Variable stiffness stem for prosthetic implants

A method for forming an implant for a bone, comprising the steps of:

• generating a first general shape for the implant having a length extending along a long axis of the bone;
• determining an area moment of inertia of a cross section of the implant, wherein the area moment of inertia of the cross section of the implant is calculated using a computer-aided design module;
• determining an area moment of inertia of a cross section of bone coplanar to the cross section of the implant, wherein the area moment of inertia of the cross section of bone coplanar to the cross section of the implant is calculated using the computer-aided design module;
• calculating a percent stress shielding from the area moments of inertia of the implant and the bone;
• comparing the calculated percent stress shielding to a preset threshold;
• and
• iteratively generating additional general shapes for the implant by locally adjusting the cross section of the implant until the calculated percent stress shielding meets the preset threshold;
• and
• wherein the percentage stress shielding is calculated using the equation:
• Percentage Stress Shielding=[1−(1/(1+N(IIZIB)))]*100;
• where N=ratio of modulus of elasticity=Eimplant/Ebone,
• II=area moment of inertia of implant, and
• IB=area moment of inertia of bone.

A system for creating an implant for implantation in a bone, comprising:

• a computer-aided design module;
• an initial representation of an implant member having an implant length configured to extend along the length of the bone, the implant member having cross sections along the implant length, the cross sections each having an area moment of inertia, wherein the area moment of inertia of the cross sections of the implant member are calculated by the computer-aided design module;
• bone information relating to the quality and dimensions of the bone, the bone information providing an area moment of inertia for a cross section of bone coplanar to a corresponding one of the cross sections of the implant member, wherein the area moment of inertia of the cross section of bone coplanar to the corresponding one of the cross sections of the implant member is calculated by the computer-aided design module, wherein a percent stress shielding using the area moment of inertia of the cross sections of the implant member and the area moment of inertia of the cross section of the bone coplanar to the corresponding one of the cross sections of the implant member is calculated and compared to a preset threshold, wherein the percentage stress shielding is calculated using the equation:
• Percentage Stress Shielding=[1−(1/(1+N(IIZIB)))]*100;
• where N=ratio of modulus of elasticity=Eimplant/Ebone,
• II=area moment of inertia of implant, and
• IB=area moment of inertia of bone;
• and
• an iterative representation of an implant member generated by locally adjusting the corresponding one of the cross sections of the implant member until the calculated percent stress shielding meets the preset threshold whereby the initial representation is changed to the iterative representation.