Med-Tech Innovation Materials
associated with this can include component loosening, malalignment, infection, pain, fracture, dislocation and metal sensitivity. It is also well known that small metallic particles can enter the patient’s bloodstream and other organs.
In 1973, Coleman et al. reported raised levels of cobalt
and chromium in the blood and urine of patients with metallic total hip replacements.2
are reported to be associated with metal ions.5 Jacobs et al. also raised
the concerns of metallic corrosion and degradation for hip implants.3,4
Recently, asymptomatic pseudotumours The real
danger concerning metal toxicity is that often patients have no symptoms that could indicate a problem. This factor, plus the fact that the level of metal debris is normally very low, make MOM hip implants “clinically” and “statistically” safe during product development and follow up clinical trials. Regardless of how low the level of the metal debris is, the ultimate goal is to eliminate this problem. How is this done? By making sure, through design, that materials are right first time (RFT), that is, by systematically conducting materials development and evaluation, a feasibility study and, finally, validation so that the goal of RFT can be achieved.
Bone is a living material Before development and design, it is important to understand the basics of bone and what is required of bone replacement materials. There are three basic layers in the structure: articular cartilage on the surface, compact bone close to the
Figure 1: Model of microfracture mechanics
surface and then spongy bone. The remodelling process is performed by bone cellular
components through resorption and deposition. Over time, as healthy bone is subject to the wear and tear of use, the bone develops nano and/or micro-fractures that weaken it. The bone reacts to this weakening by trying to repair itself, just as other damaged/defect parts of the body do.
Bone is a composite
Bone matrix is a composite consisting mainly of collagen and hydroxyapatite (HA). Collagen is an organic polymeric fibre, which provides the bone with resilience and the ability to resist stretching and twisting. HA, formed by the interaction of calcium phosphate and calcium hydroxide, is a reinforcing ceramic in the form of elongated crystals. Bone also contains smaller amounts of magnesium, fluoride and sodium. These minerals give bone its characteristic hardness and the ability to resist compression.
www.med-techinnovation.com
Bone is a viscoelastic material Bone structure looks like a reinforced polymer, that is, a bundle of polymer fibres (collagen) reinforced by hard crystals (HA) that fill all the gaps between the fibres. This unique structure exhibits viscoelastic properties. This means that the mechanical properties of bone are not physical constants, but vary with temperature and, importantly, rate of imposed stress/strain in action; the faster the action, the higher the stress, but the more brittle the bone.
Future materials development
In the past, medical implants such as hips and knees were expected to last a minimum of 15 years. With younger patients (and older patients too), the ideal lifetime of the implant should be much longer than that, indeed as long as possible. For this reason, metal was the first material considered and it has dominated the market for nearly 70 years. However, in view of what we know about bone, there is no match between the basic structures or the mechanical properties of bone and of metal. From a materials point of view, future development needs to address and study several issues thoroughly, including: • physical and mechanical properties • biocompatibility • bioactivity, that is, the body will treat the implant as its own part with equivalent or better on-site bioactivity required for body repairing functions.
Micro fracture mechanics Bone fractures are a major public health problem resulting in morbidity, mortality and substantial economic costs. Therefore, mechanical properties are factors to be considered in the development of orthopaedic implants, in particular fracture mechanics. There are three basic modes of fracture: mode I opening, mode II in-plane shearing, and mode III out- plane shearing. In reality, bone fracture is a combination of all three modes. However, mode I fracture is the most serious one and it is discussed here to explain its importance in the design and development of bone replacement materials. Figure 1 illustrates a basic model of micro fracture mechanics,6
where a is the flaw/defect
of the material, σ stress, KIC mode I fracture toughness, r yielding zone or microfracture zone ahead of the flaw tip at an angle θ and at stresses ≤ triaxial stress σh
, and v
Poisons’ ratio. Fracture toughness KIC is a basic parameter used for development, selection and design of materials. Fundamentally, micro fracture mechanics is a science of defects and their effect on deformation and fracture of a material. This is best illustrated in Figure 2 where three curves are fracture stresses as a function of flaw/defect size for three given fracture toughness KIC of 2, 4 and 6 MPa.m½
respectively.
The significant effect of the flaw/defect sizes in material is clearly shown in Figure 2. Of course, defects commonly exist in all materials in one form or another, we must therefore never assume that an implant is defect-free. The question is what would be the upper limit of defect size a shown in Figure 1.
April 2011 ¦ 21
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54