EPOCH® Hip Prosthesis
The EPOCH composite hip stem was developed as a fully porous coated bone ingrowth stem that could address issues in stress shielding. The stem has a porous surface to allow extensive fixation of the device in the femur with living, renewing tissue. In addition it is made of materials that closely match the stiffness of bone. Although traditional porous coated stems have generally had a good record of clinical success, the phenomenon known as stress shielding presents a challenge to the surgeon. Stress shielding can occur when the femoral implant is much stiffer than the femur. The implant “shields” the bone from the load placed on the implant head by the body. This bone, not being loaded as it would be in an intact femur, begins to resorb. Matching the stiffness of the implant to the stiffness of bone, as in the design of the EPOCH composite hip stem, reduces the potential for stress shielding.
To understand more about stress shielding, a simplified explanation of the anatomy and biomechanics of the femur/pelvis joint is in order. The natural joint is a ball and socket joint with the head of the femur as the ball and the acetabulum in the pelvis serving as the socket. The load from the pelvis is placed onto the head of the femur. The head of the femur is adapted to carry and transfer the load through the cartilage on the head to the more dense cortical bone of the femoral shaft.
The operated femur has very different loading conditions. The implant is now inside the shaft of the femur. The body load is still applied to the implant head but the load is now transferred from the implant, inside the femur, through the interface between the bone and implant, to the femur.
Earlier generations of hip stems were designed to be strong and to fill the relatively large cavity prepared in the femur. The result of these characteristics, in most cases, was a very stiff hip stem, much stiffer than the femur. Stems with distal end diameters smaller than 14 mm are about as stiff as a normal femur, but larger diameter stems are much stiffer. With increasing stiffness, more of the load is transferred to bone, more and more distally. Stems made from Tivanium® Ti-6A1-4V alloy with distal end diameters smaller than 14 mm are about as stiff as a normal femur. However, the larger diameter Tivanium alloy stems and all of the cobalt-chrome alloy stems are much stiffer.
The EPOCH composite hip stem was designed to have flexibility similar to the femur. This chart compares the stiffness of EPOCH hip stems (sizes 14 mm through 18 mm) to the stiffness of a normal femur. Also shown is the stiffness that the EPOCH hip would have if it were made wholly of Tivanium alloy or cobalt-chrome alloy.
One can easily see that the EPOCH composite hip stiffness is more like a normal femur. The EPOCH composite hip stem has been tested in clinical trials around the world. It has performed well and is currently offered for sale worldwide.
Design and Manufacturing
The EPOCH composite hip stem had to be built differently than any other hip stem.
The EPOCH composite hip stem design is patented in the United States and in foreign countries. This figure shows a cross section of the hip stem. The central portion, called the core, is made from a cobalt-chromium-molybdenum alloy forging. The forging is machined to shape and prepared for further processing. The outer layer of the implant is a mesh of commercially pure titanium fiber metal. The middle portion of the implant is polyaryletherketone, a high strength thermoplastic material. The stem is assembled using an injection molding process.
The central portion is tapered to provide flexibility distally and strength proximally. The fiber metal is porous to provide room for bone to grow into the implant, helping to lock the implant in place. This means of fixation also helps for load transfer from implant to bone.
