Load transmission and function

There are two pieces of in vivo evidence that support the view that load transmission with the polished Exeter stem is different from that of more conventional stems that are intended to be ‘bonded’ to the cement. First is its migration pattern as revealed by RSA, and second is its behaviour with Boneloc™ cement that was iriginally introduced in Denmark.

Recent RSA studies have made it clear that all cemented stems, of whatever design, migrate within the cement mantle in the first two post-operative years30,31. Such migration is an order of magnitude greater with the Exeter stem than with any other stem that has been studied, yet none of the RSA reports of the Exeter stem30,31,32,33 have revealed any axial migration at the cement-bone interface. In addition, its stability in torsion, as assessed with RSA, is independent of the angle of anteversion of the stem34, a finding that is not universal amongst cemented femoral components34,35.   In helping to explain these findings, there is experimental evidence that the unbonded, polished, collarless, tapered stem transmits load through the cement and into the bone more effectively than the conventional stem36,37, largely because of the relative reduction in shear and the increased compression that stems of this type generate at the interfaces and within the cement as they subside within the mantle, so contributing to both axial36,37 and torsional38 stability.

The same considerations may help to explain the fact that the polished Exeter is the only stem that has functioned satisfactorily over 5 years when used with Boneloc™ cement, a material that was associated with catastrophic results with all other stem designs39,40,41,42,43,44,45.

Experimentally, the Exeter stem fulfils the criteria for the function of a self-locking taper46 in that there is a linear relationship between the ‘push-in’ and ‘pull-out’ loads and it is this, combined with the polished surface and the absence of ‘end-bearing’ that allows it to function as a ‘force-closed’47 stem on the ‘taper-slip’ basis48. The part played by the viscoelastic behaviour of acrylic cement in this type of load transmission is still controversial 49,50, though the fact that the viscoelastic behaviour of Boneloc™ is very marked may be one of the reasons why the Exeter stem has survived uniquely well with this cement. The overall behaviour of the Exeter stem is certainly best explained on the basis of the interaction of stem shape, surface finish and the viscoelastic properties of acrylic cement.  Subsidence of this type of stem within the cement creates the circumstances under which hoop tensile stress relaxation in the cement mantle occurs and this is succeeded by radial compression 51.

There remains the question of how a stem of this type can transmit load into the proximal aspect of the femur. That it can do so is proven by histological study52 of the femoral neck, the preservation of the latter’s height and to a lesser extent, its density52,53 (Fig.11), together with the lack of diaphyseal hypertrophy when contemporary cementing is employed (under 5%)29. By contrast,  diaphyseal hypertrophy was found in 30% of the original polished Exeter stems inserted with digital packing of cement11. This difference is in part due to the greatly improved proximal filling of the femur associated with the proximal pressurisation of the cement with contemporary cementing54. In the presence of effective proximal filling of the canal, the movement of the Exeter stem towards  valgus11,34 as it subsides means that the stem is impacting itself into the proximal part of the cement mantle and axial load is being transmitted from both the lateral and the medial aspects of the upper end of the stem into the proximal end of the cement mantle both laterally and medially. Thus the lateral side of the stem is just as important as the medial side in the proximal transmission of load with this type of stem, an important matter with respect to the recovery of the lateral cortex in zone 255 following impaction grafting56.