The two cubs are now back in the paddock after two months at the vets
Paignton Zoo believe a lack of calcium from their mother has led to three rare Sumatran tiger cubs suffering from the bone condition, osteoporosis.
One of the cubs, a female, had to be put down when she had a fall and broke both shoulders.
Then two of the other cubs - a male and a female - broke bones in their legs and had to spend two months at the zoo's vet centre.
The pair have recovered and are now back with the family unit at the zoo.
The fourth cub seems unaffected and has enjoyed good health.
The cubs were born in February 2009, causing great delight at the zoo, which is a member of the European Endangered species Programme (EEP) for Sumatran tigers.
Film of the rare Sumatran tiger cubs at Paignton Zoo
The animals are a globally endangered species which, with just 400 left in the wild, is threatened with extinction.
During the summer, one of the cubs had to be destroyed because of the severity of the breaks to both her shoulders.
Soon after that, female cub Surya and male Aryo suffered breaks - Surya broke both her back legs, while Aryo broke a front leg.
Fortunately, they were clean breaks and could be healed.
"They went to the vet surgeon and that's when we found they had osteoporosis," said zoo spokesman Philip Knowling.
"There are many possible causes - it could be something to do with diet, or genetic, or not getting enough calcium from their mother, Banda. The other cub, Bintang, has been fine.
"There'll be some work done to see what we can learn from it.
Mum Banda welcomed the two cubs back
"But there should be no reason why they shouldn't grow up to be healthy, breeding animals and have a role to play in helping to ensure the future of the species."
Ghislaine Sayers, head of veterinary services at Paignton Zoo, said: "The bones have healed and look almost normal now on radiographs.
"The muscles have developed well and the joints have normal flexibility again. I am really pleased with their progress but we will keep them on their calcium supplement for quite some time yet as they still have a lot of growing to do."
Surya and Aryo were able to return to mum and brother on 27 October 2009 - dad Tenang plays no part in the rearing so is in another paddock.
The zoo's curator of mammals, Neil Bemment said: "At first we let the cubs have nose-to-nose contact with their mother to see how they would react to each other.
"Banda made nothing but positive noises towards them, so we decided to get them all back in together right away. All four are now out in the autumn sunshine, running about and chasing each other. It is fantastic to see them."
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