The clinical entity known as postoperative discitis (POD) was initially described by Frank Turnbull in the early 90s. It represents approximately 20% to 30% of all instances of pyogenic spondylodiscitis ^{[1][2][3][4][5]}[1,2,3,4,5]. Discitis or spondylodiscitis can lead to serious consequences; hence, prompt treatment is imperative. Regrettably, POD might go undetected or be diagnosed too late due to misinterpretation of postoperative clinical and imaging presentations [6].

Most reports indicate infection rates of less than 1%, which increase with the addition of fusion (without instrumentation), reaching up to 6.2% ^[1][7][8][1,7,8]. The incidence of infection after instrumented fusion is even higher, estimated up to 20% [1]. Furthermore, the impact of intraoperative use of a microscope on the rate of postoperative spondylodiscitis is not clear. Some authors have demonstrated a negative impact with up to 5% increase of the infection rate ^[9][10][9,10], while others have reported positive impact with reduction of the infection rate from 2.8% to 0.4% [11].

Minimally invasive surgeries, like percutaneous discectomy, appear to have a lower incidence of infection. The incidence according to Bonaldi et al. is 0.26% ^[12][20]. Kang TW et al. in a retrospective nationwide cohort study of patients undergoing percutaneous endoscopic lumbar discectomy in Korea reported infection rate of 0.83% ^[13][21]. A more recent article by Mahan MA in a retrospective multicenter cohort study of a total of 1277 endoscopic discectomies reported infection’s incidence 0.001% ^[14][22]. These infections usually resolve without any clinical or radiological sequelae ^[12][20].

The reported rate of disc space infections following other diagnostic or therapeutic procedures, such as discography or chemonucleolysis, stands at 2.3% ^[15][16][23,24]. The incidence of POD seems to be related to the extent of tissue damage that occurs during the surgical procedure [1].

POD is believed to occur due to direct inoculation of the avascular disc space during surgery, likely by the skin flora or environmental factors ^[17][25]. A different route of infection can be also through continuous spread during the early postoperative period from adjacent retrodiscal tissue [6]. A third source of infection is through hematogenous dissemination ^[18][26].

Injury to the disc or vascular compromise during surgery may result in the so-called aseptic discitis ^[19][27], although this entity has been seriously challenged. Fraser et al., based on animal experiments, propose that after about a 6-week period, a disc infected with Staphylococcus aureus may transition to an “aseptic” state ^[20][28]. They illustrated that vascular granulation tissue from the subchondral bone invades, absorbs, and manages the infectious process. This mechanism could elucidate the relatively mild course observed in some cases of spondylodiscitis. Table 12 summarizes the spread type of the infection.

It is widely recognized that primary hematogenous pyogenic spondylodiscitis can sometimes progress to epidural abscess and sepsis, resulting in severe consequences, including death ^[21][29]. Nevertheless, secondary post-discectomy infection typically follows a milder course compared to primary hematogenous spondylodiscitis, which is associated with higher risk factors ^[21][29].

It remains controversial whether POD is caused by an aseptic or infectious process since positive cultures are detected in only 42–73% of patients ^[17][25]. Injury to the end plate, hematoma formation, and necrotic tissue during surgical procedure provide ideal conditions for bacterial growth. POD is a mono-bacterial infection in most cases, with Staphylococcus being the predominant primary pathogen ^[17][22][25,30]. Specifically, Staphylococcus aureus is isolated in almost 50% of cases. Other common Gram-positive species include Staphylococcus epidermidis and other coagulase-negative Staphylococcus species. Common Gram-negative organisms, such as Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter cloacae, Bacteroides and Proteus species, have been also isolated from infected surgical sites ^[17][22][25,30].

Pull ter Gunne et al. presented the results of bacteria cultures in patients with PODs. Staphylococcus aureus was the most common organism (65.1%), while Enterococcus faecalis and Escherichia coli were presented in 14.5% and 10.8%, respectively ^[23][31]. Many studies have also reported the presence of fungal infections (Candida and Aspergillus fumigatus). The most precise cultures are those acquired during surgical debridement performed prior to the initiation of antimicrobial agents ^[23][24][31,32].

It has been suggested that lumbar spine spondylodiscitis has lower incidence of neurological deficits, with the exception of POD caused by Serratia marcescens, as compared to more cephalad involvement ^[21][29]. Serratia marcescens, which typically resides saprophytically in water, soil, and the human alimentary tract, has been commonly associated with nosocomial infections affecting the lungs, meninges, urinary tract, injured tissue, and occasionally, bones and joints ^[25][26][33,34]. This infection carries a high mortality rate ranging between 25% and 52%, particularly when accompanied by bacteremia. Although uncommon, spine infection post-surgery has been reported to lead to spondylodiscitis with purulent epidural abscess. Such cases are characterized by an intense clinical presentation within a week after surgery and may be complicated by paralysis ^{[27][28][29][30]}[35,36,37,38]. Eventually, discitis is expected to result in bony or fibrous ankylosis of the adjoining vertebrae. At this stage, the patient becomes completely asymptomatic. However, pain may persist if the fibrous union does not become solid. Occasionally bone destruction may occur bringing about painful kyphotic deformity.

Infections following diagnostic or therapeutic interventional spinal injections, such as intradiscal injections, epidural steroid injections (ESI), facet joint injections, and discography are reported to range from 1% to 2% ^[31][32][33][39,40,41]. Table 23 presents the incidence of infections among different types of these procedures. Fungal (Exserohilum rostratum, Aspergillus fumigatus), viral (varicella-zoster virus, HSV-1, and HSV-2), and bacterial (methicillin-resistant Staphylococcus aureus, methicillin-resistant Staphylococcus epidermidis) infections following ESI have been documented in limited studies ^{[34][35][36][37]}[42,43,44,45]. In a retrospective review of 11,980 facet joint injections, Kim et al. presented 8 cases of infections, including 1 case of systemic fungal infection (Aspergillus) that spread to the spine and 7 cases of infectious spondylitis ^[38][46]. Infections after intradiscal injections are the most common among types of interventional spine procedures ^[31][39]. Bosnak et al. mentioned 10 cases of nosocomial spondylodiscitis with Pseudomonas aeruginosa after intradiscal electrothermal therapy ^[39][47]. Additionally, infections after intradiscal injections of Cytokine Blockers (tocilizumab, interleukin-6 receptor antibody, and etanercept) have been reported in three studies, with an incidence of 2.11% ^[31][39].

The onset of POD symptoms usually occurs at 2–4 weeks (ranges 1–6 weeks) after an otherwise uneventful operation ^{[40][41][42][43][44][45]}[18,48,49,50,51,52]. The presenting signs and symptoms include acute onset of severe and continuous back pain (88%), muscle spasms (76.4%), stiffness, while sciatica and positive Lasegue’s sign are present in 87% of cases, and pseudo-Gower sign in 73% of them ^{[3][19][46][47]}[3,27,53,54]. Pain and muscle spasms are usually refractory to bed rest and analgesics ^[45][52]. At the incision site, there is frequently erythema, warmth, and often drainage of fluid. Fever may be present in 11–68% of all cases. In contrast to primary hematogenous pyogenic infection, no mortality rates have been observed by most studies, except one cohort that reported 1.4% mortality rate ^[21][48][29,55].

Staphylococcus aureus represents the most commonly isolated organism in POD. Thus, empirical treatment should always include an anti-staphylococcal antimicrobial regimen. Nevertheless, attempts to isolate the causative organism should be made, in order to proceed to targeted antimicrobial treatment ^[46][53]. There is no agreement regarding the optimal utilization of antimicrobial agents in the treatment of postoperative spondylodiscitis. Intravenous (IV) antimicrobial agents that have been successfully used as a single regimen include tobramycin, cephazolin, clindamycin and cephalothin. Antimicrobials that have been used successfully in combination include methicillin, nafcillin, rifampin, cephazolin, penicillin, vancomycin and cephalothin. Singh et al. reported their experience in 31 patients treated for POD. The duration of IV antibiotic therapy was 6 weeks in responders. Additional 3 weeks of IV antibiotic treatment was given in patients who failed to improve with conservative treatment and were taken up for surgical debridement and fixation. A combination of three antibiotics was initially used in all patients. The most common antibiotics used were vancomycin/cefepime/linezolid along with amikacin and metronidazole. Antibiotic treatment was tailored in two culture positive patients after surgical debridement. Antifungal treatment (fluconazole 150 mg PO for 3 weeks) was added in one urine culture positive patient. Six weeks of oral antibiotic treatment (linezolid 600 mg OD + ciprofloxacin 500 mg BID) were given in all patients at the time of discharge ^[42][49]. In a recent cohort study of 75 patients with POD, Meropenem, Flucloxacillin, Linezolid and Fusidic Acid were effective against Staphylococcus aureus and Staphylococcus epidermidis. Additionally, Escherichia coli, Enterobacter species and Pseudomonas aeruginosa were sensitive to Ciprofloxacin and Tobramycin. Ceftriaxone (3rd-generation Cephalosporin) was used empirically and during primary surgery but was resistant to Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli in 40% cases ^[40][18]. Patients are commenced on IV antimicrobial agents for 4 to 6 weeks, followed by per os antibiotics for another 6 weeks ^{[41][42][44][49]}[48,49,51,69]. With the early initiation of IV antibiotic treatment, the ESR typically decreases to normal levels within approximately 90 days ^[41][48]. With proper treatment, POD recurrence is uncommon, ranging from 0% to 4% ^[3][50][3,56]. The reported success rates with conservative antimicrobial treatment ranges from 35.5% to 77.8% ^[3][17][3,25].

Surgical intervention is necessary in patients failing conservative treatment. In the majority of cases, thorough surgical debridement, antibiotic therapy, and orthosis immobilization can effectively eliminate the infection ^[24][32]. Immobilization of the lumbar spine with orthotic devices is an essential component of the overall management. A meticulous surgical debridement must be performed, and the wound must be explored to remove all affected tissue. Samples from each tissue layer should be routinely sent for microbiological examination for aerobic, anaerobic, and fungal pathogens. Certain cases may necessitate even more extensive surgery ^[51][70]. The indications for surgery include severe destruction of endplates, abscess formation, chronic osteomyelitis with biomechanical instability, neurologic deficits, local kyphosis, severe pain, and pseudoarthrosis. The surgical plan encompasses, usually, posterior approach with combination of debridement and stabilization. Fusion may not be needed in all cases. The nature of the pain is highly suggestive of mechanical instability. Posterolateral fusion leads to spontaneous anterior interbody fusion, a process that is accelerated when disc space debridement is performed ^[51][52][70,71].

R. Santhanam et al. reported the results of a conservative regime in 18 patients with spondylodiscitis including strict bed rest and antimicrobial therapy (vancomycin, linezolid and cefaperazone with sulbactam). A total of 14 out of 18 patients (77.8%) improved with non-operative management. In those 4 patients, who did not respond to antibiotic treatment, surgical treatment was performed which consisted of irrigation and debridement with curettage of the disc space granulation tissue, followed by spinal fusion through transpedicular fixation (posterior approach) ^[17][25]. The authors reported that instrumentation could provide a more robust stability to the infected spine that could hasten the healing process ^[51][70].

Conservative treatment has been proven to be sufficient for the uncomplicated POD cases. However, a variable degree of disability due to pain may persist for 3–4 months.

Minimally invasive spine surgery may offer early pain relief in patients with POD and a higher diagnostic yield to targeted antibiotic treatment ^[53][72]. Early debridement through endoscopic transpedicular discectomy has demonstrated to expedite the natural healing process, halt progression to bone destruction, and prevent the formation of epidural abscesses ^[54][73]. The benefits of this minimally invasive approach include effective drainage of infected material, collection of adequate tissue samples for histological and microbiological analysis, implementation of a suction-irrigation system, rapid alleviation of pain and discomfort, and early mobilization of the patient. This procedure should be also considered as a cost-effective approach, since patient’s hospital stay that is necessary for bed rest and analgesia can be drastically curtailed to one or two days, as opposed to open surgical techniques in which several weeks may be needed ^[54][73].

Similar results have also been reported through percutaneous endoscopic discectomy ^[55][56][57][74,75,76]. Full endoscopic debridement and drainage have been employed to treat POD and psoas muscle abscesses even in elderly patients and patients with multiple medical comorbidities. The major advantage of this technique over traditional open surgery is that it can be performed even under local anesthesia ^[58][59][77,78]. Different surgical approaches can be used such as interlaminar, transforamina or bi-portal with success rates over 80% ^[58][59][77,78]. For anterior pathology, transforaminal discectomy and drainage is an optimal approach targeting anterior column directly without destructing posterior structure. While, posterior (interlaminar) approach is suitable for “posterior” epidural abscess or paraspinal abscess. In cases with both pathologies, both approaches could be used simultaneously ^[58][60][77,79]. Endoscopic drainage can be successfully combined with other minimally invasive surgical techniques with favorable results ^[61][80].

The overall reported success rates of these procedures, as indicated by level IV studies, vary from 76% to 87% ^[54][57][73,76]. A common factor contributing to the prompt healing in these procedures is the shaving or penetration of the subchondral plate of the affected intervertebral disc ^[62][81] so that vascular granulation tissue from the vertebral body can spread and be involved in the healing process.