The study was approved by the local ethics committee (VN2005/43), and informed consent was obtained from all the subjects.
Eight healthy volunteers [five male and three female, age 24.1 ± 0.8 years, body mass index (BMI) 23.8 ± 1.3 kg/m2] without chronic metabolic disease or low back pain were enrolled in this group. All the participants were non-smokers and non-medicated. There were no spinal deformities in the volunteers or local tenderness on the back by inspection and palpation. All the participants were asked to avoid excess physical activities 24 h before the experiment day.
After having a normal lunch, the experiment started at 4.00 p.m. in the outpatient operation theatre with a room temperature of 23°C. No food or water intake was allowed during the whole procedure. The experiment protocol is shown in Fig. .
Study protocol for eight healthy subjects. Three dialysates were collected in either period for one catheter
Following the local skin anaesthesia with 1% lidocaine 1–2 ml, two MD catheters were placed in the right paraspinal muscle (at L4 spinous process level, 3 cm away from midline) and the right deltoid muscle (lateral part) in a prone position. After the stabilisation period of 40 min, the subjects were asked to lie in the prone position for 1 h and in a supine position for another hour. There was a 10-min wash-out period between the two periods. MD samples (dialysates) were collected every 20 min.
Eight patients (six male and two female, median age 57.7 (37–74) years, mean BMI 28.1 ± 2.2 kg/m2) who underwent conventional instrumented posterolateral lumbar fusion were included. Patients’ diagnosis was degenerative spondylolisthesis with/without spinal stenosis. Exclusion criteria included: previous fusion, chronic metabolic disease, tumour or metastasis, and surgical complications during and postoperation. Fusion level and operative time is shown in Fig. . All the operations were performed by one of the authors (Sten Rasmussen).
Study protocol for eight patients who underwent posterior lumbar fusion surgery. Operation level and operation time were showed
Following the standard general anaesthesia, two MD catheters were placed in the paraspinal muscle at the level of midpoint of incision bilaterally (in case of catheter breakage during surgery), 3 cm away from midline. A reference catheter was placed in the deltoid muscle as above. Dialysates were collected every 20 min during whole operation period after 40-min stabilisation period.
CMA 60 (CMA/Microdialysis AB, Sweden) MD catheters, with a polyarylethersulphone membrane length of 30 mm, outer diameter 0.6 mm, and a molecular weight cut off of 20,000 Da, were used in the study. All the catheters were inserted parallel to the muscle fibres with the tip proximal.
CMA 106 (CMA/Microdialysis AB, Sweden) MD pump pumped the catheters with perfusion fluid T1 (CMA/Microdialysis AB, Sweden, Na+ 147 mM, K+ 4 mM, Ca2+ 2.3 mM, Cl− 156 mM) at a rate of 0.3 μl/min. The dialysate was analysed using the ISCUS (CMA/Microdialysis AB, Sweden) clinical MD analyser with kinetic enzymatic method immediately after dialysate collection. The concentrations of key metabolites, such as glucose, glycerol, and lactate pyruvate ratio (L/P) were measured. Quality control was carried out with control samples at the beginning of the measurement.
To ensure the position of the catheter in the muscle tissue different techniques were applied in the two groups. In the healthy group, smaller BMI and thinner thickness of subcutaneous tissue on the back were found in the subjects. Therefore, “fascial click” and “muscle spasm” were noted when the MD catheters were inserted into the paraspinal muscle [12
]. In the surgery group, magnetic resonance imaging was performed. The thickness of the subcutaneous tissue on the back in the axial plane T2 sequence image was measured. Only patients with subcutaneous tissue thickness <20 mm (CMA 60 catheter shaft length) were included.
Normal distributed data were presented as mean ± standard deviation (SD), otherwise median and range were used. SPSS statistical software 15 for Windows was used for statistical analyses. The level of significant was set to α = 0.05 (2-tailed testing). Two-way repeated measures analysis of variance was used to test for differences between two tissues (paraspina muscle and deltoid muscle) over time or different position periods. Multiple comparisons were done by followed Bonferroni post hoc analysis, if necessary. Student’s t test was used to compare the differences between the results from the eight healthy subjects and the eight patients. Mann–Whitney test was performed if the assumptions for the t test were not fulfilled.