Spatial distribution patterns of fat infiltration (FI) in the paraspinal muscles (PSMs) is a potential diagnostic biomarker that may also provide granular mechanistic insights into spine biomechanics related to chronic low back pain (cLBP), as well as advancing the use of prior summary measures limited to overall muscle FI. These are the key findings of a recent study, published in the European Spine Journal by Jeannie Bailey (University of California, San Francisco, USA) et al.
FI of the PSMs measured using magnetic resonance imaging (MRI) is an aspect of muscle quality and is considered to be worse in cLBP patients. However, the researchers note that “there is not a clear association between paraspinal muscle FI and cLBP, leaving the clinical importance of paraspinal muscle composition unestablished”.
The study authors hypothesised that paraspinal muscle fat-mapping would reveal distinct FI distribution patterns in relation to cLBP symptoms and proximity to symptomatic intervertebral disc (IVD) degeneration.
From advanced-sequence water-fat MRI of 40 axial cLBP patients and 21 controls, the research team examined the spatial distribution of paraspinal muscle FI in relation to the centre of rotation at the L4/L5 disc.
Patients with more than three months of low back pain (Visual Analogue Scale (VAS) equal to or greater than four, or an Oswestry Disability Index score of 30 or more) and between ages 18 and 70 were included. Exclusion criteria for enrolment included pregnancy, diabetes, smoking, cancer, spondylolisthesis, scoliosis, prior lumbar surgery, disc herniation, compression fractures, taking osteoporosis medication. Controls reported no prior history of back pain (VAS ≤ 1) or known spinal conditions. VAS scores were collected once, at baseline, directly prior to the subject’s MRI visit.
Using statistical parametric mapping, FI patterns for multifidus (MF), erector spinae (ES), and psoas between patients and controls, and to the presence and severity of adjacent degenerative IVD pathology, were compared.
The study results indicated that spatial distribution patterns of FI in the PSMs differ with respect to symptoms and adjacent degenerative IVD pathology; and that that the deep region of the multifidus has the highest amount of FI in relation to symptomatic IVD degeneration.
It was also shown that spatial distribution patterns of FI in the PSMs provide higher granularity and thus serve as a better diagnostic imaging-based biomarker compared to conventional summary measures for overall muscle FI percentage.
Based on these results, the researchers theorise that the presence of symptomatic degenerative disc pathology “results in arthrogenic inhibition and a selective shutting off of the local stabilising muscles, notably the deep multifidus, as a protection mechanism, resulting in disuse and eventually, elevated levels of fat infiltration localised to the shorter, deeper fascicles of the multifidus”. They add that future studies should look to collect longitudinal data to further explore causation.