BACKGROUND: Synmastia is serious condition but rare. Synmastia is a technical complication caused by over dissection of the medial pocket over the sternum. In sunken chest, the medial portion of Pectoralis muscle tends to be elevated easily from the sternocostal area. So it should be more careful to prevent from overdissection in breast augmentation of sunken chest. METHODS: An idea was obtained from buttock augmentation with implant. For buttock augmentation, implants are placed intramuscularly for the protection of sciatic nerve and for the prevention of implant displacement. Muscle splitting concept for intramuscular placing of implant in sunken chest adds tissue to sunken area and provide guarding barrier to reduce detaching tendency at the parasternal area. RESULTS: As I had the concept of intramuscular placement at the medial side for the breast augmentation in case of sunken chest, I experienced steady medial breast contour in 5 cases of sunken chest. CONCLUSIONS: Muscle splitting as coronal plane of medial portion of pectoral muscle make muscular pouch at the medial corner of subpectoral pocket which is effective for the prevention of medial displacement of implant in breast augmentation of sunken chest by guarding effect of irregularly arranged muscle fibers which resist against the detaching force of pectoral muscle from the sternocostal origin.