One of our captains had one of the cobalt chrome implants. He long struggled with the side effects from cobalt poisoning which eventually led to his death.
Titanium is great for corrosion resistance and toxicity but has terrible wear resistance. The gray sludge was likely titanium particles from erosion. Coatings are key to getting long life from titanium.
It was quite interesting very 3d, nonlinear geom, none linear materials, contact. The 2 months of experimental work on the tensile tester with pig bones then hand over to the proper Bio's for the next phase was good. Also did a vertebrae fusion yoke but that was easy comparted to the hip.