[The mechanism of fatigue fracture of the ribs].

To elucidate the mechanism of fatigue-rib-fracture, 8 rib-fractured cases were investigated. In assuming the M. serratus anterior (MSA) as the major cause from the anatomical muscle route, EMG was taken, and three-dimensional resultant forces from the muscles around the shoulder of a fresh cadaver were estimated. Most fractures were found in the sixth or seventh ribs, with a tendency to occur along the posterolateral segment. Biomechanical analysis revealed 12 and 62 kg forces applied on the scapulo-thoracic joint and gleno-humeral joint respectively by adding 90 degrees-anterior-elevation under no load. The maximum muscle-force of MSA occurring at 90 degrees-anterior-elevation was 1.2 kg in the sixth serratus in our study. These data were supported by the results with EMG. The stress on the ribs were calculated by two-dimensional finite element method, using the data. The maximum compressive stress on one rib was 4.75 kg/mm2 when 13.0 kg of MSA-force was added. In considering the limit of fatigue endurance of bones, these results suggest that muscle-force which causes fatigue fracture of ribs arises from lifting an object weighing more than 20 kg.