Ever since I knew I could find out the size and dimensions of my implant (my hip replacement) I’ve been on a journey of exploring why? Is it large or small? Cemented or cementless?
Why do I want to know?
Well I don’t really need to know, as I trust my surgeon to know what is best. But I’m nosey and why not find out.
So I sent off the form to National Joint Registry here and waited a fair few weeks to get the results. And here they are.
Pretty much confirmed what I thought I knew already. That my hip is made of Ceramic on Ceramic and that #Symbios is the manufacturer. And that I’m cementless (1 thing I wanted confirming) and then the sizes of the ball (I was really keen to find this out)
So does size matter?
The size of the ball joint of the natural hip joint (name is femoral head) actually varies and usually ranges from 40 to 54 mm (with smaller sizes in females). During the initial beginnings of hip replacement, the surgeons tried to make the implants which would mimic the natural size of the natural femoral head.
Does natural size work?
However, it was soon realised that when a large metal ball articulates with a plastic cup, the plastic part gets worn out quickly due to “volumetric wear”. To decrease the “wear” of the plastic, Sir John Charnley (who was one of the pioneers in the development of modern hip replacements), opted for a smaller 22 mm metal head (which is much smaller than the natural femoral head) in the design of Charnley hip replacement. This design was the commonest hip replacement done in the UK until recently.
Studies have shown that with this technique, there were fairly good long term results in patients who are not very young and active. However, these came with some restrictions placed, limiting what the patients could do to avoid dislocations (this is when the ball comes out of the joint) and a high dislocation rate following hip replacement has been common with smaller “ball size”
Small or larger femoral heads?
Many surgeons over the past few years have started using slightly larger size femoral head (28 mm) which is still within the range of conventional metal on plastic bearing. However, still it is smaller than the natural femoral head. The problem of wear and dislocation is still a potential concern.
Dislocation still remains a major reason (17%) for revision of hip replacement (NJR data 2008). I don’t know any hip replacement patients that aren’t worried about dislocation.
Dislocation is the main reason people need revision hip replacements. Which can result in loss of more function, which no of us want, we are all striving for more functionality.
While the cause of hip dislocations is from multi factors, it is thought using a larger ball could decrease the incident of dislocations. However, we’ve seen the size of the ball was limited by the material properties. So that was another thing that needed looking at.
How things have changed?
Also much younger and more active patients are being offered hip replacements now at much earlier times in their hip journeys. And the expectations of young and active patients are alot higher as they strive to go back to their active lifestyles and sports with little to no restrictions.
To improve the durability of the hip replacements, material scientists have come up with modern solutions (alternative bearings). As no longer could an implant last 7- 10 years, now they are required to last 30-40 years.
New materials give new dimensions?
There are essentially three types of alternative materials – cross linked poly (plastic), ceramic and metal. These alternative bearings mean they could allow larger ball sizes to be used 32 mm, 36 mm and even 40 mm. However, these sizes would be limited by the cup (socket size) and also some material allow slightly bigger ball to be used with the same cup size compared to other materials.
So to end here, size does matter.
From looking at my sizes 32mm I have a reasonably larger ball so it’s probably the reason I don’t have restrictions, it’s fitting into linear of 46/50 which they recommend for 32mm balls. Reading all this I see this hip is designed to give me as many decades as I can of activity, and more adventurous living. Obviously running would cause faster wear and tear, but overall it helps me appreciate the gift I’ve been given.