In the last few weeks I have been working with a patient who has existing upper and lower implants and implant assisted removable dentures. She is unhappy with the retention and stability of her existing reconstruction and expressed a wish that she had been more strongly encouraged to go for a fixed solution at the time she had the original treatment. She is now ready to have both the upper and lower arches converted to an implant supported hybrid style prosthesis. Thinking about this transition has brought up two important questions. The first is how much room do we need between the upper and lower gingival tissues or ridges to move forward with a fixed solution.
It has been interesting to hear the variety of answers this question generates. I have heard numbers from as low as 12mm per arch of space to a high of 20mm+. So how to make sense of it all. The first thing to know is how the numbers are being generated are we talking from the top of the fixtures or the top of the ridge. Another factor is the preference of the technician or dentist reporting the data, as well as the components that are planned for as part of the prosthesis.
The impact of the space is two fold. One is simply having enough room for all of the implant components, the bar the teeth and the acrylic. The second factor is the long-term strength of the prosthesis and cleansibility. Inadequate space can affect the amount of room between the bar and the tissue, and when this is compromised it can limit the patients ability to clean and cause tissue inflammation and irritation. The other compromise is int he amount of acrylic and the tooth placement. This can result in teeth fracturing out of the prosthesis, breaking or the acrylic fracturing.
The challenge of course comes back again to treatment planning and retrievability. I have not yet ever had a patient who asked to go from a screw retained implant supported prosthesis to a removable implant assisted prosthesis, but I do have patients who often think they will be alright with removable and then realize they want something fixed. With this in mind it is never a bad idea to plan the case so that the patient has this option int he future.