Bone Density and Cross-Sectional Area are Inversely Related in the Young Adult Distal Tibia

Abstract

Bone is posited to develop and adapt to loads by satisfying the competing demands for achieving sufficient stiffness and minimizing mass. This compromise has been demonstrated at the midshaft of the tibia by an inverse relationship between cross-sectional area and thickness of the cortical shell. The requirement for minimizing mass is likely greater at the distal tibia where inertial properties cause bone mass to be more metabolically expensive. Whether there is a similar tradeoff between bone size and the amount of bone tissue at the distal tibia remains to be determined. PURPOSE: Determine whether cross-sectional area of bone is inversely related to bone density at the distal tibia. METHODS: Six hundred seventy-three (476M, 197F) Army trainees (20.4±3.4 yrs; 1.71±0.09m; 72.5±13.3kg) underwent high resolution peripheral quantitative computed tomography (HR-pQCT, XtremeCT2, Scanco Medical AG) scans of their non-dominant distal tibias at 4% of bone length from the distal growth plate, at the start of their initial military training. Generalized linear models adjusted for sex were used to test the significance and directionality of relationships between total cross-sectional area (CSA) normalized to body mass and total volumetric bone mineral density (vBMD). RESULTS: Normalized CSA was significantly and inversely related to vBMD (p < 0.001) where a one-unit increase in normalized CSA (mm2/kg) resulted in a 5.856 mg HA/cm3 reduction in vBMD. Specifically, the lowest compared to the highest quartile of normalized CSA had 13% higher vBMD, per unit body mass. CONCLUSION: These findings support the hypothesis that bone in the distal tibial metaphysis develops in a manner which balances the need to be adequately stiff against that of minimizing the mass of a metabolically costly tissue through maintaining lower bone density relative to larger bone size.