Impression Techniques Theories Objectives Dr Muhammad Rizwan Memon

Impression Techniques, Theories & Objectives Dr Muhammad Rizwan Memon FCPS Assistant Professor

DEFINITION OF IMPRESSION a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry (GPT-6)

CLASSIFICATION OF IMPRESSION TECHNIQUES A. Based on theories of impression a) b) c) B. Based on the position of the mouth A. B. C. Mucocompressive or definite pressure or pressure theory. Mucostatic or Minimal pressure or Passive pressure theory. Selected Pressure theory. Open mouth Closed mouth Based on the method of manipulation A. Hand manipulation B. functional movements

THEORIES OF IMPRESSIONS The theories actually describe the amount of pressure required to record an impression that improves the accuracy & functions related to denture during different stages of the evolution of impression techniques. Various theories are: a) b) c) Definite-Pressure/ Muco-compressive theory Minimal- pressure/ Mucostatic theory Selective- pressure theory

Definite-Pressure/ Muco-compressive theory Based on the assumption that the retention of the CDs is tested most severely during mastication. Therefore, tissue record under functional pressure provide better support & retention. Execution of this concept is through ‘closed mouth technique’. (GRE ENE 1941)

Definite-Pressure/ Muco-compressive theory Ø ADVANTAGES: 1. Better retention & support during mastication Ø DISADVANTAGES: 1. 2. Excessive pressure could leads to alveolar bone resorption. During rest state tissue tend to rebound leads to dislodgment of denture. Alveolar ridge with sharp spine / spicules may result in pain. Overextended peripheral borders. 3. 4.

Minimal- pressure/ Mucostatic theory Ø Ø Proposed by page in 1946. Based on Pascal’s law (If a force is applied on a confined liquid it distributes equally all over the surface) Ø Ø q 1. Human body is 75% liquid which is confined mostly within the cells. When a force is applied on the underlying tissue by the dentures they act like confined tissue and distribute the force all over the denture bearing area equally According to this concept interfacial surface tension was the only significant way of retaining CDs. ADVANTAGES; High regard for tissue health & preservation.

No broad covering of the tissue is required to distribute the load. Means of retention are only cohesive and adhesive forces and not the peripheral seal and flanges. Impression of a tissue in a resting state prevents it from undesirable forces all over the day. To achieve this the impression of the arch should be made without any pressure exerting on it

Minimal- pressure/ Mucostatic theory q DISADVANTAGES; 1. This type of impression covers only firmly attached mucosal area of denture foundation results into short flanges. 2. Short flanges may results into; ü Reduced support ü Less resistance to lateral forces ü Discomfort/ irritation to tongue ü Lack of peripheral seal 3. Retaining potential of surrounding musculature is lost.

Selective- pressure theory Advocated by carl ‘o’ Boucher in 1951. This concept combines the principles of both pressure & minimal pressure techniques. Based on thorough understanding of the anatomy & physiology of the basal seat & surrounding area. Basal seat area can be divided on the basis of capacity to bear loads into; 1) 2) 3) Primary stress bearing area secondary stress bearing area Relief areas

Common clinical conditions required selective pressure technique 1. 2. 3. 4. Completely edentulous pts: Kennedy’s class-I & II pts: Displaceable/ flabby anterior ridge Fibrous/ unemployed posterior mand: ridge

Selective - pressure techniques Boucher’s technique Hobkirk’s technique Open-window technique Admix technique ( Mc Cord & Tyson 1997) 5. Fluid wax functional impression ( Applegate OC 1955) 6. Selective tissue placement / corrected impression method 1. 2. 3. 4.

PRINCIPLES AND OBJECTIVES OF IMPRESSION MAKING 1. 2. 3. 4. 5. Support Retention Stability Esthetics Preservation of alveolar ridge

Support Loss of teeth result in loss of support for the lips, cheek, oro-facial and masticatory muscles and TMJ.

Retention A phenomenon to resist the displacing force

Stability Depends upon three main factors: 1. Flange contour 2. Peripheral extension 3. Occlusal table During impression a strong consideration should be given to the extension of the flanges; ▪ extend up to muco-labial, muco-lingual, muco-buccal folds

Aesthetics It begins with the impression. Impression border should be so designed that there should be appropriate Labual & Buccal fullness A misconception: Aesthetics depends on color, size & shape of teeth alone Support of the musculature determines the overall appearance

Preservation of the remaining oral tissue: Patients already present with missing teeth and often with a diseased tissue. Further deterioration of oral structures should be avoided. (fabrication of atraumatic prosthesis) Ill fitting dentures result into alveolar resorption, hyperplasia, hypertrophies, soft tissue ischemia etc.