KALAMAZOO, Mich. (AP) -- Medical device maker Stryker Corp. said Friday it received a Food and Drug Administration warning letter over quality issues at its Hopkinton, Mass., manufacturing plant.

The warning letter to the Stryker Biotech division concerns observations made during an inspection initiated in September, the company said, and cites issues over the company's handling of a past clinical study. Other issues included the company's quality systems, such as its reporting procedures and review board.

Stryker said several corrective actions and changes were made and additional improvements are planned. No products are affected by the letter.

Stryker shares fell $1.68, or 2.6 percent, to $63.98 in afternoon trading. The stock has traded between $58.45 and $76.89 over the last 52 weeks.