Neutrocentric Concept (prosthodontics)

NEUTROCENTRIC CONCEPT

BY

Ur frnd sam

CONTENTS• Introduction• Terminology• Factors governing the position of teeth for complete dentures• Types of posterior teeth• History of the evolution of the posterior tooth form• Arranging the artificial teeth• Factors that affect occlusal balance• Types of complete denture occlusions• Neutrocentric concept• Indications of the monoplane occlusion (neutrocentric concept)• Merits and demerits• Summary

• References

• The arrangement of teeth must be Physiologically And esthetically acceptable. Physiologically , They must be in a position compatible with the lips , Tongue, and cheeks whether the mandible is in a Relaxed position or in motion.

TERMINOLOGY

• Occlusion: The static relationship between the incising

and masticatory surfaces of the maxillary or mandibular teeth or tooth analogues (GPT-8)

• Centric occlusion: The occlusion of opposing teeth when the

mandible is in centric relation. This may or may not coincide with maximum intercuspation. (GPT-8)

Maximum intercuspation:

The complete intercuspation of the opposing teeth, independent of condylar position (GPT-8)

Centric relation: The maxillomandibular relationship in which The maxillomandibular relationship in which

the condyles articulate with the thinnest the condyles articulate with the thinnest avascular portion of their respective disks avascular portion of their respective disks with the complex in the anterior-superior with the complex in the anterior-superior position against the shapes of the articular position against the shapes of the articular eminencies. eminencies.

• This position is independent of tooth This position is independent of tooth contact. contact.

• This position is clinically discernible when This position is clinically discernible when the mandible is directed superiorly and the mandible is directed superiorly and anteriorly.anteriorly.

• It is restricted to a purely rotary It is restricted to a purely rotary movement about the transverse movement about the transverse horizontal axis horizontal axis (GPT-8)

Factors governing the position of

teeth for complete dentures

• I. The horizontal relations to the residual

ridges,

• II. The vertical positions of the occlusal

surfaces and incisal edges between the

residual ridges,

• III. The aesthetic requirements, and

• IV. The inclinations for occlusion.

Requirements of complete denture occlusion

• Stability of occlusion at centric relation and in an area forward and lateral to it

• Balanced occlusal contacts bilaterally

• Control of horizontal force by buccolingual cusp reduction according to residual ridge resistance form and interarch distance

• Functional lever balance by favorable tooth-to-crest ridge position

JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture Prosthodontics

• Cutting, penetrating, and shearing efficiency of occlusal surfaces (proper sluice-way of occlusal surfaces)

• Anterior incisal clearance during all posterior masticatory movement

JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture Prosthodontics

Requirements of complete denture

occlusion

Incising units

Working occlusal

units

Balancing occlusal

units

Requirements of Incising units

These units:• Should be sharp in order to cut efficiently• Should not contact during mastication• Should have as flat an incisal guidance as

possible considering esthetics and phonetics• Should have horizontal overlap to allow for

base settling without interference• Should contact only during protrusive incising

function JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture

Prosthodontics

Requirements of Working occlusal units

• Should be efficient in cutting and grinding• Should have less bucco-lingual width – to

minimize the work load• Should function as a group with simultaneous

harmonious contacts at end of the chewing cycle • Should be over the ridge crest in the masticating

area for lever balance• Should have a surface to receive & transmit the

force vertically• Should center the work load near the

anteroposterior center of the denture JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture

Prosthodontics

Requirements of Balancing occlusal units

• Should contact on the second molars when the incising units contact in function

• Should contact at end of the chewing cycle when the working units contact

• Should have smooth gliding contacts for lateral and protrusive excursions

JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture Prosthodontics

Types of posterior teeth

• 1. An anatomic tooth is one that is designed to simulate the natural tooth form. The standard anatomic tooth has inclines of approximately 33 degree or more.

• 2. when the cusp incline is less steep than the conventional anatomic tooth of 33 degree it can be classified as a modified or semianatomic tooth. It can be considered

basically anatomic and will articulate in three dimensions.

• 3. A nonanatomic tooth is essentially flat and has no cusp heights to interdigitate with an opposing tooth and has sulci to enhance its comminuting effect on food. They articulate in only two dimensions.

History of the evolution of the posterior tooth form

• 1913

– Dr. Alfred Gysi of Switzerland

- First anatomic teeth – “True-bite”

- He also referred as normal-bite teeth - He carved the teeth to harmonize with a condylar inclination of 33 degree

• 1928 - Sears – “Channel teeth” - First non-anatomic teeth - Aim: To minimize the shifting of the denture

base & hence guard against trauma & atrophy - Maxillary teeth: Buccal & lingual plane with

deep channel run mesiodistally - Mandibular teeth: Buccolingually <1/2 of

anatomic teeth – a single central ridge that ran uninterrupted the entire length of occlusal table

• 1929

- Hall - “Inverted cusp teeth”

- He claimed that it eliminated the problem of denture instability by eliminating the cusp of teeth

- The teeth was flat with concentric cone-shaped depression on the occlusal surface liked inverted cusp

• 1930: - Avery Brothers: “Scissors bite” - The posterior occusal surfaces were locked

anteroposteriorly by grinding steps on the surface of the teeth, with angle determined by the inclination of the condylar path.

- The occlusion of these teeth scissors together & was meant to food in lateral excursions

Myerson : “True-cusp teeth”

- It was flat on the occlusal surface, with cutting blades & crushing tables incorporated in occlusal carving

- They were anatomic width & breadth

- It had a series of transverse buccal-lingual ridges with sluiceways between them

• 1934 - Nelson – “Chopping teeth” - Flat occlusal surfaces with numerous

ridges (mandibular teeth: runs transversely, maxillary teeth: runs mesiodistally)

- As they are perpendicular to one another in contact, an efficient shredding & cutting action

• 1935: French

- Maxillary teeth: Similar to Sears’s (central groove running mesiodistally) but very shallow buccolingual inclines

- Mandibular teeth: Narrower than conventional one but wider than Sears channel teeth and have the food table.(The buccal cusp is eliminated for better stability of denture base)

• 1936:

- McGrane: “Curved cusp teeth”

- These teeth constructed with curved grooves & curved cusps forming segments of an arc, concentrically arranged from a radial centre corresponding to the right & left rotation centers in the head of condyle in the glenoid fossae

• 1939

- Swenson: “Non-lock teeth" (as anteroposteriorly there was no inclination, so mandible can move forward freely)

- They were essentially flat teeth with sluiceways for shredding & allowing the food to clear the occlusal table

- They provide balancing contact, as a modest buccal & lingual incline was provided

• 1946:

• Hardy; “VO” (Vitallium occlusal)

• These were resin block of 3 posterior teeth simulating a buccal facade of 2 bicuspid & 1 molar

• A narrow zigzag of Vitallium ribbon was embedded on the occlusal surface.

• 1952:• Cook’s metal teeth • The 2nd PM & 1st M were flat stainless steel

castings with holes on the occlusal surfaces

• These teeth occluded with flat upper porcelain teeth to push the food through the holes to break it into small particles (grinder-like action)

• 1954:

• DeVan; “Neutrocentric concepts”

• Flats plane in all direction with no inclination

• 1957:

• Bader; “Cutter-bar” teeth

• In this the opposing upper porcelain cuspless teeth with a metal cutting bar replacing the 2nd PM, 1st & 2nd M

• 1961:

• Sosin; “Cross blade” teeth

• He replaced maxillary 2nd bicuspid, 1st & 2nd molars with cleat-shaped vitallium forms

• Levin : modified this scheme by reducing the size of the cross-blade to the maxillary lingual cusp

• 1967:

• Frush; “Linear occlusal concept”

• Maxillary & mandibular posteriors were flat with a single mesiodistal ridge usually on the lower

Arranging the artificial teeth

• The anterior teeth are arranged in basic

positions, principally for esthetics. The

posterior teeth are arranged in maximum

planned intercuspation (cusp form) or

occlusal contact (noncusp form) on a plane.

For developing the anterior plane of

occlusion, the plane should be located either at or slightly below the corners of the mouth.

The position of the incisal edges of the mandibular anterior teeth eventually establishes the level of the anterior plane of occlusion.

• The posterior plane of occlusion is an extension of this anterior plane level with the junction between the middle and upper third of the retromolar pads bilaterally.

• These references will place the overall plane at a level that is familiar to the tongue (2-3 mm below the dorsum of the tongue). If the plane located higher or lower, will interfere with normal tongue action.

• Maxillary posterior teeth are arranged in their correct Buccolingual position, a straightedge is placed over the crest of the mandibular ridge and a line is scratched in the wax , the lingual cusps of the maxillary teeth are placed over this line.

• The mandibular teeth then assume their buccolingual and mesiodistal position by intercupating with maxillary teeth.

• The posterior teeth are positioned in such a way that they are properly related to the bone that supports them and to the soft tissues that contact their facial and lingual surfaces. The posterior form of the arch will be determined largely by the “neutral zone” between the cheeks and tongue.

• The basic principle for the buccolingual

positioning of posterior teeth is that they

should be positioned over the residual ridge.

The tip of the canine to the middle of the

retromolar pad should provide guides for

this arrangement.

Factors that affect occlusalbalance

• 1. Condylar guidance

• 2. Incisal guidance

• 3. The plane of occlusion is established by the height of the lower cuspid and in the posterior by the height of the retromolar pad. It is also related to the ala-tragus line, or camper’s line.

• 4. The compensating curve

• 5. Cusps on teeth or the inclination of cuspless teeth

The inclination of the condylar guidance

• The sagittal condylar guidance angle (SCGA)

- The angle formed by the inclination of a condylar guide control surface of an articulator and a specific reference plane

• Not under the control of the dentist and is determined purely by the biomechanics of the joint itself.

• This is the net result of

the condyle-disc assembly

passing forwards and

downwards, under the

influence of the anterior

slope of the glenoid fossa.

Inclination of the incisal guidance

• The second end-controlling factor

Incisal guidance The influence of the contacting surfaces of the

mandibular and maxillary anterior teeth on the mandibular movements. (GPT-8)

Or

The influences by the amount of vertical and horizontal overlap of the maxillary and mandibular teeth. (GPT-8)

Incisal Guidance Angle

• The angle formed by the intersection of the plane of occlusion and a line within the sagittal plane determined by the incisal edges of the maxillary and mandibular central incisors when the teeth are in maximum intercuspation (GPT-8)

The plane of occlusion

• The plane of occlusion is established by the height of the lower cuspid and in the posterior by the height of the retromolar pad. It is also related to the ala-tragus line, or camper’s line.

compensating curve

• The anteroposterior curving (in the medial plane) and the mediolateral curving (in the frontal plane) within the alignment of the occluding surfaces and incisal edges of the artificial teeth that is used to develop the balanced occlusion(GPT-8)

TYPES OF COMPENSATING CURVES

• ANTERIOPOSTERIOR:

CURVE OF SPEE

• MEDIOLATERAL:

MONSON’S CURVE

WILSON’S CURVE

CURVE OF SPEEThe anatomic curve established

by the occlusal alignment of the teeth, as projected onto the median plane, beginning with the cusp tip of the mandibular canine and following the buccal cusp tips of the premolar and molar teeth, continuing through the anterior border of mandibular ramus, ending with the anterior most portion of the mandibular condyle.(GPT8)

WILSON’S CURVE

• In the theory the occlusion should be spherical, the curvature of the cusps as projected on the frontal plane expressed in both arches; the curve in the lower arch being concave & one in the upper arch being convex. The curvature in the lower arch is affected by an equal lingua inclination of the right & left molars so that the tips point of the corresponding cross-aligned cusps can be placed into the circumference of the circle. The transverse cuspal curvature of the upper teeth is affected by the equal buccal inclinations of their long axes (GPT-8)

MONSON’S CURVE

• For a proposed ideal curve of occlusion in which each cusp & incisal edge touches or conforms to a segment of a surface of a sphere 8 inches in diameter with its centre in the region of the glabella. (GPT-8)

• Sphere of Monson : Two curves of Spee and Wilson are studies in 3D the cusps tips of posterior teeth resting on a sphere.

CUSPS ON TEETH OR THE INCLINATION OF CUSPLESS

TEETHThese modify the effect of the plane of occlusion

and the compensating curve.

It was shown that the mesiodistal cusp heights that interdigitate lock the occlusion so that reposition of the teeth due to setting of the base cannot take place.

To prevent this all mesiodistal cusp heights be eliminated in anatomic type teeth and only buccolingual inclines need be considered as determinants of balanced occlusion.

• Anatomical teeth usually are selected for balanced occlusion

• Nonanatomical or cuspless teeth are generally the choice for monoplane occlusion, although teeth with cusps also can be used.

• For the lingualized occlusal concept, a combination of upper anatomical and lower nonanatomical teeth has been introduced.

Neutrocentric concept

• In 1954, De Van formalized guidelines for using flat teeth in his “neutrocentric concept” .

• He stated that occlusal surfaces should have

1.Flat planes in all directions with no inclination at all in respect to the underlying denture foundation.

• 2.Balance was considered unnecessary and undesirable, as the resulting inclines would create instability of the dentures.

• This concept was carried out by limiting the mesiodistal extent of the occlusal table to avoid setting the teeth over the lower molar slope inherent in the posterior portion of the residual ridge.

Indications of the monoplaneocclusion (neutrocentric concept)

• 1) Flat ridge(s)• 2) Class II jaw relations• 3) Class III jaw relations• 4) Maxillofacial patients• 5) Handicapped patients• 6) Cross bite• 7) Doubtful or Without any

perfect centric relation records

• The primary objective was to prevent the destruction of tissue and preserve the integrity of the residual ridge.

• Many dentists believe that use of monoplane occlusion , which is flat mesiodistally and buccolingually and is oriented as close as possible parallel to the upper and lower mean foundation planes, will create more stable dentures

• This term is used to denote a concept of occlusion that eliminates and anteroposterior or mediolateral inclines of the teeth and directs the forces of occlusion to the posterior teeth.

• The plane of occlusion should not be dictated by the horizontal condylar guidance and must be flat and the form of the posterior teeth is devoid of cusp.

• The horizontal and lateral condylar guidance may be set at zero.

• The direct force toward the center of the support and to reduce the frictional forces

• The buccolingual width of the teeth is reduced

• The number of teeth is reduced to direct the forces in the molar and bicuspid area of support

• To refrain from placing a tooth on the ridge incline in the second molar area

Merits

• A freedom of occlusion from centric to eccentric position.

• The elimination of inclined plane forces which create horizontal displacement of the bases during function.

• The lack of interference when the dentures settle.

Demerits

• Nonanatomic teeth occlude in only two dimensions (length and width), but mandible has an accurate three-dimentional movement due to its condylar behavior.

• The vertical component present in mastication and nonfunctional movements is not provided for, so that this form loses shearing efficiency.

• Bilateral and protrusive balance is not possible with a purely flat occlusion.

• The flat teeth do not function efficiently unless the occlusion surface provides cutting ridges and generous spillways (sluiceways).

• They cannot be corrected by much occlusal grinding without impairing their efficiency.

• Teeth look dull and unnatural which may lead to psychological problems.

• With this concept of occlusion, there is no attempt to eliminate deflective occlusal contacts in lateral or protrusive excursions.

• When the nonanatomical teeth are arranged to satisfy the monoplane occlusal concept, the condylar inclinations on the articular are set at 0 degrees. When the positioning of the maxillary posterior teeth completed, the occlusal surfaces of them should be flat against the mandibular wax occlusal rim.

Summary

Many factors enter into the arrangement of the Artificial teeth in a denture. They act as a unit; And attached to a movable base resting on movable and displaceable living tissue, which are subject to damage; therefore, they must be Arranged to function as a unit.

• However, Leverages, forces, vector of forces, discrepancies in residual ridges, maxillomandibular Relationships, residual ridge relationships, Functional and parafunctional mandibular Movements, and preferences of patients vary. The arrangement of teeth must be Physiologically And esthetically acceptable. Physiologically , They must be in a position compatible with the lips Tongue , and cheeks whether the mandible is in a Relaxed position or in motion

References • ESSENTIALS OF COMPLETE DENTURE

PROSTHODOTICS , SECOND EDITION,

by Sheldon Winkler

• BOUCHER’S PROSTHODONTIC TREATMENT FOR EDENTULOUS PATIENTS, ELEVENTH EDITION, by Hickey , Zarb and Bolender

• SYLLABUS OF COMPLETE DENTURES, by Charles M.Heartwell and Arthur O.Rahn

• JPD; 1971,Vol:25, The role of occlusion in preservation & prevention in complete denture Prosthodontics

• JPD; 1955,Vol:5,cuspless centralised occlusal pattern.

• Comparison of Zero Degree Teeth and anatomic Teeth on Complete Dentures, J. PROSTH. DENT. 17: 28-35, 1967.

• Physiologic and Neurologic Occlusion, J. PROS. DEN. 3:632-632. 1953

• J Am Dent Assoc. 1954 Feb;48(2):165-9. The concept of neutrocentric occlusion as related to denture stabilityDEVAN MM.

•