The development of bone mass and bone strength in the mandible of the female rat.

The present study provides baseline data for a number of mandibular growth dimensions, specially on bone mass and bone strength, that were collected between the 21st and the 180th days of postnatal life, which are intended as a reference for researchers designing experimental studies, specially on mandibular catch-up growth, and as an aid for clinicians who must evaluate results from published animal studies for validity and potential extrapolation to the human clinical situation. Fifty weanling female Wistar (Hsd:Wi) rats were fed ad libitum a diet previously shown to allow normal, undeformed mandibular growth. Five of them were randomly selected at different times between 21 and 180 d of life. Mandibular growth was estimated directly on the right hemimandible by taking measurements between anatomical points; mandibular bone mass (calcium mass) was estimated from the mg of calcium, determined by atomic absorption spectrophotometry, present in the ashes of the left hemimandible; and mechanical properties of the right hemimandible were determined using three-point bending mechanical test. Dimensions, bone calcium mass and bone strength of the female rat mandible increased linearly from day 21 to approximately day 90. Bone growth, as expected, was more than twice when assessed from bone weight than when derived from mandibular area, length or height when the parameters were expressed as the relative increase from the mean infant condition. The growth rate of the posterior part of the mandible (behind the third molar) was almost five times greater than that of the anterior part. The rates of growth of the studied parameters showed a marked decline after day 90. ANOVA indicated that no statistical differences were found between day 90 and day 120 values. It could be concluded that the female rat mandible attains its adult size, peak bone calcium mass and bone structural mechanical properties at some point between 90 and 120 d of postnatal life. Because of the extremely high positive correlation between mandibular bone calcium mass and both mandibular area and mandibular weight, it was possible to calculate the mandibular peak bone mass from the relations 7.69 mgCa/cm2 and 0.19 mgCa/mg bone.