My name is Charles Meister and I'm a certified dental technician specializing in complete dentures, working on certification in partial dentures. I have about 15 years clinical experience working with numerous dentists and their patients. The following is a combination of my education, clinical observations, and opinion. So don't sue me.
1. Every time you are treatment planning and prescribing a partial for a patient, you must refer to the
Kennedy method of classification of edentulous spaces and figure out which of the classifications relate to your patient. Class 1 and 2, I would never prescribe a flexible partial for, as they are not supported distally. Class 5, I would be sure to have the flexible partial clasp at least 2 teeth in the anterior, IF they are not both periodontically compromised with a hopeless prognosis. If they are, I would have a talk about extraction with a temporary acrylic partial fabricated and used until bone resorption settles down a bit all the while having the patient come in frequently for medicated temporary liners during the healing process. Now that you have those two crappy teeth out of the way, hopefully there will be now a Kennedy Class 3 or 6 classification and you can have some strong vital abutments at both sides of the partial which you can anchor to. Hopeless teeth are just going to cost both you and your patient more time and money in the long run after you need to add teeth and material to an existing partial.
2. A flexible partial, in my opinion, should only be prescribed for a totally tooth-borne replacement, meaning that it needs to be a bridge supported by teeth on both sides (Kennedy Class 3,4, sometimes 5, 6.) Biomechanics shows us time and time again that when a flexible partial is prescribed for unsupported distal extensions, this actually accelerates the rate of bone resorption due to the constant forces of mastication. That's why if I were a dentist, I would much rather prescribe a rpd metal framework with acrylic saddles. You gain the rigidity of the major connector which reduces the forces of mastication on the underlying tissue and bone, and the serviceability of an acrylic partial which can be relined by any dental lab. If esthetics are an issue, (patient doesn't want metal clasps showing) you can have flexible clasps added to the framework before the framework try-in. If the metal rpd has already been inserted and the patient doesn't like the metal clasps, they can be sectioned from the framework and flexible clasps can be added by providing mechanical retention in the existing acrylic, then injecting a thermoplastic in a matrix to form the flexible clasp. The whole shebang is going to be a bit more expensive to both you and the patient, but the benefits are unmatched by the other options. It's seriously the Cadillac of RPD's. Intra/extra coronal attachments are like your Tesla's and Jaguars, and implants are the supercars. Anything less than metal rpd as a final prosthesis is a junk car with a smelly exhaust that's going to destroy the environment if it's not properly maintained.
3.
Prepping rest seats and clasp assembly space prior to taking the final impression would be the responsible thing to do for metal and flexible partial prescription. Don't worry about the drilling into a virgin tooth thing, because partials that have rests just lying on top of the enamel create a lot of problems because they get in the way of the opposing buccal/lingual cusp that used to glide around and do its job just fine until you failed to prep rest seats before the final impression and ordered a framework to be made that a technician following the prescription, has to either call you to discuss other options of rest placement or do as the prescription asks and lay the rest on top of the enamel. Then the patient can no longer close all the way, or they change their parafunctional habits to accomodate the rest leading to tmj, muscle fatigue, headaches, malnourishment, and ultimately sadness, confusion, and distrust. Then they come back to you (maybe) and then they have all these new problems, and you have to decide to remove the rest or accomodate it by reducing the opposing cusp(s). Prepping rest seats before the final impression reduces rotational forces on abutment teeth and act as indirect retainers (clasps, I and T bars are called direct retainers) and you should have rests incorporated in the design to offset the damage that an appliance does overtime to the supportive tissues due to bone resorption. It's important to educate the patient of the importance of coming back in 4-6 months for an exam, and if warranted, a reline. You usually can have a lab reline a flexible partial, it's just a pretty tall order and some labs will put up a fight and recommend a new appliance. (Or perhaps they outsource your cases and don't have the equipment to do it)
Due to the complexity of the lab procedure, you'll probably encounter around the same lab fee for the service, so it comes down to what the patient can afford and if your lab can do it.