Vertebroplasty: Basic Science

Basic Science

Although vertebroplasty has been used with good clinical effect, the underlying basic science is poorly understood. One question concerns the mechanism for pain relief. The pain in vertebral neoplasms is thought to be due to irritation of nerve endings in the bone and compression of neural tissue by mass effect. In addition, microfracture from weakening can cause pain through stretch of these same nerve endings in Osteoporotic vertebrae or lytic tumoral lesion. The pain relief from injected PMMA has been attributed to 1) thermal necrosis of nerve endings by the exothermic cement-curing reaction, 2) toxic effect of the cement monomers on tumor cells, arresting secretion of pain-mediating substances, and 3) stabilization of microfractures and vertebral collapse. The exact role of each mechanism is unclear,¹¹ but the onset of pain relief observed clinically is rapid, usually in 2-3 days.

The volume of injected cement does not predict clinical effect. This early observation by Cotton et al.⁹ based on post-procedure CT scans led to biomechanical studies of the relationship between volume of cement injected and strength. Dean et al.¹² have shown in human adult cadaveric lumbar vertebrae that the volume of cement injected does not correlate with strength as tested on a standard materials testing machine. Tohmet et al.¹³ showed in cadaveric Osteoporotic vertebrae that unipedicular PMMA injection led to significantly increased vertebral strength over vertebra receiving no injection, and restored stiffness as effectively as bipedicular injection. This suggests that the pattern of cement distribution within the vertebral body matters just as much if not more than the total volume.^{12, 13} So far cement distribution within the vertebra has not been studied carefully, and no prospective, randomized clinical study has compared universus bi-pedicular injection.

The materials used in vertebroplasty have also been studied. Disadvantages of using PMMA center around the potential catastrophic effects of thermal damage to the dural sac, cord, and nerve roots if leakage were to occur. Surgical evacuation of leaked cement in the spinal canal has been reported.^{8, 10} The inability of PMMA to undergo remodeling, the inability to use it to deliver growth factors or chemotherapeutic agents, and the need to add radio-opaque agents and to lower viscosity (with unclear consequences on its long-term endurance)¹⁴ are all additional concerns about this material in vertebroplasty.

Alternative injectable materials are being developed for use in vertebroplasty. Osteoconductive coral granules have been studied in human cadaveric vertebrae as well as in vivo in a sheep model.¹⁵ Use of these granules led to increased bone formation and higher numbers of osteoblasts compared with control conditions. Such a material offers promise because it can act as a carrier for growth factors or cytokines. Dhert et al.¹⁶ studied a bioresorbable calcium phosphate cement (Bonesource, Stryker-Howmedica) in a burst fracture model using human non-osteoporotic cadaveric vertebrae. They used an inflatable balloon (Inflatable Bone Tamp, Kyphon, Inc.) in a transpedicular route before cement injection to compress trabecular bone within the vertebral body and to create a cavity for the cement. The clear advantages of this technique is injection of cement under less pressure into a pre-formed cavity, decreasing the change of leakage into the canal or neural foramina. Further work is needed to assess the biomechanical properties of bioresorbable cements in augmented vertebrae and in vivo performance. Finally, Belkoff et al.¹⁷ studied a glass-ceramic bioactive composite material in a cadaver study. Orthocomp (Orthovita) restored more strength and restored original stiffness, significantly better results compared to Simplex P (Howmedica) and untreated levels. Advantages of this compound include greater natural radio-opacity, and a lower temperature of setting than PMMA. Ongoing research is focused on developing optimal materials and on perfecting both delivery techniques and intraprocedure monitoring. Understanding the mechanism of action of vertebroplasty will allow us to refine the technique and better define the intra-procedure goals. These advances will improve clinical practice by reducing the time needed for the procedure, improving safety and reducing complications.