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Antibiotic impregnated PMMA beads: Use and misuse (Proceedings)

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Nov 01, 2009

In order for AI-PMMA beads to produce a positive outcome, many factors must be taken into consideration. They are not for systemic infections, antibiotic selection should be based on culture results, heat will denature some antibiotics, antibiotics will only penetrate a few mm of surrounding tissue, only antibiotics and carriers whose elution properties have been studied should be used to avoid ineffective concentrations and to minimize the risk of toxic effects to the patient, more than one antibiotic should not be mixed in the same cement, and the smallest volume of antibiotic should be used.

Because of the high recurrence rate of abscesses in rabbits, it appears that excising the abscess en bloc as one would remove a tumor is more likely to result in a cure. Reasons cited for recurrence include that the abscess wall harbors bacteria isolating them from topical treatments, the thick caseous pus does not drain adequately, and the skin wound seals before second intention healing can force out the infection. Most abscesses of the head involve bone or teeth making it difficult to remove them en bloc without rupturing them. Still, it is best to treat rabbit abscesses as tumors that are locally invasive but do not metastasize making excision with minimal margins an appropriate surgical approach. Dissect the abscess down to bone, quickly remove the abscess at the level of the bone, curette the bone and remove any teeth that are involved in the abscess, and irrigate copiously to decrease the contamination from the ruptured abscess. It may not be possible to completely excise abscesses associated with osteomyelitis or tooth abscesses; however, it is vital to remove all abnormal tissues - soft and hard. Back to the tumor analogy, at this point, treat the abscess as if you have removed gross but not microscopic disease. The next part of therapy is to treat the residual microscopic disease.

"Marsupialization" is a term used by some authors when describing their recommendation for managing residual disease in abscesses in rabbits. Utilizing this technique, the abscess is excised as described above or opened and curetted out. The skin is then sutured to the abscess wall or whatever tissue is left surrounding the excision bed circumferentially. The idea behind this technique is to delay wound healing allowing prolonged management of the open wound hoping that the infection will then be eliminated. This requires a lot of open wound management which is difficult for the rabbit and the owner. Some authors, however, claim a high success rate with this technique and it might be a method to consider if the abscess is in an anatomically difficult area where the surgeon is not comfortable trying to excise the entire capsule.

Instead of treating the wound open, an alternative is to implant antibiotic impregnated polymethylmethacrylate (AIPMMA) beads which release relatively high concentration of antibiotic locally with little systemic absorption. This allows primary closure of the site and eliminates the need for open wound care. The rabbit is still placed on systemic antibiotics for 2 weeks but long term therapy is not necessary as the beads release antibiotic for many months or even years.

The antibiotic chosen is best based on culture and sensitivity results and/or Gram's stain prior to surgery. Tyrrell showed that all bacteria isolated from rabbit jaw abscesses in their study were susceptible to clindamycin; 96% were susceptible to penicillin, ceftriaxone, and cefazolin; 86% were susceptible to azithromycin and tetracycline; only 54% were susceptible to metronidazole and ciprofloxacin (the active metabolite of enrofloxacin); and only 7% were susceptible to trimethoprim/sulfa. No aminoglycosides were tested; however, they are typically ineffective against anaerobic bacteria and Gram positive bacteria which were the main isolates from the abscesses. Based on these results clindamycin, penicillin, ceftriaxone, and cefazolin would be good antibiotic choices; however, these antibiotics have potentially fatal consequences when used parenterally in rabbits. Note that enrofloxacin and trimethoprim/sulfa would not be expected to be of much benefit.

Because the fumes are annoying and potentially damaging to contact lenses and feti, many prefer to make the beads in a hood prior to surgery. The cement comes in 20 and 40 g packets (Surgical Simplex; Howmedica, Rutherford, NJ or Bone Cement; Zimmer, Patient Care Division, Charlotte, NC) which is enough to treat several rabbit abscess. The antibiotic is mixed with the copolymer powder prior to adding the liquid monomer. Once the polymerization begins, the cement hardens within 10 minutes. Refrigerating the reagents prior to use will extend this time. It can be challenging to make all the beads that quickly and the aid of some assistants is very helpful. Beads may be rolled into spheres and strung on a fine gauge wire. Unfortunately, the size of bead that can physically be made with the fingers is often too large for use in rabbit abscesses. As an alternative, the mixture is placed in a syringe (catheter tip for larger beads and regular tip for small beads) and squirted out onto a plastic or metal surface (such as a table drape intraoperative). A scalpel is then used to cut the tube of cement into small pieces. In this manner, a group of small cylinders will be created to function as beads. These beads will be too small to string on a wire. When placing the beads, count the number implanted and record it in the patient record. A bead maker is commercially available from Orthopaedic Laboratory Biomechanics in Minneapolis at 612-336-6600 or FAX 612-336-6619. Unfortunately, the beads made with this device are generally too large for use in rabbit dental abscesses.

If removal of the beads is planned, it is ideal to place the beads on an orthopedic wire or nonabsorbable suture when making them, forming a string of beads. In this manner when the beads are to be removed the surgeon need only locate one bead and the remainder can be removed by pulling the string. Unfortunately the beads are often too small to string making bead removal more challenging.