is very likely that some parts will be over- or under-extended and therefore have to be modified prior to impression taking to ensure that the entirety of the mucosa is recorded accurately. greenstick or silicone putty can be used to extend the trays if they are under-extended; this is of vital importance, as any unsupported impression material may distort until the impressions are cast. a suitable material such as alginate can be used for this purpose. secondary (master or working) impressions as described above, special trays (acrylic or shellac) ensure that the secondary impressions accurately record the tissues whilst ensuring a uniform thickness of impression material throughout the tray. different impression materials will have different thickness requirements. alginate, for example, requires a thickness of at least 3mm to prevent distortion whereas the more elastic silicone materials can be used in thickness of 1–2 mm. therefore, when special trays are constructed, it is the responsibility of the prescribing clinician to ask for the appropriate level of spacing between the tray and the tissues. another feature which should be incorporated into special trays is tissue stops, which can be described as 2-3mm wide extensions on the impression surface of the special tray. without the incorporation of tissue stops, when the special tray is tried in the mouth to check for the accuracy of extensions, it will appear over-extended as the laboratory has extended the tray in a way that will allow the specified thickness of impression material to be accommodated. tissue stops allow the clinician to appropriately assess the extensions of the tray. the impression materials that can be used with special trays are: zinc oxide eugenol impression paste impression plaster addition silicones condensation silicones polysulphide polyether border moulding border moulding refers to the functional or manual manipulation of the cheeks and lips in order to mould the borders of the impression to that of the functional depth of the sulcus and floor of mouth. this is necessary for ensure stability and adequate retention of the complete dentures. the following steps can be carried out during impression taking: lower impression: ask patient to raise tongue to contact upper lip and move it to the right and left cheek firmly pull and relax the cheeks and lips the tray should be supported by the clinician throughout the moulding mucostatic and mucocompressive (mucodisplacive) impression techniques there are two ways in which the soft tissues can be recorded during impression taking: mucostatic impression records the soft tissues in their resting state, thus no or minimal pressure is applied during impression taking. this technique has the advantage of ensuring a close adaptation of the denture base to the entirety of the mucosa and hence enhancing retention. due to the fact that the mucosa is uneven in compressibility, however, there will inevitably be an uneven distribution of loads during masticatory function. an impression material of low viscosity (e.g. impression paste, alginate or light body silicone) is selected for this technique. a mucocompressive impression is obtained by applying some pressure to the soft tissues during impression taking, thus recording the shape of the soft tissues under masticatory loading (functional impression technique, i.e. the force is applied by asking the patient to bite down on the impression tray). consequently, the mucosa will have an even distribution of loads during function, but the retention of the denture is adversely affected as it inhibits a close adaptation of the denture base to the mucosa in the resting position, which occurs during the majority of time. such a technique, however, can be considered in patients with a history of mucosal trauma and discomfort (particularly in the lower jaw). suitable materials for this purpose include high viscosity silicone impression materials. the ultimate goal of complete dentures is to maintain oral health and function. complete dentures should be comfortable for the individual while also improving aesthetics and psychological well-being. to achieve these goals, it is important to obtain an accurate impression in order to design and create a denture that has adequate retention and stability. denture-related problems can be linked to dentist-related factors, patient-related factors or processing errors. the most common denture-related problems include insufficient retention and improper jaw relations. these are both related to the final-impression technique and the material used to create the dentures. a cochrane review in 2018 comparing final impression techniques and materials for making complete dentures concluded that further high-quality research is required as there was no clear evidence to suggest that one technique or material had a significant advantage over another. bite registration once the impressions have been cast, a set of models has been produced that provide the clinician and dental technician with a replica of the upper and lower jaws with which to work in order to produce the final complete denture. an integral part to the construction is to record how the patient is or should be biting, (i.e. the spatial relationship between the maxilla and the mandible) as well as recording all the necessary information for the next stage, the wax try-in. occlusal vertical dimension, resting vertical dimension and freeway space when setting up the teeth during construction of complete dentures, the clinician must decide a vertical height on which the patient will be biting upon; this is termed the occlusal vertical dimension (ovd). this task is particularly challenging in complete dentures, as there is no existing occlusion to which the clinician can reference to, and as a result, it is the cause of many errors in complete denture construction. the resting vertical dimension (rvd) may be defined as the vertical dimension between two points, one on the maxilla and one on the mandible, when the patient's muscles are at a relaxed position. the difference between ovd and rvd is termed the freeway space (fws). this distance should be between 2–4 mm. rvd - ovd = fws = 2–4 mm in an edentate patient, the ovd cannot be measured unless it was recorded prior to clearance of the dentition or pre-existing dentures provide a satisfactory value. in the majority of cases, however, the ovd needs to be calculated by determining the rvd and allowing for adequate fws (i.e. ovd = rvd - fws = rvd - (2 to 4mm)). the patient is asked to relax the muscles of the mandible, and the measurement for rvd is taken with a willis gauge from a point on the chin and a point underneath the nose. the record (bite) blocks record blocks are made in such a way so that the dental technician is provided with all the information necessary to provide a wax replica of the dentures. they consist of blocks of wax resting on a rigid base that can be made out of shellac, light-cured or heat-cured acrylic. the base can sometimes be made out of wax, however, such a material lacks the rigidity required to ensure accurate measures are taken. additionally, it may distort during transport and thus damage the validity of the recordings. acrylic resins demonstrate the best accuracy of fit and are therefore the most retentive, with heat-cured acrylic being superior to light-cured. the record blocks are inserted in the mouth and the following should be examined and deemed satisfactory prior to proceeding with any adjustments: retention extensions stability comfort adjusting the upper record block orientation of occlusal plane - using either a wooden spatula or a more sophisticated fox's occlusal plane indicator, the orientation of the upper occlusal plane should be parallel to both the ala-tragal line and the interpupillary line. level of occlusal plane - the block should be trimmed or added onto so that the height of the rim is aesthetically pleasing to the amount of wax shown when the patient is at rest (block should be just visible) and when the patient is asked to smile (a few mm should be visible incisally). a more thorough assessment can be performed by asking the patient to say a few sentences while the clinician concentrates on how much of the record block is visible. such adjustments will guide the dental technician to the position and length of teeth to be incorporated in the dentures. shaping of the buccal surfaces to ensure adequate lip and cheek support naso-labial angle 102–116o shaping of the palatal surface to ensure adequate tongue space mark midline, canine line and smile line adjusting the lower record block conforming to the height of desired ovd by either adding onto or removing wax from the block relationship of the buccal and lingual surfaces to the neutral zone recording the centric occlusion centric occlusion refers to teeth contact when the jaws are in centric relation (when the condyles are in the uppermost and foremost position in the glenoid fossa and when muscles are in their most relaxed state). it is sometimes referred to as the retruded jaw relationship.