? Immediately loaded orthopaedic and oral implants can undergo interfacial micromotion and fail by forming interfacial fibrous or cartilaginous tissue rather than bone. While in this instance and others it is evident that mechanical conditions affect healing of skeletal tissue (""""""""mechanobiology""""""""), specific factors influencing cell fate decisions, and mechanisms by which cells interpret and respond to changes in mechanical environment, remain unclear. This gap in understanding restricts design of conventional as well as new tissue-engineered implants. However, recent insights from fracture healing and other healing situations suggests the hypothesis that extracellular matrix remodeling and angiogenesis are central in bone healing around implants, and that tissue deformation (strain) during early implant loading affects these processes, and, in turn, cell differentiation at interfaces. To test this idea, this project will measure spatial/temporal expression of key molecular makers of angiogenesis, bone, and cartilage formation at: 1) healing sites without implants: and 2) healing interfaces where biomechanical strain conditions are varied. The project will use a novel model of the bone-implant interface in mouse tibiae, which allows experimental control of biomaterial and biomechanical factors. Also, the project will use wild-type (wt) and NEMP9-null mice (MMP9-/-), since the latter lack a key matrix metalloproteinase (MMP9, gelatinase B) involved in angiogenesis. The project will integrate data from in situ hybridization, ultrastructural analyses, immunocytochemistry and biomechanical tests to examine mechanobiology of interfacial healing.
Aim 1 will test if matrix remodeling and angiogenesis are prerequisites for osteoblast (OB) differentiation in holes without implants;
the aim will develop a """"""""molecular map"""""""" of expression of markers of angiogenesis, bone and cartilage formation during healing of empty drill holes (diam. 0.2, 0.4 and 1.0 ram) at 3, 9 and 27 days in wt and MMP9-/- mice.
Aim 2 will test if matrix remodeling and angiogenesis are essential for differentiation of mesenchymal cells into OBs in unstrained bone-implant gaps. Using wt and MMP9-/- mice, we will develop molecular maps of healing at 0, 3, 9 and 27 days at: (1) a bone-implant gap interface (BIGI) around stabilized polylactide (PLA) pins in oversized holes; and (2) a direct bone-implant interface (DBII), having a mix of direct bone-implant contact and gaps around stabilized PLA screws.
Aim 3 will control implant micromotion in its implant bed, in response to a defined load immediately post-implantation, to test whether specific strain conditions in interfacial gaps inhibit matrix remodeling, angiogenesis and differentiation of mesenchymal cells into OBs. A miniaturized rmcromotion device will be used to create implant stability vs. cyclic axial implant micromotion (100-200 (m) of pins and screws in a BIGI during healing. Interfaces will be analyzed biologically as in Aims 1-2, and interfacial strain fields will be measured directly by digital image correlation based on micro-CT images of whole bone-implant specimens. ? ?
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