| By investigating the mechanical properties and features on the surface of bonematerials at nano-level in different regions of femoral head in the elderly with femoralneck fracture, the regional deterioration patterns and material properties with agingwas studied for exploring the age-dependent fracture mechanism, and it can provide abasis for predicting age-related fracture and decreasing the early failure of fixation inproximal femur. Relationships between microstructural parameters, materialdistribution, and mechanical properties of bone materials at the macro-and micro-levels during fracture healing process were investigated. The influence oflow-magnitude high-frequency vibration (LMHFV) with different rest period insertedon bone healing was also investigated, which can provide a theoretical basis foraccelerating fracture healing in clinics.In the first section, ten femoral heads from female patients with femoral neckfractures were extracted during surgery f(ve for the Aged group, aged65-66years;fve for the Advanced aged group, aged85-95years). The femoral head was dividedinto three equal layers (anterior, central, and posterior) in the coronal view, and eachlayer was segmented intofve regions (superior, central, inferior, medial, and lateral).Nanoindentation testing and atomic force microscopy (AFM) imaging were used toinvestigate the mechanical properties and surface morphology of the specimens.Deterioration process of trabeculae about whole femoral head, different layers anddifferent regions were explored on directions with aging. No statistical differences ingrain sizes or roughnesses were found between age groups, which suggested that thenanostructure of trabeculae in the femoral heads of postmenopausalwomen cannot beused to predict age-related bone loss and fracture risk. Mechanical properties in thelongitudinal direction deteriorated more quickly than those in the transverse directionfor the whole femoral head. Comparisons between layers showed a higherdeterioration rate with aging in the anterior layer than in other layers. In different regions, mechanical properties of the medial and lateral regions deteriorated morequickly than those in the three other regions, and deterioration in the longitudinaldirection was more serious than that in the transverse direction.In the second section，eighteen sheep underwent tibial osteotomy and weresacrificed at4,8, and12weeks. Radiographs and micro-computed tomography(micro-CT) scanning of both fracture and untreated contralateral sides were taken formicrostructural assessment, material distribution evaluation, and micro-finite elementanalysis. A displacement of5%compressive strain on the longitudinal direction wasapplied to the micro-finite element model, and macro-and micro-level mechanicalproperties were calculated. Principle components analysis and linear regression wereused to establish the relationship between principle components (PCs) and mechanicalparameters. Visible bony callus formation was observed at4weeks. Macro-levelmechanical parameters increased at8weeks, but micro-level mechanical parametersdid not increase significantly until12weeks. Three PCs were extracted frommicrostructural parameters and material distribution, which accounted for87.60%ofthe total variation. The regression results showed a significant relationship betweenPCs and mechanical parameters (P<0.05). Results of this study show thatmicrostructure and material distribution based on micro-CT imaging could efficientlypredict bone strength and reflect the bone remodeling process during fracture healing,which provides a basis for exploring the fracture healing mechanism.In the third section，thirty-five three-month-old female rats were used, andtransverse fractures of bilateral tibias were established with Kirschner wire beinginserted for fixation. The animals were randomly assigned to five groups: DL (dailyloading for15min without rest days); DLR (daily loading with the loading dividedinto3bouts (5min for each bout), and separated by4hours); VL7(vibrationalloading for7day followed by7-day rest,15min/day during vibrational periods);VL7R (vibrational loading for7-day followed by7-day rest, and the loading wasdivided into3bouts (5min for each bout), and separated by4hours during vibrationalperiods), and FBC (fractured model without mechanical loading). The treatment ofLMHFV (35Hz and0.25g) started one week post-fracture. After four weeks mechanical loading, the rats were killed and the tibias were harvested. Three-pointbending test, nanoindentation testing, AFM analysis, Micro-CT scanning andhistomorphometric analysis were performed. Serum concentration of osteocalcin (OG)and tartrate-resistant acid phosphatase5b (TRAP5b) were measured. Significantlyhigher values of failure load and elastic modulus were observed in DL and DLR thanFBC (P<0.05) at macro-level. The results of nanoindentation testing showed thehighest values of indentation modulus and hardness in DLR (significantly higher thanFBC; P<0.05), besides, higher values of hardness were also observed in DL and VL7(significantly higher than FBC; P<0.05). For micro-level analyses, BMD of DLR andVL7were significantly higher than that in FBC (P<0.05), but no significantdifferences in other parameters were observed between vibrational groups and FBC.Though AFM imaging showed no significant differences in grain sizes betweenvibrational groups and FBC, roughness of DLR showed the highest value, which wassignificantly higher than that in FBC (P<0.05). There was no significant differencesof OG between vibrational groups and FBC, however, FBC showed significantlyhigher TRAP5b concentration than all vibrational groups (P<0.05). The resultsshowed that LMHFV with rest period inserted not only altered macro-and nano-levelbone mechanical parameters, but also influenced micro-level BMD and nano-levelspatial arrangement (roughness). The most efficatious effect of LMHFV protocolswith different rest period inserted was observed in DLR (daily loading with theloading divided into three bouts), which indicated that both osteogenic accumulationand cellar resensitization could be satisfied under this regimen, hence it has a greatpotential in clinical applications. |
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