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The number of loaded sutures alter the suture-holding strength in different knotless suture anchors: a biomechanical study.

  • Joe-Zhi Yen
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan
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  • Hao-Chun Chuang
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan
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  • Chih-Kai Hong
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan
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  • Kai-Lan Hsu
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan

    Department of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan
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  • Fa-Chuan Kuan
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan

    Department of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan
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  • Yueh Chen
    Affiliations
    Department of Orthopaedic Surgery, Sin Lau Christian Hospital, Tainan, Taiwan

    Institute of Allied Health Sciences, College of Medicine, National Cheng Kung University, Tainan, Taiwan
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  • Wei-Ren Su
    Correspondence
    Please address all correspondence to: Wei-Ren Su, M.D., M.Sc., Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan, No.138, Sheng-Li Road, Tainan City, Taiwan 70428. Tel:886-6-2766689; Fax:886-6-2766189;
    Affiliations
    Department of Orthopaedic Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan

    Medical Device R & D Core Laboratory, National Cheng Kung University Hospital, Tainan, Taiwan
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Open AccessPublished:November 21, 2021DOI:https://doi.org/10.1016/j.jseint.2021.10.005

      Abstract

      Background

      Due to advances in arthroscopic rotator cuff repair, a knotless suture anchor is expected to hold more than two suture limbs. The objective of this study is to investigate whether when employing different fixation mechanisms, the suture-holding strength of knotless anchors are altered by the number of loaded sutures.

      Methods

      Three types of knotless anchors (spiral core clamping, winged clamping, and spooling) were tested, with each anchor double-loaded or quadruple-loaded. Cyclic loading was applied, followed by a tensile load until failure. Clinical failure load (CFL) was defined by 3mm slippage, and ultimate failure load (UFL) was defined as a sharp deviation in the linear load-versus-displacement curve. A two-way ANOVA was performed to examine the effects of suture anchors and suture number.

      Results

      The two-way ANOVA showed significant interaction between the type of suture anchor and the number of sutures (p < 0.001). Increasing the number of sutures improved suture-holding strength of the spiral core clamping anchor and the winged clamping anchor, for which the quadruple-loaded form had a significantly higher CFL and UFL in comparison with the double-loaded form (p < 0.001, respectively). However, the UFL of the quadruple-loaded spooling anchor was significantly lower than that of the double-loaded form (p < 0.001).

      Conclusions

      The number of loaded sutures does affect the suture-holding strength of knotless suture anchors, and anchors with different fixation mechanisms are affected differently.

      Keywords

      The suture bridge technique has been the mainstream repair technique used to repair rotator cuff tears in the past decade, and multiple modified suture bridge constructs have been developed in attempts to improve clinical outcomes.[
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      ] For example, a medial suture non-tying repair has emerged as a promising alternative to prevent type 2 retears by releasing the stress concentrated on medial-row anchors.[
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      ] Also, a sizable amount of the literature has suggested that suture bridge constructs with a greater number of suture strands and suture-passed holes tend to perform better mechanically.[
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      Biomechanical analysis of four different medial row configurations of suture bridge rotator cuff repair.
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      • Shi B.Y.
      • Diaz M.
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      • Srikumaran U.
      Biomechanical Strength of Rotator Cuff Repairs: A Systematic Review and Meta-regression Analysis of Cadaveric Studies.
      ] Together, these trends indicate that lateral-row suture anchors will have to bear the extra load transferred from their medial-row counterparts and retain more sutures strands. To this end, the mechanical strength of lateral-row suture anchors is ever more critical for the stability of a suture bridge construct.
      The introduction of the knotless suture anchor was a milestone in the advancement of the arthroscopic suture bridge technique. Knotless anchors exempt orthopedic surgeons from time-consuming, technique-demanding arthroscopic knot tying procedures by eliminating knot migration and knot impingement while reportedly providing adequate mechanical stability.[
      • Lacheta L.
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      Biomechanical stability of knotless suture anchors used in rotator cuff repair in healthy and osteopenic bone.
      ,
      • Woodmass J.M.
      • Matthewson G.
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      • Bois A.J.
      • Boorman R.S.
      • Lo I.K.
      • et al.
      Suture locking of isolated internal locking knotless suture anchors is not affected by bone quality.
      ] The suture retention mechanism of a knotless anchor has been considered to be one of the critical aspects of the stability of a suture bridge repair construct.[
      • Wieser K.
      • Farshad M.
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      • Ruffieux K.
      • Gerber C.
      • Meyer D.C.
      Suture slippage in knotless suture anchors as a potential failure mechanism in rotator cuff repair.
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      • Lo I.K.
      • et al.
      Suture locking of isolated internal locking knotless suture anchors is not affected by bone quality.
      ] Several factors in a suture bridge construct have been reported to influence the outcome of rotator cuff repair, and suture slippage in lateral row knotless anchors is considered to be a potential failure mechanism in rotator cuff repair.[
      • Wieser K.
      • Farshad M.
      • Vlachopoulos L.
      • Ruffieux K.
      • Gerber C.
      • Meyer D.C.
      Suture slippage in knotless suture anchors as a potential failure mechanism in rotator cuff repair.
      ] Gap formation resulting from suture slippage in the lateral row can further lead to poor healing.[
      • Lee K.W.
      • Yang D.S.
      • Lee G.S.
      • Ma C.H.
      • Choy W.S.
      Clinical outcomes and repair integrity after arthroscopic full-thickness rotator cuff repair: suture-bridge versus double-row modified Mason-Allen technique.
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      • Park J.Y.
      • Lhee S.H.
      • Choi J.H.
      • Park H.K.
      • Yu J.W.
      • Seo J.B.
      Comparison of the clinical outcomes of single- and double-row repairs in rotator cuff tears.
      ,
      • Park M.C.
      • Cadet E.R.
      • Levine W.N.
      • Bigliani L.U.
      • Ahmad C.S.
      Tendon-to-bone pressure distributions at a repaired rotator cuff footprint using transosseous suture and suture anchor fixation techniques.
      ] Nevertheless, no previous studies have elucidated how the loading of multiple sutures influences the suture retention capacity of knotless anchors, and most product brochures specify that each anchor is designed to hold 2 sutures without suggesting that increasing the number of loaded sutures will proportionately fortify or diminish the failure load of the repair constructs.
      Despite the growing number of modified suture bridge constructs, where lateral-row suture anchors are multiple-loaded, explanations of how multiple-loading influences the fixation strength of knotless suture anchors remain controversial. As a consequence, the current study was aimed toward investigating how the number of loaded sutures interacts with different knotless suture anchor fixation mechanisms. The sturdiness of suture anchors was evaluated when loaded with either 2 or 4 sutures. We hypothesized that multiple loading would improve the suture holding strength and that an interaction exists between the suture number and the fixation mechanism.

      Method

      Experimental subjects

      In this study, 30 blocks of 60x40x40mm of synthetic bone (Sawbones, Pacific Research Laboratories, Vashon, WA, USA) were used as bone substitutes. Each block consisted of solid rigid polyurethane foam blocks (density, 0.16g/cm3) with a 2 mm layer of short-fiber-filled epoxy (density, 1.63g/cm3) on top to simulate the greater tuberosity of the human humerus.[
      • Hong C.K.
      • Hsu K.L.
      • Kuan F.C.
      • Wang P.H.
      • Hsu C.C.
      • Yeh M.L.
      • et al.
      When deadman theory meets footprint decortication: a suture anchor biomechanical study.
      ] Previous biomechanical studies have used similar foam blocks to simulate the human greater tuberosity (GT), which is believed to have a density ranging from 0.10±0.03 to 0.18±0.04 g/cm3.[
      • Tingart M.J.
      • Bouxsein M.L.
      • Zurakowski D.
      • Warner J.P.
      • Apreleva M.
      Three-dimensional distribution of bone density in the proximal humerus.
      ,
      • Woodmass J.M.
      • Matthewson G.
      • Ono Y.
      • Bois A.J.
      • Boorman R.S.
      • Lo I.K.
      • et al.
      Suture locking of isolated internal locking knotless suture anchors is not affected by bone quality.
      ]
      Three types of commercially available 4.5mm knotless suture anchors made of PEEK were purchased from respective manufacturers. All of the anchors had an internal fixation mechanism. The Footprint Ultra PK (Smith & Nephew Endoscopy, Andover, MA, USA) is a spiral core clamping anchor that employs an internal fixation mechanism at the site of the suture, clamping the sutures between an internal anchor plug and the anchor body (Figure 1A). The PopLok (ConMed Linvatec, Largo, FL, USA) is a winged clamping anchor with two wings providing secure fixation in the bone when deployed subcortically. It also employs an internal fixation mechanism, clamping the sutures between the outer and inner shafts (Figure 1B). The ReelX STT (Stryker Endoscopy, San Jose, CA, USA) is a spooling anchor that has an anchor body that expands progressively when the anchor is being deployed. It employs a spooling mechanism for internal fixation, where the sutures are incrementally tensioned and advanced for every 60º of revolution of the knob (Figure 1C).
      Figure thumbnail gr1
      Figure 1Structures of the three tested knotless suture anchors. (A) Spiral core clamping anchor (Footprint Ultra PK, Smith & Nephew, London). The anchor was deployed by revolving the core in the direction of the arrow, and the sutures were subsequently clamped between an inner plug and the anchor body. The anchor body had barbed geometry threads to prevent pullout. (B) Winged clamping anchor (PopLok, ConMed Linvatec, Largo, FL, USA). The sutures were clamped between an inner plug and the anchor body. Two wings were deployed subcortically to securely fix the anchor body in the bone. (C) Spooling anchor (Reel X STT, Stryker Corporation, Kalamazoo, MI, USA). The sutures were spooled into the anchor body, pulling in 1 cm for every full turn in the direction indicated by the arrow. The anchor body gradually expanded as it was loaded with sutures.
      A total of six constructs were tested. Each of the three types of knotless anchors were double-loaded or quadruple-loaded. In other words, they were loaded with either two or four #2 Hi-Fi® sutures. Before insertion and deployment of an anchor, a 4.5-mm unicortical drill hole was pre-drilled in the middle of the foam block. Every suture anchor was then tapped in and deployed according to the manufacturer’s instructions. The construct was firmly fixed onto the base of a material testing system (AG-X; Shimadzu, Tokyo, Japan) fitted with a 1-kN load cell. We set up a parallel system instead of a horizontal system to avoid suture cutting due to the sharp edge of drilled epoxy as well as to simulate the worst case situation.[
      • Barber F.A.
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      • Barber C.A.
      • Bynum J.A.
      • et al.
      Biomechanical analysis of pullout strengths of rotator cuff and glenoid anchors: 2011 update.
      ] (Figure 2).
      Figure thumbnail gr2
      Figure 2The setup for mechanical testing. The tested construct was fixed at the base of a material testing system, in parallel to the load actuator. Each of the suture limbs had a distinctively colored mark as a reference for the extent of suture slippage.

      Biomechanical testing setup

      Double-loaded group

      Measures were taken to ensure equivalent loading distributions on each suture limb. The limbs from the two sutures were firmly tied into a loop across a metallic ring firmly fixed to the load cell of material testing system. Each suture limb had a distinctively colored mark right beyond the surface of the foam block as a reference for the extent of suture slippage. The foam block was then positioned so that there was no initial load onto the device.

      Quadruple-loaded group

      To ensure that there was equivalent distribution loading on each of the four suture limbs, they were divided into two groups to form two loops, where each of the two loops was initially tied across a steel ring. The two steel rings were connected by a steel cable passing through the metallic ring fixed to the load cell of the material testing system. The double-loop construct ensured that the loads transduced by the two steel rings were equivalent, and the loads shared by the two suture limbs at each loop were equivalent as well. In the quadruple-loaded group, each of the four suture limbs had a distinctively colored mark right beyond the surface of the foam block as a reference for the extent of suture slippage. The foam block was then positioned so that there was no initial load on the device.

      Biomechanical testing protocol

      The biomechanical testing protocol comprised preloading, cyclic loading, and load-to-failure testing. First, the sutures were tensioned at 0.5 mm/sec under program control until a preload of 10 N was applied.[
      • Efird C.
      • Traub S.
      • Baldini T.
      • Rioux-Forker D.
      • Spalazzi J.P.
      • Davisson T.
      • et al.
      Knotless single-row rotator cuff repair: a comparative biomechanical study of 2 knotless suture anchors.
      ,
      • Woodmass J.M.
      • Matthewson G.
      • Ono Y.
      • Bois A.J.
      • Boorman R.S.
      • Lo I.K.
      • et al.
      Suture locking of isolated internal locking knotless suture anchors is not affected by bone quality.
      ] Preloading was completed after the sutures were held at 10N for 5 seconds. After the preloading, a cyclic load between 10N and 20N at 0.25Hz was applied for 50 cycles. In the subsequent 50 cycles, the maximum load was increased by 10N. In other words, the next round comprised a cyclic load ranging between 10N and 30N. The maximum load was incrementally changed until cyclic loading between 10N and 90N was completed, or the sutures were pulled out, following the protocols used in previous studies.[
      • Efird C.
      • Traub S.
      • Baldini T.
      • Rioux-Forker D.
      • Spalazzi J.P.
      • Davisson T.
      • et al.
      Knotless single-row rotator cuff repair: a comparative biomechanical study of 2 knotless suture anchors.
      ] The cyclic load was designed to simulate supraspinatus loading during activities of daily living, considering that the supraspinatus muscle was shown by Gausden et al to generate about 90N of force during activities other than lifting blocks above shoulder height.[
      • Gausden E.B.
      • McCarthy M.M.
      • Kontaxis A.
      • Corpus K.T.
      • Gulotta L.V.
      • Kelly A.M.
      Subscapularis tendon loading during activities of daily living.
      ] Eventually, the specimens that had completed all eight rounds of cyclic loading underwent destructive testing. The sutures were again tensioned at 0.5 mm/sec until being pulled out, and the maximum load recorded during the process was defined as the ultimate failure load (UFL). After the biomechanical testing, the video record was reviewed. The clinical failure load (CFL), defined by Burkhart et al as the load at which suture slippage of 3 mm occurred, was identified and documented.[
      • Burkhart S.S.
      • Wirth M.A.
      • Simonich M.
      • Salem D.
      • Lanctot D.
      • Athanasiou K.
      Knot security in simple sliding knots and its relationship to rotator cuff repair: how secure must the knot be?.
      ,
      • Bynum C.K.
      • Lee S.
      • Mahar A.
      • Tasto J.
      • Pedowitz R.
      Failure mode of suture anchors as a function of insertion depth.
      ,
      • Ono Y.
      • Woodmass J.M.
      • Nelson A.A.
      • Boorman R.S.
      • Thornton G.M.
      • Lo I.K.
      Knotless anchors with sutures external to the anchor body may be at risk for suture cutting through osteopenic bone.
      ] The clinical failure load, ultimate failure load, and ultimate failure mode of each specimen were recorded.

      Statistics

      Data were presented as means and standard deviations. An a priori power analysis was conducted using G*Power to compare the CFL and UFL in six different configurations (3 suture anchors by 2 sutures loaded), based on data from a pilot study (n = 18).[
      • Faul F.
      • Erdfelder E.
      • Lang A.G.
      • Buchner A.
      G*Power 3: a flexible statistical power analysis program for the social, behavioral, and biomedical sciences.
      ] The effect sizes, 0.93 for the CFL and 1.18 for the UFL, were calculated from the pilot study, and both were considered to be large using Cohen’s criteria. With an alpha = 0.05 and a power = 0.90, the projected sample size needed with this effect size was determined to be approximately n = 26 for the statistical comparison. Eventually, we set the sample size of each group to 5, referring to prior biomechanical studies.[
      • Woodmass J.M.
      • Matthewson G.
      • Ono Y.
      • Bois A.J.
      • Boorman R.S.
      • Lo I.K.
      • et al.
      Suture locking of isolated internal locking knotless suture anchors is not affected by bone quality.
      ] This number (n = 30) was considered to be more than adequate to meet the main objective of this study and should also have allowed for expected attrition and the additional objectives of controlling for possible mediating factors.
      Ultimately, to test our hypothesis, a two-way ANOVA was performed to identify the effects of the suture anchors (spiral core clamping anchor vs. winged clamping anchor vs. spooling anchor) and the number of sutures loaded (double vs. quadruple) with an interaction term between the suture anchor and suture number. The main effects calculated using Tukey’s test for the post-hoc analyses were reported if no statistically significant interaction was observed. Otherwise, simple main effects, computed using Fisher’s least significant difference correction for multiple comparisons, were reported when the interaction term reached statistical significance. The effect sizes and post-hoc power were calculated and assessed as well. All data in this study were analyzed using IBM SPSS, version 25 (IBM Corp., Armonk, NY, USA).

      Results

      Clinical Failure Load

      The two-way ANOVA showed a statistically significant suture anchor × loading number interaction for the CFL (p < 0.001). Specifically, the post-hoc analysis revealed that increasing the number of sutures loaded improved the clinical failure load of the spiral core clamping anchor (Footprint Ultra PK) and the winged clamping anchor (PopLok), for which the quadruple-loaded form had a significantly higher CFL in comparison with the double-loaded form (p < 0.001 and p = 0.010, respectively). However, the number of sutures loaded did not significantly affect the CFL of the spooling anchor (ReelX STT) (p = 0.516). Under the double-loaded condition, the CFLs of the spiral core clamping anchor and the spooling anchor were significantly greater than that of the winged clamping anchor (76±5.48 and 62±19.24 vs 40±10; p < 0.001 and p = 0.017, respectively). Under the quadruple-loaded condition, the CFL of the spiral core clamping anchor outperformed those of the winged clamping anchor and spooling anchor (138.1±4.73 vs. 64±13.42 and 56.32±20.20; p < 0.001 and p < 0.001). (Figure 3).
      Figure thumbnail gr3
      Figure 3Comparison of the clinical failure load between anchor types and the number of sutures loaded. Histogram plots grouped by anchor types. Error bars represent the standard deviation. Increasing the number of loaded sutures significantly increased the CFL in the FP and PL. When the anchors were double-loaded, the CFLs of the FP and RX were significantly higher than that of the PL. When quadruple-loaded, the CFL of the FP was higher than those of the other two. (FP, Footprint Ultra PK; PL, PopLok; RX, ReelX STT; n =5; *, p < 0.05; **, p < 0.01; ***, p < 0.001; p values were calculated using a two-way ANOVA and a post-hoc comparison of the estimated marginal means.)

      Ultimate Failure Load

      There was a statistically significant suture anchor × loading number interaction found for the UFL (p < 0.001). When the spiral core clamping (Footprint Ultra PK) anchor or the winged clamping anchor (PopLok) was selected, similarly, quadruple loading significantly increased the UFL (p < 0.001 and p = 0.002, respectively). Interestingly, the UFL of the quadruple-loaded spooling anchor (ReelX STT) was significantly lower than that of the double-loaded spooling anchor (p < 0.001). Under the double-loaded condition, the UFL for the spooling anchor was significantly greater than those for the spiral core clamping anchor and the winged clamping anchor (199.09±44.49 vs. 82±8.37 and 83.39±10.07; p < 0.001 and p < 0.001, respectively). Under the quadruple-loaded condition, the UFLs of the three suture anchors exhibited no statistically significant differences (142.85±4.86, 134.04±7.08, and 134.75±32.55). (Figure 4).
      Figure thumbnail gr4
      Figure 4Comparison of the ultimate failure load between anchor types and the number of sutures loaded. Histogram plots grouped by anchor types. Error bars represent the standard deviation. Increasing the number of loaded sutures significantly increased the UFLs of the FP and PL. However, it significantly decreased the UFL of the RX. When the anchors were double-loaded, the UFLs of the RX were significantly higher than those of the FP and PL. When quadruple-loaded, the differences among the UFLs of the three suture anchors did not reach statistical significance. (FP, Footprint Ultra PK; PL, PopLok; RX, ReelX STT; *, p < 0.05; **, p < 0.01; ***, p < 0.001; p values were calculated using a two-way ANOVA and a post-hoc comparison of estimated marginal means).

      Failure Mode

      All the spiral core clamping anchors and winged clamping anchors, whether double-loaded or quadruple-loaded, failed from suture slippage without obviously visible deformities or cutting of the anchor body. However, the failure pattern of the spooling anchor was different. The spooling anchors failed at the anchor bodies due to cutting through by the sutures (Table 1). The extent to which they cut through the foam was different for the double-loaded and quadruple-loaded anchors, where the former was broken unilaterally, and the latter was broken bilaterally (Figure 5).
      Table 1Failure mode of the various suture anchors
      Suture PulloutCutting of Anchor Body
      FP100
      PL100
      RX010
      FP, Footprint Ultra PK (Spiral core clamping anchor); PL, PopLok (Winged clamping anchor); RX, ReelX STT (Spooling anchor).
      Figure thumbnail gr5
      Figure 5Failure mode of the Reel X STT suture anchors. (A, B) The quadruple-loaded spooling anchor (ReelX STT) failed due to bilateral cut-through of the anchor. (C, D) On the contrary, the double-loaded anchors failed due to unilateral cut-through.

      Discussion

      In the current study, the results indicated that knotless suture anchors with different fixation mechanisms were influenced differently by increasing the number of the sutures that were loaded. Quadruple loading effectively increased the CFL and UFL in knotless suture anchors employing the clamping mechanism, considering the spiral core clamping anchor and the winged clamping anchor as the example. On the contrary, in suture anchors with the spooling mechanism, quadruple loading did not increase the CFL and significantly decreased the UFL. It has not been elucidated whether the failure load is fortified proportionately or undermined significantly by increasing the number of sutures loaded. The results of the current study provide surgeons with better understanding to the factors influencing suture holding capacity of knotless anchor.
      The principal finding in this study is that fixation mechanism is significantly associated with suture loading capacity. According to the official user instruction manual for the spooling anchor, the anchor body expands subcortically as it spools the suture materials inside. Consequently, when spooling anchor is rotated three revolutions, the degree of deformity is considerably larger in a quadruple-loaded anchor than it is in a double-loaded anchor. It is thus plausible to conclude that the spooling of four sutures exceeded the presumed expansile capacity and undermined the sturdiness of the anchor body, which accounted for the decline in the failure load by quadruple-loading of the spooling anchor. This inference was supported by the bilateral cutting-through of the quadruple-loaded spooling anchor in the current study. On the contrary, knotless suture anchors employing the clamping mechanism consistently had a higher failure load as the suture number was increased. According to the results, we suggest that surgeons take the fixation mechanism into consideration if the knotless suture anchor has to be multiple loaded. Under quadruple-loading conditions, the knotless anchors with the clamping mechanism provided more reliable results compared to anchors with the spooling mechanism.
      Whether double loaded or quadruple loaded, the spiral core clamping anchor exhibited the best clinical failure load among the three knotless anchors tested. A high clinical failure load translated into high initial fixation strength and minimal gap formation, where the mechanical stability of the repair construct was maintained after cyclic loading, thereby optimizing the biological process required for the tendon-to-bone healing process.[
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      ] In contrast, Kummer et al mentioned that inadequate suture-holding capacity could result in loosening, thus reducing the pressure and area of tendon-bone contact.[
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      • Longo U.G.
      • Ruzzini L.
      • Rizzello G.
      • Maffulli N.
      • Denaro V.
      The Roman Bridge: a "double pulley - suture bridges" technique for rotator cuff repair.
      ] Gausden et al suggested that during activities of daily living, the load transmitted through the supraspinatus tendon is reported to range between 60N and 130N.[
      • Gausden E.B.
      • McCarthy M.M.
      • Kontaxis A.
      • Corpus K.T.
      • Gulotta L.V.
      • Kelly A.M.
      Subscapularis tendon loading during activities of daily living.
      ] The results of our study indicated that the CFL of quadruple-loaded spiral core clamping anchor averaged 138.1N, which was the only sample surpassing the daily load on the supraspinatus tendon. The high initial CFL of the spiral core clamping anchor made it a promising candidate for athletic or obese patients, considering their repaired supraspinatus tendon may have to bear higher loads immediately after surgery. To summarize, the CFLs approximated in the present study could help surgeons customize a suture bridge construct of suitable strength according to the lifestyle and body weight of individual patients. If high tensile load is expected, the spiral core clamping anchor could be the knotless suture anchor of choice.
      When double loaded, the spooling anchors showed the best UFL. When quadruple loaded, no significant differences were observed among the UFLs of the three anchors. While the CFL is about maintaining the tendon and footprint in close contact, the UFL is about keeping the repair construct attached and the rotator cuff muscle loaded. Exceeding the UFL could result in complete mechanical unloading of the cuff muscles, which would lead to progression of fatty infiltration and muscle atrophy in the long run.[
      • Gerber C.
      • Meyer D.C.
      • Schneeberger A.G.
      • Hoppeler H.
      • von Rechenberg B.
      Effect of tendon release and delayed repair on the structure of the muscles of the rotator cuff: an experimental study in sheep.
      ] To avoid this issue, several previous studies recommended the use of a spooling anchor.[
      • Efird C.
      • Traub S.
      • Baldini T.
      • Rioux-Forker D.
      • Spalazzi J.P.
      • Davisson T.
      • et al.
      Knotless single-row rotator cuff repair: a comparative biomechanical study of 2 knotless suture anchors.
      ,
      • Klinge S.A.
      • Vopat B.G.
      • Paller D.
      • Avery A.L.
      • Koruprolu S.
      • Fadale P.D.
      Isolating Suture Slippage During Cadaveric Testing of Knotless Anchors.
      ] The spooling mechanism of it probably accounted for its lower CFL but higher UFL when compared with suture anchors adopting the clamping mechanism. After spooling in the sutures, the backlash in the ratcheting mechanism allowed some backward motion that resulted in earlier clinical failure. However, after reaching the stopping point of the ratchet, the pawl firmly bore against the backward sliding of the sutures and contributed to the high UFL. Interestingly, the UFL of the quadruple-loaded spooling anchor was reduced when compared with its double-loaded counterpart. The fixation strength of the spooling anchor was postulated to increase gradually with each revolution, which expanded the anchor body by spooling in additional sutures. The reduction in the UFL of the quadruple-loaded spooling anchor suggested that expansibility was limited. Spooling in excessive suture materials could be detrimental to the sturdiness of an expansible anchor, as suggested by the failure mode (sutures cutting through the anchor body). To conclude, based on the results of the current study, when the anchor is to be double-loaded and the goal is optimal UFL, a spooling anchor is recommended. When quadruple-loading is considered, however, we are not that confident with the sturdiness of suture anchors involving the spooling mechanism.
      There are some limitations in this study. First, foam blocks were used instead of cadaveric bones or animal models. As a consequence, we could not evaluate the effects of different bone mineral densities on fixation strength. Second, only the failure loads at time zero were evaluated, without factoring in contributions of tendon-to-bone healing. Third, a practical suture bridge construct may involve the use of sutures from various angles. The current condition might be too ideal, and as such, may have led to overestimation of the failure loads provided by each construct. Fourth, the vertical loading parallel with the knotless anchor is not the same as would be the case in an in vivo situation and represents a worst-case mechanical structure. Fifth, due to the design patent, we were not able to acquire the detailed structure of each anchor. Thus, we can only interpret our results based on their main suture fixation mechanisms.

      Conclusion

      The number of loaded sutures does affect the suture-holding strength of knotless suture anchors, and anchors with different fixation mechanisms are affected differently by the number of loaded sutures.

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