Spain's King Juan Carlos "is in good condition and will remain under observation for the next 24 hours" at a Madrid hospital before beginning physiotherapy following the knee operation he underwent on Friday.

The Spanish monarch had a prosthesis implanted "successfully" to replace the joint of his right knee, Javier de Joz, director of San Jose Hospital where the operation was performed, said.

As the physician explained in the first medical report after the king underwent surgery with epidural anesthesia, the operation went off with "complete normality" and lasted 90 minutes, though the king was in the operating room for almost three hours.

After the operation, the Spanish head of state "will remain under observation for the next 24 hours, before being moved to an area where he will begin specific physiotherapy."

The surgery performed by traumatologist Angel Villamor consisted of "a total arthroplasty of substitution" of the right knee joint, which had caused the king "to suffer pain for years."

"Lately it got worse so that it partially limited his physical activity," the medical report said.

It said that the pain was "chiefly the consequence of old sports injuries and was not able to heal without surgery."

The head of medical services for the royal household, Avelino Barros, signed the report together with Angel Villamor, director of the traumatology and advanced recovery unit at San Jose Hospital and medical director of the iQtra Clinic.

Queen Sofia was able to be with Juan Carlos soon after he emerged from the operating room and stayed at the hospital for almost an hour.

King Juan Carlos underwent his last annual medical examination last February at Barcelona's Planas Clinic, and the results of those tests and observations were "completely satisfactory," the medical team reported at the time.

On Sept. 30, 2010, he successfully passed his last post-operation checkup following the surgery he underwent on May 8 to have a nodule removed from the upper lobe of the right lung, which turned out to be benign.

After that exam in September, the medical team said that the king had "recovered completely" from his lung operation and required no further post-operation checkups.