[Evaluation of laxity, rigidity and compliance of the normal and pathological knee. Application to survival curves of ligamentoplasties].

PURPOSE OF THE STUDY

The aim of this prospective study was to measure the stiffness of the ACL in normal knees, ACL deficient knees and after ACL reconstructions or meniscectomies. Stiffness is a physical quantity which expresses objectively the mechanical efficiency of the ACL.

MATERIAL AND METHODS

1502 tests were performed on: 480 normal control knees, 191 acute tears and 171 chronic instabilities pre and post-operatively, 60 extra-articular plasties, 30 meniscectomies and 64 arthritic knees before and after a cruciate sparing arthroplasty. The force displacement measurements were made on radiograms of the medial and lateral compartment of each knee at 20 degrees flexion by applying a postero anterior force from 0 to 300 N with increments of 50 N. The stiffness is the slope of the F/dl curve.

RESULTS

In the normal knee the medial compartment is fixed. Its stiffness is 13.8 x 10(4) N/m and does not depend on age and sex. The diagnosis of ACL rupture is made when the right/left difference on the medial compartment is at least 4 mm at 250 N. The lowest level for an accurate diagnostic is 180 N. The positive predictive value is 99 per cent. In acute tears the stiffness is 2.5 x 10(4) N/m. It is 3.4 x 10(4) N/m in chronic instability (p = 0.0001). When a meniscectomy was performed in chronic instability the stiffness decreases significantly (M+: 3.7 x 10(4) N/m; M-: 2.1 x 10(4) N/m) (p = 0.03). After a bone-patellar tendon-bone plasty (BPTB) the stiffness was 6.0 x 10(4) N/m and did not decrease with the passage of time but after the extra-articular plasty (EAP) the stiffness was 4.7 x 10(4) N/m after 24 months and 3.0 x 10(4) N/m after 60 months. Meniscectomy decreases the stiffness in both procedures, however it remains stable after BPTB, but decreases with the passage of time after EAP.

DISCUSSION

Stiffness is a biomechanical parameter of knee ligaments. It is highly correlated with the clinical symptoms of instability (p = 0.0001). It is more accurate than laxity which is a numerical value without mechanical significance.

CONCLUSION

This method gives functional information on the ACL deficient knee. It is the more precise method for the diagnostic of ACL rupture with an efficiency of 98.5 per cent. It has a high prognostic value after ACL surgery and can be considered as the mechanical survival of the plasty.