Cement is prepared only after all needles are placed, as described earlier ("Cement Selection and Preparation"). Cement with an appropriate opacification is prepared and injected using small syringes (typically 1 mL) or devices made specifically for injection (Figure 14.11). This allows easy control of the cement introduction. Either the cement
Figure 14.11. Cement injection with a 1 mL syringe. Note bipedicular needle placement prior to beginning cement injection.
injection should be monitored in real time or small quantities (i.e., 0.1-0.2 mL) injected and the result visualized before additional cement is introduced. The latter approach, which allows one to step back from the fluoroscopy beam during visualization, minimizes radiographic exposure to the operator.
Any cement leak outside the vertebral body is an indication to stop the injection. When using a rapidly polymerizing cement (e.g., Simplex), this may be necessary only for a minute or two while the injected cement hardens. Restarting the injection may then redirect flow into other areas of the vertebra. If leakage is still seen, it is advisable to terminate the cement injection through this needle and move to the second needle. This will usually allow completion of the vertebral fill without further leakage, since the original leak now will be occluded by the initial cement, which will have hardened. One should work through a single needle at a time. This avoids contamination of both needles at once and preserves a route for subsequent injection if a leak is encountered. Injection of thick cement should be safer than a very liquid consistency. Cement can still be introduced beyond the point at which the injection devices are able to deliver it. The trocar is useful to push additional thick cement from the cannula into the vertebra. The 5 in., 13-gauge cannula holds 0.5 mL, and the 5 in., 11-gauge cannula holds 0.9 mL. Reintroducing the trocar will push the additional cement into the vertebra. This is done only if the additional amount of cement is desired. The cannula can be removed safely without reintroduction of the trocar when the cement has hardened beyond the point at which it can be injected. Simply twisting the needle through several revolutions will break the cement at the tip of the cannula and will prevent leaving a trail of cement in the soft tissues. However, removing the cannula before the cement has hardened sufficiently can allow cement to track backward from the bone into the soft tissues and may create local pain.
The amount of cement needed to produce pain relief has not been accurately documented in available clinical reports. We believe that pain relief is related to fracture stabilization, and thus the amount of cement needed to restore the initial vertebral body's mechanical integrity should also give an approximation of the quantity needed to relieve pain clinically. In an in vitro study, we showed that the initial prefracture strength and stiffness of a vertebra could be restored by injecting 2.5 to 4 mL of Simplex P in the thoracic vertebra, while 6 to 8 mL provided similar augmentation in the lumbar region.49 A reasonable guideline for the quantity of cement to be injected is the amount that is needed to fill 50 to 70% of the residual volume of the compressed vertebra. These amounts should not be taken as absolute but rather as a guide. The above described study suggests that relatively small amounts of cement are needed to restore initial biomechanical strength and that these amounts vary with the position in the spine, as well as individual vertebral body size and the degree of vertebral collapse.
We have also demonstrated that significant strength restoration is provided to the vertebral body with a unipedicular injection, where cement filling crosses the midline of the vertebral body.50 This would imply that unipedicular fills that achieve adequate cement injection volumes are likely to be successful at achieving pain relief. This fact notwithstanding, there is a higher likelihood of achieving more uniform fills, with fewer leaks, when two needles are used rather than one (Figure 14.12).
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