Rationale: Although most patients experience significant pain relief after total shoulder arthroplasty (TSA), pain persists for some patients even after surgery. The endogenous pain system may be involved in persisting postoperative pain in total knee arthroplasty (TKA) and total hip arthroplasty (THA) patients, but this has never been studies for TSA patients. Furthermore, psychological factors and coping strategies may also be of interest but have not yet been extensively studied in TSA patients.

Objective: The primary objective is to explore whether central sensitization and/or abnormal CPM responses are present in (a subgroup of) patients who will receive shoulder arthroplasty for osteoarthritis, and whether shoulder arthroplasty produces change in these measures of the endogenous pain modulatory system.

The secondary objective, in case patients with altered endogenous pain modulation are found, is to explore if change in pain over time, psychological factors and coping strategies differ between patients with and without altered pain modulation.

Study design: An exploratory prospective observational cohort study.

Study population: Patients (age 18 years or older) who are scheduled to undergo shoulder arthroplasty for osteoarthritis, at the Reinier Haga Orthopaedic Centre.

Intervention (if applicable): Not applicable.

Main study parameters/endpoints: The main study parameters are CPM and TS values at baseline and at 3 and 6 months after surgery, as well as the absence/presence of allodynia at baseline and at 3 and 6 months after surgery.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The burden of participation will consist of completing questionnaires at baseline and at three and six months after surgery. In addition, subjects will undergo psychophysical testing at 3 moments, which can lead to redness or a burning sensation of the skin during the first 24 hours after testing. The investigators do not expect any additional risks associated with participation. There is no direct benefit for the subjects.

Inclusion Criteria:

• Age ≥ 18 yrs
• Scheduled to undergo primary total shoulder arthroplasty (TSA) or reversed shoulder arthroplasty (RSA) for primary shoulder osteoarthritis or cuff tear arthropathy
• American Society of Anesthesiologists score 1, 2 or 3
• Able to provide written informed consent

Exclusion Criteria:

• Regular use of anti-depressants or anti-epileptics for any purpose, including SNRIs and gabapentinoids
• The presence of any chronic pain disorder other than osteoarthritis
• Osteoarthritis in joints other than the affected shoulder, for which arthroplasty is/will be planned in the near future
• Difficulty with or inability to perform psychophysical testing (eg. in case of cognitive or psychiatric disorders)
• Difficulty with or inability to communicate with the investigators (eg. difficulty with the Dutch language, cognitive/memory disorders)

All patients who are scheduled to undergo primary shoulder arthroplasty for shoulder osteoarthritis (OA) or rotator cuff tear arthropathy (CTA).