Impression

VARIABLE DENTURE-LIMITING STRUCTURES OF

THE EDENTULOUS MOUTH

Part II. Mandibular Border Areas*

H. R. KOLB, B.S., D.D.S.“”

Veterans Administration Center, Hampton, Vu.

T o A DENTIST, few ‘experiences are more discouraging than the discovery that a newly constructed mandibular denture lacks retention and stability. What makes

the lower arch so difficult to fit? Surely failure cannot be attributed solely to severe residual ridge resorption. A partial answer is found in a better understanding of the structures surrounding the spaces which are occupied by the denture flanges. As in Part I,l a clinical approach will be used to describe how limiting structures influence the shape of the mandibular denture borders.

THE RETROMYLOHYOID SPACE

The pouch-shaped retromylohyoid space is lined completely with loosely at- tached mucosa, which can be displaced in any direction by an overextended flange of a tray. There are no supporting structures involved here, since the medial sur- face of the mandibular body slopes obliquely outward from the mylohyoid ridge to the mandibular border, forming the submandibular fossa. The posterior wall of the retromylohyoid space (Fig. 1) is lined by the mucosal retromylohyoid curtain, the upper portion of which lies between the retromolar pad and the palatoglossal fold (anterior tonsillar pillar)2 and covers the palpable anterior border of the medial pterygoid muscle and the superior constrictor part of the buccinator-con- strictor sheet.3 The wider, lower part of the posterior wall is limited medially by the styloglossus muscle, which forms the lateral border of the tongue.

The lining mucosa which forms the lateral wall and floor of the retromylohyoid space covers portions of the retromolar pad, the vertical fibers of the mylohyoid muscle, and the submandibular gland. Distal to the mylohyoid muscle, the space dips downward and outward to permit the formation of the retromylohyoid eminence of the mandibular denture (Fig. 8). However, the denture flange should not com- pletely fill this severely undercut space. It is necessary only that the lining mucosa maintain continuous contact with the basal surface of the flange, which should not

*Part I. Maxillary Border Areas, J. PROS. DENT. 16:194, 1966. **Staff Prosthodontist.

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Fig. I.-The retromylohyoid curtain, A, lies between the palatoglossal fold, B, and the retro- molar pad, C, which is clearly distinguishable from the molar scar (arrow).

inhibit tongue movement. The external surface of the retromylohyoid eminence is in continuous contact with the lateral margin and ventral surface of the tongue, which limits flange thickness in accordance with its size and functional movements. A complaint that the tongue feels “crowded” may be due to an excessively thick flange or a space between the flange and the retromolar pad.

The mylohyoid flange of the mandibular denture (Fig. S) slopes medially and is formed by contraction of the mylohyoid muscle during simulated deglutition. Its external surface forms an “inclined plane for the tongue.“” A denture flange which does not allow for freedom of mylohyoid muscle movement can result in discomfort and difficult swallowing.

Finally, it is necessary to consider the effect of a sharp mylohyoid ridge and/or a prominent lingual tubercle (tuberosity) upon the impression procedure. These structures form the mylohyoid groove and the lingual tubercular fossa, respectively, of the mandibular denture (Fig. S) .

T:KE ALVEOLINGUAL SULCUS

The alveolingual sulcus is a short, narrow space continuous anteriorly with the sublingual crescent space and posteriorly with the retromylohyoid space. Al- though the mylohyoid fibers are more nearly horizontal in this region, the muscle is not involved in the border molding procedure, because it is covered by the easily displaceable sublingual gland.

204 KOLB J.-Pros. Den. March-April, 1966

When resorption of the mandibular residual ridge has been extreme, the mylohyoid and buccinator muscle attachments may approximate each other near the ridge crest.3r5 There is no way to avoid coverage of these attachments by the denture base, and they should not be given consideration in excess of their impor- tance. The principle of maximum extension within the limits of tissue tolerance should prevail. The mucosa which lines the lateral wall of the alveolingual sulcus is frequently displaced by the border molding material into the sublingual fossa of the mandibular body. It is not unusual to find the mucogingival line close to the residual ridge crest in this region.

THE SUBLINGUAL CRESCENT SPACE

The extreme variability of the shape of the sublingual crescent space within the same individual is dependent upon the position assumed by the tongue during any of its functional movements. Only one of its innumerable contours may be re- corded in the impression, and this one best configuration must produce a denture flange which not only permits uninhibited functional tongue movement but main- tains the border seal at the same time.6-8

The depth of the sublingual crescent space is determined by the location of the genioglossus muscle attachment relative to the height of the residual ridge crest. Of the three groups of genioglossus muscle fibers, only the anterior group acts directly on the sublingual border of the mandibular denture.g As residual ridge resorption progresses, the genial tubercles, which serve as the point of origin for the genioglos- sus muscles, become more prominent and may eventually project far above the general level of the residual ridge crest.rO

LINGUAL FRENUM

The lingual frenum is another extremely variable limiting structure that is seen when the tongue is elevated. This mucosal fold originates at the midline of the undersurface of the tongue and often terminates at the sublingual (salivary) car- uncles (Fig. 2). In other instances, it crosses and bisects the sublingual crescent space, and attaches onto the lingual aspect of the mandibular ridge. Often it fans out to find a broad insertion in the alveolar mucosa (Fig. 3). Despite the fact that the lingual frenum invades and divides the sublingual crescent space, it may be safely covered by the lingual flange without fear of denture displacement in most patients. However, this structure should be palpated for tension during the tray- adjustment procedure.

LINGUAL LEDGE

On either side of the genial eminence, a sharp bony ridge or crest, which pro- jects horizontally toward the tongue and then falls off abruptly, may be palpated. This is the lingual ledge (Fig. 4), a frequent source of annoyance to the denture wearer and consternation to the dentist, since apparently unaccountable denture abrasions and small ulcerative lesions occur in this region (Fig. 5). The ledge is a crescent-shaped prominence located bilaterally between the genial tubercle and the anterior end of the mylohyoid ridge, with which it may be continuous (Fig. 6A).

Fig. 2.-This sublingual crescent space is not divided by the lingual frenum.

206

de c,

Fig. 4.-The presence of a lingual ledge, A, is not apparent until the floor of the mouth #pressed on each side of the genioglossus muscle attachment, B. Note also the residual ridg and the mentalis muscle attachments (arrows).

is :e,

Fig. 5.-A lesion (between arrows) overlying the lingual ledge. Note also the salivary car- uncles, A, and the sublingual eminence, B.

xEr i6 DENTURE-LIMITING STRUCTURES OF EDENTULOUS kIOlJTI!i 207

but non

Fig. 6A.-A mandibular cast shows a well-defined lingual ledge, A, mylohyoid ridge, B, :cal shelf, C. Note the narrow band of masticatory mucosa which represents the crest of a lexistent residual ridge.

Fig. groove.

GB.-The arrows indicate potential sources of discomfort within the lingual ledge

208

It exists in normal mandibles as a slightly curved elevation, but becomes more and more prominent as the resorptive process reduces the mandibular ridge and body.

In mouths containing moderately resorbed residual ridges, the lingual ledge may be palpated far below the level of the floor of the mouth, and is not involved in the denture impression unless the impression tray is overextended. Where slightly resorbed, high mandibular ridges are present, the ledge is not palpable. The pres- ence of soreness and lesions in this region is explained by failure to cover the lingual ledge completely, so that the denture border rests upon it and impinges upon the thin overlying lining mucosa. On the other hand, if the lingual ledge is covered, mucosal lesions may result when border molding material is permitted to impinge upon the sharp edge of the ledge (Fig. 6B ) .

THE MUSCULOTENDINOUS REGION

The part of the denture border which occupies the space between the retro- mylohyoid space and the mandibular buccal vestibule is limited by four musculo- tendinous masses.2 When palpated from within outward, the four masses are identified as : (1) the medial pterygoid muscle, (2) the deep temporal muscle tendon attached to the temporal crest of the ramus, (3) the superficial temporal

Fig. 7.-Indelible pencil lines indicate the temporal crest and distal limit of the retromolar pad, A, and the anterior border of the masseter muscle, B. The musculotendinous border of the denture extends between these structures and tucks in under the inner cheek pad, C. Note also the medial pterygoid bulge, D.

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DENTURE-LIMITING STRUCTURES OF EDENTULOUS MOUTH 209

muscle tendon attached to the anterior border of the ramus, and (4) the anterior border of the masseter muscle (Fig. 7). The space between the superficial and deep temporal tendons is the fat-filled retromolar fossa.’ Impingement by the border of the denture upon any of these four musculote~~tlinous structures during mandibular movement may produce denture displacement or severe ulcerative lesions.

One of the widely held tenets of complete denture impressions states that the retromolar pad must be covered completely. However, some confusion exists con-

Fig. K-The landmarks of a mandibular impression include the retromylohyoid eminence, A, mylohyoid flange, B, sublingual border, C, constrictor border, D, pterygomandibular groove, 15, musculotendinous notch, F, lingual tubercular fossa, G, retromolar fossa, H, mylohyoid groove, J’, and the external oblique groove, K. The space between the right and left lingual flanges (arrows) must be widened to permit uninhibited genioglossus muscle movement.

210 KOLB J. Pros. Den.

March-April, 1966

cerning the identity of the retromolar pad. A hard elevation covered by masticatory mucosa may be palpated at the distal limit of the crest of the mandibular ridge. This remnant of the retromolar papilla has been termed the molar scar.3 A soft, easily dis- placeable bulge covered with lining mucosa is seen immediately behind and slightly lingual to the molar scar (Fig. 1). This downward extension of the palatine glan- dular mass2 is the retromolar pad. In many instances it may overlap and partially obscure the molar scar.

It is imperative that the musculotendinous border of the mandibular denture covers the molar scar completely in order to contact lining mucosa. It is desirable that the denture base cover and slightly indent the retromolar pad in order to achieve a seal. LammielO has suggested that a bead placed across the retromolar pad will ensure a border seal. However, in some instances, the interridge space in this location is so small that it necessitates shortening the mandibular denture. This may be safely accomplished up to the distal limit of the molar scar, with the realiza- tion that seal may be sacrificed.

According to Sicher,ll* the buccal fat pad is that part of the masticatory fat pad which.fills the buccal space between the masseter and the buccinator muscles and protrudes in front of the anterior border of the masseter muscle. This places the clinically palpable part of the buccal fat pad at the anterior limit of the musculo- tendinous region. The soft tissue bulge on the medial surface of the buccinator muscle which overlaps the musculotendinous and buccal borders, and which materi- ally aids in perfecting the seal, has been identified as the inner cheek pad (Fig. 7).3

THE MANDIBULAR BUCCAL VESTIBULE

The buccal shelf (Fig. 6A) has been identified as the surface of the mandible that lies between the residual ridge crest and the external oblique ridgel and is considered to be the primary stress-bearing area of the lower jaw. If a massive residual ridge is present, the mandibular buccal vestibule may be deep and narrow, and the buccal shelf may lie in a nearly vertical plane. In this instance, it is the buccal aspect of the mandibular residual ridge. As ridge resorption progresses, the buccal shelf flattens, and may eventually become a “hollow” in a severely resorbed mandible.

The external oblique ridge represents the lateral limit of the mandibular body, but it is a supporting rather than a limiting structure. The denture base must cover and extend beyond the oblique ridge in order to end in the fatty and loose connective tissues which overlie the attachment of the buccinator muscle.13 How far the man- dibular buccal flange extends beyond the external oblique ridge is determined by considerations of appearance, tissue tolerance, and patient comfort.

Border seal is enhanced by the fatty inner cheek pad which contacts the external surface of the mandibular buccal flange and creates the space frequently referred to as the “buccal pouch.”

The statements made previously with reference to the maxillary buccal frenulal apply as well to the mandibular buccal frenula. Again, the most important considera- tion is the distal slope of the mandibular labial flange.

“N%K ‘2” DENTURE-LIMITING STRUCTURES OF EDENTULOUS MOUTH 211

THE MANDIBULAR LABIAL VESTIBULE

A distinguishing feature of the mandibular labial vestibule is the presence of two soft elevations, one on each side of the labial frenum. These pads mark the origin of the mentalis muscles (Fig. 4), the fibers of which insert into the skin overlying the point of the chin and into the lower lip.llb If the mandibular labial vestibule is shallow, as it is when ridge resorption is severe, the pads may lie in the horizontal plane.lO These should be covered bv the mandibular labial flange of t:he denture.

When the lower lip is grasped between thumb and forefinger and pulled hori- zontally outward, the sickle shape of the mandibular labial frenum is accentuated. If the lip is permitted to assume a position of repose and is then gently separated from contact with the labial aspect of the residual ridge, the frenum is barely per- ceptible.

The degree of nervous tension, if not anxiety, present in an individual can often be judged by the activity of the mentalis muscle, which frequently contracts before the lip is touched by the approaching fingers of the dentist. It is the mentalis muscle, not the labial frenum, which is capable of displacing a mandibular denture.

Finally, if the masticatory mucosa covering the residual ridge crest is detached fl-om the underlying bone, the act of pulling the lower lip out and up during the final itnpression procedure will distort the basal surface of the impression.

SLiMMARY

The limiting structures of the mandibular arch exhibit greater variation than those of the maxillary arch, but the problems they present are not insurmountable. The variables considered in this article include : ( 1) the effect of tongue movement on the shape of the sublingual crescent space, (2) the prominence of the genioglos- sus attachment, (3) the prominence of the lingual ledge, (4) the proximity of the retromolar pad to the maxillary tuberosity at the established interridge distance, (5) the position of the external oblique ridge in relation to the residual ridge crest, (15) the positions of the buccinator and mylohyoid muscle attachments in relation to the ridge crest, (7) the activity of the mentalis muscle, and (8) habits and degree of neuromuscular control.

ACKNOWLEDGEMENT

The invaluable assistance of the Medical Illustration Service, Kecoughtan Veterans Admini- stration Center, is gratefuily acknowledged.

REFERENCES

I. Kolb, H. R.: Variable Denture-Limiting Stuctures of the Edentulous Mouth. Part I. Maxillary Border Areas, J. PROS. DENT. 16:194, 1966.

2!. Haines. R. W.. and Barrett, S. G. : The Structure of the Mouth in the MandibuIar Molar Region, J. PROS. DENT. 9:962. 1959.

3. Barrett, S. G., and Haines, R. W. : Structure of the Mouth in the Mandibular Molar Region and Tts Relation to the Denture, J. PROS. DENT. 12635, 1962.

4. Martone, A. L.: Clinical Applications of Concepts of Functional Anatomy and Speech Science to Complete Denture Prosthodontics. Part VII. Recording Phases, J. PROS. DENT. 13:4, 1963.

212 J. Pros. Den. March-April, 1966

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Hill, R. T. : Anatomy of Interest to the Prosthodontist, J. PROS. DENT. 5:109, 1955. Wright, C. R., Myskens, J. H., Strong, L. H., Westerman, K. N., Kingery, R. H., and

Williams, S. T.: A Study of the Tongue and Its Relations to Denture Stability, J.A.D.A. 39:269, 1949.

Shanahan, T. E. J.: Stabilizing Lower Dentures on Unfavorable Ridges, J. PROS. DENT. 12:420, 1962.

Lawson, W. A. : Influence of the Sublingual Fold on Retention of Complete Lower Dentures, J. PROS. DENT. 11:1038, 1961.

MerkefTi5H. J.: The Lingual Accessory Muscles of Mastication, J. PROS. DENT. 5:101,

Lammie, 6. A.: Aging Changes and the Complete Lower Denture, J. PROS. DENT. 6:450, 1956.

Sicher,(Fj ;Olraa Anatomy, ed. 3, St. Louis, 1960, The C. V. Mosby Company, (a) p. 466,

Edwards, L.’ F., and Boucher, C. 0. : Anatomy of the Mouth in Relation to Complete Den- tures, J.A.D.A. 29:331, 1942.

Pendleton, E. C.: Minute Anatomy of the Lower Jaw in Relation to the Denture Problem, J.A.D.A. 29:719, 1942.

VETERANS ADMINISTRATION CENTER HAMPTON, VA. 23367