الفهرس 
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Abstract
Critical sized bone defect is a major challenge for orthopedic surgeons. These defects result following any pathologic condition leading to massive bone loss. Synthetic and biological based tissue engineered biomaterials and their combinations provide a promising substitute to fill the defect site. The aim of the present study was to evaluate the osteogenic potential of 3-D printed multilayered medical grade PLLA scaffold with collagen and nano hydroxyapatite for the healing of induced critical-sized bone defect in dogs. An experimental study was conducted on 12 skeletally mature male dogs. Critical defect (25mm) was induced into the right femur of all dogs. Dogs were randomly allocated into one of the following groups (4 dogs/group). PLLA scaffold seeded with nano hydroxyapatite and collagen molded on the defect (PLLA/Collagen/nHA group), the second group; the defect seeded with collagen and nano hydroxyapatite (Collagen/nHA group), the third one was left without scaffold or additives (Sham operated group), and the operated animals were left for 12 weeks. Animals were evaluated clinically, radiographically and histopathologicaly. In (PLLA/Collagen/nHA group), all dogs showed an improvement in lameness degree from sever to apparently free from lameness. Radiography showed newly formed bone filling the defect with no inter zone. Histopathology showed more maturation of the newly formed bone in the defect site had occurred as well as defined bone trabeculae in comparing to other groups. In conclusion, 3D printed multilayered medical grade PLLA with collagen and nano hydroxyapatite provide biodegradable osteoconductive scaffold for enhancing the healing of critical sized bone defect.