Interphalangeal joints of the hand

Articular surfaces

Each interphalangeal joint is composed of the head of the more proximal phalanx and the base of its distal counterpart. This means that, for example, the proximal interphalangeal joint is formed by the union of the head of the proximal phalanx with the base of the middle phalanx. This same pattern applies to the distal interphalangeal joint, meaning that it is formed by the articulation of the head of the middle phalanx with the base of the distal phalanx.

Closer inspection of the phalangeal head reveals two curved condylar processes with a shallow groove in between. These condyles are received by two concavities of reciprocal size and shape on the base of the distally lying phalanx. Between these two concavities is a raised ridge of bone that glides within the groove of the phalangeal head, promoting intraarticular stability. These joint surfaces are covered by a layer of hyaline cartilage that extends further palmarly than dorsally, creating a proximal articular surface which is greater than the distal surface. Unlike the metacarpophalangeal joints, there is little articular surface on the dorsal aspect, and therefore little hyperextension. 

The point of articulation does not occur directly at the apex of the condylar processes, but rather on their inner sloping surfaces. This results in the radii of the condyles of the phalangeal head being greater than that of the convex surfaces of the phalangeal base, leaving the joint markedly incongruent. This incongruency manifests as a small intercondylar joint space in most people.

Joint capsule and ligaments

Surrounding each interphalangeal joint is a fibrous joint capsule, the inner lining of which is comprised of a synovial membrane. Strengthening each joint capsule are two collateral ligaments and a palmar ligament, also known as a palmar/volar plate. Dorsally, the joint capsule is strengthened by a broadening of the extensor tendons. This extensive ligamentous contribution to each joint capsule increases the articular surface of the phalangeal base, improving joint congruence.

Innervation

The interphalangeal joints of the hand receive innervation from the proper palmar digital nerves, arising from both the median and ulnar nerves. Specifically, digits 1-3 and the lateral half of digit 4 are innervated by the median nerve, while the medial half of digit 4 and the entirety of digit 5 are innervated by the ulnar nerve.

Blood supply

Each interphalangeal joint receives oxygenated blood via the proper palmar digital arteries, which are the distal extensions of the superficial palmar arch.

Movements

The morphology of the interphalangeal joints of the hand permit flexion and extension as their only active movements. However, a small degree of passive accessory movements are allowed primarily at the distal interphalangeal joints of digits 2-5.

Flexion and extension occurs in the thumb about a transverse axis that crosses through the middle of the neck of the proximal phalanx. The range of motion consists of up to about 90° flexion and 10° extension, with passive hyperextension possible when a large amount of force is applied to the distal phalanx. Strong collateral ligaments prevent any passive accessory rotational or lateral movements of the interphalangeal joint of the thumb.

Flexion and extension of digit 2, often referred to as the index finger, occurs entirely in the sagittal plane. More medially lying digits, however, have their flexion and extension occur increasingly obliquely so as to better oppose the thumb.

The degree of flexion of the proximal interphalangeal joints increases slightly across digits 2-5, however can be generally said to be in the range of 100° to 110°. In the case of the distal interphalangeal joints, digit 3 exhibits the greatest degree of flexion (80°), with digit 5 featuring the least (70°).

The strength of flexion of the interphalangeal joints of the hand are influenced by the positioning of the wrist joint. When the wrist is extended, the finger flexor muscles are lengthened, increasing the amount of tension that can be developed within them, resulting in a stronger grip. Conversely, when the wrist is flexed, these finger flexors are slackened, and their ability to generate tension is decreased. This manifests as a weaker grip.

Active extension of digits 2-5 is much more limited and can be achieved up to 2° in the proximal interphalangeal joints, and 5° in the distal.

The closed packed position of the interphalangeal joints of all digits of the hand is full extension, while the open packed (resting) position is slight flexion. These joints have a capsular pattern such that they are more limited in flexion than extension.

Passive accessory movements are limited to digits 2-5 and include anteroposterior gliding, rotation, and abduction-adduction movements. These subtle movements permit the fingers to adapt to objects of various shapes and sizes during gripping.

Muscles acting on the interphalangeal joints of the hand

The extraordinary dexterity exhibited by the fingers is reflected in the number of muscles that can act upon them. Both intrinsic and extrinsic hand muscles are responsible for producing flexion-extension movements at the interphalangeal joints of the hand.

Flexion of the interphalangeal joint of the thumb is accomplished through the action of the flexor pollicis longus muscle. The proximal interphalangeal joints of digits 2-5, meanwhile, are flexed via the flexor digitorum superficialis and flexor digitorum profundus, the latter of which also extends to the distal phalanx, and is therefore the only muscle capable of flexing the distal interphalangeal joints.

Extension of the interphalangeal joint of the thumb is performed by the extensor pollicis longus. Both the proximal and distal interphalangeal joints of digits 2-5 are extended through the actions of extensor digitorum, lumbricals, and dorsal interossei. Digit 2, the index finger, gets additional extension via the extensor indicis, while the palmar interossei assist with extension of digits 2, 4, and 5. The muscles responsible for extending digits 2-5 do so through their tendinous aponeurotic insertion into the extensor expansion. This hood-like expansion extends down the length of digits 2-5,  and is anchored on each side by the palmar ligament. It functions to maintain the direction of pull of the extensor tendons in the midline of each digit.