Types of Facebow Definition: A calliper like instrument used to record the spatial relationship of maxillary arch to some anatomic reference point or pints in the cranium and then transfer this relationship to an articulator. It orients the dental cast in the same relationship to the opening axis of the articulator. Customarily the anatomic references are the mandibular condyles transverse horizontal axis and one other selected anterior reference point (GPT-9) Parts of facebow: o U shape frame o Condylar rods or earpiece o Bite fork o Locking device o Third reference point Facebow classification 1. Kinematic 2. Arbitrary: ear piece type Fascia type 1. Kinematic facebow (mandibular) The hinge axis facebow with adjustable calliper ends that locates THE EXACT aixs of rotation of the condyles. Hinge axis of the mandible can be determined by a clutch ie., a segmented impression tray like device attached onto the mandibular teeth with a suitable rigid material such as impression plaster Uses: Locate exact terminal hinge axis before using maxillary. Facebow Aids in CR recording Indications Whenever it is critical to precisely reproduce the exact opening an closing movements of the patients to the articulator – otherwise arbitrary facebow! Drawbacks: Time consuming, expensive Method (Extra info) o Fabrication of a clutch and attaching it to lower teeth. o Assemble the hinge axis locator o Mark approximate centre of condyle on the subjects face o Place graph paper – ask patient to open and close os that only rotational movements take place o Thus exact hinge axis points is located 2. Arbitrary (maxillary) facebow Uses arbitrary or approximate points of the face as the posterior points and condylar rods are positioned on these points. The hinge axis is approximately located. This type of facebow generally locate the true hinge axis within a range of 5mm. this can lead to 0.2mm error at the second molar area. This error can be compensated in CD due to realeff effect (i.e. resiliency like effect of the mucosa – CD is tissue supported prostheses). Thus, arbitrary facebow is most commonly indicated in CD construction not ideal for fixed prosthesis work Posterior reference points o The position of the THA on either side of the face is generally taken as the posterior reference points. a) Beyron point: lies 13mm anterior to the posterior margin of the tragus of the ear on a line from the centre of tragus extending to the corner of the eye b) Bergstrom point: 10mm anterior to the centre of the spherical insert of external auditory meatus and 7mm below the Frankfort plane most frequently closed to the hinge axis – bergstrom point. next most accurate posterior point is Beyron point. Arbitrary facebow classification a. According to posterior reference indicators (facia facebow , earpiece facebow) a.1) Fascia facebow o Utilised approximate points on the skin over the temopomandibular region as the posterior reference points o Disadvantages: Placed on the skin which is movable. There is a tendency for the condylar rods to displace Requires an assistant to hold the facebow in place a.2) Earpiece facebow (convienent in 90% of patients) o Uses the external auditory meatus as an arbitrary reference point which is aligned with ear pieces similar to hose on a stethoscope o Advantages Simple to use Don’t require measurements on the face As accurate as other facebows Provides average anatomic dimensions between the external auditory meatus and horizontal axis of mandible o Disadvantages Regardless of which arbitrary position is chosen an error of 5mm from the axis can be expected, which can lead to 0.2mm error at second molar area b. According to anterior reference indicators b.1) Infraoribital pointer o Lowest margin of infraorbital rim on one side B.2) Nasion pointer o Deepest part of the midline depression below the level of eyebrows c. According to mechanism of inter-condylar distance equilibration c.1) Spring bow (hanua facebow) o Earpiece facebow made of spring steel and simply springs open and close the various head width c.2) Calibrated condylar rods o Condylar rods are graduated. They are adjusted bilaterally until equal readings are achieved on both sides c.3) Slidematic facebow o Anterior horizontal arm of the bow composed two sliding arms that allow for mediolateral movements of the bow till the two condylar rods contact the posterior reference points. Type of earpiece facebow Summary Importance of facebow 1. Ensures casts are mounted centrally in correct relation to the intercondylar axis and to the centers of lateral movements 2. The occlusal plane (orientation of upper cast) is recorded form the patient in relation to the posterior 3. Recording the inter-condyalr distance (some facebow devices) which influences the occlusal antomy of artificial teeth 4. Records the 3D position of the maxilla relative to predetermined planes. This allows the duplication of the same opening and closing arc as natural teeth (mandibular hinge axis) and then it can be transferred to the articualtor (articulator hinge axis) . this allows o Reproducing lateral movements (eccentric momvements) o Changes in VD on the articualtor can eb stimulated as the arituclator member will move in arc similar to the natural mandibular arch movements (As AHA centra is the same as MHA centre) and this important in CD and full mouth rehab.