**2.3 Experimental measurements**

Experimental studies were conducted in order to assess the taper of teeth prepared for crowns by dental students. Teeth were prepared both in the clinical setting on live patients and also on artificial plastic teeth set in a manikin phantom head jaw in a preclinical laboratory. It would then not only be possible to measure taper achieved but to also compare this with other dental schools.

Dental students are first taught to carry out crown preparations in a laboratory based course with didactic sessions of lectures and handouts. There are also practical sessions with live demonstrations projecting magnified images on television monitors via video cameras of the procedure in detail. They are taught to create a preparation with a 6° taper by holding a 3° tapered bur parallel to the long axis of the tooth while cutting each wall (Shillingburg et al., 1997). The students are then expected to perform the procedures until satisfactory and then pass practical examinations for that procedure prior to being allowed to perform the procedure on a patient.

The specimens used were obtained from both crown preparations carried out by students for their practical examinations in the laboratory and also from crown preparations done in the clinic on patients. There were 99 laboratory specimens or dies and 40 clinical dies. In order to form the replicas or dies of the tooth preparations an impression of the tooth has to be taken. This was done by using addition cured polyvinylsiloxane impression material, Reprosil (Dentsply Caulk, Milford DE, USA). The technician who fabricated the crowns used the first pour of the impression to fabricate the actual crown that was fitted onto the tooth. The impression can be used again for a second pour in order to make the die that was used to measure the taper. This second use of this type of impression has been shown to be as accurate as the first pour (Johnson & Craig, 1985). The die stone used to pour into the impression to create the die replica was Type IV (Vel Mix) (Kerr Corp., Orange CA, USA) and was mixed under vacuum and poured. The material was left to set and the standing base of the dies were trimmed specifically to be flat to the long axis of the prepared tooth and the sides of the base were also milled so that the buccal and lingual were parallel to each other and the mesial and distal parallel to each other.

The die spacer (Quick Set Die Spacer Blue, Belle de St. Claire, (Kerr Corp., Orange CA, USA) was painted over the crown preparations in 2 layers according to manufacturer's instructions, as is the usual procedure when fabricating crowns. This also overcame the incompatibility of the pink stone to the probe and was needed in order for the scanning machine to effectively read the preparation, allowing the probing tracer to follow the surface profile rather than the details of the surface texture.

The die stone replica was fixed in a clamping device and digitised by the purpose built dental scanner Incise (Renishaw, UK) with a scanning probe of SM25-1 (Renishaw, UK) shown in Figure 4. The digitisation is conducted by Tracecut Controlling Software Package (Renishaw, UK). The probe tip is made of industrial ruby of a sphere with 1mm diameter and it has been calibrated before the digitisation process. The digitisation procedure was defined as such: the sampling interval in both X and Y directions was 0.1mm, and scanning speed was 200 points per minutes, and scanning deflection is 0.5mg. The data cloud was interpolated as a surface by using a 3-D free form surface analysis software – Cloud (UCL, UK). The degree of taper of the prepared tooth was determined in a bucco-lingual (BL) plane and mesio-distal (MD) plane.

Experimental studies were conducted in order to assess the taper of teeth prepared for crowns by dental students. Teeth were prepared both in the clinical setting on live patients and also on artificial plastic teeth set in a manikin phantom head jaw in a preclinical laboratory. It would then not only be possible to measure taper achieved but to also

Dental students are first taught to carry out crown preparations in a laboratory based course with didactic sessions of lectures and handouts. There are also practical sessions with live demonstrations projecting magnified images on television monitors via video cameras of the procedure in detail. They are taught to create a preparation with a 6° taper by holding a 3° tapered bur parallel to the long axis of the tooth while cutting each wall (Shillingburg et al., 1997). The students are then expected to perform the procedures until satisfactory and then pass practical examinations for that procedure prior to being allowed to perform the

The specimens used were obtained from both crown preparations carried out by students for their practical examinations in the laboratory and also from crown preparations done in the clinic on patients. There were 99 laboratory specimens or dies and 40 clinical dies. In order to form the replicas or dies of the tooth preparations an impression of the tooth has to be taken. This was done by using addition cured polyvinylsiloxane impression material, Reprosil (Dentsply Caulk, Milford DE, USA). The technician who fabricated the crowns used the first pour of the impression to fabricate the actual crown that was fitted onto the tooth. The impression can be used again for a second pour in order to make the die that was used to measure the taper. This second use of this type of impression has been shown to be as accurate as the first pour (Johnson & Craig, 1985). The die stone used to pour into the impression to create the die replica was Type IV (Vel Mix) (Kerr Corp., Orange CA, USA) and was mixed under vacuum and poured. The material was left to set and the standing base of the dies were trimmed specifically to be flat to the long axis of the prepared tooth and the sides of the base were also milled so that the buccal and lingual were parallel to

The die spacer (Quick Set Die Spacer Blue, Belle de St. Claire, (Kerr Corp., Orange CA, USA) was painted over the crown preparations in 2 layers according to manufacturer's instructions, as is the usual procedure when fabricating crowns. This also overcame the incompatibility of the pink stone to the probe and was needed in order for the scanning machine to effectively read the preparation, allowing the probing tracer to follow the surface

The die stone replica was fixed in a clamping device and digitised by the purpose built dental scanner Incise (Renishaw, UK) with a scanning probe of SM25-1 (Renishaw, UK) shown in Figure 4. The digitisation is conducted by Tracecut Controlling Software Package (Renishaw, UK). The probe tip is made of industrial ruby of a sphere with 1mm diameter and it has been calibrated before the digitisation process. The digitisation procedure was defined as such: the sampling interval in both X and Y directions was 0.1mm, and scanning speed was 200 points per minutes, and scanning deflection is 0.5mg. The data cloud was interpolated as a surface by using a 3-D free form surface analysis software – Cloud (UCL, UK). The degree of taper of the prepared tooth was determined in a bucco-lingual (BL)

**2.3 Experimental measurements** 

compare this with other dental schools.

each other and the mesial and distal parallel to each other.

profile rather than the details of the surface texture.

plane and mesio-distal (MD) plane.

procedure on a patient.

Fig. 4. Incise dental scanner (Renishaw, UK)

All results were recorded and data were analysed by means of a one-way analysis of variance (ANOVA) and student t tests for any significant differences.
