On Measuring Implant Fixation Stability in ACL Reconstruction

Abstract

Numerous methods and devices are available for implant fixation in anterior cruciate ligament (ACL) reconstruction. Biomechanical data indicate high variability in fixation stability across different devices. This study aims to provide a better insight into measuring the structural characteristics and mechanical behavior of ACL implant fixations. Fourteen human tibial specimens with reconstructed ACLs were subjected to progressively increasing dynamic loading until failure. The motions of the tibia, the proximal and distal graft ends, as well as the testing frame and actuator, were continuously recorded via a motion tracking system. Significantly higher displacements of the machine actuator (1.0 mm at graft slippage onset, and 12.2 mm at ultimate load) were measured compared to the displacements of the proximal (0.8 and 4.3 mm, respectively) and distal graft (0.1 and 3.4 mm, respectively) ends. The displacements measured at different sites showed significant correlations. The provided data suggest significant and systematic inaccuracies in the stiffness and slippage of the fixation when using machine displacement, as commonly reported in the literature. The assessment of the distal graft displacement excludes the artifactual graft elongation, and most accurately reflects the graft slippage onset indicating clinical failure. Considering the high displacement at the ultimate load, the ultimate load could be used as a standardized variable to compare different fixation methods. However, the ultimate load alone is not sufficient to qualitatively describe fixation stability.

Biomechanical evaluation of an allograft fixation system for ACL reconstruction

“…Since any ACL graft must be considered as hyperelastic, the slippage of the graft and corresponding stiffness of the fixation are potentially different from the values generated by the displacement of the load frame. A recent study ( Benca et al, 2021 ) has shown that during cyclic loading the machine-measured displacement is mostly a result of graft strain. It was further reported that the machine-measured displacement correlated with graft slippage allowing only for some qualitative evaluation between different fixation devices but stiffness was overall underestimated by up to 400% ( Benca et al, 2021 ).…”

“…A recent study ( Benca et al, 2021 ) has shown that during cyclic loading the machine-measured displacement is mostly a result of graft strain. It was further reported that the machine-measured displacement correlated with graft slippage allowing only for some qualitative evaluation between different fixation devices but stiffness was overall underestimated by up to 400% ( Benca et al, 2021 ). Interestingly, the slippage onset, defined as an initial displacement of at least 0.1 mm, occurs already at relatively low load levels in both implant groups (64.5 and 56.0 N for the interference screw and Shark Screw ACL, respectively).…”

“…While this appears low, it is important to consider that in some specimens, graft slippage reached clinically significant values (maximum: 7.2 mm in the interference screw group and 11.3 mm in the Shark Screw ACL group) and that in clinical reality, an additional slippage at the femoral side would have to be expected at high loads. Therefore, the ultimate load alone may not be a sufficient indicator for clinical failure ( Benca et al, 2021 ). Thus, a better measure is required to evaluate fixation stability.…”

“…Relative movements of the tendon, bone, and suture at the distal end of the tibial tunnel were measured via monitoring of markers with a stereographic optical motion tracking system (Aramis SRX; GOM GmbH, Braunschweig, Germany) at a rate of up to 115 Hz. The tracking of the suture at the distal end of the graft ( Benca et al, 2021 ) enabled the monitoring of the graft slippage. Slippage onset was defined as a displacement of the distal graft end bigger than 0.1 mm.…”

“…It has to be noted that some data of fourteen specimens used in this study has been previously reported in a paper concerning general aspects in measurement of structural characteristics and mechanical behavior of ACL implant fixations ( Benca et al, 2021 ).…”