Quadriga Effect & Lumbrical-Plus Finger

One common belly. The flexor digitorum profundus (FDP) tendons to the index, middle, ring and little fingers arise from a single common muscle belly, the index often the most independent. Their excursion is largely shared, so one finger's deep flexor cannot move fully independently of the others.

A muscle that bridges flexor to extensor. There are four lumbricals, and uniquely they have no bony origin. Each arises from the FDP tendons in the palm and inserts into the radial lateral band of the extensor mechanism, running from a flexor tendon to an extensor, which is precisely why force can be rerouted from FDP to extensor.

• The radial two, to the index and middle fingers, are unipennate and arise from a single FDP tendon
• The ulnar two, to the ring and little fingers, are bipennate and arise from two adjacent FDP tendons; by linking adjacent tendons they also reinforce the shared-excursion mechanics behind the quadriga

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.

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What it is. The quadriga effect is loss of full active flexion and grip in the other fingers when one FDP is functionally too short. Because the tendons share a common belly, the over-tight tendon uses up its excursion, and the neighbouring fingers cannot complete the fist: a flexion lag and weak grip. The name comes from the Roman four-horse chariot, the quadriga, whose reins move together, so restraining one restrains all.

How the FDP is made too short.

• Over-advanced or over-tightened at repair
• Tethered by adhesion
• Sutured too proximally, or under tension, to the stump after amputation

Treatment. Release the over-tight or tethered tendon, by tenolysis or by revising an over-advanced repair or the amputation tension, to restore the excursion of the common belly.

What it is. The lumbrical-plus finger, the paradoxical extension described by Parkes, is extension of the interphalangeal 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
• A distal-finger amputation

Mechanism. Contraction intended to flex the digit instead slackens or pulls the FDP proximally. The force is transmitted through the lumbrical to the extensor mechanism, and the IP joints extend.

Why the level of division matters. Lumbrical-plus needs the lumbrical to stay attached to an FDP that has lost its distal anchor. A division distal to the lumbrical origin leaves the lumbrical pulling on a now slack, free tendon; a division proximal to the lumbrical origin removes the lumbrical's drive and does not cause it. The broader intrinsic-plus and intrinsic-minus syndromes are developed in Intrinsic-Minus and Intrinsic-Plus Hand; what matters here is the lumbrical origin and the level of division.

Treatment. Lumbrical release, dividing the offending lumbrical, or revising an over-long flexor graft to the correct length. Conservative techniques have also been described, but lumbrical release or an appropriately tensioned FDP graft are the standard surgical solutions.

In a sense the two are opposite tension errors of the same system, both produced by getting flexor tendon length or tension wrong. Quadriga is an FDP that is too short or tight, limiting the others. Lumbrical-plus is an FDP that is too long or slack, or has lost its distal insertion, so its force is rerouted through the lumbrical.

Both are diagnosed by watching an active movement rather than by imaging, and each has a specific manoeuvre. Recognise them early by testing active flexion of every finger.

Quadriga: examine the fingers that are not complaining. Ask for a full fist. The involved digit flexes normally, or is stiff for its own reasons, while the neighbouring, uninvolved fingers stop short with a flexion lag at the PIP and DIP joints, and grip strength is measurably reduced. Quantify it as the distance from each fingertip to the distal palmar crease on maximal active flexion, compared with the other hand.

Tendon or joint. A persistent gap in the uninvolved fingers with full passive flexion is the finding, because the deficit is active only. If the fingers cannot be passively flexed either, the problem is the joints, not the tendon. Passively extending the involved finger slackens the common belly and improves the others' flexion, whereas flexing it worsens them, which makes a useful demonstration.

Lumbrical-plus: watch the finger while the patient tries to make a fist. The IP joints extend, with the MCP flexing, as flexion is attempted; the patient often describes the finger "going the wrong way" or being knocked straight. The movement is paradoxical and unmistakable once seen, and active flexion effort exaggerates it.

The MCP-block manoeuvre. Blocking or holding the MCP joint prevents MCP flexion and slackens the lumbrical. This abolishes the paradoxical extension and confirms the lumbrical as the culprit. Otherwise confirmation is anatomical rather than radiological: the history contains an FDP that has lost its distal anchor.

Imaging. Neither needs imaging to diagnose. Imaging is for defining the tendon anatomy before revision surgery. General hand examination is covered in Hand Examination and Clinical Localisation; the manoeuvres above are the phenomenon-specific ones.

"Set the correct tension" is the right principle and useless as an instruction. Specific rules do the work, and each corresponds to one of the operations that causes these problems.

Grafting cuts both ways. A graft set too short risks quadriga, the opposite error to the over-long graft in the last row.

Amputation. The amputation rule and the graft rule pull in opposite directions, which is why the two phenomena are so often confused. Fixing the flexor too short causes quadriga; leaving it with no effective distal anchor causes lumbrical-plus. After a distal amputation both are possible: tenodese the flexor and the other fingers suffer, leave the profundus with nothing to pull on and this finger paradoxically extends. That is the reason the flexor is simply allowed to retract rather than being anchored under tension, and many surgeons avoid tenodesing FDP to FDS under tension. Be aware that a distal amputation can create a lumbrical-plus imbalance.

Where the rules come from. The 1 cm limit and the reason for it come from Flexor Tendon Repair Techniques; the amputation rule from Ray Amputation of the Hand and Foot; and the advancement-versus-graft decision in the avulsion setting from Jersey Finger.