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Initial Biomechanical Properties of Staple–Anchor Achilles Tendon Allograft and Interference Screw Bone–Patellar Tendon–Bone Autograft Fixation for Anterior Cruciate Ligament Reconstruction in a Cadaveric Model
Initial Biomechanical Properties of Staple–Anchor Achilles Tendon Allograft and Interference Screw Bone–Patellar Tendon–Bone Autograft Fixation for Anterior Cruciate Ligament Reconstruction in a Cadaveric Model James M. Farmer, M.D., Cassandra A. Lee, M.D., Walton W. Curl, M.D., David F. Martin, M.D., Bill Kortesis, M.D., and Gary G. Poehling, M.D.
Purpose: Anterior cruciate ligament (ACL) reconstruction is a common procedure that has a fairly high success rate. Despite such success, controversy exists with regard to fixation and graft type. The purpose of this study was to quantify the maximum load to failure for staple–anchor freeze-dried Achilles tendon allograft fixation compared with interference screw bone–pattelar tendon– bone autograft fixation at the time of insertion for ACL reconstruction. Methods: Eleven pairs of cadaveric knees were prepared for ACL reconstruction by disarticulation before graft insertion. The tibia and femur were mounted separately onto an MTS machine and were loaded to failure in line with the tunnels. Femoral fixation for the allograft was provided by a staple anchor; tibial fixation was provided by a suture anchor. Titanium interference screws on the femoral and tibial sides provided autograft fixation. A paired t test was performed to compare mechanical testing results in the 2 groups. Results: Mean maximum load to failure for the allograft was 58.7 N (range, 32.3 to 92.6 N) and 119.6 N (range, 82 to 165.9 N) for the femur and the tibia, respectively, compared with 228.2 N (range, 74.2 to 352 N) and 232.9 N (range, 65.1 to 553.1 N) for the autografts. This difference was statistically significant (P ⬍ .001) for femoral fixation, but it was not statistically significant for tibial fixation (P ⫽ .186). Conclusions: Soft tissue Achilles tendon allograft with staple fixation is a significantly weaker fixation construct when compared with autograft bone–patellar tendon– bone with interference screw fixation. Clinical Relevance: This study shows significantly weaker fixation in the staple-alograft construct and yet this construct has had at least equivalent results over a 5-year time frame, indicating that rigid femoral fixation may not be a critical factor in long-term results. Key Words: Anterior cruciate ligament reconstruction—Allograft—Ultimate strength—Nonrigid fixation.
T
he anterior cruciate ligament (ACL) is the most frequently disrupted ligament of the knee and is a common site of injury encountered by the orthopaedic surgeon.1-3 Although outcomes are successful with
From the Departments of Orthopaedic Surgery, Wake Forest University School of Medicine (J.M.F., C.A.L., D.F.M., B.K., G.G.P.), Winston-Salem, North Carolina, and the Medical College of Georgia (W.W.C.), Augusta, Georgia, U.S.A. The authors report no conflict of interest. Address correspondence and reprint requests to Gary G. Poehling, M.D., Department of Orthopaedic Surgery, Wake Forest University School of Medicine, Medical Center Bld, Winston-Salem, NC 27157, U.S.A. E-mail: [email protected] © 2006 by the Arthroscopy Association of North America 0749-8063/06/2210-3982$32.00/0 doi:10.1016/j.arthro.2006.08.004
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current surgical techniques, controversy exists with regard to type of fixation and the ideal choice for type of graft (i.e., allograft v autograft).4-8 For more than 20 years, the senior author (G.G.P.) has performed successful ACL reconstructions using staple–anchor fixation of freeze-dried Achilles tendon allograft without bone block.9 The specific aim of this study was to compare the initial biomechanical properties of our fixation technique of staple allograft reconstruction with the gold standard bone–patellar tendon– bone (BPTB) autograft fixed with interference screws in a cadaveric model. The study hypothesis was that the ultimate strength for staple fixation of allografts is significantly less than that of interference screw fixation of autografts.
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 22, No 10 (October), 2006: pp 1040-1045
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METHODS Eleven matched pairs of fresh frozen cadaveric knees were used in this study. Bone density measures of the metaphyseal bone of the tibia and femur were obtained through quantitative computed tomography scanning (Norland/Stratec XCT 3000A; Stratec Medizintechnik GmbH, Pforzheim, Germany). For preparation of the autograft, the central third of the patellar tendon, along with 25-mm bone plugs from the patella and the tibia, was harvested. Remaining soft tissues were removed for preparation of the specimen. Under direct visualization, 11-mm tibial and femoral tunnels were placed in standard position to simulate a single-incision ACL reconstruction technique with use of the Arthrex drill guide system (Arthrex, Naples, FL). Following tunnel placement, the knees were disarticulated and were further stripped of ligamentous tissue. Reconstruction methods were randomly assigned to each pair of knees: 1 knee received staple–anchor– fixed freeze-dried Achilles tendon allograft without bone block, and the contralateral side received interference screw fixation of a BPTB autograft. Allograft The freeze-dried Achilles tendon without calcaneal bone block (Musculoskeletal Transplant Foundation, Edison, NJ) was reconstituted in warm saline for a half-hour. The graft was prepared by cutting a wedge out of the distal end; the end was looped and secured to a staple (LCR 90; Instrument Makar/Smith & Nephew Endoscopy, Andover, MA) with a No. 1 Vicryl suture (Ethicon, Somerville, NJ) (Fig 1). The allograft mounted on the staple was inserted into the femoral tunnel. The graft was fixed in the tibia with a No. 2 Tevdek suture (Deknatel, Fall River, MA) on a suture anchor device (Ogden anchor; Orthofix, Bussolengo, Italy) (Fig 2). Autograft Autografts were fixed with 9 ⫻ 25-mm titanium interference screws (Arthrex) in 11-mm bone tunnels in the femur and the tibia. Biomechanical Testing Grafts were inserted into the femoral or tibial tunnel, with an end left free to be attached to the programmable servohydraulic testing apparatus (Bionix 858; MTS, Eden Prairie, MN). One fixation method (in the femoral or the tibial side) was tested at a time. The tibia or femur was attached to the load cell with a
FIGURE 1. Achilles tendon allograft looped and sutured to staple mounted onto a driver.
half-inch rod, and the free end of the graft was attached to the actuator by means of a soft tissue clamp. Each tendon was preconditioned for 1 minute with a 10-N force applied to the graft directly in line with the tunnel. Following preload, each graft was tested until failure with a load applied at 1 mm per second. Ultimate strength and mode of failure for each graft were determined. Stiffness of fixation methods was assessed from the linear region of the load elongation curve and was estimated at 80% of ultimate strength. Data were analyzed with a paired t test to compare the 2 groups by estimating the effect of the fixation technique on ultimate strength and stiffness. Statistical significance was set at P ⬍ .05. RESULTS The average age of the cadaveric specimens was 75.4 years. No statistical difference was found in average bone density between the right and left sides of the specimens. One of the specimens, along with its matched pair, was removed from analyses because the ultimate strength of the femoral interference screw was greater than 2 standard deviations above the mean (Table 1). Mean ultimate strength for the femoral staple was 58.7 N (⫾19.4; range, 32.3 to 92.6 N) compared with 228.2 N (⫾101.7; range, 74.2 to 352 N) for the femoral interference screw; this finding was statistically significant (P ⬍ .001; n ⫽ 10). Mean maximum load to failure for the tibial suture anchor fixation was
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FIGURE 3. Ultimate strength of the autograft fixed with an interference screw is significantly greater than the staple allograft construct in the femur. No statistical difference is seen on the tibial side (P ⫽ .186).
FIGURE 2. Staple Achilles tendon allograft technique with tibial suture anchor. (Reprinted with permission.37)
119.6 N (⫾24.7; range, 82 to 165.9 N) compared with 232.9 N (⫾161.2; range, 65.1 to 553.1 N) for the interference screw. This was not statistically significant (P ⫽ .186; n ⫽ 10) (Fig 3). The mode of failure for the femoral staple was located at the bone–staple interface; the mode of failure occurred at the suture– tendon interface for the tibial suture anchor. All auTABLE 1. Specimen 1 2 3 4 5 6 7 8 (removed from analysis) 9 10 11 Mean Standard deviation
Ultimate Strength of Each Specimen Femoral Staple
Femoral Screw
Tibial Anchor
Tibial Screw
58 35.04 66 36.14 71 73.6 32.3
327.1 277.7 74.2 126 120 272.3 199
99.7 112.7 112.1 133.3 82 148.2 101.7
375 65.14 293.7 375.8 92.96 88.74 117.7
24.2 56.6 92.6 65.5 58.7 19.4
362 183.49 352 350 228.2 101.7
117 165.9 112.3 128.1 119.6 24.7
566.6* 553.1 168 198.6 232.9 161.2
*Greater than 2 standard deviations from the mean.
tografts experienced failure at the bone interference– screw interface. Stiffness of the constructs was compared from the linear region on the force displacement curves (Table 2). The femoral autograph was significantly stiffer than the allograft (P ⬍ .001; mean, 27.8 ⫾ 10.7 N/mm and 7.0 ⫾ 4.6 N/mm, respectively; estimated difference, 20.8; 95% confidence interval [CI] on the difference, 13 to 28.5). Comparisons of tibial autograft interference screw construct versus allograft secured with a suture anchor revealed statistically greater stiffness of the autograft (P ⫽ .017; mean, 27.8 ⫾ 16.3 N/mm and 11.0 ⫾ 4.8 N/mm, respectively) (Fig 4). DISCUSSION Successful reconstruction of the ACL depends on several factors, including graft selection, accurate placement of the tibial and femoral tunnels, and stable fixation that allows early functional rehabilitation.5,10-13 Graft selection has been debated throughout the literature for many years.5,6,14 ACL reconstruction with a BPTB autograft fixed with interference screws is a widely used procedure because of the reliable characteristics of the graft during bone-to-bone healing and the initial strength of fixation provided by the interference screw.15,16 Despite its success, morbidity associated with autograft BPTB harvest has led to the development of alternative grafts.17,18 Allograft has become increasingly popular in its use for ACL reconstruction. Overall, allograft reconstructions result in comparable outcomes but offer several advantages over autografts, including shorter operative time, absence of donor site morbidity, and easier recovery and rehabilitation, with possibly faster return to function.6,19,20 Outcomes studies demonstrate
BPTB AUTOGRAFT FIXATION FOR ACL RECONSTRUCTION TABLE 2.
Stiffness of Fixation of Each Specimen
Specimen 1 2 3 4 5 6 7 8 (removed from analysis) 9 10 11 Mean Standard deviation
Femoral Staple
Femoral Screw
Tibial Anchor
Tibial Screw
13.45 8.38 11.96 6.76 3.32 13.17 1.81
38.7 28.65 23.33 11.28 21.02 11.84 40.41
16.59 12.72 12.67 18.42 3.01 5 7.53
47.53 15.03 43.52 31.06 20.91 3.67 20.7
4.97 2.02 3.24 5.62 7.0 4.6
N/A 28.48 36.43 37.38 27.8 10.8
9.58 11.08 12.13 10.68 11.0 4.8
56.21 51.43 33.8 10.14 27.8 16.3
equivalent results to autograft reconstructions.6,15,19-22 Graft options— both autograft and allograft— have broadened with the development of better soft tissue anchors.12,15,21 In our clinical experience, the allograft technique tested results in equal outcomes when compared with the autograft technique tested, although patients who underwent allograft reconstruction had fewer complaints of pain and fewer functional limitations during the first postoperative year.9 Because in vitro analysis reveals significantly lower loads to failure for the allograft technique, variables other than fixation strength may determine clinical outcomes of ACL reconstruction. The 3 variables addressed in biomechanical testing of the constructs in this study were ultimate failure, stiffness, and bone mineral density. Initial strength of a graft is crucial for the maintenance of stability of the knee during early rehabilitation.23 The purpose of our study was to focus on the initial ultimate strength of the staple–anchor soft tissue allograft fixation construct by applying a force in line with the bone tunnels. When a graft is loaded physiologically (i.e., with the knee in 20° of flexion), friction develops between the graft and the tunnel edge, which can stress shield the more distal site of graft fixation.18 Although the load is not representative of the shear loads applied to the graft in situ, it is consistent with many other studies of pullout strength, allowing our data to be compared with those of other studies (Tables 3 and 4).16,24-30 Straight pullout stresses are greater than the loads actually applied in vivo in that some of the force is dissipated at the bone tunnel– graft interface. The clinical success of staple-fixed soft tissue grafts may be due to the fact that ultimate strength in biomechanical
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testing does not reflect actual physiologic loads experienced by the graft construct. Our load-to-failure studies showed that stiffness of the interference screw–BPTB autograft construct was significantly greater than that of the staple and suture anchor, predicting that early displacement and failure would occur earlier for the latter construct. Although failure occurred at the bone– hardware junction, greater displacement of the staple anchor fixation construct would occur than with the interference screw, as predicted by stiffness, given the same amount of force. Both constructs were preloaded at the same rate to eliminate the factor of viscoeleastic properties of the graft– bone junction or the graft proper in the determination of stiffness. Stiffness plays a role in the rigidity of the construct. Low-tensioned grafts may emulate the normal knee more accurately than hightensioned, rigidly fixed grafts. Higher-tensioned grafts may instead overconstrain the knee in situ, leading to graft failure or loss of motion,31,32 whereas a less rigidly fixed graft may allow the graft to optimally incorporate under physiologic forces. A major concern in ACL reconstructions is bone mineral density. Ultimate strength of a construct is directly related to bone mineral density.30 The samples used in this study were not the typical age of candidates who undergo ACL reconstruction. The average age of the specimens tested was 75.4 years; this is a limitation of our study. Dual-energy x-ray absorptiometry (DEXA) scans revealed lower bone mineral density in the metaphyseal bone of our specimens than in that of younger patients. No difference in bone density was seen between matched pairs. However, because direct matched pair analysis of the fixation methods was conducted, bone density becomes a negligible factor in the comparison. The proportionate strengths of the constructs are valid. However, this
FIGURE 4. Rigid fixation of the interference screw/autograft construction creates significantly more stiffness (N/mm) than is produced by the staple allograft construct on the femoral and tibial sides (P ⬍ .001 and P ⫽ .017, respectively).
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J. M. FARMER ET AL. TABLE 3.
Comparison of Ultimate Strength of Grafts When Loads Are Applied in Line With the Bone Tunnels Femur
Tibia
Fixation Method
Mode of Failure
Not reported 527 ⫾ 189 N 344 ⫾ 227 N Not reported Not reported
Gerich36
350 ⫾ 134 N 542 ⫾ 168 N 274 ⫾ 96 N 266 N 275 N 506–758 N
Our data
229 N
Interference screw STG Interference screw BPTB Postfixation BPTB Interference screw BPTB Transfemoral screw and tibial staple STG Interference screw BPTB Femoral screw and 2 tibial staples BPTB Interference screw
Not reported Fixation site Varied Tibial fixation site Fixation site (staple ⫽ screw) Bone–tunnel interface Fixation site Fixation site
Magen25 Paschal26(porcine) Randall27
588 N 233 N
does not reflect absolute strength. It is possible that tibial fixation strength of the autograft fixed with an interference screw is significantly greater than the tibial suture anchor group that was not appreciated in this cadaveric specimen group. Similar to other studies in which elderly donors or animal models were used,25,26,30-36 absolute strength of the constructs is lower than would be expected in younger patients. Despite the osteoporotic nature of cadaveric bone, the mode of failure for the tibial allograft was the suture– tendon interface, rather than the bone–anchor interface. We investigated only initial ultimate strength; cyclic loading of the constructs was not tested, and this is a limitation of our study. Cyclic loading is thought to simulate early rehabilitation and ambulation stresses experienced by the graft. The purpose of this study was to establish the maximum load to failure time zero. It was believed that cyclic loading would add confusion to that ultimate purpose. CONCLUSIONS Soft tissue Achilles allograft with staple fixation of the femur is a significantly weaker fixation method when compared with autograft BPTB with interference screw fixation in a cadaveric model.
TABLE 4. Staple Mitek G I Mitek G II Statak LCR 90 Ogden anchor
Comparison of the Ultimate Strength of Various Suture Anchors 180 N (canine humerus) 74.4 N (human tibia) 80.9 N (human tibia) 74.7 N (human tibia) 58.7 N (human femur) 119.6 N (human tibia)
Johnson* Carpenter28 Carpenter28 Carpenter28 Our data Our data
*Data from Johnson LL, Arnoczky SP. Michigan State University.
Acknowledgment: The authors thank Arthrex, Instrument Makar/Smith & Nephew, Orthofix, and the Musculoskeletal Transplant Foundation for donating materials and supplies for this study, and Annemarie Johnson, CMI, for Figure 2.
REFERENCES 1. Eriksson E. Reconstruction of the anterior cruciate ligament. Orthop Clin North Am 1976;7:167-179. 2. Feagin JA Jr, Curl WW. Isolated tear of the anterior cruciate ligament: 5-Year follow-up study. Am J Sports Med 1976;4: 95-100. 3. Johnson RJ, Beynnon BD, Nichols CE, Renstrom PA. The treatment of injuries of the anterior cruciate ligament. J Bone Joint Surg Am 1992;74:140-151. 4. Fu FH, Bennett CH, Ma CB, Menetrey J, Lattermann C. Current trends in anterior cruciate ligament reconstruction. Part II: Operative procedures and clinical correlations. Am J Sports Med 2000;28:124-130. 5. Noyes FR, Butler DL, Grood ES, Zernicke RF, Hefzy MS. Biomechanical analysis of human ligament grafts used in knee-ligament repairs and reconstructions. J Bone Joint Surg Am 1984;66:344-352. 6. Levitt RL, Malinin T, Posada A, Michalow A. Reconstruction of anterior cruciate ligaments with bone–patellar tendon– bone and Achilles tendon allografts. Clin Orthop 1994;303:67-78. 7. Noyes FR, Barber-Westin SD. Reconstruction of the anterior cruciate ligament with human allograft. J Bone Joint Surg Am 1996;78:524-537. 8. Noyes FR, Barber SD, Mangine RE. Bone–patellar ligament– bone and fascia lata allografts for reconstruction of the anterior cruciate ligament. J Bone Joint Surg Am 1990;72:1125-1136. 9. Poehling GG, Curl WW, Lee CA, et al. Anterior cruciate ligament repair with 5-year follow-up: Allograft versus autograft. Arthroscopy 2005;21:774-785. 10. Jorgensen U, Thomsen HS. Behavior of graft within the bone tunnels following anterior cruciate ligament reconstruction, studied by cinematic magnetic resonance imaging. Knee Surg Sports Traumatol Arthrosc 2000;8:32-35. 11. Scopp JM, Jasper LE, Belkoff SM, Moorman CT 3rd. The effect of oblique femoral tunnel placement on rotational constraint of the knee reconstructed using patellar tendon autografts. Arthroscopy 2004;20:294-299. 12. Safran MR. Graft selection in knee surgery: Current concepts. Am J Knee Surg 1995;8:168-180. 13. Scheffler SU, Sudkamp NP, Gockenjan A, Hoffmann RF, Weiler A. Biomechanical comparison of hamstring and patellar tendon graft anterior cruciate ligament reconstruction tech-
BPTB AUTOGRAFT FIXATION FOR ACL RECONSTRUCTION
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niques: The impact of fixation level and fixation method under cyclic loading. Arthroscopy 2002;18:304-315. Beynnon BD, Johnson RJ, Fleming BC, et al. Anterior cruciate ligament replacement: Comparison of bone–patellar tendon– bone grafts with two-strand hamstring grafts: A prospective, randomized study. J Bone Joint Surg Am 2002;84:1503-1513. Shelton WR, Papendick L, Dukes AD. Autograft versus allograft anterior cruciate ligament reconstruction. Arthroscopy 1997;13:446-449. Hoffman FG, Peine R, Bail HJ, et al. Initial fixation strength of modified patellar tendon grafts for anatomic fixation in anterior cruciate ligament reconstruction. Arthroscopy 1999;15: 392-399. Svensson M, Kartus J, Ejerhed L, Lindahl S, Karlsson J. Does the patellar tendon normalize after harvesting its central third: A prospective long-term MRI study. Am J Sports Med 2004; 32:34-38. Steiner ME, Hecker AT, Brown CH Jr, Hayes WC. Anterior cruciate ligament graft fixation: Comparison of hamstring and patellar tendon grafts. Am J Sports Med 1994;22:240246. Harner CD, Olson E, Irrgang JJ, Silverstein S, Fu FH, Silbey M. Allograft versus autograft anterior cruciate ligament reconstruction. Clin Orthop 1996;324:134-144. Miller MD, Harner CD. The use of allograft: Techniques and results. Clin Sports Med 1993;12:757-770. Shino K, Kawasaki T, Hirose H, Gotoh I, Inoue M, Ono K. Replacement of the anterior cruciate ligament by an allogeneic tendon graft: An experimental study in the dog. J Bone Joint Surg Br 1984;66:672-681. Jackson DW, Grood ES, Goldstein JD, et al. A comparison of patellar tendon autograft and allograft used for anterior cruciate ligament reconstruction in the goat model. Am J Sports Med 1993;21:176-185. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med 1990; 18:292-299. Seitz H, Vécsei V, Menth-Chiari WA, Pichl W, Wielke B, Marlovits S. Comparison of femoral and tibial pullout forces in bone–patellar tendon– bone anterior cruciate ligament reconstructions with a new interference fixation device. Arthroscopy 1999;15:173-178. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med 1999;27:35-43. Paschal SO, Seemann MD, Ashman RB, Allard RN, Montgomery JB. Interference fixation versus postfixation of bone– patellar tendon– bone grafts for anterior cruciate ligament re-
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construction: A biomechanical comparative study in porcine knees. Clin Orthop 1994;300:281-287. Randall RL, Wolf EM, Heilmann MR, Lotz J. Comparison of bone–patellar tendon– bone interference screw fixation and hamstring transfemoral screw fixation in anterior cruciate ligament reconstruction. Orthopedics 1999;22:587-591. Carpenter JE, Fish DN, Huston LJ, Goldstein SA. Pull-out strength of five suture anchors. Arthroscopy 1993;9:109113. Nagarkatti DG, McKeon BP, Donahue BS, Fulkerson JP. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am J Sports Med 2001;29:67-71. Brand JC, Pinekowski D, Steenlage E, Hamilton D, Johnson DL, Caborn DNM. Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med 2000; 28:705-710. Dinevski E, Fowler PJ, Litchfield R, Amendola A, Brown G, Kirkley A. Randomized clinical trial assessing the effects of graft tensioning using semitendinosus and gracilis tendons in anterior cruciate ligament reconstruction. Presented at Meeting of American Academy of Orthopaedic Surgeons (AAOS); February 13-17, 2002; Dallas, Tex. Nicholas SJ, D’Amato MJ, Mullaney MJ, Tyler TF, Kolstad K, McHugh MP. A prospectively randomized double-blind study on the effect of initial graft tension on knee stability after anterior cruciate ligament reconstruction. Am J Sports Med 2004;32:1881-1886. Caborn DN, Coen M, Neef R, Hamilton D, Nyland J, Johnson DL. Quadrupled semitendinosus– gracilis autograft fixation in the femoral tunnel: A comparison between a metal and a bioabsorbable interference screw. Arthroscopy 1998;14:241-245. Rowden NJ, Sher D, Rogers GJ, Schindhelm K. Anterior cruciate ligament graft fixation: Initial comparison of patellar tendon and semitendinosus autografts in young fresh cadavers. Am J Sports Med 1997;25:472-478. Johnson LL, van Dyk GE. Metal and biodegradable interference screws: Comparison of failure strength. Arthroscopy 1996;12:452-456. Gerich TG, Kang R, Fu FH, Robbins PD, Evans CH. Gene transfer to the rabbit patellar tendon: Potential for genetic enhancement of tendon and ligament healing. Gene Ther 1996; 3:1089-1093. Koman LA, ed. Wake Forest University School of Medicine orthopaedic manual.. Winston-Salem, NC: Orthopaedic Press, 2003.
Purpose: Anterior cruciate ligament (ACL) reconstruction is a common procedure that has a fairly high success rate. Despite such success, controversy exists with regard to fixation and graft type. The purpose of this study was to quantify the maximum load to failure for staple–anchor freeze-dried Achilles tendon allograft fixation compared with interference screw bone–pattelar tendon– bone autograft fixation at the time of insertion for ACL reconstruction. Methods: Eleven pairs of cadaveric knees were prepared for ACL reconstruction by disarticulation before graft insertion. The tibia and femur were mounted separately onto an MTS machine and were loaded to failure in line with the tunnels. Femoral fixation for the allograft was provided by a staple anchor; tibial fixation was provided by a suture anchor. Titanium interference screws on the femoral and tibial sides provided autograft fixation. A paired t test was performed to compare mechanical testing results in the 2 groups. Results: Mean maximum load to failure for the allograft was 58.7 N (range, 32.3 to 92.6 N) and 119.6 N (range, 82 to 165.9 N) for the femur and the tibia, respectively, compared with 228.2 N (range, 74.2 to 352 N) and 232.9 N (range, 65.1 to 553.1 N) for the autografts. This difference was statistically significant (P ⬍ .001) for femoral fixation, but it was not statistically significant for tibial fixation (P ⫽ .186). Conclusions: Soft tissue Achilles tendon allograft with staple fixation is a significantly weaker fixation construct when compared with autograft bone–patellar tendon– bone with interference screw fixation. Clinical Relevance: This study shows significantly weaker fixation in the staple-alograft construct and yet this construct has had at least equivalent results over a 5-year time frame, indicating that rigid femoral fixation may not be a critical factor in long-term results. Key Words: Anterior cruciate ligament reconstruction—Allograft—Ultimate strength—Nonrigid fixation.
T
he anterior cruciate ligament (ACL) is the most frequently disrupted ligament of the knee and is a common site of injury encountered by the orthopaedic surgeon.1-3 Although outcomes are successful with
From the Departments of Orthopaedic Surgery, Wake Forest University School of Medicine (J.M.F., C.A.L., D.F.M., B.K., G.G.P.), Winston-Salem, North Carolina, and the Medical College of Georgia (W.W.C.), Augusta, Georgia, U.S.A. The authors report no conflict of interest. Address correspondence and reprint requests to Gary G. Poehling, M.D., Department of Orthopaedic Surgery, Wake Forest University School of Medicine, Medical Center Bld, Winston-Salem, NC 27157, U.S.A. E-mail: [email protected] © 2006 by the Arthroscopy Association of North America 0749-8063/06/2210-3982$32.00/0 doi:10.1016/j.arthro.2006.08.004
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current surgical techniques, controversy exists with regard to type of fixation and the ideal choice for type of graft (i.e., allograft v autograft).4-8 For more than 20 years, the senior author (G.G.P.) has performed successful ACL reconstructions using staple–anchor fixation of freeze-dried Achilles tendon allograft without bone block.9 The specific aim of this study was to compare the initial biomechanical properties of our fixation technique of staple allograft reconstruction with the gold standard bone–patellar tendon– bone (BPTB) autograft fixed with interference screws in a cadaveric model. The study hypothesis was that the ultimate strength for staple fixation of allografts is significantly less than that of interference screw fixation of autografts.
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 22, No 10 (October), 2006: pp 1040-1045
BPTB AUTOGRAFT FIXATION FOR ACL RECONSTRUCTION
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METHODS Eleven matched pairs of fresh frozen cadaveric knees were used in this study. Bone density measures of the metaphyseal bone of the tibia and femur were obtained through quantitative computed tomography scanning (Norland/Stratec XCT 3000A; Stratec Medizintechnik GmbH, Pforzheim, Germany). For preparation of the autograft, the central third of the patellar tendon, along with 25-mm bone plugs from the patella and the tibia, was harvested. Remaining soft tissues were removed for preparation of the specimen. Under direct visualization, 11-mm tibial and femoral tunnels were placed in standard position to simulate a single-incision ACL reconstruction technique with use of the Arthrex drill guide system (Arthrex, Naples, FL). Following tunnel placement, the knees were disarticulated and were further stripped of ligamentous tissue. Reconstruction methods were randomly assigned to each pair of knees: 1 knee received staple–anchor– fixed freeze-dried Achilles tendon allograft without bone block, and the contralateral side received interference screw fixation of a BPTB autograft. Allograft The freeze-dried Achilles tendon without calcaneal bone block (Musculoskeletal Transplant Foundation, Edison, NJ) was reconstituted in warm saline for a half-hour. The graft was prepared by cutting a wedge out of the distal end; the end was looped and secured to a staple (LCR 90; Instrument Makar/Smith & Nephew Endoscopy, Andover, MA) with a No. 1 Vicryl suture (Ethicon, Somerville, NJ) (Fig 1). The allograft mounted on the staple was inserted into the femoral tunnel. The graft was fixed in the tibia with a No. 2 Tevdek suture (Deknatel, Fall River, MA) on a suture anchor device (Ogden anchor; Orthofix, Bussolengo, Italy) (Fig 2). Autograft Autografts were fixed with 9 ⫻ 25-mm titanium interference screws (Arthrex) in 11-mm bone tunnels in the femur and the tibia. Biomechanical Testing Grafts were inserted into the femoral or tibial tunnel, with an end left free to be attached to the programmable servohydraulic testing apparatus (Bionix 858; MTS, Eden Prairie, MN). One fixation method (in the femoral or the tibial side) was tested at a time. The tibia or femur was attached to the load cell with a
FIGURE 1. Achilles tendon allograft looped and sutured to staple mounted onto a driver.
half-inch rod, and the free end of the graft was attached to the actuator by means of a soft tissue clamp. Each tendon was preconditioned for 1 minute with a 10-N force applied to the graft directly in line with the tunnel. Following preload, each graft was tested until failure with a load applied at 1 mm per second. Ultimate strength and mode of failure for each graft were determined. Stiffness of fixation methods was assessed from the linear region of the load elongation curve and was estimated at 80% of ultimate strength. Data were analyzed with a paired t test to compare the 2 groups by estimating the effect of the fixation technique on ultimate strength and stiffness. Statistical significance was set at P ⬍ .05. RESULTS The average age of the cadaveric specimens was 75.4 years. No statistical difference was found in average bone density between the right and left sides of the specimens. One of the specimens, along with its matched pair, was removed from analyses because the ultimate strength of the femoral interference screw was greater than 2 standard deviations above the mean (Table 1). Mean ultimate strength for the femoral staple was 58.7 N (⫾19.4; range, 32.3 to 92.6 N) compared with 228.2 N (⫾101.7; range, 74.2 to 352 N) for the femoral interference screw; this finding was statistically significant (P ⬍ .001; n ⫽ 10). Mean maximum load to failure for the tibial suture anchor fixation was
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J. M. FARMER ET AL.
FIGURE 3. Ultimate strength of the autograft fixed with an interference screw is significantly greater than the staple allograft construct in the femur. No statistical difference is seen on the tibial side (P ⫽ .186).
FIGURE 2. Staple Achilles tendon allograft technique with tibial suture anchor. (Reprinted with permission.37)
119.6 N (⫾24.7; range, 82 to 165.9 N) compared with 232.9 N (⫾161.2; range, 65.1 to 553.1 N) for the interference screw. This was not statistically significant (P ⫽ .186; n ⫽ 10) (Fig 3). The mode of failure for the femoral staple was located at the bone–staple interface; the mode of failure occurred at the suture– tendon interface for the tibial suture anchor. All auTABLE 1. Specimen 1 2 3 4 5 6 7 8 (removed from analysis) 9 10 11 Mean Standard deviation
Ultimate Strength of Each Specimen Femoral Staple
Femoral Screw
Tibial Anchor
Tibial Screw
58 35.04 66 36.14 71 73.6 32.3
327.1 277.7 74.2 126 120 272.3 199
99.7 112.7 112.1 133.3 82 148.2 101.7
375 65.14 293.7 375.8 92.96 88.74 117.7
24.2 56.6 92.6 65.5 58.7 19.4
362 183.49 352 350 228.2 101.7
117 165.9 112.3 128.1 119.6 24.7
566.6* 553.1 168 198.6 232.9 161.2
*Greater than 2 standard deviations from the mean.
tografts experienced failure at the bone interference– screw interface. Stiffness of the constructs was compared from the linear region on the force displacement curves (Table 2). The femoral autograph was significantly stiffer than the allograft (P ⬍ .001; mean, 27.8 ⫾ 10.7 N/mm and 7.0 ⫾ 4.6 N/mm, respectively; estimated difference, 20.8; 95% confidence interval [CI] on the difference, 13 to 28.5). Comparisons of tibial autograft interference screw construct versus allograft secured with a suture anchor revealed statistically greater stiffness of the autograft (P ⫽ .017; mean, 27.8 ⫾ 16.3 N/mm and 11.0 ⫾ 4.8 N/mm, respectively) (Fig 4). DISCUSSION Successful reconstruction of the ACL depends on several factors, including graft selection, accurate placement of the tibial and femoral tunnels, and stable fixation that allows early functional rehabilitation.5,10-13 Graft selection has been debated throughout the literature for many years.5,6,14 ACL reconstruction with a BPTB autograft fixed with interference screws is a widely used procedure because of the reliable characteristics of the graft during bone-to-bone healing and the initial strength of fixation provided by the interference screw.15,16 Despite its success, morbidity associated with autograft BPTB harvest has led to the development of alternative grafts.17,18 Allograft has become increasingly popular in its use for ACL reconstruction. Overall, allograft reconstructions result in comparable outcomes but offer several advantages over autografts, including shorter operative time, absence of donor site morbidity, and easier recovery and rehabilitation, with possibly faster return to function.6,19,20 Outcomes studies demonstrate
BPTB AUTOGRAFT FIXATION FOR ACL RECONSTRUCTION TABLE 2.
Stiffness of Fixation of Each Specimen
Specimen 1 2 3 4 5 6 7 8 (removed from analysis) 9 10 11 Mean Standard deviation
Femoral Staple
Femoral Screw
Tibial Anchor
Tibial Screw
13.45 8.38 11.96 6.76 3.32 13.17 1.81
38.7 28.65 23.33 11.28 21.02 11.84 40.41
16.59 12.72 12.67 18.42 3.01 5 7.53
47.53 15.03 43.52 31.06 20.91 3.67 20.7
4.97 2.02 3.24 5.62 7.0 4.6
N/A 28.48 36.43 37.38 27.8 10.8
9.58 11.08 12.13 10.68 11.0 4.8
56.21 51.43 33.8 10.14 27.8 16.3
equivalent results to autograft reconstructions.6,15,19-22 Graft options— both autograft and allograft— have broadened with the development of better soft tissue anchors.12,15,21 In our clinical experience, the allograft technique tested results in equal outcomes when compared with the autograft technique tested, although patients who underwent allograft reconstruction had fewer complaints of pain and fewer functional limitations during the first postoperative year.9 Because in vitro analysis reveals significantly lower loads to failure for the allograft technique, variables other than fixation strength may determine clinical outcomes of ACL reconstruction. The 3 variables addressed in biomechanical testing of the constructs in this study were ultimate failure, stiffness, and bone mineral density. Initial strength of a graft is crucial for the maintenance of stability of the knee during early rehabilitation.23 The purpose of our study was to focus on the initial ultimate strength of the staple–anchor soft tissue allograft fixation construct by applying a force in line with the bone tunnels. When a graft is loaded physiologically (i.e., with the knee in 20° of flexion), friction develops between the graft and the tunnel edge, which can stress shield the more distal site of graft fixation.18 Although the load is not representative of the shear loads applied to the graft in situ, it is consistent with many other studies of pullout strength, allowing our data to be compared with those of other studies (Tables 3 and 4).16,24-30 Straight pullout stresses are greater than the loads actually applied in vivo in that some of the force is dissipated at the bone tunnel– graft interface. The clinical success of staple-fixed soft tissue grafts may be due to the fact that ultimate strength in biomechanical
1043
testing does not reflect actual physiologic loads experienced by the graft construct. Our load-to-failure studies showed that stiffness of the interference screw–BPTB autograft construct was significantly greater than that of the staple and suture anchor, predicting that early displacement and failure would occur earlier for the latter construct. Although failure occurred at the bone– hardware junction, greater displacement of the staple anchor fixation construct would occur than with the interference screw, as predicted by stiffness, given the same amount of force. Both constructs were preloaded at the same rate to eliminate the factor of viscoeleastic properties of the graft– bone junction or the graft proper in the determination of stiffness. Stiffness plays a role in the rigidity of the construct. Low-tensioned grafts may emulate the normal knee more accurately than hightensioned, rigidly fixed grafts. Higher-tensioned grafts may instead overconstrain the knee in situ, leading to graft failure or loss of motion,31,32 whereas a less rigidly fixed graft may allow the graft to optimally incorporate under physiologic forces. A major concern in ACL reconstructions is bone mineral density. Ultimate strength of a construct is directly related to bone mineral density.30 The samples used in this study were not the typical age of candidates who undergo ACL reconstruction. The average age of the specimens tested was 75.4 years; this is a limitation of our study. Dual-energy x-ray absorptiometry (DEXA) scans revealed lower bone mineral density in the metaphyseal bone of our specimens than in that of younger patients. No difference in bone density was seen between matched pairs. However, because direct matched pair analysis of the fixation methods was conducted, bone density becomes a negligible factor in the comparison. The proportionate strengths of the constructs are valid. However, this
FIGURE 4. Rigid fixation of the interference screw/autograft construction creates significantly more stiffness (N/mm) than is produced by the staple allograft construct on the femoral and tibial sides (P ⬍ .001 and P ⫽ .017, respectively).
1044
J. M. FARMER ET AL. TABLE 3.
Comparison of Ultimate Strength of Grafts When Loads Are Applied in Line With the Bone Tunnels Femur
Tibia
Fixation Method
Mode of Failure
Not reported 527 ⫾ 189 N 344 ⫾ 227 N Not reported Not reported
Gerich36
350 ⫾ 134 N 542 ⫾ 168 N 274 ⫾ 96 N 266 N 275 N 506–758 N
Our data
229 N
Interference screw STG Interference screw BPTB Postfixation BPTB Interference screw BPTB Transfemoral screw and tibial staple STG Interference screw BPTB Femoral screw and 2 tibial staples BPTB Interference screw
Not reported Fixation site Varied Tibial fixation site Fixation site (staple ⫽ screw) Bone–tunnel interface Fixation site Fixation site
Magen25 Paschal26(porcine) Randall27
588 N 233 N
does not reflect absolute strength. It is possible that tibial fixation strength of the autograft fixed with an interference screw is significantly greater than the tibial suture anchor group that was not appreciated in this cadaveric specimen group. Similar to other studies in which elderly donors or animal models were used,25,26,30-36 absolute strength of the constructs is lower than would be expected in younger patients. Despite the osteoporotic nature of cadaveric bone, the mode of failure for the tibial allograft was the suture– tendon interface, rather than the bone–anchor interface. We investigated only initial ultimate strength; cyclic loading of the constructs was not tested, and this is a limitation of our study. Cyclic loading is thought to simulate early rehabilitation and ambulation stresses experienced by the graft. The purpose of this study was to establish the maximum load to failure time zero. It was believed that cyclic loading would add confusion to that ultimate purpose. CONCLUSIONS Soft tissue Achilles allograft with staple fixation of the femur is a significantly weaker fixation method when compared with autograft BPTB with interference screw fixation in a cadaveric model.
TABLE 4. Staple Mitek G I Mitek G II Statak LCR 90 Ogden anchor
Comparison of the Ultimate Strength of Various Suture Anchors 180 N (canine humerus) 74.4 N (human tibia) 80.9 N (human tibia) 74.7 N (human tibia) 58.7 N (human femur) 119.6 N (human tibia)
Johnson* Carpenter28 Carpenter28 Carpenter28 Our data Our data
*Data from Johnson LL, Arnoczky SP. Michigan State University.
Acknowledgment: The authors thank Arthrex, Instrument Makar/Smith & Nephew, Orthofix, and the Musculoskeletal Transplant Foundation for donating materials and supplies for this study, and Annemarie Johnson, CMI, for Figure 2.
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