PROSTHODONTICS III – LESSON 7: JAW RELATION MANDIBULAR RELATIONSHIP, Papers of Dentistry

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The Temporomandibular Joint

• Is a complex articulation of the movable mandible and the base of the skull.
• Articulation takes place in two areas:
  1. Between maxillary and mandibular teeth (interjaw dental occlusion), and
  2. Between the mandibular condyle and the temporal bone. Bone Involved in the TMJ  Condylar process or condyle of the mandible  Mandibular fossa  Articular eminence of the temporal bone Temporomandibular Joint in Action  Retraction - Posterior fibers of temporalis, deep part of masseter, and geniohyoid and digastric  Elevation - Temporalis, masseter, medial pterygoid.  Depression - Gravity, digastric geniohyoid, and mylohyoid muscles Jaw Relation  Also known as Maxillomandibular relationship  Refers to any relation/position of mandible to maxilla Orientation Jaw Relation  The jaw relation when the mandible is kept in its most posterior relation, it can rotate in a sagittal plane around an imaginary transverse axis passing through or near the condyles.  It can be recorded with a face-bow. Vertical Jaw Relation  The distance between two selected anatomic or marked points (usually, one on the tip of the nose and the other upon the chin), one on a fixed and one on a movable member.  It is the vertical measurement of the face between any two arbitrarily selected points conveniently located one above and one below the mouth usually in the midline.

Types of Vertical Relation/ Dimension

1. Vertical Dimension at Rest (VDR)
2. Vertical Dimension at Occlusion (VDO)
3. Vertical Dimension at other position A. Vertical Dimension at Rest (VDR)  vertical dimension when the mandible is in the physiologic rest position  established by muscle and gravity  used as a guide to the lost vertical dimension at occlusion (VDO)  measured when the head is upright in position and not supported by the headrest. Physiologic Rest Position  position of the mandible when all muscles that closes and opens the jaws are in a state of minimal tonic contraction sufficient only to maintain posture. B. Vertical Dimension at Occlusion (VDO)  established by the natural teeth when present and in occlusion  established by the vertical height of 2 dentures/OCR in contact  computed by the formula VDR – FWS = VDO Interocclusal Distance/ Freeway Space/ FWS  space or gap between the upper and lower teeth when the mandible is in physiologic rest position  usually 2-4mm when observed at the position of the first premolars  essential because it maintains health of periodontal tissue when teeth are present VD at rest= VD at occlusion+ FWS C. Vertical Dimension at Other Position  no significance in CD construction

4. Swallowing  presence of a very light contact at the beginning of the swallowing cycle  if denture occlusion is missing – VD maybe too small  if there is difficulty – VD is probably too great 5. Tactile Sense  Patient tactile sense o Patient is asked if the rims appear to touch too soon, or if the jaw closes too much or if it feels just right  Boos Bimeter o A device the measures the biting force o Maximum biting force ocurs at VDO  Lytle’s method o Using a central bearing plate and pin  Electromyography o Rest position determined by recording minimal activity of muscles of mastication
5. Patient Perceived Comfort Mechanical Methods Ridge Relation
   • Incisive papilla to mandibular incisors - approximately 4mm in natural dentition
   • Parallelism of ridges Measurement of Former Denture Pre-extraction Records
   1. Profile radiographs - Have been used but cannot be considered adequate.
   2. Cast of teeth in occlusion - Give an indication of the amount of space required between the ridges for the teeth of this size.
   3. Facial measurement - Use of Willis gauge Horizontal Jaw Relation - Refers to the front to back, side to side relation of the mandible to the maxilla. Classifications of Horizontal Jaw Relation
   4. Centric relation
      • The basic horizontal jaw relation
      • Bone to bone relationship
   5. Eccentric relation  Protrusion  Right and left lateral excursion  All intermediate position Centric Relation
   • The most retruded physiologic relation of the mandible to the maxilla when the condyles are in the most posterior, unstrained position in the glenoid fossa from which lateral movements can be made, at any given degree of jaw separation.

• It is a bone-to-bone relation and is classed as a horizontal relation because variations from it occur in the horizontal plane.
• A maxillo-mandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the posterior slopes of the articular eminence. Significance of Centric Relation

1. Artificial teeth are best set to occlude evenly at centric relation.
2. More definite than vertical relation and is independent of the presence or absence of teeth.
3. Recordable and reproducible over a period of time.
4. Serves as a reference for establishing an occlusion.
5. When centric relation and centric occlusion of natural teeth do not coincide, the periodontal structures around the teeth are endangered.
6. Errors in mounting the casts of the articulator can be detected, when the centric relation is used as the horizontal reference position.
7. An accurate centric relation recorded properly, orients the lower cast to the opening axis of the articulator and the mandible.
8. An accurately recorded centric relation, when transferred to the articulator, permits proper adjustments of the condylar guidance for the control of eccentric movements of the instrument.
9. When centric relation and centric occlusion of artificial teeth do not coincide there is instability of the dentures and the patient may experience pain and discomfort. Centric Relation (CR) vs Centric Occlusion (CO) vs Maximum Intercuspation (MIP) Centric Relation - This position is independent of tooth contact and is repeatable position. Centric Occlusion
   • The occlusion of opposing teeth when the mandible is in centric relation.
   • In complete denture CR=CO
   • In natural dentition, this may or may not coincide with maximum intercuspal position. Maximum Intercuspation (MIP)
   • Complete intercuspation of opposing teeth independent of the condylar position.
   • Tooth to tooth relation in maximum contact regardless of condylar position. In natural dentition… CR and MIP only coincide in about 10 percent of the population, leaving 90 percent of the population to have a slide. As a clinician, you must determine if CR is a functioning position and how the patient gets from CR to MIP. Only then can you determine if the slide is pathological or physiological. In complete dentures… Centric Relation and Centric Occlusion - If centric relation and centric occlusion do not coincide, it will result to denture instability and pain or discomfort. Methods of Retruding the Mandible
   1. Passive Method - Dentist guide mandible in terminal hinge axis movement.
   2. Active Method - Patient responds to instruction by actively retruding the mandible.

Important Eccentric Relations

1. Protrusive
2. Lateral (left and right) Importance  Necessary in order to program an adjustable articulator to simulate the patients jaw movements.  A programmed articulator is helpful in constructing a balanced denture occlusion and restorations which are in harmony, with the functional movements of the mandible. Protrusive Relation/Protruded Jaw Relation - The mandible can be protruded to a position anterior to that of the centric relation. Lateral Jaw Relation - The mandible can also be moved to the left or right sides. Recording Lateral Relations
3. Graphic method - Uses Gothic arch tracing, two separate records are made - one in the left lateral position and one in the right lateral position.
4. Lateral check records - These are made similar to the protrusive record except that the mandible is held in the lateral positron while the record is being made. Left and right lateral records are made.
5. Hanau’s Formula - Around 1 930, Hanau recommended a formula to derive the lateral inclination as follows: L = H/8 = 12 where: L = Lateral condylar inclination H = Horizontal condylar inclination