These articles demonstrate that various “commercially pure” and alloyed titanium samples contain a low but consistent percentage of contaminants that are associated with allergies. For example, traces of nickel, related to the manufacturing process were found in titanium materials used for surgical implants. “Nickel allergic patients may develop hypersensitivity reactions due to this low nickel content. Under certain circumstances, these small amounts may be enough to trigger allergic reactions in patients suffering from corresponding allergies, such as a nickel, palladium or chromium allergy.”

Titanium allergy or not? “Impurity” of titanium implant materials, 2010
Article here

Allergic potential of titanium implants, 2005
Abstract here

“According to our results, all implants immersed in human saliva already released metallic particles of titanium, nickel and vanadium after 7 days.”

Corrosion behavior of dental implants immersed into human saliva: preliminary results of an in vitro study, 2017
Article here

Nickel is released preferentially from orthopaedic devices, so even though there is only a trace it may be released first. More research in the area is needed but an interesting study looks at cobalt chromium hips which were removed and studied. Nickel makes up around 13% of this alloy but was not found in the corroded hips as it was the first metal to have corroded and moved into the body.

In vivo corrosion of 316L stainless-steel hip implants: morphology and elemental compositions of corrosion products, 1998
Abstract here

In another recently published case report, a patient reported severe pain after titanium plate placement, containing only 0.1% nickel. There was no rash, no infection and no loosening but an MRI revealed swelling, patch testing identified a nickel allergy.

The hardware was removed, and the patient’s pain resolved completely.

Could a Titanium Ulnar Shortening Plate Trigger a Metal Allergy? A Case Report, 2019
Abstract here

Titanium does not penetrate unbroken skin either as an alloy or as a nano-particle. The first article concludes “patch testing with the available Ti preparations for detection of type IV hypersensitivity is currently inadequate”. The second article looks at a symptomatic patient with no patch test reactions to metals. The patient subsequently tested positive in a lymphocyte transformation test (like MELISA), the metals were removed and the symptoms resolved

Titanium: a review on exposure, release, penetration, allergy, epidemiology, and clinical reactivity, 2016
Abstract here

Hypersensitivity to titanium osteosynthesis with impaired fracture healing, eczema, and T-cell hyperresponsiveness in vitro: case report and review of the literature, 2006
Abstract here