The mechanical property changes occurring in the semitendinosus tendon post ACL reconstruction with a hamstring autograft

Suydam, Stephen M.
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University of Delaware
The potential strength asymmetries post anterior cruciate ligament (ACL) reconstruction using a bundled semitendinosus (ST) and gracilis tendon autograft can cause asymmetry in functional tasks measuring return previous level of activity. In order to utilize the regrowth of the ST tendon for the purpose of eliminating asymmetry, it is essential to show that the tendon indeed recovers its functional properties and correlate those to bilateral strength differences. Therefore, the aims of this study are as follows: Develop a non-invasive measurement system for determining tendon functional properties, calculate the elastic properties of regrown ST tendons and correlate those properties to bilateral strength compared to the healthy limb. Continuous shear wave elastography (cSWE) was used on gel samples and compared to published standard measure of MR elastography. cSWE matched the published standard with only 8.2% difference. Echogenicity effectiveness was assessed through the use of B-mode ultrasound and isometric contractions on the Achilles tendon. Ultrasound images were taken to track the soleus junction and collect image brightness during isometric contractions. Tendon stress was then correlated to brightness change. Due to minimal correlation between echogenicity and stress (r 2 =0.05), cSWE was used to evaluate the semitendinosus tendon. Sufficient power was reached with 13 subjects, between 6-24 months post-ACL reconstruction, to measure the bilateral difference in shear elastic modulus. To establish the impact of the mechanical property recovery, the same subjects were measured for peak torque during concentric flexion, eccentric flexion, and internal rotation of the leg. Flexion strength was determined at 60o/s and 120o/s and internal rotation strength was determined at 30o/s and 60o/s. The shear elastic modulus was significantly different between the involved and uninvolved legs (p=0.004) and was strongly correlated (r=0.60) to recovery time. The recovery of the tendon's shear modulus was strongly correlated to the difference in internal rotation strength at both 30o/s and 60o/s (r=0.70,0.65). This study is the first to show remaining strength asymmetries previously demonstrated by a semitendinosus resection can be recovered with time. Knowing the tendon recovers its mechanical properties between 6 and 24 months may correlate with the recovery rate within the 2 years following ACL reconstruction.