Traction tests on soft tissues show that the shape of the stress-strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -two for the elasticity and one for the viscosity -were enough to precisely fit the non-linear stress-strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.
In a series of 30 consecutive patients who suffered from chronic instability of the knee joint, reconstruction of the torn anterior cruciate ligament was performed with a looped semitendinosus tendon, reinforced by an extra-articular anterolateral procedure. Of these 30, 27 could be followed up 9-11 years after the operation. The evaluation included the International Knee Documentation Committee (IKDC) questionnaire and was completed by testing with a Kneelax arthrometer at 132 N and by anteroposterior standing X-ray, in order to evaluate the degenerative changes. At the time of the check-up: 96% of the study group considered that they had normal or nearly normal knees, and 81% had recovered to the same sports activity level as before their injury. The degenerative changes noted at the index operation did not progress notably, except in two cases. Laxities of 7 knees were normal, with a side-to-side difference of less than 2 mm; 15 were nearly normal, with a mean difference of 3.45 mm; and 5 were abnormal, with a mean difference of 6.2 mm. The study shows that the procedure is efficient in restoring a satisfactory stability for most patients and stabilises the evolution of the degenerative lesions as shown by standing X-ray.
The purpose of this study was to compare three types of objective measurement in Lachman test position and to validate a new measurement procedure. Twenty-three patients were evaluated after anterior cruciate ligament (ACL) reconstruction at 10 years' mean follow-up. The contralateral knee had a normal ACL. Both knees were compared using Lachman test, arthrometer, radiological drawer, and electromagnetic sensing device (EMS) measurements. The values were obtained by two trained orthopaedic surgeons independently. Arthrometer assessment was done with the Kneelax 3 (Biodex) at 88 and 132 N. Radiological drawer was measured with a load of 10 kg. EMS is composed of an electromagnetic transmitter and two sensors. Displacement of the sensors was measured along the three perpendicular axes of the transmitter. Data after processing give the displacement of the sensors perpendicular to the tibial axis. The load was applied on the calf perpendicular to the tibial axis with a 10-kg counter-weight. Metallic objects must be avoided around the set-up. No significant difference between examiners was observed in clinical (P = 0.45) or objective (P > 0.3) measurements. The best correlation was obtained between arthrometer and EMS measurements (R = 0.94). Arthrometric and radiological drawer were poorly correlated (R = 0.65). The values of the medial radiological drawer were quantitatively similar to the 132 N arthrometric values. The sensitivity of EMS to skin movements was tested. The correlation between the EMS drawer and the other movements was not significant (R < or = 0.37). Quantitative correspondence with Lachman grades was only 36% of the arthrometer and EMS values and 37% of the radiological measurements. The effectiveness of EMS to evaluate anterior tibial displacement in Lachman test position after ACL reconstruction is demonstrated in this study. This method is non-invasive and comfortable for the patient, but needs a careful set-up. This method can potentially be used to assess rotational laxities.
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