NON-INVASIVE DETERMINATION OF SHOULDER KINEMATICS IN HEALTHY AND ABNORMAL SHOULDERS
Participants: J.E. Carpenter, R.E. Hughes, L.J. Huston, S. LaScalza, A. Mell, C. Hadgis
Keywords: shoulder, kinematics, instability, cuff tear, tendinopathy
Introduction
Movements of the osseous structures of the shoulder girdle ("shoulder kinematics") are important factors in upper extremity function. It has been reported that these kinematics are altered in states of shoulder dysfunction including rotator cuff disorders and glenohumeral joint instability. Traditional methods for assessing shoulder joint motions included repeated radiographs and invasive monitoring. These techniques have generally been limited to the study of static postures. Technology has been developed that may accurately assess human kinematics non-invasively in real time using an electromagnetic tracking device. This project seeks to establish the feasibility of using a non-invasive method for assessing shoulder kinematics and apply the method to clinical populations. Shoulder kinematics during functional tasks will be assessed in patients having rotator cuff tendinopathy, full-thickness rotator cuff tears, and anterior instability. Kinematics will be assessed by computing the helical axis of rotation of the humerus (Woltring et al., 1994). There are two testable hypotheses based on the measurement of the helical axis:
H1: Estimates of helical axis location and orientation using non-invasive and direct fixation techniques are identical; and
H2: The location and variability in the helical axis of the humerus is altered in shoulders with rotator cuff dysfunction and/or shoulder instability when compared to normal.
Materials and Methods
Two experiments will be conducted, and each will address one of the study hypotheses. The first experiment will determine the validity of the non-invasive electromagnetic measurement method by comparing it to direct measurement of humeral kinematics in vitro. The second experiment will evaluate the helical axis location in healthy and pathologic shoulders in vivo.
Experiment 1. Ten cadaver arms will be tested. Each test specimen will consist of the upper extremity disarticulated from the torso at the scapulothoracic and sternoclavicular articulations. Arm motion will be measured using an electromagnetic tracking device (MotionStar, Ascension Technologies, Burlington, VT). The MotionStar system consists of a transmitter that emits a DC pulse electromagnetic field and sensors that measure position and orientation with respect to the transmitter. Motion data will be collected at 100 Hz per channel. Three MotionStar sensors will be used. One will be attached to a rigid splint on the distal humerus. The splint will be adjustable to securely fit the medial and lateral humeral epicondyles. Since the splint will have to make contact with the skin over the epicondyles, the splint will maintain the elbow in a 90 o flexed posture. A second sensor will be rigidly attached to the anterior surface of the mid-diaphysis of the humerus using plastic screws. A third sensor will be attached to the medial border of the scapula using plastic screws. A test fixture will be constructed from plexiglas that will serve as a guide for controlling arm motions.
The scapula will be rigidly fixed to the test fixture using plastic screws, and MotionStar sensors will be attached to the humerus and scapula. The elbow splint and associated MotionStar sensor will be placed on the elbow. The arm will be moved through three simple motions by the hand of the experimenter: abduction, flexion, and external rotation. The specimen will also be moved through five additional arcs of motion simulating the reaching motions to be conducted by patients in the second experiment.
The helical axis of rotation of the humerus will be computed from the MotionStar sensors on the elbow splint and directly on the diaphysis of the humerus. The helical axis computed from the sensor mounted directly on the humerus will be considered to be the gold standard. The helical axis computed from the MotionStar sensor on the elbow splint is the quantity to be compared to the gold standard. The difference between the two helical axes will be used to assess the accuracy of the non-invasive method based on the elbow splint.
Experiment 2. A cross-sectional study design with four experimental groups will be used: (Group 1) healthy volunteers, (Group 2) patients having rotator cuff tendinopathy (without full thickness tears), (Group 3) patients having full-thickness rotator cuff tears, and (Group 4) patients having recurrent anterior instability of the shoulder. The inclusion criteria for Group 1 will be a negative history of shoulder dysfunction and a negative physical exam. Inclusion in Group 2 will be limited to patients diagnosed with chronic (more than 3 months) rotator cuff tendinopathy, but without a full thickness tear as seen during ultrasonographic examination. Inclusion in Group 3 will be limited to patients diagnosed with a chronic (more than 3 months) full thickness rotator cuff tear greater than 1 cm2 in size as seen during ultrasonographic examination
Testing will be conducted at MedSport. The subject will be seated on a padded wooden examining table. MotionStar sensors will be placed on the subject. The subject will perform five reaching motions: raising the hand from a point anterior and lateral to the anterior superior iliac spine; reaching from the anterior mid-sagittal plane to a point above and behind the shoulder; reaching from the anterior mid-sagittal plane to a point below and behind the shoulder; overhand throw; and across the body from contralateral to ipsilateral side. The exact locations of the beginning and end of each reach will be specified by a colored ball on a test fixture. Abduction, flexion, and external rotation will also be performed.
Helical axes will be computed and smoothed using generalized cross-validated splines (Woltring et al., 1994). The helical axes will be represented in the torso coordinate system. The intersection of the helical axes with the frontal, transverse, and sagittal planes will be computed.
Progress
The project has been funded and approved by the Institutional Review Board. Experiment 1 has been completed.
References
1. Woltring, H.J., Long, K., Osterbauer, P.J., and Fuhr, A.W. (1994) Instantaneous helical axis estimation from 3-D video data in neck kinematics for whiplash diagnostics. Journal of Biomechanics 27: 1415-1432.