Bearing Surfaces in THR — Metal, Ceramic, Polyethylene

Part of The Joint Replacement Guide — Phase 4: The Operation · Article 23 of 44 · Hip

Key Points

  • The bearing is the pair of surfaces that move against each other inside a hip replacement.
  • Modern highly cross-linked polyethylene liners, combined with either a ceramic or metal femoral head, are among the most commonly used and best-established options.
  • Ceramic-on-ceramic bearings produce very little wear but have some distinctive potential complications.
  • Conventional metal-on-metal total hip replacements are now rarely used because some designs produced excessive metal debris and unacceptably high revision rates.
  • Bearing selection depends on the complete implant combination, including head size, fixation, implant track record and patient factors. No single material is best for everyone.

What Is a Bearing?

A total hip replacement contains a femoral head—the new ball—which moves against a liner or socket surface with every step. These two moving surfaces are collectively called the bearing or bearing couple.

Common bearing combinations include:

  • ceramic-on-polyethylene;
  • metal-on-polyethylene;
  • ceramic-on-ceramic; and
  • historically, metal-on-metal.

The bearing influences wear, stability, implant choice and some of the potential reasons for revision surgery. However, it is only one part of the replacement. Fixation, component positioning, femoral-head size and the performance of the complete implant combination are also important.

Polyethylene Liners

Polyethylene is a specialised medical-grade plastic. It is descended from the material introduced into hip replacement through Sir John Charnley’s work, but its manufacture and wear resistance have improved substantially.

Modern highly cross-linked polyethylene, often abbreviated to HXLPE, is treated to reduce wear. It can be used with either a ceramic or metal femoral head and is now a well-established liner material.

Polyethylene does not eliminate wear completely. Very small particles can still be released over time and may occasionally contribute to bone loss around an implant, known as osteolysis. Modern HXLPE has substantially reduced this problem compared with earlier conventional polyethylene.[1]

A polyethylene liner also permits the use of different head sizes. A larger head can improve stability and movement before the neck of the implant contacts the socket, but it may require a thinner liner and can introduce other mechanical considerations. Bigger is therefore not automatically better.

Ceramic-on-Polyethylene

In a ceramic-on-polyethylene bearing, a ceramic femoral head moves against a polyethylene liner.

Modern ceramic is extremely hard, smooth and resistant to scratching. Laboratory studies show favourable wear characteristics, and ceramic heads may reduce some concerns associated with corrosion at the junction between a metal head and the femoral stem.

Ceramic-on-HXLPE is widely used, including in younger and more active patients. However, current clinical evidence does not establish that it is superior to metal-on-HXLPE for every patient or every outcome. Implant design, head size, fixation and patient selection can be at least as important as the head material itself.[2,3]

Fracture of a modern ceramic femoral head is possible but extremely rare. Ceramic heads are also usually more expensive than standard metal heads.

Metal-on-Polyethylene

In a metal-on-polyethylene bearing, a polished metal femoral head—usually made from a cobalt-chromium alloy—moves against a polyethylene liner.

This remains a reliable, well-established option with a long clinical history. It may be used in patients of different ages, rather than being reserved only for older or less active people.

Metal-on-polyethylene should not be confused with metal-on-metal. With metal-on-polyethylene, the moving socket surface is plastic, not metal, so it does not create the same metal-on-metal articulation.

Very small amounts of corrosion or metal debris can occasionally arise at the connection between a modular metal head and the femoral stem. Clinically significant reactions are uncommon with well-performing conventional designs, but this is one reason why the result of the complete implant combination matters rather than the bearing label alone.

Ceramic-on-Ceramic

Ceramic-on-ceramic bearings use both a ceramic femoral head and a ceramic socket liner. Their principal advantage is extremely low wear.

Potential disadvantages include:

  • an audible squeak or other noise from the hip in a minority of patients;
  • sensitivity to precise component positioning;
  • the possibility of damage if the edge of the ceramic surfaces repeatedly contacts under load; and
  • a very rare risk of ceramic fracture.

Modern ceramics are substantially stronger than earlier generations, making fracture uncommon. Nevertheless, a fractured ceramic component can be challenging to revise because small ceramic fragments may remain in the joint.

Registry and clinical evidence has not demonstrated that ceramic-on-ceramic is the best option for every young or active patient. It remains a reasonable option in selected circumstances where the surgeon has appropriate experience with the implant.[2,4]

Metal-on-Metal: Why Its Use Declined

In a metal-on-metal bearing, both moving surfaces are made from metal. These bearings were promoted particularly during the 2000s because they allowed large femoral heads and appeared to have favourable wear properties in laboratory testing.

Subsequent clinical experience showed that some metal-on-metal total hip replacements and resurfacing designs released metal particles and cobalt or chromium ions. In some patients, this produced inflammation and damage to the muscles, bone and other tissues around the hip. This is commonly described as an adverse reaction to metal debris.[5]

Some designs also had substantially higher revision rates than established alternatives. As a result, conventional large-head metal-on-metal total hip replacement has largely fallen out of routine use.

Metal-on-metal hip resurfacing has not disappeared completely. A small number of established resurfacing systems may still be considered for carefully selected patients—most commonly younger men with sufficiently large femoral heads and good bone quality—after detailed discussion of the alternatives and the need for follow-up. Resurfacing is not suitable for most patients.

If You Already Have a Metal-on-Metal Hip

Most people with a metal-on-metal hip do not develop a serious complication. However, tissue reactions can occasionally develop before marked symptoms appear.

The MHRA has issued specific follow-up guidance for people with metal-on-metal hip replacements.[5] Monitoring may include:

  • a clinical review;
  • X-rays;
  • measurement of cobalt and chromium in the blood; and
  • specialised imaging, such as a metal artefact reduction sequence MRI scan or ultrasound, where indicated.

No single blood metal-ion result automatically determines whether revision surgery is required. Symptoms, changes in metal levels, implant type, X-rays and cross-sectional imaging must be considered together.

If you have a metal-on-metal replacement, continue with the follow-up recommended by your hospital. Seek clinical advice if you develop increasing pain, swelling, weakness, clicking or a deterioration in hip function. Do not assume that the absence of symptoms means scheduled monitoring is unnecessary.

What Registries Have Taught Us

National joint registries made an important contribution to identifying higher revision rates associated with particular metal-on-metal implants and bearing combinations. Regulatory reports, clinical investigations, imaging findings and laboratory research also contributed to understanding the problem.

Registries can compare very large numbers of replacements and detect patterns that may be difficult to recognise within one hospital. They can show that a particular implant or combination is performing less well than expected, prompting more detailed investigation.

However, registry data must be interpreted carefully. A bearing cannot always be assessed separately from:

  • the design of the stem and socket;
  • fixation method;
  • femoral-head size;
  • patient age and sex;
  • the reason for surgery; and
  • differences in how particular implants were selected and used.

A registry signal raises concern but does not, by itself, prove why an implant has failed.

How Your Surgeon Chooses a Bearing

The decision may take account of:

  • your age and expected activity;
  • the size and anatomy of your hip;
  • the proposed head size;
  • the type of socket and stem;
  • the fixation method;
  • the bearing’s wear and revision evidence;
  • your surgeon’s experience with the implant;
  • any relevant metal sensitivity or previous implant reaction; and
  • the performance of the complete implant combination in national registries.

A suspected skin allergy to jewellery does not automatically mean that a conventional metal femoral head or other metallic implant cannot be used. If you have a history of significant reactions to metal or a previous implant, discuss this with your surgeon rather than arranging unvalidated allergy testing independently.

In More Depth

Bearing materials are often discussed as though one material alone determines how long a hip replacement will last. In reality, revision can be required for reasons unrelated to bearing wear, including infection, dislocation, fracture and failure of fixation.

Modern HXLPE paired with a ceramic or metal head has produced very good results, while ceramic-on-ceramic remains an alternative with low wear and its own distinct trade-offs. Evidence has not established a universally superior combination for all patients.[2,3]

The most reassuring choice is generally a well-established bearing used as part of an implant combination with strong clinical or registry evidence, selected for an appropriate patient and implanted accurately.

Questions You May Wish to Ask

  • What bearing combination are you recommending for me?
  • Why is it appropriate for my age, anatomy and activity?
  • How well established is the complete implant combination?
  • What head size will be used, and why?
  • Are there any material-specific risks relevant to me?
  • If I already have a metal-on-metal hip, what follow-up do I require?

This article provides general information and does not replace individual clinical advice. Implant and bearing selection should be discussed with your surgical team.

References

  1. Lachiewicz PF, Geyer MR. The use of highly cross-linked polyethylene in total hip arthroplasty. J Am Acad Orthop Surg. 2011;19(3):143–151. doi: 10.5435/00124635-201103000-00003.
  2. López-López JA, Humphriss RL, Beswick AD, et al. Choice of implant combinations in total hip replacement: systematic review and network meta-analysis. BMJ. 2017;359. doi: 10.1136/bmj.j4651.
  3. National Joint Registry. Outcomes after joint replacement. In: 21st Annual Report 2024. National Center for Biotechnology Information.
  4. Sedrakyan A, Graves S, Bordini B, et al. Comparative effectiveness of ceramic-on-ceramic implants in stemmed hip replacement: a multinational study of six national and regional registries. J Bone Joint Surg Am. 2014;96 Suppl 1:34–41. doi: 10.2106/JBJS.N.00465.
  5. Medicines and Healthcare products Regulatory Agency. All metal-on-metal hip replacements: updated advice for follow-up of patients. Medical Device Alert MDA/2017/018. Published 29 June 2017. MHRA guidance.
  6. National Institute for Health and Care Excellence. Total hip replacement and resurfacing arthroplasty for end-stage arthritis of the hip. Technology appraisal guidance TA304. Published 26 February 2014. NICE TA304.

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