Patients planned for shoulder surgery will be identified and contacted for participation. After informed consent is provided, patients will be randomised into one of three groups: pre-operative cutaneous disinfection with iodine (control group), pre-operative cutaneous and subcutaneous disinfection with iodine (double prevention), and pre-operative cutaneous and subcutaneous disinfection with iodine combined with a skin preparation protocol with benzoyl peroxide gel in the days prior to surgery (triple prevention). Bacterial cultures will be collected during surgery of the surgical field and tissue layers. Culture positivity for Cutibacterium Acnes will be compared between the groups as a primary outcome.

Eligible patients will be contacted for participation (see Eligibility Criteria). If subjects meet the inclusion criteria, patients will receive information regarding the study outline, associated risks, and subject rights. Informed consent must be freely given, specific, unambiguous, and may be withdrawn at any time. Subjects must be legally capable of providing consent. Consent will be and recorded in writing and stored for the duration of the study. The target for inclusion was set at 52 patients in each group, of each group 26 will be arthroscopic and 26 open surgery.

Patients will be divided into three groups of equal size:

• Triple-prevention: Benzoyl peroxide at home, subcutaneous iodine, and cutaneous iodine.
• Double-prevention: Cutaneous and subcutaneous iodine.
• Single-prevention (control group): cutaneous iodine.

Simple block randomisation will be used to prevent chronological bias and ensure equal group size. Treatments will be allocated with a 1:1:1 ratio using a block size of 12. Randomisation will be stratified by open or arthroscopic surgery to ensure an equal number in all three groups (3x2x26 patients) Random treatment allocation numbers will be generated and blinded, to be revealed after inclusion.

Benzoyl peroxide:

Patients will be instructed to apply and rub in a 5-cm strip of 5% benzoyl peroxide gel to the shoulder on dry skin in the morning and evening 2 days prior to surgery, 1 day before surgery, and a fifth time in the morning on the day of surgery.

Cutaneous iodine:

Standard pre-operative preparation including cutaneous disinfection with alcoholic iodine, sterile drapes.

Subcutaneous iodine:

Open surgery: after incision down to the fascia of the muscle layer, preparation of the whole subcutaneous layer with povidone-iodine solution will be performed and left acting during 60 seconds before full aspiration and lavage in order to not have swabs drained in povidone-iode (as it could continue to act as antiseptic during culture).

For arthroscopic procedures the disinfection and swab collection protocol will be performed when creating the first portal.

Bacterial culture swabs (n=9) will be taken during surgery from the patients' skin (before disinfection with iodine), subcutaneous tissue (after disinfection if randomised for subcutaneous disinfection), muscular layer, intra-articular/layer of the intervention (for example in case of clavicle surgery from the clavicle), surgeon's glove, outside scalpel, inside scalpel, and the retractors. For the swabs in the operative field, the applicable field is to be swabbed fully for 3 seconds. Each surgeon will follow the clear swab instructions from the microbiologist in order to obtain reliable cultures. Cultures will be analysed by the hospital's medical microbiological laboratory following standard protocol including a standard incubation time of 14 days. Treatment arm will be blinded for the laboratory employees.

Culture positivity between the groups will be analysed using chi-square tests for each culture location. In addition, a two-way ANOVA test will be performed to assess total number of positive cultures per patient. Furthermore, logistic regression models will be used to adjust for several potential confounding factors depending on the number of positive tests in the sample.

Inclusion Criteria:

• Indication for all types of surgery at the shoulder joint, acromioclavicular joint, or clavicle, including arthroscopic and open surgery.

Exclusion Criteria:

• Previous shoulder surgery;
• History of shoulder infection;
• Antibiotic treatment in the last 6 weeks;
• Allergy or hypersensitivity to one of the disinfecting agents;
• Patients with laboratory values consistent with a current infection (C-reactive protein, erythrocyte sedimentation rate, leukocyte count).

Since the disinfection protocol has to be performed by the patient and care provider, this can not be blinded. However, the laboratory staff will not know which treatment arm the patient was allocated to when assessing the bacterial cultures.