The pulley system governs the moment arm, excursion and joint rotation produced by the flexor tendons. The A2 and A4 pulleys are the most biomechanically important to these functions. A2 and A4 arise from the periosteum of the proximal half of the proximal phalanx, and the midportion of the middle phalanx, respectively. A1, A3 and A5 are joint pulleys arising from the palmar plates of the MP, PIP, and DIP joints respectively. C1, C2, and C3 are thin, condensable, cruciate sections of the flexor sheath which permit the annular pulleys to approximate each other during flexion.
Strickland JE: Flexor tendons-acute injuries, in Green DP, Hotchkiss RN, Pederson WC (eds): Green’s Operative Hand Surgery, ed 4. New York, NY, Churchill Livingstone, 1999, pp1851-1855
Miller MD (ed): Review of Orthopaedics, ed 4. Philadelphia, PA, Saunders/Elsevier, 2004, p 360