Description

• Degenerative disease of the acromioclavicular (AC) joint
  • Acute: usually due to traumatic AC separation, can lead to chronic osteoarthritis (OA) over time
  • Chronic: due to repetitive strain and degeneration over time (classic form of AC arthritis)
• Caused by mechanical friction of the articular surfaces of the distal clavicle and acromion

Epidemiology

• Acute
  • More common in adolescents and young adults
  • Due to trauma (not chronic degeneration)
  • Less common than chronic form
• Chronic
  • Typically affects middle-aged adults
  • AC joint normally degenerates over several decades. AC arthritis occurs earlier in life than glenohumeral joint arthritis or other types of OA.
  • Second most common shoulder pathology in adults >40 years (after rotator cuff pathology) (1)

Incidence

• 15/1,000 primary care patients have shoulder pain annually (1).
  • 24% of patients in PCP setting with shoulder pain have AC arthritis; 77% have >1 etiology for shoulder pain (2).
• ~20% of Americans have shoulder pain in lifetime.
  • 5% of Americans experience AC arthritis during their lifetime.
• Predominant age: 40s
• No predominant sex

Prevalence
~16% of musculoskeletal (MSK) complaints seen in PCP office are due to shoulder pain: second only to back pain (1)

Etiology and Pathophysiology

• Result of repeated movements or trauma that wears away fibrocartilaginous disk between the acromion and clavicle
• Articular disk can begin to break down by 2nd decade of life, often not symptomatic until years later.
• Prior AC separation contributes to AC joint arthritis.
• Inflammation from rotator cuff pathology can impact AC joint mechanics, contributing to arthritis.

Risk Factors

• History of trauma or contact sports, history of overhead sports (baseball, swimming, tennis), active weight lifting (bench or military press)
• Rheumatoid arthritis (RA) or other inflammatory arthritides

General Prevention

• Avoid highly repetitive motions that involve the AC joint (encourage proper technique for those who participate regularly in overhead/cross-body activities such as golf, swimming, and tennis).
• For throwing sports such as baseball, encourage pitch count in young athletes.
• For weight lifters with heavy overhead lifting, encourage use of a wider grip.

Commonly Associated Conditions

• Rotator cuff disorders (often concurrent)
• RA (consider with bilateral AC arthritis)