[ Back to index of term-papers ]
ARTIFICIAL HIP JOINTS
Binnur Özkan
ANATOMY OF THE NORMAL HIP JOINT
The hip joint is located where the thigh bone (femur) meets the pelvic bone. It is a “ball and socket” joint. The upper end of the femur is formed into a round ball (the “head” of the femur). A cavity in the pelvic bone forms the socket (acetabulum). The ball is normally held in the socket by very powerful ligaments that form a complete sleeve around the joint (the joint capsule). The capsule has a delicate lining (the synovium). The head of the femur is covered with a layer of smooth cartilage, which is a fairly soft, white substance about 1/8 inch thick. The socket is also lined with cartilage (also about 1/8 inch thick). The cartilage cushions the joint, and allows the bones to move on each other with very little friction. An x-ray of the hip joint usually shows a “space” between the ball and the socket because the cartilage does not show up on x-rays. In the normal hip this “joint space” is approximately 1/4 inch wide and fairly even in outline.
An X-Ray and Illustration Showing a Normal Hip Joint
The term “arthritis” literally means inflammation of a joint, but is generally used to describe any condition in which there is damage to the cartilage. Inflammation, if present, is in the synovium. The proportion of cartilage damage and synovial inflammation varies with the type and stage of arthritis. Usually the pain early on is due to inflammation. In the later stages, when the cartilage is worn away, most of the pain comes from the mechanical friction of raw bones rubbing on each other.
An X-Ray and Illustration Showing an Arthritic Hip Joint
People who suffer from chronic hip pain or degenerative joint disease of the hip, often have their ailing hip joint replaced with an artificial one. These artificial joints (or hip implants) are made of metal, such as titanium or stainless-steel, and have long stems which penetrate deep into the femur canal (center of the thigh bone) to hold them in place. (See Figure 1) In a surgical operation known as Primary Total Hip Replacement (PTHR), the surgeon drills a hole down the femural canal, and inserts the implant firmly into the hole, sometimes using special cement to ensure that the implant stays tightly fixed in place.
Figure shows an X-ray of a patient who has received a hip implant.
Typically, these implants begin to loosen, or otherwise fail, after 10-15 years, at which point the implant needs to be surgically replaced. This procedure is known as Revision Total Hip Replacement Surgery (RTHR). In this surgery, the failing orthopedic hip implant is replaced with a new one by removing the old implant, removing the bone cement, enlarging the implant cavity, and inserting the new implant. RTHR is more complex than PTHR: it requires more capabilities and has more uncertainty associated with it. Surgeons must plan for and remove the old implant and the old cement before cutting the new canal cavity. They must plan for the new cavity in the presence of the old implant and cement.
HISTORY OF HIP JOINT REPLACEMENT
Due to the crippling nature of arthritis, surgeons have been trying for well over a century to successfully treat this debilitating disease. It was clear that many people required surgery to relieve the terrible pain and keep their joints mobile. Initial attempts to treat arthritic hips included arthrodesis (fusion), osteotomy, nerve division, and joint debridements. The goal of these early debridements was to remove arthritic spurs, calcium deposits, and irregular cartilage in an attempt to smooth the surfaces of the joint. Indeed, there was a great search for some material which could be utilized to resurface or even replace the hip. Several proposals and trials were made including the use of muscles, fat, chromatized pig bladder, gold, magnesium and zinc. All met with failure. Surgeons and scientists were unable to find a material which was biocompatible with the body, and yet strong enough to withstand the tremendous forces placed on the hip joint.
In 1925, a surgeon in Boston, Massachusetts, M.N. Smith-Petersen, M.D., molded a piece of glass into the shape of a hollow hemisphere which could fit over the ball of the hip joint and provide a new smooth surface for movement. While proving biocompatible, the glass could not withstand the stress of walking and quickly failed. Undaunted, he pursued other materials for his "mold arthroplasty" including plastic and stainless steel. The shipping industry first used stainless steel to resist the corrosion of ocean going vessels. Its application to surgery, where it might well resist corrosion by bodily fluids, seemed natural. During the 1940's, mold arthroplasty was "state of the art."
A dramatic improvement was made in 1936 when scientists manufactured a cobalt-chromium alloy, which was almost immediately applied to orthopaedics. This new alloy was both very strong and resistant to corrosion, and has continued to be employed in various prostheses since that time. While this new metal proved to be a great success, the actual resurfacing technique was found to be less than adequate. It became clear that pain relief was not as predictable as hoped, and hip movement remained limited for many patients. Mold arthroplasty also did not allow surgeons to treat the numerous and varied arthritic deformities of the hip. The search for different types of prostheses continued.
Frederick R. Thompson of New York, and Austin T. Moore of South Carolina, separately developed replacements for the entire ball of the hip. These could be used to treat hip fractures and also certain arthritis cases. This type of hip replacement, called hemiarthroplasty, only addressed the problem of the arthritic femoral head (the ball). The diseased acetabulum (hip socket) was not replaced. The prosthesis consisted of a metal stem, which was placed into the marrow cavity of the femur, connected in one piece with a metal ball, which fit into the hip socket. While very popular in the 1950's, results remained unpredictable and arthritic destruction of the socket persisted. In addition, there was no truly effective method of securing the component to the bone. Large numbers of patients developed pain because of this loosening of the implant. The desired result was still not achieved.
As early as 1938, Dr. Jean Judet and his brother, Dr. Robert Judet, attempted to use an acrylic material to replace arthritic hip surfaces. This acrylic provided a smooth surface, but unfortunately tended to come loose. The idea did lead Dr. Edwarc J. Haboush from the Hospital for Joint Diseases in New York City to utilize a "fast setting dental acrylic" to actually glue the prothesis to the bone. A new era in fixation techniques had begun.
In England, a very innovative surgeon, John Charnley, was also attempting to solve these ongoing problems. Some of his ideas were so bold and creative that he was seriously questioned by many of his colleagues. He was relegated or banished to an isolated tuberculosis sanatorium that had been converted to a makeshift hospital. This center at Wrightington, Manchester, England, became a well-spring of knowledge for the surgical treatment of arthritis. Charnley aggressively pursued effective methods of replacing both the femoral head and acetabulum of the hip. In 1958, he addressed the eroded arthritic socket by replacing it with a Teflon implant. He hoped this would allow for a smooth joint surface to articulate with the metal ball component. When the Teflon did not achieve this goal, he went on to try polyethylene. This worked wonderfully well. In order to obtain fixation of this polyethylene socket as well as the femoral implant to the bone, Charnley borrowed polymehtylmethacrylate from the dentists. this substance, known as bone cement, was mixed during the operation then used as a strong grouting agent to firmly secure the artificial joint to the bone. Truly this was the birth of "total hip replacement."
By 1961, Charnley was performing the surgery regularly with good results. He further improved the techniques and component designs. Thousands of people were successfully relieved of their hip pain and the long term results became very predictable. The Queen of England knighted him for his immense contributions. He is now known as Sir John Charnley.
Since that time, many skilled surgeons have improved upon the concepts which started in central England. Methods of fixation and actual cementing techniques are significantly better. Refinements in the design of the prothesis have evolved to more clearly mirror the normal hip joint. Today over 100,000 hip replacements are performed annually in the United States using the principles of a low friction arthroplasty with a polyethylene socket and metal femoral prosthesis.
In the last ten years there has been considerable effort and research in trying to yet improve the methods of fixation. Occasionally it has been found that cement fixation breaks down over time. If a living type of bond could be created, this would theoretically be longer lasting and possible stronger. To this end, implants with textured surfaces that allow bone to grow into them have been developed. These have been used experimentally in animals and are now being used in humans. The results of these cementless joints look very promising when utilized in the correct circumstances.
Dr. Bertin has helped design and develops new total hip implants and instruments to use in surgery. These new innovations improved patient recovery, function and long-term results. Some of these improvements have been patented and are used by many other surgeons in the United States and around the world.
HIP JOINT REPLACEMENT
The prosthesis for total hip replacement consists of a femoral component and an acetabular component. The femoral stem is divided into head, neck, and shaft. The femoral stem is made of Ti alloy or Co-Cr alloy (316L stainless steel was used earlier) and is fixed into a reamed medullary canal by cementation or press fitting. Femoral head is made of Co-Cr alloy, alumina, or zirconia. Although Ti alloy heads function well under clean articulating conditions, they have fallen into disuse because of their low resistance to third-body wear.
The prostheses can be monolithic when they consist of one part or modular when they consist of two or more parts and require assembly during surgery. Monolithic components are often less expensive and less prone to corrosion or disassembly. However, modular components allow customizing of the implant intra-operatively and during future revision surgeries, for example, modifying the length of an extremity by using a different femoral neck length after the stem has been cemented in place or exchanging a worn polyethylene bearing surface for a new one without removing the metallic part of the prosthesis from the bone. In modular implants the femoral head is fitted to the femoral neck with a Morse taper, which allows changes in head material and diameter and neck length. Table 1 illustrates the most frequently used combinations of material in total hip replacement.
When the acetabular component is monolithic, it is made of ultra-high-molecular-weight polyethylene (UHMWPE); when it is modular, it consists of a metallic shell and an UHMWPE insert. The metallic shell seeks to decrease the microdeformation of the UHMWPE and to provide a porous surface for fixation of the cup [Skinner, 1992]. The metallic shell allows worn polyethylene liners to be exchanged. In cases of repetitive dislocation of the hip after surgery, the metallic shell allows replacing the old liner with a more constrained one to provide additional stability. Great effort has been placed on developing an effective retaining system for the insert as well as on maximizing the congruity between insert and metallic shell. Dislodgement of the insert results in dislocation of the hip and damage of the femoral head, since it contacts the metallic shell directly. Micromotion between insert and shell produces additional polyethylene debris which can eventually contribute to bone loss [Friedman et al., 1994].
Table 1. Possible Combinations of Total Hip Replacement
The hip joint is a ball-and-socket joint, which derives its stability from congruity of the implants, pelvic muscles, and capsule. The prosthetic hip components are optimized to provide a wide range of motion without impingement of the neck of the prosthesis on the rim of the acetabular cup to prevent dislocation. The design characteristics must enable implants to support loads that may reach more than 8 times body weight [Paul, 1976]. Proper femoral neck length and correct restoration of the center of motion and femoral offset decrease the bending stress on the prosthesis-bone inter- face. High stress concentration or stress shielding may result in bone resorption around the implant. For example, if the femoral stem is designed with sharp corners (diamond-shaped in a cross- section), the bone in contact with the corners of the implant may necrose and resorb.
Load bearing and motion of the prosthesis produce wear debris from the articulating surface and from the interfaces where there is micromotion. The principal source of wear under normal conditions is the UHMWPE-bearing surface in the cup. Several hundred thousands of particles are generated with each step, and a large proportion of these particles are smaller then one micron [McKellop et al., 1995]. Cells from the immune system of the host are able to identify the polyethylene particles as foreign and initiate a complex inflammatory response. This response may lead to rapid focal bone loss (osteolysis), bone resorption, loosening, and/or fracture of the bone. Numerous efforts are underway to modify the material properties of UHMWPE, to harden and improve the surface finish of the femoral head, and to develop other bearing couples, for example, ceramic-to-ceramic and metal-to-metal [Friedman et al., 1994].
TYPES OF HIP IMPLANTS
1. A Charney Stainless Steel Implants
Stainless steel is the most commonly used metal for femoral stems in hip replacements. It is an alloy of iron, chromium, nickel and molybdenum. It has extremely high resistance to corrosion, and thus does not degrade in the body. It can be shaped easily which is an important consideration for implant manufacturers who want to minimise production costs. However, problems may arise because of its relatively high stiffness, and the fact that some people may develop an allergic reaction to the nickel content. The picture shows a “Charnley” stainless steel hip implant. It is named after Sir John Charnley who pioneered in the 1960’s many of the techniques and design concepts still used today.
2. A Freeman Cobalt-chromium Implant
An alternative to stainless steel is cobalt chromium alloy (27-30% Cr, 5-7% Mo. rest Co). It has good wear properties and is more resistant to scratching. The fact that it contains no nickel means that it can be used in patients who have nickel sensitivity. The cobalt chromium implant shown here is a Freeman prosthesis designed to be used without cement and hence we can see holes for bone in-growth. The top section of the prosthesis is roughened to increase friction and hence stability. The bottom surface has been polished to prevent the stem from rubbing against the inside of the bone canal, which may lead to wear debris.
3. Johnson and Johnson titanium implants
Developed for the aerospace industry, titanium and its alloys have high strength in relation to their relatively low weight. A titanium implant has a stiffness of less than half that of stainless steel or cobalt chrome, which therefore reduces the effects of weight shielding. Its constituents give it excellent corrosion resistance, but it does suffer from a relatively low fracture toughness and poor wear properties. The two components shown are Johnson and Johnson prostheses manufactured from titanium alloy (Ti-6Al-4V). Although they are similar in geometry, the one implant would be cemented in position and the other uncemented. It can also be seen from the picture that these are modular femoral components - made up of two parts, the femoral stem and the femoral head. This allows surgeons to use finely polished wear resistant metal heads, which do not have to be the same material as the stem - an advantage considering the relatively poor wear resistance of titanium alloy.
4. PMMA Bone Cement
The second category of materials used for hip replacement are polymers. There are two main uses for them in total hip replacement. The first is as a grouting material in the form of poly(methylmethacrylate) (PMMA) bone cement. PMMA bone cement polymerises in situ. It is mixed in surgery from a polymer powder and liquid monomer, and forms a hardened material in 10-15 minutes. The main problem with PMMA bone cement is that considerable heat is released to the surrounding bone during the curing process and this causes cell death. The resulting material has poor resistance to fracture. Other problems also include the shrinkage of the cement and the release of toxic monomer into the blood stream. The other major polymer used is polyethylene. Its main advantage is its wear resistance when used as a concave acetabular cup in a total hip replacement.
5. A Ceramic Femoral Head
One means of reducing wear is to use extremely hard, polished materials that will be highly resistant to scratching and wear. Ceramics such as alumina and zirconia can be polished to produce a fine, hard surface finish. Therefore they can be used as femoral heads as shown in the picture. Another ceramic used in total hip replacements is hydroxyapatite. It consists of calcium phosphate, a mineral that forms one of the prime constituents of bone. Although it is a relatively weak and brittle material it does have good bioactivity. Therefore it can be used to coat implants, in the absence of bone cement, and achieve excellent fixation.
6. A Hybrid Implant
It is possible to combine the best mechanical properties of all the materials described and good engineering design in order to produce an implant with the optimum chance of long term clinical survival. Here is an example of such a 'hybrid' implant. It is a cobalt chromium Freeman, with a ceramic femoral head, hydroxyapatite coating and a nitrided surface finish, which hardens the surface of the stem and helps prevent scratching and the release of metal wear debris. However, there is one more parameter, which plays an important role in implant design i.e., is cost. Material scientists are constantly faced with the challenge of producing optimum material properties at minimum cost[1,2,3,4,5,6,7,8].
STUDIES ON ARTIFICIAL HIP JOINTS
There are important reasons for the failure of hip joints. One of them is friction and wear. A good material combination for total joint replacements must have a low friction and wear rate. The friction can be divided into a start up friction and steady state friction. However, the start up friction is more important than the other because it is much higher than the steady state friction and results in a much higher wear rate.
Wear rate is often substantial and the wear particles cause harmful tissue reactions that may lead to destruction of bone around the implant and consequently loosening of the component fixation. In laboratory studies on tribology of prosthetic joint materials. the role of the lubricant is naturally crucial, but there is no practical and reliable lubricant available.
Most of total hip joints are composed of ultra-high molecular weight polyethylene UHMWPE socket because of good biocompatibility, high resistance to wear and durability. However, as UHMWPE is too stable in a body, wear debris may accumulate and cause biological response such as bone absorption and loosening of prosthesis. To reduce the affect of wear debris, alumina has been used for hip prosthesis as the substitute for UMHWPE.
As Young’s moduli of ceramics are generally high, all-ceramic hip joints should have good conformity between their sockets and femoral heads and smooth surfaces to avoid stress concentration. However, the lubrication between highly conformed surfaces of ceramics under physiological condition has not been understood enough.
Zhou, Ikeuchi and Ohashi[9] studied on comparison of the friction properties of four different ceramic materials used in joint replacement. In this study, they used alumina, zirconia, silicon carbide and silicon nitride as ceramic materials. They compared the start up and steady state friction of these materials against themselves lubricated with 1 wt% water solution of carboxymethyl cellulose sodium salt (CMC-Na 1 wt% water solution). They used a pin-on-disc testing machine to study the start up and steady state friction.
They found that the coefficient of start up friction increases with increasing resting time for silicon carbide and silicon nitride, because the fluid film and boundary film are gradually squeezed out of the contact region. In contrast, the start up friction of the alumina-on alumina combination decreases with increasing resting time. The start-up friction coefficients of four ceramic materials increase with increasing load, except for silicon carbide with 3 s. of resting time. They indicated that the comparison of the start up friction of the four ceramic materials shows that the coefficient of start up friction of silicon nitride is the highest and that of silicon carbide is the lowest.
Moreover, the same results were obtained for steady state friction. They stated that, as a conclusion, silicon carbide-on-silicon carbide and silicon nitride-on-silicon nitride can be good material combinations for total hip joint replacement, from the tribological point of view.
Another study was carried on by Kusaka et.al.[10], to investigate friction and wear between nominally flat surfaces silicon carbide and silicon nitride against themselves in bovine serum solution. They used the end-face friction apparatus. Their results are similar with the previous study, which I mentioned above.
The authors investigated friction and wear of the three ceramic materials alumina, silicon carbide and silicon nitride against themselves between nominally flat surfaces in bovine serum solution. Silicon nitride shows particularly high coefficient of friction and specific wear rate due to the tribochemical reaction. Alumina, the only material currently used for all-ceramic hip joint, shows very small coefficient of friction and little wear. Silicon carbide also shows small friction and wear, though it is not used for hip joint now. And if generation of the wear debris is predicted so little, it would be excreted without accumulating in a body and the effect of wear debris would be avoided. According to their results, silicon carbide may be another candidate for all-ceramic hip joint. However, they stated that biological response to wear debris of ceramics needs further investigation.
The other study, done by Saikko and Ahlroos[11] was considered phospholipids as boundary lubricants in wear tests of prosthetic joint materials. Bovine serum and distilled water are used as lubricants in the wear tests, but they both have considerable shortcomings. With serum, the main problem is degradation, and with water, the heavy polyethylene transfer to metallic counterfaces leading to increased polyethylene wear. There is often discrepancy between laboratory and clinical wear. This has caused scepticism as to whether laboratory tests really can be used as a prediction of the tribological performance of prosthetic joint materials in the human body. Therefore, a practical and reliable "artificial joint fluid" would be most welcome. In the development of such a fluid, which is obviously a very challenging task, it must be shown that the wear tests lubricated by this fluid yield wear mechanisms and wear rates comparable to those observed in prostheses removed from patients.
Wear and friction tests, lubricated by phospholipid-water dispersions, were done with UHMWPE sliding against polished Co-Cr-Mo alloy counterfaces, the most common material combination in prosthetic joints. Three different test machines were used: a pin-on-flat reciprocator, a uniaxial hip joint simulator, and a three-axis hip joint simulator.
Four criteria, based on the known behaviour of prosthetic joint materials in the human body, were used in the evaluation of the performance of the candidate lubricants.
1. There should be no polyethylene transfer to the metallic counterface.
2. The polyethylene bearing surface should look polished, reflecting more light than the original surface.
3. Most of the polyethylene wear debris should consist of microscopic particles - 1 mm in size.
4. The wear factor should be of the order of 1 X 10-6 mm3 N-1m-1.
The phospholipids studied were DPPC and soybean lecithin. The DPPC was synthetic, 99% pure L-a-phosphatidyl-choline, dipalmitoyl, P-0763 by Sigma Chemical Co. The soybean lecithin was Lipoid S 30, manufactured by Lipoid GmbH. It is a mixture of various phospholipids, the main component (32%) being phosphatidylcholine. In addition, it contains phosphatidylethanolamine, phosphatidylinositol, lysophosphatidylcholine, and triglycerides. All of these are found in joint fluid. The following lubricants were prepared.
A. DPPC 3 mg ml-l in distilled water.
B. DPPC 3 mg ml-l in 0.lM Na2HPO4-NaH2PO4phosphate buffer, pH 7.4
C. Soybean lecithin 1 mg ml-l in distilled water
D. Soybean lecithin 3 mg ml-l in distilled water
They found that phospholipids had a remarkable effect on the wear behaviour of UHMWPE sliding against Co-Cr-Mo alloy. The prevailing lubrication mechanism was obviously of boundary or mixed type: polyethylene transfer would not have occurred if there was a hydrodynamic fluid film completely separating the surfaces. In the earlier, water-lubricated tests heavy polyethylene transfer to the Co-Cr-Mo counterface led to considerable polyethylene wear, although the amount of wear did not straight forwardly correlate with the roughness of the counterface. It was therefore highly surprising that in the present simulator tests lubricated by the soybean lecithin dispersion the wear of the UHMWPE acetabular cups was negligible although there was heavy polyethylene trans- fer to the Co-Cr-Mo femoral heads. This observation contradicts the common belief that heavy polyethylene transfer always leads to high polyethylene wear. Apparently, a boundary lubricant that is unable to prevent the polyethylene transfer can still prevent the actual wear of the polyethylene component by lubricating the contact of polyethylene against itself. In effect, in this case polyethylene slides against a Co-Cr-Mo counterface with scattered protruding lumps of polyethylene strongly adhering to the Co-Cr-Mo surface.
Williams III et. al.[12], studied the fabrication and characterization of dipalmitoylphosphatidylcholine-attracting elastomeric material for hip joint replacements. Wear debris associated with the polyethylene components of total joint replacements has been shown to induce bone resorption which contributes to implant loosening. It has been shown that elastohydrodynamic film formation in a knee prosthesis with an elastomeric tibial component is favourable compared with a polyethylene component. These elastomeric bearings have been termed cushion bearings. A hip model has been used previously by others to study the possibility of establishing a fluid film in a cushion bearing subjected to dynamic loading and oscillating motion.
In an effort to promote lubrication and reduce wear in artificial hip joints, the use of the cushion bearing concept has been proposed previously; however, an elastomeric material tested as a cushion bearing has been shown to have poor tribological properties during initiation of motion from rest. Their goal was to fabricate and characterize an elastomer that has the ability to attract to its surface naturally occurring boundary lubricants from an aqueous solution. Tecoflex SG93A (Thermedics Inc., Woburn, MA. USA), a medical grade aliphatic polyurethane, as a potential bearing material for use in the acetabular cup of hip joint prostheses. Since Tecoflex SG93A has already been proposed for use in an orthopaedic application and its tribological properties have been characterized previously in an elastohydrodynamic lubrication regime, this material was chosen as a bulk material which will impart elastomeric properties to the final experimental material. Since Tecoflex is not able to specifically attract boundary lubricants, its surface was altered to enhance its ability to attract DPPC. This surface treatment should not alter the bulk mechanical properties of the Tecoflex to the point of making it unsuitable as a cushion bearing.
The experimental material used to evaluate the concept of attracting naturally occurring boundary lubricants to enhance the tribological properties of implant materials consists of a Tecoflex SG93A solution-cast film whose surface contains poly(MPC-co-BMA).
The test elastomer and appropriate controls were characterized using fluorescence, electron spin resonance and X-ray photoelectron spectroscopy. They stated that the test elastomer was found to have an enhanced ability to attract dipalmitoylphosphatidylcholine, a known physiological boundary lubricant. A cushion bearing that also encourages boundary lubrication represents a potential improvement over currently existing orthopaedic implant-bearing materials.
Wear and the biological response to wear debris of artifical joints remain major concerns in total hip arthroplasty (THA). The long-term effects of UHMWPE wear debris are well documented and these have led to interest in alternate bearing materials for THA. Alumina ceramic-ceramic hip joints have been successfully used for more than 30 years with low wear and little incidence of osteolysis. The most common wear pattern observed on retrieved components is an elliptical wear ‘stripe’ on the heads and a corresponding worn area on the cup with an approximated wear rate of 1.5 mm3 pa. More severe wear has also occasionally occurred, usually in association with an abnormal clinical history.
Modern alumina-alumina THAs use an improved HIPed (hot isostatically pressed) alumina ceramic-bearing material which may be more resistant to severe wear. Previous in vitro simulator studies have not replicated in vivo wear rates or mechanisms.
Nevelos et. al.[13], aimed to compare previous generation non-HIPed alumina and modern HIPed alumina in a hip joint simulator under ‘normal' and ‘harsh' testing conditions. HIPed alumina was found to have a lower wear rate than non-HIPed alumina, although the difference was not statistically significant at the 95% confidence level. They stated that testing in Gelofusineâ and water lubricants did not elevate the wear rates of either material. They also found that elevated swing phase load testing also had no significant effect on the wear rates of either material. Furthermore, they stated that testing in the absence of any lubricant produced very severe wear of the non-HIPed material in one specimen only.
Hatton et. al.[14], studied on alumina-alumina artificial hip joints. Their work consists of two parts. In Part I, they studied on a histological analysis and characterization of wear debris by laser capture microdissection of tissues retrieved at revision. They investigated the tissues from uncemented Mittelmeier alumina ceramic-on-ceramic total hip replacements using histological methods and to isolate and characterise the ceramic wear debris using laser capture microdissection and electron microscopy. Tissues from around 10 non-cemented Mittelmeier alumina ceramic on ceramic THRs were obtained from patients undergoing revision surgery. Tissues were also obtained from six patients who were undergoing revisions for aseptic loosening of Charnley, metal-on-polyethylene prostheses. Tissue sections were analysed using light microscopy to determine histological reactions and also the location and content of alumina ceramic wear debris. Tissue samples were extracted from sections using laser capture microdissection and the characteristics of the particles subsequently analysed by TEM and SEM.
The tissues from around the ceramic-on-ceramic prostheses all demonstrated the presence of particles, which could be seen as agglomerates inside cells or in distinct channels in the tissues. The tissues from the ceramic-on-ceramic retrievals had a mixed pathology with areas that had no obvious pathology, areas that were relatively rich in macrophages and over half of the tissues had in the region of 60% necrosis/necrobiosis. In comparison, they found that the Charnley tissues showed a granulomatous cellular reaction involving a dense macrophage infiltrate and the presence of giant cells and <30% necrosis/necrobiosis. The tissues from the ceramic prostheses also showed the presence of neutrophils and lymphocytes, which were not evident in the tissues from the Charnley retrievals. There were significantly more macrophages (p<0.05), and giant cells (p<0.01) in the Charnley tissues and significantly more neutrophils (p<0.01) in the ceramic-on-ceramic tissues.
TEM of the laser captured tissue revealed the presence of very small alumina wear debris in the size range 5–90 nm, mean size +SD of 24+19 nm whereas SEM (lower resolution) revealed particles in the 0.05–3.2 mm size range. They said that this is the first description of nanometer sized ceramic wear particles in retrieval tissues. The bi-modal size range of alumina ceramic wear debris overlapped with the size ranges commonly observed with metal particles (10–30 nm) and particles of ultra-high molecular weight polyethylene (0.1–1000 mm). It is possible that the two size ranges of contributed to the mixed tissue pathology observed. It is speculated that the two types of ceramic wear debris are generated by two different wear mechanisms in vivo; under normal articulating conditions, relief polishing wear and very small wear debris is produced, while under conditions of microseparation of the head and cup and rim contact, intergranular and intragranular fracture and larger wear particles are generated.
In the second part of this study[15], they aimed to characterise the wear particles generated from standard simulator testing and microseparation simulator testing of hot isostatically pressed (HIPed) and non-HIPed alumina ceramic-on-ceramic hip prostheses, and compare these particles to those generated in-vivo. Until recently it was not possible to reproduce clinically relevant wear rates and wear patterns in in-vitro hip joint simulators for alumina ceramic-on-ceramic hip prostheses. The introduction of microseparation of the prosthesis components into in vitro wear simulations produced clinically relevant wear rates and wear patterns for the first time. Standard simulation conditions produced wear rates of » 0.1 mm3 per million cycles for both material types.
No change in surface roughness was detected and very few wear features were observed. In contrast, when microseparation was introduced into the wear simulation, wear rates of between 1.24 (HIPed) and 1.74 mm3 per million cycles (non-HIPed) were produced. Surface roughness increased and a wear stripe often observed clinically on retrieved femoral heads was also reproduced. Under standard simulation conditions, they found that only nanometre-sized wear particles (2–27.5 nm) were observed by TEM, and it was thought likely that these particles resulted from relief polishing of the alumina ceramic. However, when microseparation of the prosthesis components was introduced into the simulation, a bi-modal distribution of particle sizes was observed. The nanometre-sized particles produced by relief polishing were present (1–35 nm), however, larger micrometre-sized particles were also observed by both transmission electron microscopy (TEM) (0.02–1 mm) and scanning electron microscopy (SEM) (0.05–>10 mm). These larger particles were thought to originate from the wear stripe and were produced by trans-granular fracture of the alumina ceramic.
This study (Part II) has revealed that the introduction of microseparation of the prosthesis components during the swing phase of the wear simulation reproduced clinically relevant wear rates, wear patterns and wear particles in in-vitro hip joint simulators.
REFERENCES:
[1] Huddleston, H.D., Arthritis of the Hip Joint, 5th Edition 2001.
[2] Effective Health Care, Vol. 2, No.7, ISSN: 0965-0288, October 1996.
[3] http://www.hipreplacement.co.uk/Hipjnt.htm
[4] http://www.utahhipandknee.com/history.htm
[5] Handbook of Biomaterial Properties, edited by Jonathan Black, Garth Hastings, London : Chapman & Hall, 1998.
[7] Clinical Performance of Skeletal Prostheses, edited by Larry Hench, June Wilson, Chapman&Hall, 1996.
[8] Faulkner, A., Kennedy, L.G., Baxter, K., Donovan, J., Wilkinson, M., Bevan, G., Effectiveness of hip prostheses in primary total hip replacement: a critical review of evidence and an economic model. Health Technology Assessment Vol.2, No.6, 1998.
[9] Zhou, Y.S., Ikeuchi, K., Ohashi, M., Wear Vol.210, p. 171-177, 1997.
[10] Kusaka, J., Takashima, K., Yamane, D., Ikeuchi, K., Wear, Vol. 225-229, p. 734-742, 1999.
[11] Saikko, V., Ahlroos, T., Wear, Vol. 207, p. 86-91, 1997.
Wear, Vol. 225-229, p. 734-742, 1999.
[12] Williams III, P.F., Powell, G. I., Love, B., Ishihara, K., Nakabayashi, N., Johnson, R., LaBerge, M., Biomaterials, Vol. 16, p. 1169-1174, 1995.
[13] Nevelos, J.E., Ingham, E., Doyle, C., Nevelos, A.B., Fisher J., Biomaterials, Vol. 22, p. 2191-2197, 2001.
[14] Hatton, A., Nevelos, J.E., Ingham, E., Nevelos, A.B., Banks, R.E., Fisher, J., Biomaterials, Vol. 23, p. 3429-3440, 2002.
[15] Tipper, J.L., Hatton, A., Nevelos, J.E., Ingham, E., Nevelos, A.B., Fisher, J., Doyle, C., Streicher, R., Biomaterials, Vol. 23, p. 3441-3448, 2002.
