Senter for idrettsskadeforskning

Beskrivelse

Background: Loss of fixation and graft loosening are some of the common reasons for failure and revision surgery and 6-7% of all operations on the ACL in Norway are revision surgery. Fixation of soft tissue grafts has been reported to be the weakest link in ACL reconstruction. Therefore, improving fixation techniques is important to increase success rates in ACL reconstruction. It has been well documented that soft tissue ACL grafts fixed with interference screws have suboptimal biomechanical properties compared to bone plug fixation, including lower ultimate failure loads. Because of the suboptimal fixation of these devices, newer devices have been introduced. These devices have not been compared biomechanically in a controlled setting by non-industry researchers.

This study aims to determine if new soft tissue fixation devices will have better biomechanical properties than previously reported interference screw fixation in the tibia and if any of the commercially available products are superior to the others in the porcine model. This is of significant clinical relevance because failures of ACL reconstruction are often caused by fixation failure of the grafts-hardware interface before the grafts have attached themselves firmly in the bony tunnels. Biomechanical studies have suggested the graft experiences a load of approximately 300 N to 500 N when patient is walking, depending on the method being used to measure the forces. Thus, the fixation strength of the ACL construct needs to surpass this. Studies have shown that the tibia has weaker pullout values compared to the femur. The testing will be carried out on both the tibia and femur. 

Materials and methods

Specimens: Fifty-six intact fresh frozen bovine cadaveric knees, and one hundred and twelve fresh frozen human quadriceps tendon allografts will be obtained and examined grossly for previous injury, and only used if found to be free of any defect or disease.

Tibial and femoral fixation biomechanics

Specimen preparation: The tibia will be cut distal to the joint line, whereas the femur will be cut proximal to the joint line and placed in a cylindrical mold filled with polymethylmethacrylate (PMMA). Two screws will be placed into the distal tibia and the proximal femur to ensure rigid fixation. The grafts will be sized to a 9-mm diameter. 

Surgical technique: Tibias and femurs will be divided into 7 groups respectively, 8 specimens per group, for each device tested. ACL tunnels will be created with a 9 mm reamer n the tibia and femur using previously described techniques.

The quadriceps graft will be advanced from distal to proximal through the tibia and femur tunnel and randomised to different fixation devices. The graft will be looped around the Instron fixture proximally for the tibial test and distally for the femur test. The direction of pull is designed to be in-line with the tunnel to create a worst-case scenario for biomechanical testing.

Fixation devices: The fixation devices are placed under tension of the graft (60 N) with the different tensioning devices and guide wires, dilators and screws according to the manufacturer. Selected devices for testing will include one interference screw – the BIOSURE PK (Smith & Nephew Inc., Andover, MA), five suspensory devices – SUTURE WASHER 15mm (Smith & Nephew Inc., Andover, MA), Endobutton (Smith & Nephew Inc., Andover, MA). Endobutton – CL (Smith & Nephew Inc. Andover, MA.), ToggleLoc (Zimmer-Biomet, Warsaw, IN). TightRope (Arthrex Inc., Naples, FL).

Biomechanical testing methods: The specimens are locked into a fixture and into an Instron E10000 ElectroPulse Dynamic Testing System (Instron Systems, Norwood. MA, USA). The grafts are isolated and preloaded from 10 to 50 N at 0.1 Hz for 10 cycles. The grafts are then loaded for 500 cycles between 50 and 250 N at a frequency of 1 Hz. Surviving grafts are further displaced at 20 mm/min with a dynamic tensile testing machine until failure. The mechanism of failure is recorded (pullout, intrasubstance tendon stretch). Cyclic displacement (mm), maximum load at failure (N), pull-out displacement (mm), and pull-out stiffness (N/mm) is determined. In addition, load at 3 mm displacement (N) will be recorded. A 3 mm side-to-side difference in anterior translation after ACL reconstruction has been reported to be consistent with a complete ACL tear.

Power analysis: Based on previous studies, any specimen that has graft slippage of more than 2.5 mm will be considered a clinical failure. An a priori sample size calculation was done using the G*Power 2 program based on previous published data regarding differences in tibia tunnel fixation strength. Given this difference and an average of 133 N standard deviation, an alpha of 0.05 and a power 0f 0.85; 8 specimens will be needed. (G*POWER, University of Melbourne, Parkville, Victoria 3010. Australia)

Results: Yield load, ultimate failure load, cyclic displacement and removal time will be compared.