Three volar Interossei adduct the second, fourth, and fifth digits about an axis formed by the third digit. These muscles originate on the metacarpal of the digit being adducted and insert to their respective proximal phalangeal base and extensor hood. Adduction occurring along this third digit axis means that the volar interosseous muscle of the second digit is located on its ulnar border, while the interossei of fourth and fifth digit reside on their radial borders.[19] 

Four dorsal interossei work to abduct the second, third, and fourth digits while assisting with flexion of the MCPs, extension of the PIPs, and extension of the DIPs. The first dorsal interosseous muscle originates from the first and second metacarpals, inserting to the radial base of proximal phalanx of the second digit as well as its extensor hood. The second dorsal interosseous muscle originates from second and third metacarpals, inserting to the radial base of proximal phalanx of the third digit as well as its extensor hood. The third dorsal interosseous muscle originates from third and fourth metacarpals, inserting to the ulnar base of proximal phalanx of the third digit as well as its extensor hood. The fourth dorsal interosseous muscle originates from fourth and fifth metacarpals, inserting to the ulnar base of proximal phalanx of the fourth digit as well as its extensor hood.[19] 

Abductor pollicis longus (APL) originates in the forearm, at the midshaft of the radial border of ulna and the ulnar border of radius, as well as the interosseous membrane. It inserts to the radial side of the base of the first metacarpal in addition to the trapezium and opponens pollicis fascia.[20] This muscle acts to abduct the first digit.

Abductor pollicis brevis (APB) originates at trapezium and scaphoid and inserts to the lateral base of proximal phalanx of the first digit. This muscle acts in first digit abduction in addition to MCP flexion.[21]

Extensor pollicis longus (EPL) originates at the radial border of ulna and interosseous membrane, distal to the origin of APL. EPL inserts to the base of the distal phalanx and acts to extend the interphalangeal joint.[20] 

Extensor pollicis brevis (EPB) originates at the ulnar border of the radius and interosseous membrane at the level of EPL. It inserts to the base of proximal phalanx of the first digit and aids in first digit extension at the MCP.[20]

Opponens pollicis acts to pronate the first metacarpal. It originates at trapezium and inserts to the anterolateral aspect of the first metacarpal.[21]

Flexor carpi radialis (FCR) originates at the medial humeral epicondyle and inserts to trapezium, second metacarpal, and third metacarpal.  FCR acts to flex and radially deviate the wrist.[22]

Flexor pollicis brevis (FPB) has a deep and superficial head. Deep head originates at second metacarpal, while superficial head originates at transverse carpal ligament and trapezium. These insert to the lateral base of proximal phalanx of the first digit and act to flex the MCP.[21]

Flexor pollicis longus (FPL) is the primary flexor of the first digit, originating from the volar interosseous membrane and radius.  It inserts to the base of the distal phalanx.[23]

Adductor pollicis has two heads, an oblique and transverse.  The oblique head originates at the capitate, second, and third metacarpals. The transverse head originates at distal half of third metacarpal shaft.  These both insert to the medial base of thumb proximal phalanx and act to adduct first digit.[21]

Abductor digiti minimi (ADM) acts to abduct the fifth digit and has been found to originate at the pisiform, piso-hamate ligament and flexor carpi ulnaris.  It inserts to the ulnar aspect of proximal phalangeal base and extensor apparatus of the fifth digit.[24]

Flexor digiti minimi (FDM) originates from hamate, pisiform, and flexor retinaculum. Distally, it fuses with ADM to insert to the proximal phalangeal base of the fifth digit, leading to its action of MCP flexion.[24]

Extensor carpi ulnaris originates at the lateral humeral epicondyle and inserts to the dorsal base of the fifth metacarpal, acting in wrist extension and ulnar deviation.[25]

Flexor digitorum superficialis (FDS) has a broad origination from the medial epicondyle of humerus and coronoid process of the ulna to the volar aspect of the radial shaft. It then inserts to the middle phalangeal bases of second through fifth digits. This acts primarily in PIP flexion.[26]

Flexor digitorum profundus (FDP) originates from the volar-medial aspect of the ulna and interosseous membrane and inserts at the distal phalangeal bases of second through fifth digits. This performs DIP flexion.[27]

Extensor digitorum communis (central slip and terminal tendons) originates at the lateral epicondyle of humerus and inserts at the dorsal aspect of the distal phalangeal base of the second through fifth digits. There is significant anatomic variation in the accessory slips and insertions of this muscle, but the classic teaching is that there is an insertion to the middle phalanx and MCP in addition to this consistently present terminal insertion.[28]

There is a great deal of physiologic variation between individuals in the morphology of the hand bones. One of the tools utilized to discuss the variance in hand anatomy is the digit length ratio of the second to the fourth digit, otherwise referred to as the 2D to 4D ratio. This number has demonstrated to be 0.98 among the general population but shows variability between ethnicities, races, and sexes, as well as the handedness of the patient. The third digit is consistently the longest of the digits. The most significant source of variance in the hand bones is the result of differences in size.[10]

Metacarpal fractures should undergo fixation if there is any component of malrotation to ensure that patients can make a closed fist and grip properly.

Surgery is indicated in the treatment of these fractures when the fracture pattern falls outside of acceptable angulation tolerances, and thus the patient’s functional outcome would benefit from surgical fixation by lag screw fixation and/or plating techniques, or via percutaneous pinning of the fractures.[29][2] Tolerances for fractures of the shaft and neck of each metacarpal vary with the index tolerating the least angulation and the little finger tolerating the most. Fractures of the fifth metacarpal neck, otherwise known as boxer’s fractures highlight the good outcomes of nonoperative treatment of the majority of these fractures.[30][31]

include MCP or IP dislocations which are irreducible will also need to undergo surgical fixation to facilitate the restoration of function.[32] Phalanx fractures with displacement, nonunion, malunion, ipsilateral extremity fractures, pathologic fractures, or multiple unstable ipsilateral hand fractures may require surgical intervention.[33] Dorsal intraarticular fractures of the distal phalanx, otherwise known as mallet finger, rarely need to undergo surgical fixation, but intervention is indicated with greater than 30 percent involvement of the articular surface or for volar subluxation of the distal phalanx.[34] Volar interarticular avulsion fractures of the distal phalanx, also known as jersey finger, by in large require surgical fixation for the restoration of flexor digitorum profundus function.[35]

Polydactyly occurs once in every 700 to 1000 live births, making it a relatively common anatomical variant. There are a variety of types of polydactyly, and they can broadly categorize as syndromic or non-syndromic. Subtypes of polydactyly exist based upon the point of origination of the extra digit. Though the identities of the exact genetic links are still a work in progress, patients who present with polydactyly should have a careful workup to rule out underlying conditions.[36]

Small joint osteoarthritis characteristically demonstrates Heberden and Bouchard nodes of the DIP and PIP, respectively. These nodes represent the cutaneous manifestations of joint space narrowing and osteophyte formation resulting from osteoarthritis. Research shows a positive correlation between the appearance of these nodes and underlying osteoarthritis.[37]

Rheumatoid arthritis (RA) is another form of arthritis that often presents with prominent hand manifestations. Swan neck and boutonniere deformities are classically described, in addition to the ulnar deviation of the digits. The swan neck deformity is characterized by DIP flexion with PIP extension, while the inverse is true of the boutonniere deformity.  Nowadays, disease-modifying antirheumatic drugs (DMARDs) have been able to reduce the severity of hand disease seen in RA, but early intervention should be started to prevent disability and progressive joint erosion seen in the natural course of the disease.[38]