Each PIP joint has proper and accessory collateral ligaments: the proper collateral ligament (PCL) is the primary valgus-varus restraint running from the lateral condyle of the proximal phalanx to the lateral base of the middle phalanx; the accessory collateral ligament (ACL) arises just dorsal to the PCL and inserts on the volar plate rather than the phalanx. The accessory collateral ligaments check extension by supporting the volar plate, creating the dorsal extension block of the PIP joint.
Supports the volar plate at the PIP joint preventing hyperextension; contributes to the soft tissue box surrounding the PIP joint; the PIP accessory collateral ligaments are the 'check reins' that limit PIP extension beyond the neutral position.
The accessory collateral check reins are released in PIP joint contracture correction: extension contractures from intrinsic tightness (positive intrinsic tightness test) require accessory collateral and volar plate release. Flexion contractures after dorsal PIP injury or boutonniere repair require lateral band and accessory collateral release to restore extension. The accessory collaterals are the primary structure of the extension block splint mechanism — dorsal splinting at 20-30 degrees flexion relaxes the check reins and allows the volar plate to heal after volar plate injuries.
Extension contracture of the PIP joint from chronic intrinsic tightness or post-traumatic scarring tightens the accessory collateral check reins preventing full extension; surgical release of the accessory collateral check reins (check resinectomy) combined with volar plate release restores PIP extension range in established fixed flexion contractures.
