COMBINATIONS OF BEARING SURFACES FOR TH
CONTENTS
Possible combinations of material
Benefits and risk of different material combinations
Possible combinations of material for cup and ball components
In a total hip joint there are these possible combinations of bearing materials for joint surfaces:
Polyethylene cups may be coupled with metallic or ceramic balls
Moreover, polyethylene cups may be fabricated from either a cross-linked UHMWPE (ultrahigh molecular weight polyethylene) or from the conventional UHMWPE
Metallic cups may be coupled with metallic balls*)
Ceramic cups may coupled with ceramic balls.
*) recently also with ceramic balls (2007)
Picture
Combinations of bearing surfaces for total hips
Click on the icon for a full size picture
As you can see from the picture different bearing surface combinations produce different amount of wear particles (measured as cubic millimeters per year). The worst "shedder" of wear particles is a total hip where the cup made from conventional UHMWPE articulates against a metallic ball component.
Such combination produces ( when tested in hip simulator) 56 cubic millimeters of UHMWPE wear particles.
The lowest production of wear particles has a total hip where a ceramic cup articulates against the ceramic ball; it produces only 0.004 cubic millimeters of ceramic wear particles. (Heisel 2003)
Testing the wear rates in hip joint simulators
The combinations of different bearing surfaces are tested in special machines, called hip joint /knee joint simulators, and their wear is measured. The values from these simulator experiments are then used by manufacturers in advertising instead of results achieved in patients. Only one manufacturer, however, also says that " the results of in vitro tests have not been shown to correlate with clinical mechanisms" (Zimmer, 2004, advert in J Bone Joint Surg 86-B).
The volumes of wear particles are given per millions of cycles and possibly recalculated for one year's walking activity of the "normal" patient. (one million steps).
The goal of selecting materials for coupling construction
The goal is to select materials that have
the lowest production of wear particles.
the lowest friction resistance
The reasons are following
production of wear particles from the joint surfaces triggers the development of osteolysis - bone dissolving disease - that leads to loosening of the total joint. According to the current theory, the more wear particles are formed the higher the risk of osteolysis.
high friction resistance is transmitted to the interface between the skeleton and the artificial joint. If the friction is too high it may eventually cause the mechanical loosening of the total joint from its fixation with the skeleton.
Note that not all couplings made from materials with low friction produce also low quantities of wear particles. Teflon is such an example, it wears out tremendously, although it has very low friction resistance. Total hip prostheses fabricated from this material caused a patient catastrophe in the past.
Engineering practice showed that it is favorable to have the two coupled surfaces to be fabricated from two different materials. This is not always possible with construction of total joints.The convex bearing surface should be made from a harder material than the concave surface.
Occasional total hip models with ball components (convex surface) made from polyethylene ("soft top models") caused catastrophic failure rates.
Benefits and risks of different material combinations for total hip bearing surfaces:
Polyethylene cup
(articulating with metal or ceramic ball):Benefits: cheap, need not high precision with manufacture, long experience, no toxicity of wear particles
Risks: production of large number of wear particles with risk for osteolysis.
Cross-linked polyethylene cup (articulating with metal or ceramic ball):
The benefits are: high wear resistance, no toxicity of wear particles, low cost.
The risks are: reduction of other material properties (stiffness, crystallinity, etc) with risk for gross material failure, production of smaller wears particles with risk for increased osteolysis
Metal-on-metal total hip
The benefits are: long experience (although with models made from another alloys), high resistance to wear, favors larger diameters of ball and cup components
The risks are: increased metal ions blood levels with unknown general effects, development of delayed-type metal hypersensitivity.
Ceramic-on-ceramic total hip:
The benefits are: the highest wear resistance, really no toxicity of wear particles, long experience (although with older models)
The risks are: sensitive to right positioning of components, chipping of the ceramic cup liner, risk of fracture in old models. (Heisel 2003)
Newly introduced materials
The selection of the new materials for bearing surfaces of the total joints is discussed only in relation to young age patients. The reasons given are the following:Patients who are young and active require an extremely durable total joint because their high activity poses very high loads on the bearing surfaces of the artificial joint. At the same time the expected length of life of the young patients is several decades and it is desirable that the artificial joint does not fail during the patient’s lifetime.
Thus the surfaces of the total joint should be extremely wear-resistant, produce minimal quantity of wear particles, and they should perform for decades, and without suffering from the adverse effects of accumulated wear products.
During the last decade several modifications of bearing materials for artificial joints were introduced.
The new cross-linked polyethylenes (PEs)
produce (in laboratory) significantly less wear particles than the conventional polyethylene. This characteristic may be important to reduce the risk of osteolysis around total joint prostheses in young patients. The scientists believe that the risk of developing osteolysis and failure of the prosthesis increases with the quantity of produced wear particles.
The young patients will live long and the quantity of wear particles around their total joints will increase more than in old patients with a shorter life span. It is thus important that the polyethylene components in total joint for young patients are made from polyethylene with lowest possible wear.
However, choosing among the several variations of the cross-linked PEs is difficult. More cross-linked materials have certainly lower wear. On the other hand, highly cross-linked PEs are more prone to fatigue failure and have lower mechanical strength. In total joints with high localized stresses (total knees) the new cross-linked PEs may fail.
Moreover, the cross-linked polyethylenes wear less but produce very tiny wear particles. Some surgeons believe that the tinniest polyethylene wear particles are most dangerous because they cause the strongest tissue reaction and osteolysis.
Metal-on-metal bearings
Were reintroduced recently with improved alloys, design, and manufacturing. Currently metal-on-metal bearings are used for surface hip replacement and for total hip replacement. They have clinically proven wear resistance higher than that of metal-on-polyethylene bearings.
They have two other big advantages. First, the metal on metal bearing couple will polish away scratches on the surfaces, the so-called self-polishing capability.
Second, total hip joints with very large femoral ball components and thin-walled large cups may be manufactured practically only from this material. Total hips with very large femoral balls are resistant against dislocation.
There is, as yet, one disadvantage. Metal on metal couplings produce high blood levels of metal. As yet, the significance of this fact is not clear.
Ceramic-on-ceramic
They are the most expensive bearing option. They produce the lowest quantity of wear particles.The wear rate of ceramic-on-ceramic total hips is more sensitive to precise position of the components. The contact between two ceramic bearing surfaces must be on large areas, when the contact occur on small areas there arise large local stresses with considerable wear (so called stripe wear).
Long-term clinical studies are needed to show a reduction in revision surgery associated with the use of this current generation of bearings.
(Campbell P et al.: Clin Orthop 2004; 418: 98-111
Heisel C et al.: Instr Course Lect 2004; 53: 49 –65)
FRICTION
Friction is the force which brakes the movement of two surfaces gliding against each other. Higher friction between the socket and the ball of a total hip will be transmitted on the interface between the joint prosthesis and the skeleton. In theory, higher friction may damage the fixation of the prosthesis to the skeleton and accelerate loosening of the prosthesis.
All currently used coupling combinations have almost equally low friction resistance, although their friction resistance is still considerably higher than the friction resistance of the healthy hip joint.
In the past, many metal-on-metal total hips failed due to high friction. This was caused by faulty manufacturing process of the components that left no tolerance between the components. The components stuck (gal) together.
The modern metal-on-metal total hip joints have, according to the manufacturer, equally low friction resistance as all other currently used couplings.
The surgeons believe that the friction resistance in modern total hip joint prostheses is so low that it does not contribute to the mechanical loosening of the joint prosthesis.
WEAR
The bearing surfaces gliding against each other in the artificial hip joint produce a large numbers of very tiny wear particles that are torn off the artificial joint’s surface. It is always the softer material that wears out. In polyethylene-metallic or polyethylene - ceramic coupling the wear process produces polyethylene particles.
The quantity of wear particles depends on the relative travel distance between the bearing surfaces. The surface of a larger ball travels larger distance than the surface of a small ball at each step, thus the larger the ball of the total hip joint the more wear particles it will produce.
The scope of the wear problem:
An average patient with a conventional total hip (a polyethylene cup against metallic head) takes one million steps annually. At every step, he / she produces up to 400 000 tiny polyethylene particles. These particles land in the soft tissues around the total hip and some of them are transported from there further in the body. One does not know much about their fate.
If the particles are too many, they may trigger the osteolysis (bone dissolving disease) in the soft tissues around the total hip joint. Every particulate matter, not only polyethylene, may trigger the development of osteolysis. It is thus important to construct material couplings that produce the lowest rates of wear particles. According to the laboratory experiments, ceramic cups paired with ceramic heads have the lowest wear rates.
Other factors that influence the wear
There are more factors that influence the production of wear particles than only the choice of bearing materials. Size (diameter) of the femoral ball, conformity and roughness of the surfaces, patient’s weight and gender, patient's activity,and use of bone cement are some such factors. (Lewis G). There is, however, no consensus among the surgeons how important these factors are.
Femoral ball diameter
Some surgeons recommend the use femoral heads with a small (22 mm) diameter. They argue that these balls produce low numbers of polyethylene wear particles and consequently these total hip prostheses should have very low rates of failure. (On the other hand, the smaller the femoral ball the higher the risk of dislocation.)This recommendation is not substantiated by convincing statistics. There are large statistics that show no difference between the failure rates of prostheses with 22 mm and 28 mm diameter balls. (Swedish National Hip Registry, 2000). One may object that the difference in wear particles production between 22 and 28 millimeters ball components is too small and has no clinical consequence.
Patient activity
It is known that older patients have less wear of their total hips than younger patients. However, the chronological age is not the decisive factor, it is the activity of the individual patient, and this activity may be dependent on the general diseases (heart condition, e.g.), damage to other joints (rheumatoid arthritis, trauma, etc).
References:
Heisel H. et al.: J Bone Joint Surg-Am 2003; 85-A: 1366- 79
Swedish National Hip Registry, 2000 report
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