Biomechanical Evaluation of a Novel Loop Retention Mechanism for Cortical Graft Fixation in ACL Reconstruction

Abstract

Background: Implant fixation by means of a cortical fixation device (CFD) has become a routine procedure in anterior cruciate ligament reconstruction. There is no clear consensus whether adjustable-length CFDs are more susceptible to loop lengthening when compared with pretied fixed-length CFDs.

Purpose: To assess biomechanical performance measures of 3 types of CFDs when subjected to various loading protocols.

Study design: Controlled laboratory study.

Methods: Three types of CFDs underwent biomechanical testing: 1 fixed length and 2 adjustable length. One of the adjustable-length devices is based on the so-called finger trap mechanism, and the other is based on a modified sling lock mechanism. A device-only test of 5000 cycles (n = 8 per group) and a tendon-device test of 1000 cycles (n = 8 per group) with lower and upper force limits of 50 and 250 N, respectively, were applied, followed by ramp-to-failure testing. Adjustable-length devices then underwent further cyclic testing with complete loop unloading (n = 5 per group) at each cycle, as well as fatigue testing (n = 3 per group) over a total of 1 million cycles. Derived mechanical parameters were compared among the devices for statistical significance using Kruskal-Wallis analysis of variance followed by post hoc Mann-Whitney U testing with Bonferroni correction.

Results: All CFDs showed elongation <2 mm after 5000 cycles when tested in an isolated manner and withstood ultimate tensile forces in excess of estimated peak in vivo forces. In both device-only and tendon-device tests, differences in cyclic performance were found among the devices, favoring adjustable-length fixation devices over the fixed-length device. Completely unloading the suspension loops, however, led to excessive loop lengthening of the finger trap device, whereas the modified sling lock device remained stable throughout the test. The fixed-length device displayed superior ultimate strength over both adjustable-length devices. Both adjustable-length devices showed adequate fatigue behavior during high-cyclic testing.

Conclusion: All tested devices successfully prevented critical construct elongation when tested with constant tension and withstood ultimate loads in excess of estimated in vivo forces during the rehabilitation phase. The finger trap device gradually lengthened excessively when completely unloaded during cyclic testing.

Clinical relevance: Critical loop lengthening may occur if adjustable-length devices based on the finger trap mechanism are repeatedly unloaded in situ.

Keywords: ACL reconstruction; adjustable button; biomechanical testing; cortical fixation; cyclic loading.

Figure 1.

The cortical fixation devices tested included (A) a pretied fixed-length device, (B) an adjustable-length device based on the finger trap, and (C) a single-strand adjustable-length device based on the modified sling lock mechanism.

Figure 2.

Test setups for (A) isolated device testing and (B) tendon-device testing.

Figure 3.

Cumulative construct elongation during cyclic testing for the 2 test setups. Values are presented as median (line), interquartile range (box), range (error bars), and outliers (diamonds). FLD, fixed-length device; FTD, finger trap device; MSLD, modified sling lock device.

Figure 4.

With complete loop unloading, the finger trap device (FTD) displayed critical loop lengthening within the first hundreds of load cycles, whereas the modified sling lock device (MSLD) exhibited loop lengthening <1.5 mm over the 1000 test cycles.

Figure 5.

Cumulative cyclic elongation of the 2 tested adjustable-length fixation devices in high-cyclic testing. FTD, finger trap device; MSLD, modified sling lock device.

Figure 6.

Ultimate tensile and elongation test results. Significant pairwise differences are indicated with an asterisk (P < .05, Bonferroni corrected). Values are presented as median (line), interquartile range (box), range (error bars), and outliers (diamonds). FLD, fixed-length device; FTD, finger trap device; MSLD, modified sling lock device.