Simulation of Subject Specific Bone Remodeling and Virtual Reality Visualization

Abstract

Bone is a dynamic tissue. The dynamic nature is the result of the remodeling process, inwhich the skeleton is renewed continuously. The external mechanical loading affects theremodeling process which in turn affects the internal structure. German anatomist JuliusWolff first noticed this phenomenon in 1892 (Wolff, 1986). The purpose of remodeling is toprevent the accumulation of damage, adapt the internal architecture to the external loads andmaintain homeostasis (Van Der Linden et al., 2001; Pivonka et al., 2008; Lemaire et al., 2004).Compressive fractures caused by osteoporosis are common in the lower vertebrae is a frequentregion for compressive fracture due to osteoporosis (Melton et al.,1989; Melton et al.,1999;Scane,1994). Figure 1 shows the overall structure of a lumbar vertebrae. The outer dense boneis called cortical bone and the inner porous bone is called cancellous bone. To understand theremodeling process it is important to understand the role of the cells involved in the process.Osteoblasts are the cells responsible for the bone formation. They have a single eccentricnucleus. Osteoclasts are multi-nucleated cells responsible for bone resorption. Together theyare called the Basic Multicellular Unit (BMU). During bone formation some of the osteoblastsget trapped in the matrix that they secrete and become Osteocytes. Another type of cellsare lining cells which cover the entire bone surface. These are quiescent osteoblasts. Theosteocytes are connected to each other (Figure 2) and to the lining cells through narrowchannels called the Canaliculi (Parfitt, 1984).Remodeling involves the coordinated actions of osteoclasts, osteoblasts, osteocytes and bonelining cells. The remodeling process involves removal of packets of bone from the surfacefollowed by formation of new bonewithin the cavity created. This is a cyclic process involvingfive stages - quiescence, activation, resorption, reversal, formation and back to quiescence.Remodeling happens in adult animals. In younger animals in which the bone is still growingthe process is called modeling. The fundamental difference between the two processes is thatthe remodeling is a cyclic process of erosion and formation on the bone surface, whereas inmodeling, either bone resorption or formation occur continuously for a long period of timewithout interruption (Parfitt, 1984). In adults, bone is continuously remodeled at discretelocations. The balance between bone resorption and formation determines the bone mineralcontent.Osteoclast activity determines the extent of the depth of resorption and the osteoblast activitycontrols the amount of bone formation. The osteocytes, with their interconnected cellularnetwork, play an important role in communication and transportation between cells withinthe bone matrix. When there is a balance between the bone formation and bone resorption ...