Background Knee instability after Anterior Cruciate Ligament (ACL) reconstructive surgery may lead to osteoarthritis.

Objective To complete current evaluation of anterior knee laxity.

Design Cross-sectional pilot study.

Setting Physical therapy unit of the Clinique d'Eich, Centre Hospitalier de Luxembourg.

Patients 14 patients were examined 197±25 days following arthroscopic ACL reconstructive surgery (11 men and 3 women, age: 28±9 years) using the GNRB, a new knee arthrometer measuring anterior tibial displacement (ATD) when applying a standardised, progressive anterior tibial force. Inclusion criteria were: isolated ACL rupture; no prior injury in the controlateral knee.

Interventions Both knees were tested using three trials with a progressive force up to 200 N, starting with the controlateral leg. The average ATD-force curve of trials 2 and 3 were used for further calculations.

Main outcomes ATD at 134 and 200 N, and new variables: primary energy dissipation (PED: area under the ATD-force curve from 0 to 100 N), secondary energy dissipation (SED: area under the ATD-force curve from 100 to 200 N), primary compliance (PC: slope of the ATD-force curve between 30 and 50 N) and secondary compliance (SC: slope of the ATD-force curve between 100 and 200 N) were used as indicators of knee laxity.

Results The results found for the operated and the controlateral knee, respectively, were: ATD at 134 N=5.6±1.6 and 4.0±0.9 mm (difference: 42%); ATD at 200 N=7.2±1.8 and 5.3±1.0 mm (difference: 34%); PED=230±80 and 146±46 N/mm; SED=622±167 and 445±96 N/mm (difference of area between both curves at 134 N: 52%, at 200 N: 44%); PC=63±22 and 40±13 μm/N; SC=25±6 and 22±4 μm/N. Results were significantly different (p<0.05) for all indicators except SC (Student t-test).

Conclusion Anterior knee joint laxity 5 to 10 months after ACL reconstructive surgery remains greater than the non-injured contralateral knee. A complete evaluation of anterior knee laxity could help the prognostic of osteoarthritis after ACL reconstructive surgery.