Hinged external fixation of the elbow: optimal axis alignment to minimize motion resistance.

OBJECTIVE To establish an optimal single hinge axis position for application of hinged external fixation to the elbow joint. DESIGN Cadaveric biomechanical investigation. SETTING A customized motion transducer applied passive elbow motion to six cadaveric upper extremities. The instant rotation axis of the humero-ulnar articulation was determined from three-dimensional kinematic data acquired by an electromagnetic motion tracking system. For each specimen, an optimal fixator hinge position was calculated from these motion data. INTERVENTION A prototype articulated external fixator was applied to the elbow, first with its hinge aligned along the computed optimal position. Then the fixator was mounted in sixteen distinct off-axis positions. MAIN OUTCOME MEASURE Additional resistance to joint motion (in terms of energy) corresponding to deliberately introduced amounts of relative malalignment between the optimal elbow axis and the actual fixator hinge axis. RESULTS Aligning the fixator hinge along the optimized axis position resulted in a minimal amount of energy (0.15 joules) needed to rotate the elbow through a prescribed range of motion. Malpositioning the hinge by ten millimeters caused up to ten times that amount of motion resistance. CONCLUSIONS An optimal fixator hinge position can be determined to minimize the increase in motion resistance due to fixator application. The severely increased motion resistance associated with small amounts of malalignment between the fixator hinge and the anatomic elbow axis suggests the need for highly accurate fixator hinge application.