Subclinical infection of the intervertebral disc after lumbar disc herniation surgery has been correlated to chronic low back pain and vertebral endplate changes. The most commonly reported agent is Propionibacterium acnes. However, the real incidence is unclear, as it has been reported in some series ranging from 3.7% to 46%. Recently, a systematic review concluded that there is a relationship between P. acnes and endplate changes, but, there are so far no studies to verify whether the reported presence of that pathogen in the intervertebral discs is due to local infection or whether intraoperative contamination occurred during the collection of samples.

Thus, the main objective of this study is to estimate the incidence of subclinical infection in patients surgically treated for lumbar disc herniation.

To this end, a prospective cohort study will be conducted with a minimum of 95 patients between 18 and 65 years of age who have been submitted to surgery after failure of conservative treatment. The extruded disc will be removed and cultured for bacterial identification. As controls, the ligamentum flavum and the multifidus muscle, taken respectively before and after removal of the herniated fragment will also be cultured. Patients will be followed-up for a year and MRI will be done at the end of this period.

INTRODUCTION Low back pain is a frequent condition in the population, as well as vertebral endplates abnormalities, described by Modic et al.(1,2), that affect up to 6% of the general population, and, up to 46% of patients with low back pain(3). Modic type I changes are described as vertebral bone marrow edema related to acute low back pain(4). When Modic changes are detected, chances of one presenting unspecific low back pain are 4.5 times higher(1,2).

Subclinical infection caused by low-virulence pathogen can possibly lead to vertebral endplate abnormalities, detected in magnetic resonance imaging (MRI) studies, and differentiation between infection and Modic changes may be difficult(5,6). Subclinical infection can also be associated with increasing low back pain(7). Albert et al(8) reported 61 patients who had undergone surgical treatment for lumbar disc herniation where 46% of cases had a positive culture. The same authors also reported that 80% of patients with a positive culture for anaerobic pathogens presented Modic type I changes at the adjacent vertebra after a two-year follow-up, against 44% of patients with negative culture. Some studies demonstrated the presence of low-virulence pathogens in intervertebral disc tissue cultures(6-10), most commonly reported to Propionibacterium acnes.

Chronic low back pain and Modic type I changes have been treated with antibiotics for up to 100 days with superior outcomes compared to sham treatment(7). Patients were treated with amoxicillin/clavulanate (500mg/125mg)(7) based on the study where sciatica is associated with Propionibacterium acnes(8).

However, Carricajo et al(11) suggest that the presence of P. acnes in the intervertebral discs is due to either external surgical or laboratory contamination. These authors detected positive disc culture in only 3.7% of cases out of 54 patients. Furthermore, same authors demonstrated that samples of spinal muscle and ligamentum flavum collected intraoperatively at the end of procedure had positive cultures in 14.8% of cases with a negative disc culture.

A systematic review performed by Urquhart et al(12) concluded, that there is moderate evidence that a relationship between positive culture with Modic type I changes and low back pain exists, although there was low evidence for relationship of cause. For that, authors concluded that new studies should be made to determine whether pathogens in the disc are originated from external contamination or if they are truly involved in the development of chronic back pain.

HYPOTHESIS Lumbar disc herniation is related to subclinical infection of the intervertebral disc Null hypothesis: incidence of subclinical infection is the same as incidence of cases without infection in patients with lumbar disc herniation treated with surgery.

OBJECTIVES Main purpose of this study is to identify if the presence of a infection pathogen in the intervertebral disc is real or if it is intraoperative contamination.

Secondary objectives are to analyze clinical prognostic factors in patients and diagnosis of infection.The study also proposes to analyze the relationship between radiological changes (Modic I and II) and infection.

METHODS

Study design:

An open prospective cohort study will be performed at a single center, (Hospital Israelita Albert Einstein - HIAE) taking 1 year for recruiting, and ending 1 year after inclusion of last patient. Patients' data will be collected with a specific form created for this study. Patients will be summoned for a new magnetic resonance image of the lumbar spine one year after their surgical procedure.

All included patients will go through further treatment of ten sessions of postoperative physical therapy.

Population:

Patients who have failed conservative treatment for lumbar disc herniation undergoing lumbar decompression open surgery (microdiscectomy) will be consecutively included in the study.

Patient enrollment in the study:

Patients accepting their participation will date and sign the Informed Consent Form (ICF). After ICF is properly signed, patient will undergo an interview to complete initial demographic data and pretreatment forms.

Patient recruitment will be carried out for 24 months, when 95 patients shall be included (details of estimated n reported at sample size determination).

Patient allocation:

Patients will undergo surgery according to surgeons' preference. Attending surgeons will determine chosen operative technique according to their experience and preference.

Blinding:

Patients will not have access to the results of tissue cultures for pathogens, as well as the attending physician.

The radiologist that will analyze imaging studies of performed magnetic resonance will also be blinded to patients' data or laboratory culture results. A blinded investigator will analyze pain and function scores.

Early stopping of participation in the study:

Patients will be excluded from the study when:

• Withdrawn of ICF
• Diseased
• Patient selection flaw - incompatible eligibility criteria
• Lost to follow-up
• If patient presents clinical symptoms of infection such as severe lumbar or radicular pain, fever with no other detected foci, abnormal ESR, CRP, leucogram, and, altered imaging studies that leads to interruption of blinding of the results of culture exams.

Study stages

Sample collection:

Included patients will undergo standard fashion general anesthesia and prepped with clorhexidine solution. Intravenous antibiotics prophylaxis will be administered within first hour before skin incision, according to standard protocol of HIAE Infection Control Committee published at the hospital Pharmaceutics Manual.

Preoperative blood sample will be collected for leucogram, Erythrocyte Sedimentation Rate (ESR), and C-Reactive Protein (CRP). Same laboratory will be repeated at 1, 6 and 12 months time-point.

The excised herniated disc fragment will be immediately sent to microbiology laboratory analysis in a universal sterile container (screw cap tube) in no more than 30 minutes to be processed as follows. Same process will be applied to samples of deep muscle and ligamentum flavum at the end of the surgical procedure. Three cultures of the intervertebral disc will be done, as well as three of the ligamentum flavum and three of the multifidus muscle for each patient.

Search for pathogens:

Herniated intervertebral disc will be split in three equally sized fragments of over 2x2x5 mm and squashed in a laminar flow cabinet until homogeneous material is achieved. Same process will be carried out for the ligamentum flavum and multifidus muscle samples, although split in three fragments. Tissues will be cultured in specific growth medium and incubated according to the respective culture:

Similar criteria used by the Infectious Diseases Society of America (IDSA) to detect joint replacement infection will be adopted, which is the recommendation on at least two positive tissue cultures by the same pathogen to confirm diagnosis of infection(13).

• A - Aerobic culture For aerobic cultures, samples will be cultured in 5% sheep blood agar, chocolate agar and MacConkey agar plate, and will be placed in a 35°C incubator (CO2 atmosphere) for 5 days. If a positive bacteria culture is detected in the plate, the colony will be identified by MALDI TOF (Matrix Assisted Laser Desorption Ionization-Time of Flight) Microflex LT (Bruker Daltonics/BD).

• B - Anaerobic culture For anaerobic culture, tissue will be cultured in a Thioglycolate tube and incubated in a 35°C incubator for up to 21 days. If turbidity occurs at the Thioglycolate medium, material will be cultured in anaerobic blood agar and incubation at 35°C will be done in an anaerobic atmosphere. After growing of colonies, identification will be done by MALDI TOF.

• C - Histological analysis Anatomic pathology analysis of the other fragment of the herniated disc (2x2x5 mm) will be done. Sample will be transported in a universal container with tamponaded formalin (10%), followed by dehydration in alcohol diafanized in xylol and inclusion in paraffin (60-65°C), which will be stained in hematoxylin-eosin (HE) and GRAM staining.

  • HE: Histological cuts of 4μm will be performed, followed by clearing with xylol for 10 minutes twice, embedding with alcohol under increasing concentrations and stained with hematoxylin for 5 minutes, running water for 5 minutes, eosin for 1 minute and running water for 2 minutes followed by assembly of glass microscope slide with Entellan®.
  • GRAM: Another slide will be embedded with crystal violet for 1 minute, running water for 1 minute followed by lugol for another 1 minute and additional wash with running water. Unstaining of the slide will be done with alcohol 95% for 10 seconds followed by running water and then, stain with fuchsine for 30 seconds, running water wash again, drying and slide assembly with Entellan®.

• D - Molecular analysis of pathogens Positive cultures that present aerobic or anaerobic pathogens culturing, will be isolated and stowed refrigerated in -80°C freezer for posterior molecular analysis.

Molecular typing will be performed through Pulsed Field Gel Electrophoresis technique of isolated samples according to the protocol described by Oprica et al.(14) using Spe-I restriction enzyme and Bionumerics software for analysis of results.

Questionnaires See outcome measures

Imaging studies Magnetic resonance imaging studies will be performed in Siemens or General Electric 1.5T devices.

Lumbar disc herniation will be diagnosed through MRI in eligible patients. Included patients will be submitted to new MRI 12 months after surgery.

Two radiologists with expertise in musculoskeletal MRI will independently classify and perform measurements.

Confounding variables Data on the following confounding variables will be collected: age, gender, alcohol intake, smoking, body mass index (BMI), spinal injections with corticoid within 6 months before surgery, usage of oral corticoids up to 3 months before surgery, diabetes.

Since this information may change over time, data will be considered at time of last assessment before surgery.

Statistical planning Rate of subclinical infection (or Modic change) will be obtained by the ratio between number of positive cultures from surgical samples and total number of patients, and estimates will follow 95% confidence intervals. After infection cases are identified, we will investigate if there is an association between detected infection and patient outcomes by logistic regression models for Modic changes, ordinal logistic regression for Modic volume and size, and linear regression or general linear models for numeric outcomes, such as: low back pain, quality of life and function. All models will consider confounding variables such as: smoking and alcohol intake, diabetes, corticosteroids injection, BMI, gender, and age. Results will be presented as effects estimates such as odds ratio or mean ratio, 95% confidence intervals and p values. Study dropout cases, for any reason, will be considered for final analysis.

Sample size calculation Sample size was calculated to estimate of incidence of subclinical infection in patients with lumbar disc herniation. Considering that the rate of infection lies around 46%(3), we need to observe a minimum of 95 patients to achieve a 95% confidence interval with 10% absolute accuracy.

The necessary sample size to analyze secondary endpoints will depend on the observed rate of cases with subclinical infection in our study sample. If the observed rate is too small, an increase in the number of included patients will be needed. To better evaluate this, the sample size calculation will be revisited by the time we have reached half of initially planned sample size (48 patients).

Inclusion Criteria:

• Subjects between 18 and 65 years of age; both genders; with diagnose of lumbar disc herniation undergoing open decompression surgery (microdiscectomy). Patients willing and able to go through all phases of clinical investigation and rehabilitation will be included. An Informed Consent Form (ICF) must be signed.

Exclusion Criteria:

• Patients with previous lumbar disc surgery at the same level at any point of life; patients undergoing chemotherapy; patients with any immune deficiency; patients previously submitted to disc injection and/or discography; patients submitted to previous endoscopic disc surgery; patients with fusion performed at the same stage of decompression surgery; patients with any other infection within the last six months or usage of antibiotics within the last two months; patients with incomplete specific form or data; decline to participate or sign the ICF.

Patients with lumbar disc herniation who failed conservative treatment undergoing surgical treatment open decompression surgery (discectomy).