Humeral Positioning Apparatus

In designing the humeral positioning apparatus, a number of options were considered. Debski et al. (1999) designed an industrial robot based system, and Walker and Dickey

(2007) designed a custom hexapod robot system for spinal testing; however, neither type of system can decompose the glenohumeral joint’s motion into interpretable DOF during testing, and both would unduly complicate the replication of clinical assessments. Additionally, the hexapod concept could not accommodate the full range of motion of the shoulder.

With these options eliminated, a mechanical rather than mechatronic concept was pursued. Therefore, in order to satisfy the requirement of decomposing the shoulder’s motion into individual DOF, an apparatus with three distinct mechanisms (Figure 2.1) was developed to isolate each of the three rotations: axial rotation, abduction, and plane of abduction. The axial rotation DOF of the glenohumeral joint was isolated by implementing a mechanism which could mate to an intramedullary humeral rod – used in the previous version of the simulator (Kedgley, 2004) – and permit rotation about its long axis, which is coincident with the humeral axial rotation axis. A spherical bearing was selected for this component as it permitted axial rotation of the rod within the bearing and of the bearing within its casing (Figure 2.2). This bearing was also selected because it allowed the humeral rod to slide through it and incorporated two additional rotational DOF. Combining these three DOF at the rod-bearing interface enabled the humeral head – at the opposite end of the humeral rod – to freely translate in all three directions, and thus prevented the humeral positioning apparatus from having an undesirable influence on glenohumeral kinematics. This is particularly important in the event that a misalignment is present between the shoulder specimen and the apparatus as discussed below.

A mechanism to control the abduction DOF was then designed by connecting the spherical bearing to a slider mechanism that moved along a hemispherical arc connected to the existing simulator base (Figure 2.3). The slider mechanism was designed to allow the center of the spherical bearing to reach full adduction (axis of the bearing directed vertically) while also enabling it to move to a position which would simulate 130° of humeral abduction (Figure 2.4). The slider was also designed with a clamping mechanism

Figure 2.1: The humeral guide arc.

Note the slider mechanism with attached spherical bearing positioned for 90_° of humerothoracic abduction. In the bottom left image, also note the dashed line indicating the vertical hinge which controls the plane of abduction. (A) Spherical bearing, (B) slider mechanism, (C) humeral abduction guide arc, (D) vertical hinge for plane of elevation control.

Figure 2.2: Spherical bearing used to mate humeral rod to guide arc.

The degrees of freedom permitted by the bearing are (A) axial translation, (B) axial rotation, (C) two transverse rotations.

to enable the slider, bearing, and humerus to be locked at any level of abduction without influencing/constraining glenohumeral articular kinematics.

The final rotational DOF of the shoulder, plane of elevation, was also controlled through the slider and arc mechanisms by connecting the arc to the simulator base using a vertical hinge construct instead of a rigid connection. The design of the hinge mechanism enabled the humerus to be rotated by greater than 130° anterior and 45° posterior to the scapular plane, thus enabling the entire range of glenohumeral horizontal flexion-extension to be assessed (Figure 2.5) (Itoi, Morrey, & An, 2009). Therefore, in the same way that the Euler angle sequence for the glenohumeral joint defines the plane of elevation, and then abduction occurs about an axis perpendicular to that plane, rotation of the arc about the vertical hinge defines the abduction plane, and the level of abduction is defined by the slider’s position. This hinge mechanism was also designed to lock in place once the correct plane was achieved, or be left free to vary depending on the assessment being performed. To ensure the center of the glenohumeral joint could be properly aligned with the vertical hinge, the components which mate the arc to the base were designed so that the axis of the hinge lay at a distance off the simulator base that was approximately equal to half the distance between the posterior acromion and the anterior coracoid. Additionally, as discussed above, the spherical bearing permits three DOF between the humeral guide arc apparatus and the intramedullary humeral rod, thus mitigating any effects of misalignment of the vertical hinge.

Each of these mechanisms serves two roles, first to aid in the accurate orienting of the joint in its associated DOF, and second to physically resist motion in that DOF once the experimenter has specified its desired value. By combining the effect of these three components, it is possible to position the glenohumeral joint in any configuration while also allowing any individual rotational DOF to vary independently. Thus, this design enables the assessment of any conceivable position or motion and allows these assessments to be carried out in a clinically interpretable manner because the DOF of the apparatus coincide with the shoulder’s physiologic rotations.

Figure 2.3: Humeral guide arc mounted to existing simulator base

(A) Existing simulator base, (B) hinged connecting plates to mate the guide arc to existing simulator. The coloured lines: (red) the internal-external rotation DOF controlled by the bearing, (green) the abduction DOF controlled by the slider, and (blue) the plane of elevation DOF by the vertical hinge plates.

Figure 2.4: Simulator with humeral guide arc and scapular potting and rotation mechanism

In the left image, the two mechanisms are oriented for 0° of abduction while the right image shows them in their maximal abduction orientation which produces a 2:1 glenohumeral-to-scapulothoracic rhythm.

Figure 2.5: The humeral guide arc viewed from above

The range of horizontal flexion-extension permitted by the apparatus is shown. Top, maximal horizontal extension position; middle, position corresponding to humerus in scapular plane; bottom, maximal horizontal flexion position.