The Mk2 design: differences from the M kl, and mechanical design

The M kl instrumented prosthesis was the pioneer device in which the main design features of Stanmore instrumented femoral replacements were developed. Its purpose was to provide data on axial load redistribution within the prosthesis over the course o f time [#2.2]. It also proved the essential elements o f the design over extended periods. The Mk2 system has greater measurement capability but uses the same basic methods as the M kl for the instrumentation, inductive powering, telemetric technique, coil encapsulation, hermetic sealing, and data logging. However, changes were made in most of these areas, either to improve some aspect o f the design, or to allow more data channels, or because of the different mechanical considerations relevant to distal femoral replacements. Reasons for choosing the distal femur as the replacement site for the Mk2 instrumented prosthesis were given in #2.3. This section o f the Methodology and those following will report the features of the Mk2 design which differ significantly from the M kl.

4.9.1 Biomechanical aspects of distal femoral replacement

Figure 4.48 shows a typical one-half proximal and distal femoral replacement (left and centre o f figure respectively), both fixed using bone cement. Both prostheses consist o f a main bone-replacing shaft and integral intramedullary stem spigotted into the remaining femoral bone. Instead of the hip joint, in distal femoral replacement the knee joint is replaced. Traditionally at Stanmore a fixed hinge knee has been used, but this has now been replaced by the SMILES type in all but certain revision cases. The SMILES (Stanmore Modular Individualised Lower Extremity System) modular design can be adapted for use either as a primary total knee or for use with a massive implant for bone tumour cases. As in other types of artificial knee joint, this has a metal-on-plastic bearing surface to reduce fi*iction and wear, figure 4.48 (right), and is available in several standard sizes. However, the SMILES concept is unique in that it contains both a hinge and also allows rotational laxity between the femoral and tibial parts. In distal femoral

replacements, the femoral component of the SMILES knee is spigotted onto the shaft of the prosthesis, and the tibial component is located into a plastic sleeve, which forms the intramedullary stem, and is cemented into the tibia.

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Figure 4.48 Left and centre: Typical one-half proximal and distal femoral replacements, with assumed loadlines (dotted). Right: The SMILES modular design of knee component here shown as part of a distal femoral replacement. The plastic part of the tibial component (integral tray and IM stem) is not shown.

The metal tibial component bears on the contoured plastic tibial tray and is allowed to rotate in the sleeve, a design feature which offers torsional stress relief to the fixation, which is being investigated as part of the Mk2 clinical study.

Figure 4.48 also shows simplified loadlines (ignoring the effect of muscle action) for the prosthesis-bone fixation (dotted), passing through the femoral head centre

and condyles of the knee joint. The extent to which this representation reflects the actual load vector is not yet known for the proximal replacement. However, measurements from the first instrumented distal prosthesis have shown that the lever arm o f the loadline at the level o f the strain gauges is directed antero- medially and peaks at 30mm during the stance phase of gait. This is consistent with a loadline acting between the femoral head and the middle of the knee joint in this subject, thus corroborating the simple loadline model.

A comparison o f the loadline models for the distal replacement and for the proximal replacement shows the difference between the two fixations regarding bending stresses likely to be experienced at various sections along the length. For distal replacements, a longer shaft length (greater bone resection) creates higher bending stresses at the shoulder, as the distance from the midline of the prosthesis to the load line increases with shaft length. For the proximal replacement however, the reverse is true: a greater amount o f bone resection brings the loadline closer to the shoulder and therefore reduces the bending moment at the transection point. It seems reasonable to suppose that it is the cumulative bending moment along the length o f the stem and not just at the shoulder which determines the likelihood of early loosening. This simplified geometrical observation, which takes no account o f muscle action, appears to correlate with the varying survival rates o f both proximal and distal femoral replacements having different transection points. The long proximal and short distal replacements remain better fixed than the short proximals and long distals, in general (Unwin et al. 1996).

4.9.2 Structural modifications and factors of safety

The mechanical structure of the Mk2 prosthesis is shown in figure 4.49. The overall design is similar to the M kl but with several significant differences. The shaft is separated into two sections: the instrumented proximal part, including the IM stem, and the distal part, attached to the knee joint. The proximal shaft from 30mm below the shoulder consists o f two overlapping tubes, the inner one having four orthogonal flats for thin film strain gauges, and two holes through to the