Purpose: Previous studies have confirmed that Achilles tendon occurs Achilles thickening after repair surgery of the rupture. Although this mechanism has been elucidated in the laboratory, there are few reports on its impact on clinical function. We designed a retrospective study to investigate the Achilles thickening after Achilles tendon rupture repair and its correlation between the elasticity and postoperative function.

Methods: In this retrospective analysis, patients who underwent surgical treatment for acute Achilles tendon rupture from April 2016 to April 2020 were included. All the patients were regularly followed up at 3 months, 1 year, and 2 years after surgery. American Orthopaedic Foot Ankle Surgeon (AOFAS) scale and Leppilahti score were used to evaluate functional outcomes. Achilles elasticity was measured by ultrasound shear wave of elasticity. Achilles thickening was calculated as maximal transverse and longitudinal diameter in cross-sectional plane of magnetic resonance scan. Sample t-tests was used for different follow-up periods. Correlation between Achilles thickening and other factors were analyzed using Pearson's method. p < 0.05 indicates a statistically significant difference.

Results: AOFAS scale and Leppilahti score at 1 year were significantly higher than at 3 months postoperatively (both p < 0.001). These functional scales were also improved at 2-year follow-up significantly (both p < 0.001). The dorsiflexion difference showed gradually recovery in each follow-up period (t = -17.907, p < 0.001). The elasticity of the Achilles appeared to continuously decreases during the postoperative follow-up period in all position sets (p < 0.001). In thickening evaluation, the cross-sectional area of the thickest plane of Achilles was significantly higher at 1 year postoperatively (310.5 ± 25.2) mm² than that at 3 months postoperatively ((278.0 ± 26.2) mm², t = -8.219, p < 0.001) and became thinner in 2-year magnetic resonance scan ((256.1 ± 15.1) mm², t = 16.769, p < 0.001). The correlations between Achilles thickening, elasticity, and functional outcome did not show statistical significance (p > 0.05) in every follow-up period.

Conclusion: Achilles tendon thickens after surgery in the 1st year, but begins to gradually return to thinning about 2 years after surgery. There was no significant correlation between the increase and decrease of thickening and the patients' clinical function scores, Achilles elasticity, and bilateral ankle dorsiflexion difference.