Johnson & Johnson is to buy Swiss medical devices maker Synthes Inc in a deal worth $21.3 billion, its largest ever buy, boosting its orthopedics franchise and reshaping the medical technology industry.

The diversified healthcare group will pay 159 Swiss francs per share for Synthes, the two companies said on Wednesday. That is a premium of 8.5 percent over Synthes' closing share price on Tuesday.

The deal, which is expected to close in the first half of 2012, has the backing of both the Synthes and J&J boards and will give J&J a leading position in equipment to treat trauma.

Synthes, which posted sales of $3.7 billion in 2010, makes nails, screws and plates to fix broken bones, as well as artificial spine discs.

(Reporting by Katie Reid; editing by Ben Hirschler)