Fractional Lengthening of the Forearm Flexor Muscles: Anatomic Study

Forearm muscles can undergo contracture for a number of reasons, including spasticity. This deformity is amenable to surgical treatment in select cases. Among the different techniques available, fractional lengthening of the forearm flexor muscles involves multiple tenotomies at the musculotendinous junction. We studied the anatomy of the musculotendinous junction of all forearm flexor muscles to analyze the topography and extent of muscle-tendon overlapping for each muscle and to determine the area where fractional lengthening can be performed safely.

Dissections were performed on 20 fresh cadaveric upper limbs. For each muscle, we defined and measured the total overlapping zone, "corrected" overlapping zone, and useful zone (UZ), along with 3-dimensional mapping of the location of each tendon with respect to the muscles' fibers.

With regard to the wrist flexors, the average UZ was very short for the flexor carpi radialis (3.5 cm) and very long for the flexor carpi ulnaris (12.2 cm). With regard to the finger flexors, the UZ of the superficialis tendons varied greatly (2.7-5.9 cm), whereas it was relatively constant for the profundi (7.6 cm) and flexor pollicis longus (6.5 cm).

Fractional lengthening is dependent on the anatomy of the musculotendinous junction of each individual muscle. For muscles with a relatively short and variable UZ (flexor carpi radialis, flexor digitorum superficialis [FDS] II, and FDS IV), the feasibility of the procedure must be carefully evaluated intraoperatively. For FDS V, which constantly displays a very short UZ, with a thin and fragile tendon, the procedure may be risky and unreliable.

When considering fractional lengthening of the forearm muscles, differences between the tendons should be considered, and surgeons should be prepared for alternative approaches, especially for FDS V.