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

Objective: To establish an optimal single hinge axis position for applicati on 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 optima l fixator hinge position was calculated from these motion data. Interventio n: A prototype articulated external fixator was applied to the elbow, first with its hinge aligned along the computed optimal position. Then the fixat or was mounted in sixteen distinct off-axis positions. Main Outcome Measure: Additional resistance to joint motion (in terms of en ergy) corresponding to deliberately introduced amounts of relative malalign ment between the optimal elbow axis and the actual fixator hinge axis. Results: Aligning the fixator hinge along the optimized axis position resul ted 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 milli meters caused up to ten times that amount of motion resistance. Conclusions: An optimal fixator hinge position can be determined to minimiz e the increase in motion resistance due to fixator application. The severel y increased motion resistance associated with small amounts of malalignment between the fixator hinge and the anatomic elbow axis suggests the need fo r highly accurate fixator hinge application.