1. Field of the Invention
The present invention relates to orthopedics, and, particularly, to orthopedic implants.
2. Description of the Related Art
During orthopedic surgery, such as a total hip arthroplasty, orthopedic prostheses are implanted into a patient's body. For example, a femoral stem may be implanted into the intramedullary canal of the patient's femur. After the stem is inserted into the intramedullary canal at a desired depth and orientation, a femoral head may be attached to the proximal end of the stem. The femoral head fits into a substantially hemispherically shaped socket of an acetabular prosthesis and provides a surface for articulation between the femoral head and the acetabular prosthesis.
In certain embodiments, acetabular prostheses comprise two separate components, an acetabular backing component and an acetabular articulating component, which are often referred to as a shell and liner, respectively. The backing component is generally hemispherically shaped and is affixed and embedded into an acetabulum of a patient. Similarly, the liner has a hemispherical shape configured to mate with an internal cavity of the backing component. The liner further includes a socket to mate with a femoral head. The acetabular backing components are typically formed of solid metal, such as titanium or a titanium alloy, or from stainless steel. In contrast, the acetabular articulating components may be formed from an ultra-high molecular weight polyethylene polymer material (“UHMWPE”). These types of acetabular cups are often referred to in the art, and sometimes herein, as “metal backed” components.
Metal backed orthopedic components generally experience contact stresses during normal joint articulation. For example, in conventional metal backed acetabular components, previous studies have shown that polymer thickness and conformity between the femoral head and the polyethylene liner play a significant role in the level of contact stress experienced. For highly conforming acetabular component designs, the contact stress in the polymer liner is very sensitive to the polymer thickness when the polymer thickness is small. Specifically, it has been demonstrated that when the polymer thickness is less than 4-6 mm, the contact stress generally increases rapidly as the thickness decreases. Therefore, a minimum polymer thickness of 4-6 mm has been maintained for metal backed acetabular components in an attempt to lessen the contact stress. Further, having a 4 mm minimum thickness of the polyethylene liner in an acetabular component has been widely accepted as an industry design rule and has been traditionally relied upon in the manufacture of acetabular components.
Stress shielding is a phenomenon that can potentially occur with metal backed orthopedic components and can occur when a portion of the stress normally exerted on a patient's bone is instead borne by the orthopedic component, such as an acetabular cup. As result, the bone may begin to undergo atrophy and decalcification resulting in a weaker bone structure. In severe stress shielding, the bone may be resorbed by the body, decreasing the amount of bone stock at the fracture site.