Treatment with Parathyroid Hormone hPTH(1-34), hPTH(1-31), and Monocyclic hPTH(1-31) Enhances Fracture Strength and Callus Amount After Withdrawal Fracture Strength and Callus Mechanical Quality Continue to Increase

The influence of intermittent hPTH(l-34)NH₂, hPTH(1-31)NH₂, and monocyclic [Leu²⁷]cyclo(Glu²²-Lys²⁶)hPTH(1-31)NH₂ treatment on callus formation, mechanical strength, and callus tissue mechanical quality of tibial fractures in rats was investigated after 8 and 16 weeks of healing. In the 8 weeks of healing animals, the PTH-peptides were injected subcutaneously during the entire observation period (15 nmol/kg/day [hPTH(1-34)NH₂: 15 nmol = 60 µg]), and control animals with fractures were given vehicle. In the 16 weeks of healing animals, the PTH-peptides were injected only during the first 8 weeks of healing (15 nmol/kg/day), after which the animals were left untreated during the rest of the healing period. After the first 8 weeks of healing, increased fracture strength and callus volume were seen in the PTH-treated rats (ultimate load 66%, ultimate stiffness 58%, callus volume 28%), and the three peptides were equally effective. No difference in callus tissue mechanical quality was found between PTH and vehicle animals. After 16 weeks of healing, no differences in fracture strength, callus volume, or callus tissue mechanical quality were seen between PTH and vehicle. When comparing PTH-treated animals at 8 and 16 weeks, fracture strength and callus tissue mechanical quality continued to increase after the withdrawal of PTH (ultimate load 23%, ultimate stress 88%, elastic modulus 87%) and external callus volume declined during this period (27%).     

Local Anabolic Effects of Growth Hormone on Intact Bone and Healing Fractures in Rats

The local effects of rat growth hormone (rGH) injected at the surfaces of intact tibial diaphyses and healing tibial diaphysial fractures were investigated in 10-month-old female rats. Intact diaphysial bones: rats were injected daily for 14 days with vehicle, 2 µg rGH, or 20 µg rGH at the surface of the right tibial diaphysis. After 10 days of injection the animals were labeled with calcein. At the rGH-injected location, increased external diaphysial bone dimensions and increased calcein-labeled area were seen, and the responses to rGH were dose dependent. The new bone formed at the periosteal surface was woven bone. At the opposite left tibia, no systemic effect of rGH was found. rGH did not influence body weight changes during the treatment, or muscle mass or serum IGF-I at the end of the treatment. Healing diaphysial fractures: a closed fracture was made in the right tibial diaphysis, and stabilized by medullary nailing. The fractures were tested after 21 days and 98 days of healing. During the first 21 days of healing, all rats were injected daily with either vehicle or 20 µg rGH at the surface of the fracture line. In the 21-day healing rGH group, ultimate load, ultimate stiffness, external callus dimensions, and external callus volume of the fractures were increased. rGH did not affect body weight changes during this healing period or serum IGF-I at the end of the healing period. In the 98-day healing rGH group, ultimate load was still increased compared with the vehicle group, although a ninefold increase took place in the vehicle group between days 21 and 98 of healing. External callus dimensions of the fractures were increased in the rGH group, whereas body weight changes during the healing period were not affected.     

Differential Effects of Bone Structural and Material Properties on Bone Competence in C57BL/6 and C3H/He Inbred Strains of Mice

The femoral neck is a relevant and sensitive site for studying the degree of osteopenia. Engineering principles predict that bone structural parameters, like cross-sectional geometry, are important determinants of bone mechanical parameters. Mechanical parameters are also directly affected by the material properties of the bone tissue. However, the relative importance of structural and material properties is still unknown. The aim of this study was to compare bone competence and structural parameters between a murine strain showing a low bone mass phenotype, C57BL/6 (B6), and another one showing a high bone mass phenotype, C3H/He (C3H), in order to better determine the role of bone structure and geometry in bone failure behavior. Murine femora of 12- and 16-week-old B6 and 12- and 16-week-old C3H inbred strains were mechanically tested under axial loading of the femoral head. In order to assess the structural properties, we performed three-dimensional morphometric analyses in five different compartments of the mouse femur using micro-computed tomography. The mechanical tests revealed that B6 femora became stiffer, stronger, and tougher at 12-16 weeks, while bone brittleness stayed constant. C3H bone stiffness increased, but strength remained constant, work to failure decreased, and bone became more brittle. These age effects indicated that B6 did not reach peak bone properties at 16 weeks of age and C3H did reach maximal skeletal biomechanical properties before 16 weeks of age. Our investigations showed that 83% of the strength of the femoral neck in the B6 strain was explained by cortical thickness at this location; in contrast, in C3H none of the mechanical properties of the femoral neck was explained by bone structural parameters. The relative contributions of bone structural and material properties on bone strength are different in B6 and C3H. We hypothesize that these different contributions are related to differences at the ultrastructural level of bone that affect bone failure.     

Alendronate Treatment of the Brtl Osteogenesis Imperfecta Mouse Improves Femoral Geometry and Load Response Before Fracture but Decreases Predicted Material Properties and Has Detrimental Effects on Osteoblasts and Bone Formation

Long courses of bisphosphonates are widely administered to children with osteogenesis imperfecta (OI), although bisphosphonates do not block mutant collagen secretion and may affect bone matrix composition or structure. The Brtl mouse has a glycine substitution in col1a1 and is ideal for modeling the effects of bisphosphonate in classical OI. We treated Brtl and wildtype mice with alendronate (Aln; 0.219 mg/kg/wk, SC) for 6 or 12 wk and compared treated and untreated femora of both genotypes. Mutant and wildtype bone had similar responses to Aln treatment. Femoral areal BMD and cortical volumetric BMD increased significantly after 12 wk, but femoral length and growth curves were unaltered. Aln improved Brtl diaphyseal cortical thickness and trabecular number after 6 wk and cross‐sectional shape after 12 wk. Mechanically, Aln significantly increased stiffness in wildtype femora and load to fracture in both genotypes after 12 wk. However, predicted material strength and elastic modulus were negatively impacted by 12 wk of Aln in both genotypes, and metaphyseal remnants of mineralized cartilage also increased. Brtl femoral brittleness was unimproved. Brtl osteoclast and osteoblast surface were unchanged by treatment. However, decreased mineral apposition rate and bone formation rate/bone surface and the flattened morphology of Brtl osteoblasts suggested that Aln impaired osteoblast function and matrix synthesis. We conclude that Aln treatment improves Brtl femoral geometry and load to fracture but decreases bone matrix synthesis and predicted material modulus and strength, with striking retention of mineralized cartilage. Beneficial and detrimental changes appear concomitantly. Limiting cumulative bisphosphonate exposure of OI bone will minimize detrimental effects.