A novel chairside technique to assess the interocclusal clearance and abutment axial walls during tooth preparation

Abstract

The importance of a judicious and unerring abutment tooth preparation in the field of prosthodontics has always been paramount. It is not uncommon for many clinicians to face challenges during laboratory fabrication of fixed prostheses, caused due to inappropriate occlusal clearance and over axial wall taper of the abutment tooth. With evolving technologies and methods, the modus operandi for attaining such tooth preparation is varying; however, every technique has its own shortcomings. The technique mentioned in the article is cost-effective as it uses modified Heister mouth gag forceps to achieve the desired objective of evaluating the prepared abutment morphology three-dimensionally with minimum chairside time.

INTRODUCTION

Occlusal clearance is the space created between two opposing teeth when one or both are being prepared to receive a restoration. The amount of tooth structure lost while preparing the occlusal aspect is prudent and must be done cautiously. If over-prepared, it may contribute to irreversible pulp damage or loss in resistance form, and if underprepared, the structural durability of the restoration may be compromised.[¹²]

Various conventional techniques and their modifications, for verifying the occlusal clearance have been proposed using dental wax or silicone bite registration materials for interocclusal records and evaluating their thickness with an Iwanson gauge caliper or a graduated periodontal probe.[³⁴⁵⁶⁷⁸] However, positive replica models have always been more convenient and effective to assess the preparation, in terms of prepared abutment morphology and occlusal clearance, compared to their negative form. Techniques like pouring the check casts with salt incorporated dental plaster have been advocated but require an additional impression, contribute to laboratory workload, and the setting of the plaster mix is time-consuming. Recently, intraoral scanners have been successfully employed to evaluate the occlusal form three-dimensionally, but scanning is not cost-effective for every practitioner.[⁹] Therefore, the technique proposed in the article is simple and attempts to negate the previous shortcomings of verifying the abutment preparation three-dimensionally.

PROCEDURE

In this technique, a modified Heister mouth gag forceps is used, where two opposing threaded screws of 3 mm width and 5 mm height are attached at the end of forceps arms which are precisely calibrated (up to 0.5 mm) on a curved scale, and a slidable metal stopper block which engages a V-shaped groove in one of the forceps arms near its furcation junction. The metal stopper provides a standardized opening of 13 mm at the forceps end [Figure 1]. This modified instrument acts as a mini-hinged articulator and enables quick mounting of the bite record.

Figure 1: Parts of modified Heister mouth gag forceps instrument

1. Screw the forceps knob up to three turns to slightly open up the forceps arms before the clinical appointment
2. After a tentative occlusal reduction during tooth preparation, record the bite using an addition silicone bite registration material only in the region of prepared abutment [Figure 2]
3. Carefully cut the excess silicone index material adjacent to the abutment margin using a surgical blade number 22/23 [Figure 3]
4. Heat a Type 1 medium inlay wax stick over burner flame, and coat three to four layers on the abutment and opposing occlusal surface of silicone bite index [Figure 4]
5. Heat the threaded screws over the flame, orient the silicone index coated with inlay wax between them, and, immediately clamp the forceps arms until it contacts the metal stopper [Figures 5 and 6]. This enables the inlay wax to flow inside screw threads and mechanically retain on the forceps
6. After allowing the wax to cool down on its own, screw in the forceps knob to separate the silicone index from the inlay wax and clamp the forceps again until the arm rests on the stopper completely [Figure 6]. This is the 0 mm position on the graduated curved scale [Figure 7]
7. Slide the metal stopper sideways along the groove, from the forceps arm and clamp both the forceps arms together until the opposing cusps meet [Figure 8]. The reading on the scale now obtained, depicts the minimum occlusal clearance achieved in the tooth preparation [Figure 9]
8. Modify the abutment intraorally, in accordance with the measurements obtained on the scale
9. Evaluate the axial morphology of the abutment, after opening the arms of the forceps. Modify the axial walls in accordance with the undercuts, if present.

Figure 2: Selective silicone bite registration record of the prepared abutment

Figure 3: Cutting the excess silicone index material adjacent to the abutment

Figure 4: Dripping inlay wax on both sides of bite index

Figure 5: Heating the threaded screws of the instrument using a flame torch

Figure 6: Mounted bite index and inlay wax on the instrument

Figure 7: Assessment of abutment occlusal clearance with stopper in place

Figure 8: Assessing the first abutment contact after stopper removal

Figure 9: Curved metal scale depicting the minimum occlusal clearance achieved

Clinical Implication: The modified instrument design enables the clinician to visually assess the positive replica of the prepared abutment tooth three-dimensionally, along with accurately measuring its interocclusal clearance, thereby allowing the rectification of abutment morphology chairside.

SUMMARY

The novel technique described using a modified Heister forceps is beneficial, as it would aid the clinician and dental students, in effectively preparing the abutment and be more assertive regarding its laboratory phase assessment including the absence of undercut and adequate occlusal clearance. The basic materials required for the procedure are readily available and cost-effective. The technique when performed takes less than 4 min compared to the standard check cast method and is equivalent to the learning curve of the intraoral scanner.[¹⁰] As no carving or shape manipulation of wax, the pattern is done at the formative stage, no force is exerted, and minimal residual stresses are incorporated into the wax used.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Acknowledgments

The authors thank their teachers from Davangere, for their guidance and motivation, and the anonymous reviewers for their insightful suggestions.