Custom-made testing fixtures for the femur and tibia allowing for an inclined bone fixation to position the PCL aligned with the load axis were fixated to the actuator and the base plate of a tensile testing machine (ElectroPuls 10000; Instron). The femur and tibia were clamped in an anatomic position at a knee flexion angle of 90°, as this presents the worst-case scenario for PCL testing.11,28

Before testing, each graft was placed into a graft tube and preconditioned for 5 minutes at 80 N.

For PCL reconstructions with ST reinforcement (ALD-ALD ST group), the initial intra-articular distance between both bone tunnel exits was set to 40 mm. The graft and the ST were inserted into the bone tunnels without tensioning the graft. In step 1, the ST was fixated in the tibia using a 4.75-mm knotless PEEK (polyether ether ketone) anchor (Arthrex). Subsequently, the machine was moved slightly until 5 N was reached (step 2). That position was then defined as a simulated knee flexion angle of 90°, ensuring that the ST was equally loaded at 90° for all constructs.

For PCL reconstructions without ST reinforcement (ALD-ALD group), steps 1 and 2 were not performed because no ST was involved. The initial distance was set to 40 mm, which directly corresponds to a simulated knee flexion angle of 90°.

The femoral ALD was shortened until the graft was pulled into the femoral socket for a distance of 20 mm, allowing enough space for retensioning. The graft in the ALD-ALD and ALD-ALD ST groups was then slightly tensioned on the tibial site until a load of 50 N was reached and then knotted. This load level for the initial tensioning was chosen because during pretests, it was observed that moderate manually tensioning resulted in approximately 50 N.

For PCL reconstructions with ST reinforcement (Screw-ALD ST group), the initial intra-articular distance was set to 40 mm. The graft and the ST were inserted into the bone tunnels without tensioning the graft. In step 1, the ST was fixated in the tibia using a knotless anchor. Subsequently, the machine was slightly moved until 5 N was reached (step 2). That position was then defined as a simulated knee flexion angle of 90°.

For PCL reconstructions without ST reinforcement (Screw-ALD group), steps 1 and 2 were not performed. The initial distance was set to 40 mm, which directly corresponds to a simulated knee flexion angle of 90°.

For the Screw-ALD and Screw-ALD ST groups, a weight of 50 N was suspended from the whipstitching sutures, and an interference screw was inserted for tibial fixation. To ensure equal starting conditions for all screw fixation constructs, the machine’s displacement was then changed until 50 N was reached.

Subsequent to graft insertion and tibial fixation of the constructs, testing was started with 10 precycles between +0.5 and −8.0 mm of displacement, respectively (simulating ROM from 90° to 0° of knee flexion) at a frequency of 0.5 Hz (step 3). In step 4, the machine moved to a displacement of 0 mm, corresponding to a simulated knee flexion angle of 75°, and the digital displacement was set to zero. At this position, the graft was retensioned on the femoral site to approximately 200 N, and knots were tied on the femoral ALD. Then, position-controlled cyclic loading started between +0.5 and –8.0 mm of displacement (simulating ROM from 90° to 0° of knee flexion) for 1000 cycles at 1 Hz (step 5). Upon completion, load-controlled cyclic loading from 10 to 250 N and 500 N, respectively, simulating a progressive rehabilitation protocol (steps 6 and 7), was performed before a pull to failure at 50 mm/min (step 8) at a knee flexion angle of 90°.

Data were recorded at an acquisition rate of 500 Hz, and the mode of failure was noted. All specimens were constantly kept moist using a physiologic saline solution during preparation and testing.

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