Joint prosthesis with positionable head

1. A prosthesis assembly, comprising:

a stem configured to be implanted in a bone, the stem including a first coupling portion;

a head having a bearing surface configured to mate with at least one of a natural opposing joint component and a prosthetic opposing joint component, the head further having a second coupling portion;

a coupler including a third coupling portion and a fourth coupling portion, the third coupling portion configured to couple with the second coupling portion; and

an insert including (i) a fifth coupling portion configured to couple with the fourth coupling portion so that the insert may assume any one of a plurality of orientations with respect to the coupler, and (ii) a sixth coupling portion configured to couple with the first coupling portion only when the insert assumes a predetermined orientation with respect to the stem,

wherein the first coupling portion comprises a stem receptacle extending inwardly from a proximal end which opens to a surface of the stem, to a closed distal end, the first coupling portion including a first key member and a tapered wall structure between the proximal end and the distal end;

wherein the fourth coupling portion comprises a bulbous portion;

wherein the fifth coupling portion comprises a first insert receptacle extending inwardly from a distal end which opens to a surface of the insert, to a closed proximal end, the first insert receptacle including a tapered wall structure between the proximal end and the distal end;

wherein the sixth coupling portion comprises a tapered wall structure and a second key member configured to mate with the first key member,

wherein (i) the stem receptacle is defined by a receptacle structure, (ii) a distal end portion of the receptacle structure includes a distal interior surface, and (i) the distal interior surface of the receptacle structure defines the first key member, and

wherein (i) a distal end portion of the insert includes a distal exterior surface, and (ii) the distal exterior surface of the insert defines the second key member.

2. The prosthesis assembly of claim 1 , wherein:

the second coupling portion comprises a head receptacle extending inwardly from a distal end which opens to a surface of the head, to a closed proximal end, the head receptacle including a tapered wall structure between the proximal end and the distal end; and

the third coupling portion comprises a tapered wall structure.

3. The prosthesis assembly of claim 2 , wherein:

the tapered wall structure of the first coupling portion has a taper angle of about 10 degrees;

the tapered wall structure of the second coupling portion has a taper angle of about 5 degrees;

the tapered wall structure of the third coupling portion has a taper angle of about 5 degrees; and

the tapered wall structure of the sixth coupling portion has a taper angle of about 10 degrees.

4. The prosthesis assembly of claim 1 , wherein the coupler further comprises:

a neck portion adjacent to the fourth coupling portion; and

a shoulder portion adjacent to the third coupling portion.

5. The prosthesis assembly of claim 4 , wherein:

the stem receptacle has a first longitudinal axis;

the coupler has a second longitudinal axis; and

the fifth coupling portion is configured to couple with the fourth coupling portion such that the second longitudinal axis is oriented at any of a plurality of angles with respect to the first longitudinal axis, and

the angle has a magnitude between about 0 degrees and about 30 degrees.