【Introduction】
The sacrum connects the spine and pelvis, and the axial load from the spine via vertebral disc is dispersed to the bilateral iliac bones. When osteolysis is progressing by malignant sacral tumor at the first and second sacral vertebrae, the axial load from the spine will fail to sacrum U-shaped fracture meaning spinopelvic dissociation. On the other hand, particle beam therapy is interfered with instrumentation, therefore it will be required refraining from using instrumentation. This study showed the cases with primally malignant sacrum tumor causing U-shaped pathological fracture after laminectomy, and considered the mechanism.
【Methods】
Patients were the cases with sacral U-shaped pathological fracture after laminectomy for providing particle therapy to primary sacral malignant tumors. The survey items were age, gender, neurological deficits, treatments for primary sacral tumors, and periods from laminectomy. The mechanism of sacral U-shaped pathological fractures after laminectomy for preparing particle beam therapy was considered based on the items.
【Results】
Case 1: 72 years old man showed numbness from both buttock to posterior legs and difficulty urinating appeared. There was a malignant bone tumor suggesting chordoma at the first and second sacral vertebrae protruding into the spinal canal and compressing the cauda equina. Laminectomy was applied for the first and second sacral vertebrae combined with an open biopsy without inserting an implant preparing for particle beam therapy. A U-shaped sacral fracture accompanied by severe buttock pain occurred at 12 months after laminectomy, and a fusion between the fourth lumbar vertebra and the ilium was subsequently performed avoiding the irradiation field.
Case 2: 87 years old man showed both buttock to posterior leg pain and muscle weakness at toe and ankle planter flexion, as well as difficulty urinating, occurred without any apparent cause. There were osteolytic lesions in the vertebrae of the first to third sacral vertebrae, and laminectomy combined with open biopsy was performed without inserting an implant preparing for particle beam therapy. Two weeks after the laminectomy, the patient presented a U-shaped sacral fracture when he moved to a wheelchair. A fusion of the fourth lumbar vertebra and the ilium was performed at first for severe pain. Stereotactic radiation therapy was conducted after confirming the pathological diagnosis of undifferentiated pleomorphic sarcoma on behalf of particle beam therapy because instrumentation would be interfering with particle beam therapy planning.
【Discussion】
The continuity of the cortical bone at the posterior part of the sacrum from the midline sacral ridge to the sacroiliac joint may have been the final braking part against the shear force from axial load of spine in these sacral primary tumor cases occurred in the vertebral body. In the cases with primary sacral vertebral malignant tumor preparing for particle beam therapy, laminectomy without instrumentation would cause vulnerability providing pathological spinopelvic dissociation.