Detailed Anatomy and Ossification of Metacarpal and Phalangeal Bones

METACARPAL BONES

1. The five little long bones that make up the metacarpal bones are numbered from the lateral to the medial side.
2. Every bone has a base at the proximal end, a shaft, and a head positioned distally.
3. The head is rounded, and its articular surface extends more laterally than anteroposteriorly. Compared to the dorsal surface, the palmar surface is where it extends more. During flexion, the heads of the metacarpal bones form the knuckles.
   ii. The palmar surface of the shaft has a concave shape. In its distal region, the dorsal surface has a flat, triangular section.
   iii. The base has an uneven expansion

Features of Every Single Metacarpal Bone

• All of the metacarpal bones, it is the shortest and stoutest.
• A concavoconvex articular surface for the trapezium resides in the base.
• The shaft's dorsal surface exhibits uniform convexity d. Compared to the heads of other metacarpals, this one has a wider, less convex head. Sesamoid bone imprints can be seen on the palmar surface at the radial and ulnar corners.
• With respect to the other metacarpals, the first metacarpal bone, which is positioned on a more anterior plane, is rotated medially by 90°. The thumb moves at a right angle to the other digits' movements as a result of this rotation.
• There is no other metacarpal bone with which it articulates.
• The base has a backwards-facing groove from previously. The groove's medial edge is wider.
• A styloid projection protrudes from the dorsolateral corner of the base.
• The base features a single elongated facet for the fifth metacarpal on its medial side and two small oval facets for the third metacarpal on its lateral side.
• For the fourth metacarpal, the base bears an extended articular strip on its lateral side. There is a tubercle on the non-articular medial side of the base.

Side Metacarpal Determination

It is possible to identify the proximal, distal, palmar, and dorsal features of each metacarpal bone based on the information provided above. The following standards can be used to verify the lateral and medial sides.

• Greater than the anteromedial surface is the anterolateral surface.
• The base's groove's medial edge is deeper than its lateral edge.
•  An articular strip that is constricted in the middle is present on the medial side of the base.
• Dorsolateral styloid process.
  i. An articular strip with a constriction in the middle is present on the lateral side of the base.
  ii. The fourth metacarpal contains two tiny oval facets on the medial side of the base.
• The base's lateral side includes two tiny oval facets that correspond to the third metacarpal.
• An extended articular strip for the fifth metacarpal is located on the medial side of the base.
• An enlarged articular strip for the fourth metacarpal is seen on the lateral side of the base.
• The base's medial side bears a tubercle and is non-articular.

Principal Metacarpal Attachments

The palmar and dorsal interossei muscles are the primary attachment points of the metacarpal shaft. All but the third metacarpal, one bone gives birth to the palmar interossei. From the opposite sides of two metacarpals, dorsal interossei emerge. The list of other attachments is provided below.

• The radial border and the anterolateral surface of the shaft are where the opponens pollicis is placed.
• The lateral side of the base is where the abductor pollicis longus is placed.
• The ulnar side of the base gives birth to the first palmar interosseous muscle.
• A tubercle on the base's palmar surface receives the insertion of the flexor carpi radialis.
• The dorsal surface of the base is where the extensor carpi radialis longus is placed.
• The palmar surface of the base is where the adductor pollicis' oblique head originates.
• On the base's palmar surface, a slip from the flexor carpi radialis is inserted.
• Just past the styloid process on the dorsal aspect of the base, the extensor carpi radialis brevis is inserted.
  
• The palmar surface of the base is where the adductor pollicis' oblique head originates.
• The distal two-thirds of the shaft's palmar surface give rise to the adductor poll's transverse head.
• From it just the interossei emerge.
• The base of the tubercle is where the extensor carpi ulnaris is placed.
• The medial surface of the shaft is where the opponens digiti minimi is placed.

Disputes at the Bases

• A saddle-shaped joint is formed by the trapezium.
• Including the third metacarpal, the capitate, the trapezium, and the trapezoid.
• The 2nd and 4th metacarpals and the capitate.
• The third and fifth metacarpals, the hamate, and the capitate.
• The fourth metacarpal and the hamate.

Ossification

A secondary center for the head appears in the second through fifth metacarpals, while a secondary center for the base appears in the first metacarpal. • The shafts ossify from one primary center each, which appears during the ninth week of development. About 16 to 18 years old is when it merges with the shaft, appearing in the second or third year.

Medical Anatomy

Bennett's fracture is the name given to the break in the base of the first metacarpal. It is caused by a force acting along the long axis of the base and affects the anterior portion of the base. The thumb is compelled to adopt a semi-flexed posture and is incapable of resisting. It is not possible to clinch the fist. Direct or indirect trauma might potentially break the other metacarpals. The fragmented portion frequently moves forward when there is direct violence. They become reversely displaced by indirect violence.

 Because the nutritive artery splits into a plexus as soon as it reaches the medullary cavity, tubercular or syphilitic illness of the metacarpals or phalanges in children occurs in the center of the diaphysis rather than the metaphysis. However, because the nutritional artery is replaced in adults, the risk of infection is reduced. The first metacarpal has two epiphyses, one at each end, when the thumb has three phalanges. The first metacarpal sometimes bifurcates distally. Consequently, the medial branch has two phalanges and no distal epiphysis. There are three phalanges and a distal epiphysis on the lateral branch. In this scenario, there are six total digits (polydactyly).

Phalanges

• Each hand has 14 phalanges: two for the thumb and three for each finger. Every phalanx consists of a head, shaft, and base.
• The base of the proximal phalanx is identified by an incave oval facet that articulates with the metacarpal bone's head. It is distinguished by two little concave facets separated by a smooth ridge in the middle phalanx, also known as a distal phal

Shaft

Near the head, the shaft tapers. From side to side, the dorsal surface exhibits convexity. Although the palmar surface is slightly concave along its long axis, it is flattened side to side.

Head

The articular surface of the head of the proximal and middle phalanges is fashioned like a pulley. The head of the distal phalanges is non-articular and is distinguished anteriorly by a rough tuberosity in the shape of a horseshoe that holds up the delicate finger pulp.

Attachments

• The distal phalanx's base
  a. The palmar surface is where the flexor digitorum profundus is placed.
  b. Digital expansion fuse slips with two sides that should be placed on the dorsal surface. The lumbrical and interossei muscles' insertions are likewise extended by these.
• The central phalange
  A. On either side of the shaft, the two flexor digitorum superficialis slips are placed.
  b. The side of the shaft also has the fibrous flexor sheath linked to it.
  c. Dorsal digital expansion inserts a significant portion of the extensor digitorum onto the base's dorsal surface.
• The phalangeal proximum
  a. The sides of the shaft hold the fibrous flexor sheath in place.
  b. Parts of the interossei and lumbricals are inserted on either side of the base.
• The following structures are attached to the base of the proximal phalanx in the thumb.
  a. On the lateral side are inserted the abductor and flexor pollicis brevis.
  b. On the medial side are the adductor pollicis and the first palmar interosseous.
  c. The dorsal surface is occupied by the extensor pollicis brevis.
  d. The insertion of the flexor and abductor digiti minimi in the little finger is provided by the medial side of the proximal phalanx base.

Ossification:

• The distal phalanx, proximal phalanx, and middle phalanx all have major centers that emerge in the eighth, tenth, and twelveth weeks of development, respectively, from which the shaft of each phalanx ossifies.
• In 2-4 years, the secondary center emerges for the base, and in 15–18 years, it fuses with the shaft.

SESAMOID BONES

Sesamoid bones are tiny, spherical lumps of bone that are found in some tendons at high pressure areas. Their frequency of incidence varies. They are listed below.

• The flexor carpi ulnaris tendon contains the pisiform, which is frequently thought of as a sesamoid bone.
• On the palmar surface of the head of the first metacarpal bone, there are always two sesamoid bones.
• In 75% of patients, the thumb's interphalangeal joint capsule contains a single sesamoid bone.
• About 75% of participants have a single sesamoid bone on the ulnar side of the capsule containing the little finger's metacarpophalangeal joint.
• Less frequently, the lateral side of the index finger's metacarpophalangeal joint has a sesamoid bone.
• Sesamoid bone can occasionally be discovered at additional metacarpophalangeal joints.