Stress fractures of the rib arising from the manufactory assembly line

We investigated stress injuries of the rib arising from the assembly line. The patients, all males, were 20 in number and divided into two groups; those with stress fracture (n=13) and those with ''pre-fracture'' (n=7). The former group with the average age of 37 years was involved by a single fracture in 10 patients and multiple fractures in 3, ranging from 5th to 8th ribs, with the affected side on the right in 9 and the left in 4. All 13 fractures occurred in the posterior half of the rib, in which 5 were located in its lateral portion, 5 in the middle, and 3 in the posterior. The latter group with the average age of 32 years were involved on the right side in 4 and the left in 3. The working pattern causing rib injuries was either repeated raising of the upper extremity or forward pushing of the shoulder girdle. The symptom was back pain, and aggravated especially in elevation of the arm, deep breathing and sneezing. Local tenderness over the fracture site was noted in all cases. X-ray findings at initial examination revealed fractures in most cases and 3D-CT was employed in 9 questionable cases to assess fracture conditions. In the ''pre-fracture'' group, although the fracture was not recognized on X-ray, the diagnosis was made as an impending fracture based on the localized tenderness on the rib. As to treatment, bust band fixation and work load reduction were performed. Stress fractures of the rib have been occasionally reported in sports medicine, but not so much in industrial medicine. This specific injury has been treated in literature as muscle strain because of the absence of apparent trauma. It is important to recognize the underlying pathology to make the correct diagnosis. In the early stage of rib fractures, 3D-CT was more useful than X-ray in making diagnosis and assessing the bony union. As to the pathomechanism of this condition, it was postulated that repeated raising or forward pushing of the arm applying excessive traction to the origin of the serratus anterior muscle, leading to a stress fracture of the rib. It was worthy of note that the 3D-CT revealed an avulsion of the said muscle in one case. To prevent stress fractures of the rib arising from the assembly line, such measures are considered as muscle training of the shoulder girdle, especially the serratus anterior, maintenance of physical condition, meticulous work orientation, innovation of the assembly line etc. (author)