WHOLE-BODY BONE-RESORPTION IN THE GROWING PIG

Our knowledge of total body bone resorption during growth is limited. The primary purpose of this study was to determine if a commercially a vailable bone resorption assay, developed for measuring human bone res orption, could be used to measure whole body bone resorption in young, growing pigs. A secondary purpose was to evaluate if this method coul d detect changes in bone resorption in response to certain dental appl iances which have been shown to change mandibular and maxillary growth . Five growing 4-month-old male Hanford minipigs (Sur scrofa) were hou sed in metabolic cages for 24h, every other day, over a period of 1 mo nth. Three of the animals were fitted with a mandibular protrusive ort hodontic appliance. Total 24h urines were collected in which the conce ntration of creatinine and collagen type I N-telopeptide crosslinks (N Tx, a marker of bone resorption) were measured. The NTx immunoassay wa s originally developed for the analysis of human urine. Pig bone was p owdered, defatted, and decalcified, and the resulting powder digested with bacterial collagenase. The digest was screened for NTx content, i n the same fashion as the pig urines. Bone extract and pig urines were cross-calibrated to a standard of adolescent human urine. This allowe d calculation of the daily quantity of pig bone resorbed. Daily metabo lite excretion was quite variable in these growing animals; for NTx th e CV was 31%, for creatinine the CV was 25%. The mean daily quantities of bone resorption ranged between 26 and 46 grams of bone which amoun ted to 1.2-1.7% of estimated total skeletal mass. The protrusive appli ances increased bone resorption significantly during the first two wee ks of the trial. In conclusion: the NTx assay can be used to measure b one resorption in pigs; the assay is sensitive enough to indicate chan ges in bone resorption, such as those caused by an orthodontic mandibu lar protrusive appliance. During growth, bone resorption varies greatl y from day to day. On average, every 24h, 1.4% of the skeletal mass is resorbed.