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Optimal Graft Fixation Angles Proposed for ACL Reconstruction

Key findings

  • During dynamic step-up motion, ACL-deficient knees had significantly longer graft lengths compared with intact knees for all three tunnel positions studied
  • End-to-end differences between intact and deficient knees were relatively constant and did not increase with increasing flexion angles, with the most pronounced differences at lower flexion angles (<30°)
  • Fixation angles of <30° for AM, <10° for PL and <20° for SB-anatomic grafts may prevent permanent graft stretch
  • Rehabilitation approaches may need to differ according to whether the ACL has been repaired

For anterior cruciate ligament (ACL) reconstruction, improved understanding of graft length changes during knee flexion and extension could help surgeons determine the proper knee flexion angle for fixation and tensioning.

Peter D. Asnis, MD, chief of the Sports Medicine Center at Massachusetts General Hospital, and colleagues conducted the first study that assessed differences in end-to-end distances of the ACL between intact and ACL-deficient knees during dynamic, in vivo weight-bearing activity.

In The Knee, the researchers report on a study of 13 men and eight women (age range 18–59 years) who had a unilateral ACL tear and intact contralateral knee. Five patients had no significant damage to the menisci, eight had a medial meniscal tear and eight had a lateral meniscal tear.

As the patients performed a weight-bearing step-up motion, each knee was studied using combined magnetic resonance and dual fluoroscopic imaging. Using computer models, the researchers analyzed the end-to-end distances of the centers of the anatomic anteromedial (AM), posterolateral (PL) and single-bundle ACL reconstruction (SB-anatomic) tunnel positions.

The researchers compared elongation patterns in intact versus ACL-deficient knees. In addition, they calculated the deepest flexion fixation angle, based on a theoretical maximum graft length change of 6%.

The principal findings were:

  • In both intact and ACL-deficient knees, the AM, PL and SB-anatomic grafts were longest at 0° of flexion
  • Compared with intact knees, ACL-deficient knees had significantly longer end-to-end distances for the AM, PL and SB-anatomic grafts
  • In intact knees, the length of all three grafts decreased significantly with increased flexion
  • Appropriate fixation angles (not exceeding the critical threshold of 6% length change) are proposed to be about 30°, 10° and 20° for the AM, PL and SB-anatomic grafts, respectively.

The research team notes that fewer than one out of every four patients who sustain an ACL rupture undergoes ACL reconstruction within three years. The significant differences in mechanical characteristics between intact and ACL-deficient knees imply, they say, that rehabilitation strategy may need to differ according to whether the ACL has been repaired.

The study also highlights a potential role for muscle functioning in knee restraint, the researchers explain. They suggest that attention to proper muscle functioning during rehabilitation may help restore knee stability.

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