costomalacia = the softening of the sternal ends of the ribs; more generally known as "osteomalacia" (osteo=bones; -malacia=softening).
Costomalacia is not a recognized medical term. It might be a misspelling of "osteomalacia," which is a condition where there is a softening of the bones due to a deficiency in vitamin D, calcium, or phosphate. It can lead to weakening and increased risk of fractures. Treatment usually involves vitamin D and calcium supplements.