Flexor Digitorum Superficialis : Wheeless' Textbook of Orthopaedics

- See:
- Forearm Flexors
- Swan Neck Deformity following Excision of FDS
- FDS Laceration:

- Anatomy:
- origin:
- humero-ulnar head: medial epicondyle of humerus by common tendon, ulnar collateral ligament of elbow & coronoid process of ulna;
- radial: oblique line of the radial tuberosity to the insertion of pronator teres;
        - orgins are interconnected by a fibrous aponeurosis that overlies the median nerve and the ulnar artery;
    - divisions:
          - in the midpart of the forearm, the superficialis muscle divides into four bundles, which separate into a superficial and a deep layer;
- superficial layer includes tendons to the long and ring fingers
- deep layer includes tendons to the index and little fingers;
- FDS tendon to the small finger may not be present in all individuals
       - at wrist, flexor superficialis tendons are volar to tendons of FDP, and FDS covers the median nerve proximally in the forearm;
      - in mid palmar area, the FDS tendons are more volar than the flexor profundus tendons;
      - both enter the fibrous annular ligaments and the synovial sheaths;
    - distal anatomy:
          - at level of MP joints, FDS becomes more dorsal& FDP tendon more volar;
      - proximal skin creases at base of digits coincide w/ separation of FDS tendons into 2 segments, while FDP becomes subQ
          and FDS is deep;
- in this zone II region overlying the proximal phalanx, both the fds and fdp tendons are relatively avascular;
    - insertion and Camper's chiasma :
          - once they enter the flexor tendon sheath, the FDS separates into 2 segments, which pass around FDP tendon, and which then reunite at
                   Camper's chiasma (dorsal to FDP) as they insert onto the radial and ulnar aspects of proximal half of the middle phalanx;
          - four tendons (one for ea finger) divide for passage of tendons of FDP then insert as 2 slips on sides of the palmar surface ofproximal
                   half of the middle phalanx with the remainder inserting as radial and ulnar slips on the diaphysis of the middle phalanx;
    - action:
- flexes the 2nd phalanx of each finger on the proximal phalanx;
- continued action, flexes the 1st phalanx at the hand;
- hand at the wrist, flexes the forearm at the elbow;
- synergist: FDP;
- nerve supply: median, C7, C8, T1;

- Exam:
- FDP tendons of ulnar 3 digits have minimal individualization;
- ulnar three digits share common FDP muscle belly, and thus independent flexion of any
finger w/ others restrained in extension requires intact FDS functions to that finger;
- if one digit is held in extension, only limited flexion of adjacent DIP joints is possible;
- lesser number of bands extend from ulnar 3 digits to index finger;
- for this reason, the index finger functions more independently;
- FDS to the smallfinger may be absent in some individuals;
- hence to examine for FDS function, adjacent digits must be held in extension, in order to eliminate FDP motion in adjacent
fingers, which otherwise might give the impression of FDS motion in the examined finger;

- FDS Tendon Graft:
- each flexor FDS muscle tendon unit is separate from adjacent fingers, and individual action of fingers in performing flexion is possible;
- occassionally FDS tendons of ring & little fingers are removed for tendon transfer at base of ring finger, and a connecting
slip between 2 tendons prevents easy removal until the attachment in the palm is recognized;
- ring and little finger tendons usually lie in the same sheath but can be separated by traction;
- occassionally they are united in the mid palmar area;
- cautions:
- FDS harvest from the ring finger should be avoid in high ulnar nerve palsy, (w/ the goal of restoring adductor pollicis function), since this may
significantly decrease power grip;
- technique:
- tendon can be removed at the proximal phalangeal crease or at the PIP joint;
- if the tendon is removed at the PIP joint, be sure to leave one slip of the FDS as long as possible, in order to avoid swan neck deformity;
- avoidance of postop donor site complications:
- generally the digit is splinted in extension for 3 weeks, which is then followed followed by gentle flexion and extension for 3 weeks;
- swan neck deformity may follow FDS harvest;
- this is likely to occur if the joint is splinted in hyper-extension, and is more likely to occur if the harvest is performed at the PIP joint;
- this can be prevented by using one slip of the superficialis for tenodesis of the PIP joint, but this is usually not necessary;
- flexion contracture:
- may occur if the digit is splinted in flexion;
- more likely to occur if the harvest is performed at the MP joint;
- this complication is managed initially w/ manipulation of the digit, or if contracture persists, by excising remaining slips of the FDS