Fat in the lumbar multifidus muscles - predictive value and change following disc prosthesis surgery and multidisciplinary rehabilitation in patients with chronic low back pain and degenerative disc: 2-year follow-up of a randomized trial

Abstract

Background: Evidence is lacking on whether fat infiltration in the multifidus muscles affects outcomes after total disc replacement (TDR) surgery and if it develops after surgery. The aims of this study were 1) to investigate whether pre-treatment multifidus muscle fat infiltration predicts outcome 2 years after treatment with TDR surgery or multidisciplinary rehabilitation, and 2) to compare changes in multifidus muscle fat infiltration from pre-treatment to 2-year follow-up between the two treatment groups.

Methods: The study is secondary analysis of data from a trial with 2-year follow-up of patients with chronic low back pain (LBP) and degenerative disc randomized to TDR surgery or multidisciplinary rehabilitation. We analyzed (aim 1) patients with both magnetic resonance imaging (MRI) at pre-treatment and valid data on outcome measures at 2-year follow-up (predictor analysis), and (aim 2) patients with MRI at both pre-treatment and 2-year follow-up. Outcome measures were visual analogue scale (VAS) for LBP, Oswestry Disability Index (ODI), work status and muscle fat infiltration on MRI. Patients with pre-treatment MRI and 2-year outcome data on VAS for LBP (n = 144), ODI (n = 147), and work status (n = 137) were analyzed for prediction purposes. At 2-year follow-up, 126 patients had another MRI scan, and change in muscle fat infiltration was compared between the two treatment groups. Three radiologists visually quantified multifidus muscle fat in the three lower lumbar levels on MRI as <20% (grade 0), 20-50% (grade 1), or >50% (grade 2) of the muscle cross-section containing fat. Regression analysis and a mid-P exact test were carried out.

Results: Grade 0 pre-treatment multifidus muscle fat predicted better clinical results at 2-year follow-up after TDR surgery (all outcomes) but not after rehabilitation. At 2-year follow-up, increased fat infiltration was more common in the surgery group (intention-to-treat p = 0.03, per protocol p = 0.08) where it was related to worse pain and ODI.

Conclusions: Patients with less fat infiltration of multifidus muscles before TDR surgery had better outcomes at 2-year follow-up, but findings also indicated a negative influence of TDR surgery on back muscle morphology in some patients. The rehabilitation group maintained their muscular morphology and were unaffected by pre-treatment multifidus muscle fat.

Trial registration: NCT 00394732 (retrospectively registered October 31, 2006).

Trial registration: ClinicalTrials.gov NCT00394732.

Keywords: Change over time; Chronic degenerative low back pain; Multidisciplinary rehabilitation; Multifidus muscle fat; Physiotherapy; Predictive value; Surgery; Total disc replacement.

Fig. 1

CONSORT flow diagram. * Heart attack some days after randomization (n = 1), obvious exclusion criterion discovered some days after randomization (earlier large abdominal operation (n = 1)), degenerative change insufficient to satisfy inclusion criteria (n = 2) or present in more than two lower lumbar discs (n = 2)). # Changed their mind and declined surgery after randomization (3 had social reasons for not receiving treatment, 1 had work related economic reasons, and 5 wanted guaranteed success). & Changed their mind after randomization and did not attend the rehabilitation program (2 had work-related economic reasons, 1 was treated elsewhere with surgery for lumbar disc herniation, 1 had social reasons, and 2 needed to travel long distances/could not stay away from home). % Dropped out after total disc replacement (TDR) surgery (1 had serious complications with a vascular injury and leg amputation, 2 did not want to attend the follow-up and 1 could not be contacted after surgery). £ 6 patients dropped out during the rehabilitation program (1 did not find the program good enough, 1 had lumbar disc herniation during treatment and underwent microdiscectomi, 1 did not manage to go through the training program, 1 developed diabetes during or just before treatment, 1 had psychosocial reasons, and 1 had hypertension and the family doctor did not recommend training), 8 dropped out after completing the treatment (1 took part in another study, 1 patient did not complete the questionnaire, 1 patient moved, 1 patient died of cancer, 3 did not want to attend the follow-up, and there was 1 for whom the reason was unknown). $ Two patients underwent surgery with instrumented fusion before 2-year follow-up. ** One patient crossed over to surgery between 6 months and 1 year and five patients between 1 year and 2 years. Five patients underwent TDR surgery and one patient fusion. § Subjects relevant for analysis were patients with both a pre-treatment MRI and valid score for back pain, Oswestry Disability Index (ODI) score and data on work status at 2-year follow-up. Patients randomized to rehabilitation who crossed over and underwent TDR surgery before 2-year follow-up within (n = 5) or outside (n = 5) the study setting are analyzed in the surgery group, patients who refused TDR surgery and underwent rehabilitation were analyzed in the rehabilitation group (n = 2), according to as-treated principles. μ Refused surgery (n = 7), re-operated upon with a fusion (n = 2). ≠ Did not start the rehabilitation program (n = 7), received a primary fusion (n = 1). ¥ Randomized design (RCT) includes patients with MRI at both pre-treatment and 2-year follow-up. β Re-operated upon with a fusion. ∞ Crossed over to surgery (n = 5 to TDR and n = 1 to fusion), did not complete the rehabilitation program (n = 1)

Fig. 2

Grading of fat in the multifidus muscles on magnetic resonance imaging. Multifidus muscles (right, arrowheads) on axial T2-weighted images located as marked on sagittal T2-weighted images (left, lines) contain fat grade 0 at L5/S1 in one patient (a) and grade 1 at L4/L5 (b) and grade 2 at L5/S1 (c) in a different patient, whose disc prosthesis causes artefacts (arrows) that do not affect the grading. Grade 0: 0 or < 20% of total muscle cross-section (left plus right side) contains fat; grade 1: 20–50% of cross-section contains fat; grade 2: >50% of cross-section contains fat