Osteosarcoma is a malignant tumor forming tumorous bone and cartilage or osteoid. In the nationwide bone tumor registry established by the Musculoskeletal Tumor Committee of the Japanese Orthopaedic Association, 7,649 of a total of 49,768 bone tumors registered from 1972 to 2003 (32 years) were malignant (15%); 3256 of these were osteosarcomas (accounting for 42% of bone malignances). Development at 40 years of age or older is reportedly rare, but 433 (13.2%) of the 3,256 osteosarcoma patients were 40 years of age or older. Regarding the developmental site, osteosarcoma most frequently arises in the femur and tibia, followed by the humerus in 262 cases (8%), being ranked the 3rd.
It is stated in Dahlin's Bone Tumors that the incidence in those in their 40s is about 5%, and that osteosarcoma developed in the humerus in 154 of 1952 cases, accounting for 7.8% of all osteosarcoma cases .
For the differential diagnosis from metastatic bone tumor, which was a challenge in our present patient, a comprehensive judgment should be based on past medical history, age, leison, and imaging findings. In patients 60 years of age and older, cancer metastasis is initially suspected. The incidence of metastatic cancer is high in the spine and pelvis, and the periosteal reaction on plain radiographs is poor in a metastatic bone tumor. The primary lesion should also be simultaneously investigated. In our present patient, bone destruction was marked, and the periosteal reaction was poor. Arterial bone cyst, giant cell tumor of bone, conventional osteosarcoma, and telangiectatic osteosarcoma are included in the differential diagnosis. Aneurysmal bone cyst was inconsistent with age, and the developmental site was not typical for giant cell tumor of bone . Osteosarcoma, particularly, telangiectatic osteosarcoma, was strongly suspected on imaging, but a metastatic bone tumor was also considered based on the patient's age and the higher prevalence of this type of tumor. Intramedullary pinning was selected because plate fixation was difficult due to the presence of pathological fracture and the location of the lesion in the proximal humerus.
The challenge faced by surgeons after the resection of a malignant bone tumor in the proximal humerus is reconstruction of shoulder joint function. When the cuff is included in massive resection to acquire a safe resection margin, shoulder joint function is mostly lost. For reconstruction of the proximal end of the humerus, several methods employing an artificial bone head for tumor treatment , , allogeneic bone grafting , , processed bone , and vascularized bone grafting , have been reported, but reconstruction by hanging these from the acromion using an artificial ligament, fascia, or tendon is inevitably necessary. We considered the distal humerus to be contaminated with tumorcomponents via intramedullary pinning, and adopted total humerus replacement to ensure the safety of the resection margin.
For the proximal humerus, hanging from the acromion using an artificial ligament was adopted. Our resection method was corresponded closely with that for type V of the Malawer classification of proximal humeral malignant bone tumors, but we opted to perform massive resection corresponding to extra-capsular resection conserving the acromion and coracoid process because chemotherapy had been sufficiently effective . Very few cases of total humerus replacement have been reported. To our knowledge, only Grimer (1996)  and Fabroni (1999)  have reported such cases and Ayoub (1999) reported described extendable artificial joint use for the humerus . Fabroni et al. reported long-term outcomes in 3 cases, all receiving custom-made artificial joints, and the mean functional score was about 65%. The characteristics of the Howmedica Modular Reconstruction System (HMRS) for the humerus used herein include the adjustability of the stem length corresponding to the resected bone mass and the presence of a hole for attachment with soft tissue. In addition, a porous coating is applied, and the ulnar component of the elbow joint has an anatomical shape. However, the level of osteotomy for optimal setting of the ulnar component of the elbow joint is unclear, and improvement may be necessary.