Current concepts in tibial screw fixation in ACL reconstruction: A new solution

Introduction
Fixation of a graft has been shown to be one of the most important factors in determining the long-term success of ACL reconstruction. Consequently, tibial fixation is commonly considered to be more problematic than femoral fixation because the forces on the ACL substitute are parallel to the tibial tunnel¹. Also the bone quality of the tibial metaphysis is inferior to that of the femur^2,3 and the tail end of the tendon graft is more difficult to secure. Any fixation device used must provide enough strength to prevent graft slippage during rehabilitation until graft healing can occur at about 12 weeks.

With regard to interference screws in general, it has been shown that the strength of fixation of the soft tissue grafts is influenced by several variables, such as the density of the bone4, the insertion torque4, the geometry^5,6, and material of the screw⁷. However, there is controversy over the effects of screw length on graft fixation strength^7,8,9.

Controversy exists regarding the healing of tendon in a bone tunnel during ACL reconstruction. All basic science work has been done in animals, which is then extrapolated to humans. Thus, it is difficult to state how different methods of fixation affect tendon healing.

Weiler et al.¹⁰ have shown that deterioration of a soft tissue ACL graft begins immediately after implantation, leading to a significant decrease in the maximum load to failure and construct stiffness at 6-9 weeks. From 0 to 12 weeks the mode of failure is usually tendon slippage past the fixation. Rodeo et al.⁹ studied healing in dogs, and noted failure was by graft pullout up to 8 weeks post implantation.

Tendon-to-bone healing (ligamentisation) will occur in the femoral and tibial bone tunnels, depending on the fixation device used, either by indirect tendon-to-bone healing with a clear fibrous interzone between the tendon graft and bone, or by development of a direct ligament insertion where a zone of metaplastic fibrocartilage is present between the graft and lamellar bone. Interference screw fixation provides direct compression of the graft against the wall of the bone tunnel and promotes direct tendon-to-bone healing by reducing shear forces^{12, 13, 14, 15, 16, 17}. In contrast to biomechanical studies, the time frame for this process is thought to be between 12 and 24 weeks.

We aimed to design and test a new tibial fixation device that would provide rigid fixation and optimise the process of ligamentisation.

New Design
The GraftBolt™ (Arthrex; Naples, FL) is a sheath design for tibial fixation, aimed at providing concentric aperture fixation. The sheath is made of PEEK and is designed to expand on insertion of a PEEK interference screw. Expansion occurs due to distortion of 12 rectangular defects in the sheath, which correspond to 12 radially expanding tabs which protrude on screw insertion. (Figure 1)

Fig 1. Photograph of GraftBolt™

Fixation occurs by the radial compression of the graft against the wall, provided by 2mm of sheath expansion, supplemented by the secondary fixation obtained from a further 2mm of tab protrusion. The tabs are offset relative to the row above, to provide 360 degree compression.

Finite Element analysis (FEA)
A FEA based numerical model has previously been established to investigate the mechanical behavior of the bone and tendon graft in an ACL reconstructed knee using different forms of fixation¹⁸. This showed that with aperture fixation the maximum stress within the tendon graft was found to be at the proximal end of the tibial tunnel. The lowest stress was in the distal end of the tunnel.

In interference screw fixation, the stress on the tunnel wall varies between 10-20 MPa. The maximum stress occurs at the interface between the sharp threads of the screw and the tendon/wall of the tunnel indicating the cutting of the threads into the porous bone and tendon Fig 3A.

Figure 3. FEA showing the stress distribution during interference fixation (A) versus GraftBolt (B)

Modelling of the GraftBolt™ revealed a more evenly distributed compressive stress distribution on the tunnel wall, with areas of peak stress corresponding to the individual protruding tabs Fig 3B. We postulated that this would provide improved pull out strength and reduced cyclic displacement.

Mechanical testing
A mechanical analysis was performed to assess the performance of the design relative to standard Interference screw fixation.

Twenty-two (22) porcine tibia were used as a bone model and twenty-two (22) quad-stranded 8mm diameter bovine extensor tendon's were used to represent a hamstring graft. A tunnel, matching the graft diameter, was drilled through the tibia. The graft was passed through the tunnel until a 4cm gauge length was achieved. The tibia and tendons were randomly divided into 2 groups. In Group 1 the tendon's were secured with an 8mm GraftBolt™. In Group 2 the tendons were fixed with an 8mm Interference screw (Arthrex; Naples FL.). The constructs were cycled at 1Hz from 10 to 50N for 10 cycles followed by 50-250N at 1Hz for 500 cycles. Load to failure test was then carried out at a rate of 20mm/min. The ultimate load, yield load, stiffness, plastic and elastic displacement, and the mode of failure were recorded for each sample. Statistical analysis was performed using the t test.

Figure 2. Methodology for the 3D modelling of the ACL reconstructed knee

The mean ultimate tensile strength in Group 1 was 750N (SD 118) and 371.2 (82) in Group 2 p=0.01. The video tracking cyclic displacement in Group 1 was 1mm (+/- 0.3) and 3.0mm (+/- 0.3) in group 2 p=0.03. The main mode of failure in both groups was graft slippage passed the fixation.

There was a statistical significant difference between the 2 groups with GraftBolt™ outperforming the interference screw in all parameters.

Discussion
The GraftBolt™ provides significant mechanical advantages over a standard interference screw. The tendon pull-out strength is greater with reduced cyclic displacement. Thus, there is reduced likelihood of mechanical failure and early laxity induced by over aggressive rehabilitation. The improved mechanical properties is secondary to the greater compression achieved by the sheath expansion and spike protrusion. The radial compression is achieved over a greater surface area as opposed to interference screw fixation where compression is mainly at the threads, reducing tendon damage.

Direct healing is felt to be beneficial as it leads to a ligament insertion with a transition zone consisting of mineralised cartilage and fibrocartilage. This restoration of the insertion site with four zones offers a biomechanical advantage in terms of preventing stress concentration¹⁹. Weiler et al.¹⁰ suggested it was the direction of the force that was important, with perpendicular forces causing direct healing and the parallel intra-tunnel forces causing indirect healing. Following indirect healing of the graft within the tunnel, which occurs at approximately 12 weeks, our previous analysis has shown greater stress concentration in the area of the proximal tunnel at the articular surface¹⁸. This stress is created and magnified initially due to the interference screw fixation and then subsequently by intra-tunnel tendon healing, which occurs earlier than the direct surface healing. The bonding of the tendon to bone allows the forces to be dissipated over a large surface area, reducing the stress within this area of the tendon. Thus, the magnitude of the stress maybe as important as the direction in determining which type of tendon healing occurs. Aperture fixation may help promote the more beneficial direct healing.

Figure 4. Photograph of mechanical set up. (A) GraftBolt insertion in porcine tibia (B) ACL model on mechanical testing machine.

This area of increased stress may also have a potential detrimental effect. Surface healing occurs between 9 and 24 weeks, until this time this area may act as a stress riser and thus a potential point of graft failure. Weiler et al.⁷ noted in their study intra-substance failure of the tendon at this site. They suggested that interference screw fixation may have damaged the tendon at this point. This would correlate with the increased stress found on our FEA. We believe that this point of failure is caused by the combination of tendon damage and stress concentration in this area by aperture fixation and knee flexion. The tendon in this area has been shown to have the greatest motion during flexion, with tensile and compressive forces transmitted to the tendon from contact with the anterior and posterior margins of the tunnel aperture²⁰. By using a sheath design the tendon damage caused by screw threads is reduced and thus the risk of failure.

The tapered design allows for easier insertion and provides some further practical advantages which improves implant handling. On initial insertion of the screw, the distal end does not engage the proximal aspect of the sheath. This allows immediate deployment of the proximal row/s of tabs which provides rotational stability to the implant. The downfall of a tapered sheath design is the tendency for the implant to back out on screw insertion. By deploying the proximal row/s of tabs before screw engagement this tendency is reduced.

Conclusion
The GraftBolt™ provides significantly superior tibial fixation compared to interference screw fixation in an animal model of ACL reconstruction. It results in a construct, which has a greater UTS, stiffness and reduced cyclical displacement. Fixation with this type of sheath design may not only provide mechanical advantages, but also contribute to ligamentisation. The aperture fixation results in stress concentration promoting direct healing at the articular surface without the tendon damage (which can contribute to graft failure) seen in association with interference screw fixation.

References

1. Pinczewski L.A., Clingeleffer A.J., Otto D.D., et al, 1997. Integration of hamstring tendon graft with bone in reconstruction of the anterior cruciate ligament. Case report. Arthroscopy 13, 641-643.
2. Berg E.E., Pollard M.E., Kang Q., 2001. Interarticular bone tunnel healing. Arthroscopy. 17, 189-195.
3. Zantop T, Weimann A, Wolle K et al (2007) Initial and 6 weeks postoperative structural properties of soft tissue anterior cruciate ligament reconstructions with cross-pin or interference screw fixation: an in vivo study in sheep. Arthroscopy 23(1):14-20
4. Brand JC Jr, Pienkowski D, Steenlage E, et al: Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med 28: 705-710, 2000
5. Giurea M, Zorilla P, Amis AA, et al: Comparative pull-out and cyclic-loading strength tests of anchorage of hamstring tendon grafts in anterior cruciate ligament reconstruction. Am J Sports Med 27: 621-625, 1999.
6. Weiler A, Hoffmann RFG, Siepe CJ, et al: The influence of screw geometry on hamstring tendon interference fit fixation. Am J Sports Med 28: 356-359, 2000
7. Weiler A, Hoffmann RFG, Sta
   
   

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