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A Biomechanical and Radiographic Analysis of Standard and Intracortical Suture Anchors for Arthroscopic Rotator Cuff Repair
A Biomechanical and Radiographic Analysis of Standard and Intracortical Suture Anchors for Arthroscopic Rotator Cuff Repair Andrew Mahar, M.S., Darin W. Allred, M.D., Michelle Wedemeyer, B.S., Guarav Abbi, B.S., and Robert Pedowitz, M.D., Ph.D.
Purpose: To compare the fixation strength and radiographic motion of an anchor designed for intracortical (IC) fixation (FT Anchor, Arthrex, Naples, FL) with that of standard anchors used for rotator cuff repair. Type of Study: In vitro human cadaveric biomechanical study. Methods: Four types of metallic suture anchors (8 per group) were randomly inserted into human cadaveric humeri using an IC anchor and 3 types of standard anchors. Anchors were inserted 45° to the humeral head surface and 90° to the rotator cuff line of action. Anchors were tested under physiologic loads for 500 cycles followed by a failure test. The number of cycles, failure mode, and failure load were recorded. Fluoroscopy was used to measure rotation and displacement of the anchor within the humeral head during testing. Data were analyzed using a 1-way analysis of variance with a correction for multiple comparisons. Results: There were no significant differences in anchor displacement or rotation measured by fluoroscopy after cyclic loading. Total construct displacement across anchors ranged from 4.9 to 7.8 mm, well beyond the 3-mm failure criterion reported in the literature. The IC anchor had a statistically significant greater failure load than the other devices. There was no significant difference in failure load between the other 3 anchors. The anchor had the greatest number of cycles to 3 mm of failure. This was not significantly different than the TwinFix anchor (Smith & Nephew, Andover, MA), but both values were significantly greater than both the Super Revo (Linvatec, Largo, FL) and Fastin RC (DePuy Mitek, Raynham, MA) anchors. Conclusions: Anchor motion accounted for about one third of total displacement of the suture/anchor construct. IC fixation anchors performed well compared with standard anchors in human cadaveric bone. Clinical Relevance: Fluoroscopic imaging showed both rotation and displacement of the anchor within the humeral head which may contribute to early gap formation after rotator cuff repairs. Key Words: Rotator cuff— Repair—Suture anchors—Biomechanics—Implant migration.
T From the Orthopedic Biomechanics Research Center, Children’s Hospital (A.M., M.W.); Department of Orthopaedic Surgery, University of California, San Diego (A.M., R.P.); San Diego Arthroscopy and Sports Medicine Fellowship (D.W.A.); and the School of Medicine, University of San Diego (G.A., R.P.), San Diego, California, U.S.A. Supported by an unrestricted research grant from Arthrex, Inc, Naples, Florida. Address correspondence and reprint requests to Andrew Mahar, M.S., Orthopedic Biomechanics Research Center, MC5054, 3020 Children’s Way, San Diego, CA 92123, U.S.A. E-mail: [email protected] © 2006 by the Arthroscopy Association of North America 0749-8063/06/2202-05-44$32.00/0 doi:10.1016/j.arthro.2005.08.042
130
he science of arthroscopic rotator cuff repairs continues to advance, with new anchors, materials, and techniques. Despite good to excellent clinical results,1–5 postoperative imaging studies suggest there is still room for improvement in arthroscopic rotator cuff repairs.6 The quality of the rotator cuff repair may depend on the blood supply to the cuff and the quality of the tendon-bone interface.7 The repair is also clearly affected by the suture material, type of anchor, and surgical technique.8 One option available for the surgeon is to place the suture anchor deeper than recommended in an attempt to improve fixation. A recent study found that anchors placed deeper than the standard insertion depth actually may be associated with decreased biomechanical performance.9 Data
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 22, No 2 (February), 2006: pp 130-135
BIOMECHANICS OF INTRACORTICAL SUTURE ANCHORS from that study indicated that the suture material could cut through the cortical bone and cause loss of fixation. Another potential explanation for this would be anchor translation/rotation within the cancellous bone of the humeral head. It is possible that the anchor would migrate within the humeral head when loaded if the bone-implant interface does not provide adequate stability. Although it appears that deeper cancellous purchase does not provide improved fixation, anchor stabilization within the dense cortical bone of the humeral head may prove a viable option to limit anchor mobility. Adequate stabilization in this area may also prevent the potential for translation/rotation within the humeral head. For these reasons, a fully threaded intracortical (IC) anchor (FT Anchor) incorporating a recessed suture eyelet loop, which allows anchor placement flush with the cortical surface, has recently been developed by Arthrex, Inc, of Naples, Florida. The study hypothesis was that the IC anchor may provide improved biomechanical stability compared with traditional anchors used for arthroscopic rotator cuff repair. To address this hypothesis, the purpose of this study was to compare the fixation stability of a novel IC anchor compared with several standard anchors, evaluating both cyclic displacement and ultimate failure load as well as anchor motion within humeral head motion using fluoroscopy. METHODS Sixteen human cadaveric humeri (8 pairs) were sectioned at the mid-diaphysis and cleaned of soft tissue (age range, 57 to 81 years). Care was taken not to decorticate the supraspinatus tendon footprint on the humeral head. Bone density was measured with a Lunar Dual-Energy X-Ray Absorptiometry (DEXA) machine (GE Medical Systems, Waukesha, WI). These data revealed a relatively osteoporotic sample population (0.34 ⫾ 0.09 g/cm2) compared with previously reported densities (0.78 g/cm2) for young male specimens.10 Four types of anchors (n ⫽ 8 in each group) were randomly assigned to either anterior or posterior insertions within the supraspinatus footprint across specimens. Random allocation, using a Latin squares design to balance the treatment groups across the number of specimens, to anterior/posterior positions was performed to normalize for the varying bone density in the greater tuberosity.11 The Arthrex IC anchor (FT Anchor) and 3 standard cancellous type suture anchors (Fastin RC, DePuy Mitek, Raynham, MA; Super Revo, Linvatec, Largo, FL; and TwinFix
FIGURE 1.
131
Mechanical testing setup.
Ti 5.0, Smith & Nephew, Andover, MA) were all inserted using the respective manufacturers’ recommendations regarding anchor depth. Thus, each pair of shoulders received 1 type of anchor from each group (2 anchors per humerus). All anchors were metallic screw-in anchors. The IC anchor was preloaded with FiberWire suture (Arthrex) and the other anchors used No. 2 Ethibond suture (Ethicon, Somerville, NJ). Anchors within a single humeral head were separated by a minimum distance of 1.5 cm. Anchors were placed at 45° to the cortex of the supraspinatus footprint with the mechanical line of action 90° from the anchor long axis simulating the line of action of the rotator cuff (Fig 1). This insertion technique has been recommended previously to maximize anchor stability.12 The method of loading has been used previously for biomechanical testing of anchor constructs.13–15 Sutures were tied with 7 square knots backing up a surgeon’s knot over a 38-mm diameter dowel to create a closed loop. This knot was selected to minimize the risk of knot slippage during low cyclic loading. The humeri were potted in a 2-part epoxy resin (Bondo; Marhyde, Atlanta, GA) and held in place with a custom-designed fixation rig. The suture loop was then attached to an MTS machine (Materials Testing Systems, Eden Prairie, MN) to represent the loading direction of the supraspinatus tendon (Fig 1). The suture anchor construct was pretensioned to 10 N and any displacement offset eliminated. Each specimen was cyclically loaded from 10 to 45 N at 0.5 Hz to a maximum of 500 cycles. If still intact after cyclic loading, specimens were loaded at 0.5 mm/sec to failure. This biomechanical methodology has been used previously to evaluate depth of anchor insertion
132
A. MAHAR ET AL. plant angle. A second angle measurement was taken after testing using the same method. The difference between these 2 measurements then gave the relative angular change before and after cyclic loading (Fig 3). All measurements were completed by the same person for all tests to reduce variability. The suture was then marked with ink as it exited the bone hole to assist with the characterization of the mode of failure. The number of cycles to 3 mm of construct displacement, failure mode, and failure magnitude (in Newtons) were recorded for each specimen. Total construct displacement was defined as the displacement as measured by the test machine during cyclic loading and would thus include factors related to loop elongation, knot security, and implant movement. The site of failure was referenced to the anchor eyelet using the previously placed ink mark. Data were analyzed using a 1-way analysis of variance (P ⬍ .05) with a Tukey’s post hoc correction for multiple comparisons. RESULTS
FIGURE 2. men.
Fluoroscopy machine orientation relative to speci-
as well as the effects of varying suture knots and materials.9,15,16 Anchor displacement (in millimeters) and rotation (in degrees) were recorded after pretensioning but before cyclic testing using image intensified fluoroscopy. These measurements were again taken following cyclic testing. The positioning of the fluoroscopy machine relative to the specimen was kept constant between tests by using alignment marks on the testing jig. The fluoroscopy machine was also aligned perpendicular to the plane of loading to ensure accurate measurements (Fig 2). A metallic 10-mm diameter marker was rigidly fixed to the humeral head to allow for a distance calibration for each image. This calibration involved measuring the size of the 10-mm marker from the fluoroscopic image and then using that as a correction factor for the distance change of the anchor. The distance change of the implant was measured as the relative change of the center of the implant before and after cyclic loading. A K-wire was rigidly fixed to the testing jig parallel to the long axis of the humeral shaft. A line was then extended from the long axis of the implant to intersect with the a line extended from the K-wire. This gave the initial im-
There were no significant differences in radiographic measures for angle change (range, 12.6° to 21.3°) (Table 1). The IC anchor had an angle change of 12.6° ⫾ 10.6° and the Super Revo had an angle change of 15.3° ⫾ 21.9°. The TwinFix anchor had an angle of 17.0° ⫾ 15.4° and the FastinRC had an angle change of 21.3° ⫾ 23.8°. There were no significant differences between anchor types for displacement change (range, 1.3 to 2.1 mm). The IC anchor had 1.3 ⫾ 1.3 mm of displacement within the humeral head. The Super Revo anchor had 2.1 ⫾ 1.8 mm of displacement, the TwinFix had 1.3 ⫾ 0.9 mm, and the
FIGURE 3. Methods for taking distance and angle change measurements.
BIOMECHANICS OF INTRACORTICAL SUTURE ANCHORS TABLE 1.
133
Fluoroscopic Data After Cyclic Loading (Mean ⫾ SD)
Anchor Type IC Anchor Super Revo TwinFix Fastin RC
Angle Change (degrees)
Displacement Change (mm)
12.6 ⫾ 10.6 15.3 ⫾ 21.9 17.0 ⫾ 15.4 21.3 ⫾ 23.8
1.3 ⫾ 1.3 2.1 ⫾ 1.8 1.3 ⫾ 0.9 2.1 ⫾ 2.2
FastinRC had 2.1 ⫾ 2.2 mm of displacement within the humeral head. A 3-mm loss of fixation has been used previously as a criterion measure for a potentially ineffective repair construct.17 The IC fixation anchor had the greatest number of cycles to 3 mm of failure (380 ⫾ 160 cycles). This was not significantly different than the TwinFix anchor (331 ⫾ 190 cycles), but both values were significantly greater than both the Super Revo (114 ⫾ 47 cycles) and Fastin RC (54 ⫾ 53 cycles) anchors (P ⬍ .002) (Table 2). Total construct displacement after cyclic testing was not significantly different between anchors (P ⫽ .5) (Table 2). The IC anchor showed 4.9 ⫾ 2.1 mm of total construct displacement. The Super Revo construct displaced 6.3 ⫾ 2.6 mm after cyclic loading. The TwinFix anchor showed 7.4 ⫾ 7.4 mm of total construct displacement and the FastinRC showed 7.8 ⫾ 1.1 mm of total construct displacement. The IC fixation anchor loaded with No. 2 FiberWire had a statistically significant greater failure load (140 ⫾ 23 N) than the other devices, which were loaded with No. 2 Ethibond (P ⬍ .02) (Table 2). There was no significant difference in failure load between the other 3 anchors. Failure patterns were further analyzed for each group. The failure load calculated from the failure test was indicative of a structural change in the construct. This structural change was considered to be the failure mode. A follow-up correlation test did not indicate a correlation between failure load and failure mode (r ⫽ .22). For the IC anchor, 75% of constructs failed at the TABLE 2.
Mechanical Data (Mean ⫾ SD)
Anchor Type
No. of Cycles to 3 mm
Total Cyclic Displacement (mm)
Failure Load (N)
IC Anchor Super Revo TwinFix Fastin RC
379.9 ⫾ 159.5 114.0 ⫾ 46.7 331.3 ⫾ 190.3 54.2 ⫾ 52.8
4.9 ⫾ 2.1 6.3 ⫾ 2.6 7.4 ⫾ 7.4 7.8 ⫾ 1.1
139.9 ⫾ 22.7 113.2 ⫾ 12.3 92.2 ⫾ 15.9 90.3 ⫾ 7.3
FIGURE 4.
Standard anchor placement.
suture-anchor interface. The other 2 constructs had 1 example each of anchor pullout and knot slippage. For the standard type of anchors, the Fastin RC anchor had 38% failure at the suture-anchor interface, 38% knot breakage, and 25% anchor pullout. The Super Revo anchor had 38% failure at the suture-anchor interface, 38% knot breakage, and 25% anchor pullout. The TwinFix anchor had 63% failure at the suture-anchor interface, 25% knot breakage, and 13% anchor pullout. Thus, across varying anchor designs, approximately 38% to 75% of anchors failed at the sutureanchor interface. This again reinforces the idea that the suture-eyelet interface represents a weak link in the mechanical chain as has been described previously.18 –22 Anterior or posterior anchor location did not appear to affect failure modes. DISCUSSION Previous studies have reported adequate to excellent performance of suture anchors used commonly in clinical practice.1– 6,23,24 To obtain clinically relevant biomechanical data, the present study used a cyclic load protocol with oblique loads on the suture-anchor construct. The magnitude of total construct displacement of the suture-anchor construct observed in this study was concerning. The average displacement of the 3 conventional suture-anchor constructs following cyclic testing (500 cycles) was approximately 6 to 7 mm. This elongation could be attributable in part to the size of the suture loop used in the current study, which was much larger than that used clinically. The displacement could also be a combination of anchor translation/rotation, knot settling, stretch of the suture material, or the suture cutting through bone (Fig 4). Translation of suture anchors within the humeral head
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A. MAHAR ET AL.
FIGURE 5.
IC anchor placement.
has been noted in previous in vitro studies of rotator cuff repairs.25 However, anchor movement is but one manner in which repairs may fail, with suture failure and tissue failure representing other failure modes.18 –22 Anchor rotation may allow the eyelet to rotate in an arc toward the line of pull, contributing to loss of tendon-bone apposition. Thus, in the early postoperative rehabilitation period, when low cyclic loads are passed through the supraspinatus tendon, there is potential for soft-tissue retraction. This relative tissue retraction does not include any possible loss of fixation of the suture within the tendon itself. The IC anchor showed greater failure loads and similar kinematics (rotation/displacement) compared with some other anchors. These mechanical differences may be due in part to the recessed anchor eyelet that eliminates suture wear on the cortical margin (Fig 5). It may also be due in part to the cortical location of the anchor that may eliminate some of the translation and rotation possible in a deeper cancellous location. These issues related to anchor design were not part of the scope of this study but should be specifically addressed in subsequent studies. Failure load data from the current study were slightly lower than ranges reported previously of between 130 N and 180 N22,26 and are likely related to both differences in testing methodologies and in specimen quality and variability. Although the IC anchor showed significantly greater failure loads and similar rotation/translation compared with some standard suture anchors, this probably is partly the result of the difference in suture material. The IC anchor used No. 2 FiberWire whereas the other anchors were preloaded with No. 2 Ethibond. Previous research has found improved biomechanical characteristics of FiberWire compared with Ethibond for cyclic stability and failure
loads.16,17,27 In the current study, a majority of failures at peak load occurred by the suture material slipping or breaking, reinforcing the idea that the ultimate weak link in the repair usually resides in the suture or suture-eyelet interface.28,29 Many manufacturers are now producing arthroscopic rotator cuff repair anchors with stronger suture materials and an extension of the current study should consider using a single type suture material. One of the limiting factors of this study could possibly be the use of human cadaveric tissue. These tissues may have been osteoporotic or osteopenic and thus not representative of the entire population range that would undergo this method of repair. This specimen variability may have also been responsible for the differences in statistical power across tests. Some power values were as high as 0.9 whereas other suffered from relatively low power (⬃0.3). Thus, the clinical conclusions drawn from the current data must be carefully considered. It is also possible that higher loads during cyclic testing may have elucidated greater differences between groups. However, the loads used in the current study approximate values reported previously for forces delivered through a single suture anchor repair.30 Finally, there is the potential for human error when taking fluoroscopic measurements. Although this technique has been used previously to evaluate shoulder arthroplasty and shoulder function, such as scapulothoracic rhythm,31,32 to the authors’ knowledge this is the first time that fluoroscopy has been used to evaluate in vitro anchor position following suture anchor insertion. In future studies, it may be possible to use imaging programs to conduct this phase of the analysis to reduce measurement variability.
CONCLUSION Data from the current study indicate that an IC suture anchor may provide the surgeon with biomechanical stability and failure loads similar to other anchors. The total construct displacements for all anchors represent magnitudes of clinical concern. It is also of clinical concern that anchor motion within the humeral head contributed approximately one third of this total displacement. This type of analysis, with particular focus on the movement of suture anchors within the humeral head, requires further investigation to include tissue with increased bone density, suture fixation within soft tissue, altered anchor location, and different types of anchor designs.
BIOMECHANICS OF INTRACORTICAL SUTURE ANCHORS REFERENCES 1. Park JY, Chung KT, Yoo MJ. A serial comparison of arthroscopic repairs for partial- and full-thickness rotator cuff tears. Arthroscopy 2004;20:705-711. 2. Wolf EM, Pennington WT, Agrawal V. Arthroscopic rotator cuff repair: 4- to 10-year results. Arthroscopy 2004;20:5-12. 3. Kim SH, Ha KI, Park JH, Kang JS, Oh SK, Oh I. Arthroscopic versus mini-open salvage repair of the rotator cuff tear: Outcome analysis at 2 to 6 years’ follow-up. Arthroscopy 2003; 19:746-754. 4. Lo IK, Burkhart SS. Arthroscopic repair of massive, contracted, immobile rotator cuff tears using single and double interval slides: Technique and preliminary results. Arthroscopy 2004;20:22-33. 5. Burkhart SS, Danaceau SM, Pearce CE Jr. Arthroscopic rotator cuff repair: Analysis of results by tear size and by repair technique—Margin convergence versus direct tendon-to-bone repair. Arthroscopy 2001;17:905-912. 6. Galatz LM, Ball CM, Teefey SA, Middleton WD, Yamaguchi K. The outcome and repair integrity of completely arthroscopically repaired large and massive rotator cuff tears. J Bone Joint Surg Am 2004;86:219-224. 7. Reilly P, Amis AA, Wallace AL, Emery, RJ. Supraspinatus tears: Propagation and strain alteration. J Should Elbow Surg 2004;12:134-138. 8. Kim SH, Ha KI, Kim SH, Kim JS. Significance of the internal locking mechanism for loop security enhancement in the arthroscopic knot. Arthroscopy 2001;17:850-855. 9. Bynum KC, Lee S, Mahar A, Tasto J, Pedowitz R. Failure mode of suture anchors as a function of insertion depth. Am J Sports Med 2005;33:1030-1034. 10. Okamura K, Ozaki J. Bone mineral density of the shoulder joint in frozen shoulder. Arch Orthop Trauma Surg 1999;119: 363-367. 11. Liporace FA, Bono CM, Caruso SA, et al. The mechanical effects of suture anchor insertion angle for rotator cuff repair. Orthopedics 2002;4:399-402. 12. Burkhart SS. The deadman theory of suture anchors: Observations along a south Texas fence line. Arthroscopy 1995;11: 119-123. 13. Petit CJ, Boswell R, Mahar A, Tasto J, Pedowitz RA. Biomechanical evaluation of a new technique for rotator cuff repair. Am J Sports Med 2003;31:849-853. 14. Lee S, Mahar A, Bynum K, Pedowitz R. Biomechanical comparison of bioabsorbable sutureless screw anchor versus suture anchor fixation for rotator cuff repair. Arthroscopy 2005;21: 43-47. 15. Mahar A, Tucker B, Upasani V, Oka R, Pedowitz R. Increasing the insertion depth of suture anchors for rotator cuff repair does not improve biomechanical stability. J Shoulder Elbow Surg 2005;14:626-630. 16. Abbi G, Espinoza L, Odell TR, Mahar AT, Pedowitz RA. Very
17. 18. 19. 20. 21. 22.
23. 24. 25. 26. 27. 28.
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strong sutures can still slip: Evaluation of five knot types and two suture materials for arthroscopic rotator cuff repair. Arthroscopy 2005 (in press). Lo IK, Burkhart SS, Chan KC, Athanasiou, KA. Arthroscopic knots: Determining the optimal balance of loop security and knot security. Arthroscopy 2004;20:489-502. Meyer DC, Nyffeler RW, Fucentese SF, Gerber C. Failure of suture material at suture anchor eyelets. Arthroscopy 2002;18: 1013-1019. Meyer DC, Gerber C. Failure of anterior shoulder instability repair caused by eyelet cutout of absorbable suture anchors. Arthroscopy 2004;20:521-523. Barber FA, Cawley P, Prudich JF. Suture anchor failure strength—An in vivo study. Arthroscopy 1993;9:647-652. Rupp S, Georg T, Gauss C, Kohn D, Seil R. Fatigue testing of suture anchors. Am J Sports Med 2002;30:239-247. Schneeberger AG, von Roll A, Kalberer F, Jacob HA, Gerber C. Mechanical strength of arthroscopic rotator cuff repair techniques: An in vitro study. J Bone Joint Surg Am 2002;84: 2152-2160. Gartsman GM, Khan M, Hammerman SM. Arthroscopic repair of full-thickness tears of the rotator cuff. J Bone Joint Surg Am 1998;80:832-840. Jones CK, Savoie FH III. Arthroscopic repair of large and massive rotator cuff tears. Arthroscopy 2003;19:564-571. Roth CA, Bartolozzi AR, Ciccotto, MG, et al. Failure properties of suture anchors in the glenoid and the effects of cortical thickness. Arthroscopy 1998;14:186-191. Ilahi OA, Younas SA, Alexander J, Noble PC. Cyclic testing of arthroscopic knot security. Arthroscopy 2004;20:62-68. Lo IK, Burkhart SS, Athanasiou K. Abrasion resistance of two types of nonabsorbable braided suture. Arthroscopy 2004;20: 407-413. Burkhart SS, Johnson TC, Wirth MA, Athanasiou KA. Cyclic loading of transosseous rotator cuff repairs: Confirmation of the tension overload phenomenon and comparison of suture anchor fixation with transosseous fixation. Arthroscopy 1997; 13:720-724. Burkhart SS, Diaz Pagan JL, Wirth MA, Athanasiou KA. Cyclic loading of anchor-based rotator cuff repairs: Tension overload as a possible cause of failure. Arthroscopy 1997;13: 172-176. Burkhart, SS. Knot security in simple sliding knots and its relationship to rotator cuff repair: How secure must the knot be? Arthroscopy 2000;16:202-207. Mandalidis DG, McGlone BS, Quigley RF, McInerney D, O’Brien M. Digital fluoroscopic assessment of the scapulohumeral rhythm. Surg Radiol Anat 1999;21:241-246. Karduna AR, Williams GR, Williams JL, Iannotti JP. Glenohumeral joint translations before and after total shoulder arthroplasty. A study in cadavera. J Bone Joint Surg Am 1997; 79:1166-1174.
Purpose: To compare the fixation strength and radiographic motion of an anchor designed for intracortical (IC) fixation (FT Anchor, Arthrex, Naples, FL) with that of standard anchors used for rotator cuff repair. Type of Study: In vitro human cadaveric biomechanical study. Methods: Four types of metallic suture anchors (8 per group) were randomly inserted into human cadaveric humeri using an IC anchor and 3 types of standard anchors. Anchors were inserted 45° to the humeral head surface and 90° to the rotator cuff line of action. Anchors were tested under physiologic loads for 500 cycles followed by a failure test. The number of cycles, failure mode, and failure load were recorded. Fluoroscopy was used to measure rotation and displacement of the anchor within the humeral head during testing. Data were analyzed using a 1-way analysis of variance with a correction for multiple comparisons. Results: There were no significant differences in anchor displacement or rotation measured by fluoroscopy after cyclic loading. Total construct displacement across anchors ranged from 4.9 to 7.8 mm, well beyond the 3-mm failure criterion reported in the literature. The IC anchor had a statistically significant greater failure load than the other devices. There was no significant difference in failure load between the other 3 anchors. The anchor had the greatest number of cycles to 3 mm of failure. This was not significantly different than the TwinFix anchor (Smith & Nephew, Andover, MA), but both values were significantly greater than both the Super Revo (Linvatec, Largo, FL) and Fastin RC (DePuy Mitek, Raynham, MA) anchors. Conclusions: Anchor motion accounted for about one third of total displacement of the suture/anchor construct. IC fixation anchors performed well compared with standard anchors in human cadaveric bone. Clinical Relevance: Fluoroscopic imaging showed both rotation and displacement of the anchor within the humeral head which may contribute to early gap formation after rotator cuff repairs. Key Words: Rotator cuff— Repair—Suture anchors—Biomechanics—Implant migration.
T From the Orthopedic Biomechanics Research Center, Children’s Hospital (A.M., M.W.); Department of Orthopaedic Surgery, University of California, San Diego (A.M., R.P.); San Diego Arthroscopy and Sports Medicine Fellowship (D.W.A.); and the School of Medicine, University of San Diego (G.A., R.P.), San Diego, California, U.S.A. Supported by an unrestricted research grant from Arthrex, Inc, Naples, Florida. Address correspondence and reprint requests to Andrew Mahar, M.S., Orthopedic Biomechanics Research Center, MC5054, 3020 Children’s Way, San Diego, CA 92123, U.S.A. E-mail: [email protected] © 2006 by the Arthroscopy Association of North America 0749-8063/06/2202-05-44$32.00/0 doi:10.1016/j.arthro.2005.08.042
130
he science of arthroscopic rotator cuff repairs continues to advance, with new anchors, materials, and techniques. Despite good to excellent clinical results,1–5 postoperative imaging studies suggest there is still room for improvement in arthroscopic rotator cuff repairs.6 The quality of the rotator cuff repair may depend on the blood supply to the cuff and the quality of the tendon-bone interface.7 The repair is also clearly affected by the suture material, type of anchor, and surgical technique.8 One option available for the surgeon is to place the suture anchor deeper than recommended in an attempt to improve fixation. A recent study found that anchors placed deeper than the standard insertion depth actually may be associated with decreased biomechanical performance.9 Data
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 22, No 2 (February), 2006: pp 130-135
BIOMECHANICS OF INTRACORTICAL SUTURE ANCHORS from that study indicated that the suture material could cut through the cortical bone and cause loss of fixation. Another potential explanation for this would be anchor translation/rotation within the cancellous bone of the humeral head. It is possible that the anchor would migrate within the humeral head when loaded if the bone-implant interface does not provide adequate stability. Although it appears that deeper cancellous purchase does not provide improved fixation, anchor stabilization within the dense cortical bone of the humeral head may prove a viable option to limit anchor mobility. Adequate stabilization in this area may also prevent the potential for translation/rotation within the humeral head. For these reasons, a fully threaded intracortical (IC) anchor (FT Anchor) incorporating a recessed suture eyelet loop, which allows anchor placement flush with the cortical surface, has recently been developed by Arthrex, Inc, of Naples, Florida. The study hypothesis was that the IC anchor may provide improved biomechanical stability compared with traditional anchors used for arthroscopic rotator cuff repair. To address this hypothesis, the purpose of this study was to compare the fixation stability of a novel IC anchor compared with several standard anchors, evaluating both cyclic displacement and ultimate failure load as well as anchor motion within humeral head motion using fluoroscopy. METHODS Sixteen human cadaveric humeri (8 pairs) were sectioned at the mid-diaphysis and cleaned of soft tissue (age range, 57 to 81 years). Care was taken not to decorticate the supraspinatus tendon footprint on the humeral head. Bone density was measured with a Lunar Dual-Energy X-Ray Absorptiometry (DEXA) machine (GE Medical Systems, Waukesha, WI). These data revealed a relatively osteoporotic sample population (0.34 ⫾ 0.09 g/cm2) compared with previously reported densities (0.78 g/cm2) for young male specimens.10 Four types of anchors (n ⫽ 8 in each group) were randomly assigned to either anterior or posterior insertions within the supraspinatus footprint across specimens. Random allocation, using a Latin squares design to balance the treatment groups across the number of specimens, to anterior/posterior positions was performed to normalize for the varying bone density in the greater tuberosity.11 The Arthrex IC anchor (FT Anchor) and 3 standard cancellous type suture anchors (Fastin RC, DePuy Mitek, Raynham, MA; Super Revo, Linvatec, Largo, FL; and TwinFix
FIGURE 1.
131
Mechanical testing setup.
Ti 5.0, Smith & Nephew, Andover, MA) were all inserted using the respective manufacturers’ recommendations regarding anchor depth. Thus, each pair of shoulders received 1 type of anchor from each group (2 anchors per humerus). All anchors were metallic screw-in anchors. The IC anchor was preloaded with FiberWire suture (Arthrex) and the other anchors used No. 2 Ethibond suture (Ethicon, Somerville, NJ). Anchors within a single humeral head were separated by a minimum distance of 1.5 cm. Anchors were placed at 45° to the cortex of the supraspinatus footprint with the mechanical line of action 90° from the anchor long axis simulating the line of action of the rotator cuff (Fig 1). This insertion technique has been recommended previously to maximize anchor stability.12 The method of loading has been used previously for biomechanical testing of anchor constructs.13–15 Sutures were tied with 7 square knots backing up a surgeon’s knot over a 38-mm diameter dowel to create a closed loop. This knot was selected to minimize the risk of knot slippage during low cyclic loading. The humeri were potted in a 2-part epoxy resin (Bondo; Marhyde, Atlanta, GA) and held in place with a custom-designed fixation rig. The suture loop was then attached to an MTS machine (Materials Testing Systems, Eden Prairie, MN) to represent the loading direction of the supraspinatus tendon (Fig 1). The suture anchor construct was pretensioned to 10 N and any displacement offset eliminated. Each specimen was cyclically loaded from 10 to 45 N at 0.5 Hz to a maximum of 500 cycles. If still intact after cyclic loading, specimens were loaded at 0.5 mm/sec to failure. This biomechanical methodology has been used previously to evaluate depth of anchor insertion
132
A. MAHAR ET AL. plant angle. A second angle measurement was taken after testing using the same method. The difference between these 2 measurements then gave the relative angular change before and after cyclic loading (Fig 3). All measurements were completed by the same person for all tests to reduce variability. The suture was then marked with ink as it exited the bone hole to assist with the characterization of the mode of failure. The number of cycles to 3 mm of construct displacement, failure mode, and failure magnitude (in Newtons) were recorded for each specimen. Total construct displacement was defined as the displacement as measured by the test machine during cyclic loading and would thus include factors related to loop elongation, knot security, and implant movement. The site of failure was referenced to the anchor eyelet using the previously placed ink mark. Data were analyzed using a 1-way analysis of variance (P ⬍ .05) with a Tukey’s post hoc correction for multiple comparisons. RESULTS
FIGURE 2. men.
Fluoroscopy machine orientation relative to speci-
as well as the effects of varying suture knots and materials.9,15,16 Anchor displacement (in millimeters) and rotation (in degrees) were recorded after pretensioning but before cyclic testing using image intensified fluoroscopy. These measurements were again taken following cyclic testing. The positioning of the fluoroscopy machine relative to the specimen was kept constant between tests by using alignment marks on the testing jig. The fluoroscopy machine was also aligned perpendicular to the plane of loading to ensure accurate measurements (Fig 2). A metallic 10-mm diameter marker was rigidly fixed to the humeral head to allow for a distance calibration for each image. This calibration involved measuring the size of the 10-mm marker from the fluoroscopic image and then using that as a correction factor for the distance change of the anchor. The distance change of the implant was measured as the relative change of the center of the implant before and after cyclic loading. A K-wire was rigidly fixed to the testing jig parallel to the long axis of the humeral shaft. A line was then extended from the long axis of the implant to intersect with the a line extended from the K-wire. This gave the initial im-
There were no significant differences in radiographic measures for angle change (range, 12.6° to 21.3°) (Table 1). The IC anchor had an angle change of 12.6° ⫾ 10.6° and the Super Revo had an angle change of 15.3° ⫾ 21.9°. The TwinFix anchor had an angle of 17.0° ⫾ 15.4° and the FastinRC had an angle change of 21.3° ⫾ 23.8°. There were no significant differences between anchor types for displacement change (range, 1.3 to 2.1 mm). The IC anchor had 1.3 ⫾ 1.3 mm of displacement within the humeral head. The Super Revo anchor had 2.1 ⫾ 1.8 mm of displacement, the TwinFix had 1.3 ⫾ 0.9 mm, and the
FIGURE 3. Methods for taking distance and angle change measurements.
BIOMECHANICS OF INTRACORTICAL SUTURE ANCHORS TABLE 1.
133
Fluoroscopic Data After Cyclic Loading (Mean ⫾ SD)
Anchor Type IC Anchor Super Revo TwinFix Fastin RC
Angle Change (degrees)
Displacement Change (mm)
12.6 ⫾ 10.6 15.3 ⫾ 21.9 17.0 ⫾ 15.4 21.3 ⫾ 23.8
1.3 ⫾ 1.3 2.1 ⫾ 1.8 1.3 ⫾ 0.9 2.1 ⫾ 2.2
FastinRC had 2.1 ⫾ 2.2 mm of displacement within the humeral head. A 3-mm loss of fixation has been used previously as a criterion measure for a potentially ineffective repair construct.17 The IC fixation anchor had the greatest number of cycles to 3 mm of failure (380 ⫾ 160 cycles). This was not significantly different than the TwinFix anchor (331 ⫾ 190 cycles), but both values were significantly greater than both the Super Revo (114 ⫾ 47 cycles) and Fastin RC (54 ⫾ 53 cycles) anchors (P ⬍ .002) (Table 2). Total construct displacement after cyclic testing was not significantly different between anchors (P ⫽ .5) (Table 2). The IC anchor showed 4.9 ⫾ 2.1 mm of total construct displacement. The Super Revo construct displaced 6.3 ⫾ 2.6 mm after cyclic loading. The TwinFix anchor showed 7.4 ⫾ 7.4 mm of total construct displacement and the FastinRC showed 7.8 ⫾ 1.1 mm of total construct displacement. The IC fixation anchor loaded with No. 2 FiberWire had a statistically significant greater failure load (140 ⫾ 23 N) than the other devices, which were loaded with No. 2 Ethibond (P ⬍ .02) (Table 2). There was no significant difference in failure load between the other 3 anchors. Failure patterns were further analyzed for each group. The failure load calculated from the failure test was indicative of a structural change in the construct. This structural change was considered to be the failure mode. A follow-up correlation test did not indicate a correlation between failure load and failure mode (r ⫽ .22). For the IC anchor, 75% of constructs failed at the TABLE 2.
Mechanical Data (Mean ⫾ SD)
Anchor Type
No. of Cycles to 3 mm
Total Cyclic Displacement (mm)
Failure Load (N)
IC Anchor Super Revo TwinFix Fastin RC
379.9 ⫾ 159.5 114.0 ⫾ 46.7 331.3 ⫾ 190.3 54.2 ⫾ 52.8
4.9 ⫾ 2.1 6.3 ⫾ 2.6 7.4 ⫾ 7.4 7.8 ⫾ 1.1
139.9 ⫾ 22.7 113.2 ⫾ 12.3 92.2 ⫾ 15.9 90.3 ⫾ 7.3
FIGURE 4.
Standard anchor placement.
suture-anchor interface. The other 2 constructs had 1 example each of anchor pullout and knot slippage. For the standard type of anchors, the Fastin RC anchor had 38% failure at the suture-anchor interface, 38% knot breakage, and 25% anchor pullout. The Super Revo anchor had 38% failure at the suture-anchor interface, 38% knot breakage, and 25% anchor pullout. The TwinFix anchor had 63% failure at the suture-anchor interface, 25% knot breakage, and 13% anchor pullout. Thus, across varying anchor designs, approximately 38% to 75% of anchors failed at the sutureanchor interface. This again reinforces the idea that the suture-eyelet interface represents a weak link in the mechanical chain as has been described previously.18 –22 Anterior or posterior anchor location did not appear to affect failure modes. DISCUSSION Previous studies have reported adequate to excellent performance of suture anchors used commonly in clinical practice.1– 6,23,24 To obtain clinically relevant biomechanical data, the present study used a cyclic load protocol with oblique loads on the suture-anchor construct. The magnitude of total construct displacement of the suture-anchor construct observed in this study was concerning. The average displacement of the 3 conventional suture-anchor constructs following cyclic testing (500 cycles) was approximately 6 to 7 mm. This elongation could be attributable in part to the size of the suture loop used in the current study, which was much larger than that used clinically. The displacement could also be a combination of anchor translation/rotation, knot settling, stretch of the suture material, or the suture cutting through bone (Fig 4). Translation of suture anchors within the humeral head
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A. MAHAR ET AL.
FIGURE 5.
IC anchor placement.
has been noted in previous in vitro studies of rotator cuff repairs.25 However, anchor movement is but one manner in which repairs may fail, with suture failure and tissue failure representing other failure modes.18 –22 Anchor rotation may allow the eyelet to rotate in an arc toward the line of pull, contributing to loss of tendon-bone apposition. Thus, in the early postoperative rehabilitation period, when low cyclic loads are passed through the supraspinatus tendon, there is potential for soft-tissue retraction. This relative tissue retraction does not include any possible loss of fixation of the suture within the tendon itself. The IC anchor showed greater failure loads and similar kinematics (rotation/displacement) compared with some other anchors. These mechanical differences may be due in part to the recessed anchor eyelet that eliminates suture wear on the cortical margin (Fig 5). It may also be due in part to the cortical location of the anchor that may eliminate some of the translation and rotation possible in a deeper cancellous location. These issues related to anchor design were not part of the scope of this study but should be specifically addressed in subsequent studies. Failure load data from the current study were slightly lower than ranges reported previously of between 130 N and 180 N22,26 and are likely related to both differences in testing methodologies and in specimen quality and variability. Although the IC anchor showed significantly greater failure loads and similar rotation/translation compared with some standard suture anchors, this probably is partly the result of the difference in suture material. The IC anchor used No. 2 FiberWire whereas the other anchors were preloaded with No. 2 Ethibond. Previous research has found improved biomechanical characteristics of FiberWire compared with Ethibond for cyclic stability and failure
loads.16,17,27 In the current study, a majority of failures at peak load occurred by the suture material slipping or breaking, reinforcing the idea that the ultimate weak link in the repair usually resides in the suture or suture-eyelet interface.28,29 Many manufacturers are now producing arthroscopic rotator cuff repair anchors with stronger suture materials and an extension of the current study should consider using a single type suture material. One of the limiting factors of this study could possibly be the use of human cadaveric tissue. These tissues may have been osteoporotic or osteopenic and thus not representative of the entire population range that would undergo this method of repair. This specimen variability may have also been responsible for the differences in statistical power across tests. Some power values were as high as 0.9 whereas other suffered from relatively low power (⬃0.3). Thus, the clinical conclusions drawn from the current data must be carefully considered. It is also possible that higher loads during cyclic testing may have elucidated greater differences between groups. However, the loads used in the current study approximate values reported previously for forces delivered through a single suture anchor repair.30 Finally, there is the potential for human error when taking fluoroscopic measurements. Although this technique has been used previously to evaluate shoulder arthroplasty and shoulder function, such as scapulothoracic rhythm,31,32 to the authors’ knowledge this is the first time that fluoroscopy has been used to evaluate in vitro anchor position following suture anchor insertion. In future studies, it may be possible to use imaging programs to conduct this phase of the analysis to reduce measurement variability.
CONCLUSION Data from the current study indicate that an IC suture anchor may provide the surgeon with biomechanical stability and failure loads similar to other anchors. The total construct displacements for all anchors represent magnitudes of clinical concern. It is also of clinical concern that anchor motion within the humeral head contributed approximately one third of this total displacement. This type of analysis, with particular focus on the movement of suture anchors within the humeral head, requires further investigation to include tissue with increased bone density, suture fixation within soft tissue, altered anchor location, and different types of anchor designs.
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