May 25, 2011

Do hips spread with age?

My mom used to tell me that hips "spread" with age - particularly women's, particularly after childbirth.  I'd chalked this up to an old wives' tale with a teeny bit of truth: during the last stage of pregnancy, the hormone relaxin is released that causes the cartilage at the pubic symphysis (the very front of the bony pelvis) to weaken, allowing women's pelvic inlets to widen so that we can give birth to giant-brained babies (whose heads are 102% the size of our pelves - yeah, that's a fun statistic).  But after childbirth, the cartilage is strengthened again, and the pelvis returns to normal.

20-year-old's pelvis in pink,
79-year-old's pelvis in black
Today, UNC sent out a press release about a study that was published on Monday in the Journal of Orthopedic Research: "Surprising evidence of pelvic growth (widening) after skeletal maturity."  Researchers in our departments of biostatistics, orthopedics, and radiology decided to investigate the longstanding maxim that hips spread with age, which had been previously assumed to do with increasing weight with age.  Surprisingly, they found that the pelvis does widen after skeletal maturity - up to a 1 inch increase in the pelvic diameter!  To the right is the graphic from the press release.  Interestingly, the growth was seen in both males and females - so yes, women's hips do spread with age... but so do men's.

I'd initially thought that the growth would be restricted to women - after all, having a wider pelvic inlet should lead to much better outcomes in childbirth, and that makes evolutionary sense.  But it's interesting that men's pelves also grow.  I want to see a lot more of this kind of research - does it happen in other populations (that is, is this growth a result of our lifestyle of over-nutrition)?  Does growth slow down at certain times (such as during menopause for women)?  Very cool research - I can't wait to find out where researchers will go with it next.

ResearchBlogging.org

Berger AA, May R, Renner JB, Viradia N, & Dahners LE (2011). Surprising evidence of pelvic growth (widening) after skeletal maturity. Journal of orthopaedic research : official publication of the Orthopaedic Research Society PMID: 21608025

2 comments:

zinjanthropus said...

Is this just cortical drift?

Bone Girl said...

Hmmm, good question. I thought cortical drift was relevant mostly for the long (tubular) bones.

The authors of the article note:

"During childhood and young adulthood our bones grow in both length and width. Bone widens by periosteal apposition, a process which slows during aging. Bone widening also occurs in later years and this is thought to be an adaptive response to compensate for the loss of strength produced by endocortical bone loss. In a 7-year prospective study of over 600 women, Szulc et al. reported that rates of endocortical resorption increased with age, while the rate of periosteal apposition decreased with age, resulting in a decrease in the mass of the bone. Other research also supports an increase in skeletal bone width in postmenopausal women as a result of periosteal apposition."

That's all I could find in their article, though.

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