23.13 Piston in a vacuum

• The arm is the piston, the labrum as a valve

• Intraarticular pressures have been measured to -32 mmHg at rest

  • Axial traction reduces it further

• Anterior labral lesion or clinical instability inhibit the seal, removes the possibility of a vacuum

• Negative pressures are largest when:

  • The articular surface is more widespread

  • The labrum is intact

  • The capsule is not too lax or torn

  • Shoulder is in the neutral position

  • Traction is perpendicular to the glenoid plane

---