The most prominent feature of SSI in PLIF was high incidence of O/SI. O/SI is known to have a long ETD. According to the definition of the CDC, O/SI is defined as an SSI which involves any part of anatomy which was opened and manipulated during an operation other than the incision itself [
5]. The common features of SII and DII were not so different from other types of posterior instrumented surgery. However O/SI, i.e., spondylitis without incisional infection, has a range of different features from diagnosis to treatment. In the case of incisional infections, implant removal was not necessary. They were all diagnosed early and, consequently, local management began as soon as possible. Wound exploration and repeated irrigation was considered effective to prevent the infection's extension to deeper structures. Fortunately, there was no bone extension from incisional infections. Many authors have recommended the retention of implants in acute infections [
678910]. However, in other studies, implant removal was frequently requested in DII [
11121314]. In particular, late onset infections and
Propionibacterium acne (PA) infections were found to be risk factors [
13]. There was no late onset infection in the incisional infections in our cases. This is a possible reason why we were able to preserve all implants in the incisional infections. O/SI was not easily diagnosed because of its vague symptoms and an absence of local findings. Therefore, the ETD was further delayed after the true onset of infection in those cases. The O/SI almost always began from the interbody space as a spondylitis around the cages and grafted bone. It was difficult to presume that bacteria contaminated the interbody space only. Rather, it would be more plausible to presume that contaminated bacteria located in the interbody space were not easily removed by irrigation and adhered to disc remnants or foreign materials, and they gradually developed a late infection. If the contaminated bacteria were highly virulent, it is presumed that a more aggressive and acute onset infection would develop. In our cases, half of them did not undergo microbiological exams because they were managed conservatively. Intravenous antibiotics were started when laboratory and clinical manifestations strongly suggested infection. Cefolactam (Samjin Pharm, Seoul, Korea) that was comprised of cefoperazone and sulbactam was used as the intravenous antibiotic. Antibiotics were switched once the causative microorganism was determined from the sensitivity test. Among the 10 cases which underwent revision surgeries, there were no highly virulent microorganisms. Four cases showed no growth. There have been many studies that noted disc space infections after discectomy by anaerobic bacteria, especially PA [
1516171819], and the late infection in instrumented spinal surgeries by PA [
132021]. Disc space contamination by PA has been proven through other studies [
2223]. We did not perform anaerobic culturing at first and did perform it later on. PA was not identified in any case; however, some of the cases might have been PA infections. The remaining cases were also not highly virulent infections. At first, we could not appropriately diagnose O/SI. After we became aware of its clinical manifestations, the ETD was shortened. Furthermore, we realized that conservative treatment was possible if an early diagnosis was made. Maruo and Berven [
13] also noted that a late infection, in other words, late diagnosis, was a risk factor for treatment failure. The preservation of implants seriously matters in O/SI because cage removal is technically difficult [
12]. Furthermore the index segment would lose stability entirely and neurological symptoms could develop after implant removal. If staged operations were performed, patients had to stay on bed rest during the interval. That is why we could not prevent performing POSSR. As far as we have researched, there is no agreement for the treatment for infections such as O/SI. Carmouche and Molinari [
11] tried to preserve the cages but ultimately ended up removing the cages and achieved spinal fusion by posterolatral bone grafts. Ha and Kim [
12] concluded that all implants should be removed to counter any spondylitis around the cages and that the anterior approach was feasible to remove them. Sierra-Hoffman et al. [
14] and Hedequist et el. [
24] determined that, in cases caught early, the implants could be preserved, but that late cases necessitated implant removal. There are many presumptive risk factors of implant removal. However, an ETD longer than 3 months was the only independent risk factor in our study. All revision cases had implant loosening. In an image test; however, two of them appeared not having loosening. For that reason, radiological implant loosening was not counted as an independent risk factor. Therefore, a keen suspicion is the first and most important step for the successful diagnosis of an SSI, which is directly related to successful treatment. In revision surgeries, all screws and cages were loosened and grafted bone chips were already sequestrated. Cage removal is a demanding procedure, but it is not impossible. Before cage removal, all screws and rods were removed and widened holes were charged with fresh frozen allograft bone chips and bigger screws were inserted. The interbody space was distracted to make a space for the cage removal. However, we failed to remove the cage in one case. In that case, we displaced it to the opposite side and grafted an auto-iliac bone. At the final assessment, the patient achieved solid interbody fusion and the cage was embedded in the bone mass. There have been many reports stating that titanium cages can be used in the surgical treatment of spondylitis [
25262728]. We are unsure if titanium cages are safe to use in spondylitis or not. Furthermore, the titanium cage would be different from the initially applied cage because it was presumed to be covered by a biofilm of microorganisms. Though our case cannot be generalized, the meticulous debridement and copious irrigation with a pulsatile irrigator of the interbody space and auto-iliac bone grafts were considered more important than the titanium cage removal itself. While all revision cases achieved interbody fusion, two cases of spondylitis that underwent conservative treatment resulted in a nonunion state. One patient was asymptomatic, while the other had discomfort and back pain aggravated by motion but refused to receive a revision. If the causative bacteria were highly virulent, the treatment options would have been different. Tokuhashi et al. [
29] reported the successful treatment of spondylitis around cages without surgical intervention, though there was already an epidural abscess and explained it was possible because the causative bacteria were lowly virulent. POSSR might have been dangerous if the SSI was caused by highly virulent bacteria. POSSR has several drawbacks. Posterior interbody debridement might be incomplete and bicortical auto-iliac bone from the posterior iliac crest is not as strong as tricortical bone from the anterior iliac crest. It has, however, many advantages. Through a posterior approach, all implants and infected tissues in the epidural and interbody spaces could be removed. Furthermore, stability could be attained under the same anesthesia and without positional changes, minimizing neurological injury and reducing the patient's inconvenience which would result from a longer duration bed rest. At first, we thought the widened pedicle holes should be charged with auto-iliac bone. But the amount of auto-iliac bone was not enough to cover both the interbody space and the pedicle holes. Furthermore, it did not have sufficient structural hardness, due to osteoporosis. So, we cautiously used fresh frozen allogeneic bone from the bone bank of author's institute. Fortunately, there have been no adverse effects so far. Functional results were improved after treatment of the SSIs in general. Though functional improvements were not as strong in O/SI, it did not reach statistical significance. O/SI was influenced by ETD. We could say SII and DII had better functional results than O/SI; however, the early diagnosed O/SI was not inferior to SII or DII. Therefore, an early diagnosis was the key to averting implant removal and attaining a better functional result. There were several limitations in our study. As a general limitation of observational studies, the results of our case series could not represent every specific situation. In particular, our cases were mainly caused by low virulent microorganisms. A more precautious strategy would be necessary for the highly virulent aggressive SSI.