**7. 3D spine modelling for epidural training**

In order to simulate the whole epidural procedure a realistic user interface must be provided together with the flexibility of 3D visualization and haptic interaction. The 3D models for the epidural simulator were generated with an object modelling software. Each vertebra is an individual wireframe model, constructed from 514 vertices. The vertices are positioned and then wrapped by a texture. Shadows and light sources are applied through OpenGL interfaces. The spine in the simulator contains 26 separate objects for the thoracic, cervical and lumbar spinal vertebrae, sacrum and coccyx. Layers of tissue, fat, muscle and

skin were appended as layers above the bones. The different parts of the model were exported into separate format files. The format is text based with each vertex on a separate line. A custom C++ OpenGL graphics application then parses the text file to re-create each vertex. The epidural Tuohy needle was created as separate 3D models allowing it to be moved around independently. This is important to allow the operator to place the needle anywhere along the spine for training purposes.

Biomedical Engineering in Epidural Anaesthesia Research 399

enable the palpation of spinal processes and identification of the midline. Finally, the sitting position combined with maximal lumbar flexion is also used, and having the patient bend forward is advantageous to the anaesthetists because it increases the space between the

Based on this, the patient could bend the spine to various positions, so the epidural simulator is required to use computer graphics models of the human spine which can bend, flex and twist. The model can realistically duplicate the shape of the spine during various sitting positions adopted by patients during surgery and epidural anaesthesia. The extent of bending and flexing is kept within the limits of human spine flexibility. Also the model vertebrate adapt in size to match weight and height of specific patient bodies based on parametric modelling [16]. Our spine model is flexible for epidural simulation which offers

The human spine consists of twenty six vertebrae. Each of the vertebrae connects with numerous ligaments. Internally, there is a protective space running through the centre of the spine, housing the spinal cord. The column of vertebrae also provides connection points with the ribs and back muscles. The twenty six vertebrae are segmented into five regions, each with varying characteristics. From cranial to caudal there are cervical vertebrae (C1- C7), thoracic vertebrae (T1 – T12), lumbar vertebrae (L1 – L5), sacrum and coccyx. The human spine is able to bend, flex and rotate in various directions. Lumbar flexion occurs when the patient bends forwards and lumbar extension occurs when bending backwards. The spine was modelled using 3D design software, formed from 26 individual vertebrae, shown in Figure 12. The 26 vertebrae were each loaded as 3D models into a custom made software graphics application. The software renders 3D objects using vertices with the OpenGL graphics library and its utility toolkit (GLUT). The colours of each region of

vertebrae, which increases the target space for the needle to pass through.

**Figure 11.** Four common patient positions used for epidural insertion

vertebrae bone, flesh and the spinal discs were set using materials.

**Figure 12.** The model spine consisting of 26 individually rendered 3D vertebrae

accurate models of spinal vertebrae.

The 3D objects can be viewed as stereograms (Figure 10) by displaying two images of the same object side by side with slight rotation around the Y axis [14]. The epidural simulator also supports this method of stereo in addition to page-flip stereo.

**Figure 10.** Stereogram view of the spine model with two perspectives and binocular parallax

Transparency is applied to skin, subcutaneous fat, supraspinous ligament, interspinous ligament and ligamentum flavum. This allows the user to see the position of the needle tip in the tissue layers. Transparency can be adjusted during the simulation by a control on the keyboard. Rotation is enabled allowing the camera angle to rotate around the scene. This is applied by OpenGL translation and rotation which gives an effect of camera movement whilst the other objects all remain stationary. During rotation, the tip of the needle remains at the central focus point of the screen. Zoom can be applied to move closer or further away from the site of insertion in the working epidural simulator. Pan can also be applied which is a translation of the camera which allows the user to view other areas or to move up and down the spine when selecting the insertion site.

Another issue equally important is the flexibility built into the spine model. There are four common patient positions adopted during the administration of spinal or epidural anaesthesia [15]. Lateral decubitus (Figure 11) involves lying down sideways on the patients left or right, usually the right side is used for caesarean patients, because it is the opposite side from which the patient will lie on during surgery in the left lateral tilt position, which helps to increase the spread of anaesthetic. When the patient lies in lateral positions their back should be close and parallel to the edge of the bed, with their spine in a straight line. However, a variation to this position, maximal lumbar flexion in the lateral decubitus position can be used. The sitting position is preferred and often required in obese patients to enable the palpation of spinal processes and identification of the midline. Finally, the sitting position combined with maximal lumbar flexion is also used, and having the patient bend forward is advantageous to the anaesthetists because it increases the space between the vertebrae, which increases the target space for the needle to pass through.

**Figure 11.** Four common patient positions used for epidural insertion

398 Practical Applications in Biomedical Engineering

anywhere along the spine for training purposes.

down the spine when selecting the insertion site.

also supports this method of stereo in addition to page-flip stereo.

skin were appended as layers above the bones. The different parts of the model were exported into separate format files. The format is text based with each vertex on a separate line. A custom C++ OpenGL graphics application then parses the text file to re-create each vertex. The epidural Tuohy needle was created as separate 3D models allowing it to be moved around independently. This is important to allow the operator to place the needle

The 3D objects can be viewed as stereograms (Figure 10) by displaying two images of the same object side by side with slight rotation around the Y axis [14]. The epidural simulator

**Figure 10.** Stereogram view of the spine model with two perspectives and binocular parallax

Transparency is applied to skin, subcutaneous fat, supraspinous ligament, interspinous ligament and ligamentum flavum. This allows the user to see the position of the needle tip in the tissue layers. Transparency can be adjusted during the simulation by a control on the keyboard. Rotation is enabled allowing the camera angle to rotate around the scene. This is applied by OpenGL translation and rotation which gives an effect of camera movement whilst the other objects all remain stationary. During rotation, the tip of the needle remains at the central focus point of the screen. Zoom can be applied to move closer or further away from the site of insertion in the working epidural simulator. Pan can also be applied which is a translation of the camera which allows the user to view other areas or to move up and

Another issue equally important is the flexibility built into the spine model. There are four common patient positions adopted during the administration of spinal or epidural anaesthesia [15]. Lateral decubitus (Figure 11) involves lying down sideways on the patients left or right, usually the right side is used for caesarean patients, because it is the opposite side from which the patient will lie on during surgery in the left lateral tilt position, which helps to increase the spread of anaesthetic. When the patient lies in lateral positions their back should be close and parallel to the edge of the bed, with their spine in a straight line. However, a variation to this position, maximal lumbar flexion in the lateral decubitus position can be used. The sitting position is preferred and often required in obese patients to Based on this, the patient could bend the spine to various positions, so the epidural simulator is required to use computer graphics models of the human spine which can bend, flex and twist. The model can realistically duplicate the shape of the spine during various sitting positions adopted by patients during surgery and epidural anaesthesia. The extent of bending and flexing is kept within the limits of human spine flexibility. Also the model vertebrate adapt in size to match weight and height of specific patient bodies based on parametric modelling [16]. Our spine model is flexible for epidural simulation which offers accurate models of spinal vertebrae.

The human spine consists of twenty six vertebrae. Each of the vertebrae connects with numerous ligaments. Internally, there is a protective space running through the centre of the spine, housing the spinal cord. The column of vertebrae also provides connection points with the ribs and back muscles. The twenty six vertebrae are segmented into five regions, each with varying characteristics. From cranial to caudal there are cervical vertebrae (C1- C7), thoracic vertebrae (T1 – T12), lumbar vertebrae (L1 – L5), sacrum and coccyx. The human spine is able to bend, flex and rotate in various directions. Lumbar flexion occurs when the patient bends forwards and lumbar extension occurs when bending backwards. The spine was modelled using 3D design software, formed from 26 individual vertebrae, shown in Figure 12. The 26 vertebrae were each loaded as 3D models into a custom made software graphics application. The software renders 3D objects using vertices with the OpenGL graphics library and its utility toolkit (GLUT). The colours of each region of vertebrae bone, flesh and the spinal discs were set using materials.

**Figure 12.** The model spine consisting of 26 individually rendered 3D vertebrae

Initially the vertebrae are positioned in the standing position and are then adjusted by mathematical equations to match the current patient position. The curvature of the spine for four common patient positions was calculated using the equations. The shape of the spine was based on the four common patient positions used for epidural insertion. Our model's prediction for the spine shape for each of the positions is shown in Figure 13 [14].

Biomedical Engineering in Epidural Anaesthesia Research 401

As LF thickness increases, fibrosis increases and elastic tissue decreases. The dorsal side of LF contains more fibrous tissue and less elastic tissue than the dural and middle sides, as indicated by a fibrosis Score of 1.58, 1.63, and 2.63 for dural, middle, and dorsal sides respectively [18]. The loss of elastic fibres caused by increased thickness is more pronounced along the dorsal side. A single patient has several ligamentum flava, one at each spinal level between the lamina and their thicknesses vary according to the spinal level. A study of 77 patients measured LF at spinal level L2/3, L3/4, L4/5, and L5/S1, the mean LF thickness is 2.41, 3.25, 4.08, and 2.68 mm [18]. It was shown that the thickest part of ligamentum flavum is consistently at L4/5, which is the level that endures the greatest mechanical stress. LF is crescent shaped in cross section on the horizontal plane with the thickest part in the middle. It wraps around the circular epidural space and dura. It connects to lamina above and below. The elastic fibres are yellow in colour, hence 'flava' being Latin for yellow. Each flava

Object modelling software was used to create a model of the vertebrae. At the location of L2/L3 a ligamentum flavum was modelled with the thickness 2.41mm which was internally

The interior structure of the ligamentum flavum has been modelled by numerous bundles of fibres extending vertically and parallel to one another, as do the elastic and fibrous tissues in-vivo. By creating this heterogeneous model of the internal structure of ligamentum flavum, the model will describe more accurately how the material responds to a needle being inserted through it. Similar models may be created for interspinous ligament and supraspinous ligament which are also both heterogeneous in nature, consisting of over three

We have applied stereoscopic 3D computer graphics for visualization of epidural insertions. The stereoscopic images are viewed through a head mounted visor containing two OLED micro-displays in stereo using the page-flipped method. The 3D graphics are built from several vertex models of the anatomical structures as described in section 7. The stereo

is a separate ligament which is clearly seen from the side of the lamina.

**Figure 14.** The modelled ligamentum flavum between L2/L3 vertebrae.

types of elastic fibres that can used to provide realistic haptic feedback.

**9. 3D visualisation of epidural procedure** 

comprised of bundles of fibres (Figure 14).

**Figure 13.** The spine model with flexion for four common patient positions

The ability to flex and rotate the spine has provided the opportunity to simulate epidural insertions on patients in various positions. This is important because the feeling of insertion is different for each patient position. This novel aspect has not been attempted in epidural simulation before and will increase versatility of the simulation.
