Quadriga Effect & Lumbrical-Plus Finger

The FDP tendons to the index, middle, ring and little fingers arise from a single common muscle belly (the index often the most independent), so excursion is largely shared - one finger's deep flexor cannot move fully independently of the others. The lumbricals are unique: they originate from the FDP tendons in the palm and insert into the radial lateral band of the extensor mechanism, so they run from a flexor to an extensor. These two facts - shared FDP excursion and the flexor-to-extensor lumbrical - are the entire basis of the quadriga effect and the lumbrical-plus finger.

The quadriga effect is loss of full active flexion and grip in the OTHER fingers when one FDP is functionally too short: over-advanced or over-tightened at repair, tethered by adhesion, or sutured too proximally to a stump after amputation. Because the FDP tendons share a common muscle belly, the over-tight tendon 'uses up' the excursion and the neighbouring fingers cannot complete the fist - a flexion lag and weak grip. The name comes from the Roman four-horse chariot ('quadriga'): the reins move together, so restraining one restrains all. Treatment is to release the over-tight/tethered tendon (tenolysis, or revising an over-advanced repair/amputation tension).

The lumbrical-plus finger (paradoxical extension, described by Parkes) is extension of the IP joints when the patient tries to flex the finger. It occurs when the FDP loses effective distal pull while the lumbrical stays attached - classically after FDP division distal to the lumbrical origin, an over-long flexor graft, or a distal-finger amputation. Contraction intended to flex the digit instead slackens/pulls the FDP proximally, transmitting force through the lumbrical to the extensor mechanism, so the IP joints extend. Treatment is lumbrical release (dividing the offending lumbrical) or revising an over-long flexor graft to the correct length; conservative techniques have also been described.

• Flexor tendon repair/advancement: set the correct tension - over-advancing/over-tightening one FDP causes quadriga (limits the other fingers).
• Flexor tendon grafting: set the correct graft length - an over-long/slack graft causes lumbrical-plus (paradoxical extension); too short risks quadriga.
• Finger amputation: do not suture the FDP under tension to the stump (quadriga) and be aware that distal amputation can create a lumbrical-plus imbalance; many surgeons avoid tenodesing FDP to FDS under tension.
• Recognise early: test active flexion of all fingers (quadriga = the others lag/are weak) and watch for paradoxical IP extension on attempted flexion (lumbrical-plus).