Temporal changes in quadriceps tendon mechanical properties following anterior cruciate ligament reconstruction: an in vivo shear wave elastography study

Abstract In addition to hamstring and patellar tendon (PT) autografts, the quadriceps tendon (QT) has gained increasing popularity for anterior cruciate ligament reconstruction (ACLR), partly due to lower reported graft re-rupture rates. However, postoperative changes of the residual quadriceps tendon and the relationship between tendon mechanical properties and clinical outcomes remain poorly understood. The purpose of this study was to evaluate in vivo QT mechanical properties following graft harvest for ACLR and to quantify tendon mechanical properties using shear wave elastography (SWE). Twenty patients (11 males, 9 females; mean age 29.8 ± 7.2 years; mean BMI 26.8 ± 3.8 kg/m²) with anterior cruciate ligament rupture who underwent ACLR using an ipsilateral QT autograft were prospectively enrolled. In addition to standardized clinical outcome measures, participants underwent a standardized multimodal ultrasound protocol of the bilateral QT consisting of B-mode-ultrasound (B-US), Color Doppler-ultrasound (CD-US) and a SWE examination. Measurements were performed preoperatively (t0) and at 3 (t1), 6 (t2), and 12 months (t3) postoperatively. For SWE values a two-way repeated-measures analysis of variance (RM-ANOVA) was performed to evaluate differences over time and between sides. The within-subject factors were time (preoperative, 3 months, 6 months, and 12 months) and side (operated vs. contralateral control). For ROM measurements and scores a one-way repeated-measures analysis of variance (RM-ANOVA) was performed to evaluate differences over time. Effect sizes were reported as partial eta-squared (ηₚ²). Statistical significance was set at p < 0.05. Post-hoc pairwise comparisons were adjusted using the Bonferroni correction. Relationships between variables were assessed using the Pearson correlation coefficient (r). A significant main effect of time (F(3,57) = 2.86; p = 0.045; partial η² = 0.131) and a significant time-by-side interaction (F(3,57) = 3.98; p = 0.012; partial η² = 0.173) were observed. On the operated limb, a significant time effect was observed (F(3,57) = 5.14; p = 0.003), characterized by a transient increase in mechanical properties from the preoperative baseline to 3 months (Δ = −0.421; p = 0.045), followed by a significant decline from 3 to 6 months (Δ = +0.447; p = 0.003) and from 3 to 12 months (Δ = +0.368; p = 0.030). No significant difference was found between the preoperative baseline and 12 months. On the control limb, QT mechanical properties remained stable throughout the entire follow-up period (F(3,57) = 0.38; p = 0.769); none of the pairwise comparisons reached statistical significance. Over the 12-month follow-up period, significant improvements were observed in knee flexion as well as in the IKDC and Lysholm clinical scores, with the greatest gains occurring between the preoperative baseline and 6 months postoperatively. The Öhberg score exhibited a biphasic course with an initial increase at 3 months and subsequent normalisation by 12 months. Extension showed only a non-significant trend toward improvement. No statistically significant correlations were found between QT mechanical properties and clinical scores, as well as between QT mechanical properties and range of motion. Moderate correlations were observed between QT mechanical properties and IKDC ( r = 0.30, p = 0.207) and Lysholm Score ( r = 0.39, p = 0.231), without reaching significance. No significant correlations were observed between QT mechanical properties and knee flexion ( r = 0.11, p = 0.335), extension ( r = 0.10, p = 0.373), or Öhberg score ( r = -0.02, p = 0.873). This study represents the first in vivo assessment of quadriceps tendon SWE-derived mechanical properties following ACLR using a QT autograft. The findings indicate a high adaptation capacity of the QT, with transient postoperative changes in stiffness that normalize over time. Future studies incorporating longitudinal histological evaluation are warranted to further elucidate the biological mechanisms underlying QT alterations after graft harvest.