Obtain deep tissue samples from patients undergoing primary prosthetic surgery of the shoulder and patients undergoing primary instrumented surgery of the spine. Patients will be randomized to receive benzoyl peroxide 3 days prior to surgery.

Shoulder prosthetic surgery as well as interventional surgery on the spine are commonly contaminated by Cutibacterium Acnes. Although the significance of this contamination is unknown, it represents a cause for concern since in some patients, contaminating Cutibacterium Acnes from surgery may develop long-term infection of the implant. Unlike knee and hip surgeries, where the most common infection-causing germs are Staphylococcus aureus, in shoulder and spine surgery, Cutibacterium Acnes represents one of the germs most frequently associated with infections. peri implant. Skin preparation with chlorhexidine as well as antibiotic prophylaxis with cefazolin have been shown to be ineffective in eradicating Cutibacterium Acnes. Recently, different studies support the use of benzoyl peroxide in topical application, to reduce the load of Cutibacterium acnes on the skin. Despite this, it has not been shown to reduce the rate of infection, nor to reduce the phylotypes of Cutibacterium acnes associated with peri-implant infections (IA, IB and II).

Obtain deep tissue samples from patients undergoing primary prosthetic surgery of the shoulder and patients undergoing primary instrumented surgery of the spine. Patients will be randomized to receive benzoyl peroxide 3 days prior to surgery.

Inclusion Criteria:

• patients > 18 yrs.
• Primary prosthetic shoulder surgery.
• Instrumented primary surgery of the spine.

Exclusion Criteria:

• active infection.
• previous surgeries.
• interventionism prior to surgery in the last 6 months (infiltrations, arthro-CT, arthro-MRI)