Mechanical Performance of a Screw-Type Veterinary Suture Anchor Subjected to Single Load to Failure and Cyclic Loads

Effect of cyclic loading on the ultimate tensile strength of small metallic suture anchors used for attaching artificial tendons in rabbits

Background –

Suture anchor failures can lead to revision surgeries which are costly and burdensome for patients. The durability of musculoskeletal reconstructions is therefore partly affected by the design of the suture anchors.

Purpose –

The purpose of the study was to quantify the strength of different suture anchors whose sizes are suitable for attaching artificial Achilles and tibialis cranialis tendons in a rabbit model, as well as determine the effect of cyclic loading on the anchoring strength.

Method –

Four anchors (two with embedded eyelet and two with raised eyelet, n=5 per group) were tested with cyclical loading (1000 cycles and 4.5 mm/sec) and without cycling, to inform the failure loads and mode of failure of the suture anchors. An eyebolt screw with smooth eyelet was used as a control for the test groups.

Results –

All samples in all groups completed 1000 cycles and failed via suture breakage in both test conditions. All anchors had failure loads exceeding the peak Achilles tendon force in rabbits during hopping gait. The data analysis showed an effect of anchor type on the maximum tensile force at failure (F max ) in all suture categories but not an effect of loading condition. Also, the Anika anchor had a significantly less adverse effect on suture strength compared to Arthrex anchor (p=0.015), IMEX anchor (p=0.004) and Jorvet anchor (p<0.001). We observed a greater percentage of failure at the mid-section for the anchors with the raised eyelets compared to the anchors with embedded eyelets, which all failed at the knot.

Conclusion –

Anchors with embedded eyelets had clinically preferred mode of failure with less adverse effects on suture and, may be more reliable than anchors with raised eyelets for attaching artificial Achilles and tibialis cranialis tendons in rabbits.

Effect of configuration on the biomechanical performance of three suture materials used in combination with a metallic bone anchor

OBJECTIVE

To determine whether different suture configurations could improve the biomechanical performance of 3 suture materials used with bone anchors.

SAMPLES

3 suture materials (60-lb test nylon leader line, size 2 polyblend polyethylene composite suture, and 150-lb test ultrahigh-molecular weight spun polyethylene). PROCEDURES-Each suture material was looped through the eyelet of a metallic bone anchor and constructs were evaluated by use of an acute uniaxial load. Three configurations were tested for each suture material: single stranded (SS), double stranded (DS), and single stranded plus plastic insert (SSP). Force at failure, extension at failure, force at 3 mm of extension, stiffness, and site of failure of the suture were recorded for each test.

RESULTS

For all sutures, the DS configuration was the stiffest and yielded significantly higher forces at failure and forces at 3 mm of extension. The SS configuration had the lowest forces at failure. The SSP configuration yielded greater forces at failure for all suture materials, compared with the SS configuration, with a comparable stiffness. All sutures failed at the eyelet in the SS and DS configurations. In the SSP configuration, 60-lb test nylon leader line and 150-lb test ultrahigh-molecular weight spun polyethylene failed at the eyelet less frequently than did the polyblend composite suture.

CONCLUSIONS AND CLINICAL RELEVANCE

Among the tested constructs, a DS suture configuration used in combination with the metallic bone anchor gave the best biomechanical results for all suture materials. Considering that the SSP configuration yielded greater forces at failure, compared with the SS configuration, covering metallic edges in bone anchors with softer materials might protect sutures and result in increased forces at failure.

In vitro acute load to failure and eyelet abrasion testing of a novel veterinary screw-type mini-anchor design

OBJECTIVE

To determine acute load to failure (ALF) and suture abrasion (SA) at 0° and 90° for a novel screw-type mini-anchor design.

STUDY DESIGN

Biomechanical in vitro study.

SAMPLE POPULATION

Synthetic bone.

METHODS

Twenty mini-anchors were inserted into synthetic bone blocks assigned to 1 of 2 groups (0° ALF, 90° ALF). Pullout was performed at 5 mm/min. ALF, yield strength and stiffness were calculated. SA constructs were created with 4 groups of 5 anchors each with either 30 lb nylon leader line (NLL), 40 lb NLL, #2 Fiberwire or #5 Fiberwire. SA was performed at 0° and 90° with a sinusoidal wave form at 0.5 Hz and 10 N load for 1000 cycles or until failure. Data were summarized as mean ± SD. ALF data were analyzed using t-tests. SA data were analyzed using log rank, Tukey-adjusted pairwise comparisons and sign tests. Significance was set at P = .05.

RESULTS

Mean ± SD ALF at 0° and 90° was 431.8 ± 70.8 N and 683 ± 48.7 N, respectively. 90° ALF was significantly higher. Yield strength and stiffness were not significantly different at 0° and 90°. #5 and #2 Fiberwire survived significantly more cycles than 40 lb and 30 lb NLL at 90°. At 0°, 30 lb NLL survived significantly less cycles than either Fiberwire size. Suture orientation did not have a significant effect on SA for Fiberwire constructs.

CONCLUSION

The novel mini-anchor has ALF comparable to other mini-anchors. Fiberwire survived more cycles in the novel anchor eyelet than NLL and FW suture orientation in the eyelet did not affect SA.

Bearing area: a new indication for suture anchor pullout strength?

Studies performed to quantify the pullout strength of suture anchors have not adequately defined the basic device parameters that control monotonic pullout. The bearing area of a suture anchor can be used to understand and predict anchor pullout strength in a soft-bone model. First, conical-shaped test samples were varied in size and shape and tested for pullout in 5, 8, and 10 pcf sawbone models. Next, bearing area and pullout strength relationships developed from the test samples were validated against nine commercially available suture anchors, including the Mitek QuickAnchor and SpiraLok, Opus Magnum(2), ArthroCare ParaSorb, and Arthrex BioCorkscrew. The samples showed a direct correlation between bearing area and pullout strength. Increased insertion depth was a secondary condition that also increased pullout strength. The pullout strength for the suture anchors followed the predicted trends of conical devices based on their individual bearing areas. For the 5 and 8 pcf models, only two and three devices, respectively, fell outside the predicted pullout strength range by more than a standard deviation. The use of a synthetic sawbone model was validated against the pullout strength of an Arthrex Corkscrew in five fresh-frozen cadaver humeral heads. The bearing area of a suture anchor can be used to predict the pullout strength independent of design in a soft-bone model. This work helps provide a foundation to understand the principles that affect the pullout strength of suture anchors.

In vitro evaluation of screws and suture anchors in metaphyseal bone of the canine tibia

OBJECTIVE

To compare ease of insertion, load to failure, and mode of failure of cortical and cancellous screws, BoneBiter, IMEX, and TwinFix suture anchors in canine metaphyseal tibial bone.

STUDY DESIGN

Experimental biomechanical study.

ANIMALS

Canine cadaveric tibias.

METHODS

One investigator inserted all anchors and subjectively evaluated ease of placement. Anchor systems were loaded to failure along axis of insertion with audio-video recording to determine failure mode.

RESULTS

BoneBiter was the most difficult anchor to insert successfully. Mean+/-SD loads to failure were cancellous screw (711+/-193 N), IMEX 4.7 mm 18 g wire (661+/-163 N), IMEX 4.0 mm 18 g wire (661+/-165 N), cortical screw (635+/-184 N), BoneBiter #5 Kevlar suture (393+/- 109 N), and TwinFix 5.0 mm #2 polyester (267+/-73 N). No significant differences were noted among the cortical screw, cancellous screw, IMEX 4.7 and 4.0 mm, all of which were significantly (P<.001) greater than BoneBiter and TwinFix . Failure modes were pullout of bone, suture-wire breakage, eyelet breakage, or no failure to 1000 N: screws (18,0,0,2), IMEX (18,1,1,0), BoneBiter (2,8,0,0), and TwinFix (0,10,0,0).

CONCLUSIONS

Fixation devices were user friendly, with the exception of BoneBiter. Mode of failure is dependent on suture material and anchor design. Cortical and cancellous screws, and IMEX anchors with 18 g wire have significantly greater load to failure compared with BoneBiter and TwinFix suture anchors.

CLINICAL RELEVANCE

Based on load to failure, ease of use, design characteristics, and cost, IMEX anchors may have advantages over other comparable soft tissue fixation devices.