narrowing, were used for the patellofemoral compartment [11]. The radiologist was blinded to the clinical status and characteristics of all patients. A participant had to have a Kellgren and Lawrence grade of 2 or more in either knee to be classified as having osteoarthritis. Knee Pain and Function The Western Ontario and McMaster Universities Arthritis Index was used to evaluate knee pain and function [12]. This index assesses the severity of knee pain during 5 activities or situations (walking on a flat surface, going up or down stairs, at night while in bed, sitting or lying, and standing upright) and the severity of impairment of lower-extremity function during 17 activities. Pain and functional impairment were assessed in each knee separately. Responses to each question about the severity of knee pain and level of impairment were recorded on a categorical scale as none, mild, moderate, severe, or extreme. Each category was assigned a corresponding numeric score from 1 to 5 (5 = extreme). Hence, the range on the pain scale was 5 to 25 and the range on the physical impairment scale was 17 to 85 (85 = greatest functional limitation). For the purposes of analysis, participants who rated the severity of their knee pain as moderate or greater (3) with any of the 5 activities on more than half of the days in the month preceding the evaluation were considered to have knee pain. Thus, pain in the more distant past that had resolved was not included. Participants were also questioned about current and previous regular (5 times per week) or occasional use of over-the-counter and prescription analgesics and nonsteroidal anti-inflammatory drugs (NSAIDs) in the past year. Lower-Extremity Muscle Strength The strength of each leg was evaluated by using an isokinetic dynamometer (KIN-COM 500H, Chattecx Corp., Hixson, Tennessee). Peak torque was recorded in both the concentric (contractions during muscle shortening) and eccentric (contractions during muscle lengthening) modes. Participants were allowed several submaximal or maximal practice efforts to familiarize themselves with the operation of the dynamometer. Once formal testing began, the best of three maximal efforts was recorded for flexion and extension at both 60 degrees per second and 120 degrees per second. Aborted efforts were repeated in order to obtain the best possible representation of strength for each participant. Concentric and eccentric testing yielded similar results, but because of greater variability in eccentric testing, only the concentric test results are shown. Lower-Extremity Lean Tissue Mass Total-body dual-energy x-ray absorptiometry was done in all participants by using a Lunar-DPX-L instrument (Lunar Corp., Madison, Wisconsin). Results were analyzed for total and regional body composition, including body fat, mineral, and lean components (lean components were components other than fat or mineral). The right and left lower extremities were analyzed separately. The lower extremity was defined as all tissue below a diagonal line drawn outward and upward from the groin area through the femoral neck. Statistical Analysis Participants were divided into four groups on the basis of presence or absence of radiographic evidence of osteoarthritis of the knee and presence or absence of knee pain, as defined above. Men and women were compared by using the t-test. Comparisons of Arthritis Index pain and functional impairment scores were done by using nonparametric approaches. For analyses of continuous data involving more than two groups of participants (for example, osteoarthritis with or without knee pain), analysis of variance was used to determine whether an overall difference was present. The Fisher protected least-significant-difference procedure was used for pairwise comparisons. Comparisons within participants (for example, comparison of the two legs in a person with unilateral osteoarthritis of the knee) were done by using paired t-tests. Regression models were constructed with the generalized estimating equations approach of Zeger and Liang [13]. This approach inflates the standard errors to adjust for correlations in both independent variables (such as strength) and dependent variables (such as radiographic grade) within participants. Statistically significant differences (P < 0.05) in the above analyses are specifically noted below. Results The characteristics of the 462 men and women in the cohort are shown in Table 1. As expected, men were taller, were heavier, and had greater lower-extremity strength and lean tissue mass in the lower extremities compared with women (P < 0.001 for all comparisons). Table 1. Age, Height, Weight, and Lower-Extremity Strength and Lean Tissue Mass* One hundred forty-five participants (31%; 33% of the women and 30% of the men) had radiographic evidence of osteoarthritis involving the tibiofemoral compartment, the patellofemoral compartment, or both. In 62 participants (43%), the radiographic changes were unilateral. Table 2 shows the association between osteoarthritis and obesity [14-16]. Women in the cohort who had osteoarthritis were approximately 15% heavier than women with normal radiographs and no knee pain. Men with osteoarthritis were also slightly heavier than men without osteoarthritis. Table 2. Body Weight and Summed Arthritis Index Scores for Recent Pain and Function in the Left Knee in Participants with and without Radiographic Evidence of Osteoarthritis* Among those with radiographic evidence of tibiofemoral osteoarthritis, women were slightly more likely than men to report knee pain (P = 0.10; Table 3). Table 3. Radiography and Recent Pain in the Left Knee Table 2 also shows the mean summed and the distribution of scores for left knee pain and functional impairment (data for the right knee were similar). Among men and women with radiographic evidence of osteoarthritis who reported having knee pain, the mean summed pain score for the knee with osteoarthritis was approximately 12 (median score, 2 of 5). In comparison, the mean pain score of participants who reported knee pain but did not have radiographic evidence of osteoarthritis in the painful knee was approximately 10 (median score, 2 of 5)-only slightly lower than the mean pain score of participants with radiographic changes. Consistent with their relatively low pain scores, these community-dwelling participants with osteoarthritis reported moderately low use of NSAIDs (Table 4). Table 4. Participants Reporting Regular Current or Previous Use of Analgesics and Nonsteroidal Anti-inflammatory Drugs Related to the Presence of Radiographic Evidence of Osteoarthritis of the Knee and Recent Knee Pain* Arthritis Index scores for functional impairment paralleled those for pain (Table 2). Participants with osteoarthritis had the greatest functional impairment (P < 0.001 for the comparison with patients who did not have pain or radiographic evidence of osteoarthritis). Functional impairment in participants who had pain but no radiographic evidence of osteoarthrit