Influence of the use of an adhesive connection on the joint strength of modular hip endoprostheses

Abstract

Introduction: Modular hip implants enables a more precise adaptation of the prosthesis to the patient's anatomy. However, they also carry the risk of increased revision rates due to micromotion at the taper junction. In order to minimize this risk, one potential solution is to establish an adhesive bond between the metal taper junctions. Load-stable bonding techniques, already successfully employed in dentistry for connecting materials such as metals and ceramics or different alloys, offer a promising approach. Nevertheless, the bond strength of tapered adhesive bonds in modular hip implants has not been investigated to date.

Materials and methods: Twenty-eight tapered junctions, consisting of a taper (female taper) and a trunnion (male taper) were turned using TiAl6V4 ELI (n = 16) and CoCr28Mo6 (n = 12). The process parameters cutting speed (vc = 50 m/min or 100 m/min) and feed (f = 0.1 mm, 0.05 mm or 0.2 mm) were varied for the trunnions. For each set of process parameters, one trunnion and one taper were additionally subjected to sandblasting. To investigate the effect of geometry, angular mismatch in the samples were measured. The taper pairs were bonded with a biocompatible adhesive, and push-out tests were subsequently performed.

Results: The push-out forces generated from the taper connections where both tapers were sandblasted showed a mean push-out force of 5.70 kN. For the samples with only the trunnion sandblasted, the mean force was 0.58 kN, while for the samples with only taper sandblasted the mean push-out force was 1.32 kN. When neither of the tapers was sandblasted the mean push-out force was 0.91 kN. No significant effect of the process parameters on the push-out force was observed. Only the reduced valley depth Svk showed a slight correlation for the CoCr28Mo6 samples (R2 = 0.54). The taper pairs with taper mismatch (between trunnion and taper) greater than |0.1°| did not show lower push-out forces than the specimens with lower taper mismatch.

Conclusions: Sandblasted and adhesive-bonded tapered connections represent a viable suitable alternative for modular hip implant connections. Slight differences in taper geometry do not result in reduced push-out forces and are compensated by the adhesive. In mechanically joined tapers these differences can lead to higher wear rates. Further investigation under realistic test conditions is necessary to assess long-term suitability.

Fig 1. Technical drawing of the trunnion (A) and the taper (B).

Fig 2. Schematic illustration of the individual processing steps and the setup for the push-out tests.

Fig 3. Surface and profile plots of a turned sample (left in each case) and a turned and subsequently sandblasted sample (right in each case) of a trunnion.

Fig 4. Results of the push-out test for all taper connections and materials divided into groups by the sandblasting (sb) situation.

Values above and below 1.5 times the interquartile range are shown as outliers.

Fig 5. Correlation between Svk (Reduced Valley Depth) and push-out force for all TiAl6V4 ELI samples (blue) and CoCr28Mo6 samples (grey).

Dotted line shows the resulting regression line for each material type.

Fig 6. Force-path curves of the push-out tests exemplarily shown for individual samples of the different groups (CoCr28Mo6 only turned = red; TiAl6V4 ELI only turned = yellow; CoCr28Mo6 both sandblasted = light blue; TiAl6V4 ELI both sandblasted = dark blue.

The measured displacement of the testing machine was corrected for the stiffness of the setup.