Development and evaluation of a method and apparatus to measure shoulder instabilities
Creators: name: Lecarpentier, Karine
Publisher: name: University of British Columbia
Year: 2009

description: The shoulder is now subject to a rapidly increasing number and variety of surgical procedures. However, the results of these procedures in terms of improved patient life are subjectively reported. No precise, objective and non-invasive method is currently available to assess the dynamic function of the shoulder. The two major bones involved in the shoulder are the scapula (shoulder blade) and the humerus (upper arm). The extremity of the scapula in contact with the humeral head is called the glenoid. Together they form the glenohumeral joint. The articulating surface of the humeral head is spherical. The glenoid is congruent to the humeral head, but its small dimension allows small translations to occur. If translations become too large (typically 2 or 3 cm), the joint can dislocate, damaging surrounding tissues. The shoulder is then said unstable. Stability and mobility of the shoulder can be characterized by the amount and direction of translations in the glenohumeral joint. The goal of this thesis was to develop a system that gives a quantitative and precise assessment of shoulder laxity at the time of diagnosis, and again after surgical intervention. This system should be non-invasive, objective, relatively inexpensive, easy to use and to set up, and should fit any sized patient. It should give quantitative information and be repeatable and accurate within 1 or 2 mm. The system we developed measures shoulder translations using the trajectory of the elbow relative to the shoulder. An electro-magnetic sensor was incorporated in the system to measure the position of the elbow and the position of a reference point on the shoulder (acromion). We modeled the humeral head as a sphere and the glenoid as a small planar surface. From this model, we developed an algorithm that measures shoulder translations, assuming that the distance R between the sensor on the elbow and the center of the humeral head, and the orientation O of the glenoid are known. A method was developed to assess the accuracy and calibration of the electro-magnetic sensor. The maximum error was 1.5 mm. We validated the' system on an artificial model. On patients, the system showed a significant difference in the shape of the translation curve between the stable and unstable shoulder of a patient with serious shoulder instability. However, the rotation of the scapula, that we neglected, induced significant errors and prevented any useful quantitative information. The system as it stands now is a useful tool to supplement existing methods, but additional work is necessary for the system to be used routinely and replace existing methods. In particular, a more precise tracking of the scapula is necessary to measure its rotation. Immobilization of the scapula may be an alternate solution. Some suggestions were made to improve the accuracy of the electro-magnetic sensor. A better estimation of the parameter R may be obtained from radiography. Once the system gives the required quantitative information, the system will contribute to improve objectivity and accuracy in assessment which we trust will improve outcomes in shoulder surgery. The system could be adapted to measure other joint instabilities such as the knee and the hip and be the base for the development of other technologies for general improved patient outcome.