Jaw Relations Dr. Hussein KH AlmalikyThird visit in the complete denture construction (clinical step) is to register the jawrelations and transfer these relations to the articulator. Three types of jaw relationshipswill be discussed (maxillo-tempromandibular joint relationship, centric jaw relation,protrusive and lateral relationships).Orientation relationsIt means these relations that orient the maxilla to the cranium in such way that themandible is kept in its posterior position to the cranium can rotate around an imaginarytransverse axis passing through as near the condyles. This axis can be located by meansof the face-bow.Face-bow transfer: it is a caliper-like device used to record the patients maxilla/hingeaxis relationship and to transfer this relation to the articulator during mounting of themaxillary cast.Hinge axis: it is an imaginary line between the mandibular condyles around which themandible can rotate without translator movement. This term is now transferred to as thetransverse horizontal axis. If the face-bow transfers procedure properly done, the arc ofthe closure on the articulator should duplicate that exhibited by the patient.The face-bow transfer should be used when:1. Cusped posterior teeth are selected.2. Balanced occlusion in eccentric positions is desired.3. Interocclusion check records are used for verification of the jaw relationships.4. The occlusal vertical dimension is subjected to change.Some authors reported that if the cuspless teeth and a monoplane occlusion are selected, aface-bow transfer is unnecessary. Other recommends the use of the face-bow, even forthe cuspless teeth to provide more accurate placement of the casts in the articulator thanwould be obtained by simple placing arbitrarily.There are 2 types of the face-bow:Kinematic face-bow: it is initially used to accurately locate the hinge axis. It is attachedto a clutch which is in turn attaches to the mandibular teeth. As the mandible makesopening and closing movement, the condyles styli move in an arc. The kinematic face-bow generally not used for complete denture fabrication because the resiliency of the softtissues makes precise location of the rotational centers almost impossible.Arbitrary face-bow: there are several varieties of arbitrary face-bows, all are based on anaverage location of the hinge axis, and it is located over measured points on the face orby some type of earpiece. One average measurement, placing the rotational point 13 mm
anterior to the distal edge of the tragus of the ear, along a line from the superior-inferiorcenter of the tragus to the outer acanthus of the eye. The condylar styli of the face-boware then placed directly over the dots. This technique is used with Hanau H-2 andDentatus articulators.Face-bow registrationTrue hinge axis must be located on a fully-adjustable articulator.Arbitrary hinge axis is used on a semi-adjustable articulator, and this is determined by themanufacturer of the face-bow and articulator system.In order to understand the maxillomandibular relation and the concepts of the occlusion,we have to study the mandibular movements. Recording the mandibular movements isaffected by TMJ, muscular involvements, neuromuscular regulation of the mandibularmotion and ligaments.Horizontal jaw relationsIt is the relation between the mandible and the maxillae in the anteroposterior directionand media lateral direction which includes:1) Centric jaw relation: a maxillomandibular relationship in which the condyles articulatewith the thinnest avascular portion of their respective disks with the complex in theanteroposterior position against the slops of the articular eminence. The poison isindependent of tooth contact.2) Eccentric jaw relation: any position of the mandible other than centric. It includes: A. Protrusive jaw relation: resulting from protrusion of the mandible when the condyles move downward, forward. B. Lateral relation: the relation of the mandible to the maxilla in opposition to the left and right of the midsagittal plane.How does the TMJ move during function?Replaced dentures must work in harmony with the patients TMJ, which consist of thecondyles that articulate with the temporal bones and are located in the elliptical concavedepressions called the glenoid fossae in which they travel forward, from side to side andin some instances slightly backward.Between the dome-shaped concavities in the temporal bones and the condyles areinterposed the interarticular fibrocartilages, the meniscus, which are attached at their
margins to the articular capsules. The meniscus divides the joint into upper and lowercompartments. Normally, the movements in the upper compartment is chiefly glidinganteroposterior motion in which the condyles and the cartilage move as a unit, while themovement in the lower compartment is hinge-like; therefore, the condylar paths are thecontrolling factor in the mandibular movements. These movements are results of actionof muscles of mastication, suprahyoid and infrahyoid muscles.The muscles that control and move the mandible may be considered in three groups:1. Closing muscles.2. Guiding muscles.3. Opening muscles.Closing muscles include:A. Masseter muscle (muscle of mastication).B. Temporal muscle.C. Medial pterygoid.The direction of their fibers is essentially vertical. Their origin is superior to theirinsertion on the mandible.Guiding muscles include:Lateral pterygoid muscles (muscle of mastication).The action of these muscles is to pull the mandible forward if they both contractsimultaneously, or to pull the mandible laterally if they contract individually.Opening muscles include:A. Suprahyoid muscles.B. Infrahyoid muscles.Their origin below the mandible and their action is downward and backward.The teeth cannot occlude or disclude without the action of 4 paired muscles ofmastication that make it all possible.Mandibular movementsThe mandibular movements are related to 3 planes of skull (frontal, horizontal andsagittal). The point of intersection of these planes is called center of action.Basic movements of the mandible include:1) Rotational movements: these movements take place in the lower compartment of theTMJ between the superior surface of the condyles and the inferior surface of the articulardisc. Centric relation which is called uppermost, rearmost, midmost or most posteriorposition is purely rotary movement about the transverse horizontal axis.
2) Translatory movements: they are also called gliding movements of the mandible, theytake place in the upper compartment of TMJ between the superior surface of themeniscus and the inferior surface of the glenoid fossa, they are called mandibular bordermovement.Functional movements: they include combinations of rotation and translation movements:A. Opening and closing movements.B. Symmetrical forward and backward (protrusion and retraction) movements.C. Asymmetrical sidewise or lateral shift movements.Opening and closing movements are considered the most important mandibularmovements and they are divided into: 1) Habitual movement. 2) Border movement.They take place in 3 dimensional limits. The mandible can move about 10 mm laterallyand open about 50-60 mm. It protrudes approximately 9 mm and retrudes about 1 mm.These limits are known as border movements of the mandible. It is the most extremeposition to which the jaw is able to move in any direction. The border positions arelimited by nerves, bones, muscles, teeth (if present) and ligaments.Border movements are subdivided into:A. Anterior border movement: it appears as one arc in the sagittal plane. The condylesrotate and translate in this movement.B. Posterior border movements: it appears as two arcs in the sagittal plane. The condylesrotate in the opening and closing up to the point of terminal hinge opening and translateto the point of maximal opening.Sagittal plane movementsA map of the boundary of the movements of the mandible when viewed from one side ofthe head, it is tracing of the maximum vertical and anteroposterior movements of themandibular central incisors. It includes:1. Maximum intercuspation or centric occlusion (CO), this border position is usuallydefined as that point where the teeth best interdigitate.2. If the mandible is retruded farther to its posterior, most superior and most terminalhinge position, the limits of movement and the border position are determined bystructures of the TMJ. Some have also referred to this position as centric relation (CR).
Maximum hinge opening (MHO) Co ± centric occlusion B-D ± posterior border movement (two arcs) E-D ± anterior border movement (one arc) PR ± postural rest position From D to E ± protrusive closureFour movements of prime importance to complete denture service are:1) Hinge-like movement that is used in opening and closing the mouth for theintroduction of food.2) Protrusive movement that is used in the grasping and incision of food.3) Lateral a (right or left) movement that is used in the reduction of fibrous as well asother types of food.4) Bennett movement which is the bodily side shift of the mandible which when occursmay be recorded in the region of the rotating condyle on the working side. Thismovement includes: A. Functional mandibular movements that include all the natural or characteristic movements occurring during mastication, swallowing, speech and yawing. B. Parafunctional movements that occur in during clenching, tapping or grinding the teeth.Jaw registration of condylar movementsIt is a recording of the paths/positions of the condyles during border movements that arerecorded in order to program the articulator to simulate the patients condylar movements.It can be recorded by:1. Pantographic tracing to program a fully-adjustable articulator.2. Interocclusal records to program a semi-adjustable articulator.
Types of articulators:Class I ± a simple holding instrument, capable of accepting a single static registration;vertical motion is possible (nonadjustable).Class II ± an instrument that permits a horizontal as well as vertical motion but does notorient the motion to the TMJ.Class III ± an instrument that simulates the condylar pathways by using averages ormechanical equivalents for all or part the motion. It allows for orientation of the castsrelative to the joints and they may be arcon or nonarcon instruments (semi-adjustable).Class IV ± an instrument that accepts three dimensional registrations. It allows fororientation of the casts relative to the joints and simulation of the mandibular movements(fully-adjustable).