Ultrasound Imaging of Thoracolumbar Fascia: A Systematic Review

Over the past decade, there has been a notable increase in research focused on ultrasound imaging of thoracolumbar fascia (TLF). Nevertheless, published papers’ results about the application of US imaging in TLF examination are still sparse. Background and Objevtives: Hence, this systematic review was performed aiming to firstly investigate the use and the methodology of ultrasound imaging to assess pathologic and healthy TLF. Secondarily, we aim to assess intra- and inter-observer reproducibility of US imaging in TLF assessment. Materials and Methods: The search was done on PubMed and Web of Science database from inception to April 2024. Furthermore, the references of included papers were thoroughly checked to find eligible publications. The MeSH keywords used were: “Thoracolumbar fascia”, “Ultrasound Imaging”, “Ultrasound”, “Ultrasonography”, and “Ultrasound examination”. Results: Studies were aimed primarily at TLF diagnosis, treatment monitoring, or evaluating movement-related changes, underscoring the diverse clinical applications. The US parameters assessed included TLF thickness, echogenicity, stiffness, deformation, shear strain, and displacement, providing comprehensive insights into TLF features. Conclusions: Advanced US imaging holds promise as a reliable tool in musculoskeletal assessment, offering insights into TLF pathology/disfunction, treatment outcomes, and movement dynamics.


Introduction
Muscle-skeletal ultrasound (US) imaging is recognized as a safe, rapid, cost-effective, and widely accessible imaging technique, well-received by patients [1].It enables through and dynamic assessments of the musculoskeletal system across multiple planes, offering detailed visualization of soft tissues with exceptional anatomical precision [2].US use is increasingly embraced by physicians, with a continually expanding range of applications in physical and rehabilitation medicine, rheumatology, orthopedics, sport medicine, etc. Todorov et al. [3] highlighted the diagnostic role of US imaging in the evaluation of low back, reporting its function in the US examination of bony structure of the lumbar spine, intervertebral disc, sacroiliac joint, muscles, and soft tissues.
The lumbosacral spine assumes a fundamental role in maintaining the body's postural stability.However, the lumbar spine alone lacks the resilience to withstand the daily burdens it bears [4].Providing support to the lumbar vertebrae atop the sacral base necessitates the involvement of an intricate network of myofascial aponeurotic structures that envelop the trunk [5][6][7].At the core of this supportive framework along the posterior body wall Medicina 2024, 60, 1090 2 of 23 lies the thoracolumbar fascia (TLF), a harmonious aponeurotic fascia that envelops the paraspinal muscles of the lower back and sacral region [8][9][10][11].
An increasing amount of evidence underscored the clinical importance of the TLF, a complex structure composed of layers of dense connective tissue located in the lumbar region of the trunk [5][6][7][8][9][10][11]. Research suggests that the fascia is integral in transmitting forces between the lower limbs and the trunk, as demonstrated in both ex vivo cadaver studies [8][9][10][11] and in vivo research during walking [12,13].
However, there is uncertainty regarding whether medical practitioners can reach a consensus on the diverse morphological features observed in US images of the TLF.The architecture of the TLF is complex, with layers of dense collagenous connective tissue interspersed with loose connective tissue, facilitating gliding movement and contributing to trunk mobility.Continuously connected with the aponeuroses of major trunk muscles essential for movement and vertebral control, the TLF is believed to undergo fibrosis, densification, and thickening in response to inflammatory processes or soft tissue injuries [13][14][15].
Nevertheless, published papers results about the application of US imaging in TLF examination are still sparse.Hence, this systematic review was performed aiming to firstly investigate the use and the methodology of ultrasound imaging to assess pathologic and healthy TLF.Secondarily, we aim to assess intra-and inter-observer reproducibility of US imaging in TLF assessment.

Materials and Methods
We conducted a systematic literature review following the guidelines outlined in the Preferred reporting Items for Systemic reviews and Meta-analyses (PRISMA) [16].This systematic review protocol is registered in Open Science Framework registries with the registration https://doi.org/10.17605/OSF.IO/87RS5.The search for the literature was guided by the PICO (Problem/Patient; intervention/indicator, Comparison and Outcome) criteria detailed in Table 1.The search was done on PubMed, Web of Science, and Scopus database from inception to April 2024.Furthermore, the references of included papers were thoroughly checked to find eligible publications.The MeSH keywords used were as follows: "Thoracolumbar fascia", "Ultrasound Imaging", "Ultrasound", "Ultrasonography", "Ultrasound examination".The search strategy set for the topic was the following: ("Thoracolumbar Fascia") OR ("Thoracolumbar Fascia" AND "Ultrasound Imaging") OR ("Thoracolumbar Fascia" AND "Ultrasound") OR ("Thoracolumbar Fascia" AND "Ultrasonography") OR ("Thoracolumbar Fascia" AND "Ultrasound examination").All relevant English-language publications were examined for potential inclusion, provided they demonstrated the involvement of US imaging in diagnosing or evaluating TLF.Criteria for exclusion were applied to avoid peripheral content, thus focusing on primary research efforts while still allowing for review papers, case reports, etc.The exclusion criteria were (1) papers focused on US-guided injection; (2) papers focused on surgery; (3) papers that did not discuss the use of US imaging for thoracolumbar fascia; (4) papers not published in English.
We screened all studies by title and abstract, then the full text of eligible studies was checked for review, as well the references to identify any additional publications to be included.The literature search was carried out on by one reviewer (N.P.) and checked by one senior researcher (C.P.) with Ten years' experience in ultrasound imaging of fasciae.Any discrepancy was resolved by agreement among the authors (Figure 1).We screened all studies by title and abstract, then the full text of eligible studies was checked for review, as well the references to identify any additional publications to be included.The literature search was carried out on by one reviewer (N.P.) and checked by one senior researcher (C.P.) with Ten years' experience in ultrasound imaging of fasciae.Any discrepancy was resolved by agreement among the authors (Figure 1).

Risk of Bias
Two researchers evaluated study quality and differences were solved after discussion.The papers were scrutinized for quality using the Risk of Bias Assessment tool for RCTs.This tool includes different domains of bias: patient selection, index test, reference standard, flow, and timing.Each domain was judged as "low risk", "high risk" and "unclear".The observational studies and case-control studies were assessed using Newcastle-Ottawa Scales (NOS), respectively, for observational studies and for case-

Risk of Bias
Two researchers evaluated study quality and differences were solved after discussion.The papers were scrutinized for quality using the Risk of Bias Assessment tool for RCTs.This tool includes different domains of bias: patient selection, index test, reference standard, flow, and timing.Each domain was judged as "low risk", "high risk" and "unclear".The observational studies and case-control studies were assessed using Newcastle-Ottawa Scales (NOS), respectively, for observational studies and for case-control studies.The case-report studies were evaluated by JBI Critical Appraisal Checklist for Case Reports for case-report studies.

Results
Papers selected as regards the US imaging of TLF either in patients or healthy volunteers were analyzed.The agreement between the authors for including the articles was perfect (Cohen's k = 0.90).The main characteristics of the studies (published between 2011 and 2024) are summarized in Table 2 [3,.Overall, 243 records were screened and 152 were removed because duplicates.Six were discarded.The text of the remaining 85 potentially eligible papers was checked, 47 of which were not in accordance with our inclusion criteria.Finally, we included 38 studies in the review.Figure 1 shows the flow diagram of our study selection.

Type of Population
Overall, the 34 papers included in the current review involved 740 healthy volunteers and 540 patients with pathological conditions; 497 male (44.33%) and 574 females (55.67%) had been studied, with an average age of 33.26 ± 11 years old.Most of participants were healthy volunteers and the remaining participants presented some clinical conditions, such as low back pain, gluteal pain, low back pain in scoliosis, etc.

B-mode
The transducer is placed on the midline, along the spinous processes, in the longitudinal direction.Seated or bent forward or in the prone position with a cushion under the abdomen. -- To discuss a systematic approach to the ultrasonographic assessment of the lumbar spine.

Risk of Bias Assessment and Applicability Concern
The total number of RCT studies (k = 100%) were judged to be unclear for reference standard.More than half of studies (k = 55%) were assessed with unclear risk of selection bias (Figures 2 and 3).

Risk of Bias Assessment and Applicability Concern
The total number of RCT studies (k = 100%) were judged to be unclear for referenc standard.More than half of studies (k = 55%) were assessed with unclear risk of selectio bias (Figures 2 and 3).Risk of bias and applicability concerns summary about RCT studies: review authors' judgements about each domain for each included study [25,30,38,42,50].4 and 5.After evaluation by two researchers, the studies received an average NOS score of 3.0, indicative of low-quality studies (Tables 4 and 5).NOS scores of the included studies articles are shown in Tables 4 and 5.After evaluation by two researchers, the studies received an average NOS score of 3.0, indicative of low-quality studies (Tables 4 and 5).JBI Critical Appraisal Checklist for Case Reports studies was used to assess the quality of case reports (Table 6).

Discussion
To the author's knowledge, this is the first and only systematic review selecting studies about the application of US imaging in TLF examination.The present systematic review is currently the largest collection of work from inception to April 2024 on the use of US imaging for TLF examination, and includes not only RCT, but also observational studies, case-control studies, case series, case reports and reviews.Given the methodological imperfections, unclear aspects, and heterogeneity of the investigations considered, this systematic review could not draw definitive conclusions, but certainly identifies some interesting clinically relevant points on a topic which has been exponential growth in recent years [1].Thoracolumbar fascia role as tension-distributing structure means that any alterations in its mechanical properties can have significant repercussions [8][9][10][11].For instance, the weakening of the multifidus muscle, a common phenomenon in patients with LBP, can lead to a reduction in the tensile strength of the TLF [8].For this reason, the different studies included in this systematic review described various anatomical relationships involving the TLF and its correlations with surrounding structure.These included the fascial layers of TLF, the posterior layer specifically, the superficial layer of the deep fascia of the back, connections with lumbar multifidus, and erector spinae muscles, as well as the diaphragm, transversus abdominis, and internal oblique muscles [3,.Moreover, connections involving the fascia lata, plantar fascia, various components of the lumbar spine, and STF/SMF were assessed [3,.Furthermore, several terminologies were used to describe the deep fascial layers such as perimuscular connective tissue, paraspinal muscular compartment, etc.
Nonetheless, there remains a necessity for studies that examine the reproducibility across different systems and protocols to ascertain the reliability of TLS US measurements.In fact, only thirteen studies assessed the reliability of TLF Us measures, with intra-rater reliability consistently reported as good to excellent, and inter-rater reliability indicating excellent agreement.The possibility to compare the various data from different studies is tied to the need to use reliable protocols, as demonstrated by Yerli et al. [17], who used a published protocol [22], leading them to greater methodological rigor and enabling data comparison.Pirri et al. [22] reported that the intra-rater reliability for US TLF thickness assessement was for long axis TLF US examination (CNLBP: ICC 3,k : 0.91; HV: ICC 3,k: 0.92) and for transverse axis:(CNLBP: ICC 3,k: 0.88; HV: ICC 3,k : 0.88).Moreover, Yang et al. [25] showed an optimal inter-reliability ICC: 0.91.Finally, Wakker et al. [38] and Chen et al. [41] also reported an optimal inter-rater reliability in the assessment of TLF stiffness by shearwave elastography, respectively ICC: 0.93 and ICC: 0.90-0.94.Several limitations of the referenced studies should be noted.They are related to the study design and the lack of high-quality trials, as 100% of the identified RCT studies were judged to be at unclear for reference standard and more than half of studies (k = 55%) were assessed with unclear risk of selection bias.As mentioned above, most of them were carried out with different protocols.However, terminologies gap between studies seems to be a relevant limitation and makes translation of result difficult.This extreme heterogeneity leads to difficulty in carrying out a quantitative analysis and negatively affect the level of evidence.Finally, in the future, more high-quality clinical trials should be developed to ensure the viability of the proposed parameters, and to apply the assessment in the daily practice, standardizing these important technical issues.

Conclusions
In the past decade, there has been a notable increase in research focused on US imaging of TLF.The comprehensive analysis sheds light on the diverse applications and methodologies employes in TLF US research.This systematic review elucidates the utility of advanced ultrasound techniques in investigating TLF characteristics and functions.Studies aimed primarily at TLF diagnosis, treatment monitoring, or evaluating movementrelated changes, underscoring the diverse clinical applications.Advanced US imaging holds promise as a reliable tool in musculoskeletal assessment, offering insights into TLF pathology/disfunction, treatment outcomes, and movement dynamics.

Figure 1 .
Figure 1.Flow chart of study selection.

2. 1 .
Data Extraction Data concerning these parameters were collected and analyzed: a.General characteristic of the paper: first author, year of publication, study design.b.Study population characteristics.Number of patients or healthy volunteers, age, gender, and TLF status (normal vs abnormal).c.Measurements methods: type of probe, type of US imaging, positions of patients or healthy volunteer.d.Reliability.e. Outcomes: evaluated parameters.

Figure 1 .
Figure 1.Flow chart of study selection.

2. 1 .
Data Extraction Data concerning these parameters were collected and analyzed: a.General characteristic of the paper: first author, year of publication, study design.b.Study population characteristics.Number of patients or healthy volunteers, age, gender, and TLF status (normal vs abnormal).c.Measurements methods: type of probe, type of US imaging, positions of patients or healthy volunteer.d.Reliability.e. Outcomes: evaluated parameters.

Figure 3 .
Figure 3. Risk of bias and applicability concerns graph about RCT studies: review authors' judgements about each domain presented as percentages across included studies.NOS scores of the included studies articles are shown in Tables4 and 5.After evaluation by two researchers, the studies received an average NOS score of 3.0, indicative of low-quality studies (Tables4 and 5).

Figure 3 .
Figure 3. Risk of bias and applicability concerns graph about RCT studies: review authors' judgements about each domain presented as percentages across included studies.

Table 2 .
Papers on ultrasound imaging of thoracolumbar fascia.

Table 3 .
General characteristics of the 37 papers included in our analysis.

Table 4 .
Study of quality assessment using Newcastle-Ottawa scale for observational studies.Each asterisk represents if individual criterion within the subsction was fullfilled.

Table 4 .
Study of quality assessment using Newcastle-Ottawa scale for observational studies.Each asterisk represents if individual criterion within the subsction was fullfilled.

Table 5 .
Study of quality assessment using Newcastle-Ottawa scale for case-control studies.Each asterisk represents if individual criterion within the subsction was fullfilled.

Table 6 .
Study of quality assessment using JBI Critical Appraisal Checklist for Case Reports studies.