Aims: The purpose of this study is to determine muscle fibre length around the knee before and after anterior cruciate ligament (ACL) transection, and to test its ability to detect instability changes.

Material and methods: Pairs of piezoelectric crystals were inserted in four periarticular muscles (vastus lateralis -VL-, vastus medialis -VM-, lateral hamstrings -LH-, medial hamstrings -MH-) around 11 knees from 8 cats. Distance between pairs was measured while performing 10 passive repetitions of anterior tibial displacement at 90 degrees and 30 degrees flexed knee, flexion and extension, and controlled by sagittal plane video recordings. Ultrasound signal between crystals in each pair was analysed to obtain intensity and time-dependent parameters. Data from control and unstable knees were statistically compared.

Results: Fibre lengthening was observed in the four studied muscles under anterior tibial traction. Significant increases in fibre length at VL, VM (p< 0,05) and LH (P< 0,01) were seen in unstable knees versus normal knees when anterior tibial traction was performed at 90 degrees knee flexion. Flexion and extension produced passive changes in fibre length (quadriceps lengthening and hamstrings shortening for flexion, the opposite for extension), but no significant variations were measured when the ACL was sectioned.

Discussion: Ultrasonomicrometry offers a new approach to comprehend anterior instability of the knee when studying periarticular muscle fibre length. Variations in the response are specific for the anterior tibial displacement in unstable knees. This technique can be used in combination with electromyography for a better understanding of muscle behaviour in ACL deficient knees.