Per a press release from the Spanish royal court:

“During February 9 and 15, 2013, His Majesty King Juan Carlos I of Spain has been subjected to annual medical examinations.  After completed examinations, the results were satisfactory. 

However, there is still persistent aggravation of a former herniated disc in lumbo-sacral spine.  At this time the palace and His Majesty’s doctors are studying to determine the most appropriate treatment for His Majesty.”