OCCLUSION IN RELATION TO REMOVABLE PARTIAL DENTURE.

Dentists are confronted with a rather vexing problem in restoring the dentition

of a partially edentulous patient with removable partial dentures. The problem

centers mainly around the differences in characteristics of the supporting

structures of the restoration—the relatively firmly attached natural teeth on the

one hand, and the displaceable soft tissues of the residual ridges on the other

hand. Additionally, occlusal and incisal relationships for removable partial

dentures involve both the natural and artificial teeth. Factors related to both of

these conditions must be observed and correlated in creating a harmonious

occlusion with removable partial dentures. All of us recognize that the vexing

problems of occlusal rehabilitation, including maintenance, are somewhat

reduced when treating patients with fully tooth-borne removable restorations as

opposed to the distal extension type of removable partial denture.

The fourth phase in the treatment of the patient with removable partial

dentures is the establishment of a functional and harmonious occlusion.

Balanced occlusion is desirable in complete denture but in removable partial

dentures, because of the attachments of the partial denture to the abutment teeth,

occlusal stresses can be transmitted directly to the abutment teeth and other

supporting structures, resulting in the sustained stresses that may be more

damaging than those of transient stresses found in complete dentures.

Failure to provide and maintain adequate occlusion on the partial denture

is primarily a result of

1. Lack of support for the denture base.

2. The fallacy of establishing occlusion to a single static jaw relationship record.

3. An unacceptable occlusal plane.

Occlusal form of teeth on partial denture must be made to conform to an

already established occlusal pattern (Only exception being opposing complete

denture and only when anterior teeth remain in both the arches).

The establishment of a satisfactory occlusion for partial denture patient includes

the following:

1. An analysis of existing occlusion

2. The correction of existing occlusal disharmony.

3. Recording of CR or an adjusted CO.

4. The recording of eccentric jaw relations or functional eccentric occlusion.

5. The correction of occlusal discrepancies created by the fit of the framework

and in processing of the denture.

DESIRABLE OCCLUSAL CONTACT RELATIONSHIPS FOR R.P.D.

1. Simultaneous bilateral contacts of opposing posterior teeth must occur in CO.

2. Occlusion for tooth-supported partial dentures may be arranged similar to the

occlusion seen in a harmonious natural dentition.

3. Balanced occlusion in the eccentric positions should be formulated when the

partial denture is opposed by a maxillary complete denture.

4. Working side contact should be obtained for mandibular distal extension

denture. These contacts should occur simultaneously with the working side

contacts of the natural teeth to distribute the stress over the greatest possible

area.

5. Simultaneous working and balancing contact should be formulated for the

maxillary bilateral distal extension partial denture whenever possible.

6. Only working contacts need to be formulated for either the maxillary or

mandibular unilateral distal extension partial denture. Balancing side contact

would not enhance the stability of the denture because it is entirely tooth

supported by the framework on the balancing side.

7. In class IV RPD, contact of opposing anterior teeth in the planned intercuspal

position is desirable to prevent a continuous eruption of the opposing natural

incisors, unless they are otherwise prevented from extrusion by means of a

lingual plate, auxillary bar or by splinting.

8. Balanced contact of opposing posterior teeth in a straight forward protrusive

relationship and functional excursive positions is desirable only when an

opposing CD or bilateral distal extension maxillary partial denture is placed.

9. Artificially posterior teeth should not be arranged farther distally than the

beginning of a sharp upward incline of the mandibular residual ridge or over the

retromolar pad.

A harmonious relationship of opposing occlusal and incisal surfaces

alone is not adequate to ensure the stability of the distal extension removable

partial denture. In addition, the relationship of the teeth to the residual ridges

must be considered.

METHODS OF ESTABLISHING OCCLUSION:

Two methods:

1. The functionally generated path technique

2. The articulator, or, static, technique.

THE FUNCTIONALLY GENERATED PATH TECHNIQUE:

Basis: when the pathways each tooth opposed to the edentulous space makes

throughout, all functional movements of the mandible are recorded, the

artificial tooth may thus be positioned and formed so that it will remain in

harmonious contact with its antagonist at all times. The pathways are created by

the patient in a wax occlusal rim. The patient performs all the functional

excursions while the opposing teeth contact the surface of the occlusal rim. The

recording produced in the wax is actually a negative record of the movement of

each opposing tooth as the mandible executes the functional movements.

The pathways so generated are poured in hard improved stone to produce

a cast against which the artificial teeth are set. Each ridge or groove in the

resulting stone cast represents the path of a cusp- setting the teeth in contact

with the paths should result in a completely functional and harmonious

occlusion.

ADVANTAGES:

1. The greatest advantage it eliminates the need for adjusting an articulator with

the interocclusal records or a tracing device.

2. A far greater potential for error lies in attempting to adjust an articulator to

follow jaw movements precisely than in generating a functional pathway.

3. This method also eliminates the need to make a face-bow transfer, because all

the information derived from a face-bow transfer is contained in the pathway.

LIMITATIONS/DISADVANTAGES:

1. The occlusion in one of the arches must be complete before a generated

pathway can be developed.

2. If the opposing partial dentures are required, one of the partial dentures must

be completed before the other can be made.

3. This method also does not tend itself well to developing the occlusion for a

partial denture opposing a complete denture.

4. During the generation of the path in the hard inlay wax, movement of the

distal extension base carrying the occlusal rim is possible.

5. Verification of recording in mouth is difficult.

6. Masticatory cycle differs depending on the type and texture of the food being

chewed. The pattern therefore developed in the wax is accurate for the wax only

and food stuffs may fall inside or outside the particular chewing cycle.

THE ARTICULATOR OR STATIC TECHNIQUE:

If the denture base lack stability or if the patient is physically unable to form a

chew-in record, the articulator equilibration method is preferred. It is commonly

employed method. First upper cast is mounted to the articulator using face bow

with a orbitale pointer. The lower cast is related to the upper cast using a centric

interocclusal record at an acceptable vertical dimension.

The bucco-lingual relation of the lower teeth and their relation to the

upper arch are studied. A decision whether to articulate the central fossa of the

denture teeth to the lower buccal cusps or to the lower lingual cusps must be

made. Once the holding cusps have been selected, the inclines of the remaining

cusps are reduced. This allows for a cusp-to-fossa relationship. Therefore in

centric occlusion the only area of contact on the denture should be in the central

fossae.

At the time of the wax try in, eccentric records are made and the condylar

inclinations are set on the articulator. Once the centric holding cusps are

reestablished by selective grinding, eccentric balance is achieved. The end result

is a harmonious balanced occlusion that allows freedom in lateral excursion

while maintaining maximum bilateral contacts in functional and parafunctional

activities. Perfectly balanced occlusion in all eccentric positions may not be

possible in many cases when working with natural teeth in one arch.

OCCLUSAL CONSIDERATION FOR IMPLANT–SUPPORTED

PROSTHESIS:

The clinical success and longevity of the endosteal dental implants as load-

bearing abutments are controlled largely by the mechanical setting in which

they function. The treatment plan is responsible for the design, number and

position of the implants. After achievement of rigid fixation, proper crestal bone

contour, gingival health, mechanical stress, and/or strain beyond the physical

limits of hard tissues have been suggested as the primary cause of initial bone

loss around implants.

The role of occlusion is important to osseointegration prosthesis. The

choice of an occlusal scheme for implant-supported prostheses is broad and

often controversial. Almost all concepts are based on those developed with

natural teeth and are transposed to implant support systems with almost no

modification. No clinical studies have been published comparing the various

implant occlusal theories.

The restoring dentist has specific responsibilities to minimize overload to

the bone to implant interface these include a proper diagnosis leading to a

treatment plan, providing adequate support based on the patient’s individual

force factors, a passive prosthesis of adequate retention and form, progressive

loading to improve loading to improve the amount and density of the adjacent

bone and further reduce the risk of stress beyond physiologic limits. The final

element is the development of occlusal schemes that minimizes risk factors and

allows the restoration of function in harmony with the rest of the stomatognathic

system.

IMPLANT PROTECTIVE OCCLUSION:

A proper occlusal scheme is a primary requisite for long term survival,

especially when parafunction or a marginal foundation is present. A poor

occlusal scheme both increases the magnitude of loads and intensifies

mechanical stresses (and strain) at the crest of the bone.

OCCLUSAL CONSIDERATIONS

Natural teeth versus implant mobility-CHARACTERISTICS UNDER LOAD

Criterion Tooth Implant

1. Impact force decreased increased

2. Mobility variable none

3. Diameter large small

4. Cross-section not round round

5. Modulus of elasticity +/- cortical bone 5-10 times

6. Hyperemia + _

7. Orthodontic movement + _

8. Fremitus + _

9. Radiographic changes PDL, cortical bone _

10. Progressive loading since childhood shorter period

11. Wear enamel wear facets, localized minimal wear,

stress

fatigue and stress # , cervical & # of

prosthetic

abfraction, pitting on occlusal components or

cusps. Implant body.

12. Occlusal awareness high detection of premature low

(Proprioception) contacts.

13. Movement shock breaker effect of PDL capture

stress at crest

Apical intrude quickly 28µm no initial

movement

Lateral 50-108µm 10-50µm

OCCLUSION ON NATURAL TEETH AND IMPLANTS:

There has been ongoing controversy regarding whether a rigidly fixated implant

may remain successful when splinted to natural teeth. Because the implant has

no periodontal membrane, concerns centre around the potential for the non-

mobile implant to bear the total load of the prosthesis when joined to the mobile

natural tooth. In the implant-tooth fixed prosthesis, four important components

may contribute movement of the system: the implant, bone, tooth and

prosthesis.

Tooth movement:

1. 8 to 28 µm initial movement in vertical direction with 3-5 lb of load

2. similar to implant secondary movement reflects surrounding bone property.

3. 3-5 µm initial movement of implant in vertical direction

4. 56 µm combined intrusive natural tooth movement - 28µm + 28µm

5. 33µm combined intrusive movement natural tooth opposing implant - 28µm

+ 5 µm

6. 10µm combined intrusive movement implant opposing implant - 5µm + 5µm

because the initial difference in the vertical movement of teeth and implants in

the same arch may be as much as 28µm, the initial occlusal contacts should

account for this difference, or the implant will sustain greater loads than the

adjacent teeth. The existing occlusion is evaluated before implant

reconstruction. Occlusal prematurities are ideally eliminated on teeth before

implant reconstruction. Thin articulating paper (less than 25µm thickness) is

used for the initial implant occlusal adjustment in centric relation occlusion

under a light tapping force. The implant prosthesis should barely contact, and

the adjacent teeth should exhibit greater initial contacts. Only axial occlusal

contacts should be present on the implant crown, once the equilibration with a

light bite force is applied. The contacts should remain axial over the implant

body and may be of similar intensity on the implant crown and the adjacent

teeth under greater biting force to allow all elements to react similar to the

occlusal load. Hence to harmonize the occlusal forces between the implants and

the teeth, a heavy bite force occlusal adjustment is used because it depresses the

natural teeth, positioning them closer to the depressed implant position and

equally sharing the load.

In natural teeth anterior’s (healthy incisors and canines) disclude posteriors in

lateral excursions. In natural teeth having implant replacement’s anteriorly,

during lateral excursion more forces are directed on the implant because initial

movement of implant is 10 - 50µm laterally compared with natural teeth 68 -

108µm .there fore either should be splinted together or occlusal adjustment

should be carried out.

Implants do not extrude, rotate or migrate under occlusal forces. The

proposed occlusal adjustment does not encourage additional tooth movement

because regular occlusal contacts occur. The teeth opposing implants are not

taken out of occlusion. Brief occlusal contacts on a daily basis maintain the

tooth in its original position (similar to the rest of the mouth). In addition,

because most teeth occlude with the 2 teeth (with the exception of mandibular

central incisor), the opposing teeth positions are more likely to remain the same.

On the other hand, no occlusal scheme will prevent mesial drift and minor tooth

movement from occurring. An integral part of the Implant Protective occlusion

philosophy is the regular evaluation and control of occlusal contacts at each

regularly scheduled hygiene appointment. This permits the correction of minor

variations occurring during long-term function and also helps to prevent

porcelain fractures and other stress related complications on the remainder of

the natural teeth.

IMPLANT PROTECTIVE OCCLUSION:

This concept establishment is credited to Carl Misch. It is also called medial

positioned lingualised occlusion. When teeth are present, the maxillary dentate

posterior ridge is positioned slightly more facial than its mandibular

counterpart. Once the maxillary teeth are lost , the edentulous ridge resorbs in a

medial direction as it evolves from division A to B to C to D. as a result the

maxillary permucosal implant site gradually shifts towards the midline as the

ridge resorbs. As a consequence, endosteal implants are also more lingual than

their natural tooth predecessors. Although many of the occlusal concepts are

similar in removable and fixed implant restorations, several aspects are unique

to the implant supported prosthesis and there fore constitutes implant protective

occlusion.

KEY FACTORS:

1. Occlusal table width:

In IPO the width of the occlusal table is directly related to the width of the

implant body. The wider the occlusal table, the grater the force developed by

the biologic system to penetrate the bolus of food. The restoration mimicking

the occlusal anatomy of natural teeth often result in offset load (increased

stress), complicated home care, and increased risk of porcelain fracture. As a

result in non-esthetic regions of the mouth , the occlusal table should be reduced

in width compared with the natural teeth.

2. Crown contour:

in mandibular division A bone, the implant is located under the central fossa

whereas in Division B it is located under the lingual cusp region of the pre-

existing natural tooth. As a result, mandibular endosteal implants are always

positioned more medial than the original buccal cusp. All occlusal contacts are

more medial than those on the natural mandibular teeth.

3. Influence of surface area:

An important parameter of the implant protective occlusion is the adequate

surface area to sustain the load transmitted to the prosthesis. Narrow diameter

implant receive greater forces therefore either they should be splinted or

increases the number of implants. Increased loads are compensated by increase

in implant width, reduced crown height, ridge augmentation if necessary, and

increasing the number of implants.

4. Design of the weakest arch:

Any complex engineering structure will typically fail at its ‘weakest link,’ and

dental implant structures are no exception. Reduced occlusal forces with an

absence of lateral contacts in excursions are recommended on posterior

cantilevers or anterior offset pontics whenever possible. This minimizes the

moment forces on the abutments and decreases the load on the terminal implant

abutments.

5. Occlusal materials:

Occlusal materials play an important role in the transmission of forces and in

the maintenance of occlusal contacts. Commonly used materials are porcelain,

gold and acrylic resin.

Comparative characteristics:

Porcelain Gold Resin

1. Esthetics + - +

2. Impact force - + +

3. Static load +/- +/- +/-

4. Chewing efficiency + + -

5. Fracture - + -

6. Wear + + -

7. Interarch space - + -

8. Accuracy - + -

(Metal shrinkage is 10 times less in metal compared with porcelain or resin)