We studied the effects of high-dose irradiation on the mechanical properties and morphology of cortical bone in rabbits for 52 weeks after a single dose of 50 Gy of elecfron-beam to the tibia. After four weeks, the bending strength of the irradiated bone was unchanged, but at 12 weeks, the strength had decreased significantly. At 24 weeks after irradiation mean strength was less than half of confrols but by 52 weeks there was a tendency toward recovery. Similar, synchronous changes of damage and recovery were seen in cortical

Neither communication with an adjacent facet joint nor histopathology of the cyst wall provides a persuasive basis for differentiating ganglion, synovial cyst, and cyst arising from the ligaments. To accommodate the varied presentations, the authors propose a comprehensive term for these lesions: "degenerative intraspinal cyst."

These results justify and favor the use of calcium phosphate ceramics as bone graft substitutes for instrumented posterior spinal fusion in teenagers and young adults. Potentially hazardous harvesting of pelvic bone is no longer necessary for such operations.