Pedicle stress fracture: an unusual complication of pycnodysostosis

Pycnodysostosis is a rare dysplasia characterised by high bone density and susceptibility to long bone fractures caused by cathepsin K deficiency. Spinal abnormalities have rarely been described in this uncommon inherited bone dysplasia. A 28-year-old female, with a past history of pycnodysostosis and spontaneous leg fractures was hospitalized for a 2-month history of spontaneous low back pain. Physical examination revealed the typical facial and hand features of pycnodysostosis and local lumbar stiffness. No abnormalities were found in laboratory tests, particularly with regard to bone turnover markers. Fracture of the left pedicle of the third lumbar vertebra was suspected on lumbar radiographs and later confirmed by a computed tomography (CT) scan. A dramatic improvement in symptoms was achieved, thanks to a course of injectable calcitonin therapy and rest. To our knowledge, it is the first-ever reported case of pedicle stress fracture in a patient with pycnodysostosis, suggesting that spontaneous fractures resulting from this bone dysplasia do not only affect diaphysis of brittle long bones but could also affect the lumbar spine. Furthermore, the present case confirms previous observations in such patients of frequent spondylolysis, which could lead to abnormal lumbar pedicle stress. The dramatic improvement achieved by calcitonin therapy might be related to osteoclastic dysfunction in pycnodysostosis caused by a deficiency of cathepsin K, a cystein protease involved in bone matrix remodelling.