Abstract

Introduction: Exercise therapy is effective in improving symptoms of knee osteoarthritis, but its effect on structural change remains unclear.
Purpose: To describe the associations between physical activity and structural changes of the knee joint as assessed by magnetic resonance imaging (MRI) in adult male and female subjects.
Methods: A convenience sample of 325 subjects (mean age 45 yr, range 26-61) was measured at baseline and approximately 2 yr later. Measures of physical activity included questionnaire items, physical work capacity (PWC₁₇₀), and lower-limb muscle strength. Knee cartilage volume, tibial plateau area, and cartilage defect score (0-4) were determined using T1-weighted fat saturation MRI.
Results: Lower-limb muscle strength at baseline was positively associated with both percent-per-year changes in total cartilage volume (r = 0.13) and lateral and total tibial plateau area (r = 0.15 and 0.17) but not other sites. Change in muscle strength was negatively associated with annual changes in lateral and total tibial plateau area (r = -0.13 and -0.17). In females only, PWC₁₇₀ at baseline was negatively associated with percent-per-year changes in lateral and total cartilage volume (r = -0.16 and -0.17) and positively for lateral and total tibial plateau area (r = 0.18 and 0.16). Conversely, change in PWC₁₇₀ was positively associated with changes in cartilage volume at all sites (r = 0.24-0.26). For all associations, P < 0.05.
Conclusions: Overall, these associations were modest in magnitude, but they suggest that knee cartilage volume and tibial plateau area are dynamic structures that can respond to physical stimuli. Greater muscle strength and endurance fitness, especially in women, may be protective against cartilage loss, but it also may result in a maladaptive enlargement of subchondral bone in both sexes, suggesting that physical activity may have both good and bad effects on the knee.

Introduction

Osteoarthritis (OA) is a condition characterized by changes to the integrity of articular cartilage and subchondral bone. The knee is the most frequently affected joint, with a prevalence of 30% in people aged 65 yr and over ^[13]. Although exercise therapy is effective in improving symptoms of knee OA ^[30], the relationship between physical activity and structural change of the knee remains unclear.

Observational studies have suggested a higher risk of radiographic knee OA with repetitive, high-impact sports, and this risk is most strongly associated with joint injury ^[7]. A history of regular sports participation also has been shown to increase the odds of incident but not progressive radiographic OA (ROA) ^[8,14]. In contrast, moderate recreational physical exercise has been associated with decreased risk of knee OA requiring arthroplasty ^[22]. Most of these studies, however, are retrospective in nature and are subject to many biases. In randomized trials on animals, it has been repeatedly shown that exercise decreases the risk of developing OA of the weight-bearing joints ^[20,24,25]. Likewise, Slemenda et al. ^[29] have shown that increased quadriceps strength may be a protective mechanism guarding against the onset of medial-tibiofemoral OA.

Recently, novel imaging modalities such as magnetic resonance imaging (MRI) have made important contributions to our understanding of OA. We have reported both cross-sectional and longitudinal positive associations between physical activity and cartilage development in children ^[17]. Our group also has shown that increased muscle mass is strongly associated with medial tibial cartilage volume and a reduction in the loss of tibial cartilage ^[5]. On a similar note, glycosaminoglycan (GAG) content, a measure that may reflect cartilage quality, increased after 4 months of moderate exercise ^[26].

The controversy surrounding this issue warrants urgent attention because muscle-strengthening interventions are now a major component of the usual treatment program for patients with knee OA, despite the little information regarding their effects on disease prevention and/or progression. This is particularly relevant because both bone size and rate of cartilage loss have been identified as independent predictors of knee replacement in a longitudinal study ^[4]. Furthermore, there have been no studies in adults that have investigated the role of physical fitness on knee structural changes. The aim of this longitudinal study was to describe the associations between strength, endurance fitness and self-reported physical activity, and structural change of the knee joint in a convenience sample of adult male and female subjects.