Modularity, as it applies to the hip replacement implants, refers to the availability of the components in sections or parts, rather than as a single piece - the monobloc version. Historically, hip replacments were available as two "bits" - one was meant for insertion into the natural socket and the other, into the femur bone. Fairly good results were obtained by these components, particuarly with respect to long term survival. However, the philosophy was very nearly "one size fits all". This posed some practical problems by reducing the flexibility of the surgeon during the operation. For example, once the socket had been cemented in, there was no easy way to salvage the situation if the position or orientation was unsatisfactory. Similiarly, on the femoral side, once the component had been implanted, it was difficult to adjust soft tissue tension, or the length of the leg if that was neccessary.
The modular components changed this. On the socket side, the surgeon now had available to him, a metal shell that he could implant into the natural socket but still retain the ability to change the inside diameter of the bearing, it's alignment and also introduce protection against dislocation if neccessary without having to "uproot" the socket component. On the femoral side, he could change the length of the ball to adjust soft tissue tension and leg length, after fixing the component into the femur. He was able to do this, because the ball was now available in lengths with female tapers that could be "cold welded" to the male taper on the top end of the femoral component.With time, there were further developments in modularity, which is particularly handy for revision cases or the difficult and challenging primary (first time) hip replacement procedures such as in patients who have deformities in the hip joint due to abnormal development.. The current components allow the surgeon to choose not only the size and length of the ball at the final stage of the surgery, but also the length of the stem section, its shape, surface finish and curvature, the shape and size of the upper end of the femoral component, the degree of offset and the rotation or twist of the upper section of the implant in relation to the stem section as well as the surface finish of this section. The advantage with these type of components is that the final implant can be assembled in the patient (or outside prior to implantation) to closely match the patient's requirements. This in turn enables better soft tissue tension, stability, leg length equalisation, muscle function and reliablity..
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