Bearings for hip replacement

The commonest cause of failure of a hip replacement is due to wear of the moving parts of the hip (the bearing) causing bone loss (osteolysis) and loosening of the components.

Wear occurs in all hip replacements, but is very much higher in patients with high activity levels, and is obviously more common the longer the implant is in use for. Hence it is the younger patient in whom this wear is a particular problem.

To address this problem alternative bearing materials have been developed that offer lower wear rates. However, potential problems in using all these different materials need to be carefully balanced against the potential benefits. (fig 2. depicts the relative wear rates for the various materials).

Below we summarise the latest evidence on bearing materials, and the rational for how we tailor the choice to each individual patient.

UHMWP

The bearing is made up of the acetabular component (cup) and the head (ball). Traditionally, the cup was made of hard plastic material called ultra high molecular weight polyethylene (UHMWP), and the head usually of cobalt chrome. In the majority of patients requiring hip replacement, who are older than 70 years, this combination has been very successful. About 15 years ago the wear properties of UHMWP were improved by a process know as crosslinking.

This newer material has performed extremely well so far.

Fig 1: Severe wear and bone loss in a traditional hip replacement – a 6-year-old hip replacement in a young patient.

Fig 2. The relative wear rates for the various materials.

Fig 3. Highly crosslinked cup.

Benefits of highly cross linked UHMWP:

Relatively cheap
Reduces wear by 90–95% compared to normal UHMWP
Flexible options with head size and augments to improve stability
Easy to revise
Can be cemented
No issues with metal allergy.

Unknown performance after 15 years
May have increased wear rates with head sizes over 32mm.

Ceramic

Ceramic was introduced into clinical practice in France in 1970. There have been several subtle variations in the material used in the manufacture of ceramic, which cause their own unique problems.

Fig 4: Modern delta ceramic head and cup.

It is extremely hard wearing, due to its hardness, smoothness, and lubrication characteristics. A ceramic head can be used in combination with other materials such as UHMWP, ceramic cups, and most recently metal cups.

In the last few years, manufacturing techniques have dramatically improved allowing larger heads to be made, and reducing the risk of fracture.

Benefits of delta ceramic:

Lowest wear rate of any bearing when combined with ceramic cup
Inert wear debris
40 years of experience
No issues with metal allergy.

Problems:

Limited size options of the head and cup, which may compromise stability
Fracture risk (<1 in 1000)
Squeaking (<1%)
Limits options for revision due to taper damage and after fracture
Very expensive.

Metal on metal

Metal on metal (MoM) bearings are when both the cup and the socket are made of cobalt chrome. The MoM combination was first tried as far back as 1938, but was used in large numbers of hip replacements in the 1960s and 1970s. It went out of favour in the 1980s, but returned in the 1990s because of its association with the development of modern hip resurfacing.

It is a very hardwearing material, and can be safely manufactured into relatively thin cups, allowing the use of very large heads and in hip resurfacing.

There have been many concerns over the use of MoM bearings, relating to metal allergy, theoretical links with cancer, and technical problems in implantation. However, with careful patient selection and good surgical technique, the results are excellent, especially in the young active male.

Fig 5. Large head Metal on metal hip bearing.

Benefits of MoM:

Low wear
Tough
>20 years experience of modern MoM bearings
Allows use of very large heads and resurfacing.

Problems:

Metal allergy, and ALVAL/ARMD especially in young females
Unexplained pain, especially in young females
Worries over cancer risk (unfounded)
Wear particles cross the placenta
Sensitive to error in cup placement.

Resurfacing

Resurfacing is a special kind of hip replacement where the minimal amount of bone is removed from the head. It is the only hip that potentially allows the patient to return to any activity including running, climbing etc.

The surgery is more demanding, and the results are very dependent on patient selection and surgical technique.

Despite the bigger incision required for this procedure, patients recover exceptionally quickly.

Fig 6. Bilateral hip resurfacing in a 44-year-old man.

Benefits of hip resurfacing:

Benefits of MoM bearings
Very high activity possible
Bone preservation
Restores function.

Problems:

Problems of MoM bearings
Neck fracture 1%
Larger incision
Neck thinning (usually females).

Our choices in bearing materials

Young (<60yrs) active males

In general, Orthopaedics WA would offer this group resurfacing hip replacement as long as the patient has high function, minimal deformity, no metal allergy, no inflammatory arthritis.

Young males up to 65yrs

Young males up to 65yrs, with femoral deformity, large head cyst/collapse, inflammatory arthritis, and without metal allergy would be offered a large head ceramic bearing on a conventional stem.

Fig 7. Large head ceramic bearing on conventional stem.

Young females (<60yrs)

Orthopaedics WA would offer ceramic on ceramic, or ceramic on poly bearings on a conventional hip replacement.

There are many special cases, some of which require a variety of implants and bearings, and we will discuss the optimal choice for each individual before surgery.

Fig 8. Conventional hip replacement.