Mechanical factors responsible for the obstruction of the gliding mechanism of a dynamic hip screw for stabilizing pertrochanteric femoral fractures.

BACKGROUND: In treatment of pertrochanteric femoral fractures with dynamic hip screws (DHSs) (135-degree, Synthes, Bettlach, Switzerland), damage was observed in removed lag screws, leading to the conclusion that the gliding mechanism must have been obstructed as a result of either inappropriate position of the implant or insufficient medial support in the fracture zone. METHODS: The forces and moments transmitted in the screw socket are calculated using a mathematical model to find the optimal position of the implant. RESULTS: The forces and moments depend on the position and orientation of the lag screw as well as on the position of the contact point between the two main fragments. By changing the point of contact, a better decrease of the load to the DHS can be achieved than by changing the position and orientation of the screw. For a low contact point, the model shows the lowest values for the forces in the socket. CONCLUSION: Complete agreement was found between the results of the presented calculations and our own clinical experience in removed DHSs.

Enhancements in the accuracy of the center of pressure (COP) determined with piezoelectric force plates are dependent on the load distribution.

Errors up to +/- 30 mm in determining the COP with piezoelectric force plates have been reported in the literature. To compensate for these errors, correction formulas were proposed, based on measurements with single point loads. In this paper, it will be shown that the errors in the COP depend on the load distribution. Two examples are presented: (1) simulated balance study, and (2) different pressure patterns during walking. Accurate corrections can only be made for forces distributed over a small area. Errors are expected to be overcompensated if there are only a few pressure peaks separated by large distances. These errors can be as large as the statistical errors (5.8 +/- 3.7 mm) after compensation. For certain situations, it is probably better not to use correction formulas.

Parameters influencing the accuracy of the point of force application determined with piezoelectric force plates.

Errors up to +/- 30 mm in determining the point of force application with piezoelectric force plates have been reported in the literature (Kistler, 1984. Multicomponent Measuring Force Plate for Biomechanics and Industry. Kistler, Switzerland; Bobbert and Schamhardt, 1990. Journal of Biomechanics 23, 705-710; Sommer et al., 1997. Proceedings of the XVI th I.S.B. Congress). To explain the main factors influencing the systematic errors the force plate system is modeled as a two-dimensional beam structure. By this model it is strongly indicated that the cause for the errors in determining the point of force application are bending moments in the measurement posts. The main parameters influencing the shape and magnitude of the error function are the ratios between the bending stiffness of the plate and the bending and compressive stiffnesses of the measurement posts. In the current design it is therefore not possible to eliminate the cause for the errors by changing the constructive parameters. By comparing the error functions derived with the beam model to the correction formulas given in the literature an improved algorithm is proposed. This paper shall help biomechanists in understanding the basic concepts of determining the point of force application with force plates and in constructing custom-made force plates for specific applications.

Computer simulation of occlusal discrepancies resulting from different mounting techniques.

The effect of arbitrary mounting of maxillary casts on occlusal relationships was investigated in this study. Maxillary casts of 31 volunteers were mounted on an articulator by use of two split cast bases. This mounting was done first with the arbitrary face bow and second with a hinge bow. Three reference points were defined and measured on each maxillary cast with a three-dimensional digitizer. The measurements were taken from the arbitrarily mounted cast and from the cast mounted according to the hinge axis. Opening and closing movements that were transferred according to the hinge axis. Opening and closing movements that were transferred from the articulator to the mouth of the patient were simulated by a computer based on measurements of the reference points. The results revealed that the use of an arbitrary face bow causes a deviation of the hinge-axis points from the precise axis of more than 5 mm in 77% of the cases. Resulting occlusal errors depended on the angles between the arbitrary and precise axes and the direction of the axis shifts. The occlusal error is roughly proportional to the shift or tilting of the hinge axis in millimeters or degrees. For a given deviation of the arbitrary and precise axes, the occlusal error is proportional to the record height. For a record height of 2 mm or more, an occlusal error of more than 0.1 mm will occur. An average occlusal error of more than 0.1 mm would most likely lead to the necessity of extensive selective grinding of occlusal discrepancies in the patient's mouth.