Continuing periosteal apposition. I: Documentation, hypotheses, and interpretation.

Continuing periosteal apposition (CPA) of small amounts of new lamellar bone, leading to absolutely larger size, has been identified in a number of adult cranial and postcranial bones. This paper reviews 42 studies published since 1964 that have found both significant and nonsignificant age-related change in various skeletal size dimensions, e.g., length, diameter, width, and area. Also considered are four hypotheses that have, or may be, postulated for the occurrence of CPA. To date, however, these hypotheses (cohort effect, mechanical compensation, bone repair and/or mechanical response potential, and heterochrony) have not been rigorously tested, hence remain speculative. An important interpretive problem that befalls the investigation of CPA is its small effect size (i.e., the magnitude of change between observations), since most studies have restricted sample sizes. This problem is illustrated by power analysis of three reviewed studies that reported nonsignificant age-related change. The analysis indicates that these studies had very little likelihood of finding a statistically significant result, i.e., a low probability of rejecting the null hypothesis stipulating no size change with age. This finding has implications for interpreting CPA and for distinguishing between the statistical and biological significance of this phenomenon.