PCL Tunnel Placement

Teaching notes

The posterior cruciate ligament (PCL) consists of a large anterolateral bundle (ALB) and a smaller posteromedial bundle (PMB); the ALB is the primary restraint to posterior tibial translation and is tightest near 70-80 degrees of flexion. Accurate femoral tunnel placement on the lateral wall of the medial condyle is the single most consequential technical decision in PCL reconstruction, because even small errors in the proximal-distal direction alter graft length patterns far more than equivalent errors in the anterior-posterior direction.

Femoral Footprint and Tunnel Target

The PCL femoral footprint lies on the lateral surface of the medial condyle and is approximately semicircular. The center of the ALB sits 7.9 mm from the distal articular cartilage. Arthroscopically in a right knee, the isometric point is 8 mm proximal to the articular cartilage at the 1 o'clock position. During drilling, the guide pin should enter 5 to 10 mm from the intra-articular cartilage, and 1 to 2 mm of bone must remain at the articular margin after reaming.

A key asymmetry governs tunnel placement errors: a proximal-distal shift alters the graft length pattern approximately three times as much as an equivalent anterior-posterior shift. Campbell therefore states that if any error must occur, it should be in the anterior-posterior direction rather than the proximal-distal direction.

Classic Femoral Tunnel Errors

An anterior-distal femoral tunnel increases the distance between the two tunnel centers as the knee flexes, so the graft tightens progressively in flexion. The clinical result is loss of 10 to 20 degrees of flexion; if the surgeon attempts to force motion, the graft is overstretched.

A posterior-proximal femoral tunnel has the opposite effect: the inter-tunnel distance decreases with flexion, leaving the graft lax throughout the flexion arc. The tibia sags posteriorly, and posterior sag and drawer persist. On stress view, residual posterior translation greater than 4 mm defines residual posterior laxity.

Tibial Tunnel and the Killer Turn

In the transtibial technique, the graft exits the posterior tibial aperture and turns sharply toward the femoral tunnel, forming a near-right-angle bend known as the killer turn. This bend abrades and thins the graft and complicates tensioning. The tibial guide should be angled approximately 60 degrees relative to the tibial articular surface; a more perpendicular angle creates an excessively acute bend at the posterior tibia, and a tunnel started too distally may ream out the posterior tibial shelf.

Two strategies reduce the tibial-side bend: the tibial inlay technique places a bone block in the posterior trough, eliminating the exit bend and allowing a larger graft; alternatively, an anterolateral tibial tunnel starting point reduces the wrap angle. Neither approach has been proven superior in clinical outcome studies.

Graft Fixation Angle

The fixation angle determines the reference length against which all knee positions are compared. Fixation in full extension without load creates an overconstrained knee: every degree of flexion demands additional graft length beyond the fixed length, generating high graft forces and reducing total translation. Fixation at 90 degrees of flexion with an anterior drawer produces kinematics that most closely approximate the normal knee.

Two established techniques reflect this principle: in the standard technique the tibia is held at 90 degrees with an anterior drawer; in the Harner technique an Achilles allograft is fixed at 70 to 80 degrees with 10 pounds of tension, because the ALB is under maximal tension in that range.

Neurovascular Safety During Tibial Drilling

The popliteal artery and tibial nerve are at risk when the tibial tunnel is drilled. The anterior wall of the popliteal artery lies only 7 to 10 mm posterior to the PCL; this distance increases with knee flexion. Drilling at 90 degrees of flexion moves the neurovascular bundle away from the tibia. A curette or protective guide should be placed posterior to the tibia before pin advancement, and fluoroscopy should confirm pin position before reaming.

Postoperative Considerations

After PCL reconstruction, the knee is braced with the hinge locked in full extension. An extension deficit or flexion contracture appearing weeks to months postoperatively is usually caused by prolonged immobilization in flexion, not by tunnel malposition; tunnel errors principally affect either flexion range or posterior stability, not extension. A late complication of femoral-side drilling is osteonecrosis of the medial condyle, arising from subchondral vascular injury during dissection or reaming.