• Traditional hip replacement prostheses only allow a fairly narrow range of shapes and sizes
• Even small changes in hip geometry can have large effects on the function of the hip joint
• New types of hip replacement are now available that allow a far wider range of sizes and lengths
• New pre-operative 3-D imaging software can allow precise pre-operative planning of size, shape, angle and length
• For patients whose hips fall too far outside the normal range or who have severe deformity, new custom made prostheses, designed specifically for the individual patient, can now match precisely the required geometries

The ‘normal’ hip?

“Custom-made hip replacements – the latest orthopaedic technology is a leap forward for hip replacement patients.”

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 a hip replacement probably haven’t.

It is true that most patients 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 [Husmann et al. J of Arthroplasty. Vol 12 No 4 1997], 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, and
• the offset distance of the femoral head.

Valgus hip (more vertical neck)

Normal angled femoral neck

Varus hip (more horizontal neck)

Retroverted (backward angled) femoral neck

Normally anteverted (forward angled) femoral neck

Excessively anteverted (forwardly angled) femoral neck.

Hip Offset = the perpendicular distance from the middle of the femoral head to a line running down the middle of the shaft of the femur (shown on the diagram as the horizontal dotted line).

Femoral Offset

The offset is the distance from the centre of the femoral head to a line running down the anatomical long axis of the femur

Most importantly, recent research [Krishnan et al. J of Arthroplasty. Vol 21 No 3 2006] 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 overcompensate for one of the others, or vice versa.

Does any of this matter?

Decrease in the offset in the hip joint significantly weakens the abductor muscles of the hip. Research [Asayama et al. J of Arthroplasty. Vol 20 No 4 2006] has shown that a decrease hip abductor muscle strength by anything more than just 12% causes functional problems such as early fatigue and limping.

Further research [Jolles et al. J of Arthroplasty. Vol 17 Issue 3 282-288 2002] 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 – 60 degrees leads to a 7- 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 particularly poor performance!”

State-of-the-art 3D planning software now allows precise pre-operative planning, allowing the correction prosthetic opitons to be chosen to ensure anatomical reconstruction.

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 [Personal communication – Jean Pl, Symbios].

Recent research has shown that 3-D imaging software from Symbios allows unprecedented degrees of accuracy in hip replacement surgery:-

“Accuracy of reconstruction of the hip using computerised three-dimensional pre-operative planning and a cementless modular neck.”
Journal of Bone and Joint Surgery – British Volume, Vol 91-B, Issue 3, 333-340

The solution

The Symbios range of modular neck implants provide a wide range of variation for valgus/varus, ante/retroversion and neck length.

Symbios [www.symbios.ch], 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.

The Symbios Modular Neck with Ceramic Head

The ‘state-of-the-art’ Symbios Modular SPS ceramic-on-ceramic uncemented hip prosthesis

The Symbios Custom-made hip prosthesis – 3D computerised planning model shown fitting a severely deformed hip.

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 [Personal communication – Jean Pl, Symbios]. 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.

Patient experience

CLICK HERE to read a patient’s direct account of her experiences of having a Symbios Custom Made Hip Replacement for her severely deformed hip joint, by Robin Allmark, July 2012.

This video show’s Robin Allmark, a 37-year-old lady, just 6 weeks after having her complex left hip replaced with a custom-made Symbios uncemented ceramic-on-ceramic hip replacement.