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Patient Impression Guidelines
1. Polyvinyl Siloxane (PVS) impression material
Light coloured and quick setting with a plastic impression tray. For example an ideal impression material is 3M Imprint 4 Penta Quick. This will facilitate accurate capture of the occlusion with long term dimensional stability. Similar materials from other impression material manufacturers are acceptable provided they are light coloured. Alginate is not suitable.
2. Provide a precise impressionEnsure that a full arch is captured taking care in particular that the final impression you submit provides an accurate record of the crowns of the teeth without drag marks or distortions.
If you are proposing to place an implant at the end of an unbounded saddle, please take particular note to capture the saddle area distal to the proposed implant site as we will incorporate a soft tissue rest in that region into the proposed surgical guide.
3. Disinfect & Dry ImpressionsPlease disinfect and dry all impressions before sending them.
1. Field of view
The recommended field of view (FOV) is at least 12 x 12cm. At a minimum, the FOV should cover the entire arch where the implant is to be placed. A scan with a smaller FOV will compromise the accuracy of the model-CT registration. In the maxilla the FOV should cover at least half of the maxillary sinus superiorly down to the incisal/occlusal surfaces of the teeth.
In the mandible the FOV should cover the incisal/occlusal surfaces of the teeth superiorly down to the inferior border of the mandible. For the purposes of the surgical guide, the scan does not need to cover the entire skull as this can increase the radiation dose to the patient and result in a decreased image resolution.
2. ThicknessThe recommended slice thickness of the scan is 0.2mm or less. A larger slice thickness makes it difficult to visualise small vital structures.
3. Patient PositioningThe patient should place their tongue back towards the soft palate. This allows us to clearly visualise the gingiva. The patient needs to remain completely still for the duration of the scan. If possible a bite block should be used to stabilise the patient.
4. DICOM FormatThe format of the scan taken should be uploaded to a CD or USB as a set of non-proprietary single frame DICOM files.
Radiographic CT Template
Radiographic templates are utilised in cases when the patient is edentulous or close to edentulous. This allows accurate registration of the model impression scan with the CT since there are no teeth present to use as reference points.
1. Edentulous in the lower arch or bothFor patients who are edentulous in the lower arch or both arches, a radiographic template is always required.
2. Edentulous in the upper arch onlyIf the patient is edentulous in the upper arch only, the CT scan may be taken with the upper denture removed and the tongue placed towards the soft palate. This is important to ensure that the outline of the hard palate and gingiva is clearly visible in the CT for accurate model registration. The patient must remain completely still during the scan. A radiographic template is strongly recommended for patients who have tremors or have difficulty remaining still.
3. Opposing DentureIf the patient wears an opposing denture, this can be worn during the scan provided a surgical guide is not required for that arch. The wax rim in the radiographic template can be adjusted so that the patient is biting in centric occlusion.
To produce the radiographic template, we require a PVS impression of the arch. The template is worn in place of the denture during the CT and has a wax rim.