Clinical Performance of Retrieved Dynamic and Static Knee Antibiotic Cement Spacers +1Jaekel, D J; 2Klein, GR; 1,3Day J, M; 2Levine, HB; 4Parvizi, J; 4Austin M; 1,3Kurtz, S M

+1Drexel University, Philadelphia, PA; 2Hackensack University Medical Center, Hackensack, NJ; 3Exponent, Inc., Philadelphia, PA, 4Rothman Institute, Philadelphia, PA djj25@drexel.edu

Significance: Dynamic antibiotic bone cement spacers have replaced their static counterparts and have shown effectiveness in curing deep infections in past studies; however, this study aims to reveal the clinical performance of these materials as bearing surfaces. Introduction: Periprosthetic infection is a leading reason for revision of total joint replacements [1]. A commonly accepted treatment is a two-stage revision during which either a static or, more recently, a dynamic antibiotic-loaded bone cement (ABC) spacer is temporarily implanted into the joint. In contrast with a static spacer, which does not allow knee motion, a dynamic ABC spacer is fashioned with articulating surfaces that replicate the geometry of a total joint replacement to facilitate patient mobility during recovery. The clinical effectiveness in the treatment of infections has been shown in the literature, however, the mechanical performance in vivo and potential modes of damage remain poorly understood [2]. Additionally, subsurface pores and cavities can develop when mixing bone cement and effect material strength and add points of stress concentration. The purpose of this study is to characterize the clinical performance, surface damage and porous structure of dynamic ABC spacers used in the treatment of knee joint infections. We hypothesize that patients with dynamic spacers will have increased activity levels without catastrophic failure.

Table 1: Average clinical data for dynamic and static spacers Methods and Materials: 22 dynamic and 14 static knee ABC spacers were prospectively collected at revision surgeries in an IRB approved multi-center retrieval program between 2007-2010. Patient data, including UCLA activity levels, were collected. In 20/22 cases, dynamic spacers were created in the OR using StageOne Spacer Knee molds (Biomet, Inc) with either PalacosTM (Zimmer, Inc) or CobaltTM (Biomet, Inc.) bone cement. In 2/22 cases, spacers were premolded (Interspace, Exactech, Inc.) and cemented with CemexTM (Exactech, Inc). Fourteen static spacers that were molded to shape in the OR were used for comparison.

The articulating surfaces of the tibial and femoral components for each spacer were evaluated under up to 40x magnification for 7 modes of surface damage using the Hood method [3]. Each component was evaluated for damage on 8 regions of the articulating surface. Nineteen of the dynamic spacers were then scanned using μCT (Scanco, Inc.) with a resolution of 0.074 mm and evaluated using commercial software, Analyze© (Mayo Clinic) to observe the internal structure of the cement, cracking, and cavity defects. Porosity was assessed from three 5mm3 sections selected directly below the bearing surface. Three spacers were excluded from μCT analysis because of fracturing during the retrieval.

Figure 1: The UCLA activity scores for static and dynamic spacers Results: The average UCLA activity score was significantly higher (p=0.013) for dynamic spacers (5, range: 2-8) compared to static (3, range: 1-10) (Fig1.). There was no significant difference noted in BMI or age of either ABC spacer patient group (p≥0.45) (Table 1).

Wear scoring of the articulating surfaces revealed burnishing on the bearing surface of the retrieved tibial and femoral components (Fig.2). Burnishing was associated with a “wear polishing” of the surface and was the dominant mode of damage. Scratching or abrasion of the surfaces was also observed as well as embedded debris on some of the implant surfaces. There was no evidence of delamination or surface deformation in any of the cases. It was difficult to distinguish between pitting and the porous nature of the bone cement. Pitting or porous areas of the surface were prevalent in the burnished regions of the spacer. μCT analysis revealed that internal structure of the spacers was

porous and highly inhomogeneous. Regions of radiopaque material as well as a zone of irregularly mixed cement and open cavity defects, were detected within 3 mm of the articulating surface, (Fig.3). However, we found no evidence of fracture or subsurface cracking in the retrieved spacers. Porosity analysis using the μCT datasets revealed that at least 10 of the 19 scanned spacers had pores >1mm in diameter ranging from 1-15% of the total pore volume. These were present in both OR and pre-molded spacers. The average porosity was 7.5% (range: 1-29%) with a mean pore diameter of 0.305mm (range: 0.074-0.7027mm).

Figure 2: Scores for prevalent modes of damage on bearing surfaces Discussion: Historically, there have been a limited number of options available for treatment of an infected joint replacement. Static ABC spacers first emerged as a viable option, but to enhance mobility, bone cement was molded into a bearing surface, a function for which bone cement is not typically used. Nevertheless, this study supports the hypothesis that dynamic spacers allow for increased patient activity as compared static counterparts without catastrophic failure. Data from the present study suggests that patients were able to resume moderate activity levels with a dynamic spacer. Surprisingly, despite the antibiotic loading and internal structural inhomogeneity, pitting and burnishing were the prevalent damage modes that could be consistently classified for dynamic spacers with no evidence of fracture or delamination. The porous structure of the spacers also varied highly across the surfaces from 1-29% porosity without influencing the failure of the material. Large pores were also observed in the pre-molded spacers. The porosity variability and size range for pre-molded spacers are not noticeably different from that of the surgeon-molded, and exhibit cavities near the surface and comparable levels of surface damage. This retrieval collection needs to be expanded to include more pre-molded spacers, however, within this study, pre-molded spacers performed similarly to their surgeon molded counter parts.

References: [1] Bozic et al, AAHKS 2008; [2] Van Thiel et al., CORR 2011; [3] Hood et al, JBMR 1983;

Acknowledgement: NIH Grant R01 AR47904.

Figure 3: Large cavity formation in dynamic spacers

Spacer Type

Age (yrs) Ave. (Range)

Body Mass index

Ave. (Range)

Implantation Time (months) Ave. (Range)

Dynamic 69 (56-84) 32 (20-38) 3.8 (0.5-13) Static 68 (44-82) 33 (22-44) 2.9 (0.6-5.1)

Cavities

Poster No. 0915 • ORS 2012 Annual Meeting