The functional column of the thumb

The hand articulates with the distal part of the radius and ulna.

The thumb column includes the scaphoid bone, the trapezium, the^1st metacarpal and the 2 phalanges of the thumb.

The scaphoid is the most lateral bone of the proximal row of the carpus and presents on its lateral side the tubercle of the scaphoid on which the lateral collateral ligament of the carpus will be inserted.

It is a very important bone because it is the most fractured carpal bone and because of its poor vascularization, the risk of pseudarthrosis is very high.

• It articulates with its upper, convex face with the lower end of the radius
• With the lunate via its flat medial side
• The capitatum via its concave medial side
• It is via its convex lower surface that the scaphoid will articulate with the trapezium and the trapezoid

The trapezium is the lateral bone of the distal row of the carpus and one of the bones of the thumb column.

In addition to the scaphoid, it also articulates with the^1st metacarpal by its inferior face and with the trapezoid and the^2nd meta by its medial face.

On its anterior surface is a groove that is bounded laterally by the tubercle of the trapezium.

The metacarpal I, like all other metacarpals, is composed of a base, a body and a head, but at the level of the thumb it is the largest and most voluminous of the metacarpals.

The body of MI is flattened from front to back.

And its base is important because it is at this level that the fractures of the boxers are most frequently located, called Bennett's fracture.

MI articulates at its distal part with the proximal phalanx of the thumb which will itself articulate with the distal phalanx of the thumb and end with a projection called lanceolate which allows to support the nail.

The other fingers have 3 phalanges while the thumb stops at 2.

Another particularity of the thumb is the fact that it has 2 sesamoids which are constant and located at the level of the metacarpophalangeal joint. They are important because many muscles are inserted there.

The motor function of the thumb is mainly described by 3 joints

• The trapezius-metacarpal joint
• The metacarpophalangeal joint of the thumb
• The interphalangeal joint of the thumb

It is an independent, non-congruent saddle joint that joins the inferior articular surface of the trapezium to the superior articular surface of the base of MI. Its surface is more accurately termed "scoliotic saddle" because of the curvature of its groove (Kapanji 1980).

The surface of the surface is convex in the sagittal plane and concave in the frontal plane. For the metacarpal surface, it is the opposite.

The joint capsule is made up of :

• a fibrous membrane, very loose
• a synovial membrane

The ligaments maintain this joint with :

• the lateral carpo metacarpal ligament: stretched from the lateral face of the trapezium to the lateral edge of the MI
• palmar and dorsal carpometacarpal ligaments that insert on the faces of the trapezium and run obliquely to the medial edge of MI

These ligaments make sprains and dislocations of this joint very rare. If there is a dislocation, it is often associated with a fracture of the base.

Vascularization:
This joint is vascularized by arteries coming from the radial artery

Innervation:
It is innervated by nerves originating from the radial nerve for the dorsal side and the median nerve for the palmar side.

Different reference positions are described, the most common seems to describe MI at 40° of MII in the sagittal plane and at 20° of mII in the frontal plane

This joint has 2 degrees of freedom and allows the following movements

FLEXION - EXTENSION :

The flexion-extension is done along a transverse axis:

• Flexion: it brings MI closer to the axis of the hand and inclines the thumb towards the palm
• Extension: it allows the thumb to be placed in the plane of the palm

This movement is about 50° but can go up to 90°.

The motor muscles are for the flexors :

• I-flexor brevis
  • 2 leaders :
    • deep chief: fleshy fibers on the lower part of the anterior face of the trapezoid and capitate
    • superficial bundle: it originates from the retinaculum of the flexor muscles and on the lateral side of the trapezium ridge
  • The 2 chiefs run inferiorly and laterally to terminate in a common tendon on the lateral sesamoid and the lateral part of the base of the proximal phalanx of the thumb.
  • These 2 bundles form a sliding sheath for the tendon of the flexor longus muscle of the thumb.
  • It is innervated by the median nerve for the superficial head and by the ulnar nerve for the deep head
• Opponent of I
  • It originates on the upper part of the lateral side of the trapezius ridge and the retinaculum of the flexor muscles.
  • Thick and triangular, it goes down and out
  • Ends on the front, lateral edge of the^1st meta
  • It is the true flexor of the^1st meta
  • It is innervated by the median nerve
• Long flexor of the I 
  • Inserts:
    • On the upper ¾ of the anterior face of the radius
    • On the interosseous ligament
    • And sometimes even on the lateral edge of the coronoid process
  • Thick and unipinnate, its medial edge gives rise to a tendon that crosses the lateral part of the carpal tunnel and the thenar eminence between the heads of the flexor pollicis brevis muscle.
  • It ends on the palmar side of the base of P2
  • It is innervated by the median nerve C8-T1

The motor muscles for the extensors are :

• Long abductor of the I
  • fits on :
    • the posterior surface of the ulna
    • on the interosseous ligament
    • on the middle part of the posterior surface of the radius
  • fusiform, it is prolonged by a tendon oblique in bottom and outside which passes under the retinaculum of the extensors
  • ends on the lateral side of the base of the IM and often gives a fibrous expansion to the m. abductor brevis of the I
  • it is innervated by the deep branch of the radial nerve

• Extensor digitorum brevis
  • is inserted on
    • Middle 1/3 of the posterior surface of the radius
    • on the interosseous ligament
    • sometimes on the posterior surface of the ulna
  • its belly is small and will extend by an oblique tendon down and out which passes under the retinaculum of the extensor muscles
  • ends on the dorsal surface of the proximal phalanx of the thumb
  • innervated by the deep branch of the radial nerve
• Long extensor of the I
  • fits in
    • Middle 1/3 of the posterior surface of the ulna
    • about ME
  • its belly is fusiform and is prolonged by a tendon which goes down and out, which passes under the retinaculum of the extensor muscles
  • and ends at the dorsal surface of the base of the distal phalanx of the thumb
  • it is also innervated by the deep branch of the radial nerve

ABDUCTION- ADDUCTION :

• abduction spreads MI and MII
• the adduction is the opposite and thus the pulp of the thumb is against the^1st phalanx of the index

The movement is done thanks to a sagittal axis

And the overall amplitude is about 60°.

The motor muscles are :

• Abductor :
  • Short abductor of the I
    • Own muscle of the I, thenaric
    • Inserts on the anterior surface of the scaphoid tubercle and RMF
    • Heads down, out and forward
    • It is the most superficial muscle of the thenar muscles
    • Ends on the base of P1 of I and on the lateral sesamoid
    • Innervated by median N. C8-T1
  • Long abductor of the I already described
• Adductors :
  • Adductor I
    • clean muscle of the I
    • it is made up of 2 leaders
      • an oblique head that inserts on the inferior part of the natal face of the trapezoid and capitate bones and the lateral face of the base of MIII
      • transverse chief: which inserts on the anterior border of MII and MIII and the capsule of the metacarpophalangeal joints II and III
  • the 2 leaders are heading out
  • they end with a short tendon on the medial sesamoid and on the medial part of the base of the proximal phalanx of the thumb
  • it is innervated by the deep branch of the ulnar nerve C8-T1
  • when the muscle is paralyzed, the patient compensates by flexing his thumb, this is called the sign of the wheat journal 
• Incidentally: ^1st palmar interosseous
  • is inserted on the 1/2ant of the face which regrinds the axis of the hand
  • ends with 2 tendons
    • deep bundle: base of P1 on the side of the insertion of the same metacarpal
    • superficial beam

It is an ellipsoidal joint between

• The head of MI Convex and trapezoïd with dorsal base. It presents:
  • a phalangeal part for the glenoidal cavity of the phalanx
  • a palmar part with 2 tubercles that respond to the sesamoid bones
• The glenoidal cavity of the proximal phalanx: concave, oval with a large transverse axis

The glenoid fibrocartilage enlarges the glenoid palmaris, molds to M1, adheres to the flexor hallucis proprius of the thumb, and encompasses the sesamoids in its thickness.
The sesamoid bones follow the phalanx and the fibrocartilage

The means of union of this articulation are :

• a fibrous membrane that adheres dorsally to the tendons of the long and short extensor of the I
• a synovial membrane that forms 2 palmar and dorsal cul de sacs

The ligaments strengthen this joint:

• medial and lateral collateral ligaments: each ligament inserts on the medial or lateral tubercle of the head of the IM and then spreads out and ends on the base of the 1st phalanx, on the palmar ligament and the sesamoid bones
• palmar ligament, thick and encrusted with cartilage on its deep side, which contains the sesamoid bones of the thumb
• intersamoid ligament

Vascularization
This joint is vascularized by arteries coming from themain artery of the thumb and its branches

Innervation
This joint is innervated by nerves coming from the median nerve and the radial nerve.

This joint has 2 degrees of freedom and also allows F°-E° and abduction-adduction movements:

FLEXION - EXTENSION :

• the F° brings the thumb closer to the hand and the amplitude varies from 50 to 75°.
• the extension is almost null even passively, in some people it can however go up to 10°.

Muscles :

• Flexors :
  • Long flexor of the I
  • I-flexor brevis
  • ^1st palmar interosseous
• Expanders :
  • Long extensor of the I
  • Extensor digitorum brevis

ABDUCTION - ADDUCTION :

The amplitude of this movement is small compared to the other fingers.

The motor muscles :

• Abductors:
  • Long abductor of the I 
  • Short abductor of the I
• Adductor:
  • Adductor I 

The thumb has only one interphalangeal joint since it has only 2 phalanges against 3 in the other fingers.

Its axis is oblique in the opposite direction of the other fingers, i.e. inwards and downwards, which gives a pronator component to the flexion, thus allowing opposition with the other fingers

It is a synovial joint of the gingival type with the head of P1 as a pulley and the base of P2 biconcave with 2 glenoid joints separated by a median ridge

A fibro cartilage extends the 2 glenes

The means of union are :

• Capsule
• LL1 and LLE in 2 beams

This joint allows flexion-extension and a rotational movement that can be combined during opposition of the thumb

• The F° is 70°-80°.
• The active extension is 10°.
• And the passive extension is 30°.

The overall movement of the thumb is circumduction and opposition of the thumb

They result from the combination of the preceding movements and allow the opposition of the thumb which is a complex movement which allows to place the pulp of the thumb against the pulp of all the other fingers.

In this movement, all the joints of the thumb are involved to varying degrees.

  3 phases:

            1- antepulsion of M1

            2- M1 adduction and P1/M1 flexion

            3- axial rotation in pronation of M1 and P1

Marie Messager,
Osteopath in Versailles Chantiers
Yvelines - 78