Proposal For Artificial Hip Joint Engineering Essay

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Ceramic hip replacements are administered to patients to improve mobility and reduce pain. Typical patient age is constantly reducing creating a larger market whilst younger patients also need revision surgeries for resizing and damage.

Currently, 500,000 hip replacement surgeries are conducted each year at an average cost of around £5,500 including hospital fees. Whilst there are major competitors the market is large and the leading company's reputation has been sullied by a poor product.

Suitable ceramic materials must not react with the human body and must also be wear resistant, tough and hard to reduce wear particles. These include zirconia and alumina based ceramics.

Typically, hip joints are manufactured using the Bayer process to obtain the alumina powder and isostatic pressing is used to form the hip joint itself.

Single-crystal alumina based ceramics have possess all the desirable qualities mentioned above. Isostatic pressing provides the most uniform density and surface finish whilst wet-bag is less complicated than dry-bag with better results.

Investing in this market is very profitable with the potential future growth and with revenue spent on research and development RAC can gain a favourable reputation quickly. Rennib Advanced Ceramics Ltd. possesses the resources to produce the single-crystal alumina based ceramics using the Bayer process as well as manufacture the hip joint using wet-bag isostatic pressing.

What it is

Ceramic materials are traditionally associated with brittle goods such as porcelain urns and china plates. However, with the rapid development of modern technology, ceramic materials now tend to be widely used in every walk of life. Medical applications for modern ceramic materials are very popular and widespread with a large amount of research into potential applications and improvements being made by top universities, companies and hospitals. One of the successful applications in the medical field is the artificial hip joint.

The ceramic product being investigated in this report is the artificial hip joint. This is a medical product used when the natural human hip has disintegrated or failed the patient. The artificial hip is engineered to restore normal movement to an injured or damaged hip. The human hip is a ball-and-socket joint in which the roughly spherical top of the femur is able to rotate freely within a cup-shaped socket in the pelvis.1 This freedom of movement can be severely limited by osteoarthritis of the hip, with accompanying pain, stiffness and loss of mobility. Artificial hip replacement (also known as 'hip arthroplasty') plays an important role in the management of this debilitating condition.2 The main reason for the use of artificial hip joints is mainly disease. This is a big problem not only for the elder but for young people also. Some of the diseases that affect the hip include: rheumatoid arthritis (when the body's own immune system attacks the cartilage); bone tumors and; injuries to the joint.

However, the most common infection that causes the hip infection is known as osteoarthritis. This is when weakened cartilage that lines the joints leads to inflammation and changes in the ends of the bones that cause stiffness and pain. In younger people, osteoarthritis may occur following a joint injury or infection, or as a result of obesity. Artificial hip joints are made for people who suffer the pain and discomfort caused by osteoarthritis and rheumatoid arthritis.3 Artificial hip joints are implanted into these patients' bodies and allow for a normal lifestyle whilst reducing the pain. Generally the artificial hip joint includes three parts; a ball which replaces the top of the femur, a stem linked to the ball which is inserted down into the centre of marrow space of the femur and a spherical cup which is inserted separately in the bone of the pelvis. The ball is pushed into the cup, joining the leg and the pelvis. The ball can move around in the cup, so the joint can bend and rotate in a manner similar to the real thing.4

Today, many people whose hip joint has been damaged or fractured consider an artificial hip joint implant as a viable alternative to their original impaired hip joint. Artificial hip joint has evidently improved these people's life which makes them live a normal life. They can walk without wheel chairs or crutches and take part in the activities with family or friends, and more or less have slight sports for some fun. More and more disabled people are benefiting from the artificial hip joint; however there still are plenty of people who don't have the hip replacement surgery and expect to do it.

The future potential for the product depends on the patient. Older patients have a more established future with the hip joint as it will be lifetime for them. In younger people, a hip prosthesis is unlikely to last a lifetime, and other forms of treatment are often preferred. The application for an artificial hip joint is as its name implies for Hip joints. Its range is also limited as its design, shape and ergonomics are specific for the hip joint of a human being. This product cannot be used for any other application part from the hip, due to the shape and the ergonomics of the product itself.

Figure 1 An artificial hip joint in human body.5

Market Size, Projected Growth & Market Competitors

The most common need for the use of hip replacements is one in which arthritis in patients over the age of 55 has rendered the joint difficult to move or is causing the patient pain.6 Obviously arthritis does not affect every person however, the number of potential candidates is increasing rapidly. This is due to the increasing worldwide life expectancy and children from the "baby boom"5 reaching old age in the future which creates an aging population, most of whom wish to remain active for longer. With the increase in older citizens the chance of painful joint problems increases which in turn necessitates more artificial joint replacements being administered.

In addition to this, advances in medicine have caused the risk of infection to severely decrease as well as surgeons having more experience in the field. To complement the medical advance the production methods and materials used have been improved allowing for high quality parts to be manufactured more easily and at a lower cost. As a result of this progress the surgery is less exclusive and costly allowing the spectrum of potential patients to increase to more and more people including younger and injured patients. With the introduction of younger patients receiving hip replacements the number of revision surgeries to update the hip replacement will increase due to growth and the active nature of younger patients.

In 1994 an estimated 129,000 people received full hip replacements in the US7 with more having partial replacements. This rose considerably by 2005 to "800,000 units per year worldwide at an average price of €2,000 each".7 This may seem like a small increase however the number of countries in which replacement hips are administered regularly is not very large with Europe and the US accounting for most of the sales. In the US full hip placements numbered 265,345 with the total number of hip replacements recorded in the US being around 368,6598. This is translated as an increase of around 45% which shows how fast growing this market is. Furthermore, the number of hip replacement procedures in Europe in 2005 was even higher at 445,700 with the majority being primary replacements at 383,396.8 In 2009 the artificial hip joint proved to be a profitable medical product with more than 500,000 hip replacements conducted by the NHS.9 According to the National Health Service (NHS), the total cost of a hip joint replacements surgery including hospital stay and doctor's fees is estimated to range from £4,000 to £7,000.9 This shows how it has become one of the fastest growing segments in sales globally with increases each year.

With this increase in cases comes an increase in revenue. The hip replacement market is a very profitable market with prostheses sales between $600 million and $800 million in 1993.6 This grew substantially in the following years with "2005 revenues for hip implants in the US were $2 billion and $1.4 billion in Europe".7 These figures reinforce the sales data proving prosthesis to be a very profitable market and and a much more viable investment opportunity. .

However, with such a profitable market comes competing manufacturers. The main compeititors for Rennib Advanced Ceramics Ltd. are as follows:




Stryker Corporation


Smith and Nephew Richards"7


The market leader is Warsaw-based Zimmer with revenue around $300 million down from $330 million in 2008.9 Whilst this could indicate market maturity it is more likely due to a poorly designed product which caused the company problems of lawsuits in 2008. Biomet and Depuy, also based in Warsaw appear untouched over the same period with Depuy just below Zimmer in market share of around 25% each. Biomet possess almost half the market share of the market leader at $140 million revenue in 2009.

Figure 2. Hip Revenue for Competitors in 2008.9

In comparison, a fairly new company to the prosthesis manufacturing market, Exactech, had total revenue for hip implants of $20 million for 2009, up 14% from $17.5 million.10 This gives Exactech a minute market share of approximately 1.7%. This gives a better idea of where Rennib Advanced Ceramics (RAC) would place in the hip replacement market. It would seem that the market for hip replacement manufacturers is very established and to break into this market would require a lot of effort, especially in marketing and research and development to produce a ceramic product that out-performed the competitors.9

Current Materials

Interestingly, it is often found that a variety of materials are used to fabricate the hip joint in the medical industry besides ceramic materials. For instance, a commonly used artificial hip joint consists of a ceramic ball, a metal alloy stem and a spherical polyethylene cup.11 For RAC this would mean that competition would not only be from opposing ceramic manufacturers but also polymer and metal production companies. RAC would need to invest money into both research and development and marketing in order to discover and sell new materials in this competitive market.

The artificial hip joint used for medical purposes demands materials which are non- corrosive and non-toxic as it will be inside the patients' body. This therefore limits the types of materials and research shows that the most common materials used for artificial hip joint are Polythene, Metal and Ceramic. A polythene or metal cup has been used for over forty years whilst the ceramic material is a new addition to the existing two. Examples of each can be found in Fig. 2 and reasons why each material is used including advantages and disadvantages are explained below.

Figure 3 Existing or potential materials combinations used for artificial hip joint (a-d are already in market, whereas e is still in laboratory stage).

a) Charnley metal on polymer hip prosthesis;

b) McKee-Farrar metal on metal hip prosthesis;

c) alumina on alumina hip prosthesis with metal shell;

d) ceramic on polymer hip prosthesis;

e) ceramic on metal hip prosthesis.12

Normally the artificial hip joint can be made from materials with properties such as high mechanical strength, high wear resistance, self-lubrication, high chemical stability and good biocompatibility. However, the criteria used to determine the most suitable material for an artificial hip joint is not always determined by the property of the material but by can sometimes be determined by the age of the patient.

One of the primary combination of materials used in joint manufacture features metal for the ball with a polyethylene liner. The metal ball itself is made of cobalt chrome molybdenum alloy chosen for its durability and performance.  Another option would be titanium for its light weight and high strength properties. 'Polyethylene is the most understood and used of all the liner materials, offering the surgeon a range of options to obtain stability in the body while the operation is underway.' 13 Polyethylene is also a suitable choice because it is able to adapt to the shape of the human body and it is also cheap to produce.

Another arrangement is an all metal variant where both the cup and ball are constructed from metal. The metal components are made of Metal-on-Metal bearings (cobalt chromium alloy) having the potential for greatly reduced wear, with less inflammation and less bone loss.  Metal bearings are available in many sizes (28 mm to 60 mm); there are also several neck lengths available. Metal-on-metal components allow the largest heads throughout the entire range of implant sizes. Large ball heads provide increased range of motion and greater stability, which can significantly reduce the risk of hip dislocation, a crucial factor in the long term success of an implant. The disadvantage with this method is that although wear is reduced, the deposits from the wear (sub-microscopic particulates, soluble metal ions) are distributed throughout the body.  This has raised concerns about long-term bio-compatibility.  At present these are only concerns, for there have been no definitive clinical findings that these wear products are harmful. 13

The other combination of materials used in hip joint manufacture is a ball made from ceramic, again with a polyethylene or possibly a ceramic liner. Ceramic is used for the rotating part of the joint as it is harder than metal and is the most scratch resistant implant material. Because it is hard, scratch resistant and has an ultra-smooth surface the wear rate on the polyethylene bearing can be greatly reduced.  This then means that this type has an advantage over the metal and polythene option because it has fewer waste products from the constant friction between the two surfaces. However, it is more expensive than Metal-on-Polyethylene, but less than Ceramic-on-Ceramic. Also newer, stronger ceramics have resulted in reduction of fracture rates (0.01%) compared to the original brittle ceramics, therefore leading to fewer complications after operation.

The most commonly used ceramics in the medical industry are bioceramics. The bioceramics used are normally divided into two sub-categories: bioactivity and bioinertness. In the case of artificial hip joints, bioinert ceramics are favoured for their stability when exposed to the physiological environment for extended periods of time. This essentially means that the ceramic will not undergo chemical reactions with any other substances in the human body causing further discomfort or pain. A further key property of the hip joints is their load-bearing properties. The ceramic must withstand constant grinding and the possibility of sudden impacts, especially in younger patients, which means that the material chosen must exhibit good wear resistance as well as impact resistance. Generally, oxides of alumina and zirconia are widely used to fabricate the hip joint while silicon nitride and barium titanate are also used to make this artificial joint.

Additionally, α-alumina is used to make bioceramics in the form of one of the alumina allotropes. This type of alumina has good stability and polarity in the molecular bonds. Also a membrane of water molecules can form at the surface which indicates that the alumina has a hydrophilic property and this translates to good biocompatibility with the human body. Furthermore, alumina has good mechanical strength, good wear resistance and a low coefficient of friction at a relatively low cost. For these reasons Alumina is definitely an ideal material to fabricate replacement joints including the hip joint.

Recently, a new single crystal alumina has been tested for hip joint manufacture. This new material has better mechanical strength, hardness, corrosion resistance, biocompatibility, stability and wear resistance than polycrystalline alumina ceramic but is much more expensive due to its recentness. Another possible material is Zirconia which is ideal for hip joints because it has excellent biocompatibility, good fracture toughness, high fracture strength and a low elastic modulus. These properties make zirconia a suitable material to manufacture the artificial hip joint as it will have to endure high shear stresses whilst in use. However, the cost of the zirconia is much higher than the alumina and recently some research argued that zirconia bioceramics have the potential problem of radioactivity and furthermore, its wear resistance is not good as alumina. As a result, the utilization of zirconia is less extensive than alumina bioceramic.11

With regard non-oxide bioceramics, silicon nitride has good self-lubrication, wear resistance and biocompatibility. But it has high modulus which makes the material brittle and this material is expensive. Silicon nitride is not widely used for artificial hip joint. Barium titanate is used because it has good corrosion resistance for body secretions and is available for any methods of sterilization.

As mentioned previously, patients eligible for hip joints are of a variety of ages, sizes and life styles. This therefore requires constant alteration of the type of materials used for the hip artificial hip joint. One of the most common problems with replacement hips is the degradation that takes place through normal use. Over time, the body may perceive polyethylene wear particles as a source of infection. As the body starts to attack them, this leads to a disease called osteolysis, a "dissolving of the bone", which may result in having to replace the implant. This process is called a revision or a revisional surgery. 13 If this occurs then an all-ceramic hip joint or all metal hip joint is implemented which has a much stronger, wear resistant cup to reduce the wear particles produced. Depending on the size of your natural hip, the surgeon also has the option of selecting 28mm or 32mm diameter ceramic balls or even larger metal balls. Research shows that younger, more active patients have a greater chance of scratching the metal ball through sports and a more energetic lifestyle and this scratching will produce more debris.

As a result of this, new wear resistant polyethylene liners have been introduced, called "highly cross linked polyethylene."13   During the manufacturing process the plastic is treated with a small dose of radiation to help the cup resist wear.  Research shows that the wear rates of the metal-on-polyethylene implants wear at a rate of about 0.1 millimetres each year. Vitamin E, which is a natural antioxidant can are also been used which stabilizes highly cross linked polyethylene bearing material and improves the longevity of the implant bearings used in total joint replacements.

As already stated, many of the artificial hip joints produced cannot be used permanently. This is for a number of reasons: the materials can be gradually corroded by reactions with substances in the body; product damage due to either inappropriate use or faults in the product and; the introduction of younger patients who need revisional surgeries. The expected lifetime of an artificial hip joint before it needs to be renewed is roughly ten years which generates a recurring customer and guaranteed income for the manufacturer and surgeon.

However, many scientists and doctors at leading Universites such as Birmingham and Imperial College London are doing extensive research in the field to find the better materials with better corrosion resistance and biocompatibility to overcome these problems. Bioceramics can be used for several decades in the human body due to their bioinertness. This means patients only need one surgery to replace their hip joint providing they do not damage or outgrow it. However, upwards of 500,000 hip replacements are conducted by the National Health Service (NHS NHS each year. And about half of that number's hip replacements are conducted using bioceramics.8 Whilst using bioceramics saves the customer money it does not provide the guaranteed future income for RAC Ltd however, the company image would benefit greatly from using the best materials available.

Processing Methods

There are a number of steps in the production of the artificial hip joint. The processing methods are various due to the different ceramic components of the products. The most important factors in production are to ensure that the artificial hip joint will bear the load of the body and that the material will have high strength, high toughness and high wearability to reduce the daily wear.14 Because RAC specialises in advanced ceramics the processing routes used to manufacture components from metals and polymers need not be investigated.

Aluminium oxide is one of the most widely used ceramic materials to manufacture the artificial hip joint. Alumina powder is used in the manufacture of medical components, especially the artificial hip joint. Every year the medical industry requires large numbers of alumina powder and alumina-based products including the large demand of the components used to fabricate the artificial hip joint. Currently the processing route for aluminium oxide is powder processing. Alumina powder is made mostly from the mineral bauxite, which is basically colloidal aluminium hydroxide closely mixed with iron hydroxide and other impurities.11 The Bayer process is the processing route used to refine the mineral bauxite to produce the alumina. Figure 1 shows the Bayer process used to fabricate the alumina powder. In the Bayer process, the mineral bauxite is crushed through the crusher, and is then digested by mixing with caustic soda at a high temperature. This digestion causes the bauxite convert to aluminium hydroxide which is then dissolved to form the hydroxide solution. The chemical reaction is as follows:

Al(OH)3 + NaOH → Al(OH)4 + Na

The undissolved bauxite is filtered as residue to remove the solid impurities or "red mud", and the solution is therefore clarified. After that, the purified aluminium hydroxide solution is cooled and then precipitated in a precipitator. This generates fine-particle-sized aluminium hydroxide which is then heated and decomposed in the rotary kiln to form alumina powder with the water evaporated as water vapour.11 The chemical reaction is shown below:

Al(OH)4 → Al2O3

Figure 4. The Bayer Process for alumina.15

After the alumina powder is prepared, a shape-forming process is used to form the powder into the required shapes. There are many kinds of techniques used for ceramic fabrication. One widely used technique for shape forming of artificial hip joints is isostatic pressing. Isostatic pressing involves using a deformable bag, usually made from rubber, which contains the ceramic powder. The pressure is provided by gas or liquid in wet-bag pressing whilst in dry-bag pressing fluid applies to a second bag. The fluid applies equal pressure from to the part from all angles which provides a much higher quality product. Dry-bag isostatic pressing requires less fluid however, it is more complicated to achieve isostatic pressing. Wet-bag isostatic pressing is a simple way of providing uniform pressure to the ceramic powder and can be run inexpensively if water is utilised as the non-compressible fluid in which the rubber bag is submerged. The uniform pressure provided by uniaxial pressing allows for greater uniformity of compaction and increased shape capability.11 The final product would have high density without surface textures or mould scars and no density gradients.

Conversely, the only disadvantages of using isostatic pressing are that the mould is at risk of poor or incomplete filling. This causes the product to have unwanted defects such as large hollow spaces or poor density however it can be easily remedied if the mould is shaken or vibrated as the powder is poured in allowing the ceramic particles to tessellate fully. In addition, when compared to processes such as uniaxial pressing which create inferior products and have much more design limitations, the cycle time is comparatively long in the region of minutes or even hours. Despite this large numbers of products can be produced simultaneously reducing the overall production time. With this mind isostatic pressing is the most viable solution to pressing the ceramic powder.16

Figure 5. Wet-Bag Isostatic Pressing.16

Proposed Ceramic and Production Method

Drawing upon the information in the "Current Materials" section it is evident that there are many different types of ceramics and this provides both a variety of suitable materials for the consumer to choose from and the need for a decision to be made by the ceramic manufacturer as to which to material to use for the product products. The three main ceramic groups are Zirconia, Silicon Carbide and, Silicon Nitride.

Zirconia is a tough and strong ceramic. It is never used in its pure form however, with the introduction of additives such as MgO and Y2O3 its applications become more widespread and varied. Generally the additives used are oxides and this makes it an oxide ceramic; conducting iron and oxygen well. However, it is not a feasible possibility for the replacement hip joint due to its poor strength. Its common uses are parts that are subjected to large wear forces and solid oxide fuel cells.

Furthermore, silicon nitride is a non-oxide ceramic meaning that products formed from it operate poorly in air at high temperatures. These two ceramics mentioned have been examples of general ceramics and as such are unsuitable for ceramic hip replacements. For this specific application, what are known as bio ceramics need to be used. These can be used to make structural forms such as joint replacements or as ceramic coatings or as a way to encourage natural bone growth. The type of bio ceramic that will be used in for the hip replacement is known as Bioinert ceramics.

With this in mind the most suitable ceramics for production of artificial hip joints are single crystal alumina-based ceramics. This is because they present an economical alternative to more expensive ceramics such as zirconia whilst providing excellent properties. Hardness, wear-resistance and, stability are all extremely desirable properties for an hip joint and single crystal alumina ceramics excel in these areas. Alumina-based ceramics are also bio-inert which will remove complications of rejection by the body.

With the knowledge that this product is to be produced in large volumes, typically in the region of thousands or millions indicate the need for a processing route that can produce products very quickly and in numerous amounts at high quality. The processing route I will advise will be isostatic pressing since and artificial hip joint is spherical and as such requires uniaxial pressure when being constructed, "Isostatic pressing enables to produce various types of materials from powder compacts by reducing the porosity of powder mixture. The powder mixture is compacted encapsulated using isostatic pressure, it means by using pressure equally from all directions."

Conclusions and Proposal

Drawing upon the market investigation, we believe that the artificial hip replacement can be a new product manufactured in Rennib Advanced Ceramics (RAC) Company Ltd. and it there is a comparable profit in this market. The reasons as to why RAC should proceed with the production of the product are based on the following arguments.

Although several companies have already put their products into the market of artificial hip replacement and have gained a large quantity of market share, we hold the view that our new product has its unique advantages which will distinguish itself from other products, and can possess a certain market share as well albeit an initially small one. As stated previously, our new artificial hip replacement would interact with the human body very well due to our great improvement of the product's wear resistance and compatibility with the body. Therefore, a patient using our new product will feel more comfortable and experience movement ad dexterity like an able-bodied individual. Furthermore, RAC Ltd. will have the cooperation of some leading universities in the country including Loughborough University. This will allow RAC to gain a considerable advantage through further investigation into possible ceramics that ensure excellent biocompatibility and performance creating ever higher quality materials for artificial hip replacements. In addition, as an advanced ceramics manufacturing company RAC have access to the most effective and efficient manufacture equipment, this guarantees that the product can outperform the competition. Zimmer Holdings Inc.'s main problem is the poor design which lead to law suits in 2008 and damaged the company's reputation. This created a customer's deciding to take their business elsewhere and this is a great example of a way in which RAC can quickly expand its market share. In this way RAC's artificial hip replacement will give consumers more confidence and rapidly gain a strong positive reputation for a good quality product whilst at a reasonable price. Therefore, a potential increase will be observed in our market share of artificial hip joint.

In order to protect our ideas from other competitors, Rennib Advanced Ceramics Ltd. should consider the issue of the intellectual property. For example, RAC needs to apply a patent for our new ceramic product. A patent application can effectively protect ceramics and ideas developed from reproduction and plagiarism by other competitors. In addition, if RAC will be able to gain considerable income by selling the concepts to companies wishing to produce similar quality products, this income would be considerably more in the unlikely situation that the terms of the patent are broken.

With the investigation of the market of the artificial hip joint, an obvious phenomenon can be illustrated that the number of potential candidates is increasing rapidly due to the increasing worldwide life expectancy of human beings. Most patients with hip joint problems wish to remain active for longer and are receiving replacements at a younger age. This is because the artificial hip joint can magnificently improve their mobility and reduce their pain. For these reasons, the already large artificial hip joint market will continue to grow in the future, although there are a number of competitors manufacturing the products now. Currently, according to the (NHS), the total cost of a hip joint replacements surgery including hospital stay and doctor's fees is estimated to range from £4,000 to £7,0009, and with more and more younger patients requiring revisional surgery to increase the joint size the potential profit available to RAC is substantial. In conclusion, we suggest that Rennib Advanced Ceramics (RAC) Ltd Company proceed with the manufacture of the new artificial hip replacement as detailed in the "Conclusions & Proposal" section of the report.