Knee osteoarthritis is a disease characterized by deterioration and inflammation of the knee joint.6 Knee OA can be identified with radiographic evidence or by symptomology. The Kellgren-Lawrence (KL) grading scheme is the radiographic reference standard for defining knee OA.6 The severity of osteoarthritis is determined by the presence of osteophytes on the knee joint, joint space narrowing, sclerosis of the subchondral bone, and deformity of the femoral head.7 Scoring of the KL grading scheme ranges from 0 to 4. Minimal, but definitive, knee OA is defined by the presence of an osteophyte, equating to a KL grade of 2 or higher.6,7,10 Symptomatic knee OA is characterized by radiographic evidence of OA accompanied by pain, aching, or stiffness in the affected joint.
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Pain occurring on most days of a recent month is representative of symptomatic knee OA.2,6,8
Data from the 2011-2012 National Health Interview Survey (NHIS) indicates that 15.1 million United States (U.S.) adults have symptomatic knee OA. An additional 8.6 million adults are classified with KL grade 3 or 4 advanced symptomatic knee OA.1 The prevalence of knee OA has dramatically increased in the past one hundred years. Wallace et al. compared the prevalence of knee OA among American adults from the early industrial and postindustrial periods. The prevalence of knee OA (16%, 95% CI: 14-19%) was 2.6 times higher among the postindustrial sample (95% CI: 2.1-3.4) compared to the early industrial sample (6%, 95% CI: 5-7%). 10
Prevalence data for knee OA differs across studies depending on the definition of osteoarthritis. Studies using the radiographic definition of osteoarthritis trend towards higher prevalence as compared to the symptomatic definition.4,11,12 In the Framingham Osteoarthritis study, 27.4% of adults over sixty-three had radiographic knee OA. However, only 7% of adults within this sample had symptomatic OA.9,13 Similarly, the prevalence of radiographic knee OA was almost 10% higher than the prevalence of symptomatic knee OA among middle-age adults in the Johnston County OA Project (27.8% vs. 16.7%).9,11 The population being studied can also impact estimates of knee osteoarthritis. Whereas two studies found that non-Hispanic whites had the highest rates of knee OA, other articles reported highest rates in African Americans.1,9,11,13
Age and sex also influence the prevalence of knee OA. Osteoarthritis is a chronic disease whose prevalence increases across the lifespan. The prevalence of knee OA increases from 1-2% in adults 25-44 years old to 14% in adults over 65 years.1,9,13 According to Murphy et al., the overall lifetime risk of developing symptomatic knee OA by age 85 is 44.7%.2 Across age groups, females are more likely to have knee OA and have more severe knee OA when compared to men. Women over 50 years old were 1.68 (RR 95% CI 1.37-2.07) times more likely than men over 50 years old to have knee OA.5 Furthermore, diagnosed knee OA in women over 55 years old was more severe than knee OA in men of the same age range (standardized mean difference 0.20, 95% CI 0.11-0.28).14
Knee osteoarthritis places a large strain on a country’s economy. U.S. adults spend roughly 28 years living with symptomatic knee OA.15 Treatment for knee OA comes in various forms and at various price points. The minimum treatment strategy consisting of a physician visit, physical therapy, knee braces, and medications costs $684 annually.15 Many adults will spend years using medications and non-surgical treatments before seeking advanced care. The standard care treatment for end-stage knee OA is total knee arthroplasty (TKA).16 The TKA is widespread due to its life expectancy and relative safety. In 2010, 658,000 knee arthroplasties were performed, with 92% of surgeries being TKAs.16
Direct and indirect medical costs associated with knee OA create a large burden on the individual and on society. As of 2013, a primary TKA cost $20,293 in the United States.15 Individuals undergoing revision TKA spend an average of $49,360 and 5 days in the hospital.16 On a larger scale, the hospital charges associated with revision knee arthroplasties accounts for nearly $2.7 billion in U.S. healthcare spending.16 From a national viewpoint, the U.S. spends $139.8 billion dollars annually (95% CI 83.7-195.9) on direct costs attributable to knee OA.17 Indirectly, the disability associated with knee OA impacts workplace productivity. The U.S. loses nearly $164 billion each year (95% CI, 85.2-242.3) in earnings attributable to arthritis. Between direct medical costs and productivity loses, arthritis accounts for $303.5 billion in healthcare spending.17
Predisposing factors for knee OA are divided into systemic risk factors and local risk factors. Systemic risk factors include age, gender, race/ethnicity, and genetics. Age is one of the strongest systemic risk factors for knee OA due to loss of muscular strength, thinning of articular cartilage, and oxidative damage to the knee joint.6 Local risk factors for knee OA include previous injury, physical activity, and obesity. The risk of developing knee OA increases by 183% (RR 2.83, 95% CI 1.91-4.19) in adults who have suffered a previous knee injury when compared to adults who have not had a previous injury.5 Roughly 5% of cases of knee OA could be avoided if knee injuries could be prevented in adults.5
The relationship between knee OA, physical activity, and overweight/obesity is more complicated. Adults engaging in at least four hours per day of heavy physical activity were significantly more likely to have radiographic knee OA when compared to adults engaging in no heavy physical activity (OR 7.0, 95% CI 2.4-20, p<0.001).18 Further analysis of the influence of weight status revealed the highest risk of developing radiographic knee OA was in obese adults engaging in heavy physical activity (OR 13.0, 95% CI 3.3-51) .18 Although frequent and heavy physical activity may be associated with knee OA, a sedentary lifestyle and subsequent weight gain may also elevate the risk for developing knee OA.
Overweight and obesity is a strong predictor of knee OA in adults. Being overweight or becoming overweight as an adult increases the lifetime risk of developing symptomatic knee OA by nearly 30%.2 The literature often uses the World Health Organization’s (WHO) classification of overweight and obesity, which defines overweight as a body mass index (BMI) of ‰? 25 – ‰¤ 29.9 kg/m2 and obese as a BMI ‰? 30 kg/m2.19 Evidence suggests there is a curvilinear relationship between BMI and risk of knee OA. One meta-analysis quantified the pooled risk of developing knee OA as a factor of increasing BMI across twelve studies. Having a BMI of 25 kg/m2 is associated with a 59% (RR 1.59, 95% CI 1.34-1.81) increased likelihood of developing knee OA.20 A second study found one’s likelihood of developing knee OA to rise by 35% (RR 1.35, 95% CI 1.18-1.53) for each 5 kg/m2 increase in BMI.21
Disease progression and severity are also affected by weight status. A prospective study of 354 adults found overweight individuals to be 9.1 (95% CI, 2.6-32.2) times more likely to develop K/L grade 1+ knee OA within 5 years. Additionally, being overweight increased the likelihood of progressing to moderate knee OA (K/L grade 2+) within 5 years by 18.3 times (95% CI, 5.1-65.1).22 In addition to disease progression, weight status affects an individual’s symptoms. Being overweight or obese is associated with a 2.66 (95% CI, 2.03-3.48) times higher risk of being diagnosed with symptomatic knee OA.23 Overweight (OR 1.69, 95% CI 1.36-2.11) and obese (OR 2.58, 95% CI 1.74-3.58) individuals are also more likely to experience persistent knee pain than normal weight adults.23
Although being overweight or obese increases the likelihood of developing knee OA, weight reduction can improve clinical and functional outcomes. The Osteoarthritis Research International (OARSI) guidelines committee recommends weight loss as a core management strategy for overweight and obese patients with knee OA (strength of recommendation 96%, 95% CI 92-100).24 Weight loss has been shown to have positive effects on pain, joint stiffness, and disability in adults with knee OA. A meta-analysis of four randomized controlled trials (RCTs) reported a small improvement in knee pain resulting from weight loss (ES = 0.20, 95% CI 0-0.39). The literature supports a dose-response relationship between the amount of weight lost and reductions in knee pain. An intensive 18-month diet and exercise intervention found significantly lower self-reported pain scores with at least 10% (3.71, 95% CI 3.09-4.34) and 20% (2.79, 95% CI 1.87-3.71) weight loss when compared to <5% weight loss (4.46, 95% CI 3.81-5.11, p = 0.006).25
Knee joint stiffness is also alleviated by weight loss in overweight adults with knee OA. The results of two RCTs support a small decrease in joint stiffness as a result of weight loss (ES = 0.36, 95% CI -0.08, 0.80).24 Reductions joint stiffness associated with weight loss may also be related to improvements in physical function. A meta-analysis of three RCTs found an average weight loss of 6.1kg (95% CI 4.7-7.6kg) was associated with a small improvement (ES = 0.23, 95% CI 0.04-0.42) in self-reported disability.26 The relationship between weight loss and increases in physical function is also supported by objective measures. Overweight adults with knee OA who lost an average of 5.2kg (95% CI 0.85-9.55) demonstrated significant improvements in both six-minute walk distance (?” from baseline = 61.61m, 95% CI 35.90-87.32) and stair climb time (?” from baseline = -2.54s, 95% CI -4.13, -0.95).27
Weight loss should not be considered independently of physical activity. The OARSI group recommends regular aerobic activity and resistance training as an adjuvant treatment for knee OA.24 Engaging in regular aerobic activity can serve as a non-pharmacologic pain reliever for individuals with knee OA. A meta-analysis of 13 RCTs on aerobic activity has shown support for a moderate improvement in pain (ES = 0.52, 95% CI 0.34-0.70).24 Aerobic activity can also improve physical function in people with knee OA. Individuals with knee OA who take part in aerobic activity experience moderate improvements in self-reported disability (ES = 0.46, 95% CI 0.25-0.67).24 Muscle strengthening has also been identified as a core recommendation for the management of knee OA. Small reductions in pain (ES = 0.32, 95% CI 0.23-0.42) and self-reported disability (ES = 0.32, 95% CI 0.23-0.41) have been reported with quadriceps training.24
Multiple studies have examined the relationship between weight loss and clinical outcomes in overweight adults with knee OA. The Arthritis, Diet, and Activity Promotion Trial (ADAPT) was an 18-month RCT comparing standard care to dietary weight loss, exercise, or combined diet plus exercise on physical function, pain, and mobility. Individuals in the diet plus exercise group lost the largest amount of body weight (mean 5.20kg, 95% CI 0.85-9.55), followed by those in the diet only group (mean 4.61kg, 95% CI 0.38-8.84) when assessed at 18-month follow-up.27 Physical function, pain, and mobility were significantly improved for individuals in the diet plus exercise group after 18 months. Weight loss of 5% baseline body weight was associated with a 24% reduction (95% CI 2.63-8.83) in physical function scores and a 30% reduction in pain scores. Additionally, mobility improved as noted by increased 6-minute walk distance (477.76 ± 13.12m, p < 0.05) and reduced stair climb time (8.45 ± 0.81 seconds, p < 0.05).27
Whereas a weight loss goal of 5% was established for the ADAPT study, the Intensive Diet and Exercise for Arthritis (IDEA) study had a weight loss goal of at least 10% body weight.28 IDEA was an 18-month RCT that compared the effects of dietary weight loss, exercise, or diet plus exercise on clinical outcomes. Participants in the diet plus exercise group lost an average 10.6 kg (95% CI -14.1, -7.1) compared to an average 1.8 kg (95% CI, -5.7-1.8) loss in the exercise only group.28 Although IDEA participants in the diet plus exercise group had a higher percentage of body weight loss, a similar trend was reported for physical function, pain, and mobility scores after 18 months. Physical function scores decreased by nearly 42% (?”_18month = -10.3, p <0.001), pain scores decreased by 45% (?”_18month = -3.0, p = 0.02), and 6-minute walk distance increased by 15% (?”_18month = 70, p <0.001).28
Rather than analyzing the impact of weight loss on clinical and functional outcomes, one study analyzed whether weight loss could prevent the onset of knee pain in overweight adults with diabetes. White and colleagues randomized participants to one of two treatment conditions: 1) intensive lifestyle intervention focusing on aerobic activity and weight loss, or 2) diabetes education and support comparison group. Individuals randomized to the intensive exercise and weight loss program were 15% (RR = 0.85, 95% CI 0.74-0.98) less likely to develop knee pain within 1-year when compared to individuals in the comparison group.29 Participants who met the 7% weight loss target were 37% (RR = 0.63, 95% CI 0.46-0.88) and 38% (RR = 0.62, 95% CI 0.47-0.83) less likely to develop knee pain within 1 and 4 years, respectively.29 Collectively, these three studies strengthen the conclusion that the combination of dietary weight loss and exercise is associated with positive clinical outcomes.
Due to the strong evidence of the benefits of weight loss in overweight adults with knee OA, researchers have sought to identify predictors of weight loss. Predictors of weight loss can be grouped as individual factors or factors pertaining to the intervention.
Self-efficacy is the belief in one’s ability to accomplish a specific task or goal.30 Within the weight loss literature, self-efficacy can divided into exercise self-efficacy and dietary self-efficacy. Having a high exercise self-efficacy at baseline was associated with achieving the 7% weight loss target among adult participants involved in the Diabetes Prevention Program (DPP).31 Among overweight adult women, both exercise self-efficacy (r = -0.32, p < 0.001) and eating self-efficacy (r = -0.36, p < 0.001) were associated with 1-year weight change. However, only exercise self-efficacy (r = -0.32, p < 0.001) remained as a significant predictor of weight loss after 2 years.32 Not only are baseline levels of self-efficacy important, but also the change in self-efficacy can have an impact on weight loss. Individuals whose diet self-efficacy improved over 6 months were predicted to reach their target weight loss goal of 7% by the end of the study.31 Another study reported an increase in exercise self-efficacy over 12-weeks as a significant predictor of weight loss (??= -0.443, p = 0.008).33 Among overweight adults, believing in oneself despite barriers or obstacles may bolsters one's success at losing weight.
An individual’s general affect can influence their willingness to engage in behaviors that may lead to weight loss. Pre-treatment depressive symptoms may influence weight loss if the individual’s negative affect stems from unhappiness with their current weight status. Among obese adults, lower levels of baseline depressive symptoms are associated with weight loss success after 1 year.34 Additionally, baseline depression scores among overweight, older veterans with knee OA were predictive of weight loss after a 6 month intervention.35 It should be noted, however, that both studies used inventories measuring depressive symptoms rather than identifying individuals with clinical depression. Participants who are clinically depressed are often excluded from weight loss studies because they may be taking medications that impact their ability to lose weight.36
The social aspect of participation in a research study may be a contributing treatment-related factor for weight loss. Individuals who participated in a group-based commercial weight loss program lost more weight than individuals who participated in one-on-one dietary counseling sessions (-4.3 ± 6.1 kg vs. -1.3 ± 6.1 kg, p < 0.001) after one year.37 Although group support can be beneficial, an individual's desire for autonomy in regards to weight control may also be an important factor. Participants who perceived research staff to be supportive of their autonomy during the weight loss process were predicted to have a lower BMI after the 6 month intervention (?? = -0.11, p < 0.05).38 Overall, the shared experience of an intervention and the level of support provided by other participants and staff may increase an individual's willingness to continue engaging in the intervention.
The relationship between attendance at a diet or exercise intervention and weight loss is intuitive for the general population. As a result, studies have sought to find evidence to support this conclusion. Among overweight adults receiving a modified version of the Diabetes Prevention Program (DPP), attendance at counseling sessions was a significant predictor of weight loss (?? = -0.609, p = 0.003).33 Evidence from a RCT comparing self-help to a commercially based weight reduction program for adults found higher self-reported attendance at sessions was significantly associated with weight loss after 1 and 2 years (p < 0.05).37 A similar trend in attendance as a predictor of weight loss is seen among overweight adolescents who participated in a 4-month activity intervention (??= -0.28, p < 0.05).39 The evidence generated by these studies may suggest that simply being present at the intervention will be associated with weight loss.
To date, there is no consensus on the operational definition of adherence. In simplistic terms, adherence is defined as a percentage of the number of sessions completed divided by the total number of sessions offered.27,28,40 However, more complex definitions of adherence necessitate the need to first answer the question: adherence to what? Exercise adherence can be defined by attendance rates and by goal achievement. Two community-based RCTs in overweight adults with knee OA have used on-line attendance logs to track exercise adherence.27,28 Among adult participants in the Healthy Weights Initiative, meeting the goal for exercise frequency, intensity, or duration was the criteria for exercise adherence.41
Adherence to a dietary intervention depends on the behavior being measured. Multiple studies have used attendance rates to define adherence to a diet class.27,28,41 Other studies have defined adherence by the frequency of food tracking over a specific period of time. Jacobs and colleagues defined adherence as logging a meal within its specified time frame, but also separated from a consecutive record by at least 30 minutes.40 A separate study expanded the time constraint of an adherent period to no longer than 4 days between consecutive food logs.42 Aside from defining adherence by attendance rates frequency of food tracking, adherence may also be defined as an individual’s agreement to the prescribed dietary intervention. One study defined dietary adherence as the percentage of discrepancy between one’s actual daily kilocalorie loss divided by their expected daily kilocalorie loss.43 The definition of adherence is often driven by the way that adherence is measured.
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