Total Hip Replacement, hereinafter it is only named as THR, becomes a more and more important instrument since hip fractures, especially in women, exponentially increase. (WHO, Jeneva). As it has been predicted by the World Health Organization (WHO), the numbers of women undergo the hip fracture would be as many as around 2.5 millions in 2025. Since its beginning, in the late of 1950, the development of THR has made many significant improvements. However, there are still doubts that the THR materials might have adversal effects to human health (Wilkinson, J.M, et.al., 2005). It is mainly caused by the corrosive and the material debris in the long term. This essay will try to find the most appropriate THR materials that have no or minimum deleterious effects to human health. Due to the rare of THR-related-literature and short writing preparation, this essay will not involve the force working at hip in detail – its magnitude and type – neither of the THR geometry as well as it is normally needed in an ordinary designing procedure. Structurally, as an introduction, this essay will overview an anatomy description of the hip and then the material selection considerations. Assuming that several of the mechanical properties of material chosen (tensile strength, fatigue strength, yield strength, toughness and ductility) have already eligible, this essay will be concentrating on wear resistance and chemical properties only – corrosion, toxicity. In the end, it will give recommendations of material used to design THR based on the supporting reason given. In addition, since the forces working at hip are neglected, this design will be appropriate to human in general, involving young active woman.
Hip Anatomy
The acetabulum – a cup-shaped cavity within the pelvis – behaves as the socket. The joint transmits loads from the bone to another bone and undergoes wide ranges of motions such as flexion, extension, abduction, adduction, circumduction and rotation. (Pansky, B., 1975). The magnitude of the forces varies depending on the angle of femur toward vertical axis. (Oh, I. and Harris. A.H., 1978). As examined by J.P Paul, average maximum values of force working at hip is happen during fast walking condition, as much as 7.6 times of body weight (Paul, J.P., 1976). This amount may be double or even more, under running or in loaded conditions. An healthy femur has density between 1.6 and 1.7 g/cm3 and will be able to support the load either caused by gravity and activity. In case of osteoporosis, bone density is lower than 1.6 g/cm3 and micro-architectural of bone tissue deteriorated. Hence, it increases its fragility, and femur may not able to bear such the load.
THR at glance
Total hip replacement (THR) is a replacement of whole of human hip components with artificial materials (www.medicinet.com). In order for the THR to work properly, it ought to be able to support such the load, either caused by gravity of body weigh or activity. In addition, the THR should be long life and does not harmful on the body and its metabolism. In order to fulfill the above criteria, appropriate materials conspicuously have to be chosen.
Consideration of choosing materials used on THR
In essence, several characteristics of material have to be taken into account prior to design the THR. First, it has to be biocompatible with the human body. Human immunity system will deny any materials that are categorized as “foreign” matters. It will send antibody to attack such the matters and responding it with an inflammation. The extent of rejection varies from a mild irritation to death (Pansky, B., 1975). In conjunction to the THR implantation, such the response is obviously undesired. Thus, biocompatible materials will be used to minimize or even eliminate body rejection. Secondly, the materials must be anti-corrosive. Body fluids consist of NaCl (1 wt%) and other liquids that can be categorized as very corrosive solutions. It obviously leads some metals to corrode easily – not only uniform corrosion but also crevice, pitting, fretting, cracking and fatigue. (Davis, J.R., 2003). Furthermore, several materials may toxic to body, produced either by itself or its debris due to corrosion. The toxin will be washed-away by blood and may interfere other organs functions. Despite the immune system will be neutralized the toxic, it has limitation in capacity. Hence, the toxic may persist in the body due to ongoing corrosion process. (Black, J. and Hasting, G. 1998). In addition, the THR has to be able to support the load caused by the gravity or human activity and somehow do not deteriorate the bone. Hence, the chosen material have to have sufficient strength (tensile, fatigue, yield), fracture toughness and ductility). Ductility – a material property of being capable to sustain plastic deformations without fracture – is another important consideration since using very hard materials may deteriorate the bone tissue. Hence, the selected-materials should have modulus of elasticity as much as that of the bone. Since there is no materials have both strength and elasticity as well as those of the bone, material with a minimum ductility (8% EL) but strong to support the load, is recommended (Davis, J.R., 2003).
In addition, wear resistance is another significant properties that implanted-material should have. Since, the artificial ball and the cup will continuously scrape against one to another, a debris produce by friction should be as little as possible. Body has maximum capacity of overcome the accumulation, otherwise it will response with an inflammation. Hence, a very hard material, which produces minimum debris, should be selected.
Material Selection
To fulfill the aforementioned materials selection criteria, it is almost impossible to design THR that is composed by one material only. To ensure the THR running well, mechanically there are four basic elements of THR; the femoral stem, the ball attach to the stem, the acetabular cup – attach to the pelvis, and fixation units to tight the stem to the femur and the cup to the pelvis (Davis, J.R., 2003). Several materials have been well examined to construct all of the above components such as metals (or its alloy), polymers, ceramics or their combinations (Elderle, et.al, page 263).
Femur Stem and Ball
Previously, femur stem and the ball were designed by solely using one material i.e stainless steel. Due to its high sulfur content, which lead to harmful human health, another metal alloy being introduced. 316L which has 0.002%wt Sulfur to replace the previous. Another material that currently used as femoral stem is Co-Cr-Mo an Ti. It is rally tough and ductile material. However, it is dense, low fatigue strength and weak to corrosion particularly in crevice and fitting. Improving the fatigue and corrosion weaknesses of the previous substance, F75, made by hot forging that contains of 66 wt% Co, 28 wt% Cr, and 6 wt% Mo was commonly used for the stem and ball. Its premier properties are its fatigue and corrosive endurance. (Black, J. and Hasting, G. 1998). Besides fatigue and corrosive properties, Co-Cr-Mo is also superior in wear resistance whenever it co-operates with metal cup (metal-on-metal, Co-Cr-Mo). Surface roughness of head is only 0.01
