When the anterior cruciate ligament (ACL) tears again after reconstruction, surgeons pay close attention to the “bone tunnels” left by the first operation and may request three-dimensional CT. Some patients even need bone tunnel grafting first, followed by another ACL reconstruction 6–12 months later.

This is because ACL reconstruction requires tunnels in the femur and tibia to place and fix the graft. If the tunnels from the first operation are poorly positioned, markedly widened, or associated with bone defects, they can directly affect the position and fixation of the new graft during the second operation. Bone tunnel assessment is therefore an important part of planning ACL revision surgery.

The anterior cruciate ligament lies inside the knee joint and connects the femur to the tibia.

During ACL reconstruction, the surgeon creates channels in the femur and tibia, then passes and fixes the tendon or other graft used to replace the ACL through these channels. These channels inside the bone are the bone tunnels, also called tunnel tracks.

Tunnel position is important because it determines the direction of the new ligament inside the knee. If the position is appropriate, the graft can more closely reproduce the original ACL anatomy and help stabilize the knee during motion.

After the first reconstruction fails, these tunnels do not disappear. During the second ACL reconstruction, the surgeon must plan graft placement on femur and tibia that have already undergone surgery, making revision surgery usually more complex than the first reconstruction.

During ACL revision, one key question is where the new graft should be placed.

If the tunnels from the first operation are well positioned and their condition allows it, the original tunnels may be prepared and reused.

If the original tunnels are slightly offset from the ideal position, the new tunnels may need to change direction so that the graft can be placed as appropriately as possible while avoiding major interference between the old and new tunnels.

If the original tunnels are clearly outside the normal ACL position, the surgeon may need to create new anatomical tunnels.

After ACL reconstruction, some patients develop a tunnel diameter larger than the one originally created. This is usually called bone tunnel widening.

If widening is limited, the surgeon may be able to complete one-stage revision by preparing the tunnel again.

If widening is marked or a large bone defect has formed, fixation of the new graft may be affected. In this situation, bone grafting may be needed first to restore bone stock before a new ACL reconstruction.

For patients whose original tunnel is essentially in the normal anatomical position, the surgeon can identify the tunnel and prepare it again along the original direction.

In related one-stage revision strategies, surgeons gradually enlarge the drill hole according to the tunnel diameter measured on preoperative CT and “refresh” the original tunnel until cancellous bone bleeding can be seen arthroscopically on the tunnel surface. The purpose is to provide a suitable bony environment for the new graft.

For patients whose original tunnel position has some deviation, the surgeon may adjust the direction of the new tunnel rather than reconstructing the ligament exactly along the old tunnel.

If the old tunnel is clearly outside the normal position, a new tunnel can be created at a site that better matches ACL anatomy.

If the original tunnel and bone defect are not suitable for direct ACL reconstruction, the surgeon may choose staged treatment.

In the first stage, the ACL is temporarily not reconstructed. The surgeon first addresses the tunnels left by the first operation, and the implanted bone tissue also needs time to heal with the surrounding bone. For this reason, the second-stage reconstruction cannot be performed immediately.

About 6–12 months after surgery, CT is used to confirm satisfactory bony healing of the graft. The second-stage ACL reconstruction is then performed, with new femoral and tibial tunnels created and a new ligament graft placed.

Tunnel position determines whether the new graft can be placed in a relatively appropriate anatomical position. Tunnel size and bone stock affect whether the new graft can obtain reliable fixation.

For example, an old tunnel with a basically correct position may be reusable, while a clearly misplaced old tunnel may still allow the surgeon to avoid it and create a new anatomical tunnel.

If the relationship between old and new tunnels is complex, or if the remaining bone stock cannot support new fixation, the surgical plan may change.

Lower-limb alignment in the coronal and sagittal planes, cartilage injury, meniscus status, other ligament function, and bone quality should also be assessed because all may influence the surgical plan.

The patient’s activity level and the degree of recovery they hope to achieve are also part of preoperative assessment.