Measuring dynamic in-vivo glenohumeral joint kinematics: technique and preliminary results

Abstract

Rotator cuff tears are a common injury that affect a significant percentage of the population over age 60. Although it is widely believed that the rotator cuff's primary function is to stabilize the humerus against the glenoid during shoulder motion, accurately measuring the three-dimensional (3D) motion of the shoulder's glenohumeral joint under in-vivo conditions has been a challenging endeavor. In particular, conventional motion measurement techniques have frequently been limited to static or two-dimensional (2D) analyses, and have suffered from limited or unknown in-vivo accuracy. We have recently developed and validated a new model-based tracking technique that is capable of accurately measuring the 3D position and orientation of the scapula and humerus from biplane X-ray images. Herein we demonstrate the in-vivo application of this technique for accurately measuring glenohumeral joint translations during shoulder motion in the repaired and contralateral shoulders of patients following rotator cuff repair. Five male subjects were tested at 3-4 months following arthroscopic rotator cuff repair. Superior-inferior humeral translation was measured during elevation, and anterior-posterior humeral translation was measured during external rotation in both the repaired and contralateral shoulders. The data failed to detect statistically significant differences between the repaired and contralateral shoulders in superior-inferior translation (p=0.74) or anterior-posterior translation (p=0.77). The measurement technique overcomes the limitations of conventional motion measurement techniques by providing accurate, 3D, in-vivo measures of glenohumeral joint motion during dynamic activities. On-going research is using this technique to assess the effects of conservative and surgical treatment of rotator cuff tears.

Figure 1

The humerus and scapula coordinate systems were constructed from the CT models according to the convention recommended by the International Society of Biomechanics (Wu et al. 2005). Translations and rotations of the humerus were expressed relative to the scapula coordinate system. A/P = anterior-posterior axis, M/L = medial-lateral axis, S/I = superior-inferior axis.

Figure 2

Superior-inferior glenohumeral translation of the repaired and contralateral shoulders during coronal-plane elevation. Data for three trials for each subject were averaged, and then the averages and standard deviations across the five patients are plotted here. Positive values indicate superior translation of the humerus relative to the glenoid. Data are reported relative to glenohumeral elevation angle (i.e., the angle between the humerus and scapula) and not humerothoracic angle (i.e., the angle between the humerus and torso). Assuming a scapulohumeral rhythm of 2:1, these data are presented over a range that corresponds to humerothoracic angles of approximately 15° to 105°.

Figure 3

Anterior-posterior translation of the repaired and contralateral shoulders during external rotation. Data for three trials for each subject were averaged, and then the averages and standard deviations across the five patients are plotted here. Positive values indicate anterior translation of the humerus relative to the glenoid.