Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health (Bray 1987, Lahti-Koski et al. 2010). A crude population measure of obesity is the body mass index (BMI) in which a person’s weight (in kilograms) is divided by the square of his/her height (in metres). The World Health Organization (WHO) classifies obesity also on the basis of BMI. A value of BMI of 30 (kg/m2) or more is generally considered as obese. A person with BMI ≥ 25 is considered to be overweight.
Those persons with BMI ≥ 40 are morbidly obese (WHO 2004).
Obesity has reached epidemic proportions all around the world. According to WHO (2015), worldwide obesity has more than doubled since 1980 and most of the world's population live in countries where overweight and obesity kill more people than underweight and malnutrition. There are epidemiological studies emphasizing that overweight and obesity are major risk factors not only for OA but also for a number of chronic diseases including cardiovascular disease, type 2 diabetes, hypertension, cancer and premature death. The high prevalence of obesity, combined with its concomitant cardio-metabolic risks, means that it is now a major global health challenge (Abelson and Kennedy 2004, Haslam and James. 2005, Nathan 2015). In recent dynamic mathematic model study, it has been estimated that the prevalence of obesity in the United States (US) appears to be levelling off, but the reasons behind this plateau situation remain unknown.
This model predicts that by the year 2030 there will be the following prevalence rates;
overweight - 28%; obesity - 32% and extreme obesity - 9% (Thomas et al. 2014).
2.3.2 Limitations of using BMI to measure obesity
The relationship between BMI and body fatness varies according to body composition proportions (Garn et al. 1996). For example, the percentage of body fat mass is higher in women than in men with similar BMI values. BMI is also known to vary with age, therefore the age group under examination will affect the results on the prevalence of obesity. In practice, body fatness has been noted to increase with aging, meaning that BMI may correspond to a greater body fat content in elderly persons compared with younger individuals (Ross et al. 1994, Gallagher et al. 1996).
The age-related change in height can also influence BMI. In adults, height is lost with normal aging. There is an average height loss of 3 cm occurring from age 30 to 70 years and this has been estimated to account for an artificial increase in BMI of 0.7 kg/m2 for men. In women, the height loss averaged about 5 cm over the same 40-year period, accounting for an increase of 1.6 kg/m2 in BMI (Sorkin et al. 1999).
It has also been shown that young athletes who may have higher body weight due to higher lean body mass, may seem to be misclassified as obese according to their BMI value (Jonnalagadda et al. 2004). On the other hand, patients undergoing knee arthroplasty are often elderly (Koskinen et al. 2008) and presumably not very athletic (Dahm et al. 2008).
Thus, despite its limitations, BMI provides a simple, useful and widely utilized population level measure of obesity in adults (WHO 2000).
The general WHO classification of BMI is shown in table 1.
Table1. WHO classification of body mass index. Modified from WHO Global database on body mass index 2015.
WHO Classification of BMI WHO BMI cut-off points kg/m2
Under weight < 18.5
Normal range 18.5 – 24.9
Overweight ≥ 25.0
Pre-obese 25.0 – 29.9
Obese class I 30 – 34.9
Obese class II 35.0 -39.9
Obese class III ≥ 40
2.3.3 Obese patients with TKA
The prevalence of obesity has increased during the past two decades in Europe and the adverse trends in obesity indicators have continued also in Finland in the 21st century.
(Berghöfer et al. 2008, Lahti- Koski et al. 2010, Lahti-Koski et al. 2012). A recent study from the United States reported age-adjusted obesity prevalence to be 32% for men and 36% for women (Odum et al. 2013). Obesity is a major risk factor for osteoarthritis of the knee (Felson et al. 1988). Furthermore, osteoarthritis limits activity leading easily to weight gain and therefore obese patients will also be over-represented in the patient population who are candidates for knee arthroplasty surgery (Böstman 1994). In two studies originating from United States, over half of the total knee arthroplasty participants were obese (Namba et al. 2005, Fehring et al. 2007).
2.3.4 Obesity and TKA outcome
Obesity has been linked to the development of osteoarthritis of the knee and since the incidence of obesity is increasing, the need for total knee arthroplasty (TKA) is likely to increase (Fehring et al. 2007, Harms et al. 2007). Higher BMI also leads to joint replacement treatment in younger age (Guenther et al. 2015).
The increased demand for knee replacement surgery for obese patients would be predicted to lead to an increase in the number of patients experiencing major complications some of which may even require a revision operation (Cornell 2015).
Although the BMI is widely considered to be a negative factor in TKA operations, the relationship between the BMI and postoperative outcome following TKA is still controversial. Despite the increasing probability of wound infections, risk of ligament injury and technical difficulties encountered during the operation, satisfactory results have been reported also for obese patients (Spicer et al. 2001, Lozano et al. 2015). Deshmukh et al. (2002) studied a group of 180 TKA patients; in their short 1 –year follow-up analysis it was stated that BMI as a measure of obesity had no adverse influence on the outcome of TKA. However, Rodriquez-Merchan (2015) conducted a meta-review of 41 articles and he concluded that 16 studies reported no adverse association between obesity and TKA outcome but in 24 studies, it seemed that obese patients experienced a poorer TKA outcome.
There is extensive evidence that patients with a BMI ≥ 30 kg/m2 are at a higher risk of exhibiting a lower range of motion, poorer functional scores, more dissatisfaction and a higher risk of developing complications following primary TKA. For this reason, obese patients are encouraged to lose weight before the TKA procedure (Shoji et al. 1990, Namba et al.2005, Amin et al. 2006).
TKA in morbidly obese patients has been associated with an increased rate of perioperative complications, including problems with wound healing, infection, and avulsion of the MCL. It has also been demonstrated that revision rates after primary total knee arthroplasty tend to be higher in obese patients (Winiarsky et al. 1998, Foran et al.
2004, Kerkhoffs et al. 2012, Gunst and Fessy. 2015). Jämsen et al. (2010) evaluated their large and unselected consecutive 2,647 knee arthroplasty series and found that patient-related factors such as comorbidity, poor preoperative clinical state and obesity were risk factors for postoperative prosthetic infections during the first year after the operation when total rates of superficial and deep prosthetic joint infections were 2.9 % and 0.8 % respectively.
Lübekke et al. (2016) compared non-obese and obese patients in their long-term follow-up study. This follow-follow-up investigated a total of 3,438 TKA patients. In that study, BMI ≥ 35 (Obese class II) and body weight more than 100 kg were identified as the threshold for a significant increase in the early postoperative infection rate. They also suggested that BMI ≥ 35 or weight ≥ 100 kg may serve as a cutoff for higher perioperative dosage of antibiotics.
Super-obesity (BMI > 50 kg/m2) has also been described as a BMI subgroup in the literature. Werner et al. (2015) noticed that super-obesity was associated with dramatically increased rates of postoperative local and systemic complications after TKA compared to non-obese, obese and even morbidly obese patients.
2.4 BONE MINERAL DENSITY AND KNEE ARTHROPLASTY