Table 3.3 Pros and cons of implantable antimicrobial elution products.
| Product | Pros | Cons |
|---|---|---|
| PMMA beads | Readily available (especially if used for other orthopedic procedures in your facility) | Nonbiodegradable Must be removed Surgical implantation |
| Calcium‐based beads | Biodegradable – no removal required | Surgical implantation |
| Polymer gel | Can combine with dispersin B to break down biofilm Minimally invasive application – injectable Biodegradable – no removal required | Cannot flush surgical site to dilute microbial burden Location of application less precise |
| Collagen sponge | Biodegradable – no removal required | Surgical implantation Incites inflammation Causes lameness with IA application Causes renal impairment |
IA, intraarticular; PMMA, polymethylmethacrylate.
Bone cement consisting of a calcium base or polymethylmethacrylate (PMMA) may be formed into beads containing various antimicrobials and may be placed locally at the surgical site (Figure 3.5). The antimicrobial powder or liquid solution is added to the PMMA powder before the addition of the methylmethacrylate liquid. It is important that the antimicrobial of choice will remain stable during the exothermic reaction that takes place when the PMMA powder and methylmethacrylate liquid are combined [26]. Once combined, beads can be formed around a small gauge cerclage wire, to create a string of beads. In vitro evaluation identified that gentamicin‐susceptible methicillin‐resistant S. pseudintermedius (MRSP) was effectively treated with gentamicin‐impregnated beads, whereas gentamicin‐resistant MRSP was not effectively treated and silver‐impregnated beads had no effect on MRSP biofilms [11]. PMMA beads, however, are not biodegradable and require removal at a later date, whereas calcium‐based beads can be degraded locally, thus not requiring removal [27]. Therefore, when considering antimicrobial‐impregnated beads, one must determine the susceptibility of the microorganisms to effectively choose an antimicrobial and take into consideration the potential requirement for removal of beads if PMMA is chosen.
Figure 3.5 In this craniocaudal view of the antebrachium, two discontinuous strands of antimicrobial‐impregnated PMMA beads can be seen on the lateral and medial aspects of the bone.
Antimicrobial‐impregnated dextran polymer gel has been proven in vitro to elute high concentrations of antimicrobials that maintain their bioactivity after elution [28]. Gels can be injected locally, without requiring a surgical approach, and do not require removal as they are naturally resorbed [28]. A single case series of an amikacin and clindamycin‐impregnated dextran polymer gel used in combination with explantation of TPLO plates resulted in excellent clinical outcomes after 12 weeks [29]. Despite limited data existing at this time, this may be a promising option for the future and application without implant removal should be further evaluated.
Commercially available gentamicin‐impregnated collagen sponges have been placed intraarticularly in a study population of dogs with sterilely induced synovitis. A rapid elution rate was identified, resulting in a rapid rise of intraarticular gentamicin concentrations, but a rapid decline was also observed. This rapid decline may not be clinically relevant, as gentamicin is a concentration‐dependent antimicrobial and thus the high concentrations achieved may be sufficient to result in bactericidal activity.
The gentamicin‐impregnated sponge was found to incite joint inflammation, lameness, and renal impairment and was thus not recommended for clinical use [30]. However, one case report exists of its use in a clinical incidence of septic arthritis following an extracapsular stifle stabilization procedure. This case reports clinical resolution of the septic arthritis but systemic gentamicin was administered simultaneously, making it hard to determine the role of the intraarticular gentamicin [31]. A separate case series combined implant removal with an amikacin‐impregnated collagen sponge being placed at the site of implant removal for treatment of TPLO SSI, which resulted in a 96.8% long‐term resolution rate [32]. The benefit of an antimicrobial collagen sponge is its absorbable nature, thus precluding the requirement for retrieval following SSI resolution.
While all of these local antimicrobial‐eluting options provide increased levels of local therapy, differences in biodegradability may play a role in clinical decision making. If these local therapies are being employed in the face of an incompletely healed osteotomy or incomplete stabilization of an extracapsular repair, implant removal cannot be performed concurrently. Two sides of the coin can therefore be considered following complete healing of the osteotomy or adequate stifle stabilization. On the one hand, a biodegradable option does not require further surgical extraction and if the SSI is clinically resolved, the implant may not require removal. However, on the other hand, a nonbiodegradable option must be surgically removed and allows the opportunity for concurrent implant removal. With the major concern of biofilm formation on implanted materials, locally applied antimicrobial therapy may not be sufficient to eradicate the SSI and recurrence may result in further treatment and possible implant removal at a later date.
In the case of implant removal, the major microorganism burden is removed with the implant, assuming that most of these SSIs are biofilm related and as such systemic or local therapies may not be required (Figure 3.6). Of the various studies reporting treatment of SSIs with implant removal, few indicate whether postoperative systemic antimicrobial therapy was continued empirically and/or altered pending culture and susceptibility testing results. While all these studies encourage culture and susceptibility testing of implants that are removed, how this information is applied clinically is unknown. As such, evidence‐based recommendations for postoperative systemic antimicrobial use following implant removal for management of an SSI remain unclear. Logically, if the major burden of disease has been removed, monitoring for clinical recurrence of SSI signs is recommended. Thus, patients without active evidence of an SSI at the time of implant removal do not require systemic antimicrobial therapy and results of culture and susceptibility testing should be employed only in the face of recurrent signs of SSI.
Though few studies exist, when local antimicrobial therapy is combined with implant removal, such as implantation of an amikacin‐impregnated collagen sponge or amikacin and clindamycin polymer gel, SSI resolution rates ranged from 95% to 96.8%, without the concurrent use of systemic antimicrobials [29, 32].
Figure 3.6 (a) Exposed TPLO plate. (b) Screw removed. When removing screws to submit for culture, use a pair of forceps that have not touched the skin to stabilize the screw while it is being removed. Do not allow the screw to contact the skin to avoid growth of skin contaminants on your bacterial culture. (c)