Posterior Placement of Computer-Simulated ACL Grafts Yields More Anisometric Behavior During Knee Flexion
Key findings
- The least anisometric area was found in the proximal-distal direction, just posterior to the intercondylar notch
- Anteriorly positioned femoral sockets showed less length change than more posteriorly positioned sockets, which greatly decreased in length with increasing flexion
- Tibial location significantly affected graft strains for the anatomic, over-the-top and most isometric socket locations
Newer tibial-independent drilling techniques allow for more anatomic placement of anterior cruciate ligament (ACL) grafts, and allow greater graft length changes during knee flexion. Knowledge of the relationship between graft length changes and socket positioning is crucial to a successful reconstruction.
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Strains of four to six percent can result in permanent graft stretch and/or failure, so correct fixation angle and tensioning appears to be especially important in anisometric ACL reconstruction. While the isometry of the ACL has been explored in multiple cadaver studies, they do not incorporate muscle forces that control the knee during dynamic motion.
To study isometry and strain in vivo, Peter D. Asnis, MD, chief of the Sports Medicine Center at Massachusetts General Hospital, and colleagues obtained motion data from healthy volunteers and used computer simulations to analyze theoretical ACL grafts. In Arthroscopy, they report that grafts placed more posteriorly result in more anisometric behavior during knee flexion.
The researchers used dual fluoroscopy and magnetic resonance imaging to track tibiofemoral motion during step-up and sit-to-stand exercises by 12 men and six women. These kinematics were used as inputs into the computer simulation, which calculated strain in the ACL at 144 theoretical attachment sites on the femur and three theoretical attachment sites on the tibia.
Comparisons were made between the anatomic, over-the-top and most isometric locations of the femoral socket and tibial insertions. The researchers created a “heat map” to visually represent the isometric distribution over the medial aspect of the lateral femoral condyle.
The most isometric femoral socket location was approximately midway on the Blumensaat line, just posterior to the intercondylar femoral notch. This was true during both motions and for all three tibial attachment sites studied. A graft in the most isometric position underwent approximately two and six percent strain during step-up and sit-to-stand, respectively.
The strains were most affected by changing the femoral socket positions in the anterior-posterior direction. Posterior attachments resulted in decreased lengths with increasing flexion angles, and anterior-distal grafts increased in length with increasing flexion angles.
The researchers give the example that graft fixation at 30° of flexion may result in poor outcomes if the femoral socket is placed posteriorly because of repetitive stretch, shortening cycles from 30° to full extension. Such a possibility is an especially important consideration for the posterolateral socket during double-bundle ACL reconstruction.
The researchers call attention to the fact that the most isometric point on the femur was located far from the anatomic ACL insertion site. Beyond approximately 20° of knee flexion, the most isometric location and the over-the-top location had significantly higher strains than the anatomic location. These increased strains may account for the lack of improved outcomes with nonanatomic reconstructions.
The researchers also found that tibial location (anatomic, over-the-top or most isometric) significantly affected mean isometry. Other researchers recently reported that posterior positioning of the tibial socket is associated with increased risk of revision, and the authors say that further study of this issue is warranted.
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