Biomechanical analysis of the human tibia by pQCT, with potential diagnostic applications

PABLO ANDRÉS MORTARINO; GUSTAVO ROBERTO COINTRY; SARA FELDMAN; RICARDO FRANCISCO CAPOZZA; MARIANO ZAPATA; BRENDA HOMSE; JOERN RITTWEGER; JOSÉ LUIS FERRETTI

Colonia (Alemania)

Workshop; VI INTERNATIONAL WORKSHOP FOR MUSCULOSKELETAL & NEURONAL INTERACTIONS; 2008

International Society of Musculoskeletal & Neuronal Interactions (ISMNI),

BIOMECHANICAL ANALYSIS OF THE HUMAN TIBIA BY pQCT, WITH POTENTIAL DIAGNOSTIC APPLICATIONS. Mortarino P, Cointry G, Feldman S, Capozza R, Zapata M, Homse B, Rittweger J, Ferretti JL. Center of P-Ca Metabolism Studies (CEMFoC), Natl Univ of Rosario, Argentina; Inst for Biophys & Clin Res into Human Movement, Manchester Metropol Univ, Cheshire, UK.   This study aims to collect evidence of a morphological adaptation of the human tibia to its mechanical environment. It is assumed that 1. the whole body weight must be supported by the single joint surface at the heel as well as by each of the two joint surfaces at the knees, and 2. the bone loading scheme would change from a practically pure uniaxial compression close to the heel to a progressive addition of bending and torsion stresses toward the knee. It is proposed that the corresponding, adaptive features in cross-sectional geometry to that changing mechanical scheme throughout the bone length can be described by serial scan analyses performed by pQCT. With that purpose, we determined pQCT (XCT-2000, Stratec) indicators of bone mineral mass (total, cortical and trabecular BMC (ToC, CtC, TbC), cortical area (CtA)), cortical density (vCtD, corrected from the PVE as per Rittweger et al [JMNI 4:436,2004]), and metaphyseal-diaphyseal design (periosteal perimeter (PoPm), cortical thickness (CtTh), bending ant torsion moments of inertia (MI’s)) in serial scans taken at 5% intervals of the total tibia length, numbered from C1 to C19 from the heel to the knee, in 20 young healthy adults (10 males, 10 females, aged 20-40 years). Additionally, the same indicators determined at the standard 4%, 14% and 38% sites of the leg from a large sample of 300 healthy men and pre- and post-MP women were compared with the serial data for clinical purposes. Analysis of the serial scans showed that 1. CtA increased largely from C1 to C9 (r=0.965, p<0.001), then remained practically constant until C15, and fell dramatically more proximally (r=-0.798, p<0.001). 2. CtC reflected those changes closely. 3. Accordingly, the PVE-corrected vCtD values were statistically similar at every site. 4. From C5 to C16, MI’s values grew exponentially. From C5 to C9 they correlated directly with CtC or CtA, but from C10 to C16 they grew inversely with these indicators and with CtTh and directly with PoPm. 5. The proportion between MI’s (y) and CtC values (x) grew exponentially throughout the bone length (r=0.987, p<0.001). 6. ToC decayed about 33% from C1 to C3, and then it increased with a constant slope until C9, reaching the original C1 value at C7, and keeping statistically similar values from C10 to C16. These results confirm that the tibia undergoes almost pure compression distally, precisely from C1 to C3. It can be proposed that the very scarce natural stimulation in bending and torsion up to C3 would determine the maximal circularity of the diaphyseal design at that level. In agreement with this interpretation, the C3 site showed also 1. the lesser values of bone mass (practically pure cortical, about 50% more efficient in compression than the combination of cortical and trabecular bone at C1) and 2. the lesser MI’s in the whole bone. This could be explained because the most efficient factor concerning compression strength is the amount of mineralized material available in the bone cross-section (ToC at any site, CtA in pure cortical sites) rather than its spatial distribution according to bending or torsion axes (MI’s). The enhancement of bone mass from C3 proximally, with exponential increases in MI’s and PoPm and unchanged vCtD, further reveals the diaphyseal adaptation to progressively increasing bending and torsion stresses going proximally. That structural adaptation would require progressively increasing amounts of bone mass until C15. More proximally bone mass decreases, but bone diameters increase dramatically as the cortex becomes progressively thinner and MI’s grow exponentially. This situation seems to be maintained proximally as required by the anatomical constitution of an effective diaphyseal support for the double joint surface of the knee and the corresponding doubled demands of bone compression strength. The separate reference sample of normal data at the standard 4, 14 and 38% sites (close to C1, C3, and C7, respectively) corroborate these findings by showing close, linear relationships between ToC at 4% (y) and at 14% and 38% sites (x), with slopes very close to 1.5:1.0 and 1.0:1.0, respectively, for men and pre-MP women analyzed either separately or altogether. Z-scored graphs of these relationships and a software for calculation of individual Z-scores based on that reference were developed for practical purposes. This information and the derived diagnostic resources can be useful for describing the tibia structure in particular cases and evaluating any eventual change in trabecular bone mass as related to the cortical mass of the same individual, avoiding any reference to values of different, younger individuals, and following a novel biomechanical criterion.