Replication of Joints and Movements
In the natural masticatory system, the mandible moves in condylar, tooth, and neuromuscular guidance against the fixed maxilla; however, the conventional design of articulators only allows the upper arm to be moved in relation to the fixed mandibular model holder, imitating only condylar and tooth guidance. This means that relative movements with calculable variations are possible.
The conventional design of articulators only permits movement of the upper arm in relation to the fixed mandibular model holder. This mechanical simplification allows better handling and produces movements of the rows of teeth that are analogous but opposite to those of the natural dentition.
Condylar guidance is produced by mechanical replication with mainly linear forms of the condylar path, where neuromuscular guidance to Bennett movement is simulated by adjustable Bennett angles.
Joint geometry is also replicated differently in different articulators; a distinction is made between arcon and non-arcon articulators in terms of joint replication.The term arcon is formed from the words articulation and condyle.
Nonarcon articulators are devices in which the articular surfaces and condyles are transposed in comparison with the natural dentition: The condylar rollers (with condylar path) are located on the lower arm of the articulator and the condylar balls or spheres (condyles) on the upper arm. As a result, the condylar balls move in the opposite direction to the condyles in the human skull (Fig 8-4).
A condylar ball is an artificial, spherical condyle as a simplified replication of the articular condyle of the mandible (caput mandibulae). In addition, the condyles may be shaped as rollers or double spheres based on the shape of the condylar path.
In nonarcon articulators, a condylar roller represents a closed simulation of the articular surface of the TMJ on the lower arm of the device, in which the condylar ball fixed to the upper arm is accurately guided. With some condylar rollers, both the horizontal condylar inclination and the Bennett angle are adjustable; the condylar inclination is altered by twisting the roller, and the Bennett angle is changed by pivoting around a vertical axis.
Movement errors arise because, during movements of the maxilla, the condylar ball moves upward and backward instead of downward and forward. As a result, the gap between the condylar ball and the maxillary teeth is altered. In the natural masticatory system, the condyles as rotation centers are always at a constant distance from the mandibular teeth. However, if the joint parts are transposed in comparison with the anatomical model, the distance between the mandibular teeth and the rotation center changes during protrusive and lateral movements (Fig 8-5).
Arcon articulators are average-value or fully adjustable devices in which the articular surfaces and condyles are arranged in accordance with the natural model: The condylar ball (condyle) is located on the lower arm, and the condylar housing (with condylar path) is fixed to the upper part of the articulator (Fig 8-6). This design corresponds to anatomical reality and allows movements in roughly the same direction as in the masticatory organ. The mandibular movements are performed as relative movements because the upper arm (the maxilla) is moved relative to the lower arm (mandible). No movement errors caused by displacement of the rotation center compared with the mandibular teeth will occur during lateral movements (see Fig 8-5).
Condylar housings (casings or boxes) are the articular surfaces open at the bottom and located on the top part of an arcon articulator.The condyle on the lower arm of the articulator slides downward and forward on the articular surface when TMJ movements are being simulated, which corresponds to the natural pattern of movement.The condylar housing can be adjusted in accordance with the angle of the condylar path. In order to simulate Bennett movement, the condylar paths are differently shaped in the condylar housing. The articular surface can be rotated and adjusted around a vertical axis corresponding to the Bennett angle, and exchangeable inserts with curved movement paths are provided to simulate different courses of Bennett movement (see Fig 8-6). Some articulators have exchangeable condylar path inserts for individually milled condylar paths.
The condylar housing open at the bottom can be closed with a pivoting locking device on some articulators to hold the condyle on the articular surface during simulation of lateral or protrusive movements.