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The impact of childhood obesity on bones

By Peter Pollack

Tibia vara, SCFE, and fractures are common consequences

According to recent studies by the Centers for Disease Prevention and Control, one in six children (ages 2 to 19 years) is obese. The recent increase in childhood obesity has led to a number of unique orthopaedic challenges.

Speaking to members of the Pediatric Orthopaedic Society of North America at the AAOS Annual Meeting, Robert H. Lustig, MD, an endocrinologist from the University of California, San Francisco, reviewed some of the factors leading to obese conditions in children and examined some of the orthopaedic effects of that obesity.

An increase in childhood obesity has led to a variety of orthopaedic challenges.

Your body is smart
According to Dr. Lustig, a resistance to the hormone leptin is one of the major contributors to childhood obesity. Leptin—an appetite hormone manufactured by fat cells—signals the brain that the body has enough energy to maintain normal metabolic processes. Without the leptin signal, the brain sees starvation.

Eating sugar-rich foods triggers fat cells to secrete leptin. Under normal circumstances, the increase in leptin causes a person’s brain to order him or her to stop eating. Yet a variety of studies have shown that people who try to lose weight often plateau at about 20 weeks, as the body senses a decline in leptin brought on by the reduction in food intake. In essence, the brain wrongly perceives the body to be starving and shifts into a mode that encourages eating and energy storage.

“What happens is that your body is smarter than you are,” explained Dr. Lustig. “It decreases the energy expenditure to offset the decrease in caloric intake.”

Obesity and Blount’s disease
Dr. Lustig reviewed three orthopaedic conditions often associated with childhood obesity: Blount’s disease (tibia vara), slipped capital femoral epiphysis (SCFE), and fractures.

Magnetic resonance imaging shows that Blount’s disease is associated with severe growth-plate changes on both sides of the knee joint, widening in the entire proximal tibial growth plate as well as the distal femoral growth plate, and cartilage invaginations into the metaphyses.

However, a recent study comparing the metaphyseal-diaphyseal angle (MDA) to the tibial/femoral angle (TFA) in children based on their body mass index (BMI) found that MDA did correlate with BMI z-score, but in the valgus direction, not the varus, and the TFA didn’t correlate with anything at all.

“I don’t know if it’s obesity that causes Blount’s,” said Dr. Lustig, “or whether it’s Blount’s that contributes to obesity, because kids with Blount’s don’t exercise very much.”

Obesity and SCFE
Discussing the correlation of SCFE with obesity, Dr. Lustig proposed a mechanism through which changes in cartilage brought on by puberty cause softening and allow shearing forces to occur in the hip. These forces are intensified in obese children.

“Clearly, obesity and SCFE do correlate, and probably through this mechanism,” he said.

Obesity and fractures
Turning his attention to fractures, Dr. Lustig explained that finding a correlation between obesity and fractures is far more complex than most people may realize. On the one hand, obesity is commonly associated with increased bone mineral density (BMD) as the body adjusts itself to carry the extra weight. The prevalence of fractures in obese children, however, is very high.

“Nobody really knows whether obesity increases bone mass or actually decreases it,” said Dr. Lustig. “What is surprising is that obese children tend to have mostly upper extremity fractures.” He suggested that mechanical loading on the lower extremities may increase BMD in those bones, while the upper extremities, which aren’t being loaded to the same degree, have a lower BMD and a greater risk of fracture.

According to Dr. Lustig, a major problem with understanding the correlation, if any, between bone mass and obesity lies in understanding what is actually being measured. Bone density is often measured by using a dual-energy x-ray absorptiometry scan, which measures aerial bone mineral density, doesn’t factor in height, and isn’t volumetric. Weight, on the other hand, includes fat-free mass, which is probably good for the bones; subcutaneous fat, which also may be good for the bones; and visceral fat, which makes cytokines and can promote osteolysis, in addition to bone mineral content.

Dr. Lustig reported no conflicts of interest.

Peter Pollack is a staff writer for AAOS Now. He can be reached at ppollack@aaos.org

AAOS Now
September 2008 Issue
http://www.aaos.org/news/aaosnow/sep08/clinical9.asp