IntroductionProsthetic joint infection (PJI) occurs in 1-2% of primary total knee arthroplasty (TKA), and the effect of PJI has a tremendous impact on patients’ quality of life and the financial burden on the health care system [[1]Comparison of traditional molded, first-generation premolded, and second-generation premolded antibiotic-loaded polymethylmethacrylate articulating spacers for treatment of chronic prosthetic joint infection of the knee.]. Two-stage revision with antibiotic cement spacers is the gold standard treatment for chronic PJI because of high infection eradication rates of up to 80-90% [[2]Johnson A.J. Sayeed S.A. Naziri Q. Khanuja H.S. Mont M.A. Minimizing dynamic knee spacer complications in infected revision arthroplasty.]. Both static and articulating spacers have been similarly successful in terms of infection control [3Fehring T.K. Odum S. Calton T.F. Mason J.B. Articulating versus static spacers in revision total knee arthroplasty for sepsis., 4Freeman M.G. Fehring T.K. Odum S.M. Fehring K. Griffin W.L. Mason J.B. Functional advantage of articulating versus static spacers in 2-stage revision for total knee arthroplasty infection., 5Hsu Y.C. Cheng H.C. Ng T.P. Chiu K.Y. Antibiotic-loaded cement articulating spacer for 2-stage reimplantation in infected total knee arthroplasty: a simple and economic method., 6Jämsen E. Sheng P. Halonen P. Lehto M.U. Moilanen T. Pajamaki J. et al.Spacer prostheses in two-stage revision of infected knee arthroplasty., 7Ding H. Yao J. Chang W. Liu F. Comparison of the efficacy of static versus articular spacers in two-stage revision surgery for the treatment of infection following total knee arthroplasty: a meta-analysis., 8Guild III, G.N. Wu B. Scuderi G.R. Articulating vs. Static antibiotic impregnated spacers in revision total knee arthroplasty for sepsis.]; however, the articulating spacer shows promising results with regard to increased range of motion (ROM) and improved knee scores [[9]Nahhas C. Chalmers P. Parvizi J. Sporer S. Berend K. Moric M. et al.A randomized trial of static and articulating spacers for the treatment of infection following total knee arthroplasty.].The fabrication techniques for articulating spacers can be classified as prefabricated, handmade, molded, and sterilized [[10]Warth L.C. Hadley C.J. Grossman E.L. Two-stage treatment for total knee arthroplasty infection utilizing an articulating prefabricated antibiotic spacer.]. The constraint characteristics of articulating spacers can be broadly classified as either cruciate-retaining (CR) or posterior-stabilized (PS) types [[6]Jämsen E. Sheng P. Halonen P. Lehto M.U. Moilanen T. Pajamaki J. et al.Spacer prostheses in two-stage revision of infected knee arthroplasty., 11Shen H. Zhang X. Jiang Y. Wang Q. Chen Y. Wang Q. et al.Intraoperatively-made cement-on-cement antibiotic-loaded articulating spacer for infected total knee arthroplasty., 12Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection., 13Van Thiel G.S. Berend K.R. Klein G.R. Gordon A.C. Lombardi A.V. Della Valle C.J. Intraoperative molds to create an articulating spacer for the infected knee arthroplasty., 14Struelens B. Claes S. Bellemans J. Spacer-related problems in two-stage revision knee arthroplasty., 15Villanueva-Martinez M. Rios-Luna A. Pereiro J. Fahandez-Saddi H. Villamor A. Hand-made articulating spacers in two-stage revision for infected total knee arthroplasty: good outcome in 30 patients., 16Su Y.P. Lee O.K. Chen W.M. Chen T.H. A facile technique to make articulating spacers for infected total knee arthroplasty., 17Ocguder A. Firat A. Tecimel O. Solak S. Bozkurt M. Two-stage total infected knee arthroplasty treatment with articulating cement spacer., 18Hofmann A.A. Goldberg T. Tanner A.M. Kurtin S.M. Treatment of infected total knee arthroplasty using an articulating spacer: 2- to 12-year experience.]. Most spacers are CR without a post-cam design [[6]Jämsen E. Sheng P. Halonen P. Lehto M.U. Moilanen T. Pajamaki J. et al.Spacer prostheses in two-stage revision of infected knee arthroplasty., 13Van Thiel G.S. Berend K.R. Klein G.R. Gordon A.C. Lombardi A.V. Della Valle C.J. Intraoperative molds to create an articulating spacer for the infected knee arthroplasty., 14Struelens B. Claes S. Bellemans J. Spacer-related problems in two-stage revision knee arthroplasty., 15Villanueva-Martinez M. Rios-Luna A. Pereiro J. Fahandez-Saddi H. Villamor A. Hand-made articulating spacers in two-stage revision for infected total knee arthroplasty: good outcome in 30 patients.]. Some PS spacers only have a post design [[16]Su Y.P. Lee O.K. Chen W.M. Chen T.H. A facile technique to make articulating spacers for infected total knee arthroplasty., [17]Ocguder A. Firat A. Tecimel O. Solak S. Bozkurt M. Two-stage total infected knee arthroplasty treatment with articulating cement spacer.], although a post-cam design has been used in metal-on-polyethylene [[18]Hofmann A.A. Goldberg T. Tanner A.M. Kurtin S.M. Treatment of infected total knee arthroplasty using an articulating spacer: 2- to 12-year experience., [19]Haddad F.S. Masri B.A. Campbell D. McGraw R.W. Beauchamp C.P. Duncan C.P. The PROSTALAC functional spacer in two-stage revision for infected knee replacements.] and cement-on-cement spacers [[11]Shen H. Zhang X. Jiang Y. Wang Q. Chen Y. Wang Q. et al.Intraoperatively-made cement-on-cement antibiotic-loaded articulating spacer for infected total knee arthroplasty., [12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection.]. Despite the success of articulating spacers, the optimal construct has not yet been identified [[20]Spivey J.C. Guild III, G.N. Scuderi G.R. Use of articulating spacer technique in revision total knee arthroplasty complicated by sepsis: a systematic meta-analysis.]. Moreover, heterogeneous spacer mechanical complications, such as spacer loosening or fracture, joint dislocation, and extensor apparatus problems [12Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection., 13Van Thiel G.S. Berend K.R. Klein G.R. Gordon A.C. Lombardi A.V. Della Valle C.J. Intraoperative molds to create an articulating spacer for the infected knee arthroplasty., 14Struelens B. Claes S. Bellemans J. Spacer-related problems in two-stage revision knee arthroplasty.], have been reported.Theoretically, poor mechanical circumstances of the joint, such as bony loss, cruciate or collateral ligament deficiency, and defects following infection and resection arthroplasty, together with fewer constraints on the articulating spacer would cause the knee joint to become inherently unstable and lead to more spacer mechanical complications [[11]Shen H. Zhang X. Jiang Y. Wang Q. Chen Y. Wang Q. et al.Intraoperatively-made cement-on-cement antibiotic-loaded articulating spacer for infected total knee arthroplasty., [12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection., [15]Villanueva-Martinez M. Rios-Luna A. Pereiro J. Fahandez-Saddi H. Villamor A. Hand-made articulating spacers in two-stage revision for infected total knee arthroplasty: good outcome in 30 patients.]. Struelens et al. [[14]Struelens B. Claes S. Bellemans J. Spacer-related problems in two-stage revision knee arthroplasty.] and Jämsen et al. [[6]Jämsen E. Sheng P. Halonen P. Lehto M.U. Moilanen T. Pajamaki J. et al.Spacer prostheses in two-stage revision of infected knee arthroplasty.] reported high spacer mechanical complications ranging from 50.0% to 57.0% with CR spacers, whereas Shen et al. [[11]Shen H. Zhang X. Jiang Y. Wang Q. Chen Y. Wang Q. et al.Intraoperatively-made cement-on-cement antibiotic-loaded articulating spacer for infected total knee arthroplasty.], Hsu et al. [[5]Hsu Y.C. Cheng H.C. Ng T.P. Chiu K.Y. Antibiotic-loaded cement articulating spacer for 2-stage reimplantation in infected total knee arthroplasty: a simple and economic method.], and Tsai et al. [[12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection.] reported low spacer mechanical complications ranging from 4.8% to 6.3% with PS spacers. Because of the broad range of spacer styles and manufacturing techniques, the literature is heterogeneous and conclusions regarding the most efficacious design are lacking. Moreover, these complications lead to further surgery, poor clinical outcomes, prolonged treatment courses, and difficulty with reimplantation [[21]Lau A.C. Howard J.L. Macdonald S.J. Teeter M.G. Lanting B.A. The effect of subluxation of articulating antibiotic spacers on bone defects and degree of constraint in revision knee arthroplasty.]. In this regard, a study comparing the results of CR and PS spacers is critical for two-stage revision. To the best of our knowledge, no prior studies have directly compared constraint methods in articulating spacers.

The aim of this study was to compare the rate of infection eradication after two-stage revision with CR and PS spacers, spacer mechanical complications and reoperation, functional outcomes, and ROM during the interim stage. We hypothesized less spacer mechanical complications and reoperation in PS spacers but no differences in functional outcomes, ROM during the interim stage, or rate of infection eradication between CR and PS spacers.

ResultsTwo hundred and ninety-six patients who underwent two-stage revision for PJI were included in the study. A total of 155 patients were excluded: 75 had static spacers (48.4%), 48 incomplete data (30.9%), 22 fungal or tuberculous PJIs (14.2%), and 10 PJIs after revision TKA (6.5%). The baseline determinants and outcomes between the included cohort and excluded cohort with incomplete data are presented in Appendix A (Table A.1 and A.2). One hundred and forty-one patients were included at an overall mean follow-up of 57.1 months, and 56.5 % were female with a mean age of 66.4 years (95% CI, 58–85) and mean body mass index (BMI) of 25.8 kg/m2 (95% CI, 21.8–29.3). All included patients contributed one knee, with CR and PS spacers accounting for 46.8% (66/141) and 53.2% (75/141), respectively. The STROBE flowchart detailing the study design is shown in Figure 3. There were no differences between individuals receiving either CR or PS spacers in terms of age, sex, BMI, laterality, constraint of the original prosthesis, smoking, insurance [[25]The impact of insurance status on the survival outcomes of patients with renal cell carcinoma.] and socioeconomic [[26]Yan H. Liu B. Meng G. Shang B. Jie Q. Wei Y. et al.The influence of individual socioeconomic status on the clinical outcomes in ischemic stroke patients with different neighborhood status in Shanghai, China.] statuses, the Charlson comorbidity index [[27]Charlson M.E. Pompei P. Ales K.L. MacKenzie C.R. A new method of classifying prognostic comorbidity in longitudinal studies: development and validation.], McPherson host grade [[28]McPherson E.J. Woodson C. Holtom P. Roidis N. Shufelt C. Patzakis M. Periprosthetic total hip infection: outcomes using a staging system.], preoperative HSS knee score, organisms, interim period, and follow-up period (Table 1).

Figure 3STROBE flowchart detailing the design of the study.

Table 1Demographic data of patients with cruciate-retaining and posterior-stabilized spacers

Intraoperative intact PCLs, PCL insufficiency, and PCL deficiency accounted for 0% (0/27), 70.4% (19/27), and 29.6% (8/27), respectively, during explantation of original CR TKA in CR spacers, and for 0% (0/24), 62.5% (15/24), and 37.5% (9/24), respectively, in PS spacers.

The outcomes of the CR and PS spacers are documented in Table 2. The overall rate of spacer mechanical complications was lower in PS spacers (9.3%) than in CR spacers (45.5%) (PTable A.3 and A.4). One CR spacer loosening with dislodge underwent revision of the femoral spacer with splint protection (Figure 4A). In all four fractured spacers, non-weight bearing was recommended. Two fractured CR spacers and one fractured PS spacer underwent uneventful reimplantation; however, the other fractured PS spacer reinfected simultaneously, received spacer exchange, and finally retained the spacer in situ because the patient was medically unfit for further surgery. Both periprosthetic fractures were cast with immobilization and were easily reimplanted (Figure 5A).

Table 2Outcomes of patients with cruciate-retaining and posterior-stabilized spacers

CI, confidence interval; HSS, Hospital of Special Surgery; ROM, range of motion.

Figure 4A. Femoral spacer loosening with dislodge, 1.5 months after cruciate-retaining spacer insertion, which underwent femoral spacer exchange. B. Dislocated knee joint two months after cruciate-retaining spacer insertion, which underwent close reduction. C. Dislocated knee joint with tibial spacer loosening one month after cruciate-retaining spacer insertion, which underwent open reduction and thicker tibial spacer exchange.

Figure 5A. Tibial plateau periprosthetic fracture with tibial spacer loosening three months after posterior-stabilized spacer insertion. B. Femoral cam fracture with joint dislocation 2.5 months after posterior-stabilized spacer insertion, which underwent femoral spacer exchange. C. Patellar maltracking one month after posterior-stabilized spacer insertion.

Among the 20 dislocated CR spacers (Figures 4B and 4C), original CR and PS TKA accounted for 40.0% (8/20) and 60.0% (12/20), respectively (P=0.217). Original PS TKA contributed to the one dislocated PS spacer. Five dislocated CR spacers underwent closed reduction at the outpatient clinic or emergency department with a fixed-angle hinged brace. Of the remaining 15 spacers, 12 underwent open reduction with splint protection and three underwent thicker tibial spacer exchange with a fixed-angle hinged brace. One dislocation of a PS spacer was due to femoral cam fracture after the patient fell (Figure 5B), and subsequently underwent PS femoral spacer exchange. There were no joint dislocation events after intervention of these 21 dislocated spacers. There were six spacers with extensor apparatus problems, and all were patellar maltracking (Figure 5C).

The overall rate of reoperation was lower in PS (16.0%) than in CR (36.4%) spacers (P<0.001). The reoperation rate for spacer mechanical complications was lower in PS (1.3%) than in CR (24.2%) spacers (P<0.001). However, reoperation rate due to reinfection during the interim stage did not differ between the spacers. Two CR spacers and one PS spacer patient underwent above-knee amputation because of persistent sepsis with poor soft tissue status of the knee during hospitalization.

The HSS knee score was higher in the PS (72.3) than in the CR (63.8) spacers (P<0.001). ROM was greater in PS (90.3°) than in CR (80.6°) spacers (P=0.005). Moreover, the maximum flexion angle was greater in the PS (102.4°) than in the CR (89.6°) spacers (P=0.003). However, there was no difference in the maximum extension angle between spacers.

Five CR spacers and 10 PS spacers underwent spacer retention in situ because the patients were medically unfit for reimplantation surgery or felt improved function of the spacers compared with the previous prostheses. There was no recurrent infection in patients who underwent spacer retention at the final follow-up. Forty-five CR spacers (76.3%) and 62 PS spacers (96.9%) underwent reimplantation with NexGen Legacy Constrained Condylar Knees (LCCK, Zimmer, Warsaw, IN, USA) (P=0.006), whereas the remaining 14 CR spacers (23.7%, all were dislocated spacers) and two PS spacers (3.1%; one was dislocated spacer and one was spacer loosening) received reimplantation with NexGen rotating hinge knees (RHK, Zimmer, Warsaw, IN, USA) (P<0.001). Reinfection occurred in eight CR and eight PS spacers. The rate of infection eradication was comparable between CR (51/59, 86.4%) and PS spacers (56/64, 87.5%) after reimplantation (P=0.608). Of the eight reinfected revision TKAs in CR spacers, four underwent debridement, antibiotics, and implant retention (DAIR) procedure, and long-term suppressive antibiotics; three underwent resection arthroplasty; and one underwent above-knee amputation. Of the eight reinfected revision TKAs in PS spacers, five underwent DAIR, two underwent resection arthroplasty, and one underwent above-knee amputation.

Discussion

This was the first retrospective study to compare the rate of infection eradication between CR and PS spacers in two-stage revision of prosthetic knee infection, spacer mechanical complications and reoperation, functional outcomes, and ROM. Particularly, this was the first study to investigate the mechanical complications of differently constrained spacers and strictly define the position of the spacer, integrity of the spacer and bone, and the relationship between the bone and spacer of the femoral-tibial and patellar-femoral joints. Moreover, all radiographs were reviewed independently by a musculoskeletal radiologist, which minimized the underestimation of the rate of spacer mechanical complications. The results demonstrated that PS spacers had a lower overall rate of spacer mechanical complications and reoperation than did CR spacers in the interim stage. Moreover, PS spacers had a lower rate of complications in joint dislocation and of reoperation in spacer mechanical complications than did CR spacers. PS spacers had better HSS knee scores and greater ROM than did CR spacers, especially in maximum flexion angle at the time prior to implantation or at the most recent visit in patients who retained the spacer. Both CR and PS spacers could control infection after implantation.

Poor mechanical circumstances of prosthetic infected knees after resection arthroplasty, such as huge metaphyseal bony defect, and bi-cruciate or collateral ligament deficiency or defect, cause the knee to become inherently unstable in an environment similar to revision surgery [[12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection.]. In the current study, intraoperative PCL integrity in chronic PJI of original CR TKA during explantation was either PCL insufficiency or PCL deficiency without intact PCL in both CR and PS spacers. PCL deficiency was observed in all original PS TKA. Theoretically, more constrained spacers are required in an unstable knee joint [[12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection., [15]Villanueva-Martinez M. Rios-Luna A. Pereiro J. Fahandez-Saddi H. Villamor A. Hand-made articulating spacers in two-stage revision for infected total knee arthroplasty: good outcome in 30 patients., [16]Su Y.P. Lee O.K. Chen W.M. Chen T.H. A facile technique to make articulating spacers for infected total knee arthroplasty.]. Less constrained articulating spacers might result in more spacer mechanical complications, such as spacer loosening or fracture, or joint dislocation [[12]Tsai C.H. Hsu H.C. Chen H.Y. Fong Y.C. Ho M.W. Chou C.H. et al.A preliminary study of the novel antibiotic-loaded cement computer-aided design-articulating spacer for the treatment of periprosthetic knee infection., [14]Struelens B. Claes S. Bellemans J. Spacer-related problems in two-stage revision knee arthroplasty., [19]Haddad F.S. Masri B.A. Campbell D. McGraw R.W. Beauchamp C.P. Duncan C.P. The PROSTALAC functional spacer in two-stage revision for infected knee replacements.].Most cement articulating spacers are CR without a post-cam construct [[3]Fehring T.K. Odum S. Calton T.F. Mason J.B. Articulating versus static spacers in revision total knee arthroplasty for sep