Ceramics in Orthopaedic Applications - The Use of Alumina and Zirconia in Total Hip Replacements

Degenerative joint disease is recognised as an increasing problem for society, a direct result of an ageing population. During 2000, joint replacement procedures numbered more than 1.6 million, most performed as a result of arthritis.

Arthritis and Other Degenerative Joint Problems

The word arthritis literally means joint inflammation. There are several forms of arthritis, most of which affect the joints of the body. Osteoarthritis is the most common form, and the hip is one of the common joints affected by this. Osteoarthritis is a degeneration of the articular surface of the joints. It is a degenerative process, which directly results in the wearing out of the cartilage on the joint surface. Over time the joint surfaces slowly erode away until the underlying bone is exposed. Exposed bone results in a painful joint when it moves and bears weight. As a result, hip pain and stiffness increases and the bones grate together. Other hip problems that produce similar symptoms include, rheumatoid arthritis, avascular necrosis (injury and loss of blood supply to the bones).

Arthritis Occurrence Around the World

Three million Australians already suffer what is the most common disabling disease in Australia. An estimated 500 million people throughout the world suffer from arthritis - more than 17 million people young and old with rheumatoid arthritis (RA) and a further 190 million worldwide with osteoarthritis (OA), most over the age of 60.

Osteoarthritis and Joint Replacement

Osteoarthritis is the primary diagnosis leading to joint replacement. Since people over the age of 65 comprise the primary target for joint replacement, and growth of the world’s elderly population will outpace that for the overall population (200 to 400 percent over the next 30 years in emerging countries alone), the market for joint replacement products is expected to grow at a steady rate of five to ten percent annually over the next few decades.

Cost and Revenue Generated by Joint Replacement

Recent research by Access Economics put the cost of arthritis to the economy in Australia at $9 billion a year treatment, hospitalisation and lost production.

Revenues generated by sales of total orthopaedic products worldwide exceeded, US$13 billion in 2000, an increase of 12 percent over 1999 revenues.

Strongest growth in the market came in bone growth stimulators, biologies and spinal implants and instruments. Table 1 delineates orthopaedic product sales by segment and by geographic region.

Table 1. 2000 Worldwide Orthopaedic Product Sales: By Market Segment and Geographic Region (US$ Billions)

Product Segment

U.S.

Ex-U.S.

Total

Chg. Vs 1999

Reconstructive Devices

$2.4

$2.4

$4.9

9%

Fracture Fixation

$0.7

$0.7

$1.4

9%

Spinal

$1.0

$0.5

$1.5

16%

Implants/Instrumentation Arthroscopy/Soft Tissue Repair

$0.8

$0.4

$1.2

12%

Orthobiologocs

$0.8

$0.2

$1.0

44%

Other Products

$2.0

$1.1

$3.0

8%

Total Market

$7.7

$5.3

$13.1

12%

(Note: numbers may not add up due to rounding)

Explanation of market segments

        Reconstructive Devices: hip, knee, shoulder, elbow, wrist, ankle and digit implants

        Fracture Fixation: internal (plates, screws, nails, pins, wires) fixation and external fixation products

        Spinal Implants/Instrumentation: internal       fixation devices and disectomy and vertebroplasty products

        Arthroscopy/Soft Tissue repair: scopes, cameras, instruments, soft tissue implants and repair kits

        Orthobiologics: bone graft substitutes, allograft, autogenous bone and soft tissue replacement products and viscoelastics

        Other Products: power equipment, casting materials, soft goods, bracing systems, bone growth stimulators, maxillofacial fixation products, diagnostics, cement and cement mixing/delivery systems, infection control equipment, pulsed lavage/irrigation systems, continuous passive motion machines, etc.

Orthopaedic Ceramic Implants

Interest in ceramics for biomedical applications has increased over the last thirty years. The ceramics that are used in implantation and clinical purposes include aluminium oxide (alumina), partially stabilised zirconia (PSZ) (both yttria tetragonal zirconia polycrystal [Y-TZP] and magnesia partially stabilised zirconia [Mg-PSZ], bioglass ®, glass-ceramics, calcium phosphates (hydroxyapatite and ß-tricalcium phosphate) and crystalline or glassy forms of carbon and its compounds.

Materials For Total Hip Replacements

As early as 1960’s and 1970’s in Europe, Charnley, Scales, McKee, Ring and Muller developed either metal-polyethylene (M-PE) bearings for total hip replacements (THR) or all metal (M-M) cobalt-chromium-molybdenum (CoCr) heads and cups.

Alumina-Alumina Femoral Heads and Acetabular Cups

At the same period an alternative concept introduced by Boutin in France was the alumina ceramic cup combined with an alumina femoral head (A-A). Boutin was concerned about tissue reactions to both metal and plastic debris and was therefore intrigued by the reputation of alumina ceramic as a highly wear resistant bearing surface for extreme conditions.

Advantages of Using Ceramics in Total Hip Replacements

Both alumina and zirconia are currently used in total hip replacements as the femoral head and liners generating reductions in wear particles from ultra-high molecular weight polyethylene used in various other combinations.

Revision Surgery

The one thing that a surgeon can be sure of when performing a total hip replacement in a young active patient is that sooner or later revision surgeries will be necessary. When that revision surgery is performed, shortage of bone will be present and allograft bone grafting may be necessary. This generates extra pain and suffering for the patient and increased cost to the society.

The Human Hip Joint

The hip is structurally complex. It is made of bone, ligaments, soft tissue, cartilage and muscle. The hip joint is an enarthrodial, or ball and socket joint. The femur or thigh bone has a ball at the end of it, which is called the head of the femur. The socket forms part of the pelvis known as the acetabulum. For the hip to function well, a ball and socket joint is supported by a large muscle mass, known as gluteal muscle. Ligaments provide stability for the joint. Soft tissues, nerves and blood vessels provide the hip with nourishment sensation and protection. The bones that comprise the joint are surrounded with a thin lining of cartilage. This cartilage is known as articulating cartilage (cartilage that surrounds the bony surface). Articulating cartilage acts as a shock absorber and enables the joint to move smoothly in its range of movement.

Loading of the Hip Joint

The hip is designed to carry varying loads throughout the subject’s daily routine; these can be running, jumping, and stair climbing and rising out of a chair. Throughout all these activities there is some hip joint motion and forces are transferred to the joint.

Loading of a Hip Replacement Prosthesis

These joint forces produce stresses in the head and neck of the femur. Furthermore a torque about the long axis of the femur is also produced which generates additional stresses. If the hip joint has been replaced with a total hip prosthesis, these loads are invariably applied to the prosthesis of the hip.

The Affect of Osteoarthritis

The osteoarthritic process affects primarily the surface of the hip joint, both on the femoral head side, and on the acetabular side. The most logical approach would be to replace the damaged surface with articular cartilage, the same as the lost material. Current technology does not allow us to do that.

Artificial Hips

Early application of the use of this technique has led to failure, because of problems related to materials, engineering, and surgical techniques. In the past, metals have not been durable enough to allow long term use without wear, which was the most significant problem, which lead to catastrophic failure.

Advances in Prosthesis Manufacture

Engineering technology in the past was not advanced enough to allow the manufacturing of the implants on a predictable level and regular basis with good surface finish and fine tolerances to allow long term use of these components. The result has been a compromise in indicators and design of the current standard hip replacement components.

Advances in Surgical Technique

Surgical technique previously had not developed to the point where the insertion of these components could be carried out accurately and without damaging the blood supply to the proximal femur, leading to the development of avascular necrosis. These problems have now been overcome. It is now technically possible for these devicee to be implanted and hip function to be restored to a level not previously possible.

Hip Replacement Procedures

The standard hip replacement involves total removal of the femoral head and most of the neck to allow the insertion of a femoral component (total hip replacement). This might lead to problems with orientation, stress shielding (hence bone resorption and reduced bone density), and possible dislocation. Because of the wear characteristics of the materials used, it was necessary to have a small femoral head as a bearing component (to reduce the wear of the acetabular component). However, the presence of a small femoral head led to problems with stability, and the necessity to remove the femoral neck led to problems with limb length equality.

The Total Hip Replacement (THR)

There are two major types of the standard artificial hip joint: cemented and uncemented prosthesis. Each of the prostheses is made up of two parts: the acetabular component, or socket portion, which replaces the acetabulum and the femoral component, or stem portion, which replaces the femoral head.

The Femoral Components of the Total Hip Replacement

The femoral component is made of a metal stem with a metal ball on the end. Some prostheses have a ceramic ball attached to the metal stem.

The Acetabular Components of the Total Hip Replacement

The acetabular component is a metal shell with a plastic inner socket liner that acts like a bearing.

Cemented Prostheses

A cemented prosthesis is held in place by polymethylmethacrylate (PMMA) cement that attaches the metal to the bone.

Uncemented Prostheses

There are various different methods for the uncemented prosthesis. Macrotexturing with mesh or beads has been very popular. Such prostheses have a fine mesh of holes on the surface area that touches the bone. The mesh allows the bone to grow into the mesh and “becomes part of” the bone (mechanical interlock). A better alternative, which induces both chemical bonding and mechanical interlock within the pores with excellent compatability, is the biphasic hydroxyapatite coating on the stem component.

The Surgical Approach to a Total Hip Replacement

The steps for replacing the hip, begins with making an incision about 20 cm long over the hip joint. After the incision is made, the ligaments and muscles are separated to allow the surgeon access to the bones of the hip joint.

Replacing the Acetabular Component

Once the hip joint is entered, the femoral head is dislocated from the acetabulum. Then the femoral head is removed by cutting through the femoral neck with a power saw. The cartilage is removed from the acetabutum using a power drill and a special reamer. The reamer forms the bone in a hemispherical shape to exactly fit the metal shell of the acetabular component. In the uncemented variety of artificial hip replacement, the metal shell is simply held in place by the tightness of the fit or with screws to hold the metal shell in place. In the cemented variety, special PMMA type cement is used to “glue” the acetabular component to the bone.

Replacing the Femoral Component

To begin replacing the femoral head, special rasps are used to shape and hollow out the femur to the exact shape of the metal stem of the femoral component. The surgeon will also test the movement of the hip joint. Once the size and shape of the canal exactly fit the femoral component the stem is inserted into the femoral canal. The metal ball that replaces the femoral head is attached to the femoral stem.

Stabilising the Prosthesis

Stability is produced, in part, by soft tissue tension in the muscles and ligaments around the hip. Sometimes it is necessary after a total hip replacement to lengthen the leg, which tightens the soft tissues and to improve the stability of the hip. Leg length difference is usually less than 5 mm in the vast majority of cases, but can be up to 25 mm in unusual circumstances.

Implant Dislocation

Dislocation of components is a potential risk of joint replacement. Approximately 1% of components dislocate in the immediate post-operative period. These dislocations relate to problems of restoration of patient’s anatomy with a limited range of “off the shelf” total hip replacement designs. In the vast majority of these cases, treatment of this problem requires manipulation of the joint. If unsuccessful in relocating the prosthesis, a second open surgical procedure may be necessary to correct the situation. The long term studies have shown that the rate of dislocation increases 1% for every decade that the implant is in place. This is probably the result of wear of the components leading to increased laxity, and further increased laxity in the capsular structures with the ageing process.

Success of Hip Replacements in Australia

Hip replacement in Australia is less successful than reported in some other countries. Analysis of recent data suggests that the revision rate in Australia is likely to be over 20%, which contrasts with 7-8% in Sweden.

Summary

The properties of bioceramics are strongly influenced by the raw materials selected for preparation, by the method used to fabricate, processes used to consolidate and final machining processes utilised. All of these factors contribute to their final structures and hence to their long term performance as bioceramics. The optimisation of bioceramics in medical applications can be achieved by further studies of the effects of processing conditions on their structures and hence on their long term properties.

In particular, the nature and effect of additives, whether to improve biological performance or to ease processing, on the local and systemic responses needs further investigation. Sometimes improvement of one property could be detrimental to other properties.

Bioceramics – More Advanced Applications

In the early 70’s, bioceramics were employed to perform singular, biologically inert roles, such as to provide parts for bone replacement. The realisation that cells and tissues in the body perform many other vital regulatory and metabolic roles has highlighted the limitations of synthetic materials as tissue substitutes. Demands of bioceramics have changed from maintaining an essentially physical function without eliciting a host response, to providing a more integrated interaction with the host. This has been accompanied by increasing demands from medical devices to improve the quality of life, as well as extend its duration. Bioceramics potentially can be used as body interactive materials, helping the body to heal, or promoting regeneration of tissues, thus restoring physiological functions. This approach is being explored in the development of a new generation of bioceramics with a widened range of applications.

The Future for Bioceramics

Ultimately, the field of bioceramics is fundamental to advances in the performance and function of medical devices, and is a critical part of medicine and surgery. Bioceramics science is truly interdisciplinary. Therefore, the development of improved bioceramics can only be the outcome of advances in physical and biological sciences, engineering and medicine. The correlations between material properties and biological performance will be useful in the design of improved bioceramics, particularly to overcome the problems of implant rejection and related infection.

Note: A complete list of references is available by referring to the original text.

Primary author: R. Cordingley, L. Kohan, B. Ben-Nissan and G. Pezzotti.

Source: Abstracted from the Journal of the Australasian Ceramic Society, Vol. 39, No. 1, pp. 20-28, 2003.

For more information on this source please visit The Australasian Ceramic Society.

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