Master or Mistress of All You Survey
In this month’s tips I should like to return to prosthodontics and ask the potentially thorny question ‘who designs your cobalt chromium based dentures? At least 2 UK surveys have found that over 90 per cent are designed by technicians. However, it is well worth reminding ourselves that the person who is responsible for the design is always the dentist (clinical dental technicians excepted for the time being).
Why is this the case? Several reasons spring to mind from relative inexperience in design, not possessing a surveyor, a feeling that your technician will do a better job, not understanding which clasp design should be used where and when, and the time demands inherent on a heavy clinical commitment.
I hope these tips will help you take greater control of the process, so that , at the very least, you will be able to see when your technician has made errors. Start with the B set of models (see Model Practice). You can draw on these models in pencil and work out the basis for the design, remembering the ‘Rule of Thumb’ idea when it comes to designing the connector.
Decide which teeth need to be replaced. Have you considered the shortened dental arch ideas suggested by Keyser? Free end saddles, or more correctly distal extension bases, always give problems and will be discussed in more detail in a later Tips.
Look at how teeth have tipped and rotated into spaces and decide on the best line of insertion and withdrawal. Now survey the models at this position No surveyor, then make one. That is OK if you have a metal fabricating workshop and can use a lathe etc. No workshop! You can still make a near surveyor very cheaply as follows:
Take a normal HB or B pencil. Remove wood on one side down to, but not into the central pencil lead. Now reshape the point to make a chisel end. I found Stanley knives are better than scalpels, especially those with plastic handles, but do mind your fingers. You have now created a replica of the pencil lead marking end of a conventional surveyor (Fig 1).
You should be able to hold this tool at about 90 degrees to the line of insertion (Fig 2).
Absolute accuracy is not required, but you can now make survey marks on your model to give you the guidance of which clasp to use where. For a Kennedy Class I denture the line of insertion and withdrawal is parallel to the distal surface of the most distal abutment teeth. For a Kennedy Class IV it is usual to tip the model so that the relative undercut of the labial saddle can be used to help resist displacement. It is also important to define a survey line along this saddle. This line determines the maximum extension of the labial flange and will help you decide if the replacement teeth need to be gum fitted, or whether there will be space for acrylic gumwork. It is common to find that at the wax try in stage that the flange is too long, but that the wax ‘gives’ and the denture seats. If the denture is processed to this extension the chances are it will not seat down at the fit appointment, time will be wasted and your stress levels will increase!
Finally once the line of insertion and withdrawal has been decided, place three marks, one on each side and the front of the cast, to allow your technician to copy your decision on his surveyor. As these pencil marks can easily be rubbed off, it is well worth scribing these lines into the model with a sharp knife.
Plan rests seats. Most do not exist, but are so easy to prepare using a large round bur in an air turbine. Stay within enamel, and apply a touch of fluoride gel to the area at the end of the appointment. The rule is rest seats are placed next to bounded saddles, and on the mesial aspect of abutment teeth for free end saddles, as in this position, axial loads will tend to move the supporting tooth mesially where it can be buttressed by other teeth. If the rest is placed distally, next to the saddle, then forces will tend to open the contact point as tissue support is lost over time, ultimately leading to loss of this abutment tooth due to persistent overloading.
Choose your clasps. There have been many designs over the years, but the vast majority of dentures can be made sufficiently retentive with just 4 types of clasp. The choice will depend on the survey line as follows.
A. Anterior Teeth
The choice here is for a single arm clasp, either occlusally approaching or gingivally approaching ‘I’ bar, not the ugly ‘T’ shape, and which to use depends on the survey line.
Fig 3: Low distally and high mesially
– occlusally approaching clasp
Fig 4: High distally and low mesially
– gingivally approaching clasp
No undercut – consider reshaping the tooth by adding etch retained composite.
B. Posterior Teeth
For a lone standing molar, use a ring clasp (Fig 5). The more the tooth is tipped and/or rotated the greater the indication for this type of clasp. Remember, because of the curve of Monson, the clasp tip will be placed buccally on maxillary teeth, and lingually on mandibular teeth. Where there are two molar teeth consider using a three arm clasp on the anterior molar. However, there may well be a problem with a high survey line at the mesiobuccal cusp that will prevent the denture framework seating in the mouth, although it seats on the model.
The information gained so far is still insufficient for optimum denture dsign because the correct position of the clasp tip in a horizontal plane has also to be considered. Occlusally approaching clasps are less flexible than ginivally approaching clasps. The ideal undercut is:
- Occlusally approaching clasps – 0.25mm
- Gingivally approaching clasps – 0.5 mm
This can only be determined by your technician by using a surveyor with undercut gauges.
We have all seen clasps that seem to spring away from the tooth and undercut; we bend them back; they move again; the cycle is repeated until the clasp breaks. Why? This is due to a failure in detailed design.
Finally, we have all had cases where we make both maxillary and mandibular chromes. At the metal try in place one framework, bite together, correct. Remove and try the other framework with the same result. Then try both together – oops but it’s now high on the bite. Again the question is why? Each chrome framework is waxed up on a duplicate refractory model, cast, fettled and fitted to each single master cast ready for the try in stage. However, the models have not been articulated, and the occlusion will be incorrect, particularly if we have not prepared rest searts before taking the working impression. The answer is simple. Have the models articulated and this means you have to take a bite with blocks first, this short appointment is well worth the time spent as it will save you more time later.
Articulated models mean that your technician will be able to adjust the bite in the laboratory before the metal framework try in appointment.