Basic calcium phosphate (BCP) crystal deposition diseases are an uncommon form of . BCP crystals include several different types of calcium phosphate crystals, such as partially carbonate-substituted hydroxyapatite, octacalcium phosphate, and tricalcium phosphate. These crystals are deposited in joints or in periarticular structures, causing BCP-associated arthritis (including Milwaukee shoulder syndrome) and calcific tendinitis. The underlying etiology of articular and periarticular BCP crystal deposition is unknown, but risk factors include female sex, joint trauma, and various metabolic abnormalities. The presentation and management of BCP-associated osteoarthritis are similar to that of age-related osteoarthritis. Milwaukee shoulder syndrome and acute calcific tendinitis manifest with joint pain and swelling, most commonly of the shoulder or other large joints. Diagnosis is based on clinical and radiological findings. may be helpful in excluding differential diagnoses (e.g., , , ) but rarely identifies the small, nonbirefringent BCP crystals. Management is primarily supportive and includes pain relief for acute episodes, physical dissolution of crystals by various methods (e.g., needle aspiration, ultrasound therapy), and surgery in severe cases.
The underlying etiology that results in BCP crystal deposition within joints and periarticular tissue remains poorly understood. The following factors are associated with an increased risk of developing BCP crystal deposition diseases: 
The pathophysiology of BCP crystal deposition diseases is complex and not yet fully understood. BCP crystals appear to cause joint degeneration by inducing both inflammatory and catabolic processes. 
- Inflammatory processes: e.g., BCP crystals act on local fibroblasts, chondrocytes, and synovial macrophages to upregulate the expression of inflammatory cytokines and prostaglandins
- Catabolic processes: e.g., BCP crystals act on chondrocytes to upregulate the expression of enzymes that degrade cartilage, and they also inhibit antiosteoclastogenic factors → osteoclast formation
Subtypes and variants
There are three main subtypes of BCP crystal deposition disease, all of which vary in their clinical presentation and diagnosis.
Basic calcium phosphate-associated osteoarthritis
- Epidemiology: BCP-associated OA is widely prevalent. 
- Similar to
- The degree of calcification appears to correlate to the severity of OA seen on imaging. 
- Imaging: See “ ” for modalities and findings.
- Additional diagnostics: not routinely required
BCP-associated arthropathies typically affect large joints. 
Milwaukee shoulder syndrome (MSS) is a subtype of BCP-associated OA characterized by a noninflammatory osteoclast-mediated destructive arthritis of the shoulder joint and commonly accompanied by non-inflammatory joint effusion and rotator cuff deficits. 
- Epidemiology 
- Clinical features 
- Synovial fluid analysis
MSS is associated with an extensive loss of articular cartilage, destruction of the humeral head, rotator cuff defects, large effusion, and small osteophytes. Conversely, OA is typically associated with prominent osteophytes, an intact rotator cuff, and humeral head sclerosis. 
- Epidemiology 
Clinical features 
- Acute calcific tendinitis
- Rapid onset of severe pain, with mild swelling and erythema
- Range of motion may be restricted.
- Possible low-grade fever
- Symptoms are typically self-limiting, with clinical and radiological resolution in 2–3 weeks (or less, with treatment). 
- Recurrences are common. 
- Chronic calcific tendinitis: a chronic pain syndrome that may develop after recurrent acute attacks 
- Acute calcific tendinitis
- Laboratory studies: WBC and inflammatory markers (e.g., CRP, ESR) may show mild elevations.
- Imaging 
Periarticular calcium deposits may be asymptomatic and detected incidentally. 
The diagnosis of BCP crystal deposition diseases is primarily based on clinical and radiological findings. See “Subtypes and variants” section for specific diagnostic studies. 
- Assess the pretest probability of the underlying etiology based on clinical features and patient risk factors.
- Moderate or high clinical suspicion for possible septic arthritis, gout, acute CPPD disease: Perform synovial fluid analysis, as the will help narrow the diagnosis.
- Low suspicion for septic arthritis or gout: Perform imaging (beginning with x-ray of the affected joint) to evaluate for calcifications and, if the shoulder is affected, rotator cuff damage.
- If the diagnosis remains uncertain, consult rheumatology.
- Septic arthritis
- Inflammatory arthritis (other types): See “ .”
- Traumatic soft tissue or bony injury 
- Dialysis arthropathy
The differential diagnoses listed here are not exhaustive.
- The management of BCP crystal deposition diseases is primarily supportive.
- Treatment for BCP-associated OA is the same as for general OA (see “ ”).
- Treatment for other BCP crystal deposition diseases should involve consultation with a rheumatologist.
- Acute episodes: Treatment is focused on achieving pain relief.
- Chronic disease: The aim is to physically dissolve the crystals using a variety of modalities.
- Surgical intervention is a last resort.
Initial management (acute episodes) 
- Joint rest: Consider immobilization with a splint.
- NSAIDs (see “Oral analgesics”)
- Glucocorticoid injections 
- Needle aspiration :
- Physical therapy