Focus variation measurement and advanced analysis of volumetric loss at the femoral head taper interface of retrieved modular replacement hips in replica

Comparison and appraisal of techniques for the determination of material loss from tapered orthopaedic surfaces

“…Because line of sight is required, direct evaluation of the taper bore by white light interferometry is not possible without sectioning [54], which is not feasible in the case of retrievals [51]. This challenge is common to all optical methods [94]. A unique measurement system; the RedLux Artificial Hip Profiler (RedLux, Southampton, UK), operates on the principle of chromatically encoded confocal measurement [30] to record point coordinate surface information.…”

“…To curtail measurement times, however, it is important to minimise the scanning range. For taper measurement, this involves tilting of the rotational axis to orient the taper surface in perpendicularity with the optical head [94].…”

“…High-resolution replication elastomers have been used with taper bores to overcome the access limitations associated with optical systems. Compounds used in the literature claim resolution values of 0.05 -0.1 μm and shrinkage values of 0.1% [94,95], and have demonstrated the ability to duplicate fine surface details within these claimed values [97]. When replication elastomers are used to measure taper bores, discrepancies between the replicated surface and true surface have been observed [94].…”

“…With respect to Figure 8, the form defect in the unworn region results in the zero-level plane being established at a different level during volume calculation and is "largely responsible for the discrepancy in measured material loss." [94]. For these reasons, it has been suggested that replication processes may be limited with respect to resolving material loss <1 mm 3 .…”

“…For these reasons, it has been suggested that replication processes may be limited with respect to resolving material loss <1 mm 3 . [94] Notwithstanding, the use of non-contact methods could represent a significant improvement in measurement times depending on the technology used. The RedLux system claims measurement times of between a "few" and "tens" of minutes [95], substantially less than comparable contact metrology technologies.…”