- Email: [email protected]
Mechanical comparison of biodegradable femoral fixation devices for hamstring tendon graft – A biomechanical study in a porcine model
Clinical Biomechanics 24 (2009) 435–440
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Mechanical comparison of biodegradable femoral fixation devices for hamstring tendon graft – A biomechanical study in a porcine model Jia-Lin Wu a,b, Tsu-Te Yeh a, Hsain-Chung Shen a, Cheng-Kung Cheng b, Chian-Her Lee c,* a
Department of Orthopaedic Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan Orthopaedic Biomechanics Laboratory, Institute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan c Department of Orthopedics and Traumatology, Taipei Medical University Hospital, No. 252, Wu Hsing Street, Taipei 110, Taiwan b
a r t i c l e
i n f o
Article history: Received 28 October 2008 Accepted 13 February 2009
Keywords: Biomechanics Biodegradable device Hamstring graft Anterior cruciate ligament reconstruction
a b s t r a c t Background: Initial fixation strength is critical for the early post-operative rehabilitation of patients with anterior cruciate ligament reconstructions. However, even the best femoral fixation devices remain controversial. We compared the biomechanical characteristics of tendon grafts fixed by different biodegradable femoral fixation devices following anterior cruciate ligament reconstruction. Methods: The Bio-TransFix, Rigidfix, Bioscrew with EndoPearl augmentation and Bioscrew devices were used to fix porcine flexor digitorum profundus tendon grafts in 32 porcine femora. Displacement of each tendon graft was evaluated after cyclic loading testing. Stiffness, ultimate failure load and failure mode of these fixation devices were measured with load-to-failure testing. Findings: The displacement of the femur–graft–cement complex in response to cyclic loading was lower (P < 0.05) for the Bio-TransFix than the Rigidfix, Bioscrew with EndoPearl augmentation, and Bioscrew groups. The fixation stiffness values of the Rigidfix and the Bioscrew were significantly greater (P < 0.05) than that of the Bio-TransFix. The ultimate failure load was significantly greater for the BioTransFix and the Rigidfix than the Bioscrew with EndoPearl augmentation or the Bioscrew (P < 0.05). Interpretation: The Bio-TransFix provided the least graft displacement under cyclic loading. However, this device gave less stability. The Rigidfix device provided better stability and stiffness of the tendon graft among those fixation devices that showed no significant differences in graft displacement under cyclic loading. However, no single fixation device provided less displacement along with a larger failure load and stiffness in this study. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of hamstring tendons being the graft of choice for anterior cruciate ligament (ACL) reconstruction has grown in popularity and gained strong interest in recent years. It has been proved to possess biomechanical characteristics similar or superior to those of other grafts (Brown et al., 1993; Fu et al., 1999; Hamner et al., 1999). The hamstring tendon graft additionally has relative lower donor site mobility and can be able to restore its contractile capability after harvest for ACL reconstruction (Eriksson et al., 2001; Simonian et al., 1997; Yoshitsugu et al., 2006). However, hamstring tendon grafts require tendon-to-bone healing, which may lead to an extended time for graft incorporation (Blickenstaff et al., 1997; Grana et al., 1994). Thus, a much stronger and stiffer fixation should be chosen to prevent early graft slippage before biological healing has occurred. In the selection of fixation devices of the hamstring tendon grafts to the femur, several fixation meth* Corresponding author. E-mail address: [email protected] (C.-H. Lee). 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.02.003
ods have been used, including anatomic fixation with interference screws (Weiler et al., 1998a,b), extra-cortical suspension techniques with the use of buttons or a post, and transverse graft fixation techniques using cross-pin transfixation (Clark et al., 1998). Recently there has been a growing tendency toward the use of biodegradable material in graft fixation with the aim of avoiding problems that can arise from the use of metal hardware, such as the need for removal due to discomfort in the area of implantation, hardware removal during revision surgery (Abate et al., 1998; Kousa et al., 1995, 2001; Weiler et al., 2001), distortion of magnetic resonance imaging (MRI), decreased risk of graft laceration, and lack of corrosion (Brand et al., 2000). Furthermore, a number of studies have reported that the biodegradable interference screws also provide an initial fixation strength that is similar to metal interference screws (Abate et al., 1998; Kousa et al., 1995, 2001). Therefore, many biodegradable devices for soft tissue tendon fixation have been developed to facilitate initial graft fixation. The intra-articular biodegradable implant may induce inflammatory reactions of the synovium during degradation (Weiler et al., 1996, 2000). The femoral transfixation pin devices which place the pins across the femur
436
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
traversing the femoral tunnel may overcome this drawback. The Rigidfix (Mitek Products, Division of Ethicon, Inc., Westwood, MA, USA) device includes two absorbable cross-pins and penetrates the four strand hamstring graft within the femoral tunnel. The Bio-TransFix (Arthrex, Naples, FL, USA) is another choice which the graft wrapped 180° and tensioned around the pin. In studying in vitro biomechanical test according to anterior cruciate ligament graft fixation, researchers have suggested that the initial strength of hamstring tendon interference fit fixation may not allow for an accelerated post-operative rehabilitation (Aune et al., 1998; Magen et al., 1999; Weiler et al., 1998a). The studies of Nagarkatti and Harding et al. who stated that the addition of a bone disk or poly L-lactide ball substantially reduced slippage of a free tendon graft fixed with a biointerference screw (Harding et al., 2002; Nagarkatti et al., 2001). The EndoPearl (Linvatec, Largo, FL, USA), a biodegradable device with the effect of a secondary backup to augment the femoral interference screw fixation of hamstring tendon grafts has been proposed. The fixation strength after EndoPearl augmentation is increased by an interlocking effect between the graft and the tip of the interference screw (Weiler et al., 2001). Little is known about the comparative mechanical properties of these biodegradable femoral fixation devices. The efficacy of even the best fixation devices remains controversial. The study is to provide important information about the implant selection in anterior cruciate ligament reconstruction for orthopaedic surgeons. There were two specific aims of this study. The first was to use cyclic loading tests to compare the initial mechanical properties between different biodegradable femoral fixation devices. The second was to compare the ultimate failure load, stiffness and failure mode of these femoral fixation devices, using tensile load-to-failure tests. The hypothesis of the study was there would be no difference in the initial fixation strength between different femoral fixation devices using hamstring tendon graft.
2. Methods 2.1. Specimen preparation Thirty-two fresh hind limbs from adult hybrid Landrace–Yorkshire–Duroc pigs were obtained immediately after death from a local slaughterhouse. The femurs were harvested and stored at 20 °C until tested. They were then thawed at room temperature for 12 h before testing. All muscle, soft tissue, and cruciate ligaments were removed. Thirty-two fresh flexor digitorum profundus tendons from the limbs were selected for ACL tendon grafts. The mean size of each graft was 160 mm long and 4 mm in diameter. The single tendon graft was folded to achieve a two-strand graft 8 mm across and 80 mm long. Noyes et al. noted that a four strand semitendinosus-plus-gracilis tendon graft had a mean cross-sectional area of 43 mm2 (Noyes and Barber-Westin, 1996; Noyes et al., 1984). The cross-sectional area of the graft was about 50 mm2. The graft limbs were sutured together with No. 5 Ethibond sutures (Ethicon, Somerville, NJ, USA) with a whipstitch that promoted uniform load across each limb during testing. 2.2. Study groups Four groups of biodegradable graft fixation devices were investigated as above with eight test specimens in each test group. The fixation devices are shown in Fig. 1 and grouped as follows: Group A: Bio-TransFix; poly L-lactide, 5 50 mm (Arthrex). Group B: Rigidfix; poly lactic acid, 3.3 42 mm, two pins in each set (Mitek).
Fig. 1. Biodegradable implants tested: Group A, Bio-TransFix; Group B, Rigidfix; Group C, EndoPearl; Group D, Bioscrew.
Group C: Bioscrew and EndoPearl augmented; poly lactic acid, 9 35 mm augmented with a 9 mm diameter EndoPearl (Linvatec). Group D: Bioscrew alone; poly lactic acid, 9 35 mm (Linvatec).
2.3. ACL tendon graft fixation All the surgical techniques were carried out by one orthopaedic surgeon trained in sports medicine to ensure consistency. The femoral tunnel was drilled using a 10 mm coring reamer (Arthrex, Karisfeld, Germany) at the native ACL insertion area. The tubular cancellous bone was removed from the reamer for bone mineral density (BMD) measurement. This was measured by Archimedes’ principle (Beynnon and Amis, 1998). For the Bio-TransFix and Rigidfix groups, the techniques were performed according to the recommendations of the manufacturers. In the Bioscrew fixation group, a guided wire was used to prevent screw-graft divergence during screw insertion. The screw was placed between the graft and the femoral tunnel. In Group C, the EndoPearl was sutured over the top of the folded tendon graft; then the Bioscrew fixation procedures were the same as in Group D (Bioscrew alone). The Bioscrew was inserted using a torque screwdriver, and the maximal insertional torque was recorded. 2.4. Biomechanical testing Biomechanical tests were performed using a computer-controlled servo hydraulic material testing system (HT9120, Hung Ta, Taichung, Taiwan). The tensile loads were measured using a 1000 kgf load cell (EMB, Singapore) attached to the crosshead. The femoral shaft was placed in a circular plastic pipe (12 cm in diameter), which was then filled with cement to fix the free end of the tendon graft. The femur–graft–cement complex was attached to the load cell using a custom-designed fixture that allowed the graft to be loaded in vertical alignment with the motion axis of the actuator (Fig. 2). The tests were performed at room temperature, and the specimens were kept moist with saline spray during testing. The biomechanical testing protocol included both cyclic loading and load-to-failure tensile tests. A constant preload of 30 N was applied to the graft for 5 min, before performing the cyclic loading tests in the force control mode. The specimens were loaded cyclically from 30 N to 150 N at a frequency of 1 Hz, for up to 1000 cycles. These loads approximate the forces that have been measured in the ACL during terminal passive extension of the knee (Markolf et al., 1990). The loading frequency was within a physiological range of loading, assuming an average speed of one
437
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440 Table 1 Results of cyclic loading and load-to-failure tests. Group A Displacement (mm) Ultimate failure load (N) Stiffness (N/mm) BMD (g/mL) Insertion torque (Nm)
a
8.4 (0.72) 671.6 (128.7)b 63.9 (4.8)f 1.11 (0.6) NA
Group B
Group C
Group D
10.2 (1.3) 676.2 (133.7)c 81.3 (14.4)g 1.15 (0.4) NA
10.3 (2.4) 467.7 (41.5)d 77.6 (10.2) 1.17 (0.2) 1.62 (0.26)
8.9 (2.6) 471.5 (106.5)e 82.0 (16.6)h 1.14 (0.1) 1.57 (0.17)
Values expressed as mean (standard deviations). BMD: bone mineral density. a Significantly different (P < 0.05) from Groups B, C, and D. b,c Significantly different from d,e(P < 0.05). g,h Significantly different from f(P < 0.05).
Table 2 Failure modes after load-to-failure testing.
Fig. 2. All tensile loads were applied parallel to the longitudinal axis of the graft.
second per stride during walking. A loading frequency of 1 cycle per second (1 Hz) was chosen because this approximates the frequency of normal walking (Honl et al., 2002). Based on the recommendations of Beynnon and Amis (1998), we cycled each fixation technique for 1000 cycles. One thousand cycles are approximately a week of flexion–extension loading in an ACL reconstructed knee (Weiss and Paulos, 1999). When the test was finished, the displacement of the crosshead was recorded. The femur–graft–cement complex then underwent a tensile load-to-failure test at a rate of 150 mm/min. The ultimate failure load, stiffness and failure patterns were recorded. Stiffness was determined as the slope of the linear region of the load-deformation curve beyond the toe region.
Failure mode
Group A
Group B
Group C
Group D
Implant failure Graft pulling out of the femoral tunnel Graft failure
4 0 4
7 1 0
0 1 7
0 6 2
than that of three other fixation groups (P < 0.05). The ultimate failure load was significant higher in Bio-TransFix (mean 671.6 N, SD 128.7 N) and Rigidfix (mean 676.2 N, SD 133.7 N) groups than that of Bioscrew alone (mean 471.5 N, SD 106.5 N) and with EndoPearl (mean 467.7 N, SD 41.5 N) groups (P < 0.05). The stiffness was significant higher in Bioscrew alone (mean 82 N/mm, SD 16.6 N/ mm) and Rigidfix (mean 81.3 N/mm, SD 14.4 N/mm) groups than that of Bio-TransFix (mean 63.9 N/mm, SD 4.8 N/mm) group (P < 0.05). The tensile properties for each group of femur–graft–cement complexes obtained from cyclic loading and load-to-failure tests are shown in Table 1. The failure modes for transverse pin fixation devices were pin breakage (four specimens in the Bio-TransFix group and seven in the Rigidfix group), rupture of the graft (four specimens in the Bio-TransFix group) and the graft pulling out of the femoral tunnel (one specimen in the Rigidfix group). For the anatomic fixation devices, the failure modes were rupture of the graft (seven specimens in the Bioscrew with EndoPearl augmentation group and two specimens in the Bioscrew group) and the graft pulling out of the femoral tunnel (one specimen in the Bioscrew with EndoPearl augmentation group and six specimens in the Bioscrew group). The relations between the different femoral fixation devices in the study specimens and the failure mode are displayed in Table 2 and Fig. 3. There were no significant differences in bone mineral density of the distal femur between the four groups. There were no significant differences in insertion torque values needed between Groups C and D.
2.5. Statistics 4. Discussion Statistical evaluation was performed using SPSS version 10.0 for Windows (SPSS1, Chicago, IL, USA) and results are expressed as means ± standard deviations. The Kolmogorov–Smirnov test was used to test for normal distribution within the groups. Overall differences between the different test groups for BMD, displacement, ultimate failure load and stiffness were analyzed by one-way analysis of variance (ANOVA) with Bonferroni post-hoc tests for multiple comparisons; P < 0.05 was accepted as statistically significant. 3. Results The displacement of the femur–graft–cement complex was significantly lower in Bio-TransFix group (mean 8.4 mm, SD 0.72 mm)
In clinical practice, using biodegradable implants such as the Transfix Cross-pin, the Rigidfix, and the Bioscrew on the femoral side and a Bioscrew with screw posts on the tibial side for fixation of the hamstring tendon graft are common surgical techniques for ACL reconstruction (Ahmad et al., 2004; Chandratreya and Aldridge, 2004; Clark et al., 1998; Kousa et al., 2003). Different femoral fixation devices used for ACL reconstruction have differing mechanical advantages. The weakest link in soft-tissue grafts is the fixation of the graft to the bone, so it is essential that the initial mechanical characteristic of the fixation methods are optimal. The immediate stability of the reconstructed knee postoperatively depends on the initial graft fixation.
438
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
Fig. 3. Failure modes:(a) breakage of the Bio-TransFix device; (b) breakage of the Rigidfix cross-pins; (c) rupture of the Ethiboned sutures resulted in the graft pulling out of the femoral tunnel in the group treated with a Bioscrew fixation device combined with EndoPearl and (d) pulling out of the graft in the Bioscrew fixation group.
The current rehabilitation protocols after ACL reconstruction requires immediate full range motion, prompt return of neuromuscular functions and early weight bearing (Brand et al., 2000; Shelbourne and Nitz, 1990; Shelbourne and Gray, 1997). To withstand the forces generated by such rehabilitation, the secure fixation of the graft to the bone tunnel is the primary factor in limiting early rehabilitation. According to data of various biomechanical studies, the in situ forces of the graft are estimated between 30 N and 450 N depending on the activity (Brand et al., 2000; Lee et al., 2005). Therefore, the initial fixation strength of more than 450 N is integral to the success of ACL reconstruction. For simulating the post-operative loading, the reconstructed knee underwent cyclic loading, rather than single load-to-failure testing, as the former is now preferred for the biomechanical evaluation of ACL reconstruction techniques (Giurea et al., 1999; Höher et al., 2000; Nakano et al., 2000; Scheffler et al., 2002). The ACL graft under cyclic loading represents a more physiological model. Therefore, cyclic loadings between 30 N and 150 N in this study were adopted to simulate light post-operative activity, such as walking. All femoral tunnels were created to the same size but larger than the graft. To choose No. 5 Ethibond sutures over the No. 2 with a whipstitch to the graft limbs would increase the size and leverage difficulties of the construct to pass through the femoral tunnel similar to the graft. The single Bioscrew was chosen according to the typical screw size used in clinical practice. The results of our ultimate load are consistent with the report for interference
screw between 341 N and 530 N (Carborn et al., 1998). Therefore, the fixation method should be adaptable to repetitive loads on this magnitude during the post-operative rehabilitation period. Graft displacement is one cause of failed ACL reconstruction. In response to a cyclic loading test, the femur–graft–cement complex showed the displacement of the Bio-TransFix group (8.4 mm) was significantly less than the others. This difference may have been because the graft looped around the Bio-TransFix device leads to less slippage within the femoral tunnel after cyclic loads. The Rigidfix device consists of two cross-pins and passes through the fiber of the graft but does not loop around it. The graft may therefore show some slippage when subjected to repeated high tensile forces. Interference screw fixation of the soft tissue tendon graft shows similar slippage. The results of our study are consistent with those of Ahmad et al. (2004). They evaluated four different femoral soft tissue fixation devices for ACL reconstruction: the total graft slippage under cyclic loading was greater for Rigidfix and interference screws than with the Endobutton or Bio-TransFix devices. Clinically, low initial stiffness of the tendon graft may prevent it from healing to the bone tunnel. Relatively low forces may cause excessive motion of the graft in the tunnel and lead to subsequent knee laxity. In theory, the extra-cortical suspension device would not have high stiffness but high ultimate failure load. Anatomic and nearly isometric placement of the implant is needed for long-term graft survival. Outlet fixation such as the interference screw increases graft stiffness and helps reduce the ‘‘windshield
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
wiper effect”, improving graft to bone ingrowth. This may also reduce tunnel enlargement. The interference screw is the gold standard fixation device for ACL reconstruction using bone–patellar tendon–bone (BPTB) grafting. Biodegradable or titanium interference screw fixation devices are also common for femoral fixation of hamstring grafts. However, Aune et al. (1998) showed that interference screws used for ACL reconstruction with hamstring tendons have worse resistance to being pulled out and lower stiffness than BPTB grafts. Becker et al. (2001) compared TransFix (50 mm) fixation with a biodegradable interference screw (8 20 mm) for fixation of the porcine digital extensor tendon. The TransFix fixation device had greater stiffness and resistance to being pulled out than the biodegradable interference screw fixation. In our study, the stiffness found in the Bio-TransFix group was significantly lower than in the Rigidfix or Bioscrew groups, and this differed from the results of Becker et al. (2001). The interference screw chosen in our study (9 35 mm) were larger than that used by Becker et al., which may explain the different results. In our study, the Bio-TransFix and Rigidfix had significantly higher ultimate failure loads than the Bioscrew plus EndoPearl or Bioscrew alone (P < 0.05). The failure mode between the Bio-TransFix group and Rigidfix group differed. In the Bio-TransFix group, the rates of implant and graft failure were the same. However, the predominant form of failure in the Rigidfix group was implant failure. This may have been because of different compositions between these devices or the smaller diameter of the Rigidfix cross-pin than that of the Bio-TransFix. Giurea et al. (1999) reported on four devices for femoral anchorage of the hamstring tendon using a bovine model of ACL reconstruction. They found the ultimate failure load the stirrup (similar to the cross-pin) was significantly stronger than other anchorage devices (P < 0.001), including a clawed washer, a soft screw, and a round-headed screw. Ahmad et al. (2004) reported that a failure load for an interference screw (539 N) was lower than for the other techniques used (746 N for Bio-TransFix, 864 N for Endobutton, and 737 N for Rigidfix). However, Zantop et al. (2004) compared the initial fixation strength of the Rigidfix cross-pin and a biodegradable interference screw; they showed that there was no statistically significant difference between these approaches. They used a human hamstring graft and a bovine model. In our study, EndoPearl clearly enabled resistance to the tendon graft being pulled out of the femoral tunnel, because most failures occurred as ruptures at the graft–cement junction. However, there was no significant difference in the ultimate failure load in the Bioscrew–EndoPearl combined fixation group (467.7 N) compared with the Bioscrew fixation group (471.5 N). There was also no significant difference in stiffness between the Bioscrew–EndoPearl group (77.6 N/mm) and the Bioscrew group (82 N/mm). The biodegradable poly-L-lactide material EndoPearl has become available recently. This is placed proximally in the femoral tunnel and is attached to the leading end of the hamstring graft with non-absorbable sutures. The EndoPearl–Bioscrew combination decreases the probability of graft migration in the femoral tunnel and theoretically increases femoral fixation strength. Weiler et al. (2001) demonstrated that the combination of EndoPearl with a biodegradable interference screw in graft fixation showed a significantly greater maximum load to failure and stiffness than controls. In a clinical study, Arneja et al. (2004) demonstrated that using the EndoPearl in combination with a Bioscrew in the femoral tunnel for ACL reconstruction using hamstring grafts provided a significantly decreased laxity up to 18 months postoperatively, in terms of KT1000 side-to-side differences. This study had several limitations. The main difference between this study and real clinical practices was the use of a porcine knee model and the digital flexor tendon. As young human knees are
439
generally difficult to obtain for practice surgery, an animal model in this study was use to compare the initial mechanical characteristics of these four bioabsorbable femoral fixation devices. Although the values obtained for this type of model cannot be applied directly to humans, porcine models are often used to evaluate the mechanical characteristics of ACL fixation devices because they reproduce the similar size, shape and bone quality of the young human knee in satisfaction (Aerssens et al., 1998; Xerogeanes et al., 1998). The fresh flexor digitorum profundus tendons of porcine in this test were used as substitutes for human hamstring tendons because the characteristics of these tissues are very similar (Donahue et al., 2001; Miyata et al., 2000). Another limitation was the tensile force applied in line with the bone tunnel. This might not reflect the forces the graft is subjected to in vivo. If a device is tested in line with the tunnel, the worst-case scenario, the failure load may be less than if the device is loaded at an angle to the tunnel that will increase the shear forces. The rate which the graft is loaded will affect the stiffness because of viscoelastic properties of the graft-bone construct. Rehabilitation and ambulation stresses are examples of cyclic loading and are not accounted for with static testing at time zero fixation of the graft. Biomechanical analysis of the ACL and ACL-reconstructed knee has traditionally been evaluated in cadaveric studies. The results of these studies are extremely useful but limited without the information of their behavior in vivo. Therefore, it is very important in obtaining in vivo biomechanical information regarding ACL and ACL-reconstructed knees in the future. 5. Conclusion As for these four devices used in this study, no single one has the advantages of low displacement under cyclic loading and a higher stiffness and ultimate failure load under load-to-failure testing. The Bio-TransFix fixation device provided the least graft displacement under cyclic loading but less stability. Compared to the interference screw or Bio-TransFix fixation, the Rigidfix fixation device showed better stiffness of the tendon graft. References Abate, J.A., Fadale, P.D., Hulstyn, M.J., Walsh, W.R., 1998. Initial fixation strength of polylactic acid interference screws in anterior cruciate ligament reconstruction. Arthroscopy 14, 278–284. Aerssens, J., Boonen, S., Lowet, G., Dequeker, J., 1998. Interspecies differences in bone composition, density, and quality: potential implications for in vivo bone research. Endocrinology 139, 663–670. Ahmad, C.S., Gardner, T.R., Groh, M., Arnouk, J., Levine, W.N., 2004. Mechanical properties of soft tissue femoral fixation devices for anterior cruciate ligament reconstruction. Am. J. Sports Med. 32, 635–640. Arneja, S., Froese, W., MacDonald, P., 2004. Augmentation of femoral fixation in hamstring anterior cruciate ligament reconstruction with a bioabsorbable bead. Am. J. Sports Med. 32, 159–163. Aune, A.K., Ekeland, A., Cawley, P.W., 1998. Interference screw fixation of hamstring vs. patellar tendon grafts for anterior cruciate ligament reconstruction. Knee Surg. Sports Traumatol. Arthosc. 6, 99–102. Becker, R., Voigt, D., Starke, C., Heymann, M., Wilson, G.A., Nebelung, W.L., 2001. Biomechanical properties of quadruple tendon and patellar tendon femoral fixation techniques. Knee Surg. Sports Traumatol. Arthrosc. 9, 337–342. Beynnon, B.D., Amis, A.A., 1998. In vitro testing protocols for the cruciate ligaments and ligament reconstructions. Knee Surg. Sports Traumatol. Arthrosc. 6, 67–76. Blickenstaff, K.R., Grana, W.A., Egle, D., 1997. Analysis of a semitendinosus autografting a rabbit model. Am. J. Sports Med. 25, 554–559. Brand Jr., J., Weiler, A., Caborn, D.N., Brown Jr., C.H., Johnson, D.L., 2000. Graft fixation in cruciate ligament reconstruction. Am. J. Sports Med. 28, 761–774. Brown Jr., C.H., Steiner, M.D., Carson, E.W., 1993. The use of hamstring tendons for anterior cruciate ligament reconstruction. Technique and result. Clin. Sports Med. 12, 723–756. Carborn, D.N., Coen, M., Neef, R., Hamilton, D., Nyland, J., Johnson, D.L., 1998. Quadrupled semitendinosus-gracilis autograft fixation in the femoral tunnel: a comparison between a metal and a bioabsorbable interference screw. Arthroscopy 14, 241–245. Chandratreya, A.P., Aldridge, M.J., 2004. Top tips for RIGIDfix femoral fixation. Arthroscopy 20, 159–161.
440
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
Clark, R., Olsen, R.E., Larson, B.J., Goble, E.M., Farrer, R.P., 1998. Cross pin femoral fixation: a new technique for hamstring anterior cruciate ligament reconstruction of the knee. Arthroscopy 14, 258–267. Donahue, T.L., Gregersen, C., Hull, M.L., Howell, S.M., 2001. Comparison of viscoelastic, structural, and material properties of double-looped anterior cruciate ligament grafts made from bovine digital extensor and human hamstring tendons. J. Biomech. Eng. 123, 162–169. Eriksson, K., Hamberg, P., Jansson, E., Larsson, H., Shalabi, A., Wredmar, T., 2001. Semitendinosus muscle in anterior cruciate ligament surgery: morphology and function. Arthroscopy 17, 808–817. Fu, F.H., Bennett, C.H., Lattermann, C., Ma, C.B., 1999. Current trends in anterior cruciate ligament reconstruction. Part 1: biology and biomechanics of reconstruction. Am. J. Sports Med. 27, 821–830. Giurea, M., Zorilla, P., Amis, A.A., Aichroth, P., 1999. Comparative pull-out and cyclic-loading strength tests of anchorage of hamstring tendon grafts in anterior cruciate ligament reconstruction. Am. J. Sports Med. 27, 621–625. Grana, W.A., Egle, D.M., Mahnken, R., Goodhart, C.W., 1994. An analysis of autograft fixation after anterior cruciate ligament reconstruction in a rabbit model. Am. J. Sports Med. 22, 344–351. Hamner, D.L., Brown Jr., C.H., Steiner, M.E., Hecker, A.T., Hayes, W.C., 1999. Hamstring tendon grafts for reconstruction of the anterior cruciate ligament: biomechanical evaluation of the use of multiple strands and tensioning techniques. J. Bone Joint Surg. Am. 81, 549–557. Harding, N., Barber, F.A., Herbert, M.A., 2002. The effect of the EndoPearl on softtissue graft fixation. J. Knee Surg. 15, 150–154. Höher, J., Scheffler, S.U., Withrow, J.D., Livesay, G.A., Debski, R.E., Fu, F.H., Woo, S.L., 2000. Mechanical behavior of two hamstring graft constructs for reconstruction of the anterior cruciate ligament. J. Orthop. Res. 18, 456–461. Honl, M., Carrero, V., Hille, E., Schneider, E., Morlock, M.M., 2002. Bone–patellar tendon–bone grafts for anterior cruciate ligament reconstruction. An in vitro comparison of mechanical behavior under failure tensile loading and cyclic submaximal tensile loading. Am. J. Sports Med. 30, 549–557. Kousa, P., Jarvinen, T.L., Pohjonen, T., Kannus, P., Kotikoski, M., Jarvinen, M., 1995. Fixation strength of a biodegradable screw in anterior cruciate ligament reconstruction. J. Bone Joint Surg. Br. 77, 901–905. Kousa, P., Jarvinen, T.L., Kannus, P., Jarvinen, M., 2001. Initial fixation strength of bioabsorbable and titanium interference screws in anterior cruciate ligament reconstruction. Biomechanical evaluation by single cycle and cyclic loading. Am. J. Sports Med. 29, 420–425. Kousa, P., Jarvinen, T.L., Vihavainen, M., Kannus, P., Jarvinen, M., 2003. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am. J. Sports Med. 31, 174–181. Lee, C.H., Huang, G.S., Chao, K.H., Wu, S.S., Chen, Q., 2005. Differential pretensions of a flexer tendon graft for anterior cruciate ligament reconstruction: a biomechanical comparison on a pocine knee model. Arthroscopy 21, 540–546. Magen, H.E., Howell, S.M., Hull, M.L., 1999. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am. J. Sports Med. 27, 35–43. Markolf, K.L., Gorek, J.F., Kabo, M., Shapiro, M.S., 1990. Direct measurement of resultant forces in the anterior cruciate ligament. An in vitro study performed with a new experimental technique. J. Bone Joint Surg. Am. 72, 557–567. Miyata, K., Yasuda, K., Kondo, E., Nakano, H., Kimura, S., Hara, N., 2000. Biomechanical comparisons of anterior cruciate ligament: reconstruction procedures with flexor tendon graft. J. Orthop. Sci. 5, 585–592.
Nagarkatti, D.G., McKeon, B.P., Donahue, B.S., Fulkerson, J.P., 2001. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am. J. Sports Med. 29, 67–71. Nakano, H., Yasuda, K., Tohyama, H., Yamanaka, M., Wada, T., Kaneda, K., 2000. Interference screw fixation of doubled flexor tendon graft in anterior cruciate ligament reconstruction – biomechanical evaluation with cyclic elongation. Clin. Biomech. 15, 188–195. Noyes, F.R., Barber-Westin, S.D., 1996. Reconstruction of the anterior cruciate ligament with human allograft. Comparison of early and later results. J. Bone Joint Surg. 78A, 524–537. Noyes, F.R., Butler, D.L., Grood, E.S., Zwenicke, R.F., Hefzy, M.S., 1984. Biomechanical analysis of human ligament grafts used in knee-ligament repairs and reconstructions. J. Bone Joint Surg. 66A, 344–352. Scheffler, S.U., Sudkamp, N.P., Gockenjan, A., Hoffmann, R.F., Weiler, A., 2002. Biomechanical comparison of hamstring and patellar tendon graft anterior cruciate ligament reconstruction techniques: the impact of fixation level and fixation method under cyclic loading. Arthroscopy 18, 304–315. Shelbourne, K.D., Gray, T., 1997. Anterior cruciate ligament reconstruction with autogenous patellar tendon graft followed by accelerated rehabilitation. A twoto nine-year followup. Am. J. Sports Med. 25, 786–795. Shelbourne, K.D., Nitz, P., 1990. Accelerated rehabilitation after anterior cruciate ligament surgery. Am. J. Sports Med. 18, 292–299. Simonian, P.T., Harrison, S.D., Cooley, V.J., Escabedo, E.M., Deneka, D.A., Larson, R.V., 1997. Assessment of morbility of semitendinosus and gracilis tendon harvest for ACL reconstruction. Am. J. Knee Surg. 10, 54–59. Weiler, A., Helling, H.J., Kirch, U., Zirbes, T.K., Rehm, K.E., 1996. Foreign-body reaction and the course of osteolysis after polyglycolide implants for fracture fixation: experimental study in sheep. J. Bone Joint Surg. Br. 78, 369–376. Weiler, A., Hoffmann, R.F., Stahelin, A.C., Bali, H.J., Siepe, C.J., Sudkamp, N.P., 1998a. Hamstring tendon fixation using interference screws: a biomechanical study in calf tibia bone. Arthroscopy 14, 29–37. Weiler, A., Scheffler, S., Göckenjan, A., Südkamp, N.P., Hoffmann, R.F., 1998b. Different hamstring tendon graft fixation techniques under incremental cyclic loading conditions. Arthroscopy 14, 425–426. Weiler, A., Hoffmann, R.F., Siepe, C.J., Kolbeck, S.F., Südkamp, N.P., 2000. The influence of screw geometry on hamstring tendon interference fit fixation. Am. J. Sports Med. 28, 356–359. Weiler, A., Richter, M., Schmidmaier, G., Kandziora, F., Sudkamp, N.P., 2001. The EndoPearl device increases fixation strength and eliminates construct slippage of hamstring tendon grafts with interference screw fixation. Arthroscopy 17, 353–359. Weiss, JA., Paulos, LE., 1999. Mechanical testing of ligament fixation devices. Tech. Orthop. 14, 14–21. Xerogeanes, J.W., Fox, R.J., Takeda, Y., Kim, H.S., Ishibashi, Y., Carlin, G.J., 1998. A functional comparison of animal anterior cruciate ligament models to the human anterior cruciate ligament. Ann. Biomed. Eng. 26, 345–352. Yoshitsugu, T., Shinji, K., Tetsuya, M., Takeshi, H., Akira, Mi., 2006. Hamstring muscle function after tendon harvest for anterior cruciate ligament reconstruction. Am. J. Sports Med. 34, 281–288. Zantop, T., Weimann, A., Rümmler, M., Hassenpflug, J., Petersen, W., 2004. Initial fixation strength of two bioabsorbable pins for the fixation of hamstring grafts compared to interference screw fixation: single cycle and cyclic loading. Am. J. Sports Med. 32, 641–649.
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Mechanical comparison of biodegradable femoral fixation devices for hamstring tendon graft – A biomechanical study in a porcine model Jia-Lin Wu a,b, Tsu-Te Yeh a, Hsain-Chung Shen a, Cheng-Kung Cheng b, Chian-Her Lee c,* a
Department of Orthopaedic Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan Orthopaedic Biomechanics Laboratory, Institute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan c Department of Orthopedics and Traumatology, Taipei Medical University Hospital, No. 252, Wu Hsing Street, Taipei 110, Taiwan b
a r t i c l e
i n f o
Article history: Received 28 October 2008 Accepted 13 February 2009
Keywords: Biomechanics Biodegradable device Hamstring graft Anterior cruciate ligament reconstruction
a b s t r a c t Background: Initial fixation strength is critical for the early post-operative rehabilitation of patients with anterior cruciate ligament reconstructions. However, even the best femoral fixation devices remain controversial. We compared the biomechanical characteristics of tendon grafts fixed by different biodegradable femoral fixation devices following anterior cruciate ligament reconstruction. Methods: The Bio-TransFix, Rigidfix, Bioscrew with EndoPearl augmentation and Bioscrew devices were used to fix porcine flexor digitorum profundus tendon grafts in 32 porcine femora. Displacement of each tendon graft was evaluated after cyclic loading testing. Stiffness, ultimate failure load and failure mode of these fixation devices were measured with load-to-failure testing. Findings: The displacement of the femur–graft–cement complex in response to cyclic loading was lower (P < 0.05) for the Bio-TransFix than the Rigidfix, Bioscrew with EndoPearl augmentation, and Bioscrew groups. The fixation stiffness values of the Rigidfix and the Bioscrew were significantly greater (P < 0.05) than that of the Bio-TransFix. The ultimate failure load was significantly greater for the BioTransFix and the Rigidfix than the Bioscrew with EndoPearl augmentation or the Bioscrew (P < 0.05). Interpretation: The Bio-TransFix provided the least graft displacement under cyclic loading. However, this device gave less stability. The Rigidfix device provided better stability and stiffness of the tendon graft among those fixation devices that showed no significant differences in graft displacement under cyclic loading. However, no single fixation device provided less displacement along with a larger failure load and stiffness in this study. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of hamstring tendons being the graft of choice for anterior cruciate ligament (ACL) reconstruction has grown in popularity and gained strong interest in recent years. It has been proved to possess biomechanical characteristics similar or superior to those of other grafts (Brown et al., 1993; Fu et al., 1999; Hamner et al., 1999). The hamstring tendon graft additionally has relative lower donor site mobility and can be able to restore its contractile capability after harvest for ACL reconstruction (Eriksson et al., 2001; Simonian et al., 1997; Yoshitsugu et al., 2006). However, hamstring tendon grafts require tendon-to-bone healing, which may lead to an extended time for graft incorporation (Blickenstaff et al., 1997; Grana et al., 1994). Thus, a much stronger and stiffer fixation should be chosen to prevent early graft slippage before biological healing has occurred. In the selection of fixation devices of the hamstring tendon grafts to the femur, several fixation meth* Corresponding author. E-mail address: [email protected] (C.-H. Lee). 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.02.003
ods have been used, including anatomic fixation with interference screws (Weiler et al., 1998a,b), extra-cortical suspension techniques with the use of buttons or a post, and transverse graft fixation techniques using cross-pin transfixation (Clark et al., 1998). Recently there has been a growing tendency toward the use of biodegradable material in graft fixation with the aim of avoiding problems that can arise from the use of metal hardware, such as the need for removal due to discomfort in the area of implantation, hardware removal during revision surgery (Abate et al., 1998; Kousa et al., 1995, 2001; Weiler et al., 2001), distortion of magnetic resonance imaging (MRI), decreased risk of graft laceration, and lack of corrosion (Brand et al., 2000). Furthermore, a number of studies have reported that the biodegradable interference screws also provide an initial fixation strength that is similar to metal interference screws (Abate et al., 1998; Kousa et al., 1995, 2001). Therefore, many biodegradable devices for soft tissue tendon fixation have been developed to facilitate initial graft fixation. The intra-articular biodegradable implant may induce inflammatory reactions of the synovium during degradation (Weiler et al., 1996, 2000). The femoral transfixation pin devices which place the pins across the femur
436
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
traversing the femoral tunnel may overcome this drawback. The Rigidfix (Mitek Products, Division of Ethicon, Inc., Westwood, MA, USA) device includes two absorbable cross-pins and penetrates the four strand hamstring graft within the femoral tunnel. The Bio-TransFix (Arthrex, Naples, FL, USA) is another choice which the graft wrapped 180° and tensioned around the pin. In studying in vitro biomechanical test according to anterior cruciate ligament graft fixation, researchers have suggested that the initial strength of hamstring tendon interference fit fixation may not allow for an accelerated post-operative rehabilitation (Aune et al., 1998; Magen et al., 1999; Weiler et al., 1998a). The studies of Nagarkatti and Harding et al. who stated that the addition of a bone disk or poly L-lactide ball substantially reduced slippage of a free tendon graft fixed with a biointerference screw (Harding et al., 2002; Nagarkatti et al., 2001). The EndoPearl (Linvatec, Largo, FL, USA), a biodegradable device with the effect of a secondary backup to augment the femoral interference screw fixation of hamstring tendon grafts has been proposed. The fixation strength after EndoPearl augmentation is increased by an interlocking effect between the graft and the tip of the interference screw (Weiler et al., 2001). Little is known about the comparative mechanical properties of these biodegradable femoral fixation devices. The efficacy of even the best fixation devices remains controversial. The study is to provide important information about the implant selection in anterior cruciate ligament reconstruction for orthopaedic surgeons. There were two specific aims of this study. The first was to use cyclic loading tests to compare the initial mechanical properties between different biodegradable femoral fixation devices. The second was to compare the ultimate failure load, stiffness and failure mode of these femoral fixation devices, using tensile load-to-failure tests. The hypothesis of the study was there would be no difference in the initial fixation strength between different femoral fixation devices using hamstring tendon graft.
2. Methods 2.1. Specimen preparation Thirty-two fresh hind limbs from adult hybrid Landrace–Yorkshire–Duroc pigs were obtained immediately after death from a local slaughterhouse. The femurs were harvested and stored at 20 °C until tested. They were then thawed at room temperature for 12 h before testing. All muscle, soft tissue, and cruciate ligaments were removed. Thirty-two fresh flexor digitorum profundus tendons from the limbs were selected for ACL tendon grafts. The mean size of each graft was 160 mm long and 4 mm in diameter. The single tendon graft was folded to achieve a two-strand graft 8 mm across and 80 mm long. Noyes et al. noted that a four strand semitendinosus-plus-gracilis tendon graft had a mean cross-sectional area of 43 mm2 (Noyes and Barber-Westin, 1996; Noyes et al., 1984). The cross-sectional area of the graft was about 50 mm2. The graft limbs were sutured together with No. 5 Ethibond sutures (Ethicon, Somerville, NJ, USA) with a whipstitch that promoted uniform load across each limb during testing. 2.2. Study groups Four groups of biodegradable graft fixation devices were investigated as above with eight test specimens in each test group. The fixation devices are shown in Fig. 1 and grouped as follows: Group A: Bio-TransFix; poly L-lactide, 5 50 mm (Arthrex). Group B: Rigidfix; poly lactic acid, 3.3 42 mm, two pins in each set (Mitek).
Fig. 1. Biodegradable implants tested: Group A, Bio-TransFix; Group B, Rigidfix; Group C, EndoPearl; Group D, Bioscrew.
Group C: Bioscrew and EndoPearl augmented; poly lactic acid, 9 35 mm augmented with a 9 mm diameter EndoPearl (Linvatec). Group D: Bioscrew alone; poly lactic acid, 9 35 mm (Linvatec).
2.3. ACL tendon graft fixation All the surgical techniques were carried out by one orthopaedic surgeon trained in sports medicine to ensure consistency. The femoral tunnel was drilled using a 10 mm coring reamer (Arthrex, Karisfeld, Germany) at the native ACL insertion area. The tubular cancellous bone was removed from the reamer for bone mineral density (BMD) measurement. This was measured by Archimedes’ principle (Beynnon and Amis, 1998). For the Bio-TransFix and Rigidfix groups, the techniques were performed according to the recommendations of the manufacturers. In the Bioscrew fixation group, a guided wire was used to prevent screw-graft divergence during screw insertion. The screw was placed between the graft and the femoral tunnel. In Group C, the EndoPearl was sutured over the top of the folded tendon graft; then the Bioscrew fixation procedures were the same as in Group D (Bioscrew alone). The Bioscrew was inserted using a torque screwdriver, and the maximal insertional torque was recorded. 2.4. Biomechanical testing Biomechanical tests were performed using a computer-controlled servo hydraulic material testing system (HT9120, Hung Ta, Taichung, Taiwan). The tensile loads were measured using a 1000 kgf load cell (EMB, Singapore) attached to the crosshead. The femoral shaft was placed in a circular plastic pipe (12 cm in diameter), which was then filled with cement to fix the free end of the tendon graft. The femur–graft–cement complex was attached to the load cell using a custom-designed fixture that allowed the graft to be loaded in vertical alignment with the motion axis of the actuator (Fig. 2). The tests were performed at room temperature, and the specimens were kept moist with saline spray during testing. The biomechanical testing protocol included both cyclic loading and load-to-failure tensile tests. A constant preload of 30 N was applied to the graft for 5 min, before performing the cyclic loading tests in the force control mode. The specimens were loaded cyclically from 30 N to 150 N at a frequency of 1 Hz, for up to 1000 cycles. These loads approximate the forces that have been measured in the ACL during terminal passive extension of the knee (Markolf et al., 1990). The loading frequency was within a physiological range of loading, assuming an average speed of one
437
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440 Table 1 Results of cyclic loading and load-to-failure tests. Group A Displacement (mm) Ultimate failure load (N) Stiffness (N/mm) BMD (g/mL) Insertion torque (Nm)
a
8.4 (0.72) 671.6 (128.7)b 63.9 (4.8)f 1.11 (0.6) NA
Group B
Group C
Group D
10.2 (1.3) 676.2 (133.7)c 81.3 (14.4)g 1.15 (0.4) NA
10.3 (2.4) 467.7 (41.5)d 77.6 (10.2) 1.17 (0.2) 1.62 (0.26)
8.9 (2.6) 471.5 (106.5)e 82.0 (16.6)h 1.14 (0.1) 1.57 (0.17)
Values expressed as mean (standard deviations). BMD: bone mineral density. a Significantly different (P < 0.05) from Groups B, C, and D. b,c Significantly different from d,e(P < 0.05). g,h Significantly different from f(P < 0.05).
Table 2 Failure modes after load-to-failure testing.
Fig. 2. All tensile loads were applied parallel to the longitudinal axis of the graft.
second per stride during walking. A loading frequency of 1 cycle per second (1 Hz) was chosen because this approximates the frequency of normal walking (Honl et al., 2002). Based on the recommendations of Beynnon and Amis (1998), we cycled each fixation technique for 1000 cycles. One thousand cycles are approximately a week of flexion–extension loading in an ACL reconstructed knee (Weiss and Paulos, 1999). When the test was finished, the displacement of the crosshead was recorded. The femur–graft–cement complex then underwent a tensile load-to-failure test at a rate of 150 mm/min. The ultimate failure load, stiffness and failure patterns were recorded. Stiffness was determined as the slope of the linear region of the load-deformation curve beyond the toe region.
Failure mode
Group A
Group B
Group C
Group D
Implant failure Graft pulling out of the femoral tunnel Graft failure
4 0 4
7 1 0
0 1 7
0 6 2
than that of three other fixation groups (P < 0.05). The ultimate failure load was significant higher in Bio-TransFix (mean 671.6 N, SD 128.7 N) and Rigidfix (mean 676.2 N, SD 133.7 N) groups than that of Bioscrew alone (mean 471.5 N, SD 106.5 N) and with EndoPearl (mean 467.7 N, SD 41.5 N) groups (P < 0.05). The stiffness was significant higher in Bioscrew alone (mean 82 N/mm, SD 16.6 N/ mm) and Rigidfix (mean 81.3 N/mm, SD 14.4 N/mm) groups than that of Bio-TransFix (mean 63.9 N/mm, SD 4.8 N/mm) group (P < 0.05). The tensile properties for each group of femur–graft–cement complexes obtained from cyclic loading and load-to-failure tests are shown in Table 1. The failure modes for transverse pin fixation devices were pin breakage (four specimens in the Bio-TransFix group and seven in the Rigidfix group), rupture of the graft (four specimens in the Bio-TransFix group) and the graft pulling out of the femoral tunnel (one specimen in the Rigidfix group). For the anatomic fixation devices, the failure modes were rupture of the graft (seven specimens in the Bioscrew with EndoPearl augmentation group and two specimens in the Bioscrew group) and the graft pulling out of the femoral tunnel (one specimen in the Bioscrew with EndoPearl augmentation group and six specimens in the Bioscrew group). The relations between the different femoral fixation devices in the study specimens and the failure mode are displayed in Table 2 and Fig. 3. There were no significant differences in bone mineral density of the distal femur between the four groups. There were no significant differences in insertion torque values needed between Groups C and D.
2.5. Statistics 4. Discussion Statistical evaluation was performed using SPSS version 10.0 for Windows (SPSS1, Chicago, IL, USA) and results are expressed as means ± standard deviations. The Kolmogorov–Smirnov test was used to test for normal distribution within the groups. Overall differences between the different test groups for BMD, displacement, ultimate failure load and stiffness were analyzed by one-way analysis of variance (ANOVA) with Bonferroni post-hoc tests for multiple comparisons; P < 0.05 was accepted as statistically significant. 3. Results The displacement of the femur–graft–cement complex was significantly lower in Bio-TransFix group (mean 8.4 mm, SD 0.72 mm)
In clinical practice, using biodegradable implants such as the Transfix Cross-pin, the Rigidfix, and the Bioscrew on the femoral side and a Bioscrew with screw posts on the tibial side for fixation of the hamstring tendon graft are common surgical techniques for ACL reconstruction (Ahmad et al., 2004; Chandratreya and Aldridge, 2004; Clark et al., 1998; Kousa et al., 2003). Different femoral fixation devices used for ACL reconstruction have differing mechanical advantages. The weakest link in soft-tissue grafts is the fixation of the graft to the bone, so it is essential that the initial mechanical characteristic of the fixation methods are optimal. The immediate stability of the reconstructed knee postoperatively depends on the initial graft fixation.
438
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
Fig. 3. Failure modes:(a) breakage of the Bio-TransFix device; (b) breakage of the Rigidfix cross-pins; (c) rupture of the Ethiboned sutures resulted in the graft pulling out of the femoral tunnel in the group treated with a Bioscrew fixation device combined with EndoPearl and (d) pulling out of the graft in the Bioscrew fixation group.
The current rehabilitation protocols after ACL reconstruction requires immediate full range motion, prompt return of neuromuscular functions and early weight bearing (Brand et al., 2000; Shelbourne and Nitz, 1990; Shelbourne and Gray, 1997). To withstand the forces generated by such rehabilitation, the secure fixation of the graft to the bone tunnel is the primary factor in limiting early rehabilitation. According to data of various biomechanical studies, the in situ forces of the graft are estimated between 30 N and 450 N depending on the activity (Brand et al., 2000; Lee et al., 2005). Therefore, the initial fixation strength of more than 450 N is integral to the success of ACL reconstruction. For simulating the post-operative loading, the reconstructed knee underwent cyclic loading, rather than single load-to-failure testing, as the former is now preferred for the biomechanical evaluation of ACL reconstruction techniques (Giurea et al., 1999; Höher et al., 2000; Nakano et al., 2000; Scheffler et al., 2002). The ACL graft under cyclic loading represents a more physiological model. Therefore, cyclic loadings between 30 N and 150 N in this study were adopted to simulate light post-operative activity, such as walking. All femoral tunnels were created to the same size but larger than the graft. To choose No. 5 Ethibond sutures over the No. 2 with a whipstitch to the graft limbs would increase the size and leverage difficulties of the construct to pass through the femoral tunnel similar to the graft. The single Bioscrew was chosen according to the typical screw size used in clinical practice. The results of our ultimate load are consistent with the report for interference
screw between 341 N and 530 N (Carborn et al., 1998). Therefore, the fixation method should be adaptable to repetitive loads on this magnitude during the post-operative rehabilitation period. Graft displacement is one cause of failed ACL reconstruction. In response to a cyclic loading test, the femur–graft–cement complex showed the displacement of the Bio-TransFix group (8.4 mm) was significantly less than the others. This difference may have been because the graft looped around the Bio-TransFix device leads to less slippage within the femoral tunnel after cyclic loads. The Rigidfix device consists of two cross-pins and passes through the fiber of the graft but does not loop around it. The graft may therefore show some slippage when subjected to repeated high tensile forces. Interference screw fixation of the soft tissue tendon graft shows similar slippage. The results of our study are consistent with those of Ahmad et al. (2004). They evaluated four different femoral soft tissue fixation devices for ACL reconstruction: the total graft slippage under cyclic loading was greater for Rigidfix and interference screws than with the Endobutton or Bio-TransFix devices. Clinically, low initial stiffness of the tendon graft may prevent it from healing to the bone tunnel. Relatively low forces may cause excessive motion of the graft in the tunnel and lead to subsequent knee laxity. In theory, the extra-cortical suspension device would not have high stiffness but high ultimate failure load. Anatomic and nearly isometric placement of the implant is needed for long-term graft survival. Outlet fixation such as the interference screw increases graft stiffness and helps reduce the ‘‘windshield
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
wiper effect”, improving graft to bone ingrowth. This may also reduce tunnel enlargement. The interference screw is the gold standard fixation device for ACL reconstruction using bone–patellar tendon–bone (BPTB) grafting. Biodegradable or titanium interference screw fixation devices are also common for femoral fixation of hamstring grafts. However, Aune et al. (1998) showed that interference screws used for ACL reconstruction with hamstring tendons have worse resistance to being pulled out and lower stiffness than BPTB grafts. Becker et al. (2001) compared TransFix (50 mm) fixation with a biodegradable interference screw (8 20 mm) for fixation of the porcine digital extensor tendon. The TransFix fixation device had greater stiffness and resistance to being pulled out than the biodegradable interference screw fixation. In our study, the stiffness found in the Bio-TransFix group was significantly lower than in the Rigidfix or Bioscrew groups, and this differed from the results of Becker et al. (2001). The interference screw chosen in our study (9 35 mm) were larger than that used by Becker et al., which may explain the different results. In our study, the Bio-TransFix and Rigidfix had significantly higher ultimate failure loads than the Bioscrew plus EndoPearl or Bioscrew alone (P < 0.05). The failure mode between the Bio-TransFix group and Rigidfix group differed. In the Bio-TransFix group, the rates of implant and graft failure were the same. However, the predominant form of failure in the Rigidfix group was implant failure. This may have been because of different compositions between these devices or the smaller diameter of the Rigidfix cross-pin than that of the Bio-TransFix. Giurea et al. (1999) reported on four devices for femoral anchorage of the hamstring tendon using a bovine model of ACL reconstruction. They found the ultimate failure load the stirrup (similar to the cross-pin) was significantly stronger than other anchorage devices (P < 0.001), including a clawed washer, a soft screw, and a round-headed screw. Ahmad et al. (2004) reported that a failure load for an interference screw (539 N) was lower than for the other techniques used (746 N for Bio-TransFix, 864 N for Endobutton, and 737 N for Rigidfix). However, Zantop et al. (2004) compared the initial fixation strength of the Rigidfix cross-pin and a biodegradable interference screw; they showed that there was no statistically significant difference between these approaches. They used a human hamstring graft and a bovine model. In our study, EndoPearl clearly enabled resistance to the tendon graft being pulled out of the femoral tunnel, because most failures occurred as ruptures at the graft–cement junction. However, there was no significant difference in the ultimate failure load in the Bioscrew–EndoPearl combined fixation group (467.7 N) compared with the Bioscrew fixation group (471.5 N). There was also no significant difference in stiffness between the Bioscrew–EndoPearl group (77.6 N/mm) and the Bioscrew group (82 N/mm). The biodegradable poly-L-lactide material EndoPearl has become available recently. This is placed proximally in the femoral tunnel and is attached to the leading end of the hamstring graft with non-absorbable sutures. The EndoPearl–Bioscrew combination decreases the probability of graft migration in the femoral tunnel and theoretically increases femoral fixation strength. Weiler et al. (2001) demonstrated that the combination of EndoPearl with a biodegradable interference screw in graft fixation showed a significantly greater maximum load to failure and stiffness than controls. In a clinical study, Arneja et al. (2004) demonstrated that using the EndoPearl in combination with a Bioscrew in the femoral tunnel for ACL reconstruction using hamstring grafts provided a significantly decreased laxity up to 18 months postoperatively, in terms of KT1000 side-to-side differences. This study had several limitations. The main difference between this study and real clinical practices was the use of a porcine knee model and the digital flexor tendon. As young human knees are
439
generally difficult to obtain for practice surgery, an animal model in this study was use to compare the initial mechanical characteristics of these four bioabsorbable femoral fixation devices. Although the values obtained for this type of model cannot be applied directly to humans, porcine models are often used to evaluate the mechanical characteristics of ACL fixation devices because they reproduce the similar size, shape and bone quality of the young human knee in satisfaction (Aerssens et al., 1998; Xerogeanes et al., 1998). The fresh flexor digitorum profundus tendons of porcine in this test were used as substitutes for human hamstring tendons because the characteristics of these tissues are very similar (Donahue et al., 2001; Miyata et al., 2000). Another limitation was the tensile force applied in line with the bone tunnel. This might not reflect the forces the graft is subjected to in vivo. If a device is tested in line with the tunnel, the worst-case scenario, the failure load may be less than if the device is loaded at an angle to the tunnel that will increase the shear forces. The rate which the graft is loaded will affect the stiffness because of viscoelastic properties of the graft-bone construct. Rehabilitation and ambulation stresses are examples of cyclic loading and are not accounted for with static testing at time zero fixation of the graft. Biomechanical analysis of the ACL and ACL-reconstructed knee has traditionally been evaluated in cadaveric studies. The results of these studies are extremely useful but limited without the information of their behavior in vivo. Therefore, it is very important in obtaining in vivo biomechanical information regarding ACL and ACL-reconstructed knees in the future. 5. Conclusion As for these four devices used in this study, no single one has the advantages of low displacement under cyclic loading and a higher stiffness and ultimate failure load under load-to-failure testing. The Bio-TransFix fixation device provided the least graft displacement under cyclic loading but less stability. Compared to the interference screw or Bio-TransFix fixation, the Rigidfix fixation device showed better stiffness of the tendon graft. References Abate, J.A., Fadale, P.D., Hulstyn, M.J., Walsh, W.R., 1998. Initial fixation strength of polylactic acid interference screws in anterior cruciate ligament reconstruction. Arthroscopy 14, 278–284. Aerssens, J., Boonen, S., Lowet, G., Dequeker, J., 1998. Interspecies differences in bone composition, density, and quality: potential implications for in vivo bone research. Endocrinology 139, 663–670. Ahmad, C.S., Gardner, T.R., Groh, M., Arnouk, J., Levine, W.N., 2004. Mechanical properties of soft tissue femoral fixation devices for anterior cruciate ligament reconstruction. Am. J. Sports Med. 32, 635–640. Arneja, S., Froese, W., MacDonald, P., 2004. Augmentation of femoral fixation in hamstring anterior cruciate ligament reconstruction with a bioabsorbable bead. Am. J. Sports Med. 32, 159–163. Aune, A.K., Ekeland, A., Cawley, P.W., 1998. Interference screw fixation of hamstring vs. patellar tendon grafts for anterior cruciate ligament reconstruction. Knee Surg. Sports Traumatol. Arthosc. 6, 99–102. Becker, R., Voigt, D., Starke, C., Heymann, M., Wilson, G.A., Nebelung, W.L., 2001. Biomechanical properties of quadruple tendon and patellar tendon femoral fixation techniques. Knee Surg. Sports Traumatol. Arthrosc. 9, 337–342. Beynnon, B.D., Amis, A.A., 1998. In vitro testing protocols for the cruciate ligaments and ligament reconstructions. Knee Surg. Sports Traumatol. Arthrosc. 6, 67–76. Blickenstaff, K.R., Grana, W.A., Egle, D., 1997. Analysis of a semitendinosus autografting a rabbit model. Am. J. Sports Med. 25, 554–559. Brand Jr., J., Weiler, A., Caborn, D.N., Brown Jr., C.H., Johnson, D.L., 2000. Graft fixation in cruciate ligament reconstruction. Am. J. Sports Med. 28, 761–774. Brown Jr., C.H., Steiner, M.D., Carson, E.W., 1993. The use of hamstring tendons for anterior cruciate ligament reconstruction. Technique and result. Clin. Sports Med. 12, 723–756. Carborn, D.N., Coen, M., Neef, R., Hamilton, D., Nyland, J., Johnson, D.L., 1998. Quadrupled semitendinosus-gracilis autograft fixation in the femoral tunnel: a comparison between a metal and a bioabsorbable interference screw. Arthroscopy 14, 241–245. Chandratreya, A.P., Aldridge, M.J., 2004. Top tips for RIGIDfix femoral fixation. Arthroscopy 20, 159–161.
440
J.-L. Wu et al. / Clinical Biomechanics 24 (2009) 435–440
Clark, R., Olsen, R.E., Larson, B.J., Goble, E.M., Farrer, R.P., 1998. Cross pin femoral fixation: a new technique for hamstring anterior cruciate ligament reconstruction of the knee. Arthroscopy 14, 258–267. Donahue, T.L., Gregersen, C., Hull, M.L., Howell, S.M., 2001. Comparison of viscoelastic, structural, and material properties of double-looped anterior cruciate ligament grafts made from bovine digital extensor and human hamstring tendons. J. Biomech. Eng. 123, 162–169. Eriksson, K., Hamberg, P., Jansson, E., Larsson, H., Shalabi, A., Wredmar, T., 2001. Semitendinosus muscle in anterior cruciate ligament surgery: morphology and function. Arthroscopy 17, 808–817. Fu, F.H., Bennett, C.H., Lattermann, C., Ma, C.B., 1999. Current trends in anterior cruciate ligament reconstruction. Part 1: biology and biomechanics of reconstruction. Am. J. Sports Med. 27, 821–830. Giurea, M., Zorilla, P., Amis, A.A., Aichroth, P., 1999. Comparative pull-out and cyclic-loading strength tests of anchorage of hamstring tendon grafts in anterior cruciate ligament reconstruction. Am. J. Sports Med. 27, 621–625. Grana, W.A., Egle, D.M., Mahnken, R., Goodhart, C.W., 1994. An analysis of autograft fixation after anterior cruciate ligament reconstruction in a rabbit model. Am. J. Sports Med. 22, 344–351. Hamner, D.L., Brown Jr., C.H., Steiner, M.E., Hecker, A.T., Hayes, W.C., 1999. Hamstring tendon grafts for reconstruction of the anterior cruciate ligament: biomechanical evaluation of the use of multiple strands and tensioning techniques. J. Bone Joint Surg. Am. 81, 549–557. Harding, N., Barber, F.A., Herbert, M.A., 2002. The effect of the EndoPearl on softtissue graft fixation. J. Knee Surg. 15, 150–154. Höher, J., Scheffler, S.U., Withrow, J.D., Livesay, G.A., Debski, R.E., Fu, F.H., Woo, S.L., 2000. Mechanical behavior of two hamstring graft constructs for reconstruction of the anterior cruciate ligament. J. Orthop. Res. 18, 456–461. Honl, M., Carrero, V., Hille, E., Schneider, E., Morlock, M.M., 2002. Bone–patellar tendon–bone grafts for anterior cruciate ligament reconstruction. An in vitro comparison of mechanical behavior under failure tensile loading and cyclic submaximal tensile loading. Am. J. Sports Med. 30, 549–557. Kousa, P., Jarvinen, T.L., Pohjonen, T., Kannus, P., Kotikoski, M., Jarvinen, M., 1995. Fixation strength of a biodegradable screw in anterior cruciate ligament reconstruction. J. Bone Joint Surg. Br. 77, 901–905. Kousa, P., Jarvinen, T.L., Kannus, P., Jarvinen, M., 2001. Initial fixation strength of bioabsorbable and titanium interference screws in anterior cruciate ligament reconstruction. Biomechanical evaluation by single cycle and cyclic loading. Am. J. Sports Med. 29, 420–425. Kousa, P., Jarvinen, T.L., Vihavainen, M., Kannus, P., Jarvinen, M., 2003. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am. J. Sports Med. 31, 174–181. Lee, C.H., Huang, G.S., Chao, K.H., Wu, S.S., Chen, Q., 2005. Differential pretensions of a flexer tendon graft for anterior cruciate ligament reconstruction: a biomechanical comparison on a pocine knee model. Arthroscopy 21, 540–546. Magen, H.E., Howell, S.M., Hull, M.L., 1999. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am. J. Sports Med. 27, 35–43. Markolf, K.L., Gorek, J.F., Kabo, M., Shapiro, M.S., 1990. Direct measurement of resultant forces in the anterior cruciate ligament. An in vitro study performed with a new experimental technique. J. Bone Joint Surg. Am. 72, 557–567. Miyata, K., Yasuda, K., Kondo, E., Nakano, H., Kimura, S., Hara, N., 2000. Biomechanical comparisons of anterior cruciate ligament: reconstruction procedures with flexor tendon graft. J. Orthop. Sci. 5, 585–592.
Nagarkatti, D.G., McKeon, B.P., Donahue, B.S., Fulkerson, J.P., 2001. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am. J. Sports Med. 29, 67–71. Nakano, H., Yasuda, K., Tohyama, H., Yamanaka, M., Wada, T., Kaneda, K., 2000. Interference screw fixation of doubled flexor tendon graft in anterior cruciate ligament reconstruction – biomechanical evaluation with cyclic elongation. Clin. Biomech. 15, 188–195. Noyes, F.R., Barber-Westin, S.D., 1996. Reconstruction of the anterior cruciate ligament with human allograft. Comparison of early and later results. J. Bone Joint Surg. 78A, 524–537. Noyes, F.R., Butler, D.L., Grood, E.S., Zwenicke, R.F., Hefzy, M.S., 1984. Biomechanical analysis of human ligament grafts used in knee-ligament repairs and reconstructions. J. Bone Joint Surg. 66A, 344–352. Scheffler, S.U., Sudkamp, N.P., Gockenjan, A., Hoffmann, R.F., Weiler, A., 2002. Biomechanical comparison of hamstring and patellar tendon graft anterior cruciate ligament reconstruction techniques: the impact of fixation level and fixation method under cyclic loading. Arthroscopy 18, 304–315. Shelbourne, K.D., Gray, T., 1997. Anterior cruciate ligament reconstruction with autogenous patellar tendon graft followed by accelerated rehabilitation. A twoto nine-year followup. Am. J. Sports Med. 25, 786–795. Shelbourne, K.D., Nitz, P., 1990. Accelerated rehabilitation after anterior cruciate ligament surgery. Am. J. Sports Med. 18, 292–299. Simonian, P.T., Harrison, S.D., Cooley, V.J., Escabedo, E.M., Deneka, D.A., Larson, R.V., 1997. Assessment of morbility of semitendinosus and gracilis tendon harvest for ACL reconstruction. Am. J. Knee Surg. 10, 54–59. Weiler, A., Helling, H.J., Kirch, U., Zirbes, T.K., Rehm, K.E., 1996. Foreign-body reaction and the course of osteolysis after polyglycolide implants for fracture fixation: experimental study in sheep. J. Bone Joint Surg. Br. 78, 369–376. Weiler, A., Hoffmann, R.F., Stahelin, A.C., Bali, H.J., Siepe, C.J., Sudkamp, N.P., 1998a. Hamstring tendon fixation using interference screws: a biomechanical study in calf tibia bone. Arthroscopy 14, 29–37. Weiler, A., Scheffler, S., Göckenjan, A., Südkamp, N.P., Hoffmann, R.F., 1998b. Different hamstring tendon graft fixation techniques under incremental cyclic loading conditions. Arthroscopy 14, 425–426. Weiler, A., Hoffmann, R.F., Siepe, C.J., Kolbeck, S.F., Südkamp, N.P., 2000. The influence of screw geometry on hamstring tendon interference fit fixation. Am. J. Sports Med. 28, 356–359. Weiler, A., Richter, M., Schmidmaier, G., Kandziora, F., Sudkamp, N.P., 2001. The EndoPearl device increases fixation strength and eliminates construct slippage of hamstring tendon grafts with interference screw fixation. Arthroscopy 17, 353–359. Weiss, JA., Paulos, LE., 1999. Mechanical testing of ligament fixation devices. Tech. Orthop. 14, 14–21. Xerogeanes, J.W., Fox, R.J., Takeda, Y., Kim, H.S., Ishibashi, Y., Carlin, G.J., 1998. A functional comparison of animal anterior cruciate ligament models to the human anterior cruciate ligament. Ann. Biomed. Eng. 26, 345–352. Yoshitsugu, T., Shinji, K., Tetsuya, M., Takeshi, H., Akira, Mi., 2006. Hamstring muscle function after tendon harvest for anterior cruciate ligament reconstruction. Am. J. Sports Med. 34, 281–288. Zantop, T., Weimann, A., Rümmler, M., Hassenpflug, J., Petersen, W., 2004. Initial fixation strength of two bioabsorbable pins for the fixation of hamstring grafts compared to interference screw fixation: single cycle and cyclic loading. Am. J. Sports Med. 32, 641–649.