1️⃣ Definition and Indicators of Tunnel Widening
- Tunnel widening is identified as a postoperative enlargement greater than 2 mm on anteroposterior or lateral radiographs, a change that raises suspicion when observed in the parallel walls of a cone-shaped tunnel.
- This phenomenon is primarily cavitary and often peaks in the mid-zone of the tibial tunnel, occurring in the joint's plane of movement.
(Korean J Radiol 2016;17(6):919-930).
2️⃣ Causes of Tunnel Widening
- Micromotion at the graft-native tunnel interface.
- Malposition of the tibial tunnel, leading to graft micromotion.
- Graft to tunnel size mismatch, which may cause roof impingement, extension deficit, graft tear, or Cyclops lesion.
- The "windshield wiper effect," where sagittal plane micromotion of the graft in its tunnel causes bone resorption.
- "Bungee cord effect," or longitudinal elongation due to similar micromotion mechanisms
(Skeletal Radiol. 2017 Feb;46(2):161-169; Stoller's Orthopaedics and Sports Medicine: The Knee).
3️⃣ Postoperative ACL Tunnel Diameter and Graft Selection
- Immediately after ACL reconstruction surgery, the expected tunnel diameter should be around 10 mm, regardless of the graft selection.
- Drilling diameters vary by graft type, with hamstring tunnels typically made with a 7.5–9-mm drill bit and bone-patellar tendon-bone (BPTB) tunnels with a 10-mm bit
(Skeletal Radiol. 2017 Feb;46(2):161-169).
4️⃣ Graft Type and Tunnel Enlargement
- Tunnel widening is more prevalent with soft tissue grafts secured by suspensory fixation devices.
- Compared to bone-patellar tendon-bone (BPTB) autografts, hamstring (HS) autografts show significantly greater tunnel expansion, suggesting that a bone-to-bone interface might reduce graft micromotion.
- Furthermore, graft failure and tunnel expansion are more common with bioabsorbable screws than with metal screws
(Skeletal Radiol. 2017 Feb;46(2):161-169).
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