# Study Models in Orthodontics > Study models in BDS Orthodontics: diagnostic versus working casts, trimming conventions, model analysis uses, storage and the shift to digital scanning. - Canonical URL: https://prepelephant.com/topics/bds/orthodontics/study-model-ortho-bds - Exam / course: BDS · Subject: Orthodontics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Study Models in Orthodontics", PrepElephant, https://prepelephant.com/topics/bds/orthodontics/study-model-ortho-bds ## Direct answer Study models are three-dimensional records of the occlusion, poured from alginate impressions in hard plaster, and they serve four separable purposes: diagnosis (every model analysis from Carey's to Bolton's runs on them), treatment planning and simulation (including the Kesling setup), appliance construction (as working casts) and medico-legal documentation of the starting and finishing occlusion. A properly trimmed pair reads like a chart without words — the bases trimmed symmetrically about the midline, the posterior surfaces squared to it, the heels bevelled, the models articulated so the recorded occlusion is preserved and the occlusal plane sits parallel to the base. Digital scans now replicate the diagnostic functions without plaster, but the trimming conventions and the analyses they support remain standard BDS practical and viva material. ## What you must remember - **Anatomy of the cast:** the anatomical portion (teeth, alveolar process to the vestibular reflection) and the art portion or base, which is trimmed — never poured and left random. - **Trimming logic:** bases symmetric about the mid-sagittal plane, posterior surface perpendicular to it, lateral surfaces parallel to the buccal surfaces, heels bevelled for stability; conventions differ slightly between colleges, so follow your department's proforma. - **The occlusion rule:** models are trimmed only after being articulated in the recorded bite — trimming first destroys the record; the finished occlusal plane parallels the base. - **Diagnostic versus working casts:** study models document the case in hard stone; working models, on which removable appliances are fabricated, may use softer stone but need clean margins and the same faithful anatomy. - **Analyses that live on models:** arch length discrepancy (Carey's), Bolton and Pont indices, Ashley Howe's basal width, mixed dentition predictions, the Kesling setup and model surgery for orthognathic planning. - **Impression discipline:** alginate, disinfected and poured promptly (within about an hour) to avoid distortion from syneresis and imbibition; undercuts blocked where needed. - **Medico-legal weight:** models are legal documents of pre- and post-treatment status — Indian departments retain records for years, and pre-treatment casts routinely settle disputes about what was promised. - **Digital era:** intraoral scanning produces casts measurable on screen; accuracy for linear model analysis is comparable to plaster in most published comparisons, and storage collapses from cupboards to servers. ## Reading a trimmed pair like an examiner When a trimmed pair is placed on the bench, the trimming itself communicates: if the heels are asymmetric, the base asymmetry may be real anatomy or sloppy trimming, and the examiner will ask which — answered only by going back to the articulated casts before trimming. The procedural sequence taught across Indian departments runs: pour in hardened stone, trim the bases with the models hand-articulated in centric occlusion using the wax bite registration, keep the midline of the base aligned with the dental midline, and finish so that the occlusal plane lies parallel to the base and the backs sit flush against a right-angle guide. Then the analyses: run a brass wire along the arch line for available space, dividers for the summed widths, and the discrepancy writes the treatment plan. A pair trimmed this way supports every station of the university practical — display, analysis, diagnosis — and the same casts, photographed against a plain background, become part of the case record. When a case ends in dispute, the pre-treatment models, not the memory of the operator, are what the college committee reads. ## Where students lose practical marks Indian universities grade the trimmed study model as a skill station, and marks evaporate in predictable places: bases left asymmetric about the midline, the posterior surface not perpendicular, the occlusal plane not parallel to the base, and — worst — trimming before articulating, which discards the bite. The viva pairs ask for the differences between study and working models, the ideal requirements of a study model (clean, bubble-free, full vestibular depth, trimmed base, articulated accurately) and which analyses each cast supports. A favourite trap: "why pour in hard stone?" — because model analysis and repeated measurement demand an abrasion-resistant surface; soft stone abrades at the contacts and the measurements drift. Mentioning that digital scans now serve the same diagnostic role earns the modern-mark, but examiners still expect the plaster conventions by hand. ## Frequently asked questions ### What are the two portions of a study model? The anatomical portion carrying the teeth and alveolar processes with full vestibular depth, and the art portion — the trimmed base that symmetrically supports and displays it. ### Why must models be articulated before trimming? Because the trimmed bases must reproduce the recorded occlusal relationship; trimming unarticulated casts destroys the bite record the models exist to preserve. ### How do working models differ from study models? Study models are diagnostic and medico-legal records in hard stone; working models are the casts on which removable appliances are fabricated, prioritising clean margins for acrylic and wire adaptation. ### Which analyses are performed on study models? Arch length discrepancy (Carey's), Bolton and Pont tooth-size analyses, Ashley Howe's basal arch assessment, mixed dentition predictions and the Kesling diagnostic setup. ### What has digital scanning changed? Intraoral scans reproduce casts as three-dimensional files, allowing the same linear analyses on screen, instant retrieval, and storage without plaster — while trimming conventions remain a taught manual skill.