Simulation of extension, radial and ulnar deviation of the wrist with a rigid body spring model.

A novel computational model of the wrist that predicts carpal bone motion was developed in order to investigate the complex kinematics of the human wrist. This rigid body spring model (RBSM) of the wrist was built using surface models of the eight carpal bones, the bases of the five metacarpal bones, and the distal parts of the ulna and radius, all obtained from computed tomography (CT) scans of a cadaver upper limb. Elastic contact conditions between the rigid bodies modeled the influence of the cartilage layers, and ligamentous structures were constructed using nonlinear, tension-only spring elements. Motion of the wrist was simulated by applying forces to the tendons of the five main wrist muscles modeled. Three wrist motions were simulated: extension, ulnar deviation and radial deviation. The model was tested and tuned by comparing the simulated displacement and orientation of the carpal bones with previously obtained CT-scans of the same cadaver arm in deviated (45 degrees ulnar and 15 degrees radial), and extended (57 degrees ) wrist positions. Simulation results for the scaphoid, lunate, capitate, hamate and triquetrum are presented here and provide credible prediction of carpal bone movement. These are the first reported results of such a model. They indicate promise that this model will assist in future wrist kinematics investigations. However, further optimization and validation are required to define and guarantee the validity of results.

Validation of a new surgical procedure for percutaneous scaphoid fixation using intra-operative ultrasound.

A new technique for percutaneous fixation of non-displaced scaphoid fractures is described. The technique used pre-operative planning from computed tomography images, registration to intra-operatively acquired three-dimensional ultrasound images, and intra-operative guidance using an optical tracking system. Two stand-alone software applications were developed. The first one was used to determine the surgical plan pre-operatively and the second one was used to guide the surgeon during screw insertion. Laboratory validation of the technique included measurements of the inter-operator and intra-operator variability in the outcome of scaphoid fixation using the proposed procedure, and also included comparison of the performance of this procedure with the conventional percutaneous fixation technique using fluoroscopy. The results showed that the tight accuracy requirements of percutaneous scaphoid fixation were met and that the consistency was superior to the conventional technique.

In vitro analysis of colour Doppler flow with the use of proximal isovelocity surface area: improved flow estimates using a nonhemispherical model.

OBJECTIVE: To examine the geometry of the proximal isovelocity surface area (PISA) envelope and its associated isotach, and to evaluate the accuracy of two models of calculating volumetric flow by using the PISA technique. DESIGN: A new model for determining isotach geometry from the PISA envelope was developed and tested in an in vitro simulation. SETTING: Echocardiography Laboratory, Hotel Dieu Hospital, Kingston, Ontario. MATERIALS AND METHODS: PISA envelopes were visualized using an in vitro flow simulator with a series of sharp-edged orifices (2.5 to 16 mm diameter) at a range of flow rates (10 to 110 mL/s). INTERVENTIONS: Flow calculations based on the traditional hemispherical geometric assumption for the isotach and the new model were made and compared with measured flow rates. MAIN RESULTS: The hemispherical model systematically and significantly underestimated flow. The nonhemispherical model, which requires measurement of both the height (a) and lateral width (2d) of the PISA envelope, provided improved estimates of flow. CONCLUSIONS: The nonhemispherical model provides a better estimate of flow through an orifice. Flow rate Q can be calculated directly from the size of the PISA envelope and the aliasing velocity (VA) by using the relationship Q = (3.14d2 + 5.97da + 1.37a2)VA or can be read from a nomogram.