Mechanical strength of suture anchors used for attaching prosthetic tendons in rabbits

Introduction:: Suture-anchors play a crucial role in orthopedic surgery by securely attaching soft tissues (e.g., tendons) to bones, ensuring proper healing and rehabilitation after surgery [1]. Previously, we surgically replaced the Achilles tendon in rabbits using polyester silicone-coated artificial tendons [2]. A suture-anchor (IMEX, described below) was used to attach the artificial tendon to the calcaneus bone. However, the suture-anchors failed in several rabbits, with suture breakage at the mid-substance (i.e., away from the knot) being the most frequent mode of failure. It was suspected that the failure was caused by the relatively sharp edges around the eyelet of the anchor. Mechanical testing has previously been performed on several different types of suture-anchors [3, 4]. However, the differences in test variables, such as anchor size, suture type, bone size, and attachment scenario between those studies and ours make it difficult to extrapolate results. Therefore, toward establishing the cause of the suture-anchor failure in the rabbits, we tested the mechanical strength of two types of suture-anchors (Figure 1A) that we used in the surgeries and whose sizes were compatible with the rabbit model (Figure 1B). The objective of the testing was to determine the maximum tensile load to failure as well as the mode of failure of each of the suture-anchor types. Our hypothesis was that (1) the IM group would exhibit a greater average failure load compared to the IU group, and (2) the AT group would demonstrate the lowest failure load among groups due to its smaller suture size.

Materials and Methods:: One anchor type (60-27-09, 2.7 mm x 9 mm, IMEX Inc., Longview, TX) had a raised eyelet with a USP size 5 Fiberwire suture (AR-7210, Arthrex Inc., Naples, FL). The other anchor (Mini Corkscrew AR-1319FT, 2.7 mm x 7 mm, Arthrex Inc., Naples, FL) had an eyelet that was embedded within the screw and was accompanied by a USP size 0 Fiberwire suture. The suture sizes were chosen to be the largest size that each anchor could accommodate. The IMEX anchors were divided into two groups: IMEX modified (IM, n=5) and IMEX unmodified (IU, n=5). For anchors in the IM group, we manually filed and smoothed sharp edges around the eyelet using a hand-held rotary grinder and file set. Arthrex anchors (n=5) were tested in the third (AT) group. The anchors were screwed into pre-drilled holes of 5/64 inches diameter on a simulated bone block (PCF 50 (0.8g/cc), Sawbones, Vashon, WA). After the blocks were clamped in a universal material testing machine (Model 5965, Instron Inc., Norwood, MA), the suture-anchors were loaded uniaxially for 1000 cycles up to 60 N, the approximate peak Achilles tendon tensile force during hopping gait [5], then ramped to failure. The extension rate was 4.5 mm/s throughout the test. A two-tailed independent-sample Student's t-test was used to compare the failure loads between the groups. The Fisher exact test was used to compare the frequencies of the failure modes between groups, with p< 0.05 considered significant.

Results, Conclusions, and Discussions:: Results: All samples in each of the three groups completed 1000 cycles. The average failure loads of the IM, IU, and AT anchors were 400.54 ± 81.72 N, 473.24 ± 42.35 N, and 112.37 ± 4.99 N, respectively (Figure 2A). Although it was hypothesized that the IMEX modified anchor (IM group) would have a greater average load at failure than the IMEX unmodified anchor (IU group), the difference between these groups was not statistically significant. The AT anchor group had a significantly lower load at failure (p< 0.05) than both of the IMEX groups, supporting our second hypothesis. In the IU group, two out of five samples failed at the knot, and the remaining three failed around the eyelet; all five samples in the IM group failed around the eyelets, whereas all five samples in the AT group failed at the knots (Figure 2B). We found a significant difference (p< 0.05) in location of suture-anchor failure between the IM and AT groups only. Because there was no significant difference between the IMEX groups, the two groups were combined and compared to the AT group; we found a significant difference (p=0.007) in the location of failure between the IMEX groups and AT group.

Discussion: That the IMEX groups had a greater load at failure was somewhat expected since the IMEX anchor could accommodate a much larger suture, an attractive feature of the IMEX anchor. The test supported our suspicion that the eyelet of the IMEX anchor contributed to the suture failure. The AT anchors performed better than IMEX anchors, since all failures in the AT group occurred at the knot. Therefore, anchors with a buried eyelet may be superior to those with a raised eyelet for this application. By quantifying the strength of suture-anchors, our results help clinicians and engineers better understand how suture-anchors may perform in vivo, make more informed decisions about device design and use, and avoid costly revision surgeries and patient hardship.

Conclusion: The IMEX anchors with USP size 5 Fiberwire suture had a greater failure load than the Arthrex anchors with USP size 0 Fiberwire suture.


Acknowledgements (Optional): : NSF CAREER 1944001.

References (Optional): : [1] Visscher et al. (2019). Clinical Biomechanics, 61: 70–78.

[2] Hall et al. (2023). Journal of Biomechanics, 151:1-5.

[3] Barber et al. (2008). Arthroscopy: The Journal of Arthroscopic & Related Surgery, 24(8):859–867.

[4] Bardana et al. (2003). Arthroscopy: The Journal of Arthroscopic & Related Surgery, 19:274–281.

[5] West JR et al. (2004). Journal of Biomechanics, 37(11):1647–1653.