We collected 36 knee antibiotic bone cement spacers (22 dynamic, cement-on-cement, and 14 static) at revision surgeries in an institutional review board-approved multicenter retrieval program between 2007 and 2011. Retrieved dynamic and static spacers were implanted for, on average, 3.8 months (range, 0.5–13 months) and 2.9 months (range, 0.6–5.1 months), respectively (Table ). All spacers were revised as part of two-stage revisions for infected knee arthroplasties. The average age of the patients at implantation was 69 years (range, 56–84 years) and 68 years (range, 44–82 years) for dynamic and static retrievals, respectively (Table ). Twelve of 36 patients were male, 21 were female, and three were unknown. The average weight was 91 kg (range, 46–112 kg) for the patients with dynamic spacers and 85 kg (range, 55–123 kg) for patients with static spacers. After explantation, the implants were cleaned in 10% bleach solution, exposed to ultrasonication to remove loosely adhered tissue, and then stored at room temperature with components separate for further analysis. Before spacer explantation, Knee Society scores (KSS) and ROM data were collected for 18 of 22 patients with dynamic cement-on-cement spacers and eight of 13 patients with static spacers. UCLA activity scores also were collected for 20 of 22 patients with dynamic spacers and 10 of 13 patients with static spacers. Patient data on sex, height, weight, and implantation time were collected from the medical records.
Clinical data for patients with dynamic and static spacers
In 20 of 22 cases, dynamic cement-on-cement spacers were created by surgeons in the operating room using StageOneTM Knee Cement Spacer Molds (Biomet, Inc, Warsaw, IN, USA) with either Palacos® (Zimmer, Inc, Warsaw, IN, USA) or CobaltTM (Biomet, Inc) bone cement and a variety of antibiotic loadings from 2.5 to 12.2 wt/wt% (Table ). In two of 22 cases, spacers were manufactured premolded (InterSpace®; Tecres SpA, Verona, Italy) and cemented in place with CemexTM (Tecres SpA). The 14 retrieved static spacers were molded to shape by surgeons with a range of antibiotic loadings from 3.7 to 11.2 wt/wt% and used for comparison with dynamic spacers.
Antibiotics used and loadings in dynamic cement-on-cement spacers
We evaluated the articulating surfaces of the tibial and femoral components for each spacer on a scale of 0 to 3 under up to ×40 magnification for seven distinct modes of surface damage (pitting, embedded debris, scratching, delamination, surface deformation, burnishing, and abrasion) using the method of Hood et al. [10
]. We evaluated eight regions on the articulating surfaces of each component for surface damage. Nineteen of the dynamic spacers then were scanned using micro-CT (Scanco USA, Inc, Wayne, PA, USA) with a resolution of 0.074 mm and evaluated using the commercial software Analyze (Mayo Biomedical Imaging Resource, Rochester, MN, USA) to observe the internal structure of the cement, cracking, and cavity defects. Porosity was assessed from three 5-mm3
sections selected directly below the bearing surface. Three spacers were excluded from micro-CT analysis because of fracturing during retrieval. The majority of the static spacers were not imaged with micro-CT owing to the size restrictions of the equipment. We assessed the normality of the distributions of the UCLA activity scores, KSS, ROM, BMI, age, weight, porosity, and implantation time using the Shapiro-Wilk test of normality. We evaluated differences in weight, BMI, age, implantation time (normally distributed data) using the independent-sample t-test and the differences in UCLA activity score, KSS, ROM, and porosity (nonnormally distributed data) using the Mann-Whitney U test between patients with dynamic and static spacers. Correlations with porosity or UCLA scores and damage scores were assessed using a Spearman’s rho correlation test. All statistical tests were performed using SPSS®
statistics package (Version 19.0.0; IBM Corp, Chicago, IL, USA).