A Novel Instrument to Determine Pulp Proximity David G Purton*, Nicholas P Chandler † , Brian D Monteith ‡ and Alison J Qualtrough § Abstract - The aim was to determine whether extracted teeth could be used to test the Prepometer TM instrument, which indicates pulp proximity with green/amber/red light emitting diodes. Third molars were reduced to a plane in dentine and Prepometer TM readings made. Dentine was removed in 0.5 mm increments and readings made until only lights 9 or 10 (red) showed. The teeth were sectioned and the dentine thickness measured. Analysis permitted construction of a highly significant predictor-model (p < 0.01), the red/amber light boundary coinciding with a dentine thickness of 2.4 mm. The Prepometer TM was consistent in predicting pulp proximity but was more sensitive than specified. KEY WORDS: Pulp proximity; tooth preparation; alternating current; instrument; LED display * BDS, MDS † BDS, MSc, PhD ‡ BChD, MChD § MSc, PhD Eur. J. Prosthodont. Rest. Dent., Vol.17, No. 1, pp 30-34 © 2009 FDI World Dental Press Ltd. Printed in Great Britain doi:10.1922/EJPRD_761Purton05 INTRODUCTION The preparation of teeth for restorations can be hazard- ous to the dental pulp. The evidence is that a significant proportion of vital teeth prepared to retain fixed prostheses will become non-vital over succeeding years. In a study of abutments supporting crowns or fixed partial dentures, 18-23 years after placement, 15% of remaining abutments that had originally been classified as vital, were either en- dodontically treated or had periapical radiolucencies 1 . In an evaluation of fixed partial dentures 10 years following insertion, abutment teeth presumed to be vital had a 10% prevalence of radiographically visible periapical lesions 2 . From a systematic review of the literature over 50 years, Goodacre et al. 3 concluded that the single most common complication for abutments with single crowns was the need for endodontic treatment, with an incidence of 3% after an average period of 6 years. For fixed partial den- tures the need for endodontic treatment was the second most common complication, with 11% of abutments af- fected after an average of 8 years. During tooth preparation, as the distance from the pulp decreases, the permeability of the remaining dentine increases. This effect is most marked in cervical areas of the tooth, over pulp horns and on dentine that has had its smear layer removed 4,5 . It a reasonable to assume that over time, as cements and smear layers are lost beneath restorations, deep preparations place the pulp at greater risk from the diffusion of noxious agents from the external environment, than shallow preparations. To date dentists have had to use clinical experience, knowledge of dental anatomy, and dental radiographs, to keep their preparations as far as practicable away from the dental pulp, while still accommodating the mechanical re- quirements of the restoration. Patients and dentists would welcome any device that could help avoid the biological cost, the financial cost and the inconvenience of endodon- tic treatment following restorative procedures. The Prepometer TM (Fig 1) is an instrument that purports to indicate the thickness of dentine overlying a vital pulp. An electrode supplies an alternating electrical current through the dentine and pulp and completes a circuit via a lip clip electrode in the style familiar to users of electrontic apex locators. An array of light emitting diodes (LEDs) in traffic light colours, informs the operator of the proximity of the pulp. According to the manufacturer a red light indicates a dentine thickness of less than 1.4 mm over the pulp. In vitro studies have shown that the electrical resistance of dentine is affected by the age of the teeth, the thickness of dentine over the pulp 6 , the orientation of tubules 7 and the electrolyte concentration in which the teeth are stored 8 . The current path follows the orientation of the tubules 9 . The objectives of this pilot study were to: 1. investigate whether extracted teeth could serve as a model for testing the Prepometer TM . 2. assess the consistency of the Prepometer TM in determin- ing pulp proximity. 3. determine if the sensitivity of the instrument in detect- ing pulp proximity matched the claims made for it by the manufacturer. MATERIALS AND METHODS Ten disease-free third molars were placed in isotonic saline (0.9% w/v NaCl) and frozen immediately after extraction. When needed for the experiment, a tooth was removed from the freezer and high-speed diamonds and 400 grit carborundum paper were used with water coolant to remove all of the enamel from the occlusal surface, cre- ating a flat plane at a level just deeper than the deepest enamel fissure. The reduced tooth was mounted in the centre of an acrylic disc using composite resin. The disc fitted in the test apparatus shown (Fig 2). This consisted