Scientific objectives

The main overall objective was to investigate the loosening mechanism of massive femoral implants. For the M kl proximal femoral replacements, the change in the ratio o f the tip force to the shaft force, both in the short term during activity and over the longer term, would indicate the redistribution of load within the fixation. An increase in this tip/shaft ratio would be interpreted as loss of support at the shoulder and upper stem, indicating a loosening process [#1.5], and a maintenance or decrease in the ratio would indicate continuing or improved support. These measurements would be compared with radiographs, which would be studied for the particular features associated with loosening as confirmation. Shaft forces would also be compared with joint forces in total hip prostheses, for which instrumentation had been previously developed [#3].

Having established the feasibility of a ratiometric method o f determining changes in the fixation, and having gained over 4 subject-years total in vivo experience

with the M kl device in proximal femoral replacements, it was intended that the Mk2 device should be used to test two hypotheses o f clinical importance relating to the fixation of distal femoral replacements:

1 A fixed knee hinge transmits high torques to the fixation, leading to fibrous membrane formation and loosening, whereas a rotating hinge should alleviate the torque and lead to a more secure fixation,

2 Compared to an untreated cemented fixation, a cemented intramedullary design, with particular features to encourage extra-cortical bony bridging, should result in more direct stress transfer to the bone, reduce bone resorption, and hence prolong fixation.

The bony bridging has the potential to improve the fixation in two ways:

1 Since the bridging should transmit axial, bending and torsional stresses, the overall state o f stress on the stem should be reduced. The stem should therefore remain more securely fixed,

2 Increased stress should be provided to the bone at the transection site, around the root o f the stem. This would mimic the natural situation in which the bone is loaded, and avoid the stress shielding which occurs when no bridging is used.

Distal rather than proximal femoral replacements are to be used in these Mk2 studies. When the telemetry project began in 1987 proximal replacements greatly outnumbered distals because osteosarcoma (the most common type of tumour occurring in the distal femur) had then only recently begun to be treated by prosthetic replacement. Distal femoral arthroplasty is now an established practice and the incidence o f tumours requiring distal femoral replacement now outnumbers that for proximal replacements, by about 5 to 1. Moreover, loosening is observed in more distal than proximal replacements for the same postoperative stage (Unwin et al. 1996), and since the age group of patients requiring distal femoral replacement is significantly younger than that for proximals this represents a more serious long term survivorship problem (Unwin et al. 1993).

The above hypotheses will be tested by comparing the distribution o f forces and moments in each Mk2 prosthesis, during activity and over extended periods of time. It was planned that one study should compare the performance o f the Stanmore fixed hinge knee with that o f the more recent SMILES knee. This would provide design data for the development o f improved knee joints. In particular, the comparison o f shaft torque measurements between the two types o f knee joint would indicate the effectiveness or otherwise o f the additional degree o f freedom in reducing the torque transmitted to the fixation, and the measurement of tip to shaft axial force ratio would indicate the effect which this has on the integrity of the fixation over a long period. This comparison will only be possible if suitable subjects can be found for the Stanmore fixed hinge knee type. Since the development of the SMILES type, the fixed hinge is no longer used except for certain revision cases. However, even if no fixed hinge knees are used, the SMILES type will provide torque data for comparison with that for the normal anatomy from the literature (determined by calculation).

The objective of the second study is to determine the effectiveness o f coatings such as hydroxyapatite (HA) in promoting bony bridging and force transmission between the bone and the shoulder. Results will be compared with those obtained for similar non-coated prostheses. Bone remodelling in the region of the shoulder, as described in #1.5, generally results in a reduced area of contact, and eventually no contact, between the bone and shoulder. Force measurements on prostheses without coating should show how this reduces the forces transmitted through the shoulder, and hence increases the forces on the remainder o f the fixation, contributing to loosening (as currently observed in the first M kl instrumented proximal femur, #5.3). It is anticipated that HA coating in the shoulder area will be shown to maintain or even enhance the transmission o f forces through the shoulder, both axially and laterally. If this is successful further studies could include the effect o f modifying the shoulder geometry to accommodate or encourage beneficial bone remodelling. Radiographs will be used to assess the extent o f the bridging subjectively, and compared to the data. Radiographic and other analyses of the data are outside the scope o f this thesis, however.

It was planned to implant three groups o f subjects as follows:

Group A SMILES rotating hinge knee, without HA coated shoulder. Group B SMILES rotating hinge knee, with HA coated shoulder. Group C Stanmore fixed hinge knee, with HA coated shoulder.