B Masson (Toulouse)
In the sixties, Sir John Charnley introduced small femoral head size. This was to remain the trend for several decades based on the low friction theory. Today, big heads are in high demand. Wear is no longer an issue thanks to hard hard bearings and in particular the ceramic ceramic bearing
As mentioned on the latest Swedish registry report, aseptic loosening and dislocation are the main complications in Total Hip Arthroplasty (THA).
Joint stability and Range of Motion (ROM) are decisive issues, especially in younger patients with high expectations on their quality of life after a surgery. In consequence, a large variety of head diameters from 22mm to 60mm are available today.
Which ball head diameter offers the best stability without causing further complications?
Ball head diameter and stability
Larger heads increase the stability of a THA not only by reducing the risk for impingement but also by increasing the distance the head has to be lifted out of the cup
Head size directly influences the technical ROM. A benefit of 13° (from 123° to 136°) in technical ROM is achieved when the head size is increasing from 28mm to 36mm.
The ROM will obviously depend also on the design of the metal back and the neck of the stem. In this case the cup is hemispheric and the taper is a 12/14
The « real « ROM of the patient is influenced by the position of the component, the personal anatomy of the patient (soft tissue and muscles)
From a bio mechanism point of view, it is important to differentiate the « prosthetic impingement » that can lead to dislocation from the « bony impingement » that will have the same effect.
Increasing the ball head diameter is supposed to increase stability by increasing the technical ROM and therefore the « prosthetic impingement » however the « bony impingement » will occurs earlier.
The dislocation can also occur without any impingement if the « jumping distance » is small, this is called shear out dislocation. This mechanism can happen in case of special bearing and cup design.
The best compromise between several parameters such as ball head diameter, neck size, cup design should be investigate
Ball head diameter and wear
Excellent results can be achieved with ceramic-ceramic bearing total hip replacement (THR), particularly for younger patients thanks to the bearing couple’s low wear performance and the material’s biocompatibility.
Larger diameter hard on hard bearings may also change the wear characteristics due to larger wear areas or different lubrication behaviour. Many tribological tests demonstrate that a large ceramic bearing also has a very low wear rate. The influence of the clearance on the wear rate is negligible.
Using a ceramic ball head against a highly crosslinked polyethylene liner reduces the wear rate by 40% compared to metal ball heads.
Ball head diameter and design
Increasing the ball head diameter consequently increases the insert diameter then also the cup diameter. The cup diameter is limited by the physiological size of the acetabulum.
Current ceramic-on-ceramic THR bearings range from 28-36 mm to 40mm diameter but increasing the bearing diameter should not come at the expense of bone conservation,
There are several solutions to have the biggest ball head in the smallest cup. One solution is to reduce the thickness of the insert and/ or the metal back. Another solution is to change the design of the metal back from an hemispherical shape to a sub-hemispherical shape.
The limitation of the wall thickness of the ceramic insert depends on the deformability of the metal back. For a standard and direct fixation of the ceramic insert in a metal back, the minimum thickness of the ceramic insert is 3,5mm.
A new solution preassembled under standard, clean and controlled conditions can reduces both the thickness of the ceramic and the metal back. This system can offer a big ball head in an acceptable cup size with a high reliability. Preassembled inserts and metal backs eliminate the risk of misalignment or third body at the interface.
The ceramic insert is secured by the metal back. This would not be possible in surgery, giving complete seating and superior strength both of the interface and the assembled cup
The effect of larger head sizes for THA on the type of impingement, ROM, and joint stability has been reported in the literature. Results have shown that femoral heads larger than 32 mm provide greater ROM and push the limits of prosthetic impingement.
Because the increase of the ball head diameter has also some surgical consequences, clinical trials will answer for the limit in size. Based on many biomechanical tests, it seems that the use of a 36mm ball head is a good compromise between strength, stability, safety and surgical technique.