Abstract
Introduction
Dual modular hip prostheses were introduced to optimize the individual and intra-surgical adaptation of the implant design to the native anatomics und biomechanics of the hip. The downside of a modular implant design with an additional modular interface is the potential susceptibility to fretting, crevice corrosion and wear [1–2]. The purpose of this study was to characterize the metal ion release of a modular hip implant system with different modular junctions and material combinations in consideration of the corrosive physiological environment.
Methods
One design of a dual modular hip prosthesis (Ti6Al4V, Metha®, Aesculap AG, Germany) with a high offset neck adapter (CoCrMo, CCD-angle of 130°, neutral antetorsion) and a monobloc prosthesis (stem size 4) of the same implant type were used to characterize the metal ion release of modular and non-modular hip implants. Stems were embedded in PMMA with 10° adduction and 9° flexion according to ISO 7206-6 and assembled with ceramic (Biolox® delta) or CoCrMo femoral heads (XL-offset) by three light impacts with a hammer. All implant options were tested in four different test fluids: Ringer's solution, bovine calf serum and iron chloride solution (FeCl3-concentration: 10 g/L and 114 g/L). Cyclic axial sinusoidal compressive load (Fmax = 3800 N, peak load level of walking based on in vivo force measurements [3]) was applied for 10 million cycles using a servohydraulic testing machine (MTS MiniBionix 370). The test frequency was continuously varied between 15 Hz (9900 cycles) followed by 1 Hz (100 cycles). The metal ion concentration (cobalt, chromium and titanium) of the test fluids were analysed using ICP-OES and ICP-MS at intervals of 0, 5·105, 2·106 and 10·106 cycles (measuring sensitivity < 1 µg/L).
Results
Due to the additional modular interface between stem and neck adapter the total metal ion release of the modular hip endoprosthesis system increased significantly and is comparable to the coupling of a monobloc stem and a CoCrMo femoral head (Fig. 1). The application of ceramic femoral heads reduced the total cobalt and chromium release in the stem-head taper interface of non-modular and modular stems. In comparison between the four test fluids could be observed that lower pH-values and higher FeCl3-concentrations increased the metal ion release (Fig 2). In contrast, the use of bovine calf serum decreased the metal ion release of modular junctions due to the presence of proteins and other organic components.
Discussion
For testing hip implants with proximal femoral modularity according to ISO and ASTM standards, sodium chloride solutions are frequently used to determine the fatigue strength and durability of the stem-neck connection. The present study illustrate that the expansion of standard requirements of biomechanical testing and the use of alternative test fluids is necessary to simulate metal ion release by electro-chemical processes. A promising approach is the use of adapted iron-chloride solutions (10 g/L FeCl3, pH 2) to evaluate the susceptibility of modular hip junctions to fretting, crevice and contact corrosion.
