**10. Haptic interface for epidural insertion**

Haptic devices have become a more popular and accepted tool for medical simulation and provide an accurate way of re-creating the feel of surgery [21, 22]. The insertion of an epidural is a procedure which relies almost entirely upon feeling the forces on the needle. Epidural simulators are therefore ideally suited to haptic technology. This section describes methods for configuration of a haptic device to interact with 3D computer graphics as part of a high fidelity epidural simulator development program.

Haptic devices have been used in epidural simulators previously, although they are not based on measured patient data from needle insertions. Instead, they are configured by 'experts' trialling and adjusting the system. It is therefore hard to assess the accuracy of the forces generated and so creates a real potential for improvement. The haptic device has currently been set up to reconstruct the force data found during the porcine trial. The force data from the graphs were divided into sections to represent each of the tissue layers separately [23].

A haptic device has been connected and used as an input to move the needle in 3D, and also to generate force feedback to the user during insertion (Figure 16). A needle insertion trial was conducted on a porcine cadaver to obtain resultant pressure data (Section 5). The data generated from this trial was used to recreate the feeling of epidural insertion in the simulator. The interaction forces have been approximated to the resultant force obtained during the trial

representing the force generated by the haptic device. The haptic device is interfaced with the 3D graphics (see Sections 7-9) for visualization. As the haptic stylus is moved, the needle moves on the screen and the depth of the needle tip indicates which tissue layer is being penetrated. Different forces are generated by the haptic device for each tissue layer as the epidural needle is inserted. As the needle enters the epidural space, the force drops to indicate loss of resistance. An advantage to the use of haptic devices for epidural simulators is that they can accept various adjustable settings, so that patient variation including weight, height, age and sex can be accounted for, which helps to train for a range of patients. Patient variety is becoming an even more important aspect than ever since the current obesity epidemic poses great challenges for the anaesthetist. In obese patients, the depth to the epidural space is increased, anatomical landmarks are harder to feel and the midline is more difficult to locate. The resultant effect is that the risk of injury is increased.

Biomedical Engineering in Epidural Anaesthesia Research 405

Insertion force (N)

Needle depth (mm)

measured data and the aim is to develop a generic simulator based on measured data to offer a

realistic in-vitro experience before attempting the procedure on actual patients.

(mm)

**11. Human-computer interface for loss of resistance syringe** 

simulated insertion of the Tuohy epidural needle through the spinal ligaments.

With the above developed components, a hardware device has been created consisting of a regular Portex LOR syringe connected to the computer via a serial data transfer device. This allows a regular clinical syringe to be used as part of an interactive system for the epidural simulator development. The syringe was also combined with the haptic device to create a comprehensive human-computer interface. The simulator can measure force applied to the plunger and the resultant pressure of the saline inside the syringe barrel. This interface enables a real clinical syringe to interface with a 3D graphical visualization showing the

The developed hardware interface makes use of the equipment as developed in Sections 4 & 6 by incorporating custom made hardware with the developed software and the graphical visualization of the needle insertion procedure. The hardware device takes measurements of the forces applied onto the needle and the resultant pressure of the saline inside the barrel of the syringe caused by the pressure from the operators thumb on the plunger. The measurements are sent to the computer by a custom-made hardware interface device (see Sections 4 & 6). The graphical simulation uses these measurements to update the needle in the simulation and calculates the needle position. The graphical software calculates if any collisions have occurred between the needle and any bone structures, plus the resistance of insertion to saline, and the force required for the needle to move forwards through the

The developed human-computer interface uses an actual syringe and an epidural Tuohy needle as shown in Figure 17. During insertions, the LOR syringe is normally connected directly onto the Tuohy needle. We have introduced a three-way tap between the needle and syringe. This connects onto a one metre length of saline manometer tubing which runs to a disposable pressure transducer. The transducer converts the pressure of the saline into an electrical signal. The electrical signal is connected into a hardware device which amplifies and sends the pressure reading to the computer. This allows the graphics visualization to

Skin 3 0 12.9 Subcutaneous fat 6 3 6 Supraspinous ligament 4 9 9 Interspinous ligament 26 13 8 Ligamentum flavum 3 39 11.1 Epidural space 6 42 0 Dura 15 48 2.0

Porcine Tissue Layer Tissue thickness

**Table 1.** Insertion forces in porcine [23, 24]

current ligament.

**Figure 16.** The haptic device interfaced with the graphics

To apply different forces to each layer, 3D vector regions were defined within the graphics model. As the needle tip enters these regions, the software identifies which tissue layer the needle is in, based on the depth data from the trial (Table 1). The software then uses a lookup table to find the appropriate force for each layer, and instructs the haptic device to generate that force. The forces generated represent the resultant pressure on the syringe which is a sum of all resistances to insertion, which are the equal and opposite to the force applied by the user. For example, if a particular layer has insertion force of 4.3N, and the user is pressing with only 3.2N, then the haptic device exerts 3.2N, so the stylus remains stationary. Only if the user increases the force to over 4.3N the stylus will move forward. Table 1 is based on measurements taken from our porcine trial in line with [24].

The haptic device is also able to simulate palpation of the lumbar region. Palpation is the process for choosing which location to insert the needle. The haptic device was configured for palpation by creating a surface hardness profile of the lumbar region, with a hardness value for each point in the region (see Section 8). The haptic device can be used to press at any point and the user can feel the hardness at that point. This allows the user to locate landmarks and choose a point to commence needle insertion. Our advanced haptic interface is based on the


measured data and the aim is to develop a generic simulator based on measured data to offer a realistic in-vitro experience before attempting the procedure on actual patients.

**Table 1.** Insertion forces in porcine [23, 24]

404 Practical Applications in Biomedical Engineering

The resultant effect is that the risk of injury is increased.

**Figure 16.** The haptic device interfaced with the graphics

representing the force generated by the haptic device. The haptic device is interfaced with the 3D graphics (see Sections 7-9) for visualization. As the haptic stylus is moved, the needle moves on the screen and the depth of the needle tip indicates which tissue layer is being penetrated. Different forces are generated by the haptic device for each tissue layer as the epidural needle is inserted. As the needle enters the epidural space, the force drops to indicate loss of resistance. An advantage to the use of haptic devices for epidural simulators is that they can accept various adjustable settings, so that patient variation including weight, height, age and sex can be accounted for, which helps to train for a range of patients. Patient variety is becoming an even more important aspect than ever since the current obesity epidemic poses great challenges for the anaesthetist. In obese patients, the depth to the epidural space is increased, anatomical landmarks are harder to feel and the midline is more difficult to locate.

To apply different forces to each layer, 3D vector regions were defined within the graphics model. As the needle tip enters these regions, the software identifies which tissue layer the needle is in, based on the depth data from the trial (Table 1). The software then uses a lookup table to find the appropriate force for each layer, and instructs the haptic device to generate that force. The forces generated represent the resultant pressure on the syringe which is a sum of all resistances to insertion, which are the equal and opposite to the force applied by the user. For example, if a particular layer has insertion force of 4.3N, and the user is pressing with only 3.2N, then the haptic device exerts 3.2N, so the stylus remains stationary. Only if the user increases the force to over 4.3N the stylus will move forward.

The haptic device is also able to simulate palpation of the lumbar region. Palpation is the process for choosing which location to insert the needle. The haptic device was configured for palpation by creating a surface hardness profile of the lumbar region, with a hardness value for each point in the region (see Section 8). The haptic device can be used to press at any point and the user can feel the hardness at that point. This allows the user to locate landmarks and choose a point to commence needle insertion. Our advanced haptic interface is based on the

Table 1 is based on measurements taken from our porcine trial in line with [24].
