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Lower Dentures
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T h e Journal of the Am erican Dental Association and T h e Dental Cosmos
4. Positive provisions for the tooth and crown to withstand the forces o f masti cation by controlling the wedging effect of the tooth and the effect o f stress on inclined planes. 5. A preparation suitable for all teeth regardless of size, shape, thickness or in termaxillary relationship. 6. A simple technic, requiring no spe cial instruments or materials.
7. A suitable preparation for success ful cementation. 8. The minimum o f visible gold con sistent with good construction. 9. Elimination o f levers or control of their effects. 10. A shape given to the crown that enables it to withstand the forces to which it is subjected. • 110 8 West Seventh Street.
LOWER DENTURES B y M . E . N i s w o n g e r , D .D .S., Dayton, Ohio
R E V I V E D interest in the construc tion of lower dentures is indicated by the attendance at this meeting. H ave lower denture failures developed this interest? O r is it the desire to suc ceed that makes the construction o f lower dentures, as well as upper dentures, so fascinating? I wish to increase this in terest by presenting the practical experi ences of m any dentists, including m y own. Dentists have long accepted G ray’ s and Cunningham’s recorded work on mandib ular musculature as final, believing that nothing further could be added by re search. N ow another chapter, a remark able contribution o f Hugh W . M ac M il lan,1 must be added on mandibular musculature. Dentists who have accepted and made use of these new anatomic facts presented by D r. M acM illan are con structing very much better lower den tures than they had ever hoped to make.
Since the technic o f taking lower im pressions which is outlined later is largely based upon these new anatomic findings, it will be necessary to quote frequently from D r. M acM illan ’s article. T h e present custom of extending lower dentures for stability to far greater limits than is usual prompted an in quiry b y D r. M acM illan into the nature o f the structures supporting the extended dentures. A n effort was made to de termine the farthest edge or boundary to which they m ay be extended. H e w rites: The proper approach to such a study is from the standpoint of myology, not oste ology. Too many technics in dentistry, even prosthetic dentistry, have been founded on the anatomy of the dry maxillary bones. . . . As usually seen in the cadaver, the teeth of the opposing series are slightly apart—the position of rest in the living. It is important that this fact be recognized when clinical application involving anatomic mechanisms are made from dissections. It is interesting to contemplate that, in the patient, the nor Read before the Section on Full Dentures at mal relations of the parts inside the mouth the Seventy-Fourth Annual Midwinter Meeting of the Chicago Dental Society, February 16, proper cannot be observed by the examiner. As soon as the mouth is opened, the parts are 1938. put on tension, muscles have come into play 1. M a c M i l l a n , H. W .: Anatomy of Throat, Mylohyoid Region and Mandible in Relation and everything contained within the mouth to Retention of Mandibular Artificial Den assumes a relationship far different from that of the structures at rest or during functional tures. J.A .D .A ., 2 3 :1 4 3 5 , August 1936.
A
Jour. A .D .A . & D. Cos., V ol. 25, September 19 38
Niswonger— Low er Dentures excursions. These abnormal relationships are too often obtained in taking impressions. The resulting prostheses are constructed over casts that do not portray the relationships of rest or function or the happy medium.
There are two areas consisting o f muscle and soft tissues which are not generally utilized for retention of dentures. One area is termed the buccal pouch, the other the sublingual space. B U C C A L PO UCH
T h e buccal pouch is an area with little muscle tone located below the gingival region of the lower first molar. This pouch can be located by placing the forefinger in the mouth opposite the lower first molar region. When the finger is permitted to drop toward the apices of the molar, it will fall into this pouch. W ith the finger in this region, the teeth are brought into firm occlusion by con traction of the masseter muscle. Th e posterior border of the buccal pouch will be found to be the anterior fibers o f the masseter muscle. Now, the forefinger is brought forward into the buccal pouch and the fact that the anterior border is limited b y the posterior fibers of the de pressor anguli oris or triangularis be comes evident. In the average mouth, the buccal pouch will accommodate a good-sized bolus of food. Likewise, the buccal pouch m ay accommodate the finished denture over which the fibers o f the buccinator muscle will fold, aiding in retention.
I 38 5 the muscle fibers of the anterior palatine arch and can be demonstrated only with difficulty.
In one sense, this is not a space when the tissues are at rest. T h e tissues form ing the boundaries of the sublingual space m ay be described as follows: T h e sublingual space is bounded on the lin gual aspect by the tongue muscles. T h e buccal boundary o f this space is limited by the mylohyoid muscle and the m an dible. T h e inferior boundary is formed by the mucous membranes covering the submaxillary and sublingual glands. T h e posterior boundary is formed by the an terior aspects o f the glossopalatine muscle and the lingual portion o f the superior constrictor of the pharynx. T h e sublingual space m ay be demon strated by using the mouth mirror to move the tongue to one side, at the same time instructing the patient to place the tip o f the tongue lightly against the lower lip and to relax the tongue muscles as much as possible. In a cadaver, this area is distinguished and demonstrated in the best w ay. It was on a cadaver that D r. M acM illan demonstrated the posterior boundary or partition formed b y the anterior palatine arch which separates the sublingual space from the fauces. W hen the mouth is open wide and the tongue is contracted toward the throat, the sublingual space is so distorted that the anatomic boundaries of this area assume a relationship far different from that which could be satisfactorily utilized in prosthesis.
SU B LIN G U A L SP A C E
In M acM illan’ s article, we re a d : Th e “ discovery” of the muscular partition between the sublingual space and the fauces and the pouchlike formation of the posterior portion of the sublingual space has been il luminating to prosthetists. In the living pa tient, when the mouth is open wide for ex amination, the structures forming the poste rior boundary of the sublingual space are hidden from the examiner by the tension of
O V ER EX T EN SIO N AND O VERCO M PRESSIO N
Experience has taught us that m any common errors are made in lower im pressions; for example, in overextension or displacement, and overcompression of tissues. Overextension is in one sense a stretching of tissues b y the periphery of an impression or a denture which ap parently changes tissues from their nor
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T h e Journal of the American Dental Association and T h e Dental Cosmos
mal working position. Generally, the changing o f the normal working position of tissues in mouths with little or no ridge by the periphery o f the impression or finished denture results in the dislodging of the impression or denture b y these tissues. I f overextension takes place in mouths with very large ridge areas, the ridge adaptation is extensive enough to resist displacement o f the denture. In such cases, overextension o f the denture soon cuts the periphery tissues. F ew patients can or will tolerate cutting of border tissues until scar tissues are formed. Overcompression is in one sense press ing together layers of tissues covered by the denture. This will sometimes dislodge a denture, cause soreness and encourage atrophy, which results in destruction of the adaptation o f the denture. Further more, the so-called postdamming o f the lower impression, when reproduced in the denture, is only a temporary retention measure and causes both soreness and atrophy. Overextension and overcompression by use of too large an amount of impression material or an oversize lingual flange of a denture can separate the sublingual tissue folds and change the sublingual space into one that is distorted and ab normal in size and shape. Such a space is undesirable, as it interferes not only with the function and stability of the denture, but also with the normal func tions of the tissues involved. T h e posterior dislodgment or ‘ ‘kicking up” o f the heel of the impression and denture is caused m any times by stretch ing and compression of the anterior fibers of the temporal muscle by the impression or the denture. This interferes with the function o f the fibers of the temporal muscle. Sometimes, these fibers are at tached as far down on the ramus as the posterior boundary o f the retromolar triangle. Frequently, the mylohyoid fibers, which are attached to the posterior
extremity o f the mylohyoid ridge, extend upward and are attached high on the inner surface o f the ramus near the an terior border and are the antagonists of the anterior fibers of the temporal muscle. T h e stretching and compression o f these mylohyoid fibers will also cause posterior dislodgment o f the impression or denture. T h e changing of the working area of the superior constrictor muscle and the overextension and compression of the pterygomaxillary ligament which is attached to the posterior extremity of the mylohyoid ridge will likewise dislodge the impression or denture. In such cases, the impression or denture cannot be called functional. E F F E C T OF M U S C L E TO N E OR TEN SIO N ON IM PRESSIO N S
Tissues with little tone or tension are more easily overextended and overcom pressed than those with marked tissue tone or tension. Especially is this notice able in the lingual tissues. Consequently, there is not so much tone in the lingual tissues to aid in trimming the lingual periphery border o f the impression as in the tissue bordering the buccal and labial periphery o f the impression. T h e mylo hyoid muscle is of special interest because it functions below the mylohyoid ridge, whereas the genioglossus, the other im portant muscle involved in the peripheral contouring, can function above the level of the mylohyoid ridge, the same as the labial and buccal muscles. W hen the genioglossus extends the tip of the tongue forward and upward, its attachment to the genial tubercle will trim the impres sion material. Meanwhile, the mylohyoid muscle is in a noncontractile state and so lacks tone and tension that it is unable to mold a periphery o f an impression. Hence, we can expect overextension and compression o f the mylohyoid muscle, unless the periphery contouring is done by muscular action, such as opening, closing and swallowing m any times. Since
Niswonger— Lo w er Dentures the mylohyoid muscle is the principal depressor muscle involved in the opening after the external pterygoid starts the opening from centric occlusion, it will shape the periphery better if force is used to resist the opening somewhat. This is done by placing both thumbs against the lower border of the mandible and pressing upward with force, at the same time instructing the patient to open the mouth. T h e tone of the buccal and labial tis sues m ay be demonstrated as follow s: T h e mouth is opened wide and the index finger placed against the masseter and buccinator muscles opposite the lower sec ond or third molar. Evidently, there is not much tone or tension. One can read ily see how the impression material can stretch tissues in this region beyond their normal working area. T h e harder it is for the impression material to flow, the greater the stretching or oxerextending. Next, with the index finger still in the same region, the teeth are brought into centric occlusion and the masseter and buccinator muscles are contracted. T h e firm contraction of the anterior fibers of the masseter muscles is evident. T h e buc cinator likewise takes on a different form. N ow, with the teeth still in centric occlu sion, another accessory muscle o f masti cation, the orbicularis oris, is contracted. Th is too, as you know, takes on m any different forms. T h is demonstration should further con vince us of the need of taking lower im pressions under similar conditions of tis sue .contraction; which means also that centric relation can be an aid in taking impressions. Furthermore, for the best results, centric position must be estab lished at the place where it existed prior to extraction. It is necessary to digress to consider the use of the tactile muscle sense of centric occlusion in securing centric relation. T h e term “ tactile muscle sense of centric occlusion” is not generally known and
1387 not described in dental literature. I t is explained as follow s: W hen the natural teeth come into centric occlusion, reflexes stimulate the masticatory muscles until their strength is dissipated or scattered. Other reflexes stimulate the external pterygoids to action in separating the teeth from centric occlusion and starting the opening m ovem ent; after which re flexes stimulate the depressor muscles to action and complete the opening of the mandible. These reflexes also occur in crushing or attempting to crush a hard bolus of fo o d ; for example, a cherry seed or hard piece of candy, when the teeth are not in centric occlusion. This is a highly special ized arrangement. These reflexes remain after the extraction o f all the upper or the lower teeth, or both. I f these reflexes did not remain after extraction, patients with dentures could never open their mouths, regardless of where centric oc clusion was established. It is at centric occlusion that the tactile trigger-like action o f the teeth sets up the defense reflexes ending the upward movement of the mandible. Likewise, the trigger-like action will set up these reflexes at centric occlusion upon hard occlusal rims o f the impression tray or well-fitting base-plates. In the edentulous, it is through these reflexes that centric relation is easily established and used in taking functional impressions. T h e use of these reflexes is the keynote to successful functional lower dentures because it permits the maximum muscle power to adapt the impression material. This is accomplished as follows : A snap or preliminary impression is taken with the patient’ s tongue protruded and the mouth wide open, while the operator pulls upward on the buccal and labial tissues attached to and below the external oblique line. T h e better this preliminary impression, the better the final impres sion. A plaster is used that, at a certain stage, which m ay be called the whipped cream stage, is plastic enough to be read
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T h e Journal of the American Dental Association and T h e Dental Cosmos
ily molded by the tissues with a minimum amount of extension and compression. M ost compound impression material re quires considerable pressure to cause it to flow. A plaster cast is now made, over which a white vulcanite tray is to be made. T h e reason for using white rubber will be explained later. W hite rubber is dissolved in chloroform and painted on the cast; which makes it possible for the rubber to be ironed on the cast and thereby reduces the time. T h e periphery toward the crest of the ridge is strength ened for about 2 mm. with another thick ness o f vulcanite, the denture vulcanized and the plaster polished off. W e are now ready to use the tray in the mouth. T h e next step is very impor tant. W ith an indelible pencil and the mouth wide open, the mucobuccal and mucolingual folds are marked. These folds should not be distorted or stretched. T h e tray is inserted and, since the tray is white vulcanite, the outline of the indel ible pencil on the tissues will show on the tray. T h e tray is now trimmed to out line. It should be remembered that we are approaching this problem from the standpoint of myology, not osteology. Th e patient is instructed to extend the tongue forward and to relax the tongue muscles as much as possible, the operator at the same time using the mouth mirror to move the tongue to one side. This action makes it possible to mark with indelible pencil the sublingual space. T h e tray is inserted and the patient is instructed to again protrude the tongue. T h e tray is removed and trimmed to the pencil mark. This marking is repeated on the labial and opposite sides. I f this trimming is done carefully, the white vulcanite im pression tray will be seated upon the ridge and will not be dislodged with the opening of the mouth and the protruding of the tongue. Black tray compound is used for this occlusal rim. This is sealed tightly with dry heat upon the vulcanite tray. W e are
now ready to make use o f the tactile muscle sense of centric occlusion that ex isted prior to extraction, to secure centric relation. T h e procedure is as follow s: T h e edentulous patient is told to place his ja w “ where it feels most comfort able,” or to relax or to swallow. A fter he swallows, his . ja w returns to the rest position. T h e patient is told to close until he feels that he would be touching his natural back teeth. These directions are given slowly and deliberately, as this is a new experience to the patient. Th e patient is told to repeat placing the jaw in these two positions. O f course, he at first thinks that he cannot do this. He is assured that he can, to make him feel more confident. This is such a new and simple procedure that even dentists have not recognized it. A fter the patient has become acquainted with this procedure, the vulcanite tray is inserted with the compound occlusal rim constructed so that it is in even contact with the upper rim or natural teeth while the jaw is in rest position. T h e occlusal rim of the lower tray is softened with a blow torch and inserted in the mouth. Softening is very important. Compound softened in sufficiently (too hard) or oversoftened causes failure, as it interferes with the normal reflexes that were associated with the natural teeth in centric occlusion. Th e patient is told to imagine that he has all his natural teeth and to chew into the softened compound rim until he thinks that his natural back teeth would be touching, or until he can imagine him self clinching his back teeth. W e then find that his tactile muscle sense o f cen tric occlusion will stop the mandible as it did prior to extraction. W e are now ready for the functional impression to be taken with two kinds of wax, a hard w ax with a melting point around 1 3 5 0 F ., and a softer w ax with a melting point around 1 2 4 0 F . Both will soften and flow easily at body tempera ture. T h e waxes are compounded with
Niswonger— L o w er Dentures paraffin, beeswax, petrolatum and vege table dye. This procedure is called “ functional impression” taking because it will be made during m any functional ex cursions o f the mandible similar to the incising and masticating of food, swallow ing and any other opening and closing movements of the mandible in which the tissues o f the sublingual space and other oral tissues are involved. Furthermore, in these functional w ax impressions, over extension and compression are practically eliminated in mouths with a great amount o f tissue tone, and are greatly reduced in mouths with little tissue tone. A fter the w a x has been softened b y the body heat, the peripheral tissues will push the w ax out of their w ay and contour it for their movements. H ard w a x is first painted on the tray with a small brush, after the w ax has been melted with the double boiler method o f heating. More hard w ax is easily applied to secure adaptation or until the tissues have made one continu ous and smooth imprint. T h e softer w ax is next painted over the hard wax, which has a melting point of about 1 2 4 0 F. It will soften and flow easier than the hard w ax at body temperature. M ore soft w ax can be applied just as the hard w ax to secure adaptation or until it takes on a surface that is a smooth and con tinuous imprint of the fine lines of the mucous membranes covering the ridge and peripheral tissues. It is here that the vegetable dye gives the w ax a more vis ional imprint of the tissues. T h e repro duction of all the peripheral border is necessary in pouring the cast, as a guide to the finished dentures.
i 38 9 COM M ENT
Lo w er functional impression taking can be done in the following steps: 1. A thorough anatomic examination. 2. A preliminary impression in plaster or any material in order to reduce overextension and overcompression. 3. T h e making of a white vulcanite impression tray. 4. O ut lining, with soft indelible pencil, of the mucobuccal and mucolingual tissue folds bordering the ridge. 5 . Insertion of the white vulcanite tray and registration of the indelible outline on the tray. 6. Trim m ing of the white vulcanite tray to the indelible outline. 7. Building of a rigid and high fusing compound occlusal rim upon the tray. 8. Establishment of centric relation through the use of tactile muscle sense o f centric occlusion existing prior to extraction. 9. Employment of centric relation, the vulcanite tray and the w ax impression material together to secure a functional lower impression. SUM M ARY
1. Low er dentures can be more suc cessfully constructed if the approach is from the standpoint of myology instead of osteology. 2. A functional lower impression as outlined permits the peripheral and den ture bearing tissues to function histologi cally and anatomically, aiding in the denture’s stability and permanency. 3. Finally, the apparent ever-returning of lower denture patients to our offices for trimming, adjustments and com plaints is practically eliminated. 720 Fidelity Building.
T h e Journal of the Am erican Dental Association and T h e Dental Cosmos
4. Positive provisions for the tooth and crown to withstand the forces o f masti cation by controlling the wedging effect of the tooth and the effect o f stress on inclined planes. 5. A preparation suitable for all teeth regardless of size, shape, thickness or in termaxillary relationship. 6. A simple technic, requiring no spe cial instruments or materials.
7. A suitable preparation for success ful cementation. 8. The minimum o f visible gold con sistent with good construction. 9. Elimination o f levers or control of their effects. 10. A shape given to the crown that enables it to withstand the forces to which it is subjected. • 110 8 West Seventh Street.
LOWER DENTURES B y M . E . N i s w o n g e r , D .D .S., Dayton, Ohio
R E V I V E D interest in the construc tion of lower dentures is indicated by the attendance at this meeting. H ave lower denture failures developed this interest? O r is it the desire to suc ceed that makes the construction o f lower dentures, as well as upper dentures, so fascinating? I wish to increase this in terest by presenting the practical experi ences of m any dentists, including m y own. Dentists have long accepted G ray’ s and Cunningham’s recorded work on mandib ular musculature as final, believing that nothing further could be added by re search. N ow another chapter, a remark able contribution o f Hugh W . M ac M il lan,1 must be added on mandibular musculature. Dentists who have accepted and made use of these new anatomic facts presented by D r. M acM illan are con structing very much better lower den tures than they had ever hoped to make.
Since the technic o f taking lower im pressions which is outlined later is largely based upon these new anatomic findings, it will be necessary to quote frequently from D r. M acM illan ’s article. T h e present custom of extending lower dentures for stability to far greater limits than is usual prompted an in quiry b y D r. M acM illan into the nature o f the structures supporting the extended dentures. A n effort was made to de termine the farthest edge or boundary to which they m ay be extended. H e w rites: The proper approach to such a study is from the standpoint of myology, not oste ology. Too many technics in dentistry, even prosthetic dentistry, have been founded on the anatomy of the dry maxillary bones. . . . As usually seen in the cadaver, the teeth of the opposing series are slightly apart—the position of rest in the living. It is important that this fact be recognized when clinical application involving anatomic mechanisms are made from dissections. It is interesting to contemplate that, in the patient, the nor Read before the Section on Full Dentures at mal relations of the parts inside the mouth the Seventy-Fourth Annual Midwinter Meeting of the Chicago Dental Society, February 16, proper cannot be observed by the examiner. As soon as the mouth is opened, the parts are 1938. put on tension, muscles have come into play 1. M a c M i l l a n , H. W .: Anatomy of Throat, Mylohyoid Region and Mandible in Relation and everything contained within the mouth to Retention of Mandibular Artificial Den assumes a relationship far different from that of the structures at rest or during functional tures. J.A .D .A ., 2 3 :1 4 3 5 , August 1936.
A
Jour. A .D .A . & D. Cos., V ol. 25, September 19 38
Niswonger— Low er Dentures excursions. These abnormal relationships are too often obtained in taking impressions. The resulting prostheses are constructed over casts that do not portray the relationships of rest or function or the happy medium.
There are two areas consisting o f muscle and soft tissues which are not generally utilized for retention of dentures. One area is termed the buccal pouch, the other the sublingual space. B U C C A L PO UCH
T h e buccal pouch is an area with little muscle tone located below the gingival region of the lower first molar. This pouch can be located by placing the forefinger in the mouth opposite the lower first molar region. When the finger is permitted to drop toward the apices of the molar, it will fall into this pouch. W ith the finger in this region, the teeth are brought into firm occlusion by con traction of the masseter muscle. Th e posterior border of the buccal pouch will be found to be the anterior fibers o f the masseter muscle. Now, the forefinger is brought forward into the buccal pouch and the fact that the anterior border is limited b y the posterior fibers of the de pressor anguli oris or triangularis be comes evident. In the average mouth, the buccal pouch will accommodate a good-sized bolus of food. Likewise, the buccal pouch m ay accommodate the finished denture over which the fibers o f the buccinator muscle will fold, aiding in retention.
I 38 5 the muscle fibers of the anterior palatine arch and can be demonstrated only with difficulty.
In one sense, this is not a space when the tissues are at rest. T h e tissues form ing the boundaries of the sublingual space m ay be described as follows: T h e sublingual space is bounded on the lin gual aspect by the tongue muscles. T h e buccal boundary o f this space is limited by the mylohyoid muscle and the m an dible. T h e inferior boundary is formed by the mucous membranes covering the submaxillary and sublingual glands. T h e posterior boundary is formed by the an terior aspects o f the glossopalatine muscle and the lingual portion o f the superior constrictor of the pharynx. T h e sublingual space m ay be demon strated by using the mouth mirror to move the tongue to one side, at the same time instructing the patient to place the tip o f the tongue lightly against the lower lip and to relax the tongue muscles as much as possible. In a cadaver, this area is distinguished and demonstrated in the best w ay. It was on a cadaver that D r. M acM illan demonstrated the posterior boundary or partition formed b y the anterior palatine arch which separates the sublingual space from the fauces. W hen the mouth is open wide and the tongue is contracted toward the throat, the sublingual space is so distorted that the anatomic boundaries of this area assume a relationship far different from that which could be satisfactorily utilized in prosthesis.
SU B LIN G U A L SP A C E
In M acM illan’ s article, we re a d : Th e “ discovery” of the muscular partition between the sublingual space and the fauces and the pouchlike formation of the posterior portion of the sublingual space has been il luminating to prosthetists. In the living pa tient, when the mouth is open wide for ex amination, the structures forming the poste rior boundary of the sublingual space are hidden from the examiner by the tension of
O V ER EX T EN SIO N AND O VERCO M PRESSIO N
Experience has taught us that m any common errors are made in lower im pressions; for example, in overextension or displacement, and overcompression of tissues. Overextension is in one sense a stretching of tissues b y the periphery of an impression or a denture which ap parently changes tissues from their nor
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T h e Journal of the American Dental Association and T h e Dental Cosmos
mal working position. Generally, the changing o f the normal working position of tissues in mouths with little or no ridge by the periphery o f the impression or finished denture results in the dislodging of the impression or denture b y these tissues. I f overextension takes place in mouths with very large ridge areas, the ridge adaptation is extensive enough to resist displacement o f the denture. In such cases, overextension o f the denture soon cuts the periphery tissues. F ew patients can or will tolerate cutting of border tissues until scar tissues are formed. Overcompression is in one sense press ing together layers of tissues covered by the denture. This will sometimes dislodge a denture, cause soreness and encourage atrophy, which results in destruction of the adaptation o f the denture. Further more, the so-called postdamming o f the lower impression, when reproduced in the denture, is only a temporary retention measure and causes both soreness and atrophy. Overextension and overcompression by use of too large an amount of impression material or an oversize lingual flange of a denture can separate the sublingual tissue folds and change the sublingual space into one that is distorted and ab normal in size and shape. Such a space is undesirable, as it interferes not only with the function and stability of the denture, but also with the normal func tions of the tissues involved. T h e posterior dislodgment or ‘ ‘kicking up” o f the heel of the impression and denture is caused m any times by stretch ing and compression of the anterior fibers of the temporal muscle by the impression or the denture. This interferes with the function o f the fibers of the temporal muscle. Sometimes, these fibers are at tached as far down on the ramus as the posterior boundary o f the retromolar triangle. Frequently, the mylohyoid fibers, which are attached to the posterior
extremity o f the mylohyoid ridge, extend upward and are attached high on the inner surface o f the ramus near the an terior border and are the antagonists of the anterior fibers of the temporal muscle. T h e stretching and compression o f these mylohyoid fibers will also cause posterior dislodgment o f the impression or denture. T h e changing of the working area of the superior constrictor muscle and the overextension and compression of the pterygomaxillary ligament which is attached to the posterior extremity of the mylohyoid ridge will likewise dislodge the impression or denture. In such cases, the impression or denture cannot be called functional. E F F E C T OF M U S C L E TO N E OR TEN SIO N ON IM PRESSIO N S
Tissues with little tone or tension are more easily overextended and overcom pressed than those with marked tissue tone or tension. Especially is this notice able in the lingual tissues. Consequently, there is not so much tone in the lingual tissues to aid in trimming the lingual periphery border o f the impression as in the tissue bordering the buccal and labial periphery o f the impression. T h e mylo hyoid muscle is of special interest because it functions below the mylohyoid ridge, whereas the genioglossus, the other im portant muscle involved in the peripheral contouring, can function above the level of the mylohyoid ridge, the same as the labial and buccal muscles. W hen the genioglossus extends the tip of the tongue forward and upward, its attachment to the genial tubercle will trim the impres sion material. Meanwhile, the mylohyoid muscle is in a noncontractile state and so lacks tone and tension that it is unable to mold a periphery o f an impression. Hence, we can expect overextension and compression o f the mylohyoid muscle, unless the periphery contouring is done by muscular action, such as opening, closing and swallowing m any times. Since
Niswonger— Lo w er Dentures the mylohyoid muscle is the principal depressor muscle involved in the opening after the external pterygoid starts the opening from centric occlusion, it will shape the periphery better if force is used to resist the opening somewhat. This is done by placing both thumbs against the lower border of the mandible and pressing upward with force, at the same time instructing the patient to open the mouth. T h e tone of the buccal and labial tis sues m ay be demonstrated as follow s: T h e mouth is opened wide and the index finger placed against the masseter and buccinator muscles opposite the lower sec ond or third molar. Evidently, there is not much tone or tension. One can read ily see how the impression material can stretch tissues in this region beyond their normal working area. T h e harder it is for the impression material to flow, the greater the stretching or oxerextending. Next, with the index finger still in the same region, the teeth are brought into centric occlusion and the masseter and buccinator muscles are contracted. T h e firm contraction of the anterior fibers of the masseter muscles is evident. T h e buc cinator likewise takes on a different form. N ow, with the teeth still in centric occlu sion, another accessory muscle o f masti cation, the orbicularis oris, is contracted. Th is too, as you know, takes on m any different forms. T h is demonstration should further con vince us of the need of taking lower im pressions under similar conditions of tis sue .contraction; which means also that centric relation can be an aid in taking impressions. Furthermore, for the best results, centric position must be estab lished at the place where it existed prior to extraction. It is necessary to digress to consider the use of the tactile muscle sense of centric occlusion in securing centric relation. T h e term “ tactile muscle sense of centric occlusion” is not generally known and
1387 not described in dental literature. I t is explained as follow s: W hen the natural teeth come into centric occlusion, reflexes stimulate the masticatory muscles until their strength is dissipated or scattered. Other reflexes stimulate the external pterygoids to action in separating the teeth from centric occlusion and starting the opening m ovem ent; after which re flexes stimulate the depressor muscles to action and complete the opening of the mandible. These reflexes also occur in crushing or attempting to crush a hard bolus of fo o d ; for example, a cherry seed or hard piece of candy, when the teeth are not in centric occlusion. This is a highly special ized arrangement. These reflexes remain after the extraction o f all the upper or the lower teeth, or both. I f these reflexes did not remain after extraction, patients with dentures could never open their mouths, regardless of where centric oc clusion was established. It is at centric occlusion that the tactile trigger-like action o f the teeth sets up the defense reflexes ending the upward movement of the mandible. Likewise, the trigger-like action will set up these reflexes at centric occlusion upon hard occlusal rims o f the impression tray or well-fitting base-plates. In the edentulous, it is through these reflexes that centric relation is easily established and used in taking functional impressions. T h e use of these reflexes is the keynote to successful functional lower dentures because it permits the maximum muscle power to adapt the impression material. This is accomplished as follows : A snap or preliminary impression is taken with the patient’ s tongue protruded and the mouth wide open, while the operator pulls upward on the buccal and labial tissues attached to and below the external oblique line. T h e better this preliminary impression, the better the final impres sion. A plaster is used that, at a certain stage, which m ay be called the whipped cream stage, is plastic enough to be read
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T h e Journal of the American Dental Association and T h e Dental Cosmos
ily molded by the tissues with a minimum amount of extension and compression. M ost compound impression material re quires considerable pressure to cause it to flow. A plaster cast is now made, over which a white vulcanite tray is to be made. T h e reason for using white rubber will be explained later. W hite rubber is dissolved in chloroform and painted on the cast; which makes it possible for the rubber to be ironed on the cast and thereby reduces the time. T h e periphery toward the crest of the ridge is strength ened for about 2 mm. with another thick ness o f vulcanite, the denture vulcanized and the plaster polished off. W e are now ready to use the tray in the mouth. T h e next step is very impor tant. W ith an indelible pencil and the mouth wide open, the mucobuccal and mucolingual folds are marked. These folds should not be distorted or stretched. T h e tray is inserted and, since the tray is white vulcanite, the outline of the indel ible pencil on the tissues will show on the tray. T h e tray is now trimmed to out line. It should be remembered that we are approaching this problem from the standpoint of myology, not osteology. Th e patient is instructed to extend the tongue forward and to relax the tongue muscles as much as possible, the operator at the same time using the mouth mirror to move the tongue to one side. This action makes it possible to mark with indelible pencil the sublingual space. T h e tray is inserted and the patient is instructed to again protrude the tongue. T h e tray is removed and trimmed to the pencil mark. This marking is repeated on the labial and opposite sides. I f this trimming is done carefully, the white vulcanite im pression tray will be seated upon the ridge and will not be dislodged with the opening of the mouth and the protruding of the tongue. Black tray compound is used for this occlusal rim. This is sealed tightly with dry heat upon the vulcanite tray. W e are
now ready to make use o f the tactile muscle sense of centric occlusion that ex isted prior to extraction, to secure centric relation. T h e procedure is as follow s: T h e edentulous patient is told to place his ja w “ where it feels most comfort able,” or to relax or to swallow. A fter he swallows, his . ja w returns to the rest position. T h e patient is told to close until he feels that he would be touching his natural back teeth. These directions are given slowly and deliberately, as this is a new experience to the patient. Th e patient is told to repeat placing the jaw in these two positions. O f course, he at first thinks that he cannot do this. He is assured that he can, to make him feel more confident. This is such a new and simple procedure that even dentists have not recognized it. A fter the patient has become acquainted with this procedure, the vulcanite tray is inserted with the compound occlusal rim constructed so that it is in even contact with the upper rim or natural teeth while the jaw is in rest position. T h e occlusal rim of the lower tray is softened with a blow torch and inserted in the mouth. Softening is very important. Compound softened in sufficiently (too hard) or oversoftened causes failure, as it interferes with the normal reflexes that were associated with the natural teeth in centric occlusion. Th e patient is told to imagine that he has all his natural teeth and to chew into the softened compound rim until he thinks that his natural back teeth would be touching, or until he can imagine him self clinching his back teeth. W e then find that his tactile muscle sense o f cen tric occlusion will stop the mandible as it did prior to extraction. W e are now ready for the functional impression to be taken with two kinds of wax, a hard w ax with a melting point around 1 3 5 0 F ., and a softer w ax with a melting point around 1 2 4 0 F . Both will soften and flow easily at body tempera ture. T h e waxes are compounded with
Niswonger— L o w er Dentures paraffin, beeswax, petrolatum and vege table dye. This procedure is called “ functional impression” taking because it will be made during m any functional ex cursions o f the mandible similar to the incising and masticating of food, swallow ing and any other opening and closing movements of the mandible in which the tissues o f the sublingual space and other oral tissues are involved. Furthermore, in these functional w ax impressions, over extension and compression are practically eliminated in mouths with a great amount o f tissue tone, and are greatly reduced in mouths with little tissue tone. A fter the w a x has been softened b y the body heat, the peripheral tissues will push the w ax out of their w ay and contour it for their movements. H ard w a x is first painted on the tray with a small brush, after the w ax has been melted with the double boiler method o f heating. More hard w ax is easily applied to secure adaptation or until the tissues have made one continu ous and smooth imprint. T h e softer w ax is next painted over the hard wax, which has a melting point of about 1 2 4 0 F. It will soften and flow easier than the hard w ax at body temperature. M ore soft w ax can be applied just as the hard w ax to secure adaptation or until it takes on a surface that is a smooth and con tinuous imprint of the fine lines of the mucous membranes covering the ridge and peripheral tissues. It is here that the vegetable dye gives the w ax a more vis ional imprint of the tissues. T h e repro duction of all the peripheral border is necessary in pouring the cast, as a guide to the finished dentures.
i 38 9 COM M ENT
Lo w er functional impression taking can be done in the following steps: 1. A thorough anatomic examination. 2. A preliminary impression in plaster or any material in order to reduce overextension and overcompression. 3. T h e making of a white vulcanite impression tray. 4. O ut lining, with soft indelible pencil, of the mucobuccal and mucolingual tissue folds bordering the ridge. 5 . Insertion of the white vulcanite tray and registration of the indelible outline on the tray. 6. Trim m ing of the white vulcanite tray to the indelible outline. 7. Building of a rigid and high fusing compound occlusal rim upon the tray. 8. Establishment of centric relation through the use of tactile muscle sense o f centric occlusion existing prior to extraction. 9. Employment of centric relation, the vulcanite tray and the w ax impression material together to secure a functional lower impression. SUM M ARY
1. Low er dentures can be more suc cessfully constructed if the approach is from the standpoint of myology instead of osteology. 2. A functional lower impression as outlined permits the peripheral and den ture bearing tissues to function histologi cally and anatomically, aiding in the denture’s stability and permanency. 3. Finally, the apparent ever-returning of lower denture patients to our offices for trimming, adjustments and com plaints is practically eliminated. 720 Fidelity Building.