Purpose: To evaluate the post-operative functional reduction of the shoulder joint and the impacting factors to post-operative shoulder joint function in interlocking IM nailing treatment of humeral shaft fracture. Materials and Methods: From April 1999 to August 2004, 35 patients (35 cases) whom admitted to hospital for humeral shaft fracture and treated using interlocking intramedullary nail were followed up for more than 1 year. 1 year post-operative shoulder joint function were evaluated using American Shoulder Elbow Surgery Scale (ASES). Pre-operative shoulder joint pain, radiologically degenerative change and extent of nail protrusion were evaluated, and each factor was correlated with function of the shoulder joint. Results: 33 cases out of 35 cases showed union and average union period was 12 weeks. Complications consisted of 2 cases of nonunion, 1 case of infection, 1 case of loosening of distal fixing screw, 1 case of radial nerve palsy and 1 case of axillary nerve palsy. Shoulder joint function 3 months after operation : mean ASES score 78.2, 12 months after operation : mean ASES score 89.6. Pre-operative shoulder joint pain and nail protrusion showed to be statistically related to shoulder joint function. Conclusion: If the operation leaves no protrusion of intramedullary nail, it can be concluded to be relatively safe and effective.

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