Muscles Inserting onto the Intermediate Phalanges of the Hand
The flexor digitorum superficialis (FDS) is the primary muscle that inserts onto the intermediate phalanges of the hand.
Anatomical Insertion Pattern
The flexor digitorum superficialis is an extrinsic multi-tendoned muscle with four digital components, each with a tendon that inserts onto the intermediate phalanx of its corresponding finger:
- The FDS tendon splits into two slips that pass around the flexor digitorum profundus (FDP) tendon
- These slips then reunite and insert onto the volar aspect of the intermediate phalanx
- This insertion allows the FDS to flex the proximal interphalangeal joints of the four fingers
Functional Significance
The FDS has several important functional characteristics:
- It is the only muscle that directly flexes the proximal interphalangeal joints
- Each digital component can be selectively activated by volition, allowing for independent finger movement 1
- This selective recruitment is particularly important for precision grip and fine motor control
- At higher forces (approximately 50% MVC), there is increased co-activation between adjacent fingers 1
Clinical Relevance
Understanding the insertion of the FDS onto the intermediate phalanges has important clinical implications:
- Injuries to the FDS tendon can result in inability to flex the proximal interphalangeal joint while the distal interphalangeal joint remains functional (due to intact FDP)
- During clinical examination, the FDS function can be tested by holding all other fingers in extension while asking the patient to flex the finger being tested
- This prevents the action of the FDP, which inserts onto the distal phalanx 2
Anatomical Variations
Some notable variations in the FDS insertion pattern include:
- Hypoplastic or absent FDS tendons, particularly to the little finger 3
- Variable insertion patterns that may affect finger function
- These variations should be considered when evaluating tendon injuries or unexplained finger weakness
Comparative Anatomy
When compared to non-human primates:
- Humans have relatively smaller flexor ridge insertions on proximal phalanges compared to African apes 4
- This reflects the evolutionary adaptation of human hands for manipulation rather than locomotion
- The human fifth digit shows a proportionally larger flexor ridge, suggesting its importance in precision grip 4
Understanding the precise anatomy of the FDS insertion onto the intermediate phalanges is essential for accurate diagnosis and treatment of hand injuries, particularly in cases of tendon lacerations and other traumatic injuries to the hand.