Geometric Variations of Modular Head-Stem Taper Junctions of Total Hip Replacements

Abstract

Taper degradation in Total Hip Replacements (THR) has been identified as a clinical concern, and the degradation occurring at these interfaces has received increased interest in recent years. Wear and corrosion products produced at the taper junction are associated with adverse local tissue responses, leading to early failure and revision surgery. Retrieval and in-vitro studies have found that variations in taper design affect degradation. However, there is a lack of consistent understanding within the literature of what makes a good taper interface. Previous studies assessed different design variations using their global parameters assuming a perfect cone such as: taper length, cone angle and diameters. This study assessed geometrical variations of as-manufactured head and stem tapers and any local deviations from their geometry. The purpose of this study was to provide a greater insight into possible engagement, a key performance influencing parameter predicted by Morse taper connection theory. This was achieved by taking measurements of twelve different commercially available male tapers and six female tapers using a coordinate measurement machine (CMM). The results suggested that engagement is specific to a particular head-stem couple. This is subject to both their micro-scale deviations, superimposed on their macro-scale differences.Differences in cone angles between female and male tapers from the same manufacturer was found to create a predominately proximal contact. However, distally mismatched couples are present in some metal-on-metal head-stem couples. On a local scale, different deviation patterns were observed from the geometry which appeared to be linked to the manufacturing process. Future work will look at using this measurement methodology to fully characterise an optimal modular taper junction for a THR prosthesis.

Effect of head size and rotation on taper corrosion in a hip simulator

“…The axisymmetric and mirrored wear patterns may be the result manufacturing tolerances. Wade et al 20 measured the taper connections of commercially available stem and head tapers using CMM and found a maximum difference of 0.05° and 20 µm between samples from the same manufacturer and with the same design. While sample-to-sample variation is larger than that within a single sample, the axisymmetric and mirrored wear pattern depths (4 µm to 14 µm) are on the same order of magnitude as manufacturing tolerances.…”

Importance of surgical assembly technique on the engagement of 12/14 modular tapers

“…Taper geometry was measured using a coordinate measurement machine (CMM) according to the protocol described by Wade et al 23 Surface roughness parameters describing amplitude, distribution and shape of the profile was done using Vertical Scanning interferometry ( Bruker , USA) according to ISO 4288-98. 44 Outputs of this analysis are shown in Table 1.…”

“…20 Literature from the early 1900s details the extensive lengths gone to achieve a uniform distribution of stress over the whole interface, including explaining how tapers should be pressed together and not impacted with a hammer. 21 There is a wide variation in the taper junction in THR, often differing quite significantly from those used in machining processes 18,[22][23][24] ; For example, the cone angle is often steeper in THR for a 'self-locking' taper junction, often presenting a relatively rough surface topography and a level of angular mismatch. 23,25 Different designs have been found to influence performance and researchers have spent over 25 years studying what makes a 'good' taper interface, mostly failing to draw consistent conclusions.…”

“…21 There is a wide variation in the taper junction in THR, often differing quite significantly from those used in machining processes 18,[22][23][24] ; For example, the cone angle is often steeper in THR for a 'self-locking' taper junction, often presenting a relatively rough surface topography and a level of angular mismatch. 23,25 Different designs have been found to influence performance and researchers have spent over 25 years studying what makes a 'good' taper interface, mostly failing to draw consistent conclusions. These factors can be summarised into three categories: the patient (i.e.…”

Morse taper and femoral head modularity: a technical note

“…Morse tapers are not standardised in the orthopaedic industry. 1,2,4,5 The trunnion (male portion) and the bore (female portion) may appear to be compatible but might be totally incompatible. Extreme care must be taken in the…”