Bone defect repair in immobilization-induced osteopenia: a pQCT, biomechanical, and molecular biologic study in the mouse femur H. Uusitalo a , J. Rantakokko a,b , E. Vuorio a , H.T. Aro b, * a Skeletal Research Program, Department of Medical Biochemistry and Molecular Biology, University of Turku, Turku, Finland b Skeletal Research Program, Department of Orthopaedic Surgery, University of Turku, Turku, Finland Received 25 April 2004; revised 6 September 2004; accepted 17 September 2004 Available online 23 November 2004 Abstract The present study was carried out to determine whether immobilization-induced (Im) osteopenic bone possesses the same reparative capacity as normal healthy bone. Furthermore, the effects of mechanical loading versus immobilization on bone defect healing were studied. Three-week cast-immobilization was used to induce local osteopenia in mice. A standardized metaphyseal bone defect of the distal femur was created unilaterally both in immobilization-induced (Im) osteopenic mice and in nonimmobilized (Mo) age-matched control animals. After creation of the bone defect, the animals in both groups were further divided into two groups: 3-week cast- immobilization (Im-Im and Mo-Im) groups, and unrestricted weight-bearing (Im-Mo and Mo-Mo) groups. The healing process was followed up to 3 weeks using RNA analysis, histomorphometry, biomechanical testing, and pQCT measurements. At 3 weeks of healing without immobilization, bone mineral density (BMD), as well as bone bending stiffness and strength were higher in normal (Mo-Mo) than in osteopenic (Im-Mo) bone. Although the levels of mRNAs characteristic to chondrocytes (Sox9 and type II collagen), hypertrophic chondrocytes (Type X collagen), osteoblasts (type I collagen and osteocalcin), and osteoclasts (cathepsin K) during the bone defect healing exhibited similarities in their expression profiles, mechanical loading conditions also caused characteristic differences. Mechanical loading during healing (Mo-Mo group) induced stronger expression of cartilage- and bone-specific genes and resulted in higher BMD than that seen in the cast-immobilized group (Mo-Im). In biomechanical analysis, increased bending stiffness and strength were also observed in animals that were allowed weight-bearing during healing. Thus, our study shows that bone healing follows the same molecular pathway both in osteopenic and normal bones and presents evidence for reduced or delayed regeneration of noncritical size defects in immobilization-induced osteopenic bone. D 2004 Elsevier Inc. All rights reserved. Keywords: Fracture; Bone repair; Immobilization; Osteoporosis; Mouse Introduction Mechanotransduction, the transfer of mechanical load- ing-induced stimuli into chemical signals and eventually into cell and tissue response, has received considerable interest in skeletal research. In vertebrates, bone is a tissue where the effects of mechanical loading are especially well highlighted: bones not only develop as structures designed to perform mechanical tasks, but their composition and architecture are regulated by mechanical loading through- out the individual’s life span to better cope with the existing mechanical loading environment [22,31,37,44,49]. There is substantial evidence that bone cell populations responsible for adapting bone architecture are reactive, either directly or indirectly, to strain induced by prevailing mechanical loading. The molecular mechanism, whereby bone tissue detects mechanical loading, however, is still incompletely understood [44]. Immobilization is a frequently used treatment for bone injuries despite resulting osteoporosis [16,39]. In bone, 8756-3282/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2004.09.010 * Corresponding author. Department of Orthopaedic Surgery, Uni- versity of Turku, Kiinnamyllynkatu 4-6, FIN-20520 Turku, Finland. Fax: +358 2 3337690. E-mail address: hannu.aro@utu.fi (H.T. Aro). Bone 36 (2005) 142 – 149 www.elsevier.com/locate/bone