Introduction: Anterior cruciate ligament (ACL) injury is the first cause of knee instability. There is not enough evidence for the best therapeutic option, as operative and non-operative treatments of anterior cruciate ligament (ACL) injuries are often associated with a lack of proprioception and a persistent muscle weakness of unclear origin.

Material and methods: This study in the cat experimentally compares in the long-term both neural and muscular activity in the knee articular nerves (PAN and MAN), quadriceps and hamstrings, in the chronic unstable knee and the reconstructed knee. Experimental section of ACL in twenty four cat’s knees was followed by stable reconstruction in six knees and unstable reconstruction in eight knees, leaving the other ten knees unstable without reconstruction. Electrical activity from muscles and nerves was registered with Ni-Cr electrodes. Mean firing activity and peristimulus time histograms (PSTH) for each structure were obtained. Secondly, it evaluates the neuromuscular response changes due to the graft’s mechanical competence, comparing stable and unstable reconstructed knees. Two different grafts were evaluated in the reconstructed knee, pediculated extensor digitorum longus and free bone-tendon-bone from patellar tendon. Statitiscal analysis consisted in Mann-Whitney’s test with Bonferroni adjustment between groups, and a two-way ANOVA to evaluate the separate effect of graft type and graft competence.

Results: We found an increased periarticular muscle activity during anterior tibial translation in chronically unstable knees. Both reconstructed and non-reconstructed unstable knees lose the fast reactive activity in the articular nerves. When stability was recovered after reconstruction, knees showed a more adjusted, although incomplete, muscular reaction. No differences were observed among grafts, but their mechanic competence was determinant in the neuromuscular firing activity.

Discussion: ACL injured knees in the cat, with or without reconstruction, display confirmed abnormalities in neuromuscular reaction in the long-term, while to regain stability with a competent graft in the reconstructed knee is crucial to lessen this anomalous reaction.

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.