Hand · Congenital / reconstruction

Thumb Reconstruction: Pollicization & Toe-to-Hand Transfer

The thumb provides roughly 40% of hand function through opposition, pinch and grasp, so the goal of all thumb reconstruction is an opposable, sensate, stable, well-positioned post. Congenital thumb hypoplasia is graded by the Blauth (modified Blauth) classification, types I–V, and is the radial end of the radial longitudinal deficiency spectrum, strongly associated with syndromes (VACTERL, Holt-Oram, Fanconi anaemia, TAR) that must be screened for. The PIVOTAL decision is the stability of the CARPOMETACARPAL (CMC) joint: a stable CMC (types I, II, IIIA) is RECONSTRUCTED (opponensplasty, first web release, MCP ulnar-collateral stabilisation, tendon correction); an unstable or absent CMC (type IIIB and above — including the type IV floating thumb and type V aplasia) means the deficient thumb is ABLATED and the index finger POLLICIZED. Pollicization transposes, shortens, rotates and pronates the index to make an opposable thumb the brain already controls; toe-to-hand transfer is the workhorse for TRAUMATIC thumb loss (and selected congenital cases) because it adds a digit without sacrificing a finger.

~40%Of hand function is the thumb

CMC stabilityDecides reconstruct vs pollicize

Type IIIBThe reconstruct → pollicize pivot

Index → thumbPollicization principle

Bottom lineOne question decides it: is the CMC (metacarpal base) stable? Stable (I, II, IIIA) → reconstruct; unstable/absent (IIIB, IV floating thumb, V aplasia) → ablate + pollicize the index. Toe-to-hand transfer is the workhorse for traumatic thumb loss with intact fingers. Always screen for Fanconi anaemia (chromosomal breakage testing) and the other radial-deficiency syndromes before surgery, and operate in early childhood for cortical integration.

Traps that cost marks

Missing Fanconi anaemia. Thumb hypoplasia can be the first sign of Fanconi anaemia — a DNA-repair disorder causing marrow failure and malignancy that is fatal if missed. Order chromosomal breakage testing before any elective surgery; do not operate on the hand and overlook the bone marrow.
Calling the grade off the photograph. The IIIA-versus-IIIB decision is the stability of the CMC/metacarpal base (examination + radiograph), NOT how small the thumb looks. A stable base reconstructs; an unstable/absent base is pollicized.
Reconstructing a IIIB thumb. Rebuilding a thumb with no stable basal joint fails — from IIIB upward (IIIB, IV floating thumb, V aplasia) the deficient thumb is ablated and the index pollicized.
Pollicizing for an isolated traumatic thumb loss. With intact fingers, transposing one wastes a normal digit — toe-to-hand transfer is the procedure of choice; pollicization is for when a finger can be spared (congenital deficiency).
Forgetting the TAR pearl. In TAR the radius is absent but the thumb is PRESENT — a key distinction from Fanconi/Holt-Oram, where the thumb is often hypoplastic or absent.

Mnemonic

STABLEThe decision word

Stability of the CMC joint is THE decision point

Types I–IIIA stable → reconstruct (Thumb rebuilt)

Ablate + pollicize from IIIB upward (unstable base)

Blauth/modified Blauth I–V is the classification

Look for syndromes (Fanconi, Holt-Oram, VACTERL, TAR)

Early surgery in childhood for cortical integration

Hook:One word decides it: is the basal joint STABLE? Stable = build it; unstable = pollicize.

Mnemonic

POLLEXPollicization steps

Pronate the index (~120–160°) into the thumb position

Opposition — rebalance intrinsics (FDI→abductor, FPI→adductor)

Length — shorten by resecting the index metacarpal (keep the head as CMC)

Leave the neurovascular bundles intact (mobilise, don't divide)

Excise/ablate the deficient thumb first

eXcellent appearance, good function (94%/72% in series)

Hook:Pollicization makes a POLLEX (Latin for thumb) out of the index finger.

Mnemonic

FHTVScreen the radial-deficiency child

Fanconi anaemia — chromosomal breakage test (marrow failure, fatal if missed)

Holt-Oram — cardiac septal defects (TBX5); echocardiogram

TAR — thrombocytopenia–absent radius (but the thumb is PRESENT); FBC/platelets

VACTERL — vertebral, anal, cardiac, tracheo-oesophageal, renal, limb

Hook:Every hypoplastic thumb earns a syndrome screen: Fanconi, Holt-Oram, TAR, VACTERL.

Overview & epidemiology

The 40% digit

The thumb is responsible for roughly 40% of hand function. Its unique value is opposition — the combination of abduction, flexion and pronation at the CMC joint that lets the pulp meet the other digits for pinch and grasp. An effective thumb (native or reconstructed) must be mobile (especially an opposable CMC), stable (a basal joint and a competent MCP ulnar collateral for key pinch), sensate, of adequate length, and positioned with a wide first web space. Every reconstructive technique is judged against these requirements.

Congenital thumb hypoplasia is uncommon — the reported incidence of grades II–V is on the order of 1 in 10,000 newborns — and sits at the radial (preaxial) end of the radial longitudinal deficiency (RLD) spectrum. It is frequently bilateral and frequently syndromic, so the deficient thumb is assessed as part of the whole child, not in isolation. The greater the radial-forearm deficiency, the more severe the thumb deficiency tends to be.

Pathophysiology & embryology

Thumb hypoplasia is a failure of formation along the radial/preaxial axis of the limb. Upper-limb patterning is governed by three signalling centres: the apical ectodermal ridge (AER) drives proximal-distal outgrowth, the zone of polarising activity (ZPA, via Sonic hedgehog) sets the radio-ulnar (anteroposterior) axis, and the Wnt pathway the dorsoventral axis. Disturbance of radial-side formation produces a continuum from mild thenar hypoplasia, through a deficient or floating thumb, to complete absence of the thumb and the radius.

The thumb tracks the forearm. The severity of thumb deficiency correlates with the severity of the radial (forearm) deficiency — the two are graded together (modified Blauth for the thumb, Bayne-Klug for the radius).
Frequently syndromic. Named associations include Holt-Oram syndrome (TBX5 mutation; cardiac septal defects), Fanconi anaemia (a DNA-repair disorder causing progressive marrow failure and malignancy), TAR (thrombocytopenia–absent radius), and the VACTERL association. Children with a syndrome are more likely to have bilateral and combined radial-plus-thumb deficiency.
The TAR distinction. In TAR the radius is absent but the thumb is present — a key contrast with Fanconi anaemia and Holt-Oram, in which the thumb is commonly hypoplastic or absent.

Classification

The Blauth classification (with the widely used modified Blauth subdivision of type III) grades congenital thumb hypoplasia and directs treatment. The decisive feature dividing the spectrum into "reconstruct" versus "pollicize" is the stability of the carpometacarpal (CMC) joint.

Features: Minor generalised hypoplasia, all structures present
CMC joint: Stable
Treatment: Usually none

Features: Thenar hypoplasia, narrow first web, MCP UCL laxity
CMC joint: Stable
Treatment: Reconstruct (opponensplasty, web release, UCL)

IIIA

Features: Severe hypoplasia + extrinsic tendon/skeletal deficiency
CMC joint: STABLE
Treatment: Reconstruct

IIIB

Features: As IIIA but deficient metacarpal base
CMC joint: UNSTABLE / absent
Treatment: Ablate + pollicize

Features: 'Floating thumb' (pouce flottant) on a pedicle
CMC joint: Absent
Treatment: Pollicize

Features: Complete absence (aplasia)
CMC joint: Absent
Treatment: Pollicize (or toe-to-hand)

Blauth (modified) classification and treatment
Type	Features	CMC joint	Treatment
I	Minor generalised hypoplasia, all structures present	Stable	Usually none
II	Thenar hypoplasia, narrow first web, MCP UCL laxity	Stable	Reconstruct (opponensplasty, web release, UCL)
IIIA	Severe hypoplasia + extrinsic tendon/skeletal deficiency	STABLE	Reconstruct
IIIB	As IIIA but deficient metacarpal base	UNSTABLE / absent	Ablate + pollicize
IV	'Floating thumb' (pouce flottant) on a pedicle	Absent	Pollicize
V	Complete absence (aplasia)	Absent	Pollicize (or toe-to-hand)

Palmar photograph of a child's hand with a small hypoplastic thumb (type II thumb hypoplasia) — Blauth/modified Blauth type II thumb hypoplasia (palmar view): the thumb is present but small, with thenar (intrinsic muscle) hypoplasia and first web-space narrowing. Types I, II and IIIA have a stable carpometacarpal joint and are reconstructed (opponensplasty, first web release, ulnar collateral ligament stabilisation).Credit: Iris Theunis, CC BY-SA 3.0, via Wikimedia Commons

Dorsal photograph of a child's hand with a severely hypoplastic thumb — A severely hypoplastic thumb (dorsal view) with skeletal and extrinsic-tendon deficiency. Whether this is type IIIA (stable CMC → reconstruct) or IIIB (unstable/absent CMC → pollicize) is decided by examination and radiographs of the metacarpal base, NOT by external appearance — that CMC-stability assessment is the pivotal surgical decision.Credit: Iris Theunis, CC BY-SA 3.0, via Wikimedia Commons

Palmar photograph of a child's hand with a severely deficient, vestigial-appearing thumb — A severely deficient, vestigial-appearing thumb (classified by the source as Blauth type IIIB — a thumb with an unstable/absent carpometacarpal base; the IIIB-versus-IV-floating-thumb distinction is radiographic, not photographic). From the unstable-base grades upward — IIIB, the type IV floating thumb (pouce flottant) and type V aplasia — the deficient thumb is ablated and the index finger pollicized to create a functional opposable thumb.Credit: Iris Theunis, CC BY-SA 3.0, via Wikimedia Commons

Palmar photograph of a child's hand with complete absence of the thumb (type V aplasia), showing four digits — Blauth type V thumb hypoplasia: complete absence (aplasia) of the thumb — a four-digit hand with no thumb on the radial side. Like the type IV floating thumb, this is managed by pollicization of the index finger (or, where indicated, toe-to-hand transfer) to restore pinch and grasp.Credit: Iris Theunis, CC BY-SA 3.0, via Wikimedia Commons

Clinical presentation & assessment

Assessment starts with the whole child — bilaterality, the forearm and wrist (radial deviation and forearm shortening of radial longitudinal deficiency), and the systemic screen for associated syndromes — before focusing on the thumb itself.

The thumb examination characterises each component the grade depends on:

Thenar bulk and active opposition — intrinsic (opponens/abductor) function.
First web-space width — an adducted thumb in the plane of the palm cannot oppose.
MCP ulnar collateral ligament competence — a stable post for key pinch (laxity is characteristic of type II).
Extrinsic tendon function — look for pollex abductus and aberrant FPL–EPL interconnections.
The pivotal test — carpometacarpal (basal) joint stability. Stress the metacarpal base: a stable base (types I, II, IIIA) supports reconstruction; an unstable or absent base (IIIB and above) does not.

Watch how the child uses the hand: a child who bypasses the deficient thumb and pinches between the index and middle fingers is signalling a non-functional thumb — a pointer towards pollicization rather than reconstruction.

Investigations

Radiographs of the hand and forearm define the skeletal deficiency, the metacarpal base/CMC joint (the IIIA-versus-IIIB determinant), and the radius (Bayne-Klug grade of radial longitudinal deficiency). In young children the carpus and metacarpal base may be incompletely ossified, so correlate films with the clinical stability examination.
Systemic workup for associated syndromes — before any elective surgery:
- Full blood count and platelets — thrombocytopenia of TAR.
- Chromosomal breakage test (diepoxybutane / mitomycin-C) — Fanconi anaemia; missing it risks marrow failure and complicates any future surgery, chemotherapy or transplant.
- Echocardiogram — Holt-Oram and VACTERL cardiac anomalies.
- Renal ultrasound and spinal imaging — VACTERL (vertebral, anal, renal).
Refer to paediatrics, genetics, cardiology and haematology as indicated.

Caution

Order chromosomal breakage testing for Fanconi anaemia in every child with thumb hypoplasia or radial longitudinal deficiency before elective surgery. Fanconi anaemia presents with progressive bone-marrow failure and a high malignancy risk; the hand anomaly can be the presenting sign, and detecting it changes anaesthetic, surgical and oncological management.

Management

Approaching the deficient or absent thumb

1
Assess the child and screen for syndromes
Thumb hypoplasia is on the radial-deficiency spectrum — screen for VACTERL, Holt-Oram (cardiac), Fanconi anaemia (chromosomal breakage testing — fatal if missed) and TAR before any surgery.
2
Classify by Blauth (modified) I–V
Examine thenar bulk, first web, MCP UCL, extrinsic tendons and — the key — CMC/metacarpal-base stability on exam and radiograph.
3
Stable CMC (I, II, IIIA) → reconstruct
Opponensplasty (Huber/FDS), first web release, UCL stabilisation, tendon correction (EIP transfer for IIIA).
4
Unstable/absent CMC (IIIB, IV, V) → pollicize
Ablate the deficient thumb and pollicize the index (shorten, pronate, palmar-abduct, preserve NV bundles, rebalance intrinsics). For TRAUMATIC loss with intact fingers, toe-to-hand transfer instead.

Bottom lineThe whole decision turns on CMC stability. For congenital deficiency the choice is reconstruct-vs-pollicize; for traumatic loss with intact fingers it is toe-to-hand transfer (don't sacrifice a normal digit).

When the CMC joint is stable, the deficient thumb is rebuilt rather than replaced, addressing each component individually:

Restore opposition

Opponensplasty — commonly an abductor digiti minimi (Huber) transfer in children (which also adds thenar bulk) or an FDS opponensplasty — to provide the missing intrinsic opposition.

Open the first web

First web-space release/deepening (Z-plasty or local flaps) so the thumb can abduct and oppose rather than lie adducted in the plane of the palm.

Stabilise the MCP joint

Ulnar collateral ligament reconstruction/stabilisation of the MCP joint to provide a competent post for key pinch (UCL laxity is characteristic of type II).

Correct tendon anomalies

Address extrinsic tendon abnormalities (e.g. pollex abductus / aberrant FPL–EPL interconnections); an EIP transfer can restore extension/abduction where the extrinsics are deficient (type IIIA).

Two-panel diagram: a thumbless hand before pollicization, and after, with the index finger transposed into a shortened, abducted thumb position — The principle of index-finger pollicization (before / after). When there is no stable basal joint (Blauth IIIB–V), the deficient thumb is ablated and the index finger is transposed onto the radial border: it is shortened (resecting the index metacarpal, keeping the head as the new CMC), rotated and pronated (~120–160°) so its pulp faces the other digits, and set in palmar abduction with a wide first web — creating an opposable, sensate thumb the brain already controls.Credit: OrthoVellum surgical illustration

The named technique — Buck-Gramcko pollicization, and the joint/tendon conversions

The standard operation is the Buck-Gramcko pollicization, and the examiner wants the steps beyond "shorten and rotate":

The index MCP joint becomes the new basal (CMC) joint — and the single most important technical pearl is that it is fixed/sutured in full hyperextension. If it is not, the new thumb hyperextends at its new basal joint on pinch; using up the MCP's natural hyperextension makes a stable post. The metacarpal head is rotated and fixed onto the metacarpal base/remnant to act as the new trapezium, after the shaft of the index metacarpal is resected to shorten the digit.
Set the position — about 120–160° of pronation and roughly 40° of palmar abduction, so the new pulp faces the other digits, with the new thumb tip reaching about the proximal phalanx of the long finger.
Re-route the tendons to new thumb roles — the extensor indicis proprius becomes the extensor pollicis longus, the extensor digitorum communis (index) becomes the abductor pollicis longus, the first dorsal interosseous becomes the abductor pollicis brevis (opposition) and the first palmar interosseous becomes the adductor pollicis; the index FDS/FDP act as the new long flexors.
Protect the blood supply — the digital neurovascular bundles are mobilised, not divided; a dominant common digital artery to the index/middle web may require ligation of the branch to the radial side of the middle finger to free the index.

The traumatic thumb — a reconstruction ladder by level of amputation

For acquired (traumatic) thumb loss the reconstruction is chosen by how much length, and whether a mobile basal (CMC) joint and thenar muscles, remain:

Distal third (through/ distal to the IP joint) — length is usually adequate, so the need is soft-tissue/pulp coverage and a nail: local/regional flaps (Moberg volar advancement, the first dorsal metacarpal artery "kite" flap, cross-finger flap).
Middle third (proximal phalanx to the MCP) with a preserved mobile CMC and thenar muscles — the problem is length: options are first web-space deepening (phalangization), metacarpal distraction lengthening (osteogenesis), osteoplastic reconstruction (iliac bone graft plus a flap), or toe transfer.
Proximal third (at or proximal to the CMC, with loss of the basal joint and thenar motors) — a new mobile, motored basal joint is needed, so the answer is toe-to-hand transfer (or pollicization of an adjacent digit that cannot itself be salvaged).
The wrap-around (Morrison) flap — a great-toe soft-tissue/nail "wrap" around an iliac corticocancellous graft gives a good-looking, sensate thumb of the right girth without sacrificing the toe skeleton, but it does not grow (less suited to children) and provides no interphalangeal-joint motion.

Complications

Pollicization

Vascular compromise of the transposed digit (NV bundle injury)
Malposition/malrotation (inadequate pronation/abduction) limiting opposition
Stiffness, weak pinch, or an unstable new CMC
Suboptimal results where there is associated radial deficiency

Toe-to-hand transfer

Microvascular failure / flap loss (anastomotic thrombosis)
Donor-foot morbidity (gait, push-off, wound issues)
Cold intolerance, reduced sensation, or insufficient length/mobility
Need for secondary tenolysis/revision

Evidence

Association of Radial Longitudinal Deficiency and Thumb Hypoplasia: An Update Using the CoULD Registry

LoE 3

Forman M, Canizares MF, Bohn D, James MA, Samora J, Steinman S, Wall LB, Bauer AS • J Bone Joint Surg Am (2020)

Key Findings:

Congenital Upper Limb Differences (CoULD) registry: 259 patients with 383 limbs with radial deficiency (83 with a diagnosed syndrome), classified by OMT, modified Blauth (thumb) and Bayne-Klug (radius).
The severity of radial deficiency correlated with the severity of thumb deficiency (Kendall tau 0.49).
Patients with a syndrome were twice as likely to have bilateral deficiency and 2.5 times more likely to have both radial and thumb deficiency; Holt-Oram, TAR and Fanconi anaemia showed varied thumb/radial patterns.

Clinical implication: Confirms thumb hypoplasia is part of the radial longitudinal deficiency spectrum, tracks radius severity, and is frequently syndromic — mandating the systemic screen.

Limitation: Registry cross-sectional analysis; syndrome diagnosis as recorded by treating physicians.

Source: J Bone Joint Surg Am 2020;102(20):1815-1822

Verify on PubMed (PMID 33086350)

How to read this evidence

The radial-deficiency/thumb-hypoplasia association and its syndromic patterns come from Forman et al. 2020 (DOI 10.2106/JBJS.20.00281); the grade-directed algorithm and incidence from Ozols et al. 2019 (DOI 10.3390/medicina55100610); the pollicization results (72% good/excellent function, 94% appearance) from Tonkin et al. 2014 (DOI 10.1177/1753193414535177); and the paediatric toe-to-thumb data from Yoon & Jones 2022 (DOI 10.1093/jbcr/irab101). The Blauth/modified Blauth classification and the CMC-stability pivot are the standard framework these series apply.

Viva practice

The points that win marks

The thumb = ~40% of hand function; the goal is an opposable, stable, sensate, well-positioned post.
Blauth (modified) I–V; the IIIA/IIIB split is CMC (metacarpal base) STABILITY.
Stable CMC (I, II, IIIA) → reconstruct; unstable/absent (IIIB, IV floating thumb, V aplasia) → ablate + pollicize.
Pollicization: shorten, pronate, palmar-abduct the index; preserve NV bundles; rebalance intrinsics — good function, excellent appearance.
Toe-to-hand transfer for traumatic thumb loss with intact fingers (don't sacrifice a normal digit).
Screen for Fanconi anaemia (chromosomal breakage) and the radial-deficiency syndromes before surgery; remember the thumb is present in TAR.

Exam viva

Practise clinical reasoning and management decisions out loud

Viva scenarioStandard

Clinical prompt

“A 14-month-old child is referred with a small, deficient right thumb. How do you assess and classify the deformity, and what determines whether you reconstruct or pollicize?”

Viva scenarioAdvanced

Clinical prompt

“A 30-year-old loses his dominant thumb at the MCP level in a saw injury; the replant is not salvageable and the other digits are intact. Months later he wants a functional thumb. What reconstruction would you offer and why not pollicization?”

Exam cheat sheet

Exam day cheat sheet

Thumb reconstruction — one-screen revision

Principles

The thumb = ~40% of hand function; aim for a mobile, stable, sensate, well-positioned opposable post
Congenital deficiency graded by Blauth/modified Blauth (I–V); part of the radial longitudinal deficiency spectrum
Screen for syndromes: VACTERL, Holt-Oram, Fanconi anaemia (breakage testing), TAR (thumb present)

The pivot

CMC (metacarpal base) stability decides strategy
Stable (I, II, IIIA) → RECONSTRUCT (opponensplasty, first web release, UCL, tendon correction)
Unstable/absent (IIIB, IV floating thumb, V aplasia) → ABLATE + POLLICIZE

Pollicization

Transpose index: shorten (resect metacarpal, keep head as CMC), pronate/abduct, preserve NV bundles, rebalance intrinsics
Early childhood; ~72% good/excellent function, ~94% appearance
Radial anomalies worsen results but are not a contraindication

Toe-to-hand transfer

Microvascular free transfer; workhorse for TRAUMATIC thumb loss (spares the fingers)
Great toe = strength/bulk; second toe = preserves foot push-off
Restores sensate pinch/grasp/opposition; risk = microvascular failure + donor-foot morbidity

Blauth (modified) classification and treatment

Type

Features

CMC joint

Treatment

Minor generalised hypoplasia, all structures present

Stable

Usually none

Thenar hypoplasia, narrow first web, MCP UCL laxity

Stable

Reconstruct (opponensplasty, web release, UCL)

IIIA

Severe hypoplasia + extrinsic tendon/skeletal deficiency

STABLE

Reconstruct

IIIB

As IIIA but deficient metacarpal base

UNSTABLE / absent

Ablate + pollicize

'Floating thumb' (pouce flottant) on a pedicle

Absent

Pollicize

Complete absence (aplasia)

Absent

Pollicize (or toe-to-hand)

Approaching the deficient or absent thumb

1
Assess the child and screen for syndromes
Thumb hypoplasia is on the radial-deficiency spectrum — screen for VACTERL, Holt-Oram (cardiac), Fanconi anaemia (chromosomal breakage testing — fatal if missed) and TAR before any surgery.
2
Classify by Blauth (modified) I–V
Examine thenar bulk, first web, MCP UCL, extrinsic tendons and — the key — CMC/metacarpal-base stability on exam and radiograph.
3
Stable CMC (I, II, IIIA) → reconstruct
Opponensplasty (Huber/FDS), first web release, UCL stabilisation, tendon correction (EIP transfer for IIIA).
4
Unstable/absent CMC (IIIB, IV, V) → pollicize
Ablate the deficient thumb and pollicize the index (shorten, pronate, palmar-abduct, preserve NV bundles, rebalance intrinsics). For TRAUMATIC loss with intact fingers, toe-to-hand transfer instead.

When the CMC joint is stable, the deficient thumb is rebuilt rather than replaced, addressing each component individually:

Restore opposition

Opponensplasty — commonly an abductor digiti minimi (Huber) transfer in children (which also adds thenar bulk) or an FDS opponensplasty — to provide the missing intrinsic opposition.

Open the first web

First web-space release/deepening (Z-plasty or local flaps) so the thumb can abduct and oppose rather than lie adducted in the plane of the palm.

Stabilise the MCP joint

Ulnar collateral ligament reconstruction/stabilisation of the MCP joint to provide a competent post for key pinch (UCL laxity is characteristic of type II).

Correct tendon anomalies

The named technique — Buck-Gramcko pollicization, and the joint/tendon conversions

The standard operation is the Buck-Gramcko pollicization, and the examiner wants the steps beyond "shorten and rotate":

The index MCP joint becomes the new basal (CMC) joint — and the single most important technical pearl is that it is fixed/sutured in full hyperextension. If it is not, the new thumb hyperextends at its new basal joint on pinch; using up the MCP's natural hyperextension makes a stable post. The metacarpal head is rotated and fixed onto the metacarpal base/remnant to act as the new trapezium, after the shaft of the index metacarpal is resected to shorten the digit.
Set the position — about 120–160° of pronation and roughly 40° of palmar abduction, so the new pulp faces the other digits, with the new thumb tip reaching about the proximal phalanx of the long finger.
Re-route the tendons to new thumb roles — the extensor indicis proprius becomes the extensor pollicis longus, the extensor digitorum communis (index) becomes the abductor pollicis longus, the first dorsal interosseous becomes the abductor pollicis brevis (opposition) and the first palmar interosseous becomes the adductor pollicis; the index FDS/FDP act as the new long flexors.
Protect the blood supply — the digital neurovascular bundles are mobilised, not divided; a dominant common digital artery to the index/middle web may require ligation of the branch to the radial side of the middle finger to free the index.

The traumatic thumb — a reconstruction ladder by level of amputation

For acquired (traumatic) thumb loss the reconstruction is chosen by how much length, and whether a mobile basal (CMC) joint and thenar muscles, remain:

Distal third (through/ distal to the IP joint) — length is usually adequate, so the need is soft-tissue/pulp coverage and a nail: local/regional flaps (Moberg volar advancement, the first dorsal metacarpal artery "kite" flap, cross-finger flap).
Middle third (proximal phalanx to the MCP) with a preserved mobile CMC and thenar muscles — the problem is length: options are first web-space deepening (phalangization), metacarpal distraction lengthening (osteogenesis), osteoplastic reconstruction (iliac bone graft plus a flap), or toe transfer.
Proximal third (at or proximal to the CMC, with loss of the basal joint and thenar motors) — a new mobile, motored basal joint is needed, so the answer is toe-to-hand transfer (or pollicization of an adjacent digit that cannot itself be salvaged).
The wrap-around (Morrison) flap — a great-toe soft-tissue/nail "wrap" around an iliac corticocancellous graft gives a good-looking, sensate thumb of the right girth without sacrificing the toe skeleton, but it does not grow (less suited to children) and provides no interphalangeal-joint motion.

Thumb Reconstruction: Pollicization & Toe-to-Hand Transfer

Thumb Reconstruction: Pollicization & Toe-to-Hand Transfer