Custom trays.
A custom tray is what makes a definitive impression definitive. It controls material thickness, allows border moulding, and records the functional periphery rather than an arbitrary one.
Why a stock tray is not enough
A stock tray produces variable and often excessive material thickness, which increases dimensional change on setting and distortion on removal. It also cannot be border moulded, so the periphery of the resulting impression is wherever the tray happened to sit.
For a denture, the periphery is what produces the seal, and a seal recorded by accident performs accordingly.
For fixed work, excessive material thickness at a margin is where drags and voids originate.
Spacing and stops
Spacing determines how much impression material sits between tray and tissue. A close-fitting tray for a wash impression needs minimal spacing; one for a material requiring bulk needs more.
Tissue stops hold the tray at the correct distance while the material sets, preventing it being seated too deeply and squeezing material away from the areas that matter.
Say which impression material you intend to use. The spacing should suit it, and the laboratory cannot select appropriately without knowing.
Border moulding
A custom tray should be slightly short of the full sulcus depth so it can be border moulded — the periphery built up with a moulding material while the patient performs functional movements.
That records the sulcus as it behaves in function rather than at rest, which is what allows a denture periphery that seals without being displaced.
A tray extended to full depth cannot be moulded and will be overextended, producing an impression that has to be trimmed by guesswork.
Adhesive and perforation
Tray adhesive needs applying to a clean dry tray and being allowed to dry before loading. Applied and immediately filled, it remains wet and ineffective, and material pulling from the tray on removal produces distortion subtle enough to pass inspection.
Perforated trays retain material mechanically and are appropriate for some materials and situations. Say which you want.
For fixed work
Custom trays are less common in fixed restorative practice than they used to be, and they still have a place — multiple preparations in one arch, situations where a stock tray fits poorly, and cases where impression accuracy is critical.
A close-fitting custom tray with controlled wash thickness produces a measurably better record than a stock tray with a thick material bulk.
Handling the tray at the appointment
Try the tray in and adjust it before loading. Overextension is common and produces displacement of the periphery; underextension produces an impression that stops short of the sulcus.
Check that the tissue stops seat and that the tray does not rock. A tray that rocks records that movement in the impression.
Border mould in sections rather than all at once, with the patient performing the relevant functional movements for each region. Rushing this produces a periphery that is moulded in shape but not in function.
Trays for special situations
A flabby or displaceable ridge needs a tray designed to record it without compression — commonly a window tray, where the mobile area is left open and recorded separately in a low-viscosity material while the rest of the arch is captured conventionally.
A prominent torus or a painful area needs relief built into the tray so the impression material is not forced against it, which would both hurt the patient and distort the record.
For an implant overdenture, the tray has to accommodate the copings or abutments and may need to be open over them depending on the technique.
None of these are unusual requests. They do have to be described, because a standard tray will be made otherwise.
What to put on the prescription
Tick Custom Tray under Procedure, state the arch, and name the impression material you intend to use so the spacing suits it.
Say whether you want the tray perforated, whether you intend to border mould, and note any area needing relief — a prominent torus, a flabby ridge, or an area of tenderness.
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