In essence there are two basic principles by which orthopaedic hip implants are secured to the bone, ‘cemented’ and ‘uncemented’.
Traditionally acrylic bone cement is used, functionally as a grout, to lock the acetabular and/or femoral component into the patient’s bone. Typically this involves “squeezing” the acrylic bone cement into the bone, within the socket, then carefully positioning a polyethylene acetabular component into the pelvis. The component is held in the desired position firmly with pressure as the cement sets. This typically takes around ten minutes. This then establishes fixation of a cemented socket into the pelvis, ideally in perfect alignment.
On the femoral side a similar technique is utilised. A ‘restrictor’ device is positioned part- way down the femoral canal to prevent the cement from going ‘all the way’ down into the bone. Doughy cement is then injected into the femur, from the top, and then pressurised to ensure that the cement locks soundly into the bone itself.
The metal stem is then pushed down into this bone cement and maintained in perfect position until the cement has hardened.
Thus the fixation of a cemented implant is strongest immediately after surgery, allowing the patient to mobilise rapidly and typically putting full weight on to the implant unless there are other complexities that would guard against this.
An example of a cemented reconstruction is shown here. Cement has been “pressurised” into the bone, obtaining sound and solid fixation of the components within the bone with the implants perfectly positioned. The surgeon is then able to ‘balance’ the hip in terms of soft tissue tension to ensure the best possible functional outcome.
The alternative to cement is to use ‘uncemented’ fixation.
On the acetabular side, the theoretical benefits of ‘uncemented’ fixation are that this allows the surgeon to use different, potentially more “hard”, bearing options as different ‘liner options’ can be locked into the uncemented shell as demonstrated in this x-ray.
‘Uncemented’ acetabular implants have either surface coatings or a surface finish that facilitates the bone to grow into (‘ingrowth’) or on to (‘ongrowth’) the component. This secures the implant in position. Theoretically this produces a physiological and dynamic mechanism by which the implant fixes to the bone. ‘Uncemented’ fixation is used more commonly in younger patients with solid bone, allowing the surgeon to use a more hard bearing surface option while also theoretically giving a sound and dynamic bond between the component and the bony skeleton.
On the femoral side, ‘uncemented’ fixation again depends on the surface geometry of the stem. Typically there is a rough surface with an associated coating around the stem that encourages and facilitates the bone to grow ‘on to’ or ‘into’ the component. This results in solid fixation into the patient’s bone. Typically ‘uncemented’ fixation has been used in young bone but also in patients with particularly ‘strong’ femoral bone, often with a certain shape, (a so-called proximal ‘champagne flute’ shape) that can lend itself better to ‘uncemented’ rather than ‘cemented’ fixation.
There are a number of uncemented stems available with different fixation characteristics and designs. In some circumstances the surgeon may wish to achieve fixation circumferentially around the implant. In other circumstances it may well be felt to be beneficial to achieve fixation only proximally, keeping the stem ‘smooth’ distally, to facilitate bone preservation within the femur should revision surgery be likely to be required over the patient’s lifetime.
There are different ‘uncemented’ and indeed ‘cemented’ options and designs on both the femoral and acetabular side available in the circumstances of revision. A surgeon who carries out a large number of these procedures should have all of the options and techniques available to ensure the best technique is chosen in any particular patient’s circumstance.
It is difficult to compare the results of different fixation options on the acetabular or indeed femoral side without considering the bearing surface. As detailed elsewhere, it is typically the bearing surface that ‘wears’ in a hip replacement and it is frequently the wear products themselves, via a biological response to the foreign material, that can result in loosening of the implants. Clearly, therefore, it is essential that different bearing surfaces are considered when one looks at the overall results of implant fixation.
© Andrew R J Manktelow – September 2011