- Email: [email protected]
Cyclic loading of rotator cuff repairs
Cyclic Loading of Rotator Cuff Repairs: A Comparison of Bioabsorbable Tacks With Metal Suture Anchors and Transosseous Sutures Vipool K. Goradia, M.D., Daniel J. Mullen, M.D., Henry R. Boucher, M.D., Brent G. Parks, M.Sc., and John B. O’Donnell, M.D.
Purpose: The purposes of the study were (1) to compare rotator cuff repair strengths after cyclic loading of 2 bioabsorbable nonsuture-based tack-type anchors, transosseous sutures, and a metal suture-based anchor, and (2) to correlate bone mineral density with mode of failure and cycles to failure. We hypothesized that specimens with a lower bone density would fail through bone at a lower number of cycles independent of the method of cuff fixation. Type of Study: Ex vivo biomechanical study. Methods: Standardized full-thickness rotator cuff defects were created in 30 fresh-frozen cadaveric shoulders that were randomized to 1 of 4 repair groups: transosseous sutures; Mitek Super suture anchors (Mitek Surgical Products, Westwood, MA); smooth bioabsorbable 8-mm Suretacs (Acufex, Smith & Nephew Endoscopy, Mansfield, MA); or spiked bioabsorbable 8-mm Suretacs (Acufex). All repairs were cyclically loaded from 10 to 180 N; the numbers of cycles to 50% (gap, 5 mm) and 100% (gap, 10 mm) failure were recorded. Results: In comparing the repair groups, we found only 1 significant difference: the number of cycles to 100% failure was significantly higher (P ⬍ .05) for the smooth bioabsorbable tack than for the transosseous suture group. There were no statistically significant (P ⱕ .05) differences in bone mineral densities with regard to each specimen’s mode of failure. Conclusions: Our results suggested that immediate postoperative fixation provided by bioabsorbable tacks was similar to that provided by Mitek anchors and more stable than that provided by transosseous sutures. Therefore, the immediate postoperative biomechanical strength of bioabsorbable tacks seems comparatively adequate for fixation of selected small rotator cuff tears. However, additional evaluation in an animal model to examine degradation characteristics and sustained strength of repair is recommended before clinical use. Key Words: Rotator cuff tears— Absorbable anchors—Metal anchors—Cyclic loading—Cadaveric model—Bone density.
S
ince the introduction of suture anchors in 1985 by Goble et al.,1 numerous anchors have been designed and produced. Anchors come in a variety of types, including suture-based, nonsuture-based, bioabsorbable, and metal. Although some of these anchors
From the Department of Orthopaedic Surgery, The Union Memorial Hospital, Baltimore, Maryland, U.S.A. Address correspondence and reprint requests to John B. O’Donnell, M.D., c/o Lyn Camire, Editor, Union Memorial Orthopaedics, The Johnston Professional Building, #400, 3333 North Calvert St, Baltimore, MD 21218, U.S.A. E-mail: [email protected] © 2001 by the Arthroscopy Association of North America 0749-8063/01/1704-2474$35.00/0 doi:10.1053/jars.2001.21243
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may theoretically provide a technically easier method of securing soft tissue to bone, the ideal anchor for rotator cuff repair has not been developed. The ideal anchor should be easy to insert, provide adequate initial strength to permit rehabilitation exercises, and supply sustained strength of repair during the healing process.2 As arthroscopic and arthroscopically assisted mini-open techniques of rotator cuff repair are becoming more popular, the characteristics of this ideal anchor are evolving. Using suture-based anchors and an all-arthroscopic technique can be challenging. The ability to insert the anchors through a limited number of portals, pass suture through the tendon, and tie knots, requires practice before entering the operating room.3 Anchors or tacks that do not require sutures
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 4 (April), 2001: pp 360 –364
ROTATOR CUFF REPAIR: TACKS, ANCHORS, SUTURES would eliminate 2 of the more difficult steps involved in most current fixation techniques. Although many studies provide information about the initial ultimate tensile strength of rotator cuff repairs using suture anchors4-7 and anchor pull-out strength,3-8 Burkhart et al.9,10 have recommended using cyclic testing of rotator cuff repairs. These tests provide a better simulation of the in vivo mechanism of rotator cuff repair failure. To our knowledge, published reports on cyclic loading of rotator cuff repair exist only for transosseous sutures and metal suturebased anchors.9-11 The purpose of this study was 2-fold: (1) to compare the rotator cuff repair strengths after cyclic loading of 2 bioabsorbable nonsuture-based tack-type anchors, transosseous sutures, and a metal suture-based anchor; and (2) to correlate the bone mineral density for each specimen with the mode of failure and number of cycles to failure. We hypothesized that specimens with a lower bone density would fail through bone at fewer cycles independent of the method of cuff fixation. METHODS We obtained 16 pairs of fresh-frozen cadaver shoulders (mean age at death, 77 years; range, 64 to 88 years) from the Maryland State Anatomy Board in Baltimore and stored them at ⫺20°C. Each of the 32 specimens was then thawed at room temperature for 24 hours, randomized to 1 of 4 treatment groups (see below), and dissected, leaving only the intact rotator cuff, humeral head, and proximal 15 cm of the humeral shaft. At dissection, 2 shoulders (both in group 4) were found to have evidence of previous surgery or rotator cuff tears and, therefore, were discarded. As described by Burkhart et al.,9,10 a standard full-thickness defect (20 mm anterior-to-posterior and 10 mm medial-to-lateral) was then created in the supraspinatus and infraspinatus tendons of each of the 30 remaining shoulders. In group 1 (9 specimens), each rotator cuff defect was repaired with 2 No. 2 Ethibond transosseous sutures (Ethicon Inc., Somerville, NJ) using a MasonAllen stitch technique.5 These sutures were placed through 3 drill holes that were spaced 10 mm apart and 10 mm distal to the tip of the greater tuberosity.12 In group 2 (8 specimens), each defect was repaired with 2 Mitek Super anchors (Fig 1) (Mitek Surgical Products, Westwood, MA) using No. 2 Ethibond sutures with a simple stitch technique. The sutures were placed at the leading edge of the cuff tear, the tear was
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FIGURE 1. Repair devices. Left, smooth 8-mm Suretac anchor. Center, spiked 8-mm Suretac anchor. Right, Mitek Super anchor.
mobilized to close the defect, and the anchors were placed (according to the manufacturer’s guidelines) at 45° angles in the trough between the greater tuberosity and the lateral margin of the articular surface of the humeral head. In group 3 (7 specimens), each defect was repaired with 2 smooth bioabsorbable 8-mm Suretac anchors (Fig 1) (Acufex, Smith & Nephew Endoscopy, Mansfield, MA), respectively. In group 4 (6 specimens), each defect was repaired with 2 spiked bioabsorbable 8-mm Suretac anchors (Fig 1). In groups 3 and 4, the leading edges of the tears were mobilized to close the defects. The tacks were inserted (according to the manufacturer’s instrumentation and guidelines) through the tendon 5 mm from its free end and into a predrilled hole in the same trough that received the Mitek Super anchors. For each specimen, a 2.5-cm wide nylon strap9,10 was attached to the proximal rotator cuff with a modified Krackow stitch using No. 5 Ethibond sutures. The specimens were kept moist with saline during preparation and testing. Each specimen was mounted on an MTS Mini Bionix testing machine (MTS Systems, Eden Prairie, MN). The humeral shaft was clamped to a platform and the nylon strap was looped around a bar that was connected to the load cell (Fig 2). The repairs were cyclically loaded from 10 to 180 N with a 5-second cycle similar to that described by Burkhart et al.9,10 Gap formation at each repair site was measured by the same examiner using an extensometer. The number of cycles to 5-mm (50% failure) and 10-mm (100% failure) gap formation was recorded, along with the mechanism of failure.9,10 Each specimen was placed in a water bath, and the same qualified technician measured the density of the entire specimen with a radiographic bone densitometer (Hologic QDR-1000TM, Bedford, MA). Then a 1cm2 area below the cortex of the greater tuberosity of
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FIGURE 2. Cadaveric specimen mounted onto an MTS machine for cyclic loading after rotator cuff repair.
each specimen was digitally outlined, and the density of this area was quantified with the densitometer. In all specimens, fixation devices were located in the trough immediately medial to the trabecular bone being measured. The data were analyzed using a 1-way analysis of variance, with P ⬍ .05 being considered statistically significant. The Student-Newman-Keuls post hoc test was performed when differences were observed. A Pearson correlation was performed to determine the relationship between bone density and number of cycles to failure.
Five modes of failure were identified for the specimens. In group 1 (transosseous sutures), failure occurred by sutures pulling through the bone (n ⫽ 7) or sutures pulling through the rotator cuff (n ⫽ 2). In group 2 (Mitek anchors), the anchor loosened (n ⫽ 1), the suture broke at the anchor (n ⫽ 1), or the sutures pulled through the rotator cuff (n ⫽ 6). In groups 3 and 4 (smooth and spiked Suretacs, respectively), the failure patterns were identical: anchor loosening (n ⫽ 3) and anchor pulling through rotator cuff (n ⫽ 3). A failure mode was not determined for the seventh specimen in the group 3 because the nylon strap failed at 212 cycles. With the data available, we found no significant difference (P ⬎ .05) among the 4 groups in the number of cycles required to produce a 5-mm gap (Table 1) at the fixation site. For 10 mm of gap formation (Table 1), however, the number of cycles was significantly higher for group 3 (smooth Suretacs) than for group 1 (transosseous sutures) (P ⬍ .05); a significant difference was not identified among any of the other groups (P ⬎ .05). There were no significant differences (P ⫽ .93) in bone mineral density among any of the 4 treatment groups: group 1, 0.377 ⫾ 0.132 g/cm2; group 2, 0.410 ⫾ 0.200 g/cm2; group 3, 0.041 ⫾ 0.101 g/cm2; and group 4, 0.363 ⫾ 0.134 g/cm2. The bone mineral density did not correlate with the number of cycles to a 5-mm (r ⫽ ⫺0.0802, P ⫽ .674) or 10-mm (r ⫽ ⫺0.1570, P ⫽ .407) gap formation. We found no significant difference (P ⬎ .05) in bone mineral density among any of the five failure modes (Table 2). DISCUSSION Burkhart et al.9,10 reported results of cyclic loads applied to rotator cuff repairs using transosseous sutures9 and Mitek-RC anchors.10 In the first study,9 the average number of cycles to 5-mm and 10-mm gap
TABLE 1. Number of Cycles to 5- and 10-mm Gap Formation
Groups
No. of Specimens
5-mm Gap (mean cycles ⫾ SE) (range)
10-mm Gap (mean cycles ⫾ SE) (range)
Transosseous sutures Mitek Smooth Suretacs Spiked Suretacs
9 8 7 6
5.0 ⫾ 1.3 (1–12) 8.1 ⫾ 3.2 (2–30) 17.0 ⫾ 7.7 (2–60) 9.7 ⫾ 3.4 (1–22)
55.7 ⫾ 15.6 (6–125) 101.4 ⫾ 35.0 (10–250) 216.3 ⫾ 62.1 (20–464) 155.2 ⫾ 56.9 (3–332)
ROTATOR CUFF REPAIR: TACKS, ANCHORS, SUTURES TABLE 2. Bone Mineral Density by Mode of Failure Mode of Failure SBA SPTB SPTT AL APTT
No. of Failures
Mean Bone Mineral Density (g/cm3 ⫾ SE) (range)
1 7 8 7 6
0.116 0.363 ⫾ 0.048 (0.130–0.489) 0.471 ⫾ 0.055 (0.295–0.766) 0.411 ⫾ 0.033 (0.247–0.506) 0.306 ⫾ 0.048 (0.170–0.498)
Abbreviations: SBA, suture broke at anchor; SPTB, suture pulled through bone; SPTT, suture pulled through tendon; AL, anchor loosened; APTT, anchor pulled through tendon.
formation in cadaveric specimens (average age, 41.2 years) repaired with 3 simple transosseous sutures were 25 and 188, respectively. In the second study,10 the average number of cycles to 5-mm and 10-mm gap formation in cadaveric specimens (similar average age) repaired with 3 Mitek-RC anchors with a simple suture technique were 61 and 285 cycles, respectively. With each fixation method, they reported better bone fixation in cadavers less than 45 years old than in those older than 45 years. In the current study, the average numbers of cycles to 5- and 10-mm gap formation for group 1 (transosseous sutures) (5 and 56 cycles, respectively) and group 2 (Mitek Super anchors) (8 and 101 cycles, respectively) were less than those reported in the Burkhart studies9,10 because of 2 main factors. First, we used 2 sutures or 2 anchors instead of 3 anchors to repair the rotator cuff defects that were identical in size to those of Burkhart et al.9,10 Second, our cadavers were all 64 years old or older (average age, 77 years). In the study by Burkhart et al.,10 5- and 10-mm failure in specimens more than 45 years old with Mitek anchor fixation occurred at 17 and 91 cycles,10 respectively, values similar to those in the current study. Our failure modes were similar to those reported by Burkhart et al.9,10 in that the transosseous sutures primarily failed through bone and the Mitek anchor groups primarily failed through the tendons. The modified Mason-Allen stitch used in our transosseous suture group has been shown to provide better fixation under cyclic loading conditions when compared with numerous other techniques.5 Because the Mitek group failed primarily through the tendon, the modified Mason-Allen stitch should be considered with suturebased anchors. In a study by Rossouw et al.,11 a different method of cyclic loading was used than that described in the current study for rotator cuff defects repaired with Mitek GII anchors and No. 2 braided polyester sutures. Their failure mode was primarily
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suture breakage at the knots or at the anchors and not failure through the tendon. The enhanced tendon fixation may have occurred secondary to the lockingtype stitch they used. Half of our Suretac anchors failed through bone and the other half failed through the tendon. We initially hypothesized that the different modes of failure were due in part to variations in bone mineral density. However, our analysis found no relationship between bone mineral density and mode of failure or number of cycles to failure. This finding is similar to the conclusion of Barber et al.,13 who identified no correlation between bone density and pullout strength of screwtype anchors in the proximal humerus. Our results indicated that, at least in the immediate postoperative period, the initial fixation provided by Suretac anchors was adequate, similar to that of Mitek Super anchors, and more stable than that of transosseous sutures when used for rotator cuff repairs subjected to cyclic loads. A potential advantage of the Suretac anchors is that their arthroscopic placement is easier because they do not require sutures. However, because these anchors are bioabsorbable, their ability to provide stable fixation throughout the healing process is unknown. Bioabsorbable anchors are composed of a variety of materials, the most common of which are polylactic acid (PLA) and polyglycolic acid (PGA).14 The Suretac anchor is a copolymer composed of 67% PGA and 33% trimethylene carbonate; the latter degrades by hydrolysis alone and has been shown to lose 50% of its strength retention by 4 to 6 weeks of use, but it may not be completely resorbed until 6 months after implantation.15,16 Various other polymers of PGA have been shown to degrade from 2 weeks to 6 months after implantation.14,15,17,18 In general, polymers of PLA degrade slower than PGA. However, because we studied only the immediate postoperative fixation strength of PGA tacks, we believe our data regarding initial strength of repair are equally applicable to bioabsorbable PLA tacks. The anatomic site of implantation and the specific mechanical stresses applied to the implant have also been reported to affect the degradation rate.14 To our knowledge, there are no published studies that have evaluated the degradation and sustained mechanical properties of any bioabsorbable tacks used for repair of rotator cuff tears in an animal model. With the current data available on biodegradability, tacks composed of PLA polymers would seem more appropriate than the Suretac for repair of rotator cuff tears. The current study indicated that the fixation pro-
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vided by the nonsuture-based bioabsorbable tack was similar to that of the suture-based Mitek Super anchor when subjected to cyclic loading in the immediate postoperative period when there was no difference in bone density between the groups of specimens. However, because the tack may be technically easier to use than the anchor in arthroscopic repair of rotator cuff tears, we suggest caution when interpreting our findings and applying them to the clinical situation scenario. The current study provides no information about the degradation characteristics or sustained strength of repair for the Suretac anchors or other types of tacks. Bioabsorbable tacks composed of PLA polymers should be evaluated in an animal model. REFERENCES 1. Goble EM, Somers WK, Clark R, Olsen RE. The development of suture anchors for use in soft tissue fixation to bone. Am J Sports Med 1994;22:236-239. 2. Burkhart SS. Biomechanics of rotator cuff repair: Converting the ritual to a science. Instr Course Lect 1998;47:43-50. 3. Barber FA, Herbert MA, Click JN. Internal fixation strength of suture anchors—Update 1997. Arthroscopy 1997;13:355-362. 4. Craft DV, Moseley JB, Cawley PW, Noble PC. Fixation strength of rotator cuff repairs with suture anchors and the transosseous suture technique. J Shoulder Elbow Surg 1996; 5:32-40. 5. Gerber C, Schneeberger AG, Beck M, Schlegel U. Mechanical strength of repairs of the rotator cuff. J Bone Joint Surg Br 1994;76:371-380. 6. 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.
7. Reed SC, Glossop N, Ogilvie-Harris DJ. Full-thickness rotator cuff tears. A biomechanical comparison of suture versus bone anchor techniques. Am J Sports Med 1996;24:46-48. 8. Barber FA, Herbert MA, Click JN. Suture anchor strength revisited. Arthroscopy 1996;12:32-38. 9. 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. 10. Burkhart SS, Diaz Pagan JL, Wirth MA, Athanasiou KA. Cyclic loading of anchor-based rotator cuff repairs: Confirmation of the tension overload phenomenon and comparison of suture anchor fixation with transosseous fixation. Arthroscopy 1997;13:720-724. 11. Rossouw DJ, McElroy BJ, Amis AA, Emery RJ. A biomechanical evaluation of suture anchors in repair of the rotator cuff. J Bone Joint Surg Br 1997;79:458-461. 12. Caldwell GL, Warner JP, Miller MD, Boardman D, Towers J, Debski R. Strength of fixation with transosseous sutures in rotator cuff repair. J Bone Joint Surg Am 1997;79:1064-1068. 13. 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. 14. Athanasiou KA, Agrawal CM, Barber FA, Burkhart SS. Orthopaedic applications for PLA-PGA biodegradable polymers. Arthroscopy 1998;14:726-737. 15. Miller RA, Brady JM, Cutright DE. Degradation rates of oral resorbable implants (polylactates and polyglycolates): Rate modification with changes in PLA/PGA copolymer ratios. J Biomed Mater Res 1977;11:711-719. 16. Speer KP, Warren RF. Arthroscopic shoulder stabilization. A role for biodegradable materials. Clin Orthop 1993;291:67-74. 17. Bostman OM, Paivarinta U, Partio E, et al. The tissue-implant interface during degradation of absorbable polyglycolide fracture fixation screws in the rabbit femur. Clin Orthop 1992; 285:263-272. 18. Nordstrom P, Pihlajamaki H, Toivonen T, Tormala P, Rokkanen P. Tissue response to polyglycolide and polylactide pins in cancellous bone. Arch Orthop Trauma Surg 1998;117: 197-204.
Purpose: The purposes of the study were (1) to compare rotator cuff repair strengths after cyclic loading of 2 bioabsorbable nonsuture-based tack-type anchors, transosseous sutures, and a metal suture-based anchor, and (2) to correlate bone mineral density with mode of failure and cycles to failure. We hypothesized that specimens with a lower bone density would fail through bone at a lower number of cycles independent of the method of cuff fixation. Type of Study: Ex vivo biomechanical study. Methods: Standardized full-thickness rotator cuff defects were created in 30 fresh-frozen cadaveric shoulders that were randomized to 1 of 4 repair groups: transosseous sutures; Mitek Super suture anchors (Mitek Surgical Products, Westwood, MA); smooth bioabsorbable 8-mm Suretacs (Acufex, Smith & Nephew Endoscopy, Mansfield, MA); or spiked bioabsorbable 8-mm Suretacs (Acufex). All repairs were cyclically loaded from 10 to 180 N; the numbers of cycles to 50% (gap, 5 mm) and 100% (gap, 10 mm) failure were recorded. Results: In comparing the repair groups, we found only 1 significant difference: the number of cycles to 100% failure was significantly higher (P ⬍ .05) for the smooth bioabsorbable tack than for the transosseous suture group. There were no statistically significant (P ⱕ .05) differences in bone mineral densities with regard to each specimen’s mode of failure. Conclusions: Our results suggested that immediate postoperative fixation provided by bioabsorbable tacks was similar to that provided by Mitek anchors and more stable than that provided by transosseous sutures. Therefore, the immediate postoperative biomechanical strength of bioabsorbable tacks seems comparatively adequate for fixation of selected small rotator cuff tears. However, additional evaluation in an animal model to examine degradation characteristics and sustained strength of repair is recommended before clinical use. Key Words: Rotator cuff tears— Absorbable anchors—Metal anchors—Cyclic loading—Cadaveric model—Bone density.
S
ince the introduction of suture anchors in 1985 by Goble et al.,1 numerous anchors have been designed and produced. Anchors come in a variety of types, including suture-based, nonsuture-based, bioabsorbable, and metal. Although some of these anchors
From the Department of Orthopaedic Surgery, The Union Memorial Hospital, Baltimore, Maryland, U.S.A. Address correspondence and reprint requests to John B. O’Donnell, M.D., c/o Lyn Camire, Editor, Union Memorial Orthopaedics, The Johnston Professional Building, #400, 3333 North Calvert St, Baltimore, MD 21218, U.S.A. E-mail: [email protected] © 2001 by the Arthroscopy Association of North America 0749-8063/01/1704-2474$35.00/0 doi:10.1053/jars.2001.21243
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may theoretically provide a technically easier method of securing soft tissue to bone, the ideal anchor for rotator cuff repair has not been developed. The ideal anchor should be easy to insert, provide adequate initial strength to permit rehabilitation exercises, and supply sustained strength of repair during the healing process.2 As arthroscopic and arthroscopically assisted mini-open techniques of rotator cuff repair are becoming more popular, the characteristics of this ideal anchor are evolving. Using suture-based anchors and an all-arthroscopic technique can be challenging. The ability to insert the anchors through a limited number of portals, pass suture through the tendon, and tie knots, requires practice before entering the operating room.3 Anchors or tacks that do not require sutures
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 4 (April), 2001: pp 360 –364
ROTATOR CUFF REPAIR: TACKS, ANCHORS, SUTURES would eliminate 2 of the more difficult steps involved in most current fixation techniques. Although many studies provide information about the initial ultimate tensile strength of rotator cuff repairs using suture anchors4-7 and anchor pull-out strength,3-8 Burkhart et al.9,10 have recommended using cyclic testing of rotator cuff repairs. These tests provide a better simulation of the in vivo mechanism of rotator cuff repair failure. To our knowledge, published reports on cyclic loading of rotator cuff repair exist only for transosseous sutures and metal suturebased anchors.9-11 The purpose of this study was 2-fold: (1) to compare the rotator cuff repair strengths after cyclic loading of 2 bioabsorbable nonsuture-based tack-type anchors, transosseous sutures, and a metal suture-based anchor; and (2) to correlate the bone mineral density for each specimen with the mode of failure and number of cycles to failure. We hypothesized that specimens with a lower bone density would fail through bone at fewer cycles independent of the method of cuff fixation. METHODS We obtained 16 pairs of fresh-frozen cadaver shoulders (mean age at death, 77 years; range, 64 to 88 years) from the Maryland State Anatomy Board in Baltimore and stored them at ⫺20°C. Each of the 32 specimens was then thawed at room temperature for 24 hours, randomized to 1 of 4 treatment groups (see below), and dissected, leaving only the intact rotator cuff, humeral head, and proximal 15 cm of the humeral shaft. At dissection, 2 shoulders (both in group 4) were found to have evidence of previous surgery or rotator cuff tears and, therefore, were discarded. As described by Burkhart et al.,9,10 a standard full-thickness defect (20 mm anterior-to-posterior and 10 mm medial-to-lateral) was then created in the supraspinatus and infraspinatus tendons of each of the 30 remaining shoulders. In group 1 (9 specimens), each rotator cuff defect was repaired with 2 No. 2 Ethibond transosseous sutures (Ethicon Inc., Somerville, NJ) using a MasonAllen stitch technique.5 These sutures were placed through 3 drill holes that were spaced 10 mm apart and 10 mm distal to the tip of the greater tuberosity.12 In group 2 (8 specimens), each defect was repaired with 2 Mitek Super anchors (Fig 1) (Mitek Surgical Products, Westwood, MA) using No. 2 Ethibond sutures with a simple stitch technique. The sutures were placed at the leading edge of the cuff tear, the tear was
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FIGURE 1. Repair devices. Left, smooth 8-mm Suretac anchor. Center, spiked 8-mm Suretac anchor. Right, Mitek Super anchor.
mobilized to close the defect, and the anchors were placed (according to the manufacturer’s guidelines) at 45° angles in the trough between the greater tuberosity and the lateral margin of the articular surface of the humeral head. In group 3 (7 specimens), each defect was repaired with 2 smooth bioabsorbable 8-mm Suretac anchors (Fig 1) (Acufex, Smith & Nephew Endoscopy, Mansfield, MA), respectively. In group 4 (6 specimens), each defect was repaired with 2 spiked bioabsorbable 8-mm Suretac anchors (Fig 1). In groups 3 and 4, the leading edges of the tears were mobilized to close the defects. The tacks were inserted (according to the manufacturer’s instrumentation and guidelines) through the tendon 5 mm from its free end and into a predrilled hole in the same trough that received the Mitek Super anchors. For each specimen, a 2.5-cm wide nylon strap9,10 was attached to the proximal rotator cuff with a modified Krackow stitch using No. 5 Ethibond sutures. The specimens were kept moist with saline during preparation and testing. Each specimen was mounted on an MTS Mini Bionix testing machine (MTS Systems, Eden Prairie, MN). The humeral shaft was clamped to a platform and the nylon strap was looped around a bar that was connected to the load cell (Fig 2). The repairs were cyclically loaded from 10 to 180 N with a 5-second cycle similar to that described by Burkhart et al.9,10 Gap formation at each repair site was measured by the same examiner using an extensometer. The number of cycles to 5-mm (50% failure) and 10-mm (100% failure) gap formation was recorded, along with the mechanism of failure.9,10 Each specimen was placed in a water bath, and the same qualified technician measured the density of the entire specimen with a radiographic bone densitometer (Hologic QDR-1000TM, Bedford, MA). Then a 1cm2 area below the cortex of the greater tuberosity of
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FIGURE 2. Cadaveric specimen mounted onto an MTS machine for cyclic loading after rotator cuff repair.
each specimen was digitally outlined, and the density of this area was quantified with the densitometer. In all specimens, fixation devices were located in the trough immediately medial to the trabecular bone being measured. The data were analyzed using a 1-way analysis of variance, with P ⬍ .05 being considered statistically significant. The Student-Newman-Keuls post hoc test was performed when differences were observed. A Pearson correlation was performed to determine the relationship between bone density and number of cycles to failure.
Five modes of failure were identified for the specimens. In group 1 (transosseous sutures), failure occurred by sutures pulling through the bone (n ⫽ 7) or sutures pulling through the rotator cuff (n ⫽ 2). In group 2 (Mitek anchors), the anchor loosened (n ⫽ 1), the suture broke at the anchor (n ⫽ 1), or the sutures pulled through the rotator cuff (n ⫽ 6). In groups 3 and 4 (smooth and spiked Suretacs, respectively), the failure patterns were identical: anchor loosening (n ⫽ 3) and anchor pulling through rotator cuff (n ⫽ 3). A failure mode was not determined for the seventh specimen in the group 3 because the nylon strap failed at 212 cycles. With the data available, we found no significant difference (P ⬎ .05) among the 4 groups in the number of cycles required to produce a 5-mm gap (Table 1) at the fixation site. For 10 mm of gap formation (Table 1), however, the number of cycles was significantly higher for group 3 (smooth Suretacs) than for group 1 (transosseous sutures) (P ⬍ .05); a significant difference was not identified among any of the other groups (P ⬎ .05). There were no significant differences (P ⫽ .93) in bone mineral density among any of the 4 treatment groups: group 1, 0.377 ⫾ 0.132 g/cm2; group 2, 0.410 ⫾ 0.200 g/cm2; group 3, 0.041 ⫾ 0.101 g/cm2; and group 4, 0.363 ⫾ 0.134 g/cm2. The bone mineral density did not correlate with the number of cycles to a 5-mm (r ⫽ ⫺0.0802, P ⫽ .674) or 10-mm (r ⫽ ⫺0.1570, P ⫽ .407) gap formation. We found no significant difference (P ⬎ .05) in bone mineral density among any of the five failure modes (Table 2). DISCUSSION Burkhart et al.9,10 reported results of cyclic loads applied to rotator cuff repairs using transosseous sutures9 and Mitek-RC anchors.10 In the first study,9 the average number of cycles to 5-mm and 10-mm gap
TABLE 1. Number of Cycles to 5- and 10-mm Gap Formation
Groups
No. of Specimens
5-mm Gap (mean cycles ⫾ SE) (range)
10-mm Gap (mean cycles ⫾ SE) (range)
Transosseous sutures Mitek Smooth Suretacs Spiked Suretacs
9 8 7 6
5.0 ⫾ 1.3 (1–12) 8.1 ⫾ 3.2 (2–30) 17.0 ⫾ 7.7 (2–60) 9.7 ⫾ 3.4 (1–22)
55.7 ⫾ 15.6 (6–125) 101.4 ⫾ 35.0 (10–250) 216.3 ⫾ 62.1 (20–464) 155.2 ⫾ 56.9 (3–332)
ROTATOR CUFF REPAIR: TACKS, ANCHORS, SUTURES TABLE 2. Bone Mineral Density by Mode of Failure Mode of Failure SBA SPTB SPTT AL APTT
No. of Failures
Mean Bone Mineral Density (g/cm3 ⫾ SE) (range)
1 7 8 7 6
0.116 0.363 ⫾ 0.048 (0.130–0.489) 0.471 ⫾ 0.055 (0.295–0.766) 0.411 ⫾ 0.033 (0.247–0.506) 0.306 ⫾ 0.048 (0.170–0.498)
Abbreviations: SBA, suture broke at anchor; SPTB, suture pulled through bone; SPTT, suture pulled through tendon; AL, anchor loosened; APTT, anchor pulled through tendon.
formation in cadaveric specimens (average age, 41.2 years) repaired with 3 simple transosseous sutures were 25 and 188, respectively. In the second study,10 the average number of cycles to 5-mm and 10-mm gap formation in cadaveric specimens (similar average age) repaired with 3 Mitek-RC anchors with a simple suture technique were 61 and 285 cycles, respectively. With each fixation method, they reported better bone fixation in cadavers less than 45 years old than in those older than 45 years. In the current study, the average numbers of cycles to 5- and 10-mm gap formation for group 1 (transosseous sutures) (5 and 56 cycles, respectively) and group 2 (Mitek Super anchors) (8 and 101 cycles, respectively) were less than those reported in the Burkhart studies9,10 because of 2 main factors. First, we used 2 sutures or 2 anchors instead of 3 anchors to repair the rotator cuff defects that were identical in size to those of Burkhart et al.9,10 Second, our cadavers were all 64 years old or older (average age, 77 years). In the study by Burkhart et al.,10 5- and 10-mm failure in specimens more than 45 years old with Mitek anchor fixation occurred at 17 and 91 cycles,10 respectively, values similar to those in the current study. Our failure modes were similar to those reported by Burkhart et al.9,10 in that the transosseous sutures primarily failed through bone and the Mitek anchor groups primarily failed through the tendons. The modified Mason-Allen stitch used in our transosseous suture group has been shown to provide better fixation under cyclic loading conditions when compared with numerous other techniques.5 Because the Mitek group failed primarily through the tendon, the modified Mason-Allen stitch should be considered with suturebased anchors. In a study by Rossouw et al.,11 a different method of cyclic loading was used than that described in the current study for rotator cuff defects repaired with Mitek GII anchors and No. 2 braided polyester sutures. Their failure mode was primarily
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suture breakage at the knots or at the anchors and not failure through the tendon. The enhanced tendon fixation may have occurred secondary to the lockingtype stitch they used. Half of our Suretac anchors failed through bone and the other half failed through the tendon. We initially hypothesized that the different modes of failure were due in part to variations in bone mineral density. However, our analysis found no relationship between bone mineral density and mode of failure or number of cycles to failure. This finding is similar to the conclusion of Barber et al.,13 who identified no correlation between bone density and pullout strength of screwtype anchors in the proximal humerus. Our results indicated that, at least in the immediate postoperative period, the initial fixation provided by Suretac anchors was adequate, similar to that of Mitek Super anchors, and more stable than that of transosseous sutures when used for rotator cuff repairs subjected to cyclic loads. A potential advantage of the Suretac anchors is that their arthroscopic placement is easier because they do not require sutures. However, because these anchors are bioabsorbable, their ability to provide stable fixation throughout the healing process is unknown. Bioabsorbable anchors are composed of a variety of materials, the most common of which are polylactic acid (PLA) and polyglycolic acid (PGA).14 The Suretac anchor is a copolymer composed of 67% PGA and 33% trimethylene carbonate; the latter degrades by hydrolysis alone and has been shown to lose 50% of its strength retention by 4 to 6 weeks of use, but it may not be completely resorbed until 6 months after implantation.15,16 Various other polymers of PGA have been shown to degrade from 2 weeks to 6 months after implantation.14,15,17,18 In general, polymers of PLA degrade slower than PGA. However, because we studied only the immediate postoperative fixation strength of PGA tacks, we believe our data regarding initial strength of repair are equally applicable to bioabsorbable PLA tacks. The anatomic site of implantation and the specific mechanical stresses applied to the implant have also been reported to affect the degradation rate.14 To our knowledge, there are no published studies that have evaluated the degradation and sustained mechanical properties of any bioabsorbable tacks used for repair of rotator cuff tears in an animal model. With the current data available on biodegradability, tacks composed of PLA polymers would seem more appropriate than the Suretac for repair of rotator cuff tears. The current study indicated that the fixation pro-
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vided by the nonsuture-based bioabsorbable tack was similar to that of the suture-based Mitek Super anchor when subjected to cyclic loading in the immediate postoperative period when there was no difference in bone density between the groups of specimens. However, because the tack may be technically easier to use than the anchor in arthroscopic repair of rotator cuff tears, we suggest caution when interpreting our findings and applying them to the clinical situation scenario. The current study provides no information about the degradation characteristics or sustained strength of repair for the Suretac anchors or other types of tacks. Bioabsorbable tacks composed of PLA polymers should be evaluated in an animal model. REFERENCES 1. Goble EM, Somers WK, Clark R, Olsen RE. The development of suture anchors for use in soft tissue fixation to bone. Am J Sports Med 1994;22:236-239. 2. Burkhart SS. Biomechanics of rotator cuff repair: Converting the ritual to a science. Instr Course Lect 1998;47:43-50. 3. Barber FA, Herbert MA, Click JN. Internal fixation strength of suture anchors—Update 1997. Arthroscopy 1997;13:355-362. 4. Craft DV, Moseley JB, Cawley PW, Noble PC. Fixation strength of rotator cuff repairs with suture anchors and the transosseous suture technique. J Shoulder Elbow Surg 1996; 5:32-40. 5. Gerber C, Schneeberger AG, Beck M, Schlegel U. Mechanical strength of repairs of the rotator cuff. J Bone Joint Surg Br 1994;76:371-380. 6. 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.
7. Reed SC, Glossop N, Ogilvie-Harris DJ. Full-thickness rotator cuff tears. A biomechanical comparison of suture versus bone anchor techniques. Am J Sports Med 1996;24:46-48. 8. Barber FA, Herbert MA, Click JN. Suture anchor strength revisited. Arthroscopy 1996;12:32-38. 9. 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. 10. Burkhart SS, Diaz Pagan JL, Wirth MA, Athanasiou KA. Cyclic loading of anchor-based rotator cuff repairs: Confirmation of the tension overload phenomenon and comparison of suture anchor fixation with transosseous fixation. Arthroscopy 1997;13:720-724. 11. Rossouw DJ, McElroy BJ, Amis AA, Emery RJ. A biomechanical evaluation of suture anchors in repair of the rotator cuff. J Bone Joint Surg Br 1997;79:458-461. 12. Caldwell GL, Warner JP, Miller MD, Boardman D, Towers J, Debski R. Strength of fixation with transosseous sutures in rotator cuff repair. J Bone Joint Surg Am 1997;79:1064-1068. 13. 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. 14. Athanasiou KA, Agrawal CM, Barber FA, Burkhart SS. Orthopaedic applications for PLA-PGA biodegradable polymers. Arthroscopy 1998;14:726-737. 15. Miller RA, Brady JM, Cutright DE. Degradation rates of oral resorbable implants (polylactates and polyglycolates): Rate modification with changes in PLA/PGA copolymer ratios. J Biomed Mater Res 1977;11:711-719. 16. Speer KP, Warren RF. Arthroscopic shoulder stabilization. A role for biodegradable materials. Clin Orthop 1993;291:67-74. 17. Bostman OM, Paivarinta U, Partio E, et al. The tissue-implant interface during degradation of absorbable polyglycolide fracture fixation screws in the rabbit femur. Clin Orthop 1992; 285:263-272. 18. Nordstrom P, Pihlajamaki H, Toivonen T, Tormala P, Rokkanen P. Tissue response to polyglycolide and polylactide pins in cancellous bone. Arch Orthop Trauma Surg 1998;117: 197-204.