Materials selection of femoral stem component
The femoral stem component replaces a large portion of bone in the femur, and this is therefore the load-bearing part of the implant. To bear this load, it must have a Young’s Modulus comparable to that of cortical bone. If the implant is not as stiff as bone, then the remaining bone surrounding the implant will be put under increased stress. If it is stiffer than bone, then a phenomenon known as stress shielding will occur.
As discussed earlier, bones are constantly being reshaped by osteoblasts and osteoclasts, through the continual formation and resorption of bone material.
If the implant is much stiffer than the bone, then the implant will bear more of the load. Because the bone is shielded from much of the stress being applied to the femur, the body will respond to this by increasing osteoclast activity, causing bone resorption.
Due to its higher surface area, cancellous bone is more biologically active because the cells involved in the formation and destruction of bone are found on the surface only. It is therefore more quickly and more drastically affected by stress shielding, wasting away up to four times as quickly as cortical bone.
Although 70 wt% of bone material is a ceramic, hydroxyapatite ceramics are not suitable materials for femoral stem replacements, as they are much too brittle. Polymers are also unsuitable as they are prone to suffer from creep and fatigue.
Metals are generally used because they typically have a high Young’s Modulus, are tough and ductile meaning they yield before breaking, and have good fatigue resistance. They do, however, tend to be much stiffer than bone, which can lead to stress shielding.
A useful tool for comparing the mechanical properties of different materials is the materials selection map. Full instructions on how to use these are given in the TLP Optimisation of Materials Properties in Living Systems.
Below are two Materials Selection Maps showing some of the most commonly considered metals for femoral stem replacements.
As can be seen from the selection map, steel has a Young’s modulus much higher than that of bone, meaning that stress shielding is a serious issue. Stainless steel was used for the femoral component in the earliest hip replacements, and is still used in some implants today. It is an alloy of iron, chromium and usually nickel and cobalt. It is resistant to corrosion, abundant and relatively cheap and easy to produce. However, some people have allergies to nickel, which would cause extreme adverse reactions after the implant operation.
A cobalt-chromium-molybdenum alloy was later introduced as an alternative, since it had better wear properties than stainless steel. However, it is a harder metal, meaning it is more difficult to machine, and it is much more expensive.
Titanium’s modulus is only half that of stainless steel, so the remaining bone will suffer less from stress shielding. It has an excellent strength to weight ratio and an impressive resistance to corrosion. Titanium is also biologically inactive and resistant to creep deformation, and these advantages over stainless steel make it a very good choice of material for the femoral stem component. It is, however, significantly more expensive than stainless steel, costing up to five times as much per kilogram (although in making a hip implant a smaller mass of titanium would be used than steel).
Rather than using commercial purity (cp) titanium, the alloy Ti-6Al-4V (titanium alloy with 6% aluminium and 4% vanadium by weight) is often used as it gives increased toughness, as shown, and improved fatigue resistance.
This alloy can also be treated during the sintering process in a way that controls its porosity. Porosity is the ratio of the volume of pores in a material to the volume of the whole material, and the value of the Young’s Modulus decreases with increasing porosity. As can be seen from the first Materials Selection map, a porosity of 40% gives properties which match those of cortical bone extremely well.
The femoral stem component is inserted into the femur, which has been prepared to fit the component. This is traditionally bound to the bone with a polymer bone cement, polymethylmethacylate (PMMA). As well as fixing the implant in place, the cement helps distribute load more evenly between the implant and bone. The drawback with this method is that during the curing process (hardening through the cross-linking of polymer chains) a large amount of heat is released that can cause necrosis (cell death) in the bone around the implant and lengthen recovery time.
Rather than using bone cement, an alternative fixation method is to introduce a porous surface layer to the implant, which encourages bonding by allowing bone to grow into the pores. The bone and implant therefore become integrated, meaning that the implant is less prone to loosening.
A further modification is to coat the implant with a layer of hydroxyapatite. Since its chemical composition is similar to that of bone mineral, the coating enhances bone growth. Many surgeons now favour these uncemented implants as they give a quicker recovery time, with many patients able to put weight on their hip the day after surgery.
Coating with hydroxyapatite does raise some problems of its own, however. The coating is done at an elevated temperature, and as the implant cools, the alloy and the HA contract at different rates, because hydroxyapatite’s thermal expansion coefficient is higher than that of the titanium alloy. This can generate thermal stresses and cause cracking of the surface of the implant. In an attempt to match the thermal expansion coefficients (and so avoid cracking), manganese can be added to the alloy to raise its expansion coefficient.