Minimizing fiducial localization error using sphere-based registration in jaw tracking.

Some of the jaw tracking methods may be limited in terms of their accuracy or clinical applicability. This article introduces the sphere-based registration method to minimize the fiducial (reference landmark) localization error (FLE) in tracking and coregistration of physical and virtual dental models, to enable an effective clinical analysis of the patient's masticatory functions. In this method, spheres (registration fiducials) are placed on the corresponding polygonal concavities of the physical and virtual dental models based on the geometrical principle that establishes a unique spatial position for a sphere inside an infinite trihedron. The experiments in this study were implemented using an optical system which tracked active tracking markers connected to the upper and lower dental casts. The accuracy of the tracking workflow was confirmed in vitro, based on comparing virtually calculated interocclusal regions of close proximity against the physical interocclusal impressions. The target registration error of the tracking was estimated based on the leave-one-sphere-out method to be the sum of the error of the sensors, i.e., the FLE was negligible. Moreover, based on a user study, the FLE of the proposed method was confirmed to be 5 and 10 times smaller than the FLE of conventional fiducial selections on the physical and virtual models, respectively. The proposed tracking method is non-invasive and appears to be sufficiently accurate. To conclude, the proposed registration and tracking principles can be extended to track any biomedical and non-biomedical geometries that contain polygonal concavities.

A comparison of simulated jaw dynamics in models of segmental mandibular resection versus resection with alloplastic reconstruction.

STATEMENT OF PROBLEM: Composite mandibular resection resulting in mandibular discontinuity can alter jaw motion, occlusal forces, and mastication, whether or not the jaw is reconstructed. The biomechanical events associated with these changes are difficult to assess clinically and, therefore, are not well documented or researched. PURPOSE: The purpose of this study was to model movements of a mandible with a discontinuity defect, and to compare them to movements of a mandible with its continuity restored by alloplastic reconstruction. MATERIAL AND METHODS: Computational models were created with a novel simulation platform. The variables designed into the models included gravity, external forces, and jaw muscle activity. Each jaw was observed at rest, when opened by external force or by muscle drive, and during the generation of unilateral occlusal force on the nonoperated side. Scarring was simulated with springlike forces. Outputs included individual muscle forces and torques, as well as mandibular incisor and condylar motions. RESULTS: Both models displayed plausible resting postures, and jaw opening with deviation toward the defect side when scarring was simulated. Opening caused by downward force on the incisors differed from that due to muscle activation. Jaw rotations during unilateral molar contact on the unaffected side were muscle specific and influenced by mandibular discontinuity. CONCLUSIONS: Plausible jaw movements after hemimandibulectomy and/or alloplastic reconstruction could be predicted by dynamic modeling. The effect of soft tissue forces on jaw posture and movements varied with the condylar support available. In both models, different opening trajectories were produced by external force on the jaw and by jaw muscle activation. Mandibular rotation during unilateral molar contact depended on which muscles were activated, and the availability of bilateral condylar support.