Biomechanical evaluation of different anterior cruciate ligament ...

J Orthop Sci (2010) 15:125–131 DOI 10.1007/s00776-009-1417-9

Original article Biomechanical evaluation of different anterior cruciate ligament fixation techniques for hamstring graft E. MONACO, L. LABIANCA, A. SPERANZA, A.M. AGRÒ, G. CAMILLIERI, C. D’ARRIGO, and A. FERRETTI Orthopaedic Unit, “Kirk Kilgour” Sports Injury Center, Sant’Andrea Hospital, University “La Sapienza,” Via di Grottarossa 1035, Rome 00100, Italy

Abstract Background. A number of anterior cruciate ligament (ACL) fixation techniques are currently in use. Slippage or failure of the graft by excessive loading or aggressive rehabilitation may result in an unstable knee. Load and slippage of the ACL graft varies according to the fixation technique used. Methods. Graft slippage, load to failure, and stiffness were evaluated using an animal model. Six soft tissue ACL fixation techniques and bone cement as a fixation device were tested: group A, Endo Button CL-Bio RCI; group B, Swing Bridge-Evolgate; group C, Rigidfix-Intrafix; group D, Bone Mulch-Washer Lock; group E, Transfix-Retroscrew; group F, Transfix-Deltascrew; group G, Kryptonite bone cement. Maximum failure load, stiffness, and slippage at the 1st and 1000th cycles and mode of failure were evaluated. Results. The maximum failure load was significantly higher in group B (1030 N) and significantly lower in group E (483 N) than in the others. The stiffness of group B (270 N/mm) was significantly higher than the others. As for the mode of failure, group C showed failure in the femoral side in all tests (four device ruptures and two tendon ruptures on the femoral side). All failures of the other groups occurred on the tibial side except one test in group A. All failures in group G were due to slippage of the tendons. Conclusion. Load to failure and stiffness was significantly different between the ACL fixation techniques. All but one of the fixation techniques showed sufficient properties for adequate postoperative rehabilitation. Bone cement used as a fixation device in soft tissue grafts did not seem to provide adequate initial fixation suitable for early rehabilitation after ACL reconstruction.

Introduction Central-third bone–patellar tendon–bone and hamstring tendon autografts are commonly used as substiOffprint requests to: E. Monaco, Via D. Di Buoninsegna 22, Rome 00142, Italy Received: May 3, 2009 / Accepted: September 22, 2009

tutes for the anterior cruciate ligament (ACL).1 These autografts are fixed to the femur and tibia using various fixation devices. During the last two decades, the use of hamstring tendon grafts for ACL reconstruction has increased. Successful restoration of ACL function using soft tissue grafts requires rigid fixation with sufficient stiffness to withstand the repetitive loading forces that occur during routine activities of daily living and early postoperative rehabilitation to allow the graft to heal via biological integration into the native bone. Rehabilitation protocols after ACL reconstruction are usually aggressive; these forces have been estimated to range from 67 to 454 N depending on the activities involved.2 A stable mechanical environment is required to withstand this early loading before full healing occurs and is necessary for graft maturation, incorporation, and healing. Although several studies have been published about the biomechanics of hamstring fixation devices on the femur and tibia,3–9 there is a paucity of literature about the biomechanics of the complete femur–graft–tibia complex. The few previous biomechanical studies dealt only with the tensile properties of the femur–graft– tibia complex after ACL reconstruction without cyclic loading.3–9 The aim of this study was to evaluate the fixation properties of six femoral and tibial techniques and the role of a bone cement (Kryptonite Bone Cement; Doctors Research Group) as a fixation device for hamstring grafts by measuring slippage, load to failure, and stiffness. The hypothesis of the study was that the fixation complexes tested and the bone cement have sufficient biomechanical properties to resist loads during the initial postoperative rehabilitation (mechanical fixation of an ACL graft). The bone cement was tested because of its bone-like mechanical properties that allowed fixation of the tendons directly to the bone without any fixation device.

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The peak load was chosen to represent ACL forces during normal walking activity.10,11 Structural properties such as stiffness and slippage were included in our study in addition to the pull-out strength because they may affect the ability of a ligament replacement to restore and ensure stability of the reconstructed knee, especially during intensive rehabilitation.12 Methods A total of 42 fresh frozen porcine knees were used to study six commercially available fixation device combinations and one bone cement (Kryptonite). • EndoButton CL and BioRCI (Smith & Nephew, DonJoy, Carlsbad, CA, USA). EndoButton CL combines the proven EndoButton with a continuous loop of polyester tape for femoral fixation. The BioRCI is a bioabsorbable screw for soft tissue tibial fixation. • Swing Bridge and Evolgate (Citieffe, Bologna, Italy). The Swing Bridge has an eyelet, through which the tendons are directly looped, and a smooth metal half ring for cortical suspension fixation on the femoral side when the device is inserted and impacted using an out–in technique. The Evolgate is a tibial fixation device for soft tissues that is composed of three components, all made of a titanium alloy: a screw, a coil that is inserted inside the bone tunnel to reinforce the walls of the tunnel, and a washer for cortical fixation. • Rigidfix and Intrafix (Innovasive Devices; Mitek, Westwood, MA, USA). The RigidFix is a cross pin system for femoral fixation of the graft. The Intafix system consists of two components: the expansion sheath and the tapered screw. Both systems achieve fixation by placing the expansion sheath into the tibial tunnel between the soft tissue graft strands and then inserting the screw. • Bone Mulch and Washerlock (Arthrotek, Ontario, Canada). The Bone Mulch includes a two-part screw assembly that is introduced through the lateral femoral condyle. The two-part screw assembly includes an outer screw and an inner screw. The outer screw includes a threaded body and an outwardly extending nose portion whose diameter is reduced from the threaded body. The nose portion provides a mechanical fixation point by functioning as a post within the femoral tunnel. A hamstring graft is looped around this post, mechanically fixing one end of the graft. The Washerlock is composed of a screw having a head, neck, and shank and a washer disposed about the neck. The tendon is attached to the washer to ensure that the tendon is immobilized between the head of the screw and the wall of the bone hole

E. Monaco et al.: Evaluation of ACL graft fixation devices

• Transfix and Retroscrew (Arthrex, Naples, FL, USA). The TransFix technique involves insertion of a metal implant into the lateral condyle, strictly perpendicular to the axis of the femoral socket. The device passes between the two limbs of the graft. The Retroscrew is an interference screw that allows joint line fixation of soft tissue, inserting the screw with an out–in technique at the tibial side. • Transfix and Delta-screw (Arthrex, Naples, FL, USA). The Delta-screw is a tapered bio-interference bioabsorbable screw for soft tissue graft fixation. • Kryptonite (Kryptonite Bone Cement; Doctors Research Group, Southbury, CT, USA) Kryptonite Bone Cement, which is self-setting, is formed by combining and mixing three components, resulting in an exothermic polymeric reaction that forms a soft, pliable, dough-like mass and develops into a hard cement-like complex. The three components of Kryptonite Bone Cement are a liquid polyester polyol, a liquid prepolymer isocyanate mixture, and powdered calcium carbonate. It is proposed to be an osteoconductive calcified triglyceride with remarkable bone-like mechanical properties that is a nontoxic alternative to polymethylmethacrylate (PMMA). It is also suggested to be a bone void filler, an extremely adhesive cement, and an osteoconductive scaffold. In the present study, the Kryptonite was used as a substitute for an interference screw for graft fixation in an in vitro model of ACL reconstruction, resulting in a device-free fixation system. All the fixation methods used in the study are showed in Fig. 1. Six specimens were prepared for each fixation device, for a total of 42 femur–graft–tibia complexes. Animal tissues were obtained fresh from a local abattoir and then frozen at −25˚C until the day of the test, always within 2 weeks from the harvesting. Porcine knees were used in this study because they were readily available, inexpensive, and had been used in previous, similar studies.3,13,14 Construction of femur–graft–tibia complexes Soft tissue grafts for the reconstruction were obtained from bovine extensor digitorum communis tendons harvested from adult fresh-frozen bovine forelimbs. The tendons were stored at −25˚C and then thawed before use. They were kept moist until testing by being wrapped in tissue paper soaked with Ringer’s solution and stored in sealed polyethylene bags. The bifurcated tendon was divided into two halves, and the grafts were doubled and trimmed to achieve a two-stranded, 9 mm diameter graft. Skeletally mature porcine knee were cleared of all soft tissues, wrapped in saline-soaked gauze, and stored at −25˚C in sealed plastic bags until use. The


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Fig. 1. Fixation methods used in the study and the bone cement (Kryptonite)

ACL was resected from each knee and then reconstructed with the bovine graft and one of the fixation device combinations. A single orthopedic surgeon performed all 42 reconstructions. The order in which the techniques were performed was randomized. All surgical techniques were performed according to the recommendations of the manufacturer. The bone tunnels were placed in their usual surgical orientations in the femur and tibia using impaction drills that packed the bone debris into the tunnel walls rather than removing it. The diameter of the tunnel was 9 mm in all cases on both femoral and tibial sides. The diameter of the interference screws was the same of the bone tunnel diameter (9 mm) in all groups. The Kryptonite, prepared as suggested by the manufacturer, was pulled into the tunnel after the graft was inserted. Then the specimen was accurately stored at 4˚C for 12 h, maintaining the correct humidity, to be sure of the absolute hardening of the bone cement; it was then tested as for the other fixation devices. Biomechanical measurements Each reconstructed knee was mounted on a tensile machine (model Z010; Zwick-Roell, Ulm, Germany) and fixed with wires in a 50 mm diameter cylinder. The complex was mounted with specially designed grips at 45˚ of knee flexion, so the longitudinal axis of the graft coincided with the axis of the bone tunnels (Fig. 2).

Preconditioning was performed with 100 cycles of loading and unloading between the tensile loads of 10 N and 50 N at a crosshead speed of 50 mm/min. A tensile load of 90 N was then applied to the graft for 2 min as an initial graft tension, followed by 1000 cycles between 0 and 150 N with a crosshead speed of 250 mm/min and a frequency of 0.5 Hz before the final pullout. Data were recorded with Textexpert 8.1 software (Zwick-Roell) and evaluated with a load-displacement curve. Stiffness and strength were evaluated at the final pullout, as was the displacement (slippage) at cycles 1, 100, 500, and 1000. The mode of failure of each specimen was also recorded. Statistical analysis Statistical analysis was performed by statisticians of the Regional Agency of Public Health. A two-way repeated-measures analysis of variance (ANOVA) was performed, with fixation technique as one factor and cyclic load segment (cycles 1, 100, 500, and 1000) as a repeated factor. A one-way ANOVA was used to test for the effect of the ACL fixation technique on failure load. Student’s t-test was used to determine where statistical differences existed for graft slippage and failure load. Statistically significant difference was defined as P < 0.05. The STATA9 software package (Statistic Data Analysis, Statacorp, TX, USA) was used for all evaluations.

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femoral side occurred in two of six cases in the RigidfixIntrafix group. Tendon rupture on the tibial side occurred overall in 22 of the 36 complexes tested. Slippage at the tibial side occurred in two of six cases in the EndoButton-BioRCI group and in five of six cases in the Transfix-Deltascrew group. Slippage at the tibial and/or femoral side occurred in all six cases in the Kryptonite group.

Discussion

Fig. 2. Femur-graft–tibia complex mounted on the testing machine (model Z010; Zwick-Roell, Ulm, Germany)

Results Femur–graft–tibia complexes, constructed with seven fixation device combinations, were tested biomechanically with 1000 cycles of loading at 150 N. No differences between all fixation devices were found in slippage at the 1st, 100th, 500th, or 1000th cycle (P < 0.05). The group with Kryptonite fixation had significantly higher slippage that did the other groups (1.1 mm at the 1st cycle, 3.1 mm at the 100th cycle, 5.2 mm at the 500th cycle, and 7.8 mm at the 1000th cycle; P < 0.05) At the final pullout, we measured the ultimate failure load (UFL), and the load deformation curve stiffness was calculated. The UFL was significantly higher for the Swing Bridge-Evolgate combination (P < 0.05), and it was significantly lower for the Transfix-Retroscrew combination and Kryptonite fixation (P < 0.05), compared with all single comparisons with other devices. The stiffness of complexes constructed with the Swing Bridge-Evolgate device combination was significantly higher than any other value (P < 0.05). Results are summarized in Figs. 3 and 4 and Table 1. The modes of failure for each complex were classified as femoral device rupture, tendon rupture at the femoral or tibial side, and slippage at the tibial or femoral side. Femoral device rupture occurred in one of six cases in the Endo-Button-BioRCI group and in four of six cases in the Rigidfix-Intrafix group. Tendon rupture on the

The ideal fixation technique for soft tissue ACL reconstruction grafts remains controversial. The techniques currently used for tendon graft are varied and include the use of interference screws, suspension techniques, cortical techniques, anatomical fixation devices, or cross-pins. In an animal study by Rodeo et al.,15 it was shown that the primary mode of mechanical failure occurred at the soft tissue–bone interface for up to 8 weeks after reconstruction. Rehabilitation protocols after ACL reconstruction are usually aggressive, so it is essential to establish sufficient fixation strength with surgery that the graft can biologically incorporate with the native bone during the early rehabilitation protocol. We evaluated six coupled femoral and tibial devices designed for ACL reconstruction with soft tissue grafts. The devices tested included several that have a proven clinical track record but for which concerns have been raised. In addition, we biomechanically tested a bone cement (Kryptonite bone cement) used for soft tissue graft fixation. Within our protocol, we attempted to control for variables that might have affected the actual fixation with the different techniques. For example, the reconstructions were performed in a randomized manner. All tunnels and grafts were sized to 9 mm in the same manner. As in our previous studies,8,9 porcine knees were chosen because they have been previously used to evaluate ACL fixation devices and have shown bone density, morphology, and biomechanical characteristics similar to those of the human knee.16–21 Bovine extensor tendons grafts were used in this study because of their properties similar to hamstring grafts. We selected cyclic loading to model the postsurgical rehabilitation period when the ACL graft is subjected to repetitive loading, during which time the soft tissue is not yet biologically integrated into the bone. The present study shows that in all but two of the tested groups the mean strength (UFL) of the femur– graft–tibia complex was higher than the minimum required for an accelerated rehabilitation (500 N).6,22 Only in complexes assembled with the TransfixRetroscrew device combination and with the bone

N/mm (stiffness); N (failure load)


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1200

*

1000 800 600

#

400

*

200

#

0 Endo Button CL-Bio RCI

Swing BridgeEvolgate

RigidfixIntrafix

Bone Mulch- TransfixWasher Lock Retroscrew

TransfixDeltascrew

Kryptonite

Fixation Complex

mm

Failure Load

9 8 7 6 5 4 3 2 1 0

Stiffness

Fig. 3. Ultimate failure load (UFL) and stiffness of the complexes tested (mean values). *Significantly higher than the others (P < 0.05). # Significantly lower than the others (P < 0.05)

*

* Endo Button CL-Bio RCI

Swing BridgeEvolgate

RigidfixIntrafix

Bone MulchTransfixWasher Lock Retroscrew

TransfixDeltascrew

Kryptonite

Fixation Complex First cycle

1000th cycle

Fig. 4. Slippage at the 1st and 1000th cycles of the complexes tested (mean values). *Significantly higher than the others (P < 0.05)

Table 1. Slippage at the 1st and 1000th cycles: ultimate failure load and stiffness of the devices tested Fixation system EndoButton CL-BioRCl Swing Bridge-Evolgate Rigidfix-Intrafix Bone Mulch-Washerlock Transfix-Retroscrew Transfix-Deltascrew Kryptonite

Slippage at 1st cycle (mm)

Slippage at 1000th cycle (mm)

Ultimate failure load (N)

Stiffness (N/mm)

0.3 ± 0.1 0.6 ± 0.1 0.6 ± 0.4 0.3 ± 0.2 0.6 ± 0.4 0.2 ± 0.1 1.1 ± 0.4

1.6 ± 0.4 2.0 ± 0.4 2.6 ± 1.0 1.4 ± 0.5 2.4 ± 0.7 1.5 ± 0.5 7.8 ± 2.1

775 ± 220 1032 ± 301 766 ± 179 782 ± 83 483 ± 83 761 ± 167 219 ± 35

158 ± 63 270 ± 47 127 ± 54 119 ± 29 117 ± 15 144 ± 14 74 ± 12

Results are mean values

cement were the mean values (483 N and 219 N, respectively) inferior than the recommended value of 500 N. An interesting remark comes from the comparison with other studies that utilized a similar protocol.4–7,11 In fact, we found that the ultimate failure load in a “femur device–graft–tibial device” was always lower than that

obtained with a single device (tibial or femoral). We speculate that the best performance of a fixation device is achieved only if a firm point is available at the other end of the graft because testing a complete system could add biomechanical stress to the complex compared with a single device test. This phenomenon, if confirmed

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by further studies on complete constructs, should be considered when evaluating the structural properties of single fixation devices. Moreover, testing the femur device–graft–tibial device complex better reproduces the surgical procedure of ACL reconstruction. Regarding the mode of failure, our study shows that some constructs have an easily identifiable and reproducible weak point. It is well known, for example, that the loop of the EndoButton fails under a stress of more than 1000 N. In our series, the loop of the EndoButton failed in the specimens in which the tibial device resisted up to 1127 N. Therefore, should the EndoButton be used with a strong tibial fixation device, it could actually become the weak point of the construct. A similar feature was found with the Rigidfix-Intrafix device combination, where failure occurred at the femoral side due to structural and biomechanical properties of the Rigidfix compared with the stronger Intrafix. In TransfixRetroscrew complexes, failure was almost always due to slippage of the graft at the tibial side, probably because of the scanty length of the Retroscrew.22 In other constructs, the site of failure was variable, depending probably on the modality of insertion of the device and structural properties of the graft. Evaluation of the failure mode in the Kryptonite group showed, in all cases, progressive slippage of the graft inside the tunnel due to poor adhesion of the cement to the graft, although the cement was well integrated in the porous bone surrounding the tunnel. The Kryptonite, which has macroscopic trabecular aspects similar to those of bone and good mechanical properties itself, can be well used as a bone substitute for bone loss during orthopedic surgery and especially in traumatology; however, it seems to be inadequate as an ACL fixation method. It must be highlighted that biomechanical testing provides only an estimated portrayal of construct properties, as they might appear immediately after surgery. Although cyclic testing increases the validity of these results,10 it does not provide insight as to the biological behavior of graft-tunnel healing after surgery that will ultimately determine the success or failure of the ACL reconstruction.23 Despite these limitations, we believe that these data provide useful information to the knee surgeon. The first limitation of this study is that we used animal tissues. When testing graft fixation techniques with an animal model, the species is chosen for low cost and availability. We preferred porcine specimens because they can be frozen immediately after harvesting and the age of the donors and the bone quality are more uniform than in specimens obtained from human donors 3,17 Although the use of young human cadaver tibias would be the ideal material for testing tibial-side ACL graft fixation, most human cadaver tibias are from elderly donors. An article by Pena et al.24 emphasized the influ-


ence of bone mineral density (BMD) on the in vitro ultimate failure loads in human cadaver specimens. The reduced BMD decreases the validity of soft tissue ACL graft fixation measurements. When older human cadaver bone tissue is used for biomechanical testing, ultimate load-at-failure results are generally underestimated compared to the in vivo situation of ligament reconstruction in the young patient.25 Although the use of animal specimens has been criticized3, the use of porcine specimens can help minimize specimen-related bias. In fact, porcine bone models are commonly used for biomechanical graft fixation tests. Bovine tendons were used because the stiffness and viscoelastic behavior are not significantly different from a human double-looped semitendinosus and gracilis graft.21 However, the intent of this study was to compare the relative differences between various ACL fixation techniques. Therefore, although the absolute values reported here for the fixation devices may be different from what would be found had human cadaveric specimens been used, it is thought that the relative differences between ACL fixation techniques found in this study would not differ had human bones used⎯thus the conclusions of this study are valid. The second limitation of this study is that we stretched the complex along the tunnel axis to apply cyclic displacement to the complex. Therefore, we could not obtain direct information on flexion-extension motion of the knee from this study. Moreover, the results of this ex vivo study reflect only the initial mechanical characteristics of the complex for the ACL reconstruction without any biological healing or remodelling responses. Hence, caution should be used when extrapolating the results of our study to clinical estimates as we cannot assume that the structural properties of fixation devices determined in animal tissue and laboratory studies predict its performance in human knees. On the other hand, this study was performed in the laboratory of our university by a team with substantial experience in biomechanics of ligaments and tendons.6,7

Conclusion Based on our results, for the patient with good bone density, five of the seven fixation systems tested should resist enough pressure to allow early intensive rehabilitation without risk of graft-tunnel fixation failure from excessive slippage. However, the choice of the best fixation device depends on a combination of factors: the preferred surgical technique; the sex, age, and sports expectations of the patient; and last but not least, the cost of the device. Moreover, on the basis of our experience, despite the opinion of most experts,26 in some cases femoral fixation is the weak point of the reconstruction.

E. Monaco et al.: Evaluation of ACL graft fixation devices All authors have no commercial affiliation as well as consultancy, stock ownership, or patent-licensing arrangements that could be considered to raise a conflict of interest regarding this article.

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