 Hello everyone, welcome back to another session of the industry and more Today's topic in prosthodontics is theories of impression-making So what are the common theories? behind the impression-making concept the first one is muco compressive or definitive pressure technique definite pressure technique and second one is Muco static or minimal pressure. Okay, this is applying Minimal pressure and the last one we have is selective pressure technique So let's learn one by one the theories of impression-making So we'll start with the muco compressive or definite pressure technique so it was introduced by green brothers and The tissue recorded at functional pressure which they claim to have a Better support and retention for the denture and it is commonly used in closed mouth technique So we have open mouth and closed mouth technique so this muco compressive or definite pressure technique used in closed mouth and It was believed that The occlusion loading during impression-making is comparable to the occlusion loading during the function That is how they justify this technique So the first thing we take primary impression with the impression compound then the special tray making using the shellac base plate and its periphery are one eight inch shorter than the Danger outline then the second impression is made with special tray using compound Then by trims with uniform occlusion surfaces Then areas to be relieved are softened Then the impression inserted in mouth and held under biting pressure for one to two minutes Then borders are molded by asking the patient to perform functional moments Then the posterior parallel seal was obtained by making the patient swallow under biting pressure So the advantages of muco compressive techniques are better retention better support Especially during the functional moments and it provides more tissue coverage. Okay so retention better support and more tissue coverage but the disadvantages are The pressure applied can over stress the tissue so over stressing because it is Playing at a definitive pressure Technique not the minimal pressure This often results in good initial retention, but what happens is eventually there will be Bone resorption So what happens after bone resorption? There will be Losing of denture that is a loss of retention So the loss of retention during rest due to the tissue rebound So at rest there will be loss of retention due to the tissue rebound So that is the muco compressive or definite pressure Technique or theory which gives good retention and support with more tissue coverage but it gives more stress to the tissues and there will be resorption and retention loss So the second theory or technique is a muco static or minimal pressure theory. It was described by Addison but who attributed it to entry So he applied basically the Pascal's law to a soft tissues. Okay, Pascal's law So what is exactly Pascal's law? This law states that any pressure applied to a confined fluid is transmitted And diminished in all directions. Okay, so any pressure applied to a confined flow is transmitted Undiminished in all directions. So it goes to the all direction without any change So muco's are being more than 80% of water will react like a liquid in a closed vessel and which cannot be compressed so that is why he's using the minimal pressure because in a closed space the muco's are act like a Liquid so mostly it has 80 percentage of liquid the tissues having 80 percentage of water So he says that it cannot be compressed and it should be recorded at minimal pressure Okay, so before we were applying lots of pressure in muco compressive or definite pressure technique So that theory was different concept, but this theory is based on the Pascal's law. Okay, so He says that it should be recorded at minimal pressure So the impression material should record without distortion every detail of the muco's are will be taken and So the denture would fit All minute elevations and also the depressions So when we taken at a minimal pressure all the minute details such as the depressions any Elevations all will be getting So he said that it should be recorded at basing using a metal base instead of acrylics So metal base metal base should be used Not acrylic Okay to get the minimal details So this theory has regarded the interfacial surface tension as the only important retentive mechanism interfacial Surface tension it was highlighting this concept For the main course of our main reason for retention. That is a interfacial surface tension So he did not use the conventional flanges So again It was recommended in later time a short lingual flange to resist lateral displacement to be used So he was highlighting mainly on this interfacial surface tension So the advantages are The tissue health and preservation because There is very minimal pressure. So there is no chance of resorption. Okay, so the advantages are tissue health and Good preservation and also good stability not retention good stability but the problem with this technique is One is which is less Tissue coverage then Reduced retention that is what Very important, but it has no retention Lack of border molding reduces the effective border seal. There is no border seal because It's not believing in border molding or peripheral seal Border molding is not there. So there is no peripheral seal Then this lack of border seal Permits foot launchment. So there will be foot launchment because of the peripheral seal It will be a big problem Then it compromised aesthetics due to the short danger of flanges No aesthetics Then tissue variations at the time of impression making and insertion may affect the results So this is holobot mucostatic or minimal pressure theory is based on the Pascal's theory In a closed space tissue behaves like a fluid So advantages are tissue health and good preservation of structures with good stability But there are lots of disadvantages less tissue coverage Less retention. There is no border molding. So there is no peripheral seal So there will be foot launchment and compromised aesthetics The next theory is selective pressure theory. It was given by Bowsher so this is Managing the disadvantages of the first two theory that is micro compressive and micro static theory Here the principle is the mucosa over the ridge is best able to withstand the pressure and Mucosa covering the midline is thin and has little submucosa tissue So the forces acting on the danger confined to the stress bearing areas so forces are mainly on stress bearing areas and Non-stress bearing areas are relieved. Okay, that is a concept The non stress bearing areas are relieved so we learned In the maxillary and mandibular dangers, which are the stress bearing area and which are the non stress bearing area Relieving area. So all those things we learned. So the pressure is applied on the stress bearing areas And this non stress bearing areas are relieved. That is what you know as selective pressure theory So non stress bearing areas are relieved and the stress bearing areas are allowed to come in contact with the tree. So We relieve this area so this is a Bowsher's selective pressure theory The main disadvantage is it demands a firm healthy mucosa covering over the ridge So it cannot be used in flabby ridges That is the main problem It cannot be used in flabby ridges. It needs to have a good healthy very firm ridge To do this selective pressure theory. So these are the main three theories of Impression making mucostatic mucocompressive and selective pressure theory So that was all about theories of impression making. So any theory can be asked as a short note or a short essay As all together. So we learned mucocompressive or definitive pressure theory Mucostatic or minimal pressure theory and selective pressure theory. So I'll come up with a new topic in prosthorontics. Thank you