Nickel can be found in titanium implants 

These articles demonstrate that various “commercially pure” and alloyed titanium samples contain a low but consistent percentage of contaminants that are associated with allergies. 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

"The low nickel content in the implant material results from the production process. The nickel atoms are in solid solution in the titanium lattice. Nickel allergic patients may develop hypersensitivity reactions even due to this low nickel content. Hence, this reaction may be falsely attributed to the titanium material itself. "

Allergic potential of titanium implants, 2005
Abstract HERE

"Fifteen commercial titanium dental implant systems were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS) and optical emission spectrometry (ICP-OES). All investigated implant samples contained fractions of nickel, chromium, antimony and niobium."

Impurities in commercial titanium dental implants – A mass and optical emission spectrometry elemental analysis, 2022
Article HERE 

"The objective of this study was to analyze four different commercial brands in their stage of sale for clinical use. The results presented in this work showed samples where the concentration of both aluminum and nickel were exceeded in some parts of the implants."

Physical-chemical analyses of contaminations and internal holes in dental implants of pure commercial titanium, 2018
Article HERE

Nickel is released from titanium dental implants

The aim of this study was to compare levels of metallic ions and particles dissolution collected from two different dental implants surfaces immersed into human saliva.
Forty implants were immersed in 20 ml of human saliva, twenty, as a control, in sterile saline solution. ICP-MS was performed at regular intervals to detect any metallic ions released from dental implants.
According to the results, all implants immersed in human saliva already released metallic particles of titanium, nickel and vanadium after 7 days. Dental implants immersed in sterile saline solution showed no significant dispersion of metallic ions, therefore assuming an active role of natural organic and inorganic constituents of human saliva in the corrosion process.

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

Removal of titanium implants containing small amounts of nickel lead to resolution of symptoms in a nickel-allergic patient

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.
Subsequent allergy testing revealed a new diagnosis of nickel allergy. Following hardware removal, the patient had a resolution of her symptoms at 3 months.

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