Title: Medical Materials for Orthopaedic Applications: New Concept Design and Development

Xiang Zhang*, Archana Binod-Nair and Mike Salts

(*corresponding author of CERAM Research Limited, UK)

Abstract

We have all learnt lessons from medical accidents associated with orthopaedic medical devices, whether those be serious in nature or not. In general, there are three areas, in terms of root causes, that have contributed to medical accidents: choices and development of materials; the design of devices; and manufacturing-related issues. Based on the new concept of micro fracture mechanics, we will discuss the key elements associated with the design and development of innovative orthopaedic medical devices, in order to both reduce medical accidents and minimise cost.

1. Introduction

Using man-made materials for orthopaedics has been in practice for some time. Since the first

metallic hip replacement surgery was performed in 1940, technology has advanced and matured yet

still accidents occur from time to time. Unfortunately, almost all these accidents are associated with

materials or the design of the materials. Take, for example, the recent Depuy recalls (reference 1) of

its ASR XL hip implants. One major problem identified here was from debris generated from the

metal-on-metal components grinding against each other. Issues 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 (reference 2). Jacobs et al. also raised the concerns of metallic

corrosion and degradation for hip implants (references 3-4). Recently, asymptomatic pseudotumors

are reported to be associated with metal ions (reference 5). 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. And how is this done? By making sure,

through design, that materials are Right First Time (RFT) by systematically carrying out materials

development, evaluation, a feasibility study and, finally, validation the goal of RFT can be achieved.

2. Bone Basic Before development and design, it is important to understand the basics of bone and what is required

of bone replacement materials.

Bone is a living material: There are three basic layers of structures: articular cartilage on the surface,

compact bone close to the surface and then spongy bone. The remodelling process is carried out 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 which weaken the bone. The

bone reacts to this weakening in the same way it does to other repairing tasks.

Bone is a composite: Bone matrix is a composite consisting mainly of collagen and hydroxyapatite

(HA). Collagen is an organic polymeric fibre, providing 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.

Bone is a viscoelastic material: Bone structure looks like a reinforced polymer; i.e. 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.

3. New Concept for Materials Development In the past, medical implants, like 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, however, be much

longer than that – indeed as long as possible. For this reason, metal was the first material considered

and, since the 1940s, has dominated the market for nearly 70 years. However, in view of the

discussion in Section 2, neither basic structure, nor mechanical properties match between the bone

and the metal.

From a materials point-of-view, future development needs to address and study several issues

thoroughly, including:

(a) Materials physical and mechanical properties;

(b) Biocompatibility and, ideally,

(c) Bioactivity, i.e. the body will treat the implant as its own part with equivalent or better on-site

bioactivity required for body repairing functions.

3.1 Micro Fracture Mechanics Consideration.

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 particularly 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 combined in all three modes. However, Mode I fracture is the most serious one and 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 (reference 6) , where a is the flaw/defect of the

material, σ stress, KIC mode I fracture toughness, r yielding zone or mcrofracture zone ahead of the

flaw tip at an angle θ and at stresses ≤ triaxial stress σh, and ν Poisons’ ratio.

Figure 1 Model of Microfracture Mechanics

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 its 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.m ½ respectively.

Figure 2 Fracture stress as function of materials’ flaw/defect sizes for given fracture

toughness KIC (MPa.m1/2

)

.

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.

It is worth noting that a device can fail at much lower stresses in use than that shown in Figure 2; say

20% of the maximum values, under cyclic fatigue conditions. The failure is not due to the stress being

too high but rather the flaw size increasing with time under cyclic stress. Catastrophic failure

suddenly occurs when the flaw/defect grows and reaches a critical size (reference 6) and the critical

value of fracture toughness of that material, governed by the law of fracture mechanics:

KIC = σ (πa)1/2 (1)

The basic principle worth bearing in mind is that fracture toughness is a material constant (for a given

condition/environment). All variations observed in fracture toughness are actually variations of

fracture stresses and of flaw/defect sizes that either exist in the finished products or develop later

during use.

3.2 Development of Toughened Ceramics

Ceramics are well known brittle materials. However, most recently ceramics have been used as hip

components with success. This is an important breakthrough after the use of metals and, later,

polymers as implant materials. Ceramics have several advantages over metals and polymers. They

are the most chemically and biologically inert of all materials. They are also strong and hard. To date,

almost all reported results demonstrate that ceramics produce the lowest rates of wear particles

(reference 7-15) in comparison with metal and/or PE. However, the main disadvantage of medical

ceramic materials is their fragility. Unlike metals and polymers, ceramic materials cannot deform

under stress. When the stress acting on medical ceramic materials exceeds a certain limit, the

material breaks. Therefore, toughening ceramic will be a challenging task but an important one for

materials scientists for the future of orthopaedic materials development. There are several ways to do

this. One of them is to use one ceramic to toughen another ceramic and another way is to make

ceramic polymer hybrids.

3.2.1 New Concept: Ceramic Toughening Ceramic

Many types of ceramics are potentially applicable for the development of medical materials for

orthopaedic applications. To date, alumina and zirconia-toughened alumina have been explored. One

key parameter to evaluate a ceramic as good or bad is the fracture toughness KIC. However, it is a

most difficult parameter to be measured with great accuracy and with confidence. For example, the

3-point bending test is commonly used to measure fracture toughness. However, it is very difficult to

make an ideal pre-crack with sharp ending tip in a ceramic before testing, as shown in Figure 3 (1).

The basic theory of fracture toughness is based on that sharp crack. In the Instead of a sharp crack,

a 3-point bending specimen is machined to produce a notch with a blunt end, as shown in Figure 3 (2),

for the measurement of fracture toughness (reference 16). It is obvious that, for the same crack size

a, the two specimens in Figure 3 will give rise to very different fracture toughness values. In addition

the that, the quality of the tool and the speed to produce the notch, etc. will all have influences on the

reproducibility of the measured KIC results, and will seriously affect accuracy.

It is important to accurately measure fracture toughness KIC for the development of new ceramics for

orthopaedic applications. The fracture stress and maximum defect size allowed can be accurately

predicted by using Equation (1). The investigation of testing methods of fracture toughness and

other mechanical properties precisely and accurately will be discussed and published in a separate

paper. The following gives one example of zirconia-toughening alumina to explain the new concept

regarding design and development of these kinds of materials.

Figure 3 specimens to measure fracture toughness

Figure 3 shows the microstructure of one material developed at CERAM using zirconia to toughen

alumina (ZTA), where bright phase of the microscopy is zirconia and the dark continue phase alumina.

Figure 4 shows a typical fracture surface microscopy for both ZTA and the control. It is clearly shown

that the control is more brittle than the ZTA because the former has less deformed features than the

latter. This is further confirmed by fracture toughness KIC results shown in Figure 5. All ZTA has

much higher KIC values than the control. The highest KIC value as achieved at CERAM is 7.2

MPa.m1/2

and the potential exists to achieve greater fracture toughness than that. These and other

complete results will be published in a separate paper.

Figure 3 microscopy of zirconia toughened alumina,

(bright phase = zirconia and dark phase = alumina)

Figure 4 Fracture surface of (a) alumina control and (b) zirconia toughened alumina,

Figure 5 Fracture toughness KIC of zirconia toughened alumina and alumina control

3.2.2 New Concept Toughening Mechanisms

The basic model of micro fracture mechanics shown in Figure 1 contains the following equation

Where rh is classically called ‘yielding’ zone, or plastic zone. With respect to ceramics, it is better

called ‘micro deformation/fracture’ zone. The size of rh, fracture toughness KIC and the critical triaxial

stress σh are influenced among these three parameters. To maximise fracture toughness KIC, it is

necessary to enlarge the micro deformation/fracture zone rh. The effect of this enlargement has

turned a sharp crack tip (refer to Figure 1) to an effectively blunted crack front; the bluntness is

defined by rh, i.e. extended micro deformation and/or micro fracture zone. The effect of this is to ease

the stress concentration at the sharp crack/defect tip. It also absorbs/consumes a large amount of

energy during forming micro deformation and/or micro fracture. Both these effects will make

catastrophic failure less likely to occur, hence increased fracture toughness of that ceramic. This is

what is seen in Figure 4 (the toughened alumina) and (the effect) in Figure 5 - its KIC is increased

significantly in comparison of the control alumina.

3.2.3 New Concept: Further Development of Ceramic Polymer Hybrids

Based on the above micro fracture mechanics model, we have, at CERAM, developed a range of

toughened ceramics in combination with other materials, such as polymers, to form hybrid composites.

Figure 6 is one example of ceramic hybrids. This ceramic compound is composed of a ceramic foam

(brighter phases, highly loaded) and mixture of polymers (darker phases). This kind of ceramic hybrid,

having the advantage of biocompatibility and bioactivity and the hardness of ceramics with the

flexibility of the toughened polymer, is easier to design and to develop with good control of the

microstructure - achieving biomaterials requirements and toughening of the ceramic and hence great

potential for orthopaedic applications. For example, employing a toughened polymer makes the

ceramic hybrid - we can make rh, i.e. ‘yielding’ or ‘micro deformation/fracture’, much larger than that

of ceramic toughening ceramic as discussed in Section 3.2.1. Therefore, the new material no longer

has the problem of brittle fracture as in the case of ceramic hip joints. It will not be sensitive to sharp

crack/defects. In addition, the new ceramic hybrids can be designed to possess better

biocompatibility and, most importantly, bioactivity with tailored properties to meet different application

needs. Various forms of ceramic hybrid compounds can be made into different microstructures for

different applications, potentially including, but not limited to, spinal fusion, suture anchors, fixation

and trauma screws, femoral implants, dental implants, total and partial joint replacement.

Figure 6: microstructure of ceramic and polymer hybrids developed at CERAM

(porous ceramic is brighter phase and polymer darker phase)

Bioactive materials will be the new materials technology in the future. The body treats all artificial materials as ‘enemies’. Metal and polyethylene, used in hip and knee joints, are classified ‘safe’, but are, in fact, no longer bio-inert but rather potential hazards when their debris migrates into other parts of body. This is obviously a major concern. Therefore, biocompatibility is, at the very least, a basic requirement and bioactivity is critical for the future development of orthopaedic implants. Reference:

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