Comparative Cohort Study Of Paraspinal Muscle Volume Change Between Uniportal Full Endoscopic And Minimamally Invasive Open Posterolateral Transforaminal Lumbar Interbody Fusion

DOI: https://doi.org/10.21203/rs.3.rs-1943082/v1

Abstract

Background

Recent literature of uniportal full endoscopic posterolateral transforaminal lumbar interbody fusion demonstrates good clinical and radiological outcomes with minimally invasive benefits potentially preserving paraspinal muscle. There is no literature on paraspinal muscle volume change between the endoscopic and microscopic minimally invasive interbody fusion.

Methods

We included patients who met the indication criteria for lumbar fusion and underwent either uniportal full endoscopic posterolateral transforaminal lumbar interbody fusion or open transforaminal lumbar interbody fusion. Clinical parameters of visual analog scale and Oswestry disability index were measured at preoperative, postoperative 1 week, 3 months postoperative and final follow up. Magnetic Resonance Imaging measurement of preoperative and postoperative Kjaer grade, right and left psoas muscle mass area, right and left paraspinal muscle mass area were performed.

Results

74 levels of Endo-TLIF and 42 patients with minimally invasive Open TLIF were included. There was statistically significant greater improvement in VAS and ODI in Endo-TLIF cohort at 1 week significant improvement of Kjaer grade at postoperative 1 year in Endo-TLIF compared to MIS-TLIF. There is statistically significant improvement in paraspinal muscle mass area in Endo-TLIF (104.83 ± 316.45) mm2 compared to MIS-TLIF (89.88 ± 185.14) mm2

Conclusion

Uniportal Endoscopic Posterolateral Lumbar Transforaminal Interbody Fusion achieved improved paraspinal and psoas muscle bulk and less fatty infiltration in the operated level as compared to Minimally Invasive Open Transforaminal Lumbar Interbody Fusion while both cohorts achieved equivalent positive clinical outcomes.

Introduction

Aging population leads to increase incidence of degenerative spinal conditions. 1 Lumbar spinal fusion is an effective treatment strategy for selected patients with degenerative spinal conditions.24 Open and minimally invasive transforaminal lumbar interbody fusion had shown good long term clinical outcomes and fusion rate, while minimally invasive option had less soft tissue injury, length of stay and blood less .5 Over the past 2 decades there is increasing literature on various endoscopic spinal techniques ranging from lumbar discectomy, decompression and lately fusion. 6 Interlaminar endoscopic lumbar discectomy, lumbar endoscopic unilateral laminotomy with bilateral decompression and interlaminar contralateral endoscopic lumbar foraminotomy had been described to address various lumbar degenerative conditions requiring decompression and discectomy.710 Endoscopic fusion is one of the latest technique being described in the endoscopic literature with a relatively steep learning curve. 11, 12.Two types of endoscopic lumbar interbody fusion were described in the literature, the first type described earlier in the literature used the safe corridor of Kambin’s Triangle ventral to the facet joint with or without foraminoplasty using small diameter transforaminal endoscope for disc preparation and cage insertion. Several authors had good clinical results with this technique of uniportal endoscopic trans-kambin facet sparing approach for transforaminal lumbar interbody fusion. 1315 There were recent spinal endoscopic publications on interlaminar posterolateral approach to perform transforaminal lumbar interbody fusion using the same anatomical corridor described by Harms et al for open transforaminal lumbar interbody fusion .16 This approach is popular among biportal endoscopic assisted posterolateral lumbar interbody fusion. 17, 18 Kim and Wu et al published several reports on uniportal endoscopic interlaminar posterolateral approach for transforaminal interbody fusion (Endo-TLIF) applications on graded 2 spondylolisthesis, foraminal stenosis, and scoliosis. 1922 Paraspinal muscle bulk has shown to have important correlation to prevention of disc degeneration and had been used as a surrogate marker for prognosis in back pain management. 2326 There were limited literature on comparative study on paraspinal muscle bulk in patients who had undergone Endo-TLIF as compared to minimally invasive open TLIF. We hypothesized that Endo-TLIF being a minimally invasive procedure with soft tissue preservation can conserve paraspinal and psoas muscle bulk better than minimally invasive open TLIF. We performed a retrospective comparative study on prospectively collected data to evaluate this hypothesis.

Materials And Methods

Indication, Inclusion and Exclusion Criteria

This retrospective study was approved by institutional review board, Nanoori Research Ethical Committee-IRB2021-007. Informed consents were obtained from all patients participated in study.

We retrospectively evaluated a prospectively collected data of patients who underwent single level Endo-TLIF and minimally invasive open TLIF (MIS-TLIF). From the MIS-TLIF and Endo-TLIF data base, we included symptomatic patients who suffered lumbar claudication and back pain and failed minimum 6 weeks of conservative treatment with the following clinical diagnosis of grade 2 and below spondylolisthesis and spinal stenosis with segmental instability. We excluded patients who had spinal fusion surgery due to trauma, revision surgery, tumor, infection, pseduoarthrosis, congenital spinal deformity, sagittal malalignment and coronal deformity with more than 20 degrees coronal curve. We changed our practice to evolve from minimally invasive open TLIF to Endo-TLIF in July 2019. Patients were enrolled from December 2017 to June 2019 for MIS-TLIF and from July 2019 to December 2020 for Endo-TLIF

For postoperative clinical evaluation in this cohort of patients, we evaluated clinical outcomes of Visual Analogue Scale and Oswestry Disability Index at preoperative, 1 week postoperative, 3months postoperative and final follow up.27 Postoperative complications were documented. Successful operation percentage defined as number of patients with good to excellent MacNab criteria/ total number of patients at final follow up was calculated. For radiological evaluation, CT lumbar spine evaluation was done on final follow up to evaluate the fusion grade using Bridwell Grade. Preoperative and Postoperative 1 year MRI mid disc axial cut follow up Kjaer grade, psoas and paraspinal muscle cross sectional area were measured with INFINITT PACS M6 Version (INFINITT Healthcare Corporation, Seoul, Republic of Korea) for cross sectional area measured in mm2. (Fig. 1).

We evaluated the amount of fat in the lumbar multifidus muscle on axial T1 weighted MRI scans with grading by Kjaer grading system.26 Kjaer Grade 0 denotes there is normal condition(0–9%), grade 1 there is slight fat infiltration (10–50%) and grade 2 there is severe fat infiltration (> 50%) in the lumbar multifidus muscle.

Surgical Technique

Description of Technique

Uniportal Full Endoscopic Posterolateral Facet Sacrificed Transforaminal Lumbar Interbody Fusion (Endo-tlif)

This technique of Endo-TLIF had been described in literature by Kim and Wu et al.2022, 28, 29 This is summarized here. Patient underwent regional anesthesia or general anesthesia and positioned on Wilson Frame over a radiolucent table. Skin is marked over the intended pedicle screws trajectory under fluoroscopic guidance. The endoscopic portal skin incision and docking is aimed at the ipsilateral upper mid pedicle for the level of ETLIF on anteroposterior (AP) view of intraoperative fluoroscopy (right L4 mid pedicle for right L4/5 ETLIF for example).The skin marking is typically 3 cm to 4cm from midline and 1-1.5cm long incision is made on the skin with fascia incised, sequential dilation with final endoscope working channel of outer diameter 13.7mm. The working channel target is ipsilateral facet through the Wiltse muscle splitting approach between multifidus and longissimus muscle. The author uses endoscope has 15 ° viewing angle, outer diameter of 10 mm, working channel diameter of 6 mm and working length 125 mm. Soft tissue dissection done with radiofrequency ablator, isthmus and inferior articular process is exposed and bony drilling is done on the isthmus and inferior articular process for inferior articular facetectomy which is harvested as bone graft.22 Superior articular facet is drilled under endoscopic guidance and harvested as bone graft. We subsequently remove ligamentum flavum and expose the ipsilateral disc space and the traversing and exiting nerve root. Working channel is rotated with bevelled facing lateral and inferior direction protecting the exiting and traversing nerve root.20 Epidural vessels hemostasis performed over the disc space. Radiofrequency ablation is performed over the region of sinuvertebral nerve and basivertebral nerve at the region adjacent to endplate and near the pedicle with the working retractor cannula protecting traversing nerve root out of harm’s way. 30 Disc and end plate preparation is performed with radiofrequency ablator, forceps and endoscopic drill under direct endoscopic vision without damaging the end plate. After trial of appropriate size cage, a 3D printed cage is inserted under working cannula of size 13.7mm outer diameter. Autologus graft mixed with allograft is tamped into the disc space under image fluoroscopy guidance. Single large 3D printed cage with demineralised bone matrix (DBM) is inserted as oblique as possible and final position of cage is checked under both fluoroscope as well as endoscope, making sure the cage is in satisfactory position and neural elements are decompressed. Surgical drain is inserted and endoscope and working cannula is withdrawn with skin closure in layers.

Minimally Invasive Open Microscopic Transforaminal Lumbar Interbody Fusion (Mis-tlif)

Patient is positioned prone on Wilson frame and underwent general anesthesia. TLIF procedure was performed on the symptomatic side. After a vertical skin incision in midline, subdermal dissection to the lateral aspect of symptomatic pedicle. Wiltse paraspinal approach to the facet was performed using self-retaining retractors. After a complete facetectomy with burr and Kerisson rongeur, the ligamentum flavum was removed to expose the lateral border of the ipsilateral traversing nerve root. The retractor was angled medially, the patient was tilted laterally to decompress the contralateral side if necessary. Extensive decompression was performed, which included decompression of the central stenosis and contralateral side. A discectomy was also performed under microscopy. Hemostasis is performed at the epidural vessel but not to region of sinuvertebral and basivertebral nerve as there is no effect way to protect the traversing nerve root. A 3D printed interbody cage filled with only autologous local bone was inserted. After interbody fusion, the retractor was removed, and the same procedure was repeated for each segment. Percutaneous pedicle screws were inserted under fluoroscopic guidance. Epidural catheter insertion for postoperative pain control was done prior to closure. Closure in layers done with wound drain placed.

Statistical Analysis

Data was analyzed with SPSS version 18 statistical analysis software (IBM Corporation, New York). The continuous variables were expressed as mean and standard deviation (SD). The paired t test is used for comparison of MRI, pre-operative and post-operative 1year, Psoas and Parasagittal cross sectional area. Clinical visual analogue scale(VAS), were measured at pre-operative, 1 day post-operative, 1 week post-operative and final follow reported by the patients were analysed with paired t test. A value of (p < 0.05) considered significant. Comparative data between Endo-TLIF and MIS-TLIF was performed with independent T test.

Results

Baseline Demographics

In the period from December 2017 to June 2019 for MIS-TLIF and from July 2019 to December 2020 for Endo-TLIF, a total of 42 patients underwent single level MIS-TLIF and 74 patients underwent single level Endo-TLIF. In MIS-TLIF, their mean age was 68.67 (41−86) years old with a mean follow up of 39.2 (24-70) months. In Endo-TLIF, their mean age was 64.76 (39−82) years old with a mean follow up of 19.00 (12−31) months. All the patients underwent general anesthesia for the surgery. There was statistically significant longer follow up in minimally invasive open TLIF than Endo-TLIF (Table 1)

Table 1. Baseline demographics data and clinical parameters of minimally invasive open transforaminal lumbar interbody fusion( MIS-TLIF) and endoscopic posterolateral transforaminal lumbar interbody fusion( Endo-TLIF).

 

MIS TLIF

Endo-TLIF

P value

Number of patients

42

74

N/A

Number of Patients with Level Lumbar Two Three 

2

5

N/A

Number of Patients with Level Lumbar Three Four 

12

16

N/A

Number of Patients with Level Lumbar  Four Five

23

42

N/A

Number of Patients with Level Lumbar Five Sacral One

5

10

N/A

Age (mean, range in years)

68.67 (41−86)

64.76 (39−82)

0.060

F/U Period (mean, range in years)

39.2 (24-70)

19.00 (12−31)

0.018

Male : Female Ratio

9:33

19:55

0.607

Complication Rate 

1

2

0.916

Bridwell Grade

1.67 ± 0.09

1.72 ± 0.07

0.659

Preoperative MRI axial cut spinal canal area of right psoas muscle at mid disc level (mean , SD ) mm2

836.00 ± 52.28

878.46 ± 39.38

0.518

Postoperative 1 year MRI axial cut spinal canal area of right psoas muscle at mid disc level (mean , SD ) mm2

813.72 ± 51.63

966.67 ± 38.90

0.020

Preoperative MRI axial cut spinal canal area of left psoas muscle at mid disc level (mean , SD )

891.18 ± 56.46

908.34 ± 42.54

0.809

Postoperative 1 year MRI axial cut spinal canal area of left psoas muscle at mid disc level (mean , SD ) mm2

875.67 ± 60.25

984.62 ± 45.39

0.151

Preoperative MRI axial cut spinal canal area of right paraspinal muscle at mid disc level (mean , SD ) mm2

1741.86 ± 71.58

2171.59 ± 53.92

<0.001

Postoperative 1 year MRI axial cut spinal canal area of right paraspinal muscle at mid disc level (mean , SD ) mm2

1651.98 ± 80.58

2276.43 ± 60.71

<0.001

Preoperative MRI axial cut spinal canal area of left paraspinal muscle at mid disc level (mean , SD ) mm2

1677.82 ± 68.65

2138.69 ± 51.72

<0.001

Postoperative 1 year MRI axial cut spinal canal area of left paraspinal muscle at mid disc level (mean , SD ) mm2

1545.08 ± 75.30

2295.66 ± 56.73

<0.001

Preoperative VAS (mean , SD )

7.81 ± 0.18

7.80 ± 0.13

0.956

Postoperative VAS at 1 week(mean, SD )

3.26 ± 0.10

3.27 ± 0.08

0.947

Postoperative VAS at 3 months(mean, SD )

2.71 ± 0.12

2.72 ± 0.09

0.990

Postoperative VAS at final follow up(mean, SD)

2.52 ± 0.13

2.24 ± 0.10

0.086

Preoperative ODI(mean, SD)

76.10 ± 1.26

75.00 ± 0.95

0.490

Postoperative ODI at 1 week(mean, SD)

32.67 ± 0.85

32.97 ± 0.64

0.773

Postoperative ODI at 3 months(mean, SD)

28.38 ± 0.81

28.54 ± 0.61

0.875

Postoperative ODI at final follow up(mean, SD)

26.62 ± 0.82

25.97 ± 0.62

0.531

Percentage MacNab Good To Excellent Outcome(%)

97.30

97.62

0.916

 

Clinical And Radiological Outcomes 

In terms of complication, in Endo-TLIF cohort, we had one incidental durotomy in a patient who required dural patch blocking repair.31 No revision surgery was required for this patient who had incidental durotomy without neurological sequelae and was allowed to mobilize two day postoperatively. There was one case of drain tip retention in the wound of the patient who required an additional local anesthesia procedure for removal of the drain tip. In the minimally invasive open TLIF cohort, there was one case of early symptomatic adjacent level prolapsed intervertebral disc which was treated conservatively. None of the patient had neurological complications after surgery. (Table 1)

In MIS-TLIF cohort, there was statistically significant improvement in VAS score with 4.55+/- 1.02, 5.10+/-1.25, 5.29+/-1.04 at postoperative one week, 3months and final follow up respectively , p<0.05. There was statistically significant improvement in ODI score with 43.43+/- 7.79, 47.71+/-8.57, 49.48+/-8.09 at postoperative one week, 3months and final follow up respectively , p<0.05. (Table 2)

Table 2: Clinical And Radiographic parameters of minimally invasive open transforaminal lumbar interbody fusion (MIS-TLIF), p value was calculated with paired T test

MIS-TLIF

Mean

Std. Deviation

P value

VAS improvement at 1 weeks

-4.55

1.02

<0.001

VAS improvement at 3 months

-5.10

1.25

<0.001

VAS improvement at final follow up

-5.29

1.04

<0.001

ODI improvement at 1 weeks

-43.43

7.79

<0.001

ODI improvement at 3 months

-47.71

8.57

<0.001

ODI improvement at final follow up

-49.48

8.09

<0.001

Change in Kjaer Grade after operation at 1yr

0.00

0.00

N/A

Change in MRI axial cut cross sectional area of right psoas muscle at mid disc level (mean , SD )

-22.28

135.97

0.295

Change in MRI axial cut cross sectional area of left psoas muscle at mid disc level (mean , SD )

-15.52

135.30

0.462

Change in MRI axial cut cross sectional area of right paraspinal muscle at mid disc level (mean , SD )

-89.88

185.14

0.003

Change in MRI axial cut cross sectional area of left paraspinal muscle at mid disc level (mean , SD )

-132.74

237.15

<0.001


In Endo-TLIF cohort, there was statistically significant improvement in VAS score with 4.53+/- 1.42, 5.08+/-1.42, 5.55+/-1.36 at postoperative one week, 3months and final follow up respectively , p<0.05. There was statistically significant improvement in ODI score with 42.03+/- 10.69, 46.46+/-10.40, 49.03+/-9.58 at postoperative one week, 3months and final follow up respectively , p<0.05. (Table 3)

Table 3: Clinical And Radiographic parameters of Uniportal Full Endoscopic Posterolateral Facet Sacrificed Transforaminal Lumbar Interbody Fusion (Endo-TLIF) , p value was calculated with paired t test

Endo TLIF

Mean

Std. Deviation

P value

VAS improvement at 1 weeks

-4.53

1.42

<0.001

VAS improvement at 3 months

-5.08

1.42

<0.001

VAS improvement at final follow up

-5.55

1.36

<0.001

ODI improvement at 1 weeks

-42.03

10.69

<0.001

ODI improvement at 3 months

-46.46

10.40

<0.001

ODI improvement at final follow up

-49.03

9.58

<0.001

Change in Kjaer Grade after operation at 1yr

-0.05

0.23

0.045

Change in MRI axial cut cross sectional area of right psoas muscle at mid disc level (mean , SD )

88.20

134.50

<0.001

Change in MRI axial cut cross sectional area     of left psoas muscle at mid disc level (mean , SD )

76.28

114.15

<0.001

Change in MRI axial cut cross sectional area of right paraspinal muscle at mid disc level (mean , SD )

104.83

316.45

0.006

Change in MRI axial cut cross sectional area of left paraspinal muscle at mid disc level (mean , SD )

156.97

299.24

<0.001


Comparative data showed no statistical significant difference between the MIS-TLIF and Endo-TLIF for VAS and ODI at postoperative one week, 3 months and final follow up. (Table 4). 

Radiological evaluation of minimally invasive open TLIF showed there was no significant change in Kjaer grade and bilateral psoas muscle cross sectional area. There was statically significant decrease in cross sectional area of at mid disc measure at postoperative one year for right paraspinal muscle,  -89.88+/-185.14 and left paraspinal muscle -132.74+/-237.15, p<0.05. (Table 2)

Radiological evaluation of Endo-TLIF showed there was statistically significant change in Kjaer grade , -0.05+/- 0.23, p<0.05. There was statistically significant increase in cross sectional area muscle measure at mid disc axial cut at postoperative one year, with 88.20+/- 134.50, 76.28 +/- 114.15, 104.83+/- 316.45 and 156.97+/- 299.24 for right psoas, left psoas, right paraspinal and left paraspinal muscle respectively, p<0.05 (Table 3)

Comparative data between the 2 cohorts at postoperative one year as compared to preoperative state showed Endo-TLIF statistically significant improved Kjaer grade of -0.05+/-0.18, right psoas muscle of -110.5+/-135.0, left psoas muscle of -91.8+/-122.2, right paraspinal muscle of -194.7+/-276.5 and left paraspinal muscle of 289.7+/- 278.5, p<0.05 (Table 4) There was no statistical difference in terms of Bridwell grade of fusion between Endo-TLIF and MIS-TLIF (Table 1). 

Table 4: Comparative Clinical And Radiographic parameters of endoscopic posterolateral transforaminal lumbar interbody fusion and open transforaminal lumbar interbody fusion, p value was calculated with Independent T test

Parameters

Endo  TLIF

MIS TLIF

Difference ETLIF –Open TLIF

P value

VAS improvement at 1 weeks

4.53 ± 1.42

4.55 ± 1.02

-0.02 ± 1.29

0.928

VAS improvement at 3 months

5.08 ± 1.42

5.10 ± 1.25

-0.01 ± 1.36

0.957

VAS improvement at final follow up

5.55 ± 1.36

5.29 ± 1.04

0.27 ± 1.25

0.270

ODI improvement at 1 weeks

42.03 ± 10.69

43.43 ± 7.79

-1.40 ± 9.75

0.419

ODI improvement at 3 months

46.46 ± 10.40

47.71 ± 8.57

-1.25 ± 9.78

0.508

ODI improvement at final follow up

49.03 ± 9.58

49.48 ± 8.09

-0.45 ± 9.07

0.798

Change in Kjaer Grade after operation at 1yr

-0.05 ± 0.23

0.00 ± 0.00

-0.05 ± 0.18

0.045

Change in MRI axial cut spinal canal area of right psoas muscle at mid disc level (mean , SD )

-88.20 ± 134.50

22.28 ± 135.97

-110.5 ± 135.0

<0.001

Change in MRI axial cut spinal canal area of left psoas muscle at mid disc level (mean , SD )

-76.28 ± 114.15

15.52 ± 135.30

-91.8 ± 122.2

<0.001

Change in MRI axial cut spinal canal area of right paraspinal muscle at mid disc level (mean , SD )

-104.83 ± 316.45

89.88 ± 185.14

-194.7 ± 276.5

<0.001

Change in MRI axial cut spinal canal area of left paraspinal muscle at mid disc level (mean , SD )

-156.97 ± 299.24

132.74 ± 237.15

-289.7 ± 278.5

<0.001

Discussion

Paraspinal fat infiltration and cross sectional area had been recently been investigated as one of the key parameters associated with lower back pain. 3235 High amount of fat in paraspinal muscle (> 50%) had an increased risk of high-intensity pain and disability. 32 In a systematic review, multifidus cross sectional area was negatively associated with and predictive of lower back pain up to 12months.35 Psoas muscle cross sectional area could be affected with sciatica and radicular pain on the affected side. 36 While there is still growing and inconclusive evidence regarding the effect of paraspinal muscle cross sectional and fat infiltration relationship with lower back pain, most authors agreed that a higher paraspinal muscle and lower fat infiltration tends to be associated with improved back pain and disability parameters.

There is evidence that minimally invasive decompression increased while open decompression decreased paraspinal muscle cross sectional area.3739 The authors attributed the decreased in paraspinal muscle area in open approach to resection and retraction of paraspinal muscle.

There are limited studies on the effect of fusion surgery on fat infiltration, paraspinal and psoas cross sectional area.39, 40 There is suggestion that minimally invasive tubular or miniopen Wiltse paraspinal approach TLIF has better protection of paraspinal muscle cross sectional area and improved lower back pain, ODI score and higher patient satisfaction rate compared to conventional open TLIF. However there is no study on the effect of endoscopic fusion and minimally invasive open Wiltse paraspinal approach TLIF. In our study, we found there is statistically significant improvement in change of Kjaer grade in Endo-TLIF compared to MIS-TLIF group. This suggests that in Endo-TLIF, there is a trend of decrease amount of fat infiltration in the paraspinal muscle in postoperative one year MRI scan. Similar findings are found in bilateral psoas muscle and paraspinal muscle cross sectional area in postoperative one year mid-disc axial cut MRI scan, Endo-TLIF had statistically significant improved cross sectional area compared to MIS-TLIF.

Endoscopic spine surgery is gaining traction among the spine community as a minimally invasive spine surgery technique to treat degenerative spinal conditions such as disc herniation and spinal stenosis. 6, 9, 4143. Lumbar endoscopic fusion is one of the latest technique of endoscopic spine surgery. The early results of endoscopic fusion is promising.13, 17, 18, 44. There are 3 main subtypes of endoscopic fusion: 1) uniportal transforaminal transkambin endoscopic fusion which operates within a small safe corridor of Kambin’s triangle with or without foraminoplasty using a narrow width cage in order to fit through the small safety corridor and deployed in the intervertebral disc space, this technique is performed with small diameter transforaminal endoscope.13, 44, 45 2) Biportal endoscope assisted interbody fusion which involves full ipsilateral facet resection and interbody fusion with the aid of an arthroscope. 17, 46 3) Uniportal full endoscopic posterolateral route transforaminal interbody fusion using a similar approach to biportal endoscopic interbody fusion, the facets were resected and harvested as bone graft and a large interbody cage was inserted after disc preparation with the aid of large stenosis endoscope.19, 20, 22 The advantage of endoscopic fusion with full facet resection is that large interbody cage can better stabilize the anterior column of lumbar spine as a load sharing device, this helps to promote fusion and prevents implants failure due to reversed bending movement generated if there is insufficient anterior column support. 47 In our Endo-TLIF cohort we used the technique of uniportal full endoscopic posterolateral route transforaminal interbody fusion which allowed large cages used in MIS-TLIF to be inserted in our patients who underwent Endo-TLIF.

Several etiologies of mutifidus muscle atrophy in lower back pain are discussed in literature such as disuse atrophy48, reflex inhibition49 and dorsal ramus syndrome.50 The link between sinuvertebral and basivertebral nerve and paraspinal muscle bulk as well as fat infiltration is still under investigation. There are several reports of ablation of sinuvertebral and basivertebral nerves for treatment of back pain. The radiofrequency ablation was performed through a transpedicular, extrapedicular approach or applied on the dorsal aspect of the disc and endplates to relieve of back pain and decrease paraspinal muscle spasm and subsequent contracture. 30, 5153

In anatomical and electrophysiological study, paraspinal muscle are supplied by polysegmental innervation. 54 There are studies which showed alleviation of paravertebral spasm by radiofrequency ablation of hypersensitive sinuvertebral and basivertebral nerves which can relieve inhibitory mechanism of multifidus muscles.30, 55 The dorsal ramus given out as a branch of exiting nerve root passes through the intertransverse ligament and divide into medial ramus which innervates the multifidus muscle, the intermediary ramus which innervates the longissimus muscle and lateral ramus which innervates the iliocostal muscle. While ventral ramus gives a branch to supply prevertebral flexor, psoas muscle. We illustrated the nerves supply to the psoas and paraspinal muscle in Fig. 2 and spatial relationship of the spinal nerve branches in Fig. 3. (Figs. 2 and 3)

In our technique of Endo-TLIF, the authors typically applied radiofrequency ablation on the area of sinuvertebral and basivertebral nerves prior to end plate preparation and insertion of interbody cage, but the authors did not do that when we performed MIS-TLIF. There is a plausible correlation with this practice of radiofrequency ablation and paraspinal muscle bulk increment in our series of Endo-TLIF as compared to MIS-TLIF which had decreased paraspinal muscle bulk. There was also better Kjaer grade in Endo-TLIF while there was no change in MIS-TLIF despite both surgeries were done in paramedian approaches. Other technical differences are direct docking on lamina and facet joint in Endo-TLIF as compared to dissection and retraction of paraspinal mucle through Wiltse approach in MIS-TLIF. It is interesting that although Endo-TLIF and MIS-TLIF was performed on one side, with the contralateral side incision used for pedicle screws only; both side paraspinal muscle and psoas muscle had increment in cross sectional muscle area in Endo-TLIF and decrement in MIS-TLIF, which could not be fully explained by the application of radiofrequency ablation or muscular dissection alone. More basic science and clinical studies are required to evaluate this phenomenon. (Fig. 4)

Despite the radiographic difference in paraspinal and psoas muscle bulk and Kjaer grade, there was no significant difference in VAS, ODI and successful operation (good to excellent outcome in MacNab’s score). Both Endo-TLIF and MIS-TLIF had shown to be effective operation in selected patients who required lumbar spinal fusion procedure.

Limitations

The data was obtained as a retrospective evaluation with patients who had undergone Endo-TLIF and MIS-TLIF. There could be inherent selection and performance bias in the study. Pre-operative data such as comorbidities, Charlson Morrison Index, length of operations time were not collected which might introduce confounders in the study. We limited these confounding factors by having the same team of anesthetists and surgeons for all the operations performed in the data set. The follow up were relatively short in Endo-TLIF and we continued to follow up on these patients with a view to evaluate the long term results in the future. A prospective study and randomized controlled trial would be more ideal to eliminate these bias.

Conclusion

Uniportal Endoscopic Posterolateral Lumbar Transforaminal Interbody Fusion achieved improved paraspinal and psoas muscle bulk and less fatty infiltration in the operated level as compared to Minimally Invasive Open Transforaminal Lumbar Interbody Fusion while both cohorts achieved equivalent positive clinical outcomes.

Declarations

Ethical approval and consent to participate: All procedures performed in studies involving human participants were in accordance with the ethical standards of the Nanoori Hospital’s Ethics Committee and the national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. 

Consent from patients: All patients had given their informed consent for photographs ,videos and images for publication. Informed consent was obtained from all individual participants included in the study

Consent to publish: The authors give full consent to publisher to publish the article

Availability of data and materials: Data and materials are available upon request to corresponding author

Competing interest /Conflict of interest: all co-authors have no conflict of interest.

Funding: No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. 

Authors’ contribution: Conceptualization, HSK, and PHW.; methodology, HSK, PHW,; software, HSK, PHW.; validation, HSK, JWA,MK,IL, JSP, JHL, SK, JL, YJY and ILJ.; formal analysis, HSK, PHW.; investigation, MK,IL, JSP, JHL.; resources, ITJ;  data curation, MK,IL, JSP, JHL.; writing—original draft preparation, HSK, and PHW.; writing—review and editing, HSK, and PHW.; visualization, HSK, and PHW.; supervision, HSK, and PHW.; project administration, HSK, and PHW.; funding acquisition, ITJ. All authors have read and agreed to the published version of the manuscript

Acknowledgements

Both Dr. Hyeun Sung Kim and Dr. Pang Hung Wu contribute equally to be first co-author for the paper. We would like to acknowledge scientific team members, Ms Seonghee Park and Mr. Kyeong Rae Kim for providing assistance in statistical support, acquiring full text articles and managing digital works.

Compliance with Ethical Standards Funding: No funds were received in support of this work. 

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