Does Subacromial Injection With Glutamate Receptor Antagonist, Ketamine, Attenuate Pain in Rotator Cuff Tendinopathy?

Inclusion Criteria: * diagnosis of rotator cuff tendinopathy by exercise-related shoulder pain with positive impingement tests as described by Hawkins and Neers, and MRI findings consistent with tendinopathy. * Symptom duration at least 1 year to ensure neuronal ingrowth and NMDAR expression Exclusion Criteria: * previous surgery in any shoulder. * previous cortisone use, either as injections or orally * symptoms or signs of cervicobrachialgia or polyneuropathy * full thickness rotator cuff ruptures verified by MRI * primary inflammatory mediated pain, hence, patients with glenohumeral arthrosis, glenohumeral arthritis or systemic disorders predisposing for arthritis * a central component of pain perception manifested by radiating pain in the involved limb; implying worse outcome after subacromial decompression. * pregnancy * breastfeeding * reduced liver function (Increased serum bilirubin, ASAT or ALAT), decompensated heart failure (NYHA class 3-4) * increased intracranial pressure or disease of the central nervous system (CNS) * chronic alcoholism * epilepsy * psychiatric disease, increased intraocular pressure * acute intermittent porphyria * hyperthyroidism * use of thyroid hormones * upper respiratory tract infections * pneumonia * intracranial lesions * acute head injuries * ocular injuries * hydrocephalus * risk factors predisposing for intra-articular bleeding * increased risk of infection