Reverse shoulder prosthesis and related methods

A prosthetic joint assembly for joining a humerus bone to a scapula bone, the humerus bone having a humeral head radius distance, the humerus bone having a natural center of rotation (nCOR) with respect to the scapula bone, the prosthetic joint assembly comprising:

• a humeral component having two opposite ends, the first end comprising a humeral stem adapted for rigid engagement with the humerus bone and the second end comprising a concave dish;
• and
• a scapular component having two opposite sides, the first side comprising a scapular baseplate adapted for rigid engagement with the scapula bone, the second side comprising a convex surface adapted to engage the concave dish;
• wherein the geometry of the humeral component and the scapular component is such that when the humeral stem is engaged with the humerus bone, the scapular baseplate is engaged with the scapula bone, and the concave dish and the convex surface are engaged, the humerus bone freely swivels with respect to the scapular component about a prosthetic center of rotation (pCOR), and the humerus bone is shifted to a relocated position with respect to the scapula bone;
• and
• wherein the geometry of the humeral component and the scapular component is such that the relocated position of the humerus bone with respect to the scapula bone is fixed by (a) calculating a humeral reference point in a direction that is 30 to 60 degrees above horizontal and medial to the nCOR and a distance from the nCOR approximately equal to between 60% and 80% the humeral head radius distance;
• (b) calculating a scapular reference point that is in a direction from the nCOR that is approximately normal and medial to the direction of the humeral reference point and at a distance from the nCOR approximately equal to between 60% and 80% the humeral head radius distance;
• (c) moving the humerus bone so that the humeral reference point is superimposed on the scapular reference point and establishing the pCOR at the scapular reference point.

A baseplate for engagement with a scapula bone, the scapula bone having a glenoid cavity, the baseplate comprising:

• a vertical dimension;
• a first side comprising a mating surface with a scapular stem, the mating surface adapted to cooperate with the glenoid cavity, the scapular stem adapted for insertion into, and rigid engagement with, the scapula bone;
• and
• a second side opposite of the first side comprising a trunnion offset inferiorly relative to a center of the vertical dimension, the trunnion being adapted to engage with a glenosphere component.