Sternotomy scars, which are localized in a high-tension area are often dystrophic, hypertrophic or keloidal, and can be difficult to accept to patients who already endured a heavy intervention with high psychologic impact. The investigators wish to evaluate immediate post-surgical injection of botulinum toxin to improve scar aspect and impact. A randomized, placebo-controlled, split-scar clinical trial will determine the improvement of scar aspects assessed by standardized scores (SBSES, POSAS), patient satisfaction and tolerance.

Adult wound healing does not restore the original skin architecture, and results in fibrotic scars. Specifically in high-tension areas and due to activation of fibrogenesis pathways, wound healing processes can lead to pathological scarring, hypertrophic scars, or keloids. The anterior thoracic region is one the body area suggested to the highest tensions, and sternotomies are performed perpendicularly to the lines of skin tension. Therefore, sternotomy scars are often dystrophic, inaesthetic and fibrous, or even hypertrophic. Depending on geographical distribution, sternotomy scars are found hypertrophic in 10 % to more than 50 % of cases. Aesthetic impact or related symptoms such as pruritus and pain impair psychological state and alter quality of life of these patients who endured a frequently difficult to handle surgical procedure.

Botulinum toxin type A is a neurotoxin derived from Clostridium botulinum culture. Through inhibition of pre-synaptic acetylcholine release, it induces local muscle paralysis that is reversible in a few months. Its interest is widely described in urology and neurology. Its dermatologic uses include the cosmetic purpose to reduce face wrinkles and the functional purpose to reduce hyperhidrosis. Its safety is well established. It has been proposed in the prevention and treatment of hypertrophic scars and keloids, and has been shown to reduce TGFß1, a main actor of fibrogenesis and pathological scarring. Through reduction of myofibroblastic differentiation, it downregulates mechanisms leading to dystrophic scars.

It was proposed in the improve scar aspect after surgical procedures, and early post-operative local injections showed efficacy in thyroidectomies, mammoplasties, or face surgeries to enhance scar aspect. Regarding sternotomy, a single study in China with a small sample (17 patients) studied post-operative injections (8 to 14 days after) of botulinum toxin. This split-scar study reported significant improvement of Vancouver Scar Scale, scar width and patient satisfaction in the treated site, at 6 months.

The authors aim to evaluate immediate post-operative botulinum toxin after sternotomies, to improve scar aspect through standardized scores (Stony Brook Scar Evaluation Scale, Patient and Observer Scar Assessment Scale) and impact through patient satisfaction at 6 and 18 months. The double-blind randomized placebo-controlled trial will be conducted in split-scar method, with administration for each patient of treatment (botulinum toxin) or placebo (saline) in the upper or lower part of scar, according to randomization.

Each patient will serve as its own control and will be evaluated for up to 18 months by a dermatological team, following standardized monitoring, including objective and subjective scores, as well as tolerance data.

Inclusion Criteria:

• Patient aged 18 years or older
• Scheduled sternotomy at Bordeaux University Hospital within the next 4 months
• Person affiliated with or benefiting from a social security system
• Patient able to attend follow-up visits required by the study
• Free, informed, and written consent signed by the participant or an impartial witness (in case the participant is unable to write) and the investigator (no later than the day of inclusion)

Exclusion Criteria:

• Known allergy to botulinum toxin type A or any excipient in BOTOX® (human albumin, sodium chloride)
• Neuromuscular transmission disorder (myasthenia gravis or Lambert-Eaton syndrome)
• Motor peripheral neuropathy (such as amyotrophic lateral sclerosis or motor neuropathy)
• Infection at the injection sites
• Pregnancy or breastfeeding
• Person under protection measures

The data is available from Dr ZAGALA. An email can be sent to her so that she can contact the data manager and owner, the Bordeaux University Hospital. The data can be made available after contractual agreement.

Only the Methodology Center and the Bordeaux University Hospital Pharmacy will have access to the correspondence list between the patient's number, the type of product and the area treated (lower or upper part of the scar). At no time during the study will investigators or patients have access to this list.