Philip Loveday, 59, injured his neck playing rugby when he was 16 after a player fell on him while playing a match for the Army.
X-rays at the time failed to reveal he had a broken vertebrae.
Loveday, from Bridgend, found out only when he went for an MRI scan after dislocating his shoulder.
He believes an active lifestyle, including continuing playing rugby for a number of clubs, had prevented him suffering problems because of the long-term injury.
He said he had been told he would have to live with the damage as doctors were unable to repair the damage to the C3 vertebrae.
“It is incredible that I’m still here and walking,” the BBC quoted him as saying.
“I can only think that my strong neck muscles during my 20-year Army career and a lifetime playing rugby has kept my head on my shoulders,” he said.
Describing the incident from his youth, he said he remembered going over the line to score a try and an opposition player falling on his neck. He heard a snap, and was taken to hospital.
“As far as I was concerned everything was OK but looking back you have to remember that X-rays then weren’t as clear as they are today.
“The Army had told me that if I wanted to stay in the job I’d have to bulk up.
“I went from being a seven-stone weakling with a tiny neck when I joined up to having a 22-inch neck. My neck muscles got bigger and stronger and supported my head and neck,” he said.
Loveday thinks his strengthened neck muscles took the pressure off his spinal column. He remained in the dark about his condition until his visit to the hospital.
“I had an MRI scan of my shoulders and neck and the doctor asked me if I knew I had a fractured neck. I said no.
“But the fracture was still there and visible on the scan. It was shocking and I’m just so glad to be here,” he said.