Viscoelastic properties of the semitendinosus and gracilis tendons in reconstruction of the ACL: an in vivo evaluation.

Tensioning of the tendons of the semitendinosus and the gracilis used to reconstruct the anterior cruciate ligament (ACL) of the knee is one of the main factors to take into consideration during surgery that aims at obtaining good postoperative joint stability. Some authors have shown in vitro that the viscoelastic nature of the patellar tendon is responsible for its gradual release (loss of tension). It is the purpose of this study to verify in vivo the biomechanical behavior of the semitendinosus and gracilis tendons. A total of 40 consecutive patients were submitted to reconstruction of the ACL with flexor tendons of the knee using a method that allows for the gradual tensioning of the doubled semitendinosus and gracilis tendons using the Swing Bridge method. Patients were divided into 2 groups: in the first 20 patients (group I), after adequate manual pretensioning and tibial and femoral stabilization, the tendons were twisted using a dynamometer to measure torsion forces until tensioning equal to 2.5 N was achieved. The control group (group II) was made up of the successive 20 patients in whom tensioning of the tendons at 2.5 N was carried out in many phases. Five minutes after initial tensioning at 2.5 N the decrease in tension was recorded that returned to initial values. A similar procedure was carried out after 5 more minutes. The patients in both groups were evaluated using a KT-1000 arthrometer one year after surgery. The patients in group II showed a decrease in torsion equal to 37.2% after the first 5 minutes, and a further 26.8% after 5 more minutes, and measurement by means of KT-1000 revealed mean values for S/S 30 lb. and S/S MM of 1.3 mm and 1.7 mm, respectively, that proved to have a statistically significant difference (p < 0.05) as compared to the patients in group I (S/S 30 lb.: 2.0 mm; S/S MM: 2.4 mm). Although these data do not provide indications on the amount and duration of pretensioning and definitive tensioning to be applied to the transplant, they do clearly show the viscoelastic properties of the tendons. These properties require a more in-depth study to better determine the exact tensioning of the neoligament.