No abstract

PurposeThe biarticular anatomy of the gastrocnemii is an important mechanism of knee-ankle coupling and diferential elongation may afect this function leading to weakness of the push-of phase during the gait. Achilles tendon ruptures may cause detachment of the gastrocnemius tendon from the soleus aponeurosis with subsequent diferential elongation of the individual subtendons. This study investigated the efects of such detachment by investigating tendon fusion levels of the two muscle groups, and the efect of sequential diferential elongation of the gastrocnemius on the Achilles tendon resting angle (ATRA) and to the knee-ankle coupling. Methods Conjoined tendon length (CTL) was measured in 23 cadavers. ATRA in knee extension (ATRA 0) and 90-degree knee lexion (ATRA 90) was measured with the gastrocnemius tendons (GT) intact, transected and with the gap reduced in 5-mm increments. In 15 specimens, knee-ankle coupling was examined. Results Considerable anatomical variation was present with CTL ranging from 2 to 40% of ibular length. In the intact triceps, surae ATRA 0 difered from ATRA 90 by 6 degrees (p < 0.001). Cutting the gastrocnemius caused an immediate separation of the tendon ends by 19 mm. ATRA 0 and ATRA 90 increased 8 and 4 degrees (p < 0.001), signiicantly larger increase for ATRA 0 (p < 0.001). Lengthening the gastrocnemius 10 mm altered the coupling point 10 degrees towards dorsilexion.Transixing the gastrocnemius at the level of the gap where the Achilles was sectioned, decoupled the knee-ankle coupling in all but two specimens. A moderate correlation between CTL and length of the medial gastrocnemius tendon was found. Conclusions A greater relative ATRA 0 than relative ATRA 90 indicates diferential elongation of the gastrocnemius. By elongating the gastrocnemius the knee-ankle coupling point shifts dorsally, and 20 mm elongation completely decouples the knee-ankle coupling. Independent lengthening of the gastrocnemius may explain the loss of power experienced by some patients following acute Achilles tendon rupture despite what would appear to be appropriate approximation of the ruptured tendon ends. Recognizing this occurrence is crucial when treating Achilles tendon ruptures and such patients require surgical correction in order to avoid long-term weakness of push-of strength.