Utilization of biometric denture space: an approach beyond implants and surgeries

INTRODUCTION

Permanent teeth are guided by the Gubernacular canal and erupt by resorption of primary teeth under the influence of muscular environment created by forces exerted by tongue, cheeks and lips, apart from the genetic factors. These forces influence the position of the resultant arch form, and occlusion. After the loss of teeth, as impression surface decreases (due to alveolar ridge resorption), retention and stability are compromised and rely on correct positioning of the teeth and contours of the external or polished surface of the dentures. Therefore, these surfaces should be harmoniously contoured with the horizontally directed forces applied by the peri-denture muscles enabling seating of denture in a well-balanced muscular space known as neutral zone (NZ).¹

The NZ has been defined as the potential space between the lips and cheeks on one side and the tongue on the other; that area or position where the forces between the tongue and cheeks or lips are equal.² Historically, different terminologies have been loosely associated with this concept, including dead zone, stable zone, zone of minimal conflict, zone of equilibrium, zone of least interference, biometric denture space, denture space, and potential denture space.³

The lower denture commonly presents with difficulties like pain and loose denture.⁴ This is because the mandible atrophies at a greater rate than the maxilla and has less residual ridge for retention and support.⁵ The NZ technique is most effective for patients who have had numerous unstable, unretentive lower complete dentures, in patients with partial glossectomy, mandibular resections or motor nerve damage of the tongue.⁶

This article describes the fabrication of a complete denture using NZ impression technique for enhanced stability and masticatory efficiency with lingualized occlusion (LO).

Case Report

A female patient aged 71 years reported to the department of Prosthodontics with the chief complaint of loose, ill-fitting and unstable lower denture. On examination, mandibular residual ridge was atrophic with loss of vertical dimension, collapsed facial profile, sunken cheeks and loss of muscle tonicity. After thorough evaluation of the patient’s history and existing clinical conditions, a special functional impression technique was used to record the final impression of the mandibular arch followed by a functional moulding of surrounding structures and muscles.

Procedure

• The preliminary impressions were made using irreversible hydrocolloid impression material and special trays were fabricated.

• The maxillary border moulding was done in a conventional manner and final impression was made.

• Denture bearing area of the mandible was recorded using a functional method whereby, impression compound and green stickwas used in ratio of 3:7 by weight (Admixed technique)⁷ and wash impression was made. (Figure 1)

• Maxillary rim was oriented intraorally, facebow transfer was carried out (Figure 2) and cast the maxillary cast mounted on Hanau wide vue articulator.

• The mandibular rim was adjusted to the correct occlusal vertical dimension and centric jaw relation was recorded and mounted.

• The wax was removed from the mandibular denture base and retentive wire loops were attached to the record base to aid in retention of the neutral zone recording material. The admixed technique material combination was used to form into a roll according to the shape of the crest and attached to the wire loops on the mandibular record base.

• The compound was reheated and placed in patient’s mouth and the patient was asked to perform a series of actions like swallowing, speaking, sucking, pursing lips and slightly protruding the tongue several times which simulated physiological functioning. The compound was moulded into the shape of the NZ by patient. (Figure 3)

• The obtained NZ impression was placed on the master model and index was prepared in plaster to help in preservation of the NZ space.

• Wax was then poured into the space previously occupied by moulding compound giving an exact representation of the NZ in wax. Teeth arrangement was done in LO scheme and then it was checked by replacing the plaster index. (Figure 4)

• The trial was done to check for retention, stability, esthetics and speech. (Figure 5)

• At the same appointment, cheek plumpers were made in wax and were attached to the upper waxed-up denture to evaluate for a fuller appearance.

• After satisfactory try-in, the dentures and the cheek plumpers were processed and finished.

• A pair of commercially available Neodymium Iron Boron (NdFeB) magnets was employed to retain the cheek plumper with final prosthesis and the patient was educated about the same. (Figure 6 & 7)

The dentures provided the patient with improved facial appearance, stability and retention during function. (Figure 8)

DISCUSSION

Management of resorbed ridges had always been a challenge in fabrication of complete dentures.⁸ In highly atrophic mandible, muscular control over the denture is the main retentive and stabilising factor during function. The ultimate objective of prosthodontics is to restore form, function and aesthetics and to fit the denture in biometric denture space to ensure that the muscular forces work effectively in harmony to stabilize the denture. The function of lips, cheeks and tongue and their controlling action on the dentures during function is a fundamental principle behind NZ concept.

Impression plaster as advocated by Johnson is messy and cumbersome when compared with material of choice for this case. Other materials most commonly discussed in literature are tissue conditioner which lacks strength and viscosity to withstand muscle forces and requires incremental addition of materials in layers over the retentive loops.⁹

According to Payne,¹⁰ LO provides distinct advantages as it yields cross-arch balance resulting in improved denture stability and enhanced patient comfort. The potentially damaging lateral forces are reduced because maxillary lingual cusps provide the sole contact with mandibular posterior teeth. Vertical forces could be centred upon the mandibular residual ridges which is considered advantageous for denture stability and maintenance of the supporting hard and soft tissues. Kawai y et al.¹¹ suggested that the LO scheme with artificial teeth is more efficient for patients with severely resorbed mandibular ridges. Madalli P et al¹² compared the pressure values on the supporting tissue using three different posterior occlusal schemes and concluded that LO scheme transfers stresses from working side to non-working side to stabilize the mandibular denture.

CONCLUSION

This method provides the patient with a great degree of comfort and confidence. When bone resorption is significant, this technique allows for functional stability imparting confidence to patient with atrophic ridges and facilitates adaptation to the new prosthesis.