prostho sheet # 10 - Ziad Al-zo3bi

Theories concerning the behavior of the hard and soft tissues available in the mouth will affect the techniques devised to utilize the construction of the secondary cast.
- I produce the secondary cast by taking a secondary impression.

- At the beginning when I examine the patient, I take a primary impression in order to produce a diagnostic cast, and as we took before, the diagnostic cast helps me in analysis, surveying, detect the available distances intraorally, examine the mobility, teeth drifting and spaces between teeth. All these variables are quite important in establishing the most suitable design satisfying all the desired criteria.

- The diagnostic cast ideally should be mounted on the articulator, with indices so as I can restore the proper orientation in case I removed it from the articulator. (This is called spread cast mounting).

- In the clinic I start to apply the preparations I’ve determined on the diagnostic cast intraorally, which are the guide lines, rest seats and any restorations needed, and by this, I’m ready for taking the secondary impression.

- Many techniques used to obtain the secondary impression for a cobalt chromium framework of the RPD, and each one depends on the behavior of the tissues available in the patient’s mouth.

- Q: what are the tissues available in the oral cavity?

A: 1- hard tissues: natural teeth with the associated alveolar bone. And all have certain displace-ability through the periodontal ligaments.

2- soft tissues: maybe with high compressibility or little one, however, the movement of soft tissues is greater than that of hard tissues.

- The partial denture will rest on bone, natural teeth and on the soft tissues of the residual tissue, so if the secondary impression was not calibrated in a way, this will lead to rocking of the denture, so I should analyze and determine from where perfectly I can get my support, will it be from the hard tissues, soft tissues, or from both? , and standing on that , I’ll decide which theory I’ll follow in making the secondary impression with a lesser amount of denture rocking on the bearing area.


 The techniques :

• 1st:
- Called the corrected cast impression techniques.
- The secondary impression stage of the RPD construction can be carried out immediately following the mouth preparations.
- Now it could be after the production of the metal framework.
- All the impressions start after mouth preparations, I can take a quick non-accurate anatomical impression, but then I’ll rely on another correction step after producing the metal framework, an accurate one.
- Such technique can be used when the rocking movement is minimally expected, as in class 3, or class 4, where the RPD is resting on rest seats on the natural teeth, so I don’t expect that much movement.
 Note: rocking resulted from what’s called dual support, when the denture gets its support from both; natural teeth and the adjacent soft tissues, and as mentioned, the compressibility of the soft tissue is much higher that what it is in the hard tissues, so when the patient bites on an area supported by soft tissues, it will be compressed more than the area supported by the hard tissue, consequently, rocking occurs.
 Teeth movement could reach at maximum 0.02 mm, while in soft tissues ranges from 0.5 up to 2 mm if they were flabby.

- So option #1 : severe movement and rocking are not expected.
- Option #2 : I expect rocking, so I take an accurate anatomical impression, and construct a metal frame work that has a specific relationship with the natural teeth through the rest seats, called rest seats relationship.
- This framework has a perfect relationship between the rests and the rest seats; they fairly fit the teeth, while the saddles usually are distal extension bases, constructed over the residual ridge with a RELIEF, means that the saddle is not directly seated over the ridge, it’s a bit elevated.
- To overcome the problem of the saddle in being suspended over the ridge, I construct a special tray on these saddles, and then I depend on these trays in taking the secondary impression, not on the previous one.
- This impression I took is made in a way to prevent any sort of rocking under load, how can I achieve this?
I achieve this by using a mucocompressive material, this material assures that the soft tissues are will always be under load, so an further loads exerted on them, won’t yield in any sinking or rocking of the saddle in the distal extension base area.
- Option 3: at the time of denture insertion, I make a functional re-line with a mucocompressive material and later I process it using heat cure acrylic resin.
- So as a conclusion:
Anatomic impression: used in case of bounded saddles, when the support is mainly taken from the definite relation between the rests and the rest seats.
Functional impression: used in case of dual support; obtained from both hard and soft tissues.

- The anatomic impression is taken by a stoke tray with alginate, the thickness of the alginate should be around 3-4 mm to give the appropriate details.
- If the thickness is greater than that, in particularly on the distal extension area, it won’t give me the detail accurately.
- To overcome this problem, I heat a small piece of modeling compound material – similar to what we used In taking the primary impression in the complete denture, and apply it on the area of the tray corresponding to the distal extension area inside the mouth, then take the impression by alginate, so on the distal extension area, there will be two materials; the compound and alginate over it.
- By this, guarantee not to place large thickness of alginate on the saddle area, and to get the final details accurately.





o Tissue classification:
- Q: what are the primary stress bearing areas?
A: the buccal shelf in lower and the residual ridge in upper.

- The palate can provide support to the denture in the upper jaw, so it’s not a stress bearing area.
- According to the anatomy of the jaws, I can classify the tissues into primary, secondary stress bearing areas, and non-contributing areas.
- non-contributing areas are those which we can’t get neither support nor bearing stresses from them.
- For example: the lingual flange in the lower is a non-contributing area, because they often have undercuts and any downward forces will result in more space between the denture base and the tissues.
- Many other areas are considered as so, as in the incisal papillae, the ridge in the lower if it was thin, flabby tissues on the upper ridge, all these are non-contributing area.

• When I have a distal extension base, I construct a metal framework and focus my attention on the construction of the acrylic part of this saddle, because before doing this, I have to modify the cast by :
1- Closing the undercuts (and mostly they’re lingually) , that would interfere with the remove of the tray that I’ll use in taking the impression, - doing this closure- using a base plate wax.
2- I apply a separating media, seat the metal framework, then produce a tray over the saddle area.
3- I remove any excess material using surgical scumble.
4- Finishing.
5- Clinically, I add this step, which is the border molding using green stick.
- After I took the final properly molded impression, I put it on the modified cast, then I turn it upside down, to be sure that the metal framework is well seated over the cast.
- Then I do beading and boxing, in order to pour it with stone or whatever material, and get a new modified cast, in which the saddle area was taken by a mucocompressive technique.
- Q: what are the materials used in taking such impressions?
A: 1- zinc oxide euginol impression.
2- wax impression, with an extra advantage; that when I take this mucocompressive compressive impression, the excess wax when left over time, it will distort according to the anatomy of the tissues there, therefore; less pressure will be exerted, and less traumatizing to the underlying tissues.



The End
Date of the lecture: 19/4/2011
Dr. Osama.

Zaid M.Al-Zu’bi
Best wishes for all,