The long flexor muscles of the fingers originate at the medial epicondyle of the humerus. Two tendons attach to each finger. The superficial flexor tendon splits into two slips and ends at the base of the middle phalanx, while the deep flexor tendon passes through the split superficial tendon and attaches to the palmar base of the distal phalanx.
The flexor tendons are encased in tendon sheaths in the palm and fingers. They allow frictionless gliding with high amplitude and ensure a constant bone-near position of the flexor tendons, even during finger movement. Integrated into the tendon sheaths are reinforcing annular ligaments.
Radial and ulnar to the flexor tendon sheath runs the digital vessel/nerve bundle. At the level of the distal palm, the nerves are located palmar in the transverse plane, and the arteries dorsal. They are found 2 to 5 mm lateral to the flexor tendon sheath and are thus at risk of injury during surgical procedures.
The annular ligament system consists of a synovial-lined sheath around the flexor tendons, extending approximately from the neck of the metacarpal bone to the distal joint. The sheath is stiffened by fibrous reinforcements (stratum fibrosum), which are found at various intervals along its entire length.
These structures are oriented either transversely or obliquely. Typically, the transverse reinforcements are referred to as "annular ligaments" (Lig. anulare) and the oblique ones as "cruciate ligaments" (Lig. cruciforme), regardless of whether the oblique reinforcements are truly cruciform (x-shaped) or consist only of oblique fibers in a single orientation.
Numerous anatomical studies have confirmed that an annular ligament is always present on the palmar plate of the individual finger joints. Ring-shaped ligament structures are also regularly found on the shafts of the proximal and middle phalanges. These are designated from proximal to distal as A-1, A-2, A-3, A-4, and A-5. The cruciate ligaments are also numbered, from proximal to distal as C-1, C-2, and C-3. C-1 and C-3 are located distal to the A-2 and A-4 bands, respectively, and are attached proximally to the phalanx and distally to the palmar plate. The cruciate ligaments located proximal to the A-2 and A-4 annular ligaments are less regularly found. A distinct gap between the A-1 and A-2 annular ligaments is found in 40 to 90% depending on the cited study and is sometimes referred to in the literature as C-0. The C-3 cruciate ligament occurs more regularly than C-0 but varies in its form.
The A-2 annular ligament is the longest band and constitutes about 40% of the length of the proximal phalanx.
The A-1 annular ligament measures on average about 6 mm (4-8 mm), corresponding to about 20% of the proximal phalanx length.
The function of the annular ligaments is to closely guide the flexor tendons to the bone skeleton, thereby ensuring optimal force transmission and mobility of the fingers. The A-2 annular ligament plays the most important biomechanical role as the longest. The cruciate ligaments prevent collapse and allow the expansion of the tendon sheath during digital movements. They facilitate the approximation of the annular ligaments during maximum flexion.
This finely tuned movement system is massively disrupted in stenosing tenosynovitis, the so-called trigger finger. A nodule formation of the flexor tendons directly proximal to the tendon sheath entrance leads to mechanical irritation of the A-1 annular ligament with consequent stenosis. The gliding ability of the flexor tendons is impaired, resulting in a triggering phenomenon.
The primary therapeutic approach is conservative and involves the local infiltration of cortisone in the area of the A-1 annular ligament. This leads to good success, especially at the beginning of the disease, the tendon nodule becomes smaller, and the tendon runs smoothly again in the tendon canal. However, if mechanical narrowing has already developed, cortisone therapy is only of short duration. Surgical opening of the narrowing, the A-1 annular ligament release, is required to eliminate the entrapment.