Neutral zone/ orthodontic seminars

NEUTRAL ZONEINDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.comwww.indiandentalacademy.com

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INTRODUCTION The goal of dentistry is for patients to keep all of their teeth throughout their lives in health and comfort. If the teeth are lost despite all efforts to save them, a restoration should be made in such a manner as to function efficiently and comfortably in harmony with the muscles of the stomatognathic system and the temporomandibular joints



The stable position of the teeth represents equilibrium of all the forces acting on them. If that position of equilibrium namely the neutral zone, is not found, the resulting dentition will not last long and will not be esthetically pleasing and the patients use of



functional efficiency, maximum length of use and pleasing esthetics will not have been met.To understand the stable position of teeth, the concept of neutral zone is important.



Neutral Zone is defined as that area in the mouth where, during function, the forces of the tongue pressing outward are neutralised by the forces of the cheeks and lips pressing inwards.



FORM AND FUNCTION: In orthodontics and prosthodontics the basic principles of treatment are identical. They revolve around the relationship between form and function..



Function determines form. Abnormal functions such as deviate swallowing pattern, mouth breathing and thumb sucking, will modify or dictate the form of the dental arches



It is equally true that alteration of the form of a growing individual will alter function. If the alteration of form is an improvement, there will be a concomitant improvement in function. Ideally, both the function and form should be corrected together for optimal results, and this correction should take place before growth has ceased.



Prosthodontic treatment in complete dentures is also influenced by the concept that form follows function. Complete dentures that are constructed by concepts that do not take into consideration the unique functioning of the individual patients musculature are over looking this basic law of physiology.



If form is dictated by function, then in complete denture construction, the operator must shape and form dentures to be in harmony with function



In all areas of dentistry, the ultimate problem in maintaining the health of the stomatognathic system throughout life is one of harmonious pressure distribution. The primary function of this system is the application, distribution and dissipation of the pressure of the bite and of the muscles of the lips, cheeks and tongue.



To put it more simply, the primary function of the stomatognathic system is mastication.The prosthodontist who is unaware of the effect of muscle function will be faced with cases of prosthodontic relapse unstable dentures.



THE NEUTRAL ZONE AND DENTURE SPACE In completely edentulous patients there exists within the oral cavity a void that may be called the potential denture space. The denture space is bounded by the maxilla and soft palate above, by the mandible and floor of the mouth below,



by the tongue, medially or internally, and by the muscles and tissues of the lips and cheeks laterally or externally. Within the denture space there is an area that has been termed the neutral zone.



Weath (1970) has demonstrated that there is a difference in the shape of the denture space and resultant arch form at rest as compared to the denture space and arch form established by function.



FACTORS AFFECTING THE NEUTRAL ZONE



Muscles and the neutral zone:Dentures should occupy a position in the mouth where all the forces during function are neutralized. Otherwise, denture stability will be decreased proportionately to the difference in the amount of the opposing forces.Neutral zone in complete dentures is in fact the zone previously occupied by the natural teeth.



MUSCLES OF THE CHEEK The outer limits of the neutral zone are determined by the perioral musculature.BUCCINATORThe main determinant of length, strength and position of the perioral musculature is the buccinator muscle. The buccinator is a thin, flat muscle composed of three bands.



The combined width of the three bands covers the entire outer surface of the dento alveolar structures, that is the teeth, alveolar process and gingival tissues.

The upper and lower bands are continuous from side to side without decussation. The middle band fibers decussate and joint into the fibers of the orbicularis oris.www.indiandentalacademy.com


Because the muscle fibers form a continuous band, the size of the arch is limited by the length of the muscles when they are contracted repetitiously. Regardless of the reason for variations in muscle tonus in different patients, the strength of the contractile force, at the length of the muscle during contraction, forces can inviolate outer limit for arch size.



The effects of neutral zone confinement on the dentoalveolar structures can also play a critical role as a determinant of facial profile. A restrictive perioral musculature may prevent the dentoalveolar arches from expanding to a normal alignment with the skeletal base.



The commonpractice of centralization, or lingualization of occlusion, prevents the buccinator from performing its proper function in two ways. First, lingualization of occlusion creates a space between the cheek and the teeth and the external surface of the denture, where food tends to accumulate and it becomes more difficult for the cheek to place the food back onto the occlusal surfaces of the teeth.



Secondly, the space resulting from lingualization prevents the buccinator from neutralizing the lateral forces of the tongue during function.



MASSETER: The masseter muscle has no influence on the neutral zone. It only affects the distobuccal border of the denture.



MUSCLES OF THE LIPSOrbicularis oris : To a great extent forms the lips. In function, as in chewing, smiling and swallowing, it exerts force against the teeth and denture flanges, which is counteracted by the tongue.



Canine muscle : This together with other muscles, pulls the lower lip up and in sucking and swallowing helps to pull the lips forward, thus exerting forces on the teeth and labial denture flange.



The greater zygomatic muscle pulls the angle of the, mouth upward and backward.

The risorius muscle retracts the corner of the mouth.

The mentalis muscle turns the lower lip outward and in contracting makes the lower labial vestibule shallow.



The triangular muscle contracts during sucking to exert pressure on the teeth and the denture flanges.



The Modiolus : Because of the strength and variability of movement of the area, the modiolus is extremely important in relation to the stability of the lower denture. Unless the teeth and external surface of the denture are properly positioned and contoured by narrowing in the premolar area, the modiolus may constantly unseat the lower denture.



MUSCLES OF THE TONGUEThe tongue is composed of intrinsic muscles that lie within the tongue itself and extrinsic muscles that insert into the tongue.The function of the extrinsic muscles the styloglossus, palatoglossus, hyoglossus, and genioglossus is to move the tongue into various positions



. The tongue is capable of many varied shapes and positions during speech, mastication, and swallowing and in all of these functions is in constant contact with the lingual surface of the teeth, the lingual flange of the lower denture and the palatal surface of the upper denture.



Because of this contact, the tongue is a dominant factor in establishing the neutral zone and therefore in the stability or .lack of stability of the lower denture.



The common practice of lingualization is probably one of the greatest influencing factors in lower denture instability, because it violates the neutral zone and encroaches on the tongue space.



DENTURE SURFACES

The dental profession has always been concerned with equalizing the vertical forces that are delivered by the occlusal surfaces of the teeth and that are counteracted by the vault and the ridges. It has ignored the importance of the horizontal forces exerted on the polished or external surface of the denture



Thus the dental profession has been concerned mainly with two surfaces the occlusal and the impression surfaces.Sir Wilfred Fish in 1948 described a denture as having three surfaces, with each surface playing an independent and important role in the overall fit, stability and comfort of the denture.



The first surface, the impression surface, is that part of the denture in contact with the tissues and on which the denture rests and determines retention of the denture,The second surface, the occlusal surface is that area in contact with the teeth, either natural or artificial, of the opposite jaw.



The stability of the denture when the teeth are in contact is determined by the fit of the impression surface against the tissues and the fit of the occlusal surfaces against each other.



The third surface, the polished or external surface as termed by Fish, is all the rest of the denture that is not part of the other two surfaces. It is mostly denture base material, but it consists also of those surfaces of the teeth that are not contacting or articulating surfaces.



The buccal and lingual surfaces of the posterior teeth and the labial and lingual surfaces of the lower anterior teeth are not part of the occlusal surface but are part of the polished surface of the denture. The upper anterior teeth actually belong to two surfaces, both the occlusal and the polished surfaces.



When the teeth are in contact, the lingual surfaces of the upper anterior teeth are part of the occlusal surface. When the teeth are apart, as in speaking as at rest, these surfaces are part of the polished surface.



The external surface is in contact with, the cheeks, lips and tongue. One can visualize that, based on a square unit of area; the external surface is as large as or larger than the impression and occlusal surfaces combined, depending on the anatomic structures.



INFLUENCE OF FORCES ON DENTURE SURFACESThe more the ridge loss, the less the area of the denture base and the less the influence the impression surface area will have on the stability and retention of the denture.



As the surface area of the impression surface decreases and the external surface area increases, the development and contour of the external surface becomes more critical.



In other words, where more of the ridge has been lost, the more the denture stability and retention is dependent on the external surface than on the impression surface. Many unstable lower dentures are caused by the external surface not being properly formed and the teeth not positioned in the neutral zone.



The forces on the external surface are constantly changing in magnitude and direction during swallowing, speaking and mastication. It is only when the mouth is completely at rest, that the forces are constant.



If a person's teeth were in contact all the time, the external surface would be relatively unimportant in denture stability. Conversely if a person never brought his teeth into contact, the occlusal surface would be relatively unimportant and the stability would be dependent on, the forces on the external surface as transmitted to the impression surface.



The only time teeth are in contact is, during mastication and swallowing. This means that the patient will only make tooth contact during normal function. But the lips, cheeks and tongue are constantly in function. This stresses the significance of the horizontal forces exerted by the lips, cheeks and tongue.



In order to construct dentures that function properly not only in chewing but also in speaking and swallowing, we must develop the fit and contour of the external surface of denture just as accurately and meticulously as the fit and contour of the impression surface and the occlusal surface.



The lower posterior teeth are drastically affected by the position of the tongue. If the lower posterior teeth are lingualized excessively, normal tongue function will immediately unseat the denture. The tongue cannot and should not be restricted by the position of the posterior teeth.



MUSCLE INFLUENCE ON THE DEVELOPMENT OF THE DENTAL ARCHES Teeth erupt into the mouth under the influence of muscular environment. This environment which is created by the, forces between the tongue, cheeks and lips has a definite influence on the position of the erupting teeth, the resultant arch form and occlusion.



However, the muscular forces alone do not always determine the developing dental arch form. There is genetic factor which cannot be overlooked. This internal factor, along with the local environmental forces, combines their influences uniquely to determine the final arch form and tooth position.



It would stand to reason that when the teeth are erupting into the mouth during childhood and adolescence (2 - 14 year group) the muscular activity and habits that develop will continue through life.



Even after the teeth are lost, the forces created by these habits and actions still persist and will have a great influence on any complete or extensive partial removable prosthesis that is placed into the mouth.



It is therefore extremely important that the teeth be placed in that part of the mouth and with an arch form that falls within the area formed by muscular forces.



Our objective is to utilise the information on denture space and muscle function so as to, position the teeth and the external surfaces of the denture that the force the musculature exerts, instead of having a negative influence, will favourably affect the dentures and tend to seat or stabilize the dentures.



This can be accomplished through awareness of the neutral zone and by positioning the teeth and developing external surfaces of the denture so that all the forces exerted are neutralized and the denture is in a state of equilibrium.



DIRECTION OF FORCES

For the muscular forces to be of a stabilizing nature, the dentures must be so constructed that they will receive these forces at the proper angle. Dr. Fish (1948) described the cross section of stable dentures in the molar area to be triangular in shape, with the tooth being the apex and the denture periphery the base of a triangle.



A force exerted on an inclined plane may be broken down into two components. One component acts in the direction parallel to the inclined plane. The other component, called normal force, acts perpendicularly to the inclined plane. If the inclined planes of the external surface are properly fashioned and the forces are of equal magnitude, the resultant normal force will be in a seating direction



By the same token, if the dentures are triangular but not properly located within the neutral zone, the lateral force will be unequal and not provide the equilibrium necessary for a stable denture. This will result either in the dislodgement of the denture or unequal pressure on the ridge.



NEUTRALIZATION OF FORCESThe theory of the neutralization of forces that stabilize dentures and the rationale involved was one of the major contributions made by Dr.Russel Tench and his co worker, Dr.A.A.Cavalcarti.



lips, cheeks and tongue in the passive and functioning state exert forces on the natural teeth. In the natural dentition, arch integrity and tooth position are maintained when all the forces generated by the musculature are neutralized.



Any changes in the forces generated by the musculature because of increased size, altered muscle function, or abnormal habit patterns will upset the equilibrium and result in alteration of tooth position and arch form.



If we accept the assumption that the teeth are positioned and maintained in a neutral state by all the forces exerted against them by the musculature, it seems reasonable that when the dentures are made, the artificial teeth should be placed in the same relative position to the musculature as the natural teeth.



. The term "relative position" rather than exact position" is used because age, tonus, ridge resorption and other factors may modify or alter the denture space and neutral zone so that the artificial teeth, should not necessarily be in the exact same position as the natural teeth.



If the teeth are placed too far lingually in the molar region, they will encroach on the tongue space. Dr.Mayskens estimates that if the sizes of the mandibular teeth are too large or if the posterior teeth are set 1 mm lingually, the tongue is deprived of approximately 1000 cubic mm of functional space. This can force the tongue into an abnormal retracted position.



In summary, the neutral zone philosophy is based on the concept that for each individual patient there exists within the denture space a specific area where the function of the musculature will not unseat the denture, and at the same time where the forces generated by the tongue are neutralized by the forces generated by the lips and cheeks Furthermore, denture stability is as much or more influenced by tooth position and flange contour as by any other factors.



In other words, we should not be 'dogmatic and insist that the tooth should always be placed over the crest of the ridge, or lingual to the ridge or buccal to the ridge. Placement of the teeth should be detected by the musculature and will vary for different patients.



DETERMINATION OF THE NEUTRAL ZONEDIAGNOSIS AND TREATMENT PLANNING

Success in complete denture prosthetics is frequently dependent on what is done prior to the construction of the dentures as much as or more than on the skill and meticulous care utilized in the actual construction of dentures.



Examination, diagnosis, and treatment planning for complete dentures should be as meticulous and detailed as for any other branch of dentistry.After a proper examination and preparation of diagnostic casts we go in for the treatment proper.



Reversed Sequence in Denture Construction



First, the bases are very carefully adjusted in the mouth to be sure that they are not overextended and that they, are stable during mouth opening, swallowing and speaking.



Then modelling compound instead of wax, is used to fabricate occlusion rims. These rims which are moulded by muscle function, locate the patients neutral zone,.



After tentative vertical dimension and centric relation are `established, the final impressions are made with a closed mouth procedure. Only then, when the final impression is completed, are the vertical dimension and centric relation refined and finalized.



DIAGNOSING THE LOWER DENTURE PROBLEM The premise behind the rationale is that, in our thinking and procedures, we should separate the denture base from that which rests on the denture base - the body of the denture. With the neutral zone approach, the impression surface is called the base and the polished surface is called the 'body' of the denture.



Once the operator begins to think in terms of first creating a stable, base and then placing on that base, teeth and flange contours that will not unseat the denture base, the problem and its solution become apparent and comparatively simple to solve.



EFFECT OF VERTICAL AND HORIZONTAL FORCESwww.indiandentalacademy.com


The borders of the dentureThe body of the denture consists of the external or polished surface and the labial, buccal and lingual surfaces of the teeth

The occlusal surfaces and incisal edges

of the tooth on a denturewww.indiandentalacademy.com


IDENTIFYING CAUSES OF INSTABILITYwww.indiandentalacademy.com


First, an acrylic base is constructed,



If we now place on this stable base the body of the denture,



We now have an upper and lower baseand body. If we instruct the patient to bring the jaws together and the denture bases are dislodged, it can only be caused by the occlusion and the occlusion should be corrected.



CLINICAL AND LABORATORY PROCEDURES



PRIMARY IMPRESSIONS AND CONSTRUCTION OF ACRYLIC TRAYSwww.indiandentalacademy.com


SELECTION OF STOCK TRAYS www.indiandentalacademy.com


SHAPING THE TRAYSwww.indiandentalacademy.com


Fingers in crisscross position & close the lips around the fingers, seal off the air, & draw in as if sucking through a straw & then swallowwww.indiandentalacademy.com


The patient is instructed to seal off the air by closing the lips around the operators finger & to suck & swallow vigorously.www.indiandentalacademy.com


CONSTRUCTION OF THE ACRYLIC BASEwww.indiandentalacademy.com


CREATING STABLE BASES



Stability of base is tested by 2 methods1 base in patients mouth & operators fingers are placed on bicuspid area to firmly seat the base

If squishing sound is heard as the base is seated or eases up or pops upwww.indiandentalacademy.com


2 ask the patient to open wide, purse the lips as in sucking, wet the lips & speak normally if any unstability observed by the operator or the patient himself



Methods for locating areas of overextension of the base

visual observation or eyeballing use of disclosing material www.indiandentalacademy.com


visual observation or eyeballingwww.indiandentalacademy.com


use of disclosing materialwww.indiandentalacademy.com


Mandibular- 6 sections

1 labial or buccal open widely several times, then purse the lips as in sucking, & then swallow vigorously base is removed and examined. at least 1-2mm thickness of disclosing wax, smooth & rounded should be seen.www.indiandentalacademy.com


2 lingual

swallow vigorously several times, wet the lips with the tongue & and count from 1 to 10www.indiandentalacademy.com


Retromylohyoid fossa

lingual to retromolarpad and posterior to mylohyoid ridge-undercut areawww.indiandentalacademy.com


Maxillary- 4 sections (excluding posterior border)

frequently under extended area in maxillary base is around the tuberosity in the buccal space area. functional movements opening wide, pursing the lips as in sucking, bringing the upper lip down, swallowing & moving the upper jaw from side to side (thickness of flange over tuberosity to be sure that the coronoid process of the mandible doesnot dislodge the upperbase. www.indiandentalacademy.com


Locating the posterior borderPosterior border should terminate at the flexion line between the movable & nonmovable tissue of the palate.

vibrating line/blowdown line 3 marks should be made after drying one on each hamular notch & third in the midline at flexion line. www.indiandentalacademy.com


LOCATING THE NEUTRAL ZONE AND PLANE OF OCCLUSION

Materials:Locating the neutral zone for the lower denture is one of the most important factors in achieving stability of the lower denture. To locate the neutral zone and form the body of the denture, if is necessary to use materials that can be moulded by the horizontal forces of the tongue, lips and cheeks.



Buchman, Gelb, Lott and Levin, and Russell have described the use of waxes in locating the neutral zone.Kline of France has advocated the use of self curing acrylic to form a 'piezograph', which is his term for the registration of the neutral zone.Health of England has, for experimental purposes used a gel that is a polymer of dimethyl silocane filled with 17% calcium silicate.



Tench has suggested the use of modelling compound, which has worked well. Korr's low fusing gray and green compound is best suited for this purpose. It permits the patient to mold the compound into the neutral zone with the least amount of time and effort.



LOCATING THE NEUTRAL ZONE FOR THE LOWER ARCH www.indiandentalacademy.com


ESTABLISHING THE 0CCLUSAL PLANEIn the natural dentition, the lower occlusal plane runs from the incisal edges of the lower anterior teeth through the tips of the cusps of the posterior teeth to a point approximately two thirds of the height of the retromolar pad



To locate the occlusal plane, place the rim back into the mouth and use a sharp pointed pencil to mark the commisures of the lip and the height of the lower lip at rest. These three points are connected by a line that is continued on each side to a point one half to two thirds the height of the retromolar pad. The excess compound is trimmed to this line.



To further check the correctness of the height of the occlusal plane, observe its relationship to the lateral borders of the tongue. With the tongue at rest, the height of the occlusal plane should be 12 mm below the greatest convexity of the lateral borders of the tongue. If necessary, the compound rim can be, modified by reduction or addition.



TESTING THE STABILITY OF THE LOWER OCCLUSION RIM

The lower occlusion rims is placed back into the patients mouth and checked for stability by having the patient open wide, wet the lips with the tongue, count from 1 to 100, and say exaggerated "oh" "ahs" and "ees".



The next procedure is to test the outer edge of the rim with the tip of the index finger in the bicuspid and incisor regions. If pressure on the outer edges causes the opposite side of the rim to lift up, then the rim must be narrowed from the labial or buccal to where the vertical pressure will not cause the rim to tilt.



This will occur where there has been extensive ridge resorption and where the residual ridge is narrow buccolingually and labio-lingually. If this is not corrected and the teeth placed at this position, then the vertical forces as in mastication will tilt the denture.



The final test is to have the patient speak, swallow, wet the lips and open wide without the rim moving or being dislodged. We have therefore created a tray or base that is not dislodged by muscle function and have placed on it a body that is also not displaced by muscle function.



LOCATING THE NEUTRAL ZONE FOR THE UPPER ARCH

The tray is firmly seated, and with the operator's fingers out of the mouth, the patient is instructed to suck and swallow.



A line is scribed about 2 mm below the upper lip at rest and the compound is trimmed to this line. The excess in the posterior is cut away parallel to the ridge. The upper rim is now tentatively completed and will be further modified when determining vertical dimension and registering centric relation.



VERTICAL DIMENSION:www.indiandentalacademy.com


FINAL IMPRESSIONS

The advantage with the use of a closed mouth technique are:

A more accurate functional molding of the borders can be obtained, especially in the lower arch.

By having the patient to close gently and swallow, there is more even distribution of pressure and impression material with less likelihood of excessive pressure in one area or another.www.indiandentalacademy.com


TECHNIQUE :With the procedure to be described, two impression pastes of contrasting colors are used. This is called a colorcoded impression procedure, the purpose of which is to locate areas of tissue displacement. The impression trays and the two zinc oxide eugenol paste should be of three different colors.



LOWER SECONDARY IMPRESSION:The material is mixed and placed evenly over the lower tray and the tray is placed carefully in the mouth. Slight pressure is applied with the forefingers in the bicuspid area until the paste is seen to exude from the tray around the peripheries.



The upper rim, which has been lubricated, is placed into the mouth, and the patient is guided into a hinge closure and instructed not to exert any pressure after light contact is made. After the initial contact, the patient is directed to swallow and remain closed. In about 30 seconds, the patient is asked to open and wet the corners of the mouth with the tongue, purse the lips, such in as drawing through a straw and then close and swallow again. At no time should excessive pressure be applied.



When material has set, the lower rim and tray are removed from the mouth and inspected. If there are areas of the tray showing through the impression material, it indicates areas of excessive pressure that will cause tissue displacement. These areas are relieved to a minimum depth of 0.5 mm.



The impression material covering the peripheris is cut away with a sharp knife to prevent excessive pressure that will cause tissue displacement. These areas are relieved to a minimum depth of 0.5 mm. The impression material covering the peripheris is cut away with a sharp knife to prevent excessive build up of the borders, which would result in overextension when the correctiveimpression is made.



The tray is now ready for a second impression. The material of choice is Krex, which is white soft thin, free flowing and of a contrasting color to the zinc oxide eugenol used. The Krex is mixed evenly and spread over the entire tray and borders and the procedure as mentioned before is repeated. After it is set the tray is removed and the impression is checked for defects.



Upper impression: Prior to the making of the upper impression, several holes are drilled in the ruage area. The zinc oxide impression paste is mixed and placed over the tray and borders. The tray is carried into the mouth and centered over the ridges.



The anterior part of the tray is seated with a light pressure to position it properly. With the fore finger applying pressure in the molar area, the posterior part is seated until the material starts to flow out from the posterior border.



The completed lower impression is inserted into the mouth and the patient is guided into a hinge closure, avoiding excessive pressure. The patient is instructed to swallow and remain closed. In 30 seconds the patient is directed to open the mouth, move the jaw from side to side, purse the lips as in sucking, bring theupper lip down hard, and then swallows and close without pressure



. When the material has set the tray is removed and the impression is inspected for pressure areas.If pressure areas are present, Krex is used for corrective impression and the above procedure is carried out to get a corrected impression.



Centric relationThe centric relation is recorded with the same completed final impression trays and compound rims.After the vertical relation is rechecked and corrected for any changes occuring after the final impression making the centric relation is recorded by the check bite procedure or the nick and notch method.



After the centric relation is recorded a facebow recording is made. After this the facebow assemblage, upper and lower final impressions, and occlusion rims are now ready to be sent to the laboratory.



LABORATORY PROCEDURESwww.indiandentalacademy.com


FABRICATION OF TONGUE, LIP AND CHEEK MATRICES



SELECTION AND ARRANGEMENT OF ANTERIOR TEETHThe neutral zone developed by each individual patient is usually not a narrow restricted area and therefore permits some latitude for positioning of the anterior teeth to obtain adequate lip support for optimum facial appearance.



This is especially true, with the upper neutral zone, since it is not nearly as critical for denture stability as is the lower neutral zone.After the shade, shape, size and tooth material are selected the arrangement of the anterior teeth are done.



SELECTION AND POSITIONING OF POSTERIOR TEETH

With the neutral zone concept, either anatomic or non anatomic forms can be used. The neutral zone only indicates the labiolingual or buccolingual position of the teeth. The occlusal forms of the teeth to be used are essentially the operator's choice.



After selection of the proper size, occlusal morphology and material of the posterior teeth to be used, we go in for the positioning or arrangement of teeth. The following is a step by step sequence for arrangement of anterior and posterior teethThe lower anterior teeth are set to the height of the labial matrix and to the labial limit of the neutral zone.



The upper anterior teeth are set against the labial limits of the upper matrix.The lower posterior teeth are set against the tongue matrix and against the template occlusally.The upper posterior teeth are set, to the buccal limits of the neutral zone.



The matrices are removed, and the upper bow of the articulator is closed in order to evaluate the relationship of the upper and lower posterior teeth.The upper posterior teeth will have to be rearranged to assure maximum contact with the lower posterior teeth.



The upper and lower posterior teeth are checked for the buccal and lingual relationship to each other.In order to avoid an edge to edge relationship which might lead to check biting, the lower posterior teeth may be moved buccally within the neutral zone, resulting in a crossbite relationship.



THE TRIAL DENTURE

The purpose of the trial denture is to check the following.Stability and retention of the bases Vertical dimensionPhoneticsCentric relation Esthetics



PREPARATION OF THE TRIAL DENTURE FOR INVESTINGThe laboratory procedures for investing, packing and processing of dentures when using the neutral zone technique is generally the same as for conventional dentures. However, because of the materials used for the external impressions, it is necessary to be especially careful in some of the procedures.



Great care must be taken to be sure that none of the external impression material has flowed under the bases of the trial denture. When zinc oxide eugenol paste has been used for the external impression, the flasks should not be allowed to remain in the boilout tank for more than 5 minutes, because the zinc oxide eugenol paste will liquefy and attack the stone, resulting in a bleached appearance to the processed acrylic.



After processing the dentures are remount on the articulator. Occlusal discrepancies are checked for with the template and carbon paper. They are corrected, the dentures are finished, polished and insertion is done.



A clinical remounting is done and the dentures are inserted and checked for any discrepancies. The patient is given post insertion instructions which are similar to that of conventional complete dentures.



CONCLUSION

In summary, the neutral zone philosophy is based on the concept that for each individual patient there exists within, the denture space, a specific area where the function of the musculature will not unseat the denture, and at the same time where the forces generated by the tongue are neutralized by the forces generated by the lips arid cheeks.



Furthermore, denture stability is as much or more influenced by tooth position and flange contour as to any other factor.In other words, we should not be dogmatic and insist that the teeth should always be placed over the crest of the ridge,or lingual to the ridge or buccal to the ridge! Placement of the teeth should be dictated by the musculature and will vary for different patients.



REFERENCES

Beresin VE, Schiesser FJ. The neutral zone in complete dentures. J Prosthet Dent. 1976 Oct;36(4):356-67.

Neutral zone approach for denture fabrication for a partial glossectomy patient: a clinical report. : J Prosthet Dent. 2000 Oct;84(4):390-3

Bocage M, Lehrhaupt J. Lingual flange design in complete dentures. : J Prosthet Dent. 1977 May;37(5):499-506



Walsh JF, Walsh T. Muscle-formed complete mandibular dentures. : J Prosthet Dent. 1976 Mar;35(3):254-8.Orstavik JS, Floystrand F. Retention of complete maxillary dentures related to soft tissue function . Acta Odontol Scand. 1984 Oct;42(5):313-20.Niedermeier W, Hofmann M. The effect of the arrangement of the artificial sets of teeth on the physical stability of total protheses . Dtsch Zahnarztl Z. 1979 Aug;34(8):616-8.



Demirel F, Oktemer M. The relations between alveolar ridge and the teeth located in neutral zone. : J Marmara Univ Dent Fac. 1996 Sep;2(2-3):562-6.Suenaga K, Sato T, Nishigawa G, Minagi S. Relationship between size of denture foundation area and resorption of alveolar ridge in the edentulous mandible. : J Oral Rehabil. 1997 Apr;24(4):315-9.



Khamis M, Razek A, Abdalla F. Two-dimensional study of the neutral zone at different occlusal vertical heights. J Prosthet Dent. 1981 Nov;46(5):484-9.Alfano SG, Leupold RJ. Using the neutral zone to obtain maxillomandibular relationship records for complete denture patients. : J Prosthet Dent. 2001 Jun;85(6):621-3.



REVIEW OF LITERATURE Beresin VE, Schiesser FJ. In 1976 concluded that the neutral-zone philosophy is based upon the concept that for each individual patient there exists within the denture space a specific area where the function of the musculature will not unseat the denture and where forces generated by the tongue are neutralized by the forces generated by the lips and cheeks.



Walsh JF, Walsh T. in 1976 presented a method whereby a patient's musculature is used to indicate the position of the teeth and to develop the shape and thickness of the denture base. Of 30 patients tested, 28 experienced improved stability of their lower dentures.



Bocage M, Lehrhaupt J. in 1977 concluded that the lingual design advocated for complete lower dentures involves no changes in current concepts regarding minimum-pressure, functional impression techniques. The sublingual horizontal extension suggested is placed in a biologically acceptable fashion by increasing the area of the denture, which enhances retention and stability.



Niedermeier W, Hofmann M. in 1979 demonstrated the effect of masticatory forces within and outside the stable stress-bearing areas of the complete upper denture on the pressure gradient and the flow of saliva into the space under the prosthesis and showed that masticatory stress within the stable area led to a



continual increase in pressure in the space under the prosthesis. Masticatory force outside the stable area led to a decrease in pressure and an equalization of pressure in this space as well as dislocation and tilting of the complete denture. Due to masticatory force outside the stable area, the shape of the ridge influences the stability of the complete denture.



Khamis M, Razek A, Abdalla F. in 1981 developed a new technique to locate the neutral zone. The neutral zone was studied on two groups of patients with prominent or flat residual alveolar ridges. The neutral zone was determined for each patient at three vertical heights. It was studied both vertically and horizontally



The width of the neutral zone was measured at different regions with these conclusions: 1. The width of the neutral zone is minimum at the level of the occlusal plane and increases gradually as it goes up and down. 2. The width of the neutral zone is also minimum at the posterior (molar) region and increases gradually toward the anterior.



3. There is no significant difference in the width of the neutral zone in patients with prominent or flat alveolar ridges. 4. The width of the neutral zone increases as the vertical dimension of occlusion increases and decreases as the vertical dimension of occlusion decreases.



Orstavik JS, Floystrand F. in 1984 investigated the influence of free tongue, lip, and cheek function on the retention of complete maxillary dentures. Test dentures were designed with full palatal coverage and functionally determined filling in of the vestibular sulcus.



The front teeth were arranged primarily to meet cosmetic demands--that is, anterior to the top of the residual alveolar ridge. Retention was measured as resistance to dislodgement-provoking loads applied vertically to the incisive edge of the central incisors, using a miniature bite force recorder.



All the participants were able to load their front teeth with 35 N or more without loss of retention. None of them experienced denture dislodgement provided the tongue, lips, and cheeks wee allowed to act freely. If the peripheral soft tissues were separated from the vestibular denture flange, no obvious effect on denture retention could be detected. Physically preventing the tongue from pressing against the posterior part of the denture reduced the retention significantly.



Measurements of anterior loads tolerated after stepwise reductions of the denture extension indicated that the tongue acted primarily by pressure against the tuber regions. Tongue pressure against the central parts of the palate and lip or cheek pressure against the vestibular flange seemed to be of less importance.



Demirel F, Oktemer M. in 1996 defined the neutral zone as the area where the forces of the tongue pressing outward are neutralized by the forces of the cheeks and lips pressing inward. In this study records of neutral zones of 30 edentulous patients were used to establish the relation between teeth arrangement according to neutral zone principles and teeth arrangement according to crest of ridge.



These 30 patients were classified according to their ages, edentulous periods and denture experience. The results have shown that the lower molars were positioned a little bit closer to lingual with respect to crest of ridge; premolars were positioned either close to crest of ridge or they were coincided on it. In the anterior zone, the teeth were in accordance with known principles of positioned of anatomic landmarks.



Suenaga K, Sato T, Nishigawa G, Minagi S. in 1997 investigated the relationship between the size of the denture foundation area and the resorption of the alveolar ridge. The denture foundation area was recorded using a modelling compound impression technique with border moulding.



Both sides of each edentulous mandible were examined separately, making a total of 110 experimental sides in the study. On a stone cast made from each impression, the size of of the superficial denture foundation area and of the projected denture foundation area on the tentative plane of occlusion of each anatomical zone were measured.



The vertical height of the alveolar ridge at the lateral incisor and first molar region was also evaluated. The size of the superficial denture foundation area in the antero-lingual and postero-lingual zones showed no significant correlation with the degree of alveolar ridge resorption. The size of the projected denture foundation area on the tentative plane of occlusion in the anterior section showed negative significant correlation with the degree of alveolar ridge resorption.



Alfano SG, Leupold RJ. In 2001 presented a technique for obtaining maxillomandibular registration for complete denture patients.. The maxillary rim is formed with the use of conventional techniques. The mandibular rim is made from modeling plastic impression compound on a record base formed by the patient into the neutral zone.



The mandibular rim then is reheated, and the patient determines the occlusal vertical dimension by swallowing. An imprint of the maxillary rim is made on the mandibular rim at the occlusal vertical dimension. The posterior extent of the mandibular rim is relieved 1 mm. Orientation notches are placed in both rims, and centric relation is recorded with a fast-setting vinyl polysiloxane material.



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