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 femur2,3 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 geometry5,6, and material of the screw7. However, there is controversy over the effects of screw length on graft fixation strength7,8,9.
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 forces12, 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™ |
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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 threa