**2. Related works**

Biomechanic models can be divided into four categories: physical, *in-vitro*, *in-vivo* and computer models. Computer models have been extensively used because they can provide information that cannot be easily obtained by other models, such as internal stresses or strains. They can also be used for multiple experiments with uniform consistency, lowering experimental cost, and simulating different situations easily and quickly. Multi-body and finite element models, or a combination of the two are the most popular simulation tools that can contribute significantly to our insight of the biomechanics of the spine.

Development of a Detailed Human Spine Model with Haptic Interface 167

years, haptics has been widely applied in numerous VR environments to increase the levels of realism. Especially, haptics has been investigated at length for medical education and surgical simulations, like surgical planning and laparoscopic surgical training (Basdogan et al., 1998; Forest et al., 2004; Gorman et al., 2000; Seitz et al., 2004; Williams et al., 2004).

Haptics has been widely utilized in medical fields, but little has been applied to spinal diseases. Integrating haptics into spine models means surgeons can investigate biodynamic responses of whole human spine which either have not been investigated enough in the literature or are limited to partial spine segments. Understanding biodynamic behaviour of the whole human spine is beneficial to wheelchair design applications for the disabled. When applying forces to a certain vertebra of the spine under fixed constraints on sacrum and selected vertebrae, users such as surgeons or clinicians can feel force feedback from the spine as well as examine its locomotion. These results are useful for designing suitable and comfortable wheelchairs for the disabled with specific abnormal spinal configurations. In addition, by simulating in a haptically integrated graphic environment, orthopaedic surgeons can gain insight into the planning of surgery to correct severe scoliosis. Different rod and brace designs can be experimented with using this virtual environment. Furthermore, the surgeons may be able to understand the change in force distribution following spine fusion procedures, which can also assist in post-operative physiotherapy. The objective of this chapter is to present the development of a detailed human spine model with a haptic interface, which can be

The spinal column (Figure 1) extends from the skull to pelvis and is made up of 33 individual vertebrae that are stacked on top of each other. The spinal column can be divided into 5 regions: 7 cervical (C1-C7), twelve thoracic (T1–T12), and five lumbar vertebrae (L1– L5) in the lower back, five bones (joined together in adults) to form the bony sacrum, and 3-

Intervertebral discs (Figure 2) are soft tissue structures situated between each of the 24 cervical, thoracic, and lumbar vertebrae of the spine. Their functions are to separate consecutive vertebral bodies. Once the vertebrae are separated, angular motions in the sagittal (forward, backward bending) and coronal planes (sideway bending) can occur.

Facet joints are paired joints which are found in the posterior of the spinal column (Figure 3). The surfaces of each joint are covered by cartilage which helps to smooth the movement between two vertebrae. Certain motions are facilitated by these joints, such as: bending forward, bending backward and twisting. In addition, people can feel pain if the joints are damaged because of the connected nerves. Some experts believe that these joints are the

The neural elements (Figure 4) consist of the spinal cord and nerve roots. The spinal cord runs from the base of the brain down through the cervical and thoracic spine. The spinal cord is surrounded by spinal fluid and by several layers of protective structures, including the dura mater, the strongest, outermost layer. At each vertebral level of the spine, there is a

pair of nerve roots. These nerves go to supply particular parts of the body.

useful for investigating various medical applications.

5 bones fused together to form the coccyx or tailbone.

most common reasons for spinal discomfort and pain.

**3. Overview of human spine structure** 

Finite element models (FEMs) are helpful to understand underlying mechanisms of injury and dysfunction, leading to improved prevention, diagnosis and treatment of clinical spinal problems. FEMs often provide estimates of parameters that *in-vivo* or *in-vitro* experimental studies either cannot or are difficult to obtain accurately. FEMs are divided into two categories: dynamic study and static study models. Models for static study generally are more detailed in representing spinal geometries (Greaves et al., 2008; Kumaresan et al., 1999; Natarajan et al., 2007; Teo & Ng, 2001; Yoganandan et al., 1996). They can predict internal stresses, strains and other biomechanical properties under complex loading conditions, but they generally only consist of one or two motion segments and do not provide effective insight for the whole column. Dynamic study models generally include a series of vertebrae connected by ligaments and disks modeled as springs (Maurel et al., 1997; Ng & Teo, 2005; Pankoke et al., 1998; Schmidt et al., 2008; Seidel et al., 2001; Zander et al., 2002). They predict locally the kinematic and dynamic responses of a certain part of the spine under load. FEMs have also been widely used to study scoliosis biomechanics (Aubin, 2002; Gignac et al., 2000; Lafage et al., 2004; Schlenk et al., 2003). Understanding of the biomechanics of spine deformation helps surgeons formulate treatment strategies for surgery and design and develop new medical devices. Being complex, FEMs of scoliotic spines are usually restricted to 2D models or simplified 3D elastic beam element models. Although preliminary results achieved are promising, extensive validation is necessary before using the models clinically.

Multi-body models have advantages like less complexity, less demand on computational power, and relatively simpler validation requirements. They possess potential to simulate both kinematics and kinetics of the spine effectively. Rigid bodies are interconnected by various joints. Multi-body models can also include many anatomical details while being computationally efficient. The head and vertebrae are modeled as rigid bodies and soft tissues are usually modeled as massless spring-damper elements. Such models are capable of producing biofidelic responses and can be broken down into two categories: car collisions and whole-body vibration investigations. In the former, displacements of the head with respect to the torso, accelerations, intervertebral motions, and neck forces/moments can provide good predictions for whiplash injury (Esat & Acar, 2007; Garcia & Ravani, 2003; Jager et al., 1996; Jun, 2006; Linder, 2000; Stemper et al., 2004; Van Der Horst, 2002). In the latter, multi-body models are helpful for determining the forces acting on the intervertebral discs and endplates of lumbar vertebrae (Fritz, 1998; Luo & Goldsmith, 1991; Seidel & Griffin, 2001; Verver et al., 2003; Yoshimura et al., 2005). In both cases, multi-body models focus either on the cervical or the lumbar spine. Since spine segments are partially modeled in detail, it is impossible to investigate the kinematics of the thoracic spine region. In other words, global biodynamic response of the whole spine has not been studied thoroughly.

Although finite element and multi-body models are powerful tools used to study intrinsic properties of injury mechanisms, other techniques have been developed to obtain deeper understanding of biomechanical properties of medical diseases. One such technique is *computer haptics*. The word *haptics* was introduced in the early 20th century to describe the research field that addresses human touch-based perception and manipulation. In the early 1990s, the synergy of psychology, biology, robotics and computer graphics made computer haptics possible. Much like computer graphics is concerned with rendering visual images, computer haptics is the art and science of synthesizing computer generated forces to the user for perception of virtual objects through the sense of touch. Simulation with the addition of haptics may offer better realism compared to only a visual interface. In recent

Finite element models (FEMs) are helpful to understand underlying mechanisms of injury and dysfunction, leading to improved prevention, diagnosis and treatment of clinical spinal problems. FEMs often provide estimates of parameters that *in-vivo* or *in-vitro* experimental studies either cannot or are difficult to obtain accurately. FEMs are divided into two categories: dynamic study and static study models. Models for static study generally are more detailed in representing spinal geometries (Greaves et al., 2008; Kumaresan et al., 1999; Natarajan et al., 2007; Teo & Ng, 2001; Yoganandan et al., 1996). They can predict internal stresses, strains and other biomechanical properties under complex loading conditions, but they generally only consist of one or two motion segments and do not provide effective insight for the whole column. Dynamic study models generally include a series of vertebrae connected by ligaments and disks modeled as springs (Maurel et al., 1997; Ng & Teo, 2005; Pankoke et al., 1998; Schmidt et al., 2008; Seidel et al., 2001; Zander et al., 2002). They predict locally the kinematic and dynamic responses of a certain part of the spine under load. FEMs have also been widely used to study scoliosis biomechanics (Aubin, 2002; Gignac et al., 2000; Lafage et al., 2004; Schlenk et al., 2003). Understanding of the biomechanics of spine deformation helps surgeons formulate treatment strategies for surgery and design and develop new medical devices. Being complex, FEMs of scoliotic spines are usually restricted to 2D models or simplified 3D elastic beam element models. Although preliminary results achieved are promising, extensive validation is necessary before using the models clinically. Multi-body models have advantages like less complexity, less demand on computational power, and relatively simpler validation requirements. They possess potential to simulate both kinematics and kinetics of the spine effectively. Rigid bodies are interconnected by various joints. Multi-body models can also include many anatomical details while being computationally efficient. The head and vertebrae are modeled as rigid bodies and soft tissues are usually modeled as massless spring-damper elements. Such models are capable of producing biofidelic responses and can be broken down into two categories: car collisions and whole-body vibration investigations. In the former, displacements of the head with respect to the torso, accelerations, intervertebral motions, and neck forces/moments can provide good predictions for whiplash injury (Esat & Acar, 2007; Garcia & Ravani, 2003; Jager et al., 1996; Jun, 2006; Linder, 2000; Stemper et al., 2004; Van Der Horst, 2002). In the latter, multi-body models are helpful for determining the forces acting on the intervertebral discs and endplates of lumbar vertebrae (Fritz, 1998; Luo & Goldsmith, 1991; Seidel & Griffin, 2001; Verver et al., 2003; Yoshimura et al., 2005). In both cases, multi-body models focus either on the cervical or the lumbar spine. Since spine segments are partially modeled in detail, it is impossible to investigate the kinematics of the thoracic spine region. In other words, global biodynamic response of the whole spine has not been studied thoroughly.

Although finite element and multi-body models are powerful tools used to study intrinsic properties of injury mechanisms, other techniques have been developed to obtain deeper understanding of biomechanical properties of medical diseases. One such technique is *computer haptics*. The word *haptics* was introduced in the early 20th century to describe the research field that addresses human touch-based perception and manipulation. In the early 1990s, the synergy of psychology, biology, robotics and computer graphics made computer haptics possible. Much like computer graphics is concerned with rendering visual images, computer haptics is the art and science of synthesizing computer generated forces to the user for perception of virtual objects through the sense of touch. Simulation with the addition of haptics may offer better realism compared to only a visual interface. In recent years, haptics has been widely applied in numerous VR environments to increase the levels of realism. Especially, haptics has been investigated at length for medical education and surgical simulations, like surgical planning and laparoscopic surgical training (Basdogan et al., 1998; Forest et al., 2004; Gorman et al., 2000; Seitz et al., 2004; Williams et al., 2004).

Haptics has been widely utilized in medical fields, but little has been applied to spinal diseases. Integrating haptics into spine models means surgeons can investigate biodynamic responses of whole human spine which either have not been investigated enough in the literature or are limited to partial spine segments. Understanding biodynamic behaviour of the whole human spine is beneficial to wheelchair design applications for the disabled. When applying forces to a certain vertebra of the spine under fixed constraints on sacrum and selected vertebrae, users such as surgeons or clinicians can feel force feedback from the spine as well as examine its locomotion. These results are useful for designing suitable and comfortable wheelchairs for the disabled with specific abnormal spinal configurations. In addition, by simulating in a haptically integrated graphic environment, orthopaedic surgeons can gain insight into the planning of surgery to correct severe scoliosis. Different rod and brace designs can be experimented with using this virtual environment. Furthermore, the surgeons may be able to understand the change in force distribution following spine fusion procedures, which can also assist in post-operative physiotherapy. The objective of this chapter is to present the development of a detailed human spine model with a haptic interface, which can be useful for investigating various medical applications.
