Spinal Deformity In Osteogenesis Imperfecta

John P. Lubicky, M.D.
Chief of Staff, Shriners Hospital for Children, Chicago, Professor of Orthopaedic Surgery, Rush Medical College

Osteogenesis Imperfecta is a heritable disease characterized by an abnormality of the collagen matrix -- one of the main building blocks of bone. The net result is the formation of structurally weak bone, both appendicular and axial, that is accompanied by a degree of ligamentous laxity and hypotonia. All these factors contribute to the development of the various spinal conditions seen in O.I.

In a number of studies, it has been shown that the incidence and severity of spinal deformity correlates with the severity of the correlate O.I. overall. The most accepted O.I. classification, that of Sillence, is useful in prognosticating severity and in anticipating certain deformities. For example, Type III is associated with elongated pedicles and posterior rib angulation. Also, dentinogenesis imperfecta has been shown to be associated with higher incidence of scoliosis.

The etiology of the various spinal deformities as mentioned is due to a combination of weak bone and ligamentous laxity. Multiple and repeated microfractures of the vertebral bodies lead to vertebral deformity and possible damage to the vertebral growth plates, further deforming and altering the growth of the affected vertebrae and perpetuating the deformity. Newer imaging techniques like DEXA scans confirms the decreased density of O.I. bone and low Z scores have been shown to be associated with more severe scoliosis. While the most common spinal deformity is scoliosis, the pathoetiology described here also explains other deformities such as spondylolysis and spondylolisthesis, as well as a number of craniocervical conditions.

The treatment of spinal deformities in O.I. has traditionally involved observation, bracing and surgery. However, it is becoming clear that preventive and therapeutic treatment of the severe bone weakness by such drugs as pamidronate may really change the natural history of spinal deformities in O.I. Recent reports have shown that the cyclical administration of pamidronate have improved bone density and strength throughout the skeletons of treated patients, but this was most remarkable in the spine. Even with the use of these drugs, the traditional methods of treatment as mentioned are still necessary if deformities develop.

A number of studies have shown that brace treatment for patients with O.I. is not only ineffective, but can be deleterious because the pressure required on ribs and pelvis by the brace to effect correction and control of the curves may only serve to cause deformities of those areas, rather than improving the spinal deformity. In essence, bracing does not affect the natural history of spinal deformities in O.I.

Surgical treatment has consisted of spinal fusion and instrumentation whenever possible. However, in those children who have very severe O.I. the bone is very weak and may not be able to support instrumentation. In these situations, even if instrumentation can be implanted, little correction can be expected because any forces placed through the instrumentation may just fracture the bone rather than effect correction. Those children who have milder forms of O.I. can be treated like their counterparts with idiopathic scoliosis. Posterior fusion with segmental instrumentation, with possible supplementary anterior fusion to either prevent crankshafting or improve correction should be the standard of care for the treatment of spinal deformities in O.I. The supplemental use of methylmethacralate to help with instrumentation fixation is probably not necessary with the use of segmental instrumentation. Those patients with very severe O.I. who develop severe curves, but who either don't sit or sit in a very supported environment, probably are not candidates for surgery.

Similar techniques and strategies can be applied to the treatment of spondylolysis, spondylolisthesis and craniocervical abnormalities.

In the future, treatment algorhythmns may include pre-treatment of weak bone with pamidronate or similar drugs prior to any surgical intervention, to minimize instrumentation problems and to allow better correction of deformities.

Reference: Proceedings of the 7th International Conference on Osteogenesis Imperfecta. Montreal, Canada, 1999.