Medical equipment manufacturer Stryker Orthopedics has recalled its two hip implants voluntarily

Frankly, they're defective. This info had been announced awhile ago (2013).

Type keyword phrase "Stryker Orthopedics has recalled its two hip implants" in Google or any search engine and you will see many links. Here's one linked article that explains the recall:
http://www.drugwatch.com/hip-replacement/recalls/

Did they repossess the ones in service?

I can't say for sure. Frankly, it's not Hip!