Shoulder and Elbow - Shawn W. O'Driscoll

This image shows the micromotion machine and its experimental setup (left) and a geometric model used to convert plate motion to stem micromotion (right).

The tilt was assumed to occur at the center of the component. In a right triangle, the tilting angle (α) was calculated from the laser displacement (D), with sin α = D/L. In an isosceles triangle, the stem micromotion at the level of isthmus (dis) and micromotion at the stem tip (dt) were calculated from α (sin α/2 = [dis/2]/lis = [dt/2]/lt).

L is the radius of the plate (45 mm), lis is the length from the center to the isthmus (13 mm) and lt is the length of the stem (25 mm)

This image shows the opposite effects of eccentric loads or subluxation on monopolar and bipolar radial heads.

The bipolar head tilts under such conditions. Once tilted, the compressive force on the bipolar radial head can be broken down into two force vectors, one of them being tangential to the joint surface. This tangential force facilitates subluxation.