Discovery Health no cover for my opertion on my leg

I pay R2100 every month for medical aid. I went to a specialist. The specialist sent them (discovery health) a motivation letter which says "me (Mr JH du Preez) is known with a massive osteochondroma on the posterior aspect of his proximal femur encroaching the scaitic nerve and also restricting his range of motion." I had to make a loan of R55000 by my employer to pay for the operation. Medical paid R0. I also had to go for a mri scan which cost me R12 500. I feel this is unfair. Why do I have to pay for medical aid and now I have make a loan to get the surgery done. Its a total cost of R67 500.