title: Healing of non-displaced fractures produced by fatigue loading of the mouse ulna.
abstract: We developed a fatigue loading protocol in mice to produce a non-displaced ulnar fracture in vivo, and characterized the early healing response. Using adult (5 month) C57Bl/6 mice, we first determined that cyclic compression of the forelimb under load-control leads to increasing applied displacement and, eventually, complete fracture. We then subjected the right forelimbs of 80 mice to cyclic loading (2 Hz; peak force approximately 4N) and limited the displacement increase to 0.75 mm (60% of the average displacement increase at complete fracture). This fatigue protocol created a partial, non-displaced fracture through the medial cortex near the ulnar mid-shaft, and reduced ulnar strength and stiffness by >50%. Within 1 day, there was significant upregulation of genes related to hypoxia (Hif1a) and osteogenesis (Bmp2, Bsp) in loaded ulnae compared to non-loaded, contralateral controls. The gene expression response peaked in magnitude near day 7 (e.g., Osx upregulated 8-fold), and included upregulation of FGF-family genes (e.g., Fgfr3 up 6-fold). Histologically, a localized periosteal response was seen at the site of the fracture; by day 7 there was abundant periosteal woven bone surrounding a region of cartilage. From days 7 to 14, the woven bone became denser but did not increase in area. By day 14, the woven-bone response resulted in complete recovery of ulnar strength and stiffness, restoring mechanical properties to normal levels. In the future, the fatigue loading approach can be used create non-displaced bone fractures in transgenic and knockout mice to study the mechanisms by which the skeleton rapidly repairs damage.
authors.affiliation: Department of Orthopaedic Surgery, Washington University, St. Louis, Missouri, USA. mario.d.martinez@gmail.com
authors.forename: Mario D
authors.initials: MD
authors.lastname: Martinez
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Schmid
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: McKenzie
authors.forename: David M
authors.initials: DM
authors.lastname: Ornitz
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Silva
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D055396:Bone Morphogenetic Protein 2 / Q000235:genetics; D019245:Compressive Strength / Q000502:physiology; D017102:Fracture Healing / Q000502:physiology*; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000235:genetics; D058574:Integrin-Binding Sialoprotein; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012795:Sialoglycoproteins / Q000235:genetics; D014057:Tomography, X-Ray Computed; D014457:Ulna / Q000000981:diagnostic imaging / Q000293:injuries* / Q000378:metabolism* / Q000502:physiology; D014458:Ulna Fractures / Q000000981:diagnostic imaging / Q000378:metabolism* / Q000503:physiopathology
chemical_list: D055396:Bone Morphogenetic Protein 2; C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; C546872:Ibsp protein, mouse; D058574:Integrin-Binding Sialoprotein; D012795:Sialoglycoproteins