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Suture Anchors—Update 1999
Suture Anchors—Update 1999 F. Alan Barber, M.D., and Morley A. Herbert, Ph.D.
Summary: New suture anchors continue to become available. Our prior reports on the pullout strength of over 50 different anchors is supplemented by a similar test conducted on 25 additional new anchors. This anchor comparison, using an established protocol in fresh porcine femurs, recorded failure strength, failure mode (anchor pullout, suture eyelet cutout, or wire breakage), eyelet size, minor and major diameters, and drill hole sizes. These new anchors were tested in diaphyseal cortex, metaphyseal cortex, and a cancellous trough. Tensile stress parallel to the axis of insertion was applied at a rate of 12.5 mm/sec by an Instron 1321 until failure and mean anchor failure strengths were calculated. Anchors tested included DePuy 4.5 prototypes D1, D2 Catera 4.5, and D3; DePuy 3.5 prototypes D4- Catera 3.5, D5, and D6; Mainstay 2.7, 3.5, 4.5; ROC EZ 2.8, EZ 3.5, and XS 3.5; Ultrafix RC and Ultrafix MiniMite; 1.3 MicroMitek, Panalok 3.5, and Tacit 2.0; Umbrella Harpoon; PeBA 2.8, 4.0, 6.5; and Stryker 1.9, 2.7, 3.4, and 4.5 prototypes. Screw anchors still tend to have higher values, but for the newer nonscrew designs this distinction is less apparent. The new biodegradable anchors were all composed of poly L-lactic acid suggesting a trend away from other polymers, and these new biodegradable anchors showed load-to-failure strengths comparable to others in their class. All anchors were stronger than the suture for which they are designed to accommodate. Key Words: Suture anchor—Ultimate load—Poly L-lactic acid—Biomechanics.
S
uture anchors allow the easy and consistent attachment of sutures to bone, and are widely accepted for use in glenohumeral instability and rotator cuff repairs.1-10 Anchors are used for reattachment of tendons and ligaments in the hand and elbow,11-13 foot,14-16 and knee17 as well. Anchors are also finding application in other surgical disciplines such as bladder suspensions in urology. Many options are available providing variations in size, shape, composition (including bioabsorbability), insertion technique, and radiopacity. Since our last report, many new anchors have become available. The purpose of this study is to evaluate anchors recently introduced for load-to-
From the Plano Orthopedic and Sports Medicine Center, Plano (F.A.B.); and Advanced Surgical Institutes, Medical City Dallas Hospital, Dallas (M.A.H.), Texas, U.S.A. Address correspondence to F. Alan Barber, M.D., Plano Orthopedic and Sports Medicine Center, 5228 West Plano Pkwy, Plano, TX 75093, U.S.A. r 1999 by the Arthroscopy Association of North America 0749-8063/99/1507-2025$3.00/0
failure (pullout) strength, failure mode, and suture size acceptance to provide the surgeon some objective data for anchor selection.
METHODS AND MATERIALS The protocol for this study followed that used by our previous studies.18-20 Fresh, never frozen, porcine femurs were used as the test environment. Approximately 10 samples of each anchor tested were inserted into the three test environments: diaphyseal cortex (usually 3 to 4 mm thick), metaphyseal cortex (usually 1 to 2 mm thick), and a cancellous bone trough created by decortication of the metaphyseal cortex with an electrically powered burr. Most anchors were threaded with stainless steel wire 0.018 inch (0.46 mm) or greater so the likely failure mechanism would be by anchor pullout or anchor eyelet cutout rather than wire breakage; however, some anchors were threaded with the suture material with which they were packaged.
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 15, No 7 (October), 1999: pp 719–725
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One anchor of each sample style was placed in each femur with rotation of the insertion positions to average out bone thickness variations. Each anchor was at least 1 cm from the other anchors to prevent crack propagation between drill holes during testing. All anchors were inserted according to the manufacturer’s instructions and often by the product manager for the anchor in question. Anchor insertion and pullout testing were conducted with the bones at room temperature to avoid any temperature-dependent variables in anchor component performance and to reduce variations in porcine femur response. Anchors tested included DePuy-Orthotech 4.5 prototypes D1, D2 Catera 4.5, and D3; DePuy 3.5 prototypes D4- Catera 3.5, D5, and D6 (DePuy-Orthotech,Warsaw, IN, Fig 1); Mainstay 2.7, 3.5, 4.5 (Howmedica, Rutherford, NJ, Fig 2); ROC EZ 2.8, EZ 3.5, and XS 3.5 (Innovasive Devices, Marlborough, MA, Fig 3); Ultrafix RC and Ultrafix MiniMite (Linvatec, Largo, FL, Fig 4); 1.3 MicroMitek, Panalok 3.5, and Tacit 2.0 (Mitek, Westwood, MA, Fig 3); Umbrella Harpoon (Arthrotek, Ontario, CA, Fig 4); PeBA 2.8, 4.0, 6.5 (Orthopedic Biosystems, Scottsdale, AZ, Fig 2); and
FIGURE 2. Anchors shown include in the top row from left to right the PeBA 2.8, 4.0, and 6.5; in the middle row from left to right Mainstay 4.5, 3.5, and 2.7; and in the bottom row from left to right the Stryker 1.9, 2.7, 3.4, and 4.5 anchors.
Stryker 1.9, 2.7, 3.4, and 4.5 prototypes (Stryker Endoscopy, Santa Clara, CA, Fig 2). A servo-hydraulic materials testing machine (model 1321; Instron Corp, Canton, MA) was used to deter-
FIGURE 1. The DePuy anchors tested include 4.5-mm major diameter versions from left to right across the top: the D1 (threaded through the anchor center), D2 (Catera 4.5 anchor), D3 (threaded through the anchor center); and across the bottom from left to right, the 3.5-mm major diameter versions: the D4 (Catera 3.5 anchor), D5 and D6 (both with sutures through the center of the anchor).
FIGURE 3. From left to right are the Innovasive Devices ROC EZ 2.8, ROC EZ 3.5, and the ROC XS 3.5.
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FIGURE 4. Anchors shown in the left column are the Mitek Tacit 2.0 on top and the MicroMitek 1.3 mm anchor on the bottom left. The center column includes the Umbrella Harpoon in top with the suture and below it the Mitek Panalok. The right column includes the Ultrafix MiniMite (undeployed) on top, the Ultrafix RC (deployed), and below it the Ultrafix RC (undeployed).
mine failure strength. A displacement rate of 12.5 mm/second was used to be consistent with our previous studies.18-20 The sampling rate for force and position data was 50/second, downloaded into a spreadsheet, normalized, and graphically analyzed. The bones were held in place with a specially prepared aluminum box. Each bone was tightly secured within the box with an infusion pressure bag that allowed the bone to be supported and self centered so that the anchor was always directly under the actuator of the Instron, and the pullout stresses parallel to the axis of anchor insertion to provide the worst case failure strength (ultimate load).21,22 The wire sutures were bent into a J shape for better fixation and clamped by the upper hydraulic fixture. The failure mode for each test (anchor pullout, eyelet wire suture cutout, or wire suture breakage) was recorded. Anchor drill hole size, minor and major diameters for the screw anchors, or bone defect size was recorded. Statistical analysis was performed with SAS software (Cary, NC). Data were subjected to descriptive statistical analysis, analysis of variance,
Some anchors had fewer than 10 valid tests in a particular bone environment. Because of the density of the porcine bone, some anchors failed to insert, and others were not tested in all environments. Mode of failure for each environment was recorded (Table 1). Grouped for all bone locations, anchor failure occurred 39% of the time by anchor pullout, 35% of the time by wire breakage, and 26% of the time by anchor eyelet cutout. The overall failure mode varied considerably with the bone environment into which they were implanted, but the principal mode of failure was not wire breakage. The average load-to-failure strength for each anchor is reported for the three bone environments studied: the diaphyseal cortex, metaphyseal cortex, and cancellous trough. These three environments were selected to be consistent with prior testing to allow direct comparisons with other anchors. The diaphyseal cortex (Fig 5) is thick cortical bone that may be considered similar to that of the humeral or femoral shaft. The thinner metaphyseal cortical environment (Fig 6) is similar to that of the anterior glenoid labrum. The cancellous trough (Fig 7) is designed to simulate the rotator cuff reattachment site. The dense cortical bone along the porcine diaphysis proved difficult for the insertion of some anchors; however, of those inserted, only 24% failed by anchor pullout. Most obtained enough purchase in the bone that wire breakage was the predominant mode of failure. Of those implanted in the metaphyseal cortex, anchor failure occurred most often by anchor pullout (40%) and wire suture breakage (37%), whereas those anchors inserted into the cancellous bone troughs failed half the time by anchor pullout (Table 1). Suture size capacity varied among the anchors tested. Some anchors are prethreaded and normally do not allow suture selection. The anchors that have smaller suture size capacity are designed for use with smaller sutures. In all cases, the anchors are clearly stronger than the breaking strength of the suture appropriate for the specific anchor (Table 2). TABLE 1. Mode of Failure
Diaphysis Cancellous Metaphysis
Pullout
Wire Break
Eyelet Cutout
24% 50% 40%
44% 30% 37%
32% 20% 23%
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FIGURE 5.
Pullout force in pounds: diaphyseal cortex.
The defect made in the bone by an anchor is determined by the drill size and for screw anchors, the minor (or core) diameter of the screw (Table 2). These were recorded for each anchor tested and are included as well. Of the 25 different anchors included in this study, 16 were inserted into the diaphyseal bone, 22 were tested in the metaphyseal cortex, and 24 in the cancellous trough. The biodegradable anchors tested in this round (Mitek Panalok and Innovasive Devices anchors) (Figs 3 and 4) were more durable than some previous biodegradable anchors and cautious handling was not required. Failure because of biodegradable material was not evident. The biodegradable screw anchors did require predrilling, and the proper use of insertion
FIGURE 6.
instruments was important. However, when the proper insertion techniques were followed, anchor insertion went smoothly. Insertion difficulties occurred with several metal screw anchors in the diaphyseal bone because the torsional force necessary to advance the anchor into the dense cortical bone exceeded the strength of the screw hub causing shearing off to occur, but this is a reflection of the test environment and not a defect in the anchor design.
DISCUSSION The continued development of new anchors challenges surgeons to assess objective measures of anchor
Pullout force in pounds: metaphyseal cortex.
SUTURE ANCHORS—UPDATE 1999
FIGURE 7.
Pullout force in pounds: cancellous trough.
performance before selecting an anchor. This study evaluated 25 new anchors in a way that allows comparison with previously tested anchors. Soft tissue reattachment has many components and the strength of the suture anchor is but one. The soft tissue strength, the manner in which the suture is passed through the tendon, how the knots are tied, and postoperative rehabilitation are major factors in clinical success. TABLE 2. Drill Hole Size in Millimeters Anchors
Major
Minor
DePuy D1 4.5 DePuy D2 4.5 DePuy D3 4.5 DePuy D4 3.5 DePuy D5 3.5 DePuy D6 3.5 Mainstay 2.7 Mainstay 3.5 Mainstay 4.5 ROC EZ 2.8 ROC EZ 3.5 ROC XS 3.5 Ultrafix RC Ultrafix MiniMite MicroMitek 1.3 Panalok 3.5 Tacit 2.0 Umbrella Harpoon PeBA 2.8 PeBA 4.0 PeBA 6.5 Stryker 1.9 Stryker 2.7 Stryker 3.4 Stryker 4.5
4.5 4.5 4.5 3.5 3.5 3.5 2.7 3.5 4.5
2.7 2.7 2.7 2.0 2.0 2.0 1.5 2.0 2.4
Variable Variable
2.0 Variable 2.8 4.0 6.5 1.9 2.7 3.4 4.5
Drill
Suture
2.8 3.5 3.5 2.9 2.4 1.3 3.5
2 2 2 2 2 2 0 2 5 2 2 2 5 2 4-0 2 2-0 5 2 5 (3) 5 (3) 2 2 5 5
1.2 3.4
1.4 1.7 2.6 2.6
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Suture anchors may provide better strength and more consistent results than bone tunnels.19 Cyclic loading data show that sutures in bone tunnels can fail when subjected to as few as 25 cyclic loads at physiologic levels and that, in the clinical setting, a cause of repair laxity may be the suture cutting through the bone tunnel.23 A suture anchor would not respond in the same way and movement of the suture in the eyelet would not cut through the eyelet in the same way it might through bone. The bone into which the anchor is inserted is critical. Bone density studies indicate areas in the humeral head where bone mineral concentration is greatest.24,25 While potentially failure can occur at the anchor, the tendon, or the suture,21,22 the weakest point is the suture-tendon interface.26 With the selection of a strong suture, the most likely failure is from the suture cutting through the tendon. This is supported by the lack of difference in clinical outcomes between repairs performed with anchors and those performed with bone tunnels.5,9 The ‘‘no’’ profile of anchors buried in the bone and covered by soft tissue eliminates the problems seen with other more prominent fixation devices.26-28 Consequently, the suture anchor may not only be easier and quicker to insert, but a more reliable form of fixation than a bone tunnel. Clinical reports of anchors pulling out of the bone are noticeably lacking in the literature. The evaluation of the in vitro anchor load-to-failure strength makes the mode of failure significant. Mode-of-failure analysis in this study test environment indicated that only 35% failed by wire breakage. This indicates that this testing substantially reflects the load-to-failure re-
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sponse of the anchor in bone rather than merely the wire suture strength. The load-to-failure strength data for all three environments are consistent with prior anchor testing. These anchors perform well in this in vitro setting. It should be remembered that this is a worst case failure scenario because the direction of pull is in the axis of insertion. This type of force is unlikely to occur in the clinical setting because most anchors are placed, like a tent stake, at an angle to the direction of pull. While some newer anchors have eyelets that limit the suture size with which they can be tested, or have lower load-to-failure levels, all anchors were significantly stronger than the suture for which they are designed. Examples include the 1.3 MicroMitek and Tacit 2.0 (Fig 4). The MicroMitek and Tacit 2.0 are not designed for use in the rotator cuff or Bankart repairs. They are designed to use No. 2-0 or 4-0 suture (and do not accommodate larger sutures). This small size makes them very well suited for the designed use in hand surgery. Suture capacity was recorded for these anchors (Table 2). The surgeon should be aware of the load to failure characteristics and other properties of the suture selected to work with the anchor. For instance, there is not a considerable difference in strength between No. 1 and No. 2 suture. A doubled strand of No. 2 braided polyester suture will fail at about 30 lb while a doubled strand of No. 1 braided polyester suture fail at about 25 lb. Knots in braided suture hold better than those in monofilament suture. However, monofilament sutures slip more easily than braided sutures so tying slip knots is easier. If multiple sutures are passed through a single eyelet, as is possible with the PeBA anchors (Fig 2), the first suture can be tied with a slip knot, but subsequent sutures will not slide and require a different approach. Drill hole size for nonscrew anchors and minor diameter size for screw anchors reflect the bone defect created by the anchor. These data are tabulated for reference. Although some larger anchors may have higher load-to-failure values, this potential benefit may be offset by the larger defect created in the bone. It should be emphasized that higher load-to-failure strengths may be of questionable importance because the limiting factor is still the suture strength. Choosing an anchor with a pullout strength of 125 lb over another anchor with a pullout strength of 60 lb solely because of the load to failure data ignores the fact that a No. 2 suture placed in either anchor will break by the time the load on it reaches 30 lb; but in all likelihood, before the suture broke, it will have torn through the
soft tissue first. If there is a failure of tendon fixation and a revision is contemplated, a large anchor (either removable or not removable) will be harder to work around than a smaller one. The new biodegradable suture anchors tested here were quite comparable in load-to-failure strength and mode of failure to previous anchors of that class. They required predrilling before insertion. The nonscrew biodegradable anchors tested this round, the Panalok and ROC anchors (Figs 3 and 4), were not as strong as the metal screw anchors, but were quite comparable to the other biodegradable anchors such as the GLS and the Linvatec Bio-Anchor. One advantage of these anchors is that the biodegradable anchor eyelet is soft when compared with a metal anchor eyelet, and should not fray the associated suture while the knot is being tied. The material for all the new biodegradable anchors tested is poly L-lactic acid and reflects the current trend away from the polyglycolic acid implants. These polyglycolic acid implants are associated with inflammatory reactions in the shoulder and other bones,29-33 and are implicated in other reactions as well.34 Additionally, these biodegradable anchors offer easier revision, better imaging, eventual anchor resorption, and use in pediatric applications not found with nonbiodegradable materials. In summary, the suture anchor and suture are only two of the various interfaces the arthroscopic surgeon must consider when attaching soft tissue to bone. When appropriately selected, they should not be the weakest link in the repair. The key steps in arthroscopic anchor use include site exposure, drill hole placement, anchor insertion, suture placement, and knot tying. These new anchors further expand the options available to the surgeon and reflect continued improvements in this technology. Acknowledgment: The authors wish to recognize the technical assistance of James N. Click, P.A.-C, Gwenn Freeman, and Nancy Alexander in the materials preparation and testing.
REFERENCES 1. Richmond JC, Donaldson WR, Fu F, Harner CD. Modification of the Bankart reconstruction with a suture anchor report of a new technique. Am J Sports Med 1991;19:343-346. 2. Field LD, Bokor DJ, Savoie FH. Humeral and glenoid detachment of the anterior inferior glenohumeral ligament: A cause of anterior shoulder instability. J Shoulder Elbow Surg 1997;6: 6-10. 3. Wolf EM. Arthroscopic capsulolabral repair using suture anchors. Orthop Clin North Am 1993;24:59-69. 4. Bacilla P, Field LD, Savoie III FH. Arthroscopic Bankart repair in a high demand patient population. Arthroscopy 1997;13: 51-60.
SUTURE ANCHORS—UPDATE 1999 5. Paulos LE, Evans IK, Pinkowski JL. Anterior and anteriorinferior shoulder instability: Treatment by glenoid labrum reconstruction and a modified capsular shift procedure. J Shoulder Elbow Surg 1993;2:305-313. 6. Levine WN, Richmond JC, Donaldson WR. Use of the suture anchor in open Bankart reconstruction a follow-up report. Am J Sports Med 1994;22:723-726. 7. Barnes SJ, Coleman SG, Gilpin D. Repair of avulsed insertion of biceps. A new technique in four cases. J Bone Joint Surg Br 1993;75:938-939. 8. Wolf EM, Wilk RM, Richmond JC. Arthroscopic Bankart repair using suture anchors. Oper Tech Orthop 1991;1:184191. 9. Norlin R. Use of Mitek anchoring for Bankart repair: A comparative, randomized, prospective study with traditional bone sutures. J Shoulder Elbow Surg 1994;3:381-385. 10. Tamai K, Sawazaki Y, Hara I. Efficacy and pitfalls of the Statak soft-tissue attachment device for the Bankart repair. J Shoulder Elbow Surg 1993;2:216-220. 11. Bovard RS, Derkash RS, Freeman JR. Grade III avulsion fracture repair on the UCL of the proximal joint of the thumb. Orthop Rev 1994;167-169. 12. Hallock GG. The Mitek mini GII anchor introduced for tendon reinsertion in the hand. Ann Plastic Surg 1994;33:211-213. 13. Rehak DC, Sotereanos DG, Bowman MW, Herndon JH. The Mitek bone anchor: application to the hand, wrist and elbow. J Hand Surg Am 1994;19:853-860. 14. Pederson B, Wertheimer SJ, Tesoro D, Coraci M. Mitek anchor system: A new technique for tenodesis and ligamentous repair of the foot and ankle. J Foot Surg 1991;30:48-51. 15. Chen DS, Wertheimer SJ. Centrally located osteochondral fracture of the talus. J Foot Surg 1992;31:134-140. 16. Schon LC, Clanton TO, Baxter DE. Reconstruction for subtalar instability: A review. Foot Ankle 1991;11:319-325. 17. Gillquist J. Reconstruction of the anterior cruciate ligament: Indications, operative technique, and rehabilitation. Oper Tech Orthop 1992;2:104-111. 18. Barber FA, Herbert MA, Click JN. The ultimate strength of suture anchors. Arthroscopy 1995;11:21-28. 19. Barber FA, Herbert MA, Click JN. Suture anchor strength revisited. Arthroscopy 1996;12:32-38. 20. Barber FA, Herbert MA, Click JN. Internal fixation strength of suture anchors—Update 1997. Arthroscopy 1997;13:355-362.
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21. Carpenter JE, Fish DN, Huston LJ, Goldstein SA. Pull-out strength of five suture anchors. Arthroscopy 1993;9:109-113. 22. Hecker AT, Shea M, Hayhurst JO, Myers ER, Meeks LW, Hayes WC. Pull-out strength of suture anchors for rotator cuff and Bankart lesion repairs. Am J Sports Med 1993;21:874-879. 23. Burkhart SS, Johnson TC, Wirth MA, Athanasiou KA. Cyclic loading of transosseous rotator cuff repairs: Tension overload as a possible cause of failure. Arthroscopy 1997;13:172-176. 24. Saitoh S, Nakatsuchi Y, Latta L, Milne E. Distribution of bone mineral density and bone strength of the proximal humerus. J Shoulder Elbow Surg 1994;3:234-242. 25. Barber FA, Feder SM, Burkhart SS, Ahrens J. The relationship of suture anchor failure and bone density to proximal humerus location: A cadaveric study. Arthroscopy 1997;13:340-345. 26. Shea KP, O’Keefe RM Jr, Fulkerson JP. Comparison of initial pull-out strength of arthroscopic suture and staple Bankart repair techniques. Arthroscopy 1992;8:179-182. 27. Edwards DJ, Hoy G, Saies AD, Hayes MG. Adverse reactions to an absorbable shoulder fixation device. J Shoulder Elbow Surg 1994;3:230-233. 28. Shall LM, Cawley PW. Soft tissue reconstruction in the shoulder comparison of suture anchors, absorbable staples, and absorbable tacks. Am J Sports Med 1994;22:715-718. 29. Hollinger JO, Battistone GC. Biodegradable bone repair materials synthetic polymers and ceramics. Clin Orthop 1986;207: 290-305. 30. Berg EE, Oglesby JW. Loosening of a biodegradable shoulder staple. J Shoulder Elbow Surg 1996;5:76-78. 31. Bostman OM. Intense granulomatous inflammatory lesion associated with absorbable internal fixation devices made of polyglycolide in ankle fractures. Clin Orthop 1992;278:178199. 32. Edwards DJ, Hoy G, Saies AD, Hayes MG. Adverse reactions to an absorbable shoulder fixation device. J Shoulder Elbow Surg 1994;3:230-233. 33. Fraser RK, Cole WG. Osteolysis after biodegradable pin fixation of fractures in children. J Bone Joint Surg Br 1992;74: 929-930. 34. Cheng JC, Wolf EM, Chapman JE, Johnston JO. Pigmented villonodular synovitis of the shoulder after anterior capsulolabral reconstruction. Arthroscopy 1997;13:257-261.
Summary: New suture anchors continue to become available. Our prior reports on the pullout strength of over 50 different anchors is supplemented by a similar test conducted on 25 additional new anchors. This anchor comparison, using an established protocol in fresh porcine femurs, recorded failure strength, failure mode (anchor pullout, suture eyelet cutout, or wire breakage), eyelet size, minor and major diameters, and drill hole sizes. These new anchors were tested in diaphyseal cortex, metaphyseal cortex, and a cancellous trough. Tensile stress parallel to the axis of insertion was applied at a rate of 12.5 mm/sec by an Instron 1321 until failure and mean anchor failure strengths were calculated. Anchors tested included DePuy 4.5 prototypes D1, D2 Catera 4.5, and D3; DePuy 3.5 prototypes D4- Catera 3.5, D5, and D6; Mainstay 2.7, 3.5, 4.5; ROC EZ 2.8, EZ 3.5, and XS 3.5; Ultrafix RC and Ultrafix MiniMite; 1.3 MicroMitek, Panalok 3.5, and Tacit 2.0; Umbrella Harpoon; PeBA 2.8, 4.0, 6.5; and Stryker 1.9, 2.7, 3.4, and 4.5 prototypes. Screw anchors still tend to have higher values, but for the newer nonscrew designs this distinction is less apparent. The new biodegradable anchors were all composed of poly L-lactic acid suggesting a trend away from other polymers, and these new biodegradable anchors showed load-to-failure strengths comparable to others in their class. All anchors were stronger than the suture for which they are designed to accommodate. Key Words: Suture anchor—Ultimate load—Poly L-lactic acid—Biomechanics.
S
uture anchors allow the easy and consistent attachment of sutures to bone, and are widely accepted for use in glenohumeral instability and rotator cuff repairs.1-10 Anchors are used for reattachment of tendons and ligaments in the hand and elbow,11-13 foot,14-16 and knee17 as well. Anchors are also finding application in other surgical disciplines such as bladder suspensions in urology. Many options are available providing variations in size, shape, composition (including bioabsorbability), insertion technique, and radiopacity. Since our last report, many new anchors have become available. The purpose of this study is to evaluate anchors recently introduced for load-to-
From the Plano Orthopedic and Sports Medicine Center, Plano (F.A.B.); and Advanced Surgical Institutes, Medical City Dallas Hospital, Dallas (M.A.H.), Texas, U.S.A. Address correspondence to F. Alan Barber, M.D., Plano Orthopedic and Sports Medicine Center, 5228 West Plano Pkwy, Plano, TX 75093, U.S.A. r 1999 by the Arthroscopy Association of North America 0749-8063/99/1507-2025$3.00/0
failure (pullout) strength, failure mode, and suture size acceptance to provide the surgeon some objective data for anchor selection.
METHODS AND MATERIALS The protocol for this study followed that used by our previous studies.18-20 Fresh, never frozen, porcine femurs were used as the test environment. Approximately 10 samples of each anchor tested were inserted into the three test environments: diaphyseal cortex (usually 3 to 4 mm thick), metaphyseal cortex (usually 1 to 2 mm thick), and a cancellous bone trough created by decortication of the metaphyseal cortex with an electrically powered burr. Most anchors were threaded with stainless steel wire 0.018 inch (0.46 mm) or greater so the likely failure mechanism would be by anchor pullout or anchor eyelet cutout rather than wire breakage; however, some anchors were threaded with the suture material with which they were packaged.
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One anchor of each sample style was placed in each femur with rotation of the insertion positions to average out bone thickness variations. Each anchor was at least 1 cm from the other anchors to prevent crack propagation between drill holes during testing. All anchors were inserted according to the manufacturer’s instructions and often by the product manager for the anchor in question. Anchor insertion and pullout testing were conducted with the bones at room temperature to avoid any temperature-dependent variables in anchor component performance and to reduce variations in porcine femur response. Anchors tested included DePuy-Orthotech 4.5 prototypes D1, D2 Catera 4.5, and D3; DePuy 3.5 prototypes D4- Catera 3.5, D5, and D6 (DePuy-Orthotech,Warsaw, IN, Fig 1); Mainstay 2.7, 3.5, 4.5 (Howmedica, Rutherford, NJ, Fig 2); ROC EZ 2.8, EZ 3.5, and XS 3.5 (Innovasive Devices, Marlborough, MA, Fig 3); Ultrafix RC and Ultrafix MiniMite (Linvatec, Largo, FL, Fig 4); 1.3 MicroMitek, Panalok 3.5, and Tacit 2.0 (Mitek, Westwood, MA, Fig 3); Umbrella Harpoon (Arthrotek, Ontario, CA, Fig 4); PeBA 2.8, 4.0, 6.5 (Orthopedic Biosystems, Scottsdale, AZ, Fig 2); and
FIGURE 2. Anchors shown include in the top row from left to right the PeBA 2.8, 4.0, and 6.5; in the middle row from left to right Mainstay 4.5, 3.5, and 2.7; and in the bottom row from left to right the Stryker 1.9, 2.7, 3.4, and 4.5 anchors.
Stryker 1.9, 2.7, 3.4, and 4.5 prototypes (Stryker Endoscopy, Santa Clara, CA, Fig 2). A servo-hydraulic materials testing machine (model 1321; Instron Corp, Canton, MA) was used to deter-
FIGURE 1. The DePuy anchors tested include 4.5-mm major diameter versions from left to right across the top: the D1 (threaded through the anchor center), D2 (Catera 4.5 anchor), D3 (threaded through the anchor center); and across the bottom from left to right, the 3.5-mm major diameter versions: the D4 (Catera 3.5 anchor), D5 and D6 (both with sutures through the center of the anchor).
FIGURE 3. From left to right are the Innovasive Devices ROC EZ 2.8, ROC EZ 3.5, and the ROC XS 3.5.
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FIGURE 4. Anchors shown in the left column are the Mitek Tacit 2.0 on top and the MicroMitek 1.3 mm anchor on the bottom left. The center column includes the Umbrella Harpoon in top with the suture and below it the Mitek Panalok. The right column includes the Ultrafix MiniMite (undeployed) on top, the Ultrafix RC (deployed), and below it the Ultrafix RC (undeployed).
mine failure strength. A displacement rate of 12.5 mm/second was used to be consistent with our previous studies.18-20 The sampling rate for force and position data was 50/second, downloaded into a spreadsheet, normalized, and graphically analyzed. The bones were held in place with a specially prepared aluminum box. Each bone was tightly secured within the box with an infusion pressure bag that allowed the bone to be supported and self centered so that the anchor was always directly under the actuator of the Instron, and the pullout stresses parallel to the axis of anchor insertion to provide the worst case failure strength (ultimate load).21,22 The wire sutures were bent into a J shape for better fixation and clamped by the upper hydraulic fixture. The failure mode for each test (anchor pullout, eyelet wire suture cutout, or wire suture breakage) was recorded. Anchor drill hole size, minor and major diameters for the screw anchors, or bone defect size was recorded. Statistical analysis was performed with SAS software (Cary, NC). Data were subjected to descriptive statistical analysis, analysis of variance,
Some anchors had fewer than 10 valid tests in a particular bone environment. Because of the density of the porcine bone, some anchors failed to insert, and others were not tested in all environments. Mode of failure for each environment was recorded (Table 1). Grouped for all bone locations, anchor failure occurred 39% of the time by anchor pullout, 35% of the time by wire breakage, and 26% of the time by anchor eyelet cutout. The overall failure mode varied considerably with the bone environment into which they were implanted, but the principal mode of failure was not wire breakage. The average load-to-failure strength for each anchor is reported for the three bone environments studied: the diaphyseal cortex, metaphyseal cortex, and cancellous trough. These three environments were selected to be consistent with prior testing to allow direct comparisons with other anchors. The diaphyseal cortex (Fig 5) is thick cortical bone that may be considered similar to that of the humeral or femoral shaft. The thinner metaphyseal cortical environment (Fig 6) is similar to that of the anterior glenoid labrum. The cancellous trough (Fig 7) is designed to simulate the rotator cuff reattachment site. The dense cortical bone along the porcine diaphysis proved difficult for the insertion of some anchors; however, of those inserted, only 24% failed by anchor pullout. Most obtained enough purchase in the bone that wire breakage was the predominant mode of failure. Of those implanted in the metaphyseal cortex, anchor failure occurred most often by anchor pullout (40%) and wire suture breakage (37%), whereas those anchors inserted into the cancellous bone troughs failed half the time by anchor pullout (Table 1). Suture size capacity varied among the anchors tested. Some anchors are prethreaded and normally do not allow suture selection. The anchors that have smaller suture size capacity are designed for use with smaller sutures. In all cases, the anchors are clearly stronger than the breaking strength of the suture appropriate for the specific anchor (Table 2). TABLE 1. Mode of Failure
Diaphysis Cancellous Metaphysis
Pullout
Wire Break
Eyelet Cutout
24% 50% 40%
44% 30% 37%
32% 20% 23%
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F. A. BARBER AND M. A. HERBERT
FIGURE 5.
Pullout force in pounds: diaphyseal cortex.
The defect made in the bone by an anchor is determined by the drill size and for screw anchors, the minor (or core) diameter of the screw (Table 2). These were recorded for each anchor tested and are included as well. Of the 25 different anchors included in this study, 16 were inserted into the diaphyseal bone, 22 were tested in the metaphyseal cortex, and 24 in the cancellous trough. The biodegradable anchors tested in this round (Mitek Panalok and Innovasive Devices anchors) (Figs 3 and 4) were more durable than some previous biodegradable anchors and cautious handling was not required. Failure because of biodegradable material was not evident. The biodegradable screw anchors did require predrilling, and the proper use of insertion
FIGURE 6.
instruments was important. However, when the proper insertion techniques were followed, anchor insertion went smoothly. Insertion difficulties occurred with several metal screw anchors in the diaphyseal bone because the torsional force necessary to advance the anchor into the dense cortical bone exceeded the strength of the screw hub causing shearing off to occur, but this is a reflection of the test environment and not a defect in the anchor design.
DISCUSSION The continued development of new anchors challenges surgeons to assess objective measures of anchor
Pullout force in pounds: metaphyseal cortex.
SUTURE ANCHORS—UPDATE 1999
FIGURE 7.
Pullout force in pounds: cancellous trough.
performance before selecting an anchor. This study evaluated 25 new anchors in a way that allows comparison with previously tested anchors. Soft tissue reattachment has many components and the strength of the suture anchor is but one. The soft tissue strength, the manner in which the suture is passed through the tendon, how the knots are tied, and postoperative rehabilitation are major factors in clinical success. TABLE 2. Drill Hole Size in Millimeters Anchors
Major
Minor
DePuy D1 4.5 DePuy D2 4.5 DePuy D3 4.5 DePuy D4 3.5 DePuy D5 3.5 DePuy D6 3.5 Mainstay 2.7 Mainstay 3.5 Mainstay 4.5 ROC EZ 2.8 ROC EZ 3.5 ROC XS 3.5 Ultrafix RC Ultrafix MiniMite MicroMitek 1.3 Panalok 3.5 Tacit 2.0 Umbrella Harpoon PeBA 2.8 PeBA 4.0 PeBA 6.5 Stryker 1.9 Stryker 2.7 Stryker 3.4 Stryker 4.5
4.5 4.5 4.5 3.5 3.5 3.5 2.7 3.5 4.5
2.7 2.7 2.7 2.0 2.0 2.0 1.5 2.0 2.4
Variable Variable
2.0 Variable 2.8 4.0 6.5 1.9 2.7 3.4 4.5
Drill
Suture
2.8 3.5 3.5 2.9 2.4 1.3 3.5
2 2 2 2 2 2 0 2 5 2 2 2 5 2 4-0 2 2-0 5 2 5 (3) 5 (3) 2 2 5 5
1.2 3.4
1.4 1.7 2.6 2.6
723
Suture anchors may provide better strength and more consistent results than bone tunnels.19 Cyclic loading data show that sutures in bone tunnels can fail when subjected to as few as 25 cyclic loads at physiologic levels and that, in the clinical setting, a cause of repair laxity may be the suture cutting through the bone tunnel.23 A suture anchor would not respond in the same way and movement of the suture in the eyelet would not cut through the eyelet in the same way it might through bone. The bone into which the anchor is inserted is critical. Bone density studies indicate areas in the humeral head where bone mineral concentration is greatest.24,25 While potentially failure can occur at the anchor, the tendon, or the suture,21,22 the weakest point is the suture-tendon interface.26 With the selection of a strong suture, the most likely failure is from the suture cutting through the tendon. This is supported by the lack of difference in clinical outcomes between repairs performed with anchors and those performed with bone tunnels.5,9 The ‘‘no’’ profile of anchors buried in the bone and covered by soft tissue eliminates the problems seen with other more prominent fixation devices.26-28 Consequently, the suture anchor may not only be easier and quicker to insert, but a more reliable form of fixation than a bone tunnel. Clinical reports of anchors pulling out of the bone are noticeably lacking in the literature. The evaluation of the in vitro anchor load-to-failure strength makes the mode of failure significant. Mode-of-failure analysis in this study test environment indicated that only 35% failed by wire breakage. This indicates that this testing substantially reflects the load-to-failure re-
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F. A. BARBER AND M. A. HERBERT
sponse of the anchor in bone rather than merely the wire suture strength. The load-to-failure strength data for all three environments are consistent with prior anchor testing. These anchors perform well in this in vitro setting. It should be remembered that this is a worst case failure scenario because the direction of pull is in the axis of insertion. This type of force is unlikely to occur in the clinical setting because most anchors are placed, like a tent stake, at an angle to the direction of pull. While some newer anchors have eyelets that limit the suture size with which they can be tested, or have lower load-to-failure levels, all anchors were significantly stronger than the suture for which they are designed. Examples include the 1.3 MicroMitek and Tacit 2.0 (Fig 4). The MicroMitek and Tacit 2.0 are not designed for use in the rotator cuff or Bankart repairs. They are designed to use No. 2-0 or 4-0 suture (and do not accommodate larger sutures). This small size makes them very well suited for the designed use in hand surgery. Suture capacity was recorded for these anchors (Table 2). The surgeon should be aware of the load to failure characteristics and other properties of the suture selected to work with the anchor. For instance, there is not a considerable difference in strength between No. 1 and No. 2 suture. A doubled strand of No. 2 braided polyester suture will fail at about 30 lb while a doubled strand of No. 1 braided polyester suture fail at about 25 lb. Knots in braided suture hold better than those in monofilament suture. However, monofilament sutures slip more easily than braided sutures so tying slip knots is easier. If multiple sutures are passed through a single eyelet, as is possible with the PeBA anchors (Fig 2), the first suture can be tied with a slip knot, but subsequent sutures will not slide and require a different approach. Drill hole size for nonscrew anchors and minor diameter size for screw anchors reflect the bone defect created by the anchor. These data are tabulated for reference. Although some larger anchors may have higher load-to-failure values, this potential benefit may be offset by the larger defect created in the bone. It should be emphasized that higher load-to-failure strengths may be of questionable importance because the limiting factor is still the suture strength. Choosing an anchor with a pullout strength of 125 lb over another anchor with a pullout strength of 60 lb solely because of the load to failure data ignores the fact that a No. 2 suture placed in either anchor will break by the time the load on it reaches 30 lb; but in all likelihood, before the suture broke, it will have torn through the
soft tissue first. If there is a failure of tendon fixation and a revision is contemplated, a large anchor (either removable or not removable) will be harder to work around than a smaller one. The new biodegradable suture anchors tested here were quite comparable in load-to-failure strength and mode of failure to previous anchors of that class. They required predrilling before insertion. The nonscrew biodegradable anchors tested this round, the Panalok and ROC anchors (Figs 3 and 4), were not as strong as the metal screw anchors, but were quite comparable to the other biodegradable anchors such as the GLS and the Linvatec Bio-Anchor. One advantage of these anchors is that the biodegradable anchor eyelet is soft when compared with a metal anchor eyelet, and should not fray the associated suture while the knot is being tied. The material for all the new biodegradable anchors tested is poly L-lactic acid and reflects the current trend away from the polyglycolic acid implants. These polyglycolic acid implants are associated with inflammatory reactions in the shoulder and other bones,29-33 and are implicated in other reactions as well.34 Additionally, these biodegradable anchors offer easier revision, better imaging, eventual anchor resorption, and use in pediatric applications not found with nonbiodegradable materials. In summary, the suture anchor and suture are only two of the various interfaces the arthroscopic surgeon must consider when attaching soft tissue to bone. When appropriately selected, they should not be the weakest link in the repair. The key steps in arthroscopic anchor use include site exposure, drill hole placement, anchor insertion, suture placement, and knot tying. These new anchors further expand the options available to the surgeon and reflect continued improvements in this technology. Acknowledgment: The authors wish to recognize the technical assistance of James N. Click, P.A.-C, Gwenn Freeman, and Nancy Alexander in the materials preparation and testing.
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