Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 33

Major factors in materials design and selection for the femoral stem are mechanical properties, biocompatibility, and ease of fabrication. Mechanical properties are important because the stem supports physiological stresses imposed on the femoral head due to walking, running, and jumping, composed predominantly of repeated cyclic stresses of a compressive and bending nature. The peak force on the hip joint has been measured as three and five times the body weight during walking and running, respectively, resulting in a stress on the femur of an adult of ~10–15 MPa. Additionally, the number of gait cycles in one year is ~10⁶ for patients after hip joint replacement. At a minimum, therefore, the selected material should have compressive and bending strengths far in excess of the maximum stress on the femur, high elastic modulus to prevent permanent deformation, and good fatigue resistance to withstand repeated cyclic loading under physiological stresses. These requirements eliminate the use of ceramics, polymers, and polymer matrix composites.

Because hip implants are designed to have a long lifetime, the stem should maintain its mechanical properties and biocompatibility with the surrounding physiological environment over this period. This means that the selected metal should not corrode in the physiological environment. Because the noble metals do not have the requisite mechanical properties, are expensive, or a combination of the two, we are left with the passivated metals. Candidate metals are Ti, certain Ti alloys, stainless steel, and a Co–Cr alloy composed of cobalt (Co), chromium (Cr) and other elements. While high purity or even commercial purity Ti does not have the requisite mechanical properties for this application, a Ti alloy that is widely used in the aerospace and other industries is known to have far better mechanical properties and it is also corrosion resistant. This Ti alloy, containing ~6% aluminum (Al) and ~4% vanadium (V) by weight, is commonly abbreviated Ti6Al4V. While stainless steel and Co–Cr have desirable mechanical properties and corrosion resistance, their densities are approximately twice the value for Ti6Al4V (~4.4 g/cm³). Based on a combination of mechanical property requirements, corrosion resistance, and weight, femoral stems are now manufactured almost exclusively formed from Ti6Al4V.

Upon implantation, the stem is stabilized (that is, fixed in position) to prevent motion relative to the femur. One method of fixation involves using a polymer (PMMA) cement, often referred to as bone cement. While this cement works well and is commonly used in hip joint surgery, it suffers from a few limitations such as minor toxicity, temperature increase upon setting which can have an adverse effect on the surrounding tissue, and some degree of brittleness, particularly after an extended period.

An alternative is to roughen the surface of the stem to allow bone to grow into the rough regions and stabilize the implant. Another method is to coat the roughened surface with a layer of hydroxyapatite particles. Hydroxyapatite is bioactive and stimulates cells to create new bone, leading to the formation of a strong bond at its interface with host bone. However, due to the difference in thermal expansion coefficient between hydroxyapatite and Ti6Al4V, a strongly adherent and long lasting hydroxyapatite coating on Ti6Al4V has been difficult to achieve. Overall, femoral stems that are not cemented in place often have a rough surface, or a rough surface coated with a hydroxyapatite layer.