Abstract
Clinical assessment of elbow deformity in children at present is mainly based on physical examination and plain X-ray images, which may be inaccurate if the elbow is not in fully supination; furthermore, the rotational deformity is even harder to be determined by such methods. Morrey suggested that the axis of rotation of the elbow joint can be simplified to a single axis. Based on such assumption, we are proposing a method to assess elbow deformity using rotational axis of the joint, and an optimized calculation algorithm is presented.
The rotation axis of elbow in respective to the upper arm can be obtained from the motion tract of markers placed at the forearm. Cadaver study was done, in which three skeletal motion trackers were placed over both the anterior aspect of humerus, as well as distal ulna. Osteotomy was created at the supracondylar region of humerus through lateral approach, and the bone fragments were stabilized with a set of external skeletal fixator, leaving the soft tissue intact. The amount of deformity was created manually by adjusting the position of the distal fragment via skeletal fixator. Ultrasound 3D motion tracking system from Zebris® was used in this study, and the program was developed under the Matlab environment. Cycles of passive elbow flexion/extension motion were carried out for each set of deformity. The data were initially transformed to humerus coordinate, and since the upper arm was not absolutely stationary, its influence on the measured position of ulna was adjusted. With this adjusted data, a best fit plane that would include most of the ulna positions (MU) within a minimal distance was obtained. The rotation axis was calculated as the normal vector to this plane, and the carrying angle could subsequently be assessed according to the relationship between this axis and the x-axis on the xy-plane as well as on the xz-plane.
Fresh frozen cadaver study was conducted in the Medical Simulation Center at Tzu-Chi University. After adjustment of the raw data to eliminate the influence of humerus motion, the ulna motion could be narrowed down from a band of 10mm to 3mm, with a significant smaller standard deviation. The rotation axis was obtained by the normal vector to the best fit plane. Two different approaches were attempted to find the plane. In the first method, the plane was obtained via least square method from the adjusted ulna positions, and the second method found the plane via RANSAC method. Calculations were repeated several times for each method, and the results showed a variation of 5 degrees in the first method and about 2 degrees in the second method.
Rotational axis can be used to define the 3-dimensional deformity of elbow joint; however, it is difficult to obtain such axis accurately due to hypermobility and multi-directional motion of the shoulder joint. In this study, we have developed another method to assess the elbow deformity using motion analysis system instead of the conventional image studies, and this may be applicable clinically if the facility becomes more accessible in the future. (This research was supported by the project TCRD-TPE-99-30 granted by the Buddhist Tzu-Chi General Hospital, Taipei Branch).
