Abstract
Summary
Micromotions between stem and neck adapter depend on prosthesis design and material coupling. Based on the results of this study, the amount of micromotion seems to reflect the risk of fretting-induced fatigue in vivo.
Introduction
Bimodular hip prostheses were developed to allow surgeons an individual reconstruction of the hip joint by varying length, offset and anteversion in the operation theatre. Despite these advantages, the use of these systems led to a high rate of postoperative complications resulting in revision rates of up to 11% ten years after surgical intervention. During daily activities taper connections of modular hip implants are highly stressed regions and contain the potential of micromotions between adjacent components, fretting and corrosion. This might explain why an elevated number of fretting-induced neck fractures occurred in clinics. However, some bi-modular prostheses (e.g. Metha, Aesculap, Ti-Ti) are more often affected by those complications than others (e.g. H-Max M, Limacorporate, Ti-Ti or Metha, Ti-CoCr) implying that the design and the material coupling have an impact on this failure pattern. Therefore, the purpose of this study was to clarify whether clinical successful prostheses offer lower micromotions than those with an elevated number of in vivo fractures.
Materials and Methods
Two different bimodular hip designs (Metha and H-Max M, n = 6 each) were tested in vitro. Embedded Ti6Al4V (Ti) stems were assembled with Ti or CoCr29Mo (CoCr) necks and sinusoidally loaded (f = 1 Hz, 10,000 cycles) ranging from 0.23 to 4.30 kN (peak to peak, represents going upstairs) using a servohydraulic testing machine (MiniBionix II, MTS). Based on the results of four eddy-current sensors, micromotions were assessed in the region of the crack origin of fractured prostheses (lateral radius). Due to the test set-up, the recorded displacement includes, beside the real micromotions, the elastic deformation between sensor holder and reflector. The amount of the elastic deformation was determined using the finite-element technique. For statistical analyses Twoway-ANOVAs were performed (α = 0.05).
Results
The H-Max M prostheses exhibited significantly lower micromotions compared to Metha prostheses (1.8 ± 2.2 µm vs. 4.1 ± 3.2 µm, p = 0.03). For Ti-Ti couplings, Metha prostheses showed a trend towards higher micromotions compared to H-Max M (6.5 ± 1.6 µm vs. 3.6 ± 1.5 µm, p = 0.08). Independent of the design, prostheses with Ti neck adapters caused significantly higher micromotions than those with CoCr adapters (5.1 ± 2.1 µm vs. 0.8 ± 1.6 µm, p < 0.01). No differences between the clinically used Metha prostheses with CoCr neck adapters and H-Max M prostheses with Ti necks were found (2.6 ± 2.0 µm, p = 0.25).
Discussion
Both, the material coupling and the design influence the interface micromotions. The magnitude of micromotions might explain why bimodular hip systems are susceptible to fretting-induced fractures; however, the threshold for critical micromotions is still not known. The results of this study indicate that the amount of micromotion at taper interfaces could be directly linked to the risk of clinical failure.
