May 2015

Obese children develop knee malalignment as they mature

5PEDS-news-obesity-iStock29518762-copyEffect might increase future OA risk

By Emily Delzell

As obese children undergo the rapid physical changes of puberty, they develop knee malalignment that could potentially contribute to development of knee osteoarthritis (OA), according to recent research from Nationwide Children’s Hospital in Columbus, OH.

“Children start in varus and, typically, around age seven or eight, progress to normal alignment. Our study found that obese children begin with slightly higher varus [than nonobese children] that moves to greater valgus alignment in late puberty,” said study lead author Sharon Bout-Tabaku, MD, assistant professor of pediatrics at Nationwide Children’s.

She and colleagues pooled data from three groups of children and adolescents: healthy participants aged 4 to 17 years; children aged 4 to 17 years taking part in a weight management program; and obese adolescents aged 13 to 17 years enrolled in a clinical trial of the weight loss drug sibutramine.

The investigators stratified participants into nonobese and obese groups based on body mass index z-scores (BMI-Z), or standard deviations, from a reference population. They defined obesity as a BMI Z-score at or above the 95th percentile for the reference standard and ranked pubertal age by Tanner stage. They assessed body composition and knee alignment with DEXA (dual-energy x-ray absorptiometry), measuring metaphyseal-diaphyseal angle (MDA) and anterior tibiofemoral angle (ATFA).

Obese children begin with greater varus than nonobese children, which shifts to greater valgus alignment in late puberty.

More than half (52%) of the 320 participants were obese. Although obese children were older overall (11.9 vs 10.5 years) obese participants at Tanner stages 3 and 4 were significantly younger than their nonobese counterparts.

Mean ATFA and MDA angles didn’t differ according to obesity status, and neither angle was correlated with BMI-Z score or fat mass Z-scores. MDA values, however, did vary by Tanner stage and obesity status.

Obese Tanner stage 1 participants had significantly greater MDA, indicating less valgus alignment, while obese Tanner stage 4 and 5 adolescents had significantly lower MDA, compared with nonobese children. Sex-stratified analyses showed heavier girls had greater variability in knee alignment than those weighing less.

The Journal of Rheumatology published the study in January.

“It may be that the extra weight children carry puts more pressure on developing joints so they don’t straighten out as they grow, but we need to follow children longitudinally to see whether excess weight actually changes the way knees and bones develop,” Bout-Tabaku said.

Sarah Shultz, PhD, ATC, a lecturer in biomechanics at the School of Sport and Exercise at Massey University in Wellington, Australia, has studied dynamic alignment in obese prepubertal children, finding they often use a more valgus gait to prevent additional loading on the medial compartment of the knee.

“This difference could mean needing a wider stance, or trying to shift forces more laterally through the knee,” she said. “Regardless, the fact that prepubertal children are choosing this pattern supports the idea that a valgus gait promotes abnormal bone growth due to abnormal mechanical loading, and could lead to a more permanent static alignment in pubertal obese children.”

Whether—and how early—this increases risk of OA in children is still unknown, said Bout-Tabaku, but a Austrian study epublished in May 2014 by Knee Surgery, Sports Traumatology, Arthroscopy did identify signs of early OA in extremely heavy teenagers.

Magnetic resonance imaging scans of 39 morbidly obese adolescents with and without knee pain revealed that all but one had a marked cartilage lesion in at least one knee region, and the group reporting pain had significantly more lesions.

The next step for Bout-Tabaku is seeing whether losing weight restores normal alignment, which she and her colleagues will look at in an upcoming study.

She pointed out that the major limitation of her current study was the use of supine DEXA scans rather than standing x-rays to measure knee alignment and that had children been weight-bearing, the significant differences between the obese and nonobese groups likely would have been amplified.

Shultz, who in her own research has used DEXA to measure ATFA, noted its use might be why, in contradiction with other studies of static alignment, the current investigation didn’t find greater valgus alignment in younger obese children.

Bout-Tabaku agreed DEXA needs further validation as a measure of knee alignment, but sees potential value for the modality.

“DEXA exposes children to less radiation and allows us to look at adiposity, which causes inflammation that may be causative for OA,” she said.

Sources:

Bout-Tabaku S, Shults J, Zemel BS, et al. Obesity is associated with greater valgus knee alignment in pubertal children, and higher body mass index is associated with greater variability in knee alignment in girls. J Rheumatol 2015;42(1):126-133.

Shultz SP, D’Hondt E, Fink PW, et al. The effects of pediatric obesity on dynamic joint malalignment during gait. Clin Biomech 2014;29(7):835-838.

Widhalm HK, Seemann R, Hamboeck M, et al. Osteoarthritis in morbidly obese children and adolescents, an age-matched controlled study. Knee Surg Sports Traumatol Arthrosc 2014 May 20. [Epub ahead of print]

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