Articulator Introduction
The oral cavity is confined by maxilla and mandible with two temporomandibular joints (TMJs). During the fabrication of prosthesis, some procedures are carried out in the oral cavity and some are performed in the dental laboratory in the absence of the patient. A mechanical device stimulating the two jaws and the TMJs is needed for ease of work. This device is an “articulator”.
Table of Contents
Articulator Definition
An articulator is a mechanical device that represents the TMJs and the jaw members to which maxillary and mandibular casts may be attached to simulate some or all the mandibular movements.
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Mean Value Articulator Classification
Condylar Guidance In Mean Value Articulator
Articulators are classified in several ways. The most common classification of articulator is based on their adjustability. The adjustability of the articulator is judged based on their ability to accept face bow and various occlusal records which modifies condylar and incisal guidance accordingly.
Parts Of Mean Value Articulator
Articulators can be classified according to:
- According to theories of occlusion
- Based on the type of interocclusal records used
- Based on the adjustability of the articulators
- Based on the University of Michigan workshop.
Classification According to Theories of Occlusion:
Bonwill Articulator
- Bonwill theory articulator: This articulator was made by WGA Bonwill. This articulator is based on Bonwill’s theory of equilateral triangle. According to this theory, the teeth move in relation to each other as guided by the condylar and the incisor guidance. The equilateral triangle is formed between two condyles and condyles to the midincisal point (of the mandible). Theoretically, the dimension of an equilateral triangle is 4 inches.
- Conical theory articulator: This theory was proposed by RE Hall. According to this theory, the lower teeth move over the surface of upper teeth over the surface of the cone generating an angle of 45° with the central axis of cone tip 45° to the occlusal plane.
- Spherical theory articulator: This articulator is based on Monson’s theory of spherical occlusion.
The theory states that the lower teeth move over the surface of a sphere of 8 inches which has its center at the glabella. The surface of the sphere passes through the glenoid fossa along and articulating eminence.
Articulator Classification
Based on the Type of Interocclusal Records Used:
- Interocclusal records adjustment: These devices are programmed based on interocclusal records made up of plaster, bite registration polyvinyl siloxane, zinc oxide eugenol (ZOE) paste, and base plate wax.
- Graphic records adjustment: These articulators are capable of producing accurate jaw movements. A radiographic record can be used to accurately locate the hinge axis on an articulator.
Classification According to Adjustability of Articulators:
- Nonadjustable articulators
- Semi adjustable articulators
- Fully adjustable articulators.
Nonadjustable Articulators:
Bonwill Articulator
These are simple devices to mount the casts. They are a simple mechanical device that opens and closes in one direction. They have a fixed horizontal axis of rotation. The upper and lower members are joined together with the hinge-like mechanism. Examples are slab articulator and hinge articulator, mean value articulator.
Condylar Guidance In Mean Value Articulator
Semi-adjustable Articulators:
They have adjustable horizontal condylar paths. They also have adjustable lateral condylar paths and incisal guide tables, and adjustable intercondylar distance. They accept the face bow record.
These articulators are divided into two types based on the arrangement of condylar guidance assembly.
1. Arcon articulator: The term arcon was coined by Bergstrom by combining two words, i.e. articulator and condyle. In this type of articulator, the arrangement of condylar guidance and condylar element is similar to the arrangement in TMJ. The condylar guidance which represents the glenoid fossa is attached to the upper member of the articulator. The condylar element which represents the human condyle is attached to a lower member, e.g. Hanau wide-vue.
Arcon articulator Advantages: The face bow transfer, occlusal plane, and the relationship of opposing cast are preserved when the articulator opens and closes.
Articulator Classification
2. Nonarcon articulators: In this type of articulator the position of condylar guidance and elements is reversed compared to human TMJ. The condylar guidance is attached to the lower member and the condylar element is attached to the upper member, e.g. Hanau H2.
Bonwill Theory Articulator
Fully Adjustable Articulators:
They have the ability to adjust according to the patient’s mandibular movements. They have provision to customize condylar guidance.
They are highly precise in recording and understanding mandibular movements. Not widely used due to its complexity. An example is Stuart’s instrument stethoscope.
Based on University of Michigan Workshop (Based on Ability to Simulate Jaw Movements):
Articulators may show single-dimensional movement or they can show complex three-dimensional movements. The workshop on complete denture occlusion at the University of Michigan in 1972 classified articulators based on their ability to simulate jaw movements.
Class 1:
A simple device, capable of accepting a single static record. Only vertical movement is possible.
Class II:
These articulators perform horizontal and vertical movements but cannot orient the movement to TMJ with a face bow.
Class IIa:
Limited eccentric motion is possible which is based on average value.
Class IIb:
Limited eccentric motion is possible which is based on theories of arbitrary motion.
Class IIc:
Limited eccentric motion is possible which is based on engraving records obtained from the patient.
Class 3:
They accept the face bow records but the facility is limited. They permit horizontal and vertical movements.
Class 3 a:
They accept the face bow records. They accept static protrusive records.
Class 3 b:
They accept static lateral, protrusive records.
Class 4:
These articulators accept three-dimensional dynamic records. The point orientation of the cast can be achieved using face bow
transfer.
Class 4 a:
The condylar path is determined by engraving records produced by the patient.
Class 4 b:
These articulators are similar to type IVa but they allow angulations in customization of condylar path.
Uses:
- To diagnose the condition of occlusion in both natural and artificial dentition.
- To plan the treatment based on the relationship of opposite natural or artificial teeth.
- To aid in the fabrication of restoration and prosthesis.
- To correct and modify complete restorations.
- To arrange artificial teeth.
Advantages:
- Better visualization of cast and restorations in occlusion, especially from the lingual side.
- Chairside and patient appointment times are reduced.
- No interference due to saliva, tongue, and cheek.
- Realeff effect eliminated (discrepancy due to resiliency of mucosa).
- Transfer of work from clinician to technician through articulators and vice versa.
Limitations:
Since it is a mechanical instrument, it is subjected to errors resulting from fatigue and wear. It can simulate but not duplicate the jaw movements/relation.
Ideal Requirements:
- It should hold casts in the correct horizontal and vertical relation.
- Provision for removal and reattachment of casts should be there.
- It should provide a positive anterior vertical stop.
- It should accept a face bow transfer record using an anterior reference point.
- It should open and close in hinge movement.
- It should be made of noncorrosive and rigid material that resists wear and tear.
- It should not be bulky and heavy.
- There should be adequate space between the upper and lower members.
- The moving parts should move freely without any friction.
- The condylar guide should allow protrusive and lateral movements.
- Condylar guidance should be adjustable.
- The articulator should be adjustable in the horizontal direction.
- The incisal guide table should be customizable (adjustable).
Mean Value Articulator
A mean value articulator is called as “three-point articulator” or “free plane articulator”. It is a nonadjustable and nonarcon type of articulator. The mean value articulator is based on Bonwill’s theory of occlusion. According to this theory, an equilateral triangle of 4 inches is formed between two condyles and from each condyle to the midincisal point of mandibular incisors. It is capable of lateral and protruding movements
based on the fixed average value in addition to hinge movements. Thus, these cannot be programmed to suit the individual requirements of a patient. The mean value of incisal guidance is fixed at 5°. Condylar guidance is fixed at 33° for the mean value articulator.
Bonwill Theory Articulator
Various Models of Mean Value Articulator:
In the mean value articulator, the intercondylar distance is 110 mm, condylar guidance is 30–33°, and incisal guidance is 5°. These are
mean (average) values adopted for the fabrication of articulators so known as mean value articulators.
Parts of Mean Value Articulators:
parts of mean value articulator – Incisal Pin/Vertical Rod:
It helps to keep a fixed distance between upper and lower members. It has a pointed tip resting on the center of the incisal guide table. The upper flat surface should be flush with the superior surface of the upper member.
parts of mean value articulator – Condylar Shaft:
This is a horizontal shaft attached to the upper member of the articulator. This is attached in condylar slots. The length is 110 mm which represents intercondylar distance.
Values Of Mean Value Articulator
Parts of Mean Value Articulator – Midincisor Pin/Central Pin:
It is the anterior reference point. This is located at the midpoint on the incisal pin/rod. Relate to anterior occlusal plane. During the mounting of wax occlusal rims, this pin coincides with the midline of occlusal rims. It helps during the arrangement of maxillary central incisors according to the midline and occlusal plane.
Parts Of Mean Value Articulator – Horizontal Bar.
Two horizontal bars are attached in the posterior region of the articulator. The upper bar is the supporting bar and the lower bar is known as the horizontal orientation bar/rod. The orientation bar along with the midincisal pin form the plane of the articulator. The plane of occlusion coincides with the plane of the articulator.
Parts Of Mean Value Articulator – Upper Member:
This represents the maxilla on which the maxillary cast may be attached (through the mounting ring). It is triangular-shaped. In the anterior part incisal pin/vertical rod is attached and in the posterior part, two extensions corresponding to condyle elements are attached.
Lower Member:
It is an “L”-shaped frame with vertical and horizontal arms. The vertical part of the “L” frame: The vertical arm of the “L” frame is attached to the upper member through condylar slots.
The horizontal bar is also attached to this part of the lower member. Horizontal part of the “L” frame: In the anterior region, the anterior guide table is fixed. In the center, a mounting rim is attached to which a mandibular cast is attached.
Incisal Guide Table/Anterior Guide Table:
It has a concave upper surface which gives the incisal guidance. The concavity is designed in such a way that it gives anterior guidance equal to 5°. The incisal pin/vertical rod is in contact with the center of the anterior guide table. The value of incisal guidance is fixed in the mean value articulator.
Values Of Mean Value Articulator
Condylar Guidance:
In the mean value, articulator condylar guidance is represented by a slot known as condylar slot/track. Condylar slot together with condylar elements make condylar guidance. Condylar elements are fixed in condylar slots.
The spring assembly is attached in condylar slots/track to permit movements of condylar elements, when not in action it holds condylar elements in the most posterior parts of the slots. In mean value, articulator condylar guidance is fixed at 33°
Mounting Rings/Plates:
They are two in number, each attached to the upper and lower members. Mounting plates/rings are attached to the upper and lower members by thumb screws.
Maxillary and mandibular casts are mounted on mounting rings/plates using dental plaster. These mounting rings are removable so that the same articulator can be used with other cases or patients with another set of mounting rings.
Stabilizing Rod:
It is located in the anterior part of the upper member opposite to the vertical rod. It prevents the fall of the upper member of the articulator while working on it. It stabilizes the upper member.
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