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Paller, David
Biomedical Engineering Program
Objective (1): The primary objective of this investigation involved determining the accuracy of uniplanar RSA measurements in a system that was subject to a combination of rotations and transhtions. It was hypothesized that motion of a reference based on a randomly placed bead cluster (off axis) would produce different results compared to a reference that coincided with the axes (on axis) of the calibration systen. It was also hypothesized that mathematically transfening the off axis cluster to coincide with the centroid of the on axis cluster would improve accuracy. Design: The testing device involved an Actylite cube within a gimbaled fixture mounted to a rotary positioning stage and linear translation stage. Within the cube, two separate clusters of metal beads were implanted. Each bead cluster represented a different rigid body, one coinciding with the center of rotation of the cube and the other arbitrarily off-axis. This cube was configured with varying degrees of orientation in two planes. This facilitated accuracy testing of the off and on axis reference systems during combined rotation and translation using the UmRSA software. Outcome Measurements: Error values were calculated by subtracting the measured value calculated from UmRSA from the "gold standard" value (actual translation and/or rotation of the testing system). Average error values were compared between the on axis, off axis, and mathematically transferred off axis bead clusters. Results: The average error values determined for translations were ±165, ±307, and ±238 micrometers in the on axis bead cluster, ±3434, ±5073, and ±10183 micrometers for offaxis cluster, and ±425, ±333, and ±33 micrometers for the transferred off-axis cluster along the x, y, and z-axes respectively. The average rotation error values were recorded as ±0.3°, ±0.34°, and ±0.092° for the on axis cluster and ±0.35°, ±0.32°, and ±0.034° for the off axis and transferred bead clusters about the x, y, and z axes. Conclusion: The accuracy of the data from the on axis cluster located within the Acrylite block produced accuracy measurements from UmRSA similar to previous reports in the literature. The accuracy values obtained for the translations of the off axis bead cluster were substantially greater than normal accuracy measurements reported with the use of RSA. Upon performing a point transformation of the centroid of the off-axis bead cluster, mean error values were greatly reduced to values similar to those reported in the literature.
Objective (2): The objective of this study was to measure the six degree of freedom kinematics of distal femoral fractures instrumented with different methods of distal fixation coupled with a retrograde intramedullary nail during catastrophic loading. Design: Forty (20 matched pair) osteopenic fresh frozen cadaveric femora were randomly assigned into two groups of paired specimens; locked versus unlocked and locked versus washer. The first type of distal fixation was the standard statically instrumented nail ("unlocked"). The advanced locking fixture contained two transverse fully threaded screws locked directly to the intramedullary nail by an advanced compression setscrew and an endcap ("locked"). The final type of fixation consisted of a proximal fully threaded transverse screw in conjunction with a bolt affixed with condylar washers of each end ('washer") at the distal screw site. This method of fixation was affixed directly to the intramedullary nail in the same fashion as the advanced locking system. Intervention: All instrumented femurs were mounted on a servo-hydraulic testing machine and tested in axial compression at a rate of three millimeters per minute until catastrophic failure occurred or the limit of 3000 Newtons was reached. Outcome Measurements: Load at clinical failure was determined for each group. Resultant translations and angulations of all construct components (distal and proximal bone locks, screws, nail) were measured using RSA. Results: The mean load at clinical failure for the lecked group was 1338 ± 822 N and 1738 ± 953 N for the washer group (p = 0.066). No significant differences existed between (p = 0.60) the mean varus angulation of the washer group (4.41 ± 2.82) and the locked group (3.79 ± 1.36°). The mean load at clinical failure for the unlocked group was 1165 ± 772 N versus 1609 ± 667 N for the leeked group (p = 0.09). The mean yarns angulation at clinical failure in the unlocked group was 5.41 ± 3.20° and 5.2 ± 2.8° in the locked group (p = 0.89). Conclusion: Upon completing this study with the RSA system, it can be concluded that the failure mechanism for a supracondylar fracture can not be analyzed accurately with a one dimensional measurement. The most common failure mechanism was medial translation and varus angulation. Although statistically significant differences were not found between the clinical failure load values for each type of fixation, possible clinically significant differences might exist. Key Words: Distal femur fractures, Intramedullary nail, Distal locking setscrews, RSA, Accuracy, Cadaveric Bone.