The first bone cement used for PV was the PMMA Simplex P (Stryker-Howmedica-Osteonics; Rutherford, NJ).13 This is the only cement approved by the U.S. Food and Drug Administration (FDA) for use in the treatment of pathological fractures in the spine. It is not specifically approved for PV. Multiple other PMMA cements have been used for PV and seem to have similar clinical results.16,17,33 It is important to note that bone cement is not treated as a pharmaceutical by the FDA but rather as a device. Alterations in the composition are therefore equivalent to making a new (nonapproved) material. It has been suggested by other authors that such alterations constitute "off-label" use.39 Off-label use would be correctly applied if an unaltered cement were used in a nonindicated application or location. Alteration in the ratio of monomer to copolymer (liquid to powder) or addition of other materials (opacification agents or antibiotics) results in the creation of a new material, and FDA approval no longer exists. Patients should be informed that such alterations in the cement are to be used and the reasons and consequences behind these changes should be discussed.
Inherent in performing PV safely is the need to accurately monitor the injection of cement in real time.33 This is usually accomplished with fluoroscopy and requires that the cement be opacified so that it may be adequately seen in small quantities during introduction. It has been determined that barium sulfate, in quantities of 30% by weight mixed with the PMMA, will provide an appropriate level of opacifica-tion.33,40,41 Simplex P as supplied contains only 10% by weight of barium sulfate, therefore additional barium sulfate needs to be added to obtain an adequate mix for visualization. In vitro, biomechanical evaluations have been performed that demonstrate that this change alters the handling and mechanical properties of the cement minimally. However, a more significant mechanical alteration occurs with changes in the ratio of liquid to powder.40-42 I use two methods to slow the polymerization of Simplex P. First, 20 mL of powder is removed from a full dose package (40 g) of powder, discarded, and replaced by 12 g of barium sulfate to bring the barium load above 30% by weight. During mixing, all the monomer (20 mL) is added, having been chilled near 0°C for 24 hours or more. The second technique used to retard polymerization involves chilling the cement once mixed. Immediately after mixing, all syringes to be used for injection are placed in a bath of chilled, sterile normal saline (Figure 14.7). The change in monomer-to-powder
ratio and chilling of the mixed cement will give a working time for Simplex P of over 15 minutes. Clinical studies using the modified cements have reported uniformly positive results.16,7,33 Indeed, the safety of the procedure seems mainly to depend on preventing cement leaks, rather than on which cement is used.33 No untoward results related to cement alterations have been reported clinically.
Some investigators add antibiotics routinely to PMMA prior to injection, the most common antibiotic being tobramycin.16,33 However, the infection rate with PV is very low, and the efficacy of adding antibiotics to the cement has not been scientifically substantiated in normal, uninfected patients. One report in the orthopedic literature did show reduced infection rates in hip replacement in which cement containing antibiotics was used for immunosuppressed patients.43 For these reasons, I do not recommend the addition of antibiotics to cements except in the situation of immunocompromise.
Adequate precaution should be used during cement mixing to maintain sterility. Cement manufacturers provide closed, vacuum mixing materials that aid in maintaining a sterile environment. Open mixing, which increases the risk of cement contamination and reduces the cement strength by the inclusion of air bubbles and may produce inho-mogeneous mixing of opacifiers with the cement, should be avoided whenever possible.
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