Mechanical Testing of Absorbable Suture Anchors
Abstract & Commentary
Synopsis: Even if absorbable suture anchors offer advantages in term of complications, they may be the weakest link in the fixation of soft tissue to bone.
Source: Meyer DC, et al. Mechanical testing of absorbable suture anchors. Arthroscopy. 2003;19(2):188-193.Absorbable suture anchors are attractive implants because there is little risk of mechanical complications. Few data are available to help the surgeon choose an appropriate anchor. Bench testing was performed for most of the available absorbable anchors with attention to eyelet design and effects on suture. Mechanical properties were correlated to their design and weight.
Twelve absorbable anchor models were tested: Panalok 3.5 mm and Panalok RC (Mitek), Bio Roc EZ 2.8 mm and 3.5 mm (Innovasive), Rotorloc (Acufex), Bio-FASTak (Arthrex), Bio-Anchor (Livantek), Biophase (Biomet Merck), Tag Wedge 3.7 mm and Tag Rod II (Acufex), BioStatak (Zimmer), and BioCorkscrew 5.0 mm (Arthrex). All were loaded with No. 2 Ethibond suture.
Anchors were tested in a 37°C water-bath by insertion into polyurethane test bone. Mechanical testing included pull-out testing in line of the anchor axis, as well as time to failure under a constant 100 N load. Drawing the suture through the eyelet 25 times assessed the effects of suture abrasion on strength.
Mechanical testing revealed that the anchor fixation was not the weakest link. Load to failure ranged from 124 to 244 N (5 failed by eyelet cutout and 6 by suture breakage). Static loading at 100 N revealed widely varying time to failure ranging from 0.1 to more than 300 hours. Eight failed from eyelet cutout, while the same anchor (Tag Wedge 3.7 mm) was the only one to pull out of bone, and 3 did not fail after 300 hours. Mechanical strength under both conditions correlated with anchor weight but not with crystallinity. Unlike metal anchors, suture abrasion did not seem to be a major problem for absorbable anchors.
Comment by J.P. Warner, MD, and Phillipe Clavert, MD
Few data about absorbable anchors are available. The major finding of this study was that the anchors failed predominantly by cut-through at their eyelet. As the properties of absorbable polymers may differ as a function of temperature, the experiments were performed at 37°C. This is a major strength of this paper. Absorbable polymer strength is increased by high crystal content, which may lead to inflammatory foreign-body reactions during absorption. No correlation of mechanical performance between both tensile strength or static loading and crystallinity was found. The weight of the anchors had a significant, but not decisive, influence on the mechanical performance of the anchor. Meyer and colleagues recommend designing absorbable implants as small as possible but with sufficient mechanical strength, to create a small drill hole, and to implant as little absorbable material as possible.
Double-loaded anchors failed in tensile load at only slightly higher load than the breaking strength of a single suture loop. Absorbable suture anchors are sensitive to permanent load: They creep under static tension. In 6 of 12 anchors, the initial breaking strength of the suture anchor was lower than the tensile strength of the suture material. Compared to metal anchors, the suture anchor eyelet allows suture gliding without relevant abrasion (except for 2 of them).
Development of absorbable suture anchors is increasing because of potential risks of metallic injury to the glenoid cartilage, loosening, and migration. This paper gives us a lot of new considerations about these anchors. It would be interesting for Meyer et al to have studied the cyclic load of these anchors until the failure of the eyelet or the suture because during the passive rehabilitation time after surgery, repairs are cycled and sutures may have to slide in the eyelet.
Dr. Warner is Associate
Professor of Orthopaedics, Harvard Medical School, Boston, MA.
Dr. Clavert is a Fellow at Harvard Medical School.
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