Our understanding of the etiology of knee osteoarthritis is quickly evolving. Knee osteoarthritis is a complex, and multi-factorial disease. One of the more robust risk factors for the development of knee osteoarthritis is obesity. This has been extensively examined over the last twenty years. Here is a brief review of some of the key papers published:
Gelber et al. (1999):
- 1180 male medical students
- Each 8 kg increase in weight as a young adult (aged 20-29 years) was associated with a 70% increase in risk of clinical knee OA more than 30 years later
Murphy et al. (2008):
- Longitudinal study of 3068 black and white women and men age ≥45 years living in rural North Carolina
- Radiographic, sociodemographic, and knee symptom data were measured at baseline (1990-1997) and first follow-up (1999-2003)
- Lifetime risk of symptomatic radiographic knee OA increased by 30.3% in individuals who were obese compared with those who had a normal BMI
Lohmander et al. (2009):
- Large, Swedish prospective cohort study (n = 27 960)
- BMI, waist circumference, waist-hip ratio, body weight and body fat percentage were monitored over 11 years
- A categorical score of ‘overweight’ in any of the above measurements was associated with the incidence of knee osteoarthritis, with BMI eliciting the strongest association.
- Relative risk ratio from 2.2-8.1
Blagojevic et al. (2010):
- Examined 36 papers reporting on BMI
- All studies demonstrated that being overweight or obese were risk factors for knee osteoarthritis
- Effect size for obesity as a risk factor for knee OA was reported to be I2 = 97%
- Random effects pooled odds ratio for obesity compared to normal weight was 2.63
Toivanen et al. (2010):
- Large, population based prospective study (n=823) with follow-up of 22 years
- Risk for knee osteoarthritis was 7 times greater for people with BMI > 30 kg/m2 compared to the control group with a BMI < 25
Mechanisms Linking Obesity & Knee OA
The mechanisms behind the association between obesity, knee OA and knee pain are multifactorial, involving mechanical load through the knee joint, as well as the systemic inflammation caused by cytokine production from adipose tissue (Roos & Arden, 2016).
In obese patients, excess weight increases joint loading. Excessive and abnormal joint loading have been suggested to be an important risk factor for the development of knee osteoarthritis (Andriacchi et al., 2015). Abnormal loads have been shown to alter the composition, structure, and mechanical properties of hyaline cartilage, leading to erosion of the surface cartilage (Sowers & Karvonen-Gutierrez, 2010). As a result, it is possible that the additional body mass can stress articular cartilage beyond the biological regeneration capabilities, potentially initiating degenerative changes over time.
Mechanoreceptors exist on the surface of chondrocytes, and it is speculated that the mechanical activation of these mechanoreceptors may increase expression of cytokines, growth factors and metalloproteinases. The latter would in turn produce mediators such as prostaglandins and nitrous oxide, and while not fully understood, these processes may lead to additional oxidative stress, inflammation, and tissue breakdown at the knee joint (Teichtahl et al. 2008).
There is also an important systemic component linking obesity and knee OA. Obesity is characterized as a state of chronic inflammation. C-reactive protein (CRP) is a marker of low-grade inflammation commonly associated with obesity. CRP is also associated with decreased cartilage volume and disease progression of OA (Spector et al., 1997; Sharif et al., 2000; Hanna et al., 2008). Moreover, Interleukin-6 and Interleukin-1, which are primary regulators of CRP, are also positively associated with knee joint space narrowing, another measure of structural degeneration (Stannus et al., 2010). Therefore, systemic inflammation caused by obesity may play an important role in OA progression. This could also potentially explain the positive association between obesity and hand OA, despite the hands not being weight-bearing joints (Carman et al., 1994; Oliveria et al., 1999).
Lastly, fat mass has been shown to be a risk factor for cartilage defects, which is a feature of early knee OA (Berry et al., 2010). This would suggest that fat mass, rather than lean mass, might drive the association. This is not entirely surprising based on the aforementioned relationship between obesity and knee OA. Adipose tissue is an endocrine organ that can secrete substances such as cytokines and adipocytokines (Pottie et al. 2006; Sowers and Karvonen-Gutierrez 2010). Elevated concentrations of adipocytokines such as leptin, resistin and adiponectin have been identified in the synovial fluid and plasma of osteoarthritis patients. The latter suggests the potential for these adipocytokines to influence cartilage homeostasis and therefore, have a role in the relationship between obesity and OA of the knee (Lee & Kean, 2012).
Does Weight Loss Improve OA?
Weight loss has been shown to be an effective strategy to reduce the risk and progression of knee OA, improve pain and function, and reduce certain inflammatory mediators. It has been proposed that, at the population level, approximately 30% of knee OA is avoidable with the reduction of BMI (Zhang et al., 2010). Unlike some unmodifiable risk factors (age, genetics, joint alignment), body weight is ‘easily’ (more on this below) modifiable and has the potential for a high impact at the population level.
Even modest reductions in BMI have been demonstrated to produce great reductions in disease risks. A two-unit drop in BMI has been shown to lower knee OA risk by 50% (Felson et al., 1999). Similarly, Aaboe et al. (2011) showed that for every 1 kg of weight loss, the peak knee load was diminished by 2.2 kg at a given walking speed. Since there is a clear association between knee joint load and disease progression (Miyazaki et al., 2002; Bennell et al., 2011), this is a promising finding for the beneficial impact weight loss may have on knee OA.
Weight loss also seems to have a protective effect on cartilage loss. A recent study demonstrated that participants who lost weight over 48 months showed significantly lower cartilage degeneration, as assessed with MRI. This indicates that rates of disease progression can be lower in those who achieve greater weight loss (Gersing et al., 2017).
As previously mentioned, weight loss can significantly improve pain and function in patients with knee osteoarthritis. A study by Messier et al. (2004) demonstrated that moderate exercise and weight loss significantly reduced OA associated pain while improving performance (i.e. 6-minute walk distance and stair-climb-time) and self-reported physical function (Messier et al., 2004). In addition, a systematic review and meta-analysis by Christensen et al. (2007) found that weight reduction significantly improved disability in obese patients with knee osteoarthritis, and also showed clinical efficacy on pain reduction. Lastly, among overweight and obese adults with knee OA, participants in the diet + exercise and diet groups had more weight loss and greater reductions in IL-6 levels (inflammatory mediator) than those in the exercise group after 18 months (Messier et al., 2013).
Given the above information, weight loss (if applicable) is an effective intervention that should be considered as first-line treatment for patients with knee OA given its potential to reduce the risk and progression of knee OA, improve pain and function, and reduce certain inflammatory mediators. That being said, if weight loss is not applicable to the patient (i.e. they already have a healthy BMI and/or body weight), education regarding the importance of weight management should be included nonetheless as part of the treatment.
Guiding thoughts for successful weight loss
Weight loss can be a daunting task for many individuals. Anecdotally, more people fail than succeed when it comes to losing weight and keeping it off, though some research studies claim to counter this narrative (Wing & Phelan, 2005). The research outlined above demonstrates compelling evidence that achieving weight loss in individuals with knee OA can be a impactful treatment. To round out this discussion we have briefly discussed the literature regarding weight loss intervention success, with the aim of providing a guiding set of principles that may help you implement weight loss with patients/clients of your own. Importantly, this will not be a discussion of the specifics regarding a weight loss regimen, but more of an overview of factors that can influence the success of weight loss and maintenance.
A review in 2005 consolidated many factors that can influence the success or failure of weight loss interventions (Elfhag & Rossner, 2005). Factors such as goal setting, early weight loss, physical activity levels, motivation and consistency were important in determining the success of a weight loss program. Additionally, factors like eating patterns, self-efficacy and stress are certainly worth considering but are beyond the scope of this article. Importantly, aim to find the factors that you feel comfortable influencing and the tools that make the biggest difference in success for the smallest investment. The 20% of the changes that lead to 80% of the results.
Weight lost regimens require the same realistic goal setting that is paramount to any successful rehabilitation program. In a qualitative report of 76 obese women, it was observed that maintaining weight after a weight loss period was strongly influenced by whether or not the individual reached a self-determined goal weight (Byrne, Cooper, & Fairburn, 2003). The take home message here is: aim to set specific, achievable and measurable goals with your patients. It is not uncommon for unrealistic goals to be set by patients (Foster et al, 1997), conscious or subconsciously. Therefore, be prepared to have a frank conversation and gently nudge them towards a more manageable goal, since achieving that goal can influence long term success.
Losing significant weight early in the plan can be a powerful motivator. In fact, successful weight loss and subsequent weight loss maintenance is predicted by relatively rapid initial weight loss (Nackers, Ross, & Perri, 2010). Those who lost weight at a rate of ≥0.63kg/week were 5.1 times more likely to achieve the goal of a 10% reduction in body weight by the 18-month follow-up (Nackers, Ross, & Perri, 2010). However, the challenge is to balance sustainability with early success, which is not easy. Consider starting by adding one component at a time (e.g. more physical activity) and wait for that to become consistent or less effective before adding more.
Although nutrition is incredibly important in a weight loss program, not everyone may be comfortable providing general nutritional guidance. However, physical activity is a powerful component that physical therapists can leverage to help their patients achieve weight loss. If you can, providing guidance on both topics or outsourcing the nutritional component to an expert will be most beneficial. A systematic review published in the International Journal of Obesity identified that using both diet and exercise in the weight loss intervention resulted in 20% more initial weight loss compared to diet alone (Curioni & Lourenco, 2005). Using both physical activity with basic nutritional guidance may allow for greater weight loss while reducing the magnitude of change required in each area of an individual’s lifestyle. This may be important for individuals who are resistant to large changes in lifestyle or behaviour. Smaller changes (eat more vegetables and try to exercise twice a week) can compound into meaningful weight loss over time and may feel like less of a burden. Finding the balance of additive (eat more veggies) and subtractive (eat fewer calories) approaches is no doubt an art, and it likely varies between individuals.
Motivation to lose weight can be transient, but if carefully stoked it can help the individual find success over the long term. Autonomous motivation, feeling intrinsically driven to achieve the weight loss goals set forth, is a predictor of adherence, initial weight loss and weight loss maintenance (Williams, Grow, Freedman, Ryan, & Deci, 1996). One way this could be fostered is to discuss the tangible benefits to weight loss. In knee OA populations, this could be as simple as discussing that achieving weight loss has a high probability of reducing knee pain, which is likely a reason they came to you in the first place. This of course, is on top of the astonishingly extensive list of other benefits that come with weight loss. But, knee pain is likely a proximate complaint and an easy lever to pull on with your patients.
Consistency in behaviour is a vital component to weight loss success. Two studies (Gorin, Phelan, Wing, & Hill, 2004; Wing & Phelan, 2005) reported that maintaining a dieting approach across the entire week is superior to intermittent dieting (e.g. diet during the week days but not the weekends). In practicality it is imperative that you allow for some planned leeway and anticipate social events that may make adhering to diet and exercise difficult. Thus, it may be useful to aim for ‘average’ goals (e.g. 3 servings of vegetables a day on average; 3 days of exercise per week on average). More importantly, emphasizing that small deviations from their weight loss plan is not synonymous with failure. In fact, this is very natural. As such, discussing tactics for quickly returning to their plan after a deviation is paramount. In general, using guilt or ‘punishment’ after your patient deviates from the plan is not a good approach.
It is clear, that encouraging weight loss as a means of conservative treatment for knee OA is a good idea. The more difficult question is how to go about guiding someone toward sustainable weight loss in a healthy manner. Using the power of physical activity in addition to healthy eating habits is an effective means of achieving this. However, the path to sustained weight loss is nonlinear and it is prudent to be clear about that early on. This involves setting expectations, discussing perceived barriers, and outlining achievable goals in the short and long term. Though directly counselling someone in weight loss may be on the fringe of a physical therapist’s scope, it is a powerful approach you can use. Done with empathy and conviction, it can make a world of difference in someone’s life, arthritic or not.
- Weight loss (if applicable) is an effective intervention that should be considered as a first-line treatment for patients with knee OA given its potential to reduce the risk and progression of knee OA, improve pain and function, and reduce certain inflammatory mediators.
- If weight loss is not applicable to the patient (i.e. they already have a healthy BMI and/or body weight), education regarding the importance of weight management should be provided.
- Weight loss is difficult and nonlinear. Spend time discussing this with your patients.
- Identify possible pitfalls and tools to avoid these situations or get back on track.
- Help set realistic goals.
- Leverage tangible motivations (i.e. reductions in knee pain, improvements in physical function, etc.).
- Help identify tools to be consistent in different environments (e.g. work, home and vacation).
If you liked what you read here and want to know more about the tools you can use to help your patients with weight loss, give this a share and comment below! Let us know your questions so we can address them in future articles.
Jesse Charlton, CSCS, MSc
PhD Student in Rehabilitation Sciences
Motion Analysis and Biofeedback Lab, University of British Columbia
Anthony Teoli MScPT
Registered Physiotherapist & Founder of InfoPhysiotherapy
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