Restorability is a fairly nebulous term that many may consider subjective and individual to the clinician, the tooth and the patients perception of what they would accept as an outcome. I liken attempts at rationalising the process of the assessment of restorability to hand writing a map of the cosmos and solar system, you can outline the factors but it can never be comprehensive enough to make what may be considered the right decision or destination. This hasn’t stopped many publications from attempting to create indices that will make the process ‘easier’. Having assessed restorability many times for countless individuals and teeth I must say that neither myself or the good colleagues I have worked with in the past have ever used these indices or ‘guides’. Those with the experience and nous appreciate how subjective and multifactorial the assessment can be.
What baffles me much more so, and I hope you may nod with some sentiment, that there is a lack of attribution to occlusal forces in the assessment of teeth. Force applied through a tooth is likely to effect the longevity of the restoration quite significantly, some may say more so than the volume of tooth remaining !
Have a look at these articles from a wide variety of sources, all of whom tend to emphasise different factors. The Bandlish paper is especially impressive in that they relate pre-op tooth volume to post restoration preparation, not an easy undertaking but a very interesting read.