Callam said: "I underwent a routine scan on an injury and discovered a bone cyst in my hip; we were very lucky that we found it. It is a rare injury to find in a young sportsman - they are not very common.
"I had an MRI scan two years ago and there was nothing there, but for whatever reason it had developed over that period of time. I then underwent a biopsy to ascertain what the cyst was and that was why there was almost a two-month wait to determine the next step forward.
"Initially I was devastated, but fortunately we found out when we did because it could have had serious repercussions on my rugby career."
Callam underwent an operation at the Spire Murrayfield and he praised the treatment he received at the nearby hospital. He added: "I got looked after fantastically well and the surgeons did a superb job. I was lucky to receive such wonderful attention.
"Now after undergoing the operation I will hopefully be back playing again next season. I've got eight weeks on crutches. Hopefully after four months I should be able to start training again and then it will be a case of building myself up from there."
Head coach Andy Robinson added: "It was a very unfortunate injury and we all feel for Dave at this time. Obviously the recovery period now is very important for him.
"We are hoping David comes back stronger than ever in time for next season. On behalf of the club, I would like to thank the surgeons at Spire Murrayfield for the superb treatment Dave received."