Assessment of the psoas muscle changes following the oblique lateral interbody fusion (OLIF) approach: A prospective observational study

Mantu Jain; Auroshish Sahoo; Suprava Naik; Pankaj Kumar; Dipun Mishra

doi:10.1016/j.jor.2022.07.007

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33 (); 60-65

doi:

10.1016/j.jor.2022.07.007

Assessment of the psoas muscle changes following the oblique lateral interbody fusion (OLIF) approach: A prospective observational study

Mantu Jain^a,, Auroshish Sahoo^a, Suprava Naik^b, Pankaj Kumar^c, Dipun Mishra^a

a Department of Orthopedics, AIIMS Bhubaneswar, Odisha, 751019, India

b Department of Radiology, AIIMS Bhubaneswar, Odisha, 751019, India

c Department of Surgery, AIIMS Bhubaneswar, Odisha, 751019, India

∗Corresponding author: Mantu Jain. montu_jn@yahoo.com

Received: 2022-5-25, Accepted: 2022-7-11, Published: 2022-7-14

Professor P K Surendran Memorial Education Foundation

Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Traditional open posterior fusion techniques involve stripping of the posterior paraspinal muscles with protracted retraction, which can lead to ischemia and denervation. This may result in poor clinical outcomes despite a well-performed surgery. Oblique lumbar interbody fusion (OLIF) is a minimally invasive modified retroperitoneal anterior approach in the corridor between the psoas major (PM) and great vessels. The purpose of this research was to study changes in psoas properties and clinical outcomes in terms of the loss of hip flexion in patients undergoing OLIF surgery.

Patients with lumbar pathologies who underwent instrumented OLIF at our center were included. These patients were examined clinically at baseline and 6 months postoperatively. The Oswestry disability index (ODI) and visual analog scale (VAS) scores for back pain and leg pain were noted. Magnetic resonance imaging (MRI) scans were obtained preoperatively and postoperatively (minimum 6 months) to compare the cross-sectional areas (CSAs) of the PM.

In total, 17 patients (male: female = 8:9) with a mean age of 46.06 ± 10.49 years were included. The operation time was 154.94 ± 32.33 min, estimated blood loss was 190 ± 56 mL, and mean CSAs of the right and left psoas were, respectively, 9.94 ± 3.19 and 10.65 ± 3.74 cm² preoperatively and 10.00 ± 3.06 and 8.53 ± 2.81 cm² at follow-up. Qualitative measurements revealed that muscle atrophy on the left side (approach side) was 19.12% ± 3.14% and fatty degeneration had occurred in 13 of 17 (76.5%) patients. Postoperatively, all patients had significant improvement in the VAS (back and leg) and ODI scores. Furthermore, 3 of 17 (17.6%) patients had mild hip weakness (4/5) on the left side immediately postoperatively, but it resolved in 6 months. Moreover, 2 of 17 (11.8%) patients complained of paraesthesia.

OLIF does cause injury to the PM but it is clinically insignificant according to MRI scans and clinical evaluations.

Keywords

Psoas muscles

Abnormalities

OLIF

Degeneration

Atrophy

Spondylolisthesis

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1 Introduction

Lumbar spinal fusion surgery is a widely employed procedure for several types of spinal ailments. Conventionally, this fusion is achieved using the posterior open approach through the posterior lumbar interbody fusion or transforaminal lumbar interbody fusion methods. Studies have shown that the elevation of paraspinal muscles and continuous wide retraction result in ischemia, denervation, and malfunction, which cause chronic pain and inferior postoperative clinical outcomes.^1–3 This iatrogenic muscle injury has become a major concern for practicing spine surgeons, particularly because of the reduction in trunk muscle strength and the late onset of spinal instability, causing failed back syndrome.⁴^,⁵ Radiologically, magnetic resonance imaging (MRI) has shown muscle degeneration, atrophy, and fatty tissue accumulation, particularly in the multifidus.⁶^,⁷ Alarmingly, approximately 30% of the patients exhibited worsened backache compared with their preoperative status.⁶

In recent years, oblique lumbar interbody fusion (OLIF) is gaining popularity among spine surgeons as it is a minimally invasive surgical (MIS) procedure to approach the anterolateral part of the intervertebral disc between the psoas major (PM) and great vessels through the oblique lateral corridor.^8–10 This can be combined with a percutaneously placed posterior pedicle screw or an open screw through the Wiltse approach. The Wiltse approach reduces the injury to the posterior paravertebral muscles, reducing the incidence of chronic postoperative low back pain (LBP).¹¹

Studies concerning the physical properties of the PM and clinical symptoms arising due to them following OLIF are scanty¹² and lacking in the Indian population. We, therefore, studied the changes in PM properties and clinical outcomes in terms of the loss of hip flexion in patients undergoing OLIF surgery.

2 Methods

2.1

2.1 Patient population

The study was approved by our institutional ethics committee (IEC/T/IM-NF/Ortho/21/166). Written informed consent was obtained from all our patients. From January to December 2021, 17 consecutive patients who underwent instrumented OLIF surgery were prospectively enrolled. All surgeries were conducted by one team of surgeons (MJ, PK, and AS) at our institution. The patient's chief complaints were LBP, neurogenic claudication, and differing radiating pain. All patients had received a prior failed conservative treatment of minimum 6 months. The principal indications for surgery were spinal stenosis with collapsed disc space or low-grade spondylolisthesis (grades I and II) confirmed through standing anteroposterior, lateral, and dynamic (flexion-extension) radiographs.

2.2

2.2 Surgical technique

Under general anesthesia, the patients were positioned in a right lateral position with the left side on the top. The target vertebral levels were identified through fluoroscopy. An oblique skin incision of approximately 5 cm was made, and subsequently, the retroperitoneal space was opened through splitting muscles with blunt finger dissection. Fingers were moved anteriorly to the psoas muscle, retracting the peritoneum and great vessels anteriorly and docking on the disc space. The disc space was verified through fluoroscopy and visualized using a tubular retractor system. Discectomy was completed, and vertebral endplates that were prepared for an intervertebral cage filled with autograft (obtained from the adjacent iliac crest) were inserted. The patient was turned to a prone position, and pedicle screws were inserted using the open posterior approach in the initial cases and using the Wiltse approach in the later cases.¹³

2.3

2.3 Clinical assessment

All clinical and operative variables were recorded. The operative variables consisted of the duration of operation time (in min) and intraoperative blood loss (in mL). Functional assessment was performed using the visual analog scale (VAS) score for back pain and leg pain along with Oswestry disability index (ODI) evaluation. Any weakness in hip flexion on the approached side was evaluated at the final follow-up along with any hypoesthesia over the thigh.

2.4

2.4 Evaluation of back muscle injury

An MRI was executed on a 1.5 T System (Siemens Magnetom Aera) with the patients in the supine position with knees relaxed over a pillow. Sagittal and axial T2 weighted image (T2WI) and sagittal and axial T1 weighted image (T1WI) MRI of the lumbosacral spine were acquired for all the patients by using the following parameters: sagittal T2WI: TR-3800, TE-99, FOV-320, X-100, and NEX-3; axial T2WI: TR-3800, TE-99, FOV-200, X-75, and NEX-4; sagittal T1WI: TR-580, TE-9, FOV-320, X-100, and NEX-4; and axial T1WI: TR-6000, TE-10, FOV-200, X-75, and NEX-3. The slice thickness was 3 mm, and interslice gap was 0.3 mm. WARP sequences were obtained from postoperative patients to reduce metallic artifacts from implants. Axial sections were obtained in slabs at the intervertebral disc level in alignment with the disc. The measurement of the cross-sectional area (CSA) of the PM was performed at the disc level preoperatively and 6 months postoperatively of the OLIF surgery.

A qualified musculoskeletal radiologist (SN) measured all the parameters. Anatomic markers and locating lines on the sagittal plane scans were used to handpick the most similar preoperative and 6-month follow-up axial images, at the same spinal level, for comparison. Measurements were obtained in a Syngo Via workstation. The area of the PM was obtained through manual placement of the freehand region of interest along the margin of the PM at the postoperative level. The signal intensity of the PM of the left side (operated side) was also compared qualitatively with that of the right side on both axial T1WI and T2WI for any fatty degeneration.

For a single-level surgery, the reading obtained was the final value. However, for a two-level surgery, the readings were obtained at all the operated levels, and the mean of all values was considered as the final reading. The percentage change in the volume reflected atrophy. All data were entered in an MS Excel sheet.

2.5

2.5 Statistical assessments

The data were assessed using SPSS version 20 software (Microsoft, Chicago, USA). The outcome data were checked for normality. Descriptive statistics was used for analyzing demographic data. All categorical variables are expressed as percentages, and numerical variables that were parametric are expressed as the mean ± standard deviation. The paired t-test was used for comparing the variables of the cohort before and after the surgery. A p value of <0.05 was regarded statistically significant.

3 Results

3.1

3.1 Patient demographics

Nine women and eight men with an average age of 46.06 ± 10.49 years (range, 28–65 years) were included in the study (Table 1). The operative pathologies were lumbar spondylolisthesis and lumbar spinal stenosis in 14 and 3 of 17 patients, respectively. We conducted 14 single-level surgeries (13 at the L4/5 level and 1 at the L3/4 level) and 3 two-level fusion surgeries (at L3/4–L4/5 levels). We used the left-side approach for the cage placement of all patients, and screws were placed through the traditional open posterior approach (n = 5) or Wiltse approach (n = 12). For surgical findings, the operation time was 154.94 ± 32.33 min, and the estimated blood loss was 190 ± 56 mL.

Table 1 Demographic data of the patient.

Variable	Mean ± SD	Range
No of Patients	17
Age in years	46.06 ± 10.49	28–65
Gender	8/9 (M/F)
Preoperative Diagnosis
Lumbar canal stenosis	3
Spondylolisthesis	14
Height	164.1 ± 6.9 cm	152–174 cm
Weight	68.3 ± 8.3 kg	58–94 kg
BMI	25.6 ± 3.1 kg/m²	21–28 kg/m²
Level of Fusion	14/3 (Single/Double)
L3/4	4 (1/3)
L4/5	16 (13/3)
Follow-up in months	9 ± 0.25	6–12

3.2

3.2 Muscle volume of the PM

The mean CSAs of the right and left psoas were 9.94 ± 3.19 and 10.65 ± 3.74 cm² immediately after surgery and 10.00 ± 3.06 and 8.53 ± 2.81 cm² at 6 months after surgery (approach side; p < 0.01), respectively. However, the contralateral side showed no difference (Table 2). Muscle atrophy of approximately 19.12% ± 3.14% of muscle volume was observed on the left side. Fatty degeneration of the left-sided PM was observed in 13 of 17 (76.5%) patients. No correlation was observed between changes in the PM and surgery duration (Pearson correlation value = 0.168; p value = 0.519).

Table 2 Changes in the clinico-radiological parameters following OLIF surgery.

S·NO	Variables	Preoperative findings mean ± SD	Postoperative findings mean ± SD	Change (preoperative -postoperative) mean ± SD	P value
1.	ODI	66.94 ± 6.75	32.35 ± 7.91	34.59 ± 11.50	<0.01
2.	VAS (Back)	7.88 ± 0.78	3.24 ± 0.56	4.65 ± 0.86	<0.01
3.	VAS (Leg)	7.71 ± 1.40	3.00 ± 0.79	4.71 ± 1.31	<0.01
4.	Psoas (Right) in cm²	9.94 ± 3.19	10.00 ± 3.06	−0.06 ± 0.56	0.67
5.	Psoas (Left) in cm²	10.65 ± 3.74	8.53 ± 2.81	2.12 ± 1.69	<0.01

3.3

3.3 Clinical outcome

Postoperatively, all patients demonstrated an improvement in the VAS scores for back and leg. Similarly, the ODI scores had improved (p < 0.01; Table 2). Furthermore, 3 of 17 (17.6%) patients had mild hip weakness (4/5) on the left side immediately after the operation, but it resolved within 6 months at the final follow-up. Moreover, 2 of 17 (11.8%) patients complained of paraesthesia. A few illustrations are presented in Figs. 1–5.

T2 weighted axial cut MRI of a patient showing preoperative and postoperative Psoas muscle outlined.

T2 weighted axial cut MRI of a patient showing atrophy of left Psoas muscle.

A patient with double level OLIF showing T2 weighted axial cut MRI Psoas muscle atrophy.

Another patient showing atrophy of left Psoas muscle following OLIF surgery.

T2 weighted axial cut MRI of a patient showing changes in left Psoas morphology.

4 Discussion

We found significant postoperative changes in the PM muscle in terms of its CSA on the approaching side at 6 months after surgery. Furthermore, fatty degeneration was observed in the psoas muscle, although no major clinical implications were indicated.

Approach-related morbidity in lumbar spine fusion, such as iatrogenic paraspinal muscle injury, is a major concern for surgeons performing lumbar spine fusion by using the traditional open posterior approach.^1–3 This has led to advancements such as lumbar spinous process-splitting laminectomy, the Wiltse approach of pedicle screw placement, and several MIS techniques.^14–16 OLIF is a mini-open anterior retroperitoneal lumbar interbody fusion method established to overcome the shortcomings of the extreme lateral interbody fusion (XLIF) or direct lateral interbody fusion.¹⁷ This approach is rapidly gaining popularity because of the retained advantages of lateral approaches and nonrequirement of neuromonitoring. However, despite several advantages, the complications cannot be ignored. Changes in the physical properties of psoas have been extensively studied exclusively by Inoue et al.¹²

Conventionally, an open posterior spinal fusion surgery demands continuous retraction of paraspinal muscles, which reduces capillary perfusion and possible avascular changes within the muscle.¹⁴ This effect is profound if the retraction time exceeds 2 h.¹⁸ Kawaguchi et al. determined that damage to the back muscle is related to exposure, retraction time, and pressure on the paravertebral muscles.¹⁹ Paraspinal muscle dysfunction is caused by degeneration in the postoperative period, which plays a vital role in the progression to failed back syndrome. In OLIF surgery, tubular retractors are placed anterolaterally between the PM and peritoneum to expose and curette out the disc and fix a cage. In contract to open surgery where retraction of paraspinal muscles is prolonged, the retraction of PM in our procedure (duration and force) was significantly less. Therefore, we had a transient clinical weakness of hip flexor (17.6% cases), but, radiologically, due to affection of microcirculation of the PM, there was some atrophy noted to some extent.

Inoue et al. noted an increase in the CSA of the affected psoas after 1 week of OLIF surgery.¹² The authors used computed tomography (CT)-based software and subjected patients to CT at 1 week, 3 months, and 6 months. They observed an increase in CSA of the PM at 1 week, which they attributed to edema or hematoma occurring due to surgical manipulation. However, at the end of 1 year postoperatively, the authors did not observe any degeneration of the PM in MRI images. Conversely, we observed both atrophy and degeneration (76.5%) at 6-month follow-up in our patients. Fatty infiltration of the lumbar multifidus muscles in posterior surgeries is associated clinically with LBP.²⁰ The infiltration is greater in the open techniques than in minimally invasive posterior techniques.²¹

Clinically, we evaluated psoas muscle injury based on ipsilateral side hip flexion weakness and any associated sensory loss over the thigh caused by injury to the lumbar plexus traversing through the PM. Tohmeh et al. reported a transient postoperative iliopsoas weakness in 27.5% of the patients and a sensory loss in the upper medial thigh in 17.6% of the patients following an XLIF fusion.²² Furthermore, we noted transient hip flexor weakness and paraesthesia in 17.6% and 11.8% of the patients, respectively. Although OLIF does not directly harm the psoas, a strong posterior retraction of the psoas can indirectly cause temporary dysfunction. Similarly, Kim et al. and Abe et al. reported transient postoperative PM weakness or numbness on the approach side in OLIF surgery.²³^,²⁴

The strength of our study is the use of MRI for the evaluation of muscle injury. MRI is a well-established procedure to identify the volume and quality of muscles.²¹ However, its only limitation is the additional time and effort required to assess muscle quality, such as fatty infiltration. This study also has some limitations. Hip flexor muscles consist of iliopsoas, pectineus, rectus femoris and adductor longus. We evaluated hip flexion weakness as a measure of PM damage, but the muscle action could not be isolated. Second, a histopathological evaluation of the PM was not performed, and we relied only on MRI for analyzing muscle degeneration. Third, in a few of our initial patients, pedicle screws were placed using the open posterior approach and later by using the Wiltse approach. No further subgroup analysis was performed in these patients because our primary focus was on changes in the psoas muscle. However, the operative time, blood loss, and functional score at 6 months (ODI) may have been affected because of the choice of the screw placement method. Furthermore, our sample size was small and the follow-up period was short. Therefore, multicentric studies with large sample sizes are warranted to validate our findings.

5 Conclusions

OLIF does cause injury to the PM but it is clinically insignificant according to MRI scans and clinical evaluations.

Institutional ethical committee approval

IEC/T/IM-NF/Ortho/21/166.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Authors contribution

MJ conceived the idea with AS and got ethical clearance for the study. MJ, PK, AS were involved in sample collection. AS and DM followed up the cases and compiled the data. SN did the radiology assessment. MJ and DM wrote up the manuscript where other authors provided the critical inputs. All authors have read and agree to content of the manuscript.

Informed consent of patient

Taken.

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