An incision less than half the length of the extension was made. However, if the expanding tendon is deep (such as in psoas major muscle, or iliacus muscle), the incision may be made wider than usual. After identifying the tendon, it was exposed and stabilized using sterilized spitz tubes (Fig. 1a). Correcting the twist of the tendon was an important part of this process. Both ends of the tendon were marked depending on the length of the extension and overlap (Fig. 1b). For example, an α mm extension with a β mm overlap would require markings with a length of α + β mm. The tendon was threaded, as shown in Fig. 1c, d. At the time, the length of each loop had to be extended to the amount of tendon extension. It is particularly important to make the second loop length accurate as the locking mechanism mainly functions in the second loop. To accurately measure the loop length (α mm), the loop was made parallel to the tendon. The distance between the apex of the loop and the center of the tendon of each loop was adjusted using a Vernier caliper so that the distance was approximately half the amount of extension (1/2 α mm) (Fig. 2a–c). Symmetrical transverse incisions were made up to half the tendon width according to the two markings (Fig. 1e). Finally, the tendon was carefully extended to create a tense loop. After extending the tendon, the locking mechanism provided accurate volume extension and prevented overextension (Fig. 1f).

The suture procedure for tendon lengthening using the sliding-lengthening with a locking mechanism. a The tendon is exposed and stabilized using sterilized spitz tubes. b The markings show the total length of the extension and overlap. c, d Threading of the tendon. The arrows indicate the direction of threading. e Cutting of the tendon. The two markings indicate where the symmetrical transverse incisions are made, which are up to half the tendon width. f Extension of the tendon to create a tense loop

Accurate loop length measurement. a–c The loop is positioned parallel to the tendon. The distance between the apex of the loop and the center of the tendon of each loop should be half the amount of extension (1/2 α mm). The distance is adjusted using a Vernier caliper

Complications associated with this technique include cut-out of the thread and rupture of the tendon at the half-section. The following pitfalls are important: (1) correcting the torsion of an exposed tendon; (2) making the second loop length more precise, as it is primarily a locking mechanism; and (3) carefully extending the tendon so that it does not break during tendon extension.

The patient was a 6-year-old boy with CP, right lower limb monoplegia, and gross motor function classification system (GMFCS) level 1. Owing to the high tension of the flexor hallucis longus (FHL), we performed FHL-only lengthening. The preoperative extension angle of the big toe was -45 degrees at the metatarsophalangeal (MTP) joint and -30 degrees at the interphalangeal (IP) joint in passive extension. Only a 15 mm skin incision was made posterior to the medial malleolus of the ankle joint, as shown in Fig. 3a. We planned to perform a 15 mm extension and 15 mm overlap, and therefore, we pulled out 30 mm of the FHL and marked it (Fig. 3b). Loop length was determined to be 15 mm using a Vernier caliper (Fig. 3c). We then performed tendon lengthening and confirmed that the overlap was 15 mm (Fig. 3d). The postoperative passive extension angle of the big toe was up to 30 degrees at the MTP joint and 10 degrees at the IP joint.

A 6-year-old boy with cerebral palsy with right lower limb monoplegia. a Only 15 mm skin incision was made posterior to the medial condyle of the ankle joint. b Flexor hallucis longus was pulled out. c Loop length measurement using a Vernier caliper. d Immediately after lengthening