Jaw Relationships
Jaw registration has to be done at 2 clinical stages
To mount primary casts in correct relationship for occlusal analysis and design
To mount working casts prior to setting up teeth
For CoCR RPDs, the record rims are constructed on the metal framework for stage 2
Intro
In order to establish a functional and harmonious occlusal relationship between opposing dentitions, the maxillary and mandibular casts must be mounted on an articulator in the correct jaw relationship
Maxillo-Mandibular Relationship (MMR)
(def.) The positional relationship between the mandible and maxilla
Consists of 2 interdependant components
Jaw relationship
Vertical
Horizontal
Antero-posterior
Lateral
The mandible is capable of adopting an infinite number of positions in both vertical and horizontal planes
In the construction of denture, it is essential that the required relationship is established accurately
Once established, the jaw relationship is registered/ recorded, so that the casts of the patient's jaw can be related in a similar manner to that of the patient's mouth
This registration is then transferred to an articulator for the setting up of teeth
Establishing the Jaw Relationship
Consists of 2 stages
ESTABLISHING THE VERTICAL DIMENSION OF OCCLUSION (OVD)
ESTABLISH A REPRODUCIBLE HORIZONTAL RELATIONSHIP WITH THE MANDIBLE(CR/ RCP)
REST VERTICAL DIMENSION (RVD)
OCCLUSAL VERTICAL DIMENSION (OVD)
FREEWAY SPACE (FWS)
The vertical dimension of the face when the teeth are fully intercuspated (MI)
This position is established by the TEETH
The vertical dimension of the face with the jaws in the physiologic rest position (RP)
RP is the habitual postural position of the mandible when the patient is resting comfortably in the upright position with the condules in a neutral unstrained position in the glenoid fossa
Factors which influence RVD
Short term factors
Long term factors
Head posture (decreases when head is tilted forward)
Stress (decreases with stress)
Presence of absence of teeth (decreases following extraction)
Body posture (decreases in supine position)
Age (increases with age due to alveolar growth-- lengthening of lower third of the face)
Chronic neuromuscular disorders e.g. paralysis
Parafunction e.g. bruxism decreases RVD
Drugs (e.g, increases with diazepam, decreases with caffeine)
Establishing vertical component
When natural teeth are present in the jaws and in a state of normal occlusion -- contact between the opposing teeth determines the OVD
When no contacts are present due to the absnece of teeth, the OVD must be estimated
Basis for estimation of OVD is the RVD
RVD - FWS = OVD (FWS usually about 2-4mm)
(def.) the space between upper and lower teeth when mandible is in resting position
3 common clinical situations in RPD cases
1) when there are natural tooth contacts at the desired vertical dimension
2) when there is natural tooth contact at a reduced vertical dimension (must determined OVD by measuring RVD; FWS is more in these cases)
3) When there is no natural tooth contact (must determine OVD by measuring RVD and subtracting the FWS)
3 common LAB situations in RPD cases
1) when there are enough teeth for the two casts to be articulated (stable MI) -- have both CR/MI and OVD
2) When there are insufficient teeth to obtain a stable MI -- no MI but have OVD
3) When there are insufficient teeth to determine jaw position or vertical dimension -- no MI and no OVD
How is the vertical dimension assessed clinically?
Facial proportions
Swallowing
Speech, closest speaking space
Patient's tactile sense and percieved comfort
Willis guage
Calipers
What is the OVD you have chosen is excessive?
Trauma to the denture bearing tissues; both soft tissue and alveolus
Clicking during speech
Difficulty in pronouncing words starting with 'p', 'b' and 'm'
Strained appearance
TMJ dysfunction
What if the vertical dimension you have chosen is inadequate?
Lack of facial support; wrinkled
Reduced masticatory effiieicny
Dribbling of saliva
Angular chelitis
Cheek biting
TMJ disorder
Once the correct OVD is established (with allowance for necessary FWS), the jaw relation in the horizontal plane must be established
Centric Relation-- the most retruded unstrained position of the mandible from which lateral movements can be made at an established vertical dimension -- CLINICALLY REPRODUCIBLE
2 situations to be considered
Dentate patients with sufficient and stable natural tooth contacts-- MI is the horizontal relationship to be recorded
Patients lacking acceptable tooth contact-- the retruded jaw relationship (CR) must be recorded at the established OVD
Reasons for using CR
1) it is reproducuble
2) apparatus used for reproducing some jaw movements and setting the teeth operate from the retruded positon (CR)
3) denture instability results when teeth of denture are set up in other than CR
4) OVD may be altered during function if the teeth are set in other than CR
5) May result in TMJ dysfunction when patient attempts to modify chewing action to accomodate incorrect jaw relationships
Retruded Contact Position-- position of the mandible when its condyles are in their retruded position in the glenoid fossa and there is occlusal contact of the teeth or occlusion rims
How Jaw Relationship affects Treatment Planning
1) When there are enough tooth contacts to give a stable MI
2) When tooth contact determines the vertical dimension but unstable MI
3) Vertical dimension with excessive freeway space (natural tooth contacts present at a lower OVD e.g. due to severe attrition)
Set up the rest of the occlusion to maximally intercuspate with the natural teeth
There should be no lateral interferences
4) When there are insufficient teeth to determined the vertical dimension
Take CR record in this position (at OVD determined by tooth contact)
Set up the rest of the teeth to maximally intercuspate with the natural teeth and with each other
There should be no lateral interferences in centric and eccentric positions
Establish the correct vertical dimension (determine RVD and then estimate OVD. It should be 2-4mm less than RVD)
Register the MMR with the mandible in CR at the determined OVD
Establish the rest of the occlusion around that in the same way as for the other two cases
** if patient is over-closed by a large amount, do not attempt to raise OVD to normal at once
Establish the correct occlusal vertical dimension
Register the MMR in retruded contact position or CR at the established OVD
Establish the rest of the occlusion around that. Sometimes you are able to obtain balanced occlusion
Aims of the Jaw Registration (MMR) Stage
1) Determine the horizontal position of the upper false teeth and lower false teeth
2) Determine position of the occlusal plane in relation to the maxilla
3) Determine the occlusal vertical dimension
4) Record the MMR at the selected OVD with the mandible in its most retruded position
5) Determine the midline or the centre-line around which the teeth are to be arranged
6) Select the mould and shade of artificial teeth to be used for the prosthesis
Wax Wafer Technique
Trim the record block until mandible and maxilla touch at the desired OVD and antero-posterior position. Cut V shaped grooves on the U/ record block
Cut off 1-2mm wax from the height of the wax rim. Warm a piece of remodelling wax, soften it uniformly and attach it to the record block
Guide the mandible to close in the most retruded position (ask patient to rause tongue to touch the palate). Hold the mandible in this position until wax wafer has hardened
Transfer of Jaw Relationships
After establishing vertical and horizontal components of the jaw relationship, it is essential to transfer this accurately to an articulator
Any errors in this stage will be retained in subsequent stages and appear as occlusal errors in the complete denture
A face-bow record will have to be taken if using an adjustable articulator
An articulator is an instrument in which the casts of the upper and lower dentate or edentulous jaws can be attached for reproducing jaw relations and movements of the lower jaw relative to the upper
Types of articulators
Hinge
Free plane (average value)
Semi-adjustable (Dentatus)
Fully adjustable (Dentar)