Silicate-substituted Calcium Phosphate as a Bone Void Filler After Kyphoplasty in a Young Patient With Multiple Compression Fractures due to Osteogenesis Imperfecta Variant

Mitchell A. Hardenbrook, M.D., L.C.D.R., M.C., U.S.N.; Sergio R. Lombardo, D.O., L.T., M.C., U.S.N.

Disclosures

Neurosurg Focus. 2006;21(6) 

In This Article

Discussion

There were few good treatment options for this relatively young patient who presented with debilitating back pain due to multiple compression fractures and severe osteoporosis. Posterior lumbar fusion was a poor option for this patient, given his inferior quality of bone and his young age. Kyphoplasty was a viable option for treating the compression fractures, but there is debate about the appropriate bone void filler.

Traditionally, PMMA is used in kyphoplasty to provide immediate structural support from within the VB. The benefits of PMMA are that it hardens quickly after insertion, is relatively inexpensive, and is easy to inject. However, PMMA has many potential drawbacks. There is concern for extravasation of PMMA outside the confines of the VB, with potential injury to nerves and vascular structures.[7,12] The release of methyl methacrylate monomer into the vascular system during injection can result in acute hypotension, and there have been reports of cardiovascular collapse.[14] Also, PMMA cannot be incorporated biologically into the VB and remains, in essence, a foreign substance within the VB. There is also concern that the difference in modulus of elasticity of PMMA compared with native and osteoporotic bone could result in additional compression fractures at adjacent vertebrae.[5] These potential side effects of PMMA are a concern in all patients, but they are particularly salient in this young one.

Although there is a case report describing the use of PMMA augmentation in treating a single-level compression fracture due to osteogenesis imperfecta,[18] that patient was 57 years old. Our patient, on the other hand, was 25 years old and had multiple levels requiring treatment. We believed that a better choice would be a more biological bone void filler with the potential to form new bone and incorporate into the VB.

There are reports of alternative materials considered for use in patients undergoing kyphoplasty or vertebroplasty, including calcium phosphate and calcium sulfate.[13] Although calcium phosphate can provide structural support and osteoconduction, there are concerns about its ability to produce healing in metabolically diseased bone. Combining bone morphogenetic protein–2 with the calcium phosphate[22] could improve its osteoinductive properties, but this substance is very expensive. Calcium sulfate can provide an osteoconductive scaffold, but it tends to be rapidly resorbed, limiting its usefulness in vertebral augmentation.[25]

Silicate-substituted calcium phosphate has unique characteristics that make it the most appropriate selection. Silicate-substituted calcium phosphate is a phase-pure 80% porous calcium phosphate material in which the phosphate groups are selectively replaced with silicate ions. Whereas the calcium phosphate scaffolding has the structural support and osteoconduction of traditional calcium phosphate bone filler, the addition of silicate ions in creases its osteoinductive properties. Studies have shown that silicon, in vivo, enhances osteoblastic differentiation and stimulates the formation of bone.[15,17] Moreover, in some reports it has been suggested that silicon is a significant dietary trace element for bone formation and remodeling.[2,10,20,21]

These osteoinductive properties combined with the os teoconductive microstructure of the calcium phosphate lattice provide a rich environment for rapid bone incorporation. This was radiographically evident in our patient on the postoperative x-ray films and CT scan obtained 9 months postoperatively, which showed incorporation of bone within the VB. Additionally, the granules of silicate-substituted calcium phosphate provided structural support of the compressed vertebrae. This was clinically evident in the improvement of the patient's back pain at the initial postoperative visit at 4 weeks.

The use of silicate-substituted calcium phosphate after kyphoplasty allowed this patient with compression fractures to gain enough structural support to provide early relief of back pain while providing a substrate for incorporation and healing within the bone of the VB. Through this approach, we were able to treat multiple compression fractures successfully in a young adult with severe osteoporosis and avoid using PMMA, with all of its potential short- and long-term complications.

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