**5.3 Discretizing the default spine segments**

To achieve a more detailed spine model, the improvement of the default spine model mentioned above is required and can be done in the three following steps: refining the spine segments, reassigning muscle attachments and creating the spinal joints.

From the base human model, the segments may be broken down into individual bones for greater model fidelity. Every bone in the human body is included in the generated skeletal model as a shell model. To discretize the spine region, the standard ellipsoidal segments representing the cervical (C1-C7), thoracic (T1-T12) and lumbar (L1-L5) vertebral groups are firstly removed. Based on input such as center of mass location and orientation of each vertebra, the individual vertebra segment is then created. Figure 11 shows all ellipsoidal segments of 24 vertebrae in the cervical, thoracic and lumbar regions after discretizing.

The muscles are attached to the respective bones based on geometric landmarks on the bone graphics. With the new vertebra segments created, the muscle attachments to the original segment must be reassigned to be more specific to the newly created vertebra segments. The physical attachment locations will remain the same. Figure 12(a) and (b) shows the anterior

Development of a Detailed Human Spine Model with Haptic Interface 177

It is necessary to create individual non-standard joints representing intervertebral discs between newly created vertebrae. The spinal joints are modeled as torsional spring forces and the passive 3 DOFs jointed action can be defined with user-specified stiffness, damping, angular limits and limiting stiffness values. These joints are used in an inverse dynamics analysis to record the joint angulations while the model is being simulated. The properties of the joints can be found in the literature (Moroney et al., 1988; Panjabi et al., 1976; Schultz

To stabilize the spine model, interspinous, flaval, anterior longitudinal, posterior longitudinal and capsule ligaments are created. Figure 13 displays various types of

Multifidus muscle: The multifidus muscle is divided into 19 fascicles on each side according to descriptions by the group of Bogduk (Bogduk et al., 1992a; Macintosh & Bogduk, 1986). The multifidus can be modeled as three layers with the deepest layer having the shortest fibres and spanning one vertebra. The second layer spans over two vertebrae, while the third layer goes all the way from L1 and L2 to posterior superior iliac spine (Zee et al., 2007). The rather short span of the multifidus fascicles makes it possible to model them as line

Erector spinae muscle: According to (Macintosh & Bogduk, 1987; 1991), there are four divisions of the erector spinae: longissimus thoracis pars lumborum, iliocostalis lumborum pars lumborum, longissimus thoracis pars thoracis and iliocostalis lumborum pars thoracis. The fascicles of the longissimus thoracis and iliocostalis lumborum pars lumborum originate from the transverse processes of the lumbar vertebrae and insert on the iliac crest close to the posterior superior iliac spine (Zee et al., 2007). The fascicles of the longissimus thoracis pars thoracis originate from the costae 1-12 close to the vertebrae and insert on the spinous process of L1 down to S4 and on the sacrum. The fascicles of the iliocostalis lumborum pars thoracis originate from the costae 5–12 and insert on the iliac crest. Since muscles of the two pars thoracis are automatically generated by LifeMOD, only muscles of the two pars

lumborum need to be added to the model as shown in Figure 14(b).

et al., 1979; Schultz & Ashton-Miller, 1991).

**5.4 Creating the ligamentous soft tissues** 

ligaments attached to vertebrae in the cervical spine region

Fig. 13. Various types of ligaments in the cervical spine

**5.5 Implementing lumbar muscles** 

elements without via-points (Figure 14(a)).

and posterior view of several muscles in neck/trunk regions. Table 1 lists attachment locations of these muscles.

Fig. 11. Front and side view of the complete discretized spine

Fig. 12. Neck and trunk muscle set: (a) Anterior view; (b) Posterior view


Table 1. Attachment locations of neck and trunk muscle set

It is necessary to create individual non-standard joints representing intervertebral discs between newly created vertebrae. The spinal joints are modeled as torsional spring forces and the passive 3 DOFs jointed action can be defined with user-specified stiffness, damping, angular limits and limiting stiffness values. These joints are used in an inverse dynamics analysis to record the joint angulations while the model is being simulated. The properties of the joints can be found in the literature (Moroney et al., 1988; Panjabi et al., 1976; Schultz et al., 1979; Schultz & Ashton-Miller, 1991).
