By Erin Boutwell
Osteoarthritis (OA) risk factors and symptoms seen in adults have been found in obese kids, who often have musculoskeletal pain. Weight loss may help, but preventing OA may also require gait and exercise interventions, particularly those that reduce pain that leads to inactivity.
Osteoarthritis is an extremely common musculoskeletal disorder. In 2008, an estimated 27 million US adults had clinical OA in one or more joints.1 OA may be caused by mechanical, systemic, and genetic factors.2 It represents the failure of a joint caused by abnormal mechanical loads, and the ability to comprehend this disorder requires a biomechanical perspective.3
No cure exists for OA,4 though clinicians often emphasize weight loss and exercise because of the clear association between musculoskeletal issues and obesity in adults.5-7 However, the applicability of adult-centered research findings to the treatment of obese children—who do not yet have OA but appear to be at increased risk—is problematic.
Simone Gill, PhD, OTR/L, assistant professor in the Department of Occupational Therapy and director of the Motor Development Laboratory at Boston University, outlined a major issue in extending adult OA research findings to a pediatric population. “Children are still growing and developing, and we know very little about how obesity impacts changes in their development,” she said.
Because the rising worldwide population of persons classified as “overweight” (body mass index [BMI] ≥ 25 kg/m2 and < 30 kg/m2) or “obese” (BMI ≥ 30 kg/m2) is expected to contribute to increased OA incidence,4 it’s important to consider early intervention in heavy children and adolescents. Assessment of body fat in children is not calculated using an absolute BMI, as in adults, but rather using age- and gender-specific percentiles; in children aged 5 to 19 years, “overweight” typically corresponds to a BMI between the 85th and 94th percentile, and “obese” is typically associated with a BMI greater than the 95th percentile.8
No data have established a direct link between OA and obesity in children, but researchers are beginning to uncover evidence of musculoskeletal complaints and injuries that are precursors to adult OA in the younger obese population.
Obesity in children
Currently, the connection between OA later in life and childhood obesity is primarily hypothetical. Marienke van Middelkoop, PhD, assistant professor at the Erasmus MC Medical University Department of General Practice in Rotterdam in the Netherlands, offered an explanation of the mechanism by which obesity could be a risk factor for development of musculoskeletal issues in children.
“Obesity could indeed be a cause of increased strain on joints during weight bearing,” she said. “The [same] mechanism has also been suggested for the association between obesity and osteoarthritis of weight-bearing joints such as the hip and the knee.”
Research has linked knee OA to excessive forces transmitted across a joint because of high body weight, potentially leading to cartilage breakdown and progressing to OA.9 Loading alone, however, doesn’t explain why hand OA is also more prevalent in obese individuals.9
“There are also other potential disease pathways suggested [for obesity-related osteoarthritis], including inflammation and hormone secretion,” van Middelkoop said.
Children, in general, are at a high risk for musculoskeletal pain; studies have reported that between 18% and 32% of children experience pain at least once a week.10-12 The exact mechanism behind this pain is unknown, but it may be related to overuse.13 A systematic review coauthored by van Middelkoop suggests the occurrence of musculoskeletal pain may be even higher in heavier children than in the general population.5 In the review, van Middelkoop found the overall prevalence of musculoskeletal pain was 26% higher in overweight children than in normal-weight children, and that low back pain in particular was 42% more common in overweight children.
Perhaps the strongest link between obesity and musculoskeletal degeneration comes from a series of studies using magnetic resonance imaging (MRI) to examine the knee joints of morbidly obese children and adolescents.14,15 (Morbid obesity was defined as BMIs exceeding the 99.5th percentile for participants’ age and gender.) These studies, coauthored by Harald Widhalm, MD, of the Department of Trauma Surgery at the Medical University of Vienna in Austria, demonstrate the possibility that children aged as young as 9 years may show symptoms of joint degeneration. In a 2012 study, Widhalm and colleagues found that 100% of 20 morbidly obese children aged 9 to 19 years had cartilage lesions in at least one knee compartment.
“These [morbidly obese] patients often present with severe deviations of the leg, showing cartilage pathologies…in the medial joint,” said Widhalm, who went on to warn, “The fact that these patients are often developing cartilage lesions even at that young age is a big problem. We also do not know…if cartilage lesions in these patient types are reversible.”
Articular cartilage lesions have been related to long-term joint issues such as adult OA.16 While a direct causative link between these lesions and obesity has not been established, Widhalm et al reported their prevalence seems to correlate with musculoskeletal pain symptoms commonly found in obese children.17
Poor frontal-plane knee alignment can play a large role in joint loading. In a 2014 Rheumatology study van Middelkoop and others noted a significant increase in the risk of developing radiographic knee OA in middle-aged overweight women with knees in varus alignment (odds ratio of 3.3), finding a more modest association in women with valgus knee alignment (odds ratio of 2.8).18
Sarah Shultz, PhD, ATC, a lecturer in exercise and sport science at Massey University in Wellington, New Zealand, noted, “If the child is choosing to be in genu valgum in order to relieve some pressure medially…then they are inadvertently placing themselves at greater risk for osteoarthritis to the lateral aspect of the knee.”
Nor is the knee the only critical joint to consider in obese children. “The foot and ankle [complex] is the first line of defense against the extra ground reaction forces; combined with the architecture of the foot [small bones, ligaments, arches], it makes the foot highly susceptible to changes due to mass,” she said.
Another potential issue in obese children is that their increased body mass may result in gait compensations that exacerbate these lower limb malalignments and may increase the risk of OA.19 Gill elaborated, “Obese children tend to walk more slowly, and then try to stabilize their bodies…by keeping their feet on the ground for longer periods of time during the gait cycle.” Previous work documents that obese children may walk at slower speeds,20 increase their step widths,20 and experience higher joint powers at the hip and knee during loading response.21
Weight loss. Weight loss is commonly recommended in adults as a method of reducing abnormal loading on lower limb joints,9 and many doctors and scientists advocate multidisciplinary weight-loss approaches. Widhalm coordinates such weight reduction programs. “It is very important to have a multiprofessional team; otherwise, there is no chance to help these patients,” he said.
However, is weight loss alone enough?
Sharon Bout-Tabaku, MD, assistant professor of pediatrics at Nationwide Children’s Hospital in Columbus, OH, said, “The question is: Can you can make people’s outcomes even better in regards to pain and risk of developing osteoarthritis, if—along with weight loss—you can strengthen their muscles or if you can help train them to walk better?”
Exercise & physical therapy. Bout-Tabaku gave an example of a weight-loss concurrent therapy, suggesting, “Obese kids have a lot of truncal mass, and it’s possible that truncal mass, even though it improves with weight loss, still causes some instability. Thus, strengthening of the core and lower extremity muscle groups may help the downstream effects of injury to the knee.”
Gill also suggested that gait pattern deviations could be addressed through exercise interventions. “Changes in muscle quality can happen with strength training,” she said, then highlighted the importance of cardiovascular fitness. “Another possible benefit in terms of aerobic activity in particular is that it not only helps burn fat, but also contributes to increased blood flow in the brain, which might be related to some of the issues of motor planning in that population.”
Shultz observed, “I think it’s important to include physical activity in any weight-management program, but physical limitations should be considered. I am a big believer in resistance training for obese youth and think that activities like boxing or aquatics are great alternatives to the more conventional activities [such as running].” Alternative activities are important to consider in obese children because the higher mechanical joint powers [the rate of mechanical work performed at the joint] they experience during exercises such as running may reduce their motivation for regular participation.21
Orthotic devices. Studies have evaluated various orthotic interventions for reducing pain and redistributing load within the knee joint in adults with OA, but the efficacy of such devices in adult knees is inconclusive at best. Several studies have investigated the effects of lateral wedges on peak knee external adduction moment, a measure often used to approximate knee loading,22 and have reported peak moment reductions of 5.8%23 and 8.7%24 with a laterally wedged insole compared with a neutral insole during walking in subjects with medial knee OA. The effects of lateral wedges on pain, however, have been inconsistent.25,26 Also, there are few high-quality studies that have evaluated the use of knee bracing in patients with knee OA;27 however, some investigations have reported improved walking distance28,29 and pain relief28-30 with a knee brace compared with no brace.
The potential influence of orthotic devices is an as yet unexplored avenue of research in correcting joint malalignment or preventing OA risk factors in children in adolescents. “Both knee braces and wedged insoles reduce the medial knee joint load [in adults], but the preventive effects have never been studied,” noted van Middelkoop. “It is so far unknown if you can prevent osteoarthritis with these interventions in high-risk groups.”
Reversibility. The ability to reverse obesity-related disease states that occur in children and adolescents is an important topic to explore. Bout-Tabaku said drawing a distinction between musculoskeletal complaints and OA in children is key. She queried, “What does ‘disease’ mean? Does ‘disease’ mean just having pain? Or does ‘disease’ mean having osteoarthritis?”
She added, “We don’t have any data saying that kids already have osteoarthritis.” All that has been reported, she said, is the existence of “some lesions in obese children that may be similar to what people see in adults who have very early stage osteoarthritis,” a reference to Widhalm’s previously cited detection of articular cartilage lesions in morbidly obese children.14,15
Therefore, “reversing” OA in children may not be a meaningful goal. It may be possible, however, to reverse some of the musculoskeletal symptoms demonstrated by obese children, particularly those that have been associated with OA in adults: pain, cartilage damage, and gait deviations. But how reversible are these symptoms?
Investigators like Gill are optimistic about the ability of weight loss and exercise regimens to reverse some of the musculoskeletal disease and damage reported in obese children and adolescents, including poor muscle quality and gait abnormalities. Improvements in gait (eg, increased step lengths, increased walking speeds, and reduced step width) have been reported in adults who have undergone bariatric surgery in as little as three months postsurgery.31
Regarding the reversal of cartilage damage, though, Gill was not as optimistic. “If you decrease the load that is being applied to those joints, then you might be able to minimize the wear and tear that we all experience in terms of lost cartilage over time. Because obesity exacerbates that,” she said. “That [cartilage loss] might not be reversible, per se, but you can slow how much that is happening.”
Gill’s thoughts build on the findings of a seminal 2005 Arthritis & Rheumatism study in which the authors found that in adults, for each pound of weight loss, four fewer pounds of force are applied to the knee joint.32 However, a direct causative link between reduction in knee load and slowed disease progression has yet to be established.
Nor is it clear exactly where the threshold between reversible and permanent joint damage lies. Bout-Tabaku asked, “When you’re done growing, does obesity confer greater or less of a risk than when you’re still growing?”
The idea that a child’s stage of development could affect obesity-related risk factors is a finding of Bout-Tabaku’s 2015 Journal of Rheumatology paper, in which she reported that pubertal stage (measured by Tanner stage) influenced knee alignment in obese children.33 In particular, knee alignment was comparable between obese and normal-weight children at Tanner stages 2 and 3, but, at Tanner stages 4 and 5, she found a greater valgus alignment in obese compared with normal-weight children (See “Obese children develop knee malalignment as they mature,” LER: Pediatrics, May 2015, page 5).
Bout-Tabaku emphasized the importance of pain reduction in obese kids, saying, “Pain itself is a disease state. We know that obese kids have more prevalent pain than nonobese kids.” She found that 76% of obese adolescents participating in the Teen–Longitudinal Assessment of Bariatric Surgery (Teen-LABS) study had pain at baseline.34
The next, ongoing phase of the Teen-LABS study involves investigating whether weight loss can reduce the pain incidence in these children. Bout-Tabaku said, “If kids are having pain, they then go into this vicious cycle of being more sedentary and deconditioned, putting them at risk for falls, abnormal joint mechanics, and injury to their joints.” In adults, weight loss has been associated with a reduction in joint loading,35 inflammation,35,36 and pain.31,35,36
The big picture
A common theme among all researchers and clinicians in this area was the information gap with regard to OA risks in this young population. Even more mysterious is how to identify root causes versus downstream effects.
Gill is convinced longitudinal studies are needed to parse some of the cause-and-effect relationships proposed between obesity and poor motor function over time. Bout-Tabaku added that a longitudinal study tracking obese and nonobese individuals over time who start with a normal knee alignment could be extremely informative.
These numerous unknowns, combined with the detrimental effects of OA, make it clear that intervening early, before musculoskeletal problems develop, is key. As Widhalm puts it, “For sure, prevention of morbid obesity is the best strategy.”
Erin Boutwell is a freelance writer based in Chapel Hill, NC.
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