Overview

Basic Calcium Phosphate (BCP) disease is a crystal-associated arthropathy characterized by deposition of BCP crystals in periarticular and intra-articular tissues. A severe clinical presentation of BCP disease is Milwaukee Shoulder Syndrome, which typically affects elderly women, leading to rapid joint destruction, massive rotator cuff tears, and non-inflammatory joint effusion. It often involves the shoulder, but other joints like the hip and knee may be affected.

Anatomy and Physiology

• Shoulder Joint (Glenohumeral): Ball-and-socket joint stabilized by the rotator cuff tendons and labrum
• Rotator Cuff (SITS): Supraspinatus, Infraspinatus, Teres minor, Subscapularis — commonly disrupted in MSS.
• BCP crystals deposit in periarticular soft tissues, tendons, and joint cartilage, triggering matrix degradation and joint destruction.

Aetiology and Risk Factors

Aetiology

• Abnormal crystal formation and deposition due to joint aging, cartilage degeneration, or microtrauma
• Enzymatic activity (e.g. collagenase, MMPs) leads to structural breakdown

Risk Factors

• Age > 70
• Female sex
• Long-standing rotator cuff tear
• Osteoarthritis
• History of joint trauma or surgery
• Crystal arthropathies (e.g. CPPD, gout)

Pathophysiology

• BCP crystals form in joint or periarticular tissues due to altered phosphate metabolism or cartilage damage.
• Crystals stimulate macrophages and fibroblasts to release degradative enzymes (e.g. collagenase, elastase).
• Results in:
  • Rotator cuff rupture
  • Joint instability
  • Joint space narrowing and erosions
  • Massive joint effusion with low cellularity
• Crystals are too small for polarised microscopy, making diagnosis challenging.

Clinical Manifestations

Milwaukee Shoulder Syndrome (classic presentation)

• Elderly woman with painless or mildly painful swelling of shoulder
• Large non-inflammatory effusion
• Marked shoulder instability and crepitus
• Loss of active motion (due to rotator cuff tear), preserved passive motion
• Pain and loss of function as destruction occurs of joint

Other features

• Recurrent joint effusions
• Affects shoulder > hip > knee
• May resemble neuropathic (Charcot) joint

Diagnosis

No formal criteria; diagnosis is clinical + imaging + synovial fluid + exclusion of other causes

Synovial fluid analysis

• Large-volume effusion, low WBC count (<2000/mm³)
• No crystals seen on polarised light
• Can use alizarin red staining to identify calcium-containing crystals
• Electron microscopy or X-ray diffraction can identify BCP (rarely used clinically)

Imaging

• X-ray
  • Superior migration of the humeral head
  • Severe humeral head and glenoid destruction
  • Soft‑tissue calcifications and joint space narrowing
  • Intra‑articular loose bodies and periarticular osteopenia
• MRI
  • Massive rotator cuff tear – especially supraspinatus and infraspinatus, often with tendon retraction
  • Superior migration of humeral head
  • Large joint effusion
  • Articular cartilage loss and bony erosions – destructive arthropathy of humeral head and glenoid
  • Minimal synovial thickening
• Ultrasound
  • May show calcification and effusion

Crystal Arthropathy Clinical Comparison Table

Treatment

Conservative (mainstay)

• Joint aspiration (for symptomatic relief)
• NSAIDs (for mild pain)
• Physiotherapy (passive ROM to prevent contractures)
• Intra-articular corticosteroids (if inflammation suspected)

Surgical (if severe)

• Reverse total shoulder arthroplasty in cases of joint collapse or significant dysfunction
• Rotator cuff repair rarely indicated due to chronicity

Complications and Prognosis

Complications

• Progressive joint destruction
• Loss of function and disability
• Recurrent large joint effusions
• Prosthetic failure if arthroplasty performed late

Prognosis

• Prognosis poor without early supportive management
• Disease is not systemically inflammatory, but causes major mechanical disability

References

1. Dieppe PA, Swan A. Identification of calcium crystal deposition diseases. Best Pract Res Clin Rheumatol. 2005;19(6):983–1000.
2. Halverson PB, et al. Milwaukee shoulder syndrome: clinical and laboratory findings in 38 patients. Arthritis Rheum. 1986;29(5):567–574.
3. Tanimoto K, et al. Milwaukee shoulder syndrome: destructive shoulder arthropathy associated with apatite crystals. Mod Rheumatol. 2006;16(2):86–92.
4. Oliviero F, et al. Basic calcium phosphate crystals in osteoarthritis: morphology, localization and association with clinical parameters. Clin Exp Rheumatol. 2008;26(3):410–416.
5. McCarty DJ, et al. BCP crystal-associated arthritis: pathogenesis and management. Curr Rheumatol Rep. 2003;5(3):221–227.