Studying the therapeutic effect of paravertebral injections of BoNT-A (botulinum toxin A) requires further studies to confirm the reported short-term therapeutic effect and to determine potential predictive factors of efficacy.

This study was a randomized, double-blinded, placebo-controlled phase 3 trial comparing BoNT-A Type A injections to a placebo in patients with chronic Low-back Pain (LBP). This superiority trial obtained support from the French Hospital Clinical Research Project (PHRC) and the approval of a French ethics committee (2003/02) ("Comité Sud-Ouest et Outre-Mer III", consent obtained in February 2003).

The number of participants included in the study was chosen to be similar as those included in previous studies, that showed a strong positive effect of BoNT-A injections on LBP. Furthermore, the design of our study (i.e. a crossover) increased the power of the statistical analysis. A scientific committee has been solicited at this step, after including 19 patients, to assess the results.

Inclusion Criteria:

• LBP defined as a pain located between the thoracic lumbar hinge and the gluteal sulcus, where pain had evolved over a period of 6 months despite well conducted medical treatment, self-assessed lumbar pain intensity over 50 millimeters long on a visual analogue scale of 100 millimeters (0=no pain; 100=maximal pain),
• having been on sick leave for 60 or more days in the year preceding the inclusion (in order to include patients with high consequences of chronic low-back pain on their work),
• same long-term chronic pain treatment for at least 6 weeks

Exclusion Criteria:

• age under 18 or over 55 years (to avoid secondary causes of low back pain, like spinal tumor),
• ongoing pregnancy or breast-feeding,
• a neuromuscular pathology (myasthenia gravis, amyotrophic lateral sclerosis, myopathy, polymyositis), aminoglycoside treatment at the time of inclusion,
• skin infection at injection points,
• diabetes and alcoholism (in order to avoid other etiologies of chronic pain),
• a history of injecting BoNT-A A,
• anticoagulation treatment, sciatica,
• suspected spinal inflammatory disorder (spondylitis, inflammatory rheumatism, tumoral pathology),
• a failed back surgery syndrome (when surgery failed to relieve low-back pain), - incapacity to stand, cardiorespiratory deficiency which does not allow the isokinetic exploration of the spinal muscles,
• cognitive disorders limiting patient participation,
• conflicts of interest owing to existing pain (unconsolidated work accident, ongoing damage compensation).
• Spine infection, tumour or trauma had been specifically excluded by an MRI done by all patients before the inclusion in the present study.
• No patient was allowed to take opiates during the time of the study,
• facet joint injections were also not permitted during the study period.