Effect of dexamethasone on mandibular bone biomechanics in rats during the growth phase as assessed by bending test and peripheral quantitative computerized tomography

Long-term glucocorticoid administration to growing rats induces osteopenia and alterations in the biomechanical behavior of the bone. This study was performed to estimate the effects of dexamethasone (DTX), a synthetic steroid with predominant glucocorticoid activity, on the biomechanical properties of the mandible of rats during the growth phase, as assessed by bending test and peripheral quantitative computed tomographic (pQCT) analysis. The data obtained by the two methods will provide more precise information when analyzed together than separately. Female rats aged 23 d (n=7) received 500μg.kg-1 per day of DXT for 4 weeks. At the end of the treatment period, their body weight and body length were 51.3% and 20.6% lower, respectively, than controls. Hemimandible weight and area (an index of mandibular size) were 27.3% and 9.7% lower, respectively. The right hemimandible of each animal was subjected to a mechanical 3-point bending test. Significant weakening of the bone, as shown by a correlative impairment of strength and stiffness, was observed in experimental rats. Bone density and cross-sectional area were measured by pQCT. Cross-sectional, cortical and trabecular areas were reduced by 20% to 30% in the DTX group, as were other cortical parameters, including the bone density, mineral content and cross-sectional moment of inertia. The "bone strength index" (BSI, the product of the pQCT-assessed xCSMI and vCtBMD) was 56% lower in treated rats, which compares well with the 54% and 52% reduction observed in mandibular strength and stiffness determined through the bending test. Data suggest that the corticosteroid exerts a combined, negative action on bone geometry (mass and architecture) and volumetric bone mineral density of cortical bone, which would express independent effects on both cellular (material quality) and tissue (cross-sectional design) levels of biological organization of the skeleton in the species.

La administracion cronica de glucocorticoides a ratas en fase de crecimiento induce osteopenia y modificaciones negativas del comportamiento biomecanico del hueso. El estudio presente fue realizado para estimar los efectos de dexametasona (DTX), esteroide sintetico con actividad glucorticoide predominante, sobre las propiedades biomecanicas de la mandibula de ratas durante la fase de crecimiento, estimacion realizada mediante el ensayo de flexion a tres puntos, por un lado, y tomografia periferica cuantitativa computarizada (pQCT), por el otro. Los datos obtenidos mediante los dos metodos citados brindaran informacion mas precisa cuando son analizados en forma conjunta que cuando son analizados separadamente. Ratas hembras de 23 d de edad (n = 7) recibieron 500μg.kg-1/d por via subcutanea durante 4 semanas. El peso y la longitud corporales mostraron una disminucion del 51.3% y 20.6%, respectivamente, en las ratas tratadas con respecto a las controles (n = 7). El peso de la hemimandibula derecha y el area mandibular (indice del tamano del hueso) disminuyeron 27.3% y 9.7%, respectivamente. La hemimandibula derecha de cada animal fue analizada biomecanicamente en el test de flexion a tres puntos. Se observo un significativo debilitamiento del hueso, demostrado por la disminucion correlativa de la resistencia (a la fractura) y de su rigidez estructural (medida en la fase elastica de deformacion) en los animales experimentales. La densidad osea y el area de seccion transversal fueron estimadas mediante pQCT. Las areas de seccion transversal, cortical y trabecular, mostraron una reduccion significativa de entre 20% y 30%, asi como la densidad osea, su contenido mineral y el momento de inercia de la seccion transversal. El BSI (indice de resistencia osea), el producto de xCSMI y vCtBMD (medidos topograficamente), disminuyo un 56% en las ratas tratadas, valor semejante al 54% y 52% observado en la resistencia y rigidez mandibulares determinadas mediante el test de flexion. El analisis de los resultados obtenidos sugiere que DXT ejerce una accion negativa y combinada sobre la mandibula, sobre su geometria (masa y arquitectura) y sobre su densidad mineral volumetrica del tejido cortical, acciones que expresarian efectos independientes sobre los niveles celular (calidad material) y tisular (diseno arquitectonico) de organizacion biologica del esqueleto en la especie estudiada.

Growth inhibition in rats fed inadequate and incomplete proteins: repercussion on mandibular biomechanics / Growth inhibition in rats fed inadequate and incomplete proteins: repercussion on mandibular biomechanics.

This study describes the effects of feeding growing rats with a diet containing inadequate and incomplete proteins on both the morphological and the biomechanical properties of the mandible. Female rats aged 30 d were fed freely with one of two diets, control (CD, 301 Cal/100g) and experimental (ED, 359 Cal/100g). CD was a standard laboratory diet, while ED was a synthetic diet containing cornflower supplemented with vitamins and minerals. Both diets had the same physical characteristics. Control (C) and experimental (E) animals were divided into 4 groups of 10 animals each. C40 and E40 rats were fed CD and ED, respectively, for 40 d; C105 were fed the CD for 105 d; and E105 were fed the ED for 40 d and then the CD for the remainder of the experimental period (65 d). Mandibular growth was estimated directly on excised and cleaned bones by taking measurements between anatomical points. Mechanical properties of the right hemimandible were estimated by using a 3-point bending test to estimate the structural properties of the bone. Geometric properties of both the entire bone and the cross-section were determined. Bone material properties were calculated from structural and geometric properties. The left hemimandibles were ashed and the ash weight obtained. Rats fed the ED failed to achieve normal body weight gain. Complete catch-up was observed at the end of nutritional rehabilitation. Mandibular weight and length were negatively affected by the ED, as were the cross-sectional area, the mineralized cortical area, and the cross-sectional moment of inertia. All of these parameters showed incomplete catch-up. The structural bone mechanical properties indicative of strength and stiffness were negatively affected. Intrinsic material properties, as assessed by the modulus of elasticity and maximal elastic stress, were within normal values. In summary, the experimental bone was weaker than the control and structurally incompetent. The bone considered was smaller than the control bone, showing a significant reduction in the cross-sectional area and the moment of inertia. However, material properties as well as the ash fraction and degree of mineralization were similar in E and C bones. Therefore, the E bone was weaker than the C bone because of its smaller bone mass, which appears to have been negatively influenced by the ED in relation to its effects on overall body mass.

Bone mineral density and bone strength from the mandible of chronically protein restricted rats

The present investigation was performed to assess the biomechanical repercussion of protein malnutrition imposed on rats between the 26th and 135th days of postnatal life on the mandible, which is not a weight-bearing bone but supports the loads related to the masticatory activity. Female Wistar rats aged 26 d (n=14) were placed on either a 4%-protein diet (ICN 960254, P4 group) or a 20%-protein diet (ICN 960260, P20 group) and killed 111 d later. Both body weight and length were recorded regularly. The mandibles were dissected and cleaned of adhering soft tissue. Mandibular growth was estimated directly by taking measurements between anatomical points. Areal Bone Mineral Density (BMD) was estimated using a bone densitometer (LUNAR DPX-L). Mechanical properties of the right hemimandible were determined using a three-point bending mechanical test to obtain the load/deformation curve and estimate the structural properties of the bone. Results were summarized as means ± SD. Comparisons between parameters were performed by Student's t test. A 75% reduction in body weight and a 32% reduction in body length were observed in P4 rats. Like body size, mandibular weight, length, height and area (index of mandibular size) were negatively affected by P4 diet, as was the posterior part of the bone (posterior to molar III). The anterior part (alveolar and incisor alveolar process) was not affected by age or diet. The "load capacity" extrinsic properties of the mandible (load fracture, stiffness, yielding point) were between 43% and 64% of control value in protein restricted rats. BMD was similar in both groups of animals. Conclusion: 1) Chronic protein malnutrition imposed on rats from infancy to early adulthood reduces the growth of the posterior part of the mandible without inducing changes in the anterior part, which produces some deformation of the bone in relation to age-matched rats; and 2) the significant reduction of strength and stiffness of the mandible seem to be the result of an induced loss of gain in bone structural properties as a consequence of a correlative loss of gain in both growth and mass, yet not in bone material properties.

La investigación presente fue diseñada con el objeto de evaluar la repercusión biomecánica de la malnutrición proteica impuesta a ratas entre los días 26º y 135º de edad sobre la mandíbula (M), hueso que no soporta carga relacionada con el peso corporal sino con las fuerzas masticatorias. Ratas Wistar hembras de 26 d de edad (n=14) fueron alimentadas con dietas conteniendo 4% (grupo P4) (ICN 960254) o 20% (grupo P20) (ICN 960260) de caseína y sacrificadas 111 d después. Peso y longitud corporales fueron registrados regularmente. Las mandíbulas fueron disecadas y liberadas de tejido blando. Se realizaron mediciones entre diversos puntos anatómicos para estimar la morfometría del hueso. La Densidad Mineral Osea (DMO) fue determinada en un densitómetro LUNAR DPX-L. La M derecha de cada animal fue sometida al test de flexión a 3 puntos para obtener la curva carga/deformación y estimar las propiedades estructurales del hueso mandibular. Los resultados (X±DS) fueron analizados estadísticamente mediante test t de Student. El peso y la longitud corporales fueron menores en el grupo P4 que en el P20 (-75% y -32%, respectivamente). Longitud de la base, altura y área mandibular (índice del tamaño de M) fueron afectados negativamente por la dieta P4, lo mismo que la porción posterior de M (posterior al molar III). La porción anterior (procesos alveolar e incisivo) no fueron afectadas por dieta o edad. Todas las propiedades biomecánicas de M (carga de fractura, resistencia en fase elástica, límite elástico) fueron 43-64% menores en grupo P4 que en grupo P20. El valor de DMO fue similar en ambos grupos. CONCLUSION: 1) La malnutrición proteica crónica impuesta a ratas desde la infancia hasta la adultez reduce el crecimiento de la porción posterior de la mandíbula sin inducir cambios en su porción anterior, lo que produce una cierta deformación del hueso en comparación con animales de la misma edad; y 2) la importante disminución de la resistencia a fractura y de la rigidez durante el período elástico sería el resultado de una reducción de ganancia de las propiedades estructurales óseas como consecuencia de una reducción correlativa de ganancia de masa ósea, con mantenimiento de la normalidad de las propiedades óseas intrínsecas.