Modelling changes in modular taper micromechanics due to surgeon assembly technique in total hip arthroplasty

Abstract

Aims: The aim of this study was to develop a novel computational model for estimating head/stem taper mechanics during different simulated assembly conditions.

Methods: Finite element models of generic cobalt-chromium (CoCr) heads on a titanium stem taper were developed and driven using dynamic assembly loads collected from clinicians. To verify contact mechanics at the taper interface, comparisons of deformed microgroove characteristics (height and width of microgrooves) were made between model estimates with those measured from five retrieved implants. Additionally, these models were used to assess the role of assembly technique-one-hit versus three-hits-on the taper interlock mechanical behaviour.

Results: The model compared well to deformed microgrooves from the retrieved implants, predicting changes in microgroove height (mean 1.1 μm (0.2 to 1.3)) and width (mean 7.5 μm (1.0 to 18.5)) within the range of measured changes in height (mean 1.4 μm (0.4 to 2.3); p = 0.109) and width (mean 12.0 μm (1.5 to 25.4); p = 0.470). Consistent with benchtop studies, our model found that increasing assembly load magnitude led to increased taper engagement, contact pressure, and permanent deformation of the stem taper microgrooves. Interestingly, our model found assemblies using three hits at low loads (4 kN) led to decreased taper engagement, contact pressures and microgroove deformations throughout the stem taper compared with tapers assembled with one hit at the same magnitude.

Conclusion: These findings suggest additional assembly hits at low loads lead to inferior taper interlock strength compared with one firm hit, which may be influenced by loading rate or material strain hardening. These unique models can estimate microgroove deformations representative of real contact mechanics seen on retrievals, which will enable us to better understand how both surgeon assembly techniques and implant design affect taper interlock strength. Cite this article: Bone Joint J 2020;102-B(7 Supple B):33-40.

Keywords: Contact mechanics; Finite element modeling; Microgrooves; Surgeon assembly; Tapers; Total hip arthroplasty.

Figure 1:

Finite element model of the head/stem modular taper (Left) with representative modeling of microgroove surfaces (inset).

Figure 2:

Typical loading profiles (left) for head/stem assembly with 1 or 3 mallet hits. Load profiles used to drive computational finite element models of the head/stem taper interface at varying axial forces ranging from 4, 8, and 12 kN (center). Respective microgroove plastic strain within contact area (right) show differences between one-hit and three-hit simulations.

Figure 3:

Comparison between undeformed and deformed microgroove peak (top) measured by white light interferometry, and undeformed and deformed microgroove peak as simulated by our FEA model (bottom). Comparable microgroove flattening is observed.

Figure 4:

Comparison of undeformed and deformed taper microgroove heights from both retrievals and computational model.

Figure 5:

Comparison of change in stem taper microgroove width from both retrievals and computational model.

Figure 6:

Representative contact pressure distributions over the stem taper length for tapers assembled at 4kN (red) and 8kN (purple) applied in one-hit (solid) or 3-hit (dashed) sequences.

Figure 7:

Comparison of average plastic strain (left) and total number of stem taper microgrooves (right) undergoing permanent deformation at the different load magnitudes and load sequences.