UVM Theses and Dissertations
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.