Clinical guide to border moulding and secondary impression in complete dentures

For complete denture fabrication, the following two impressions are mandatory:

Steps in making impressions

Preliminary examination of the patient

The operator must record detailed case history of the patient including extra-oral and intra-oral findings to enable choose a suitable impression material and technique.

Position of the patient

Patient to be made to sit in upright position to avoid gag and the impression material. The head and neck of the patient to be in line with the trunk to avoid neck pain. The occlusal surface of the patient to be at the level of the elbow of the operator. This allows comfortable and a clear working view for the operator.4 Chair height is modified as follows: for the mandibular impression - the patient’s mandibular arch should be at the level with operator’s elbow, for the maxillary impression-the patient’s maxillary arch should be at a level with the operator’s elbow.

Position of the operator: (Figure 1)

The primary cause of operator's musculoskeletal problems is faulty operator working posture.5

Figure 1

Operator position while making impressions

Therefore it is important to follow the correct operator position as mentioned.

For the mandibular impression, the operator should be in front of the patient and on the right side at 7 o’clock position for right handed operator. Left side at 5 o'clock position for left handed operator.

For the maxillary impression, the operator should be to the right and a little behind the patient at 11 o’clock position for right handed operator. Left side at 2 o’clock position for left handed operator.

Selection of primary impression tray/stock tray

An impression tray is the primary factor in accurate impression making. It is a receptacle which is used to cany, confine, control and make the impression. Primary impressions are recorded with stock trays which follow the outline of the denture-bearing area. 6 The aim is to gain proper extension without distorting movable tissues and to provide uniform thickness of the impression material. If the impression material between tissues and tray is too thin, it may get distorted on removal or during cast pouring. However, if it is too thick, it may displace or distort movable tissues. Selection and adjustment of the tray plays a significant role in making an impression. 6

The parts of an impression tray includes body and handle. The body consists of floor and flanges.

Stock trays can be classified as:

These can be made of either metal or plastic. Metal is preferred over plastic as rigidity of the impression tray is the prime requirement for a dimensionally stable impression.

Figure 2

a: Stock trays (Perforated); b: Stock trays (Non perforated)

To select the correct stock tray, a divider can be used. For the maxillary arch, the divider tips are placed in disto-buccal vestibule next to the tuberosity and the same width stock tray is selected. For the mandibular arch, the divider tips are placed in disto-lingual vestibule under the retro molar pad and the same width stock tray is selected. The stock tray should cover all the anatomical landmarks and should provide 4-5mm space for the impression material

Selection of Primary impression material

Selection of materials for primary impression making for complete denture depends on the preferred impression technique customised for the patient:

Making of primary impression of maxillary and mandibular arch

Primary impression is made with stock trays using mucostatic or mucocompressive impression technique with appropriate impression material as required. Primary cast is made from this impression using dental plaster/dental stone.

Fabrication of custom tray

A custom (special) tray should be constructed, according to the guidelines for border moulding. (Figure 3)

Custom trays can be made of

For ease of custom tray removal from the primary cast, all deep undercuts and tissue irregularities if present should be adequately relieved or blocked out using baseplate wax. This is often referred to as blocking out of primary cast.

Figure 3

Custom trays

Space for the secondary impression material has to be created. This wax is commonly called as " spacer." 7 Different spacer designs like full spacer and partial spacer (I spacer, T spacer, mushroom spacer) have been advocated by various authors. The most commonly used spacer design is Boucher’s spacer design for selective pressure technique which advocates application of one of baseplate wax thickness on the entire basal seat area except posterior palatal seal area and on mandibular ridge except buccal shelf area and retromolar pad of the primary cast. J.J.Sharry advocates placement of four tissue stops, 2 mm in width extend across the crest of the ridge from buccal to palatal / lingual aspect located in molar and cuspid regions. These tissue stops are intended to help in orientation of the custom trays and to maintain uniform thickness of secondary impression material.Additional wax called “relief wax” is applied in areas where excessive relief is required (eg. flabby tissues, deep tissue undercuts etc.,)

An outline denoting tray extension is marked on the primary cast 2mm short of the vestibule another line 4mm short from the sulcus depicting spacer outline is marked. The tray should be about 2 -3 mm thick with rounded edges. Tray handle should be 3-4 mm thick, 8-10mm in height, 8-10mm in width. The angulation of tray handle should mimic the position of anterior teeth. For maxillary tray, the handle should be angulated to about 10-15⁰ and for mandibular at 90⁰. Horizontal grooves may be made on the handle for better handling and grip.

Clinical verification of custom tray extensions: 8

The custom tray is placed in the patient’s mouth and checked for:

Border moulding of maxillary and mandibular arch:8

Border moulding is defined as shaping of the border areas of an impression tray by functional or manual manipulation of the tissues adjacent to the borders to duplicate the contour and size of the vestibule.

Border moulding can be performed by two techniques:9

Single step or simultaneous border moulding:

In this technique, putty consistency of polyether or polyvinyl siloxane can be used.

The border moulding material is placed on the entire periphery of the custom tray and is refined in a single step.

Advantages:

Less clinical time.

Less possibility of error if placed correctly.

Disadvantages:

It requires skill and expertise (once moulded cannot be re-moulded/ modified).

Incremental or sectional border moulding:

In this technique, low fusing compound (green stick) is the material of choice. Other materials like tissue conditioners, waxes and resins can also be used. The border moulding material is placed incrementally over the periphery of the custom tray and refined sectionally. The sequence of border moulding is as follows.

Border moulding of maxillary arch:10 (Figure 4)

Figure 4

Steps in maxillary border moulding

The maxillary arch border moulding can be completed in 6 steps as follow: (Table 1)

Border moulding of Mandibular arch: 10 (Figure 5)

Figure 5

Steps in mandibular border moulding

The mandibular arch border moulding can be completed in 8 steps as follows: (Table 2)

Check the border moulding: (Figure 6)

Smooth, uniform, rounded and matt appearance of green stick indicates correct border moulding. If shiny surface, it indicates improper moulding. Correction to be done by either resoftening the green stick material or by extending the borders - whichever is required.

Figure 6

Border moulded custom trays

After completion of border moulding, the patient should be able to touch the entire upper lip with the tongue without displacement of the tray.

To check the retention after border moulding:

Materials for secondary impression making are

The materials used for secondary impression are

Zinc oxide eugenol impression paste is the most popular material for recording secondary impression in complete dentures.

Making of secondary impressions of maxillary and mandibular arch

Secondary impression

The impression that represents the completion of the registration of the surface or object. It is an impression made for the purpose of fabrication of the master cast on which the prosthesis is fabricated. This impression is made with custom tray / individualised tray (Figure 3) using selective pressure technique.

Mixing, loading and impression making

Zinc oxide eugenol impression material or light body consistency of addition polyvinyl siloxane material can be used according to manufacturer's instructions. The homogenous mixture is loaded in to the special tray and covering the borders. The tray with loaded impression material is inserted into the patient’s mouth, correctly centered (using the tray handle and labial frenum as a reference) and seated in position. Tissue function is simulated by repeating all the border moulding movements and pressure is maintained till the material sets. The tray is withdrawn and the impression is rinsed in water, followed by careful examination of the impression surface. (Figure 7, Figure 8) If accurate, then the impression is disinfected before proceeding for pouring of dental stone to obtain a master cast.

Figure 7

Secondary impression with Zinc oxide eugenol impression paste

Figure 8

Secondary impression with light body material

Recent advances in materials

Recent advances in making secondary impression utilizing digital intraoral scanner

Computer-aided design and computer-aided manufacturing (CAD/CAM) technology is used in fabricating complete dentures. A special scan retractor is used in retracting the mobile tissues of lips, cheeks and vestibule while taking a digital impression. Complete dentures can also be fabricated with CAD/CAM technology using a traditional clinical recording method and scanning them.11