FRACTURE HEALING IN A MOUSE MODEL FOR SENILE OSTEOPOROSIS

Abstract

Introduction: Osteoporosis, a major public health burden, is associated with increased fracture risk. Fracture healing in osteoporosis is altered with reduced callus formation and impaired biomechanical properties of newly formed bone leading to high risk of fixation failure. Experimental data have shown decreased healing potential in aged animals and in animal models of post-menopausal bone loss. It is unclear whether fracture healing is similarly impaired in senile osteoporosis. The objective of this study is to investigate fracture healing in a small animal model of senile osteoporosis, senescence-accelerated mouse prone 6 (SAMP6).

Materials & Methods: A mid-femur osteotomy was created in SAMP6-mice (n=24) and senescence-resistant inbred strains (SAMR1) (n=24) were used as controls. The osteotomy was rigidly fixed using a newly developed screw-plate-implant (MouseFix). Fracture healing was evaluated at 7, 14, 28 and 42 days after surgery using micro-CT and histomorphometry. Biochemical marker for bone formation (osteocalcin) and bone resorption (TRAP5b) were evaluated from serum samples. MSC were extracted from the femurs of mice and cultured in vitro and differentiated into either osteoblasts or adipocytes using standard induction media.

Results: Studies carried out in vitro confirmed that MSC isolated from the bone marrow of SAMP6 mice had a reduced tendency to differentiate toward the osteoblast cell lineage as previously reported in human osteoporotic patients. Although osteoblastogenesis was clearly impaired, the formation of new bone in SAMP6 mice was comparable to that observed in SAMR1 mice. Similar results were found for histomorphometry data analyzing the degree of bone mineralisation. Interestingly, osteocalcin levels were significantly increased in serum samples from osteoporotic mice at day 7 and 14 following fracture.

Discussion: The data presented here indicates that fracture healing proceeds normally in a mouse model for senile osteoporosis. This finding supports the clinical observation that although fracture fixation is difficult in osteoporosis, healing potential seems to be unchanged. MSC from osteoporotic patients as wells as from SAMP6-mice show reduced proliferation rate together with adipogenic rather than osteogenic differentiation pattern. However, decreased cell dynamics seems not to influence diaphyseal fracture healing. Other sources of MSC other than bone marrow-derived MSC may therefore be pivotal in determining the outcome of intramembranous bone repair in both normal and osteoporotic bone.