Additively manufactured calcium phosphate reinforced CoCrMo alloy: Bio-tribological and biocompatibility evaluation for load-bearing implants

Abstract

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to find an alternative to CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.

Keywords: CoCrMo alloys; load-bearing implants; metal ion release; surface modification; tribofilms.

Figure 1.

Electron micrographs of CCM_1LP, a) low magnification view of entire melted cross section; b) heat affected zone (HAZ); c) interface of two solidification fronts; d) radial region of one solidification front and e) superficial surface of the melted region.

Figure 2.

X-Ray diffraction (XRD) spectra of CCM and CCM_1LP samples.

Figure 3.

Cross-sectional hardness profile of CCM_1LP, from substrate to surface hardness.

Figure 4.

a) Instantaneous normalized wear rate of CCM₀, CCM₃, CCM_1LP and CCM, b) instantaneous contact resistance of CCM₀, CCM₃ and CCM_1LP during tribological testing.

Figure 5.

Electron micrograph of wear track scar for, a) CCM, b) CCM_1LP, c) CCM₀ and d) CCM₃ samples.

Figure 6.

a) MTT assay results showing the optical density values after 3 and 11 days for all samples measured at a wavelength of 570 nm, showing increase in cell density values up to 11 days indicating no cytotoxic effects. Note (*) p-value < 0.001. b-e) Confocal microscopy images showing the ALP expression after 11 days where protein expression is indicated by green fluorescence and cell nuclei by blue fluorescence for, b) Cp-Ti, c) CCM₀, d) CCM₁, e) CCM₂.

Figure 7.

a-d) Histological evaluation and a1-d1) SEM characterization of a) dense Ti, b) CCM₀, c) CCM₁, d) CCM₂, e) CCM₃, after 5 weeks in rat distal femur bone showing osteoid-like new bone formation and interfacial bonding between the implant and the surrounding bone tissue area.

Figure 8.

a-d) Histological evaluation and a1-d1) SEM characterization of a) dense Ti, b) CCM₀, c) CCM₁, d) CCM₂, e) CCM₃, after 12 weeks in rat distal femur bone showing osteoid like new bone formation and interfacial bonding between the implant and the surrounding bone-tissue area

Figure 9.

EDS images of Cp-Ti control, CCM₀, CCM₁, CCM₂ and CCM₃ samples indicating absence of Ti, Co and Cr ions around the implant-tissue interface and the surrounding area, respectively.

Figure 10.

Histological evaluation and SEM characterization of CCM₀ wear tested samples after 5 and 12 weeks showing osteoid like new bone formation and interfacial bonding between the implant and the surrounding bone-tissue area.

Figure 11.

Histological evaluation and SEM characterization of CCM₃ wear tested samples after 5 and 12 weeks showing osteoid like new bone formation and interfacial bonding between the implant and the surrounding bone-tissue area.

Figure 12.

Histological evaluation and SEM characterization of CCM₀ and CCM₂ rabbit study samples after 5 weeks showing osteoid like new bone formation and interfacial bonding between the implant and the surrounding bone-tissue area.

Figure 13.

Histological evaluation and SEM characterization of CCM₀ and CCM₂ rabbit study samples after 12 weeks showing osteoid-like new bone formation and interfacial bonding between the implant and the surrounding bone-tissue area.