Angular Dimensions of Retromolar Pad: An Observational Study

Introduction

Retromolar pad (RMP) is a pear shaped, soft, glandular elevation of the oral mucosa that lies distal to mandibular third molar or distal to the termination of mandibular edentulous residual ridge. The contents of the retromolar pad include glandular tissue, buccinator muscle fibres, superior constrictor muscle fibres, fibres of pterygomandibular raphe and the terminal part of the tendon of temporalis muscle, all of which are covered by non-keratinised loose alveolar mucosa.¹

RMP is a relatively stable landmark as compared to the residual alveolar ridge even in cases with extremely resorbed mandibular ridges due to the attachments of  these muscles and dense cortical bone underlying RMP which makes it relatively resistant to resorption.² Also, the muscles attached to the retromolar pad limit the denture base extension and prevent placement of undue pressure on the distal part of the retromolar pad during impression procedures. So, the denture base should extend up to approximately one half to two thirds over the retromolar pad.³

A well formed RMP acts as a cushion and provides an excellent seal when the denture border crosses its surface.1 The denture base must cover the retromolar pad to achieve adequate basal seal and stability to resist movements by adding another plane.⁴ Mandibular first molar corresponds to the level of halfway up to two thirds of the height of the retromolar pad. So, retromolar pad acts as one of the most reliable guides for the arrangement of the mandibular teeth in the complete denture cases as it dictates the posterior occlusal plane.^3,5

Posterior displacement of the denture is prevented by the inclined plane as formed by the retromolar pad with the ridge. This can be seen as an angle that the retromolar pad makes with the residual alveolar ridge, especially in extremely resorbed mandibular ridges.^2,5

Placement of denture teeth over the slope formed by the retromolar pad through crest of the residual alveolar ridge acts as a source of denture instability. So, denture teeth must end just before the residual ridge slopes vertically towards the retromolar pad.⁴ If a line is drawn on the cast from the lingual of the retromolar pad, passing anteriorly lingual to the crest in the premolar region, it helps in establishing the lingual extent of the occlusal rim and also in determining the position of lingual surfaces of the mandibular posterior teeth.⁴

Retromolar pad is at an incline in relation to the residual alveolar ridge, which is even more discernible with advanced resorption cases.⁵ This angle needs to be recorded properly, right from primary impression till the final denture base formation for achieving excellent seal, stability and support to the denture base. Use of existing straight stock trays leads to undue pressure in the RMP region, often requiring arbitrary and inadequate modification of tray (Figure 1). Excessive pressure in RMP region during impression may lead to soreness and a rebound effect leading to lower denture instability. 

The current study aimed to record the angle formed by the incline of retromolar pad in relation to residual ridge resorption, so as to formulate such stock trays that will facilitate the impression recording and the final denture outcomes

Methods

The present observational study measured the angle made by the retromolar pad with the residual alveolar ridge in 280 mandibular master casts of edentulous patients. The inclusion criteria were completely edentulous patients of both the genders with well healed ridges and patients consenting to participate in the study. The exclusion criteria were patients with unhealed extraction sockets or history of recent extractions, patients with neuromuscular disorders, partially edentulous mandible and patients with a history of maxillofacial trauma and mandibular fracture. On each master cast, a line was marked using two different coloured markers, one over the retromolar pad and the other over the crest of the distal ridge joining with the retromolar pad (Figure 2a). Then, the angle formed by both of these coloured markers was measured using a protractor kept at the level of eye (Figure 2b). It was observed that more the residual ridge resorption, steeper was the angle. 

Results

The SPSS software was used for all the data calculations and statistics. The mean RMP angle on the right side was 152.50 ± 12.90 degrees (Range: 105.00-175.00 degrees) and it was 154.27 ± 11.80 degrees on the left (Range: 125.00-180.00 degrees). The paired sample t-test gave a p-value of 0.024, which suggested a statistically significant difference between the right and the left side. Only four subjects out of 280 showed RMP angle between 105° and 130° i.e. 105°, 125°, 128° and 130°. These observations were taken as outliers. Resultant average range was 135°-175° with a mean of 152.12 ± 13.10° which can be prepared to be incorporated in an impression tray to accommodate the critical incline of RMP. Since the lower limit was 135°, incorporating an angle of 135° would be adequate to accommodate higher angles up to 175° irrespective of differences in RMP angle on the right and left sides (Table 1).

Discussion

Various factors like sequelae of tooth loss, duration of edentulism, duration of denture wear, gender, genetics, systemic conditions, etc. determine the amount of remodelling of the edentulous jaws. After the distal-most tooth is lost, the surrounding bone remodels to merge with the retromolar pad.⁶ RMP is an important peripheral seal and limiting structure for lower denture base. Its inclination provides resistance to posterior displacement of denture. Literature lacks reports regarding this important angular dimension of retromolar pad. Present study reported the most acute angle of 135 degrees between RMP and residual ridge, which was taken as the standard angle for the stock trays. The clinical explanation for incorporation of this angle is that the most common material used for primary impression making in edentulous ridges is impression compound, which is a rigid material and can support itself while making impression in ridges with most obtuse angles. Also, the trays comprising of this incline will help in avoiding undue pressure, soreness and instability of lower denture.

Conclusion

Modification in the stock trays to provide a means to record this crucial angle will help in fabrication of custom trays for creating a strong foundation for a good complete denture prosthesis in patients with extremely resorbed completely edentulous mandibular ridges. Based on the findings of the present study, it is recommended to incorporate a mean RMP angle of 135 degrees in mandibular stock trays.

Conflict of interest

Nil