Custom-made hip replacements

Every year thousands of people undergo hip replacement surgery in the UK. Whilst it is true that most people can benefit from standard hip prostheses, the latest orthopaedic technology can prove to be a leap forward for some patients. This article looks at some recent advances in hip replacement research and techniques.

Are you average? Do you think you look the same as the person sitting next to you? Do you have an arthritic hip? Is your hip the same size and the same shape as the person sitting next to you? Will your hip be ‘OK’ with a standard off-the-shelf replacement, or could you, possibly, need something different from ‘the norm’?

These are all questions that you should ask yourself, but which no doubt most patients needing hip replacements probably haven’t.

This article on custom made hip replacements is written by Mr Ian McDermott, Consultant Orthopaedic Surgeon & Honorary Professor Associate, Brunel University School of Sport & Education.

The ‘normal’ hip?

It is true that most people can be served perfectly well by the standard hip replacement prostheses currently in use in the UK. However, there is data available from large banks of CT scans of hips which show clearly that there is actually great variation between individuals with respect to:-

• the width and internal dimensions of the bones,

• the valgus / varus angle of the hip

• the anteversion / retroversion angle of the hip

• the offset distance of the femoral head

Most importantly, further recent research has shown that each of these variables can alter independently of each other. This means that surgically implanting a bigger prosthesis to correct for one large variable may actually over compensate for one of the others, or vice versa.

Does any of this matter?

Decrease in the offset in the hip significantly weakens the abductor muscles of the hip. Research has shown that a decrease in hip abductor muscle strength by anything more than just 12% causes functional problems such as early fatigue and limping.

Research has also shown that implanting a hip where the femoral stem angle plus the cup angle cumulatively fall outside of the “safe range” of 40^o – 60^o leads to a seven times greater risk of post-operative dislocation of the hip replacement.

Attempts to appropriately correct all of the different variables in hip geometry with standard hip replacements can potentially lead to surgeons inserting the hip prosthesis in a position that is good for the hip joint itself, but which unfortunately leads to either shortening or lengthening of the overall leg length. Leg length discrepancies in the region of just 1cm are noticeable to patients, and leg length inequality can cause a variety of problems, including limping and low back pain. Small leg length discrepancies can be corrected for by patients using heel raise inserts inside their shoes. However, leg length differences of more than 1cm normally require a built-up shoe on the short side, which can be very restrictive in terms of footwear.

“Proper pre-operative planning prevents poor performance!”

One the main difficulties lies with the fact that simple X-rays are notoriously bad at measuring true hip dimensions and geometry, and they can often be  misleading.

Fortunately, the latest technology that has been specially developed in Switzerland allows for extremely accurate 3-dimensional mapping and measurement of the hip joint using rapid CT-scanning, with a radiation dose not significantly different from a simple plain X-ray. The data from the scan is interpreted by software that allows 3-dimensional reconstruction of a patient’s bones and hip joints on a computer. This then allows measurements of relevant distances, dimensions and angles to determine exactly what offset, neck angles and neck lengths are required to anatomically reconstruct the patient’s hip joint.

The software then allows the surgeon to see whether or not a standard off-the-shelf prosthesis would be appropriate or adequate for surgical use.

Data has shown that for somewhere in the region of 50% of patients, their joint geometry falls outside the anatomical reconstructive capabilities of a standard off-the-shelf hip replacement.

The solution

Symbios, an Orthopaedic company based in Switzerland, has developed a range of uncemented femoral stems for hip replacements with what are described as ‘modular’ femoral necks. This allows for great variation in neck length, neck valgus/varus angle and neck anteversion/retroversion – all independently. Thus, true anatomic reconstruction of all the different joint variables can be achieved in the large majority of patients, without having to compromise on loss of offset and without creating leg length discrepancies.

There is, however, a small number of patients in who even the new generation of modular implants does not have the sufficient range of shapes and sizes to adequately restore appropriate anatomy. Clinical experience has demonstrated that somewhere in the region of 5% of patients may fall into this category. For such patients as this, a new generation of custom-made hip prostheses are now available, which are designed specifically for the size and shape of an individual’s anatomy, allowing for relatively easy reconstruction even in patients with significant deformity of the hip and the proximal femur.

3-D computer planning for a custom-made hip replacement 

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