There are many reasons for the bone protruding from the lower side of the left breast, such as normal phenomenon, rib ectopia, tumor, etc. You should consult a doctor in time to clarify the cause of the disease and carry out treatment. 1. Normal phenomenon: due to the anatomical structure of human body, if the body is thin, there may be bone protruding, usually no special discomfort, no special treatment. 2. Rib protrusion: teenagers have calcium deficiency due to insufficient calcium supplement, which causes developmental deformity and leads to protruding ribs under the chest, they should supplement calcium in time and eat more calcium-rich food, such as milk, fish and so on. 3. Tumor: patients may have tumor, there may be primary bone tumor, causing one side of the bone protruding, or there may be bone metastasis, caused by the transfer of tumor from other places. There may be other reasons for the protruding bone on the left side of the lower breast, so you should go to the hospital in time to clarify the cause of the disease and carry out the corresponding treatment.