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Biomechanical analysis of two different tibial fixation methods for anterior cruciate ligament reconstruction using soft tissue graft: An experimental study in sheep knees.

The performance of tibial fixation methods of soft tissue graft (ACL) using interference screw and suture disc is studied in the paper. Tendoachillis graft was harvested from 16 fresh-frozen hind limbs from mature sheeps and double folded graft was used for fixation through a tibial tunnel. Two groups of mechanical test specimens of 8 each according to the fixation technique are made, as group A (suture disc) and group B (interference screw). The first 4 specimens were used for tensile test and the rest was used for cyclic fatigue test, with specimens undergoing a preconditioning procedure before actual testing. The study and test results shows that the mean yield load for graft fixed with interference screw is 246.5N and was comparable with the yield load for the graft fixed with suture disc which is 222.7 N. But the mean elongation for suture disc and interference screw is 17.71 and 12.19 respectively. So the interference screw fixation for soft tissue graft at tibial side has comparable pull out strength and stiffness as that of suture disc. (p > 0.05) The elongation for the graft fixed with the suture disc is more than that of the graft fixed with interference screw. This difference in elongation between suture disc group and interference screw group is statistically significant. (p=0.02, Unpaired T test) This indicates that the chance of residual laxity and slippage is more in suspensory fixation modality. Interference screw fixation for soft tissue graft has less elongation and residual laxity and could be an ideal fixation device for tibial fixation of soft tissue graft.

Introduction

Anterior Cruciate Ligament (ACL) tear has been the commonest ligament injury in the knee. ACL insufficiency has the greatest potential to cause both short term and long term disability. Most of the ACL deficient people are predisposed to subsequent degenerative changes of the knee joint [1]. These are due to the loss of the primary function of the ACL that is to prevent the anterior subluxation of the tibia. Surgical reconstruction of the ACL is a common practice to treat the "chronic instability" of knees due to ACL insufficiency. [1].

The type of graft that the surgeon chooses for ACL reconstruction has evolved over the past few decades. The two most commonly used grafts are the central third of the patellar ligament and the hamstring (gracilis and semitendinosus) tendons [2]. The method used to fix an ACL graft must be stiff enough to restore the load-displacement response (that is, stability) of the knee to normal, strong enough to avoid failure, and secure enough to resist slippage under cyclic loading during the first 1 to 2 months after reconstruction [5].

The outcome of anterior cruciate ligament (ACL) reconstruction depends on a great number of variables concerning mechanics, biology, surgical technique and rehabilitation. Among these factors, graft fixation seems to be a determinant of successful ACL reconstruction [3]. The popularity of hamstring soft tissue graft for ACL reconstruction has been increased in recent years. Direct and strong fixation of hamstring graft to bone is key to successful ACL reconstruction using soft tissue graft [4]. Also Recent studies [5,6,8,11] have demonstrated that the mechanical properties of doubled semitendinosus and gracilis (DSTG) tendon graft are higher than those of the normal ACL and patellar tendon (PT) graft.

Tibial fixation of the graft seems to be the weakest link in the graft-bone construct. In recent years, a great emphasis has been put on improving graft properties, attachment sites, fixation methods and capability to withstand better tension by the grafts to improve the performance of the ACL reconstruction in terms of side-to-side laxity difference [3]. Literature mentions different tibial fixation modalities for soft tissue graft [5]. These include mainly two modalities-one is distal or suspensory fixation while other is aperture fixation. In distal fixation graft is fixed to tibia with help of suture linkages between graft and the fixation implant that is fixed on the distal cortex (suture disc or staple). Aperture fixation uses interference screw that is placed inside the tunnel and at the anatomical position of the ACL insertion. This modality gives cotico-cancellous fixation.

Any fixation method with poor mechanical properties of stiffness, strength, or slippage has the potential to compromise the clinical outcome [6]. The mechanical and material properties of the reconstruction play an important role in the possible failure of the newly reconstructed ACL. [3] The most popular tibial fixation techniques use the suture disc and the interference screw with double folded tendon graft. But unfortunately orthopaedicians aren't sure of the mechanical advantages among either of these techniques. The results of the simulated mechanical tests are to be further compared with clinical observations.

The objective of this investigation was to study the residual laxity and slippage in suspensory modality of the tibial fixation methods of soft tissue graft (ACL) using suture disc and interference screw by comparing the results of tensile and fatigue tests.

Materials and Methods

Sixteen fresh-frozen hind limbs from mature sheeps weighing approximately 25kg were used in this study. Tendoachillis graft was harvested from each specimen with average length of 12 cm. Graft was prepared and ends of the graft were tied with no.5 ethibond suture with continuous locking stitch for 1.5 cm on each side using double folded graft for fixation. Figure 1 depicts the soft tissue graft (sheep tendoachilis) with sutured at the end.

[FIGURE 1 OMITTED]

The mean thickness of each double graft was 7 mm. Upper end tibia was dissected and removed of all soft tissues. A tunnel of same diameter that of tendon i.e. 7 mm was made in the direction of anatomical ACL with length of tibial tunnel 2.5 cm. Specimens were divided into two groups (8 for each group), according to the fixation technique, as group A and group B. For group A, stainless steel 316L suture disc and stainless steel 316L non-absorbable interference screw (7X20 mm) for group B was used. The specimens in both the groups A and B were further subdivided into subgroups of four specimens each. The first four specimens were used for tensile test and the next four were used for cyclic fatigue test. Interference screws were always placed in anatomic position. Suture wheel was tied on the cortical bone with help of tied sutures taken at the ends of tendon graft. In both groups 2 cm of graft was inside the tunnel.

[FIGURE 2 OMITTED]

The specimens were set on a Instron 6021 tensile & fatigue testing machine (fig2). This machine has a loading resolution of 10 N, which enabled the test specimens to be preconditioned at lower loads. The tibia was oriented along the axis of the tunnel and fixed to the inferior of the machine with help of transfixing steinmann pin in the shaft of tibia (fig3).

A specially designed clamps was used to avoid soft-tissue slippage or failure at the clamp-tendon interface. Two different series of tests were performed on samples: load to failure test and a cyclic loading test .The fixation complex was preconditioned before the actual testing to remove any wraping of the graft.

Figure 2 shows Instron testing machine with a special hook made for holding soft tissue graft loop proximally and distal metal frame to hold bone and stienmann pin assembly. Figure 3 showing graft bone construct mounted on the machine for testing. The pretensioning was done for a load range between 0 and 25N for 10 cycles at a crosshead speed of 50mm/min for over 10minutes. Tensile Test was carried out with an initial tensile load of 10N with strain rate of 200mm/min. till failure. For the fatigue specimens for 1000 cycles between the range 10N and 100N with strain rate of 200mm/min. Mean load and standard deviation ([+ or -] S.D.) were calculated for failure load (N), yield load (N) and deformation at the yield point(mm). Linear stiffness N/mm) was considered in the linear elastic region of the load / deformation curve. Then from the fatigue testing for 1000 cycles on elastic region, the graft elongation was determined as the difference measured in millimeters between length at the beginning of the test and after at 10N at 1000 cycles.

[FIGURE 3 OMITTED]

The data from both these test for the two devices were analysed using statistical software (SPSS 10.0., SPSS, Chicago, IL.)

Results and Statistical Analysis

Results of load-to-failure (on A1 specimen) series for specimen of group A--Suture disc fixation are as follows. Mean failure load = 232.33 [+ or -] 40.0 N, Mean yield load = 222.75 [+ or -] 42.3, Mean stiffness = 16.28 [+ or -] 3.47 N and Mean deformation = 17.71 [+ or -] 2.81 mm. The results of cyclic loading test (on A2 specimen); Mean Elongation = 17.71 [+ or -] 2.81 mm.

[FIGURE 4 OMITTED]

The results of load-to-failure (on B1 specimen) series for specimen of group B with Interference screw fixation are as follows. Mean failure load = 262.5 [+ or -] 83.54 N, Mean yield load = 246.55 [+ or -] 73.42 N, Mean stiffness = 25.37 [+ or -] 12.51 N and Mean deformation = 10.83 [+ or -] 3.11 mm. The results of cyclic loading test (on B2 specimen); Mean Elongation = 12.19 [+ or -] 2.58 mm. listed in Table 3 and Table 4 The load displacement plot for a typical tensile test specimen with tibia fixation for suture disc is shown in figure 4 and that with interference screw in figure 5. The interference screw elongation was 12mm whereas the elongation of suture disc fixation was 18mm, which shows around 50% variation between them.

[FIGURE 5 OMITTED]

The mean yield load for graft fixed with interference screw is comparable with the yield load for the graft fixed with suture disc. (p=0.5). The mean maximum load and mean stiffness for both the groups is also statistically comparable. (p=0.5, 0.1 respectively, Unpaired t test) This indicates that the stiffness, yield and load to failure for both fixation modalities are comparable when both constructs were tested for load to failure.

The elongation for the graft fixed with the suture disc is more than that of the graft fixed with interference screw. This difference between suture disc group and interference screw group is statistically significant p=0.02. (Unpaired t test). The increased elongation in the suture disc group to cyclic loading indicates that there is more chance of residual laxity of graft fixed with suture disc. Interference screw fixation reduces effective length of graft available for elongation. Aperture fixation makes the graft construct fixation more anatomic. The study of load displacement plots in figure 4 and figure 5 indicate a difference in performance of the fixations even though the overall ultimate failure strength is comparable.

Discussion

The popularity of hamstring soft tissue graft for ACL reconstruction has been increased in recent years. Initial reports showed inferior clinical outcomes but recent investigations found that with improvement in fixation modalities, the results of soft tissue graft has been comparable with conventional bone patellar graft [2]. However the patellar tendon is considered superior graft tissue because of tendency towards residual laxity among the knees reconstructed with soft tissue grafts.

Conventionally soft tissue grafts were fixed with suspensory type of fixation using staples, suture disc or screw posts. Recently, the interference screw with soft threads is also being used to fix soft tissue grafts [7]. The literature about this kind of construct (soft tissue graft--interference screw) still shows controversial opinions but it has been widely accepted that the biomechanical properties of such a kind of construct has adequate stability and pull out strength [8]. In our particular study we compared distal and aperture fixation modality for tibial fixation of soft tissue graft.

The study showed for both fixations, the mean failure load and the deformation for the tensile tests and elongation for the fatigue testing showed comparable results for mean values of maximum failure load, yield load and stiffness and comparable to results of previous studies [11]. The elongation of the graft during the cyclic loading was significantly high in suture disc device than in interference screw. This increased amount of elongation may allow increased translation resulting in a clinical failure with a positive Lachman, anterior drawer, and pivot shift, although the graft may remain structurally intact, but non-functional.

This indicates that the chance of residual laxity and slippage is more in suspensory fixation modality. The cause for increased elongation in suture disc group is probably related to the longer length of tendon which is available to stretching. Intra tunnel part of tendon is also available for lengthening. Other factor that also plays role in elongation and slippage of the graft was cinching of the graft by sutures under cyclic loading and adding to slippage. On the contrary in aperture fixation with interference screw, 2 cm of graft within the tunnel was compressed by screw and unavailable for stretching. Secondly as this graft does not depend on suture linkages for fixation and hence interference screw showed higher resistance to elongation.

It has been shown that the fixation level of an ACL graft has a significant influence on anterior knee stability and that anatomic graft fixation at the original ACL insertion site (aperture fixation) is the most preferable [9]. Post ACL reconstruction widening of tunnels is reported more commonly in ACL reconstruction done with hamstring graft. Even-though the exact cause is not clear-it is generally accepted that the micro-motion of the graft inside the tunnel triggers this phenomenon. This is more when graft is fixed with suspensory fixation methods like suture wheel, staple than anatomic interference screw. Anatomical fixation and more rigid tibial fixation methods may reduce this problem [10].

The limitation of this study included limited number of specimens, different mechanical properties of sheep bones and tendo-achilis graft compared with human specimens. Inspite of this, the present study carried out on an animal bone, it does have clinical relevance [11]. And moreover the material properties of the animal tendon are nearly similar to the human tendon [12].

Conclusion

The present study carried out on an animal bone, does have clinical relevance and the material properties of the animal tendon are nearly similar to the human tendon. The interference screw fixation for soft tissue graft at tibial side has comparable pull out strength and stiffness for the suspensory fixation modality for soft tissue graft. Interference screw fixation for soft tissue graft has less chance of elongation and residual laxity. Interference screw could be ideal fixation device for tibial fixation of soft tissue graft.

Acknowledgements

Authors are much grateful to Karunya University, Coimbatore for funding and motivation to carry out this work. We are extremely grateful to GKNM Hospitals, Coimbatore for their support to carry out the clinical procedures utilizing their facilities and Yarn testing lab, SITRA, Coimbatore for their support in utilizing their Instron test facility. We also extent our thanks to the following engineering undergraduate students Mr. Yogeshwaran S, Mr. Kumaran K, Mr. Prashanth S.D. and Mr. Samiksha Rai, for their help during the experiments.

Received 20 August 2007; published online 18 December 2007

References

[1.] Nebelung W, Wuschech H. (2005), Thirty-five years of follow-up of anterior cruciate ligament-deficient knees in highlevel athletes, Arthroscopy. Jun;21(6):696-702

[2.] Aglietti P, Giron F. et.al. (2004), Anterior cruciate ligament reconstruction: bone-patellar tendon-bone compared with double semitendinosus and gracilis tendon grafts. A prospective, randomized clinical trial, J Bone Joint Surg Am. Oct; 86-A (10): 2143-55.

[3.] E.Pefia, M.A.Martinez. et al. (2005), A finite element simulation of the effect of graft stiffness and graft tensioning in ACL reconstruction, Clinical Biomechanics, Jul; 20(6): 636-44

[4.] Steiner ME, Hecker AT. et.al. (1994), Anterior cruciate ligament graft fixation. Comparison of hamstring and patellar tendon grafts, Am J Sports Med. Mar-Apr; 22(2): 240-6; discussion 246-7.

[5.] Magen, H.E., Howell, (1999), Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts, Am. J. Sports Med. 27 (1), 35-43

[6.] Miyata, K., Yasuda et.al., (2000), Biomechanical comparisons of anterior cruciate ligament: reconstruction procedures with flexor tendon graft. J. Orthop. Sci.5 (6), 585-592.

[7.] Wagner M, Kaab MJ. (2005), Hamstring tendon versus patellar tendon anterior cruciate ligament reconstruction using biodegradable interference fit fixation: a prospective matched-group analysis, Am J Sports Med. Sep; 33(9): 1327-36.

[8.] Brown CH Jr, Steiner ME. et.al. (1993), The use of hamstring tendons for anterior cruciate ligament reconstruction. Technique and results, Clin Sports Med. Oct; 12(4): 723-56. Review.

[9.] Weiler A, Hoffmann RF et.al. (1998), Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone. Arthroscopy. Jan-Feb;14(1):29-37.

[10.] Fauno P, Kaalund S. (2005), Tunnel widening after hamstring anterior cruciate ligament reconstruction is influenced by the type of graft fixation used: a prospective randomized study, Arthroscopy. Nov; 21(11): 1337-41.

[11.] Carlo Fabbriciani, Pier Damiano Mulas et al., (2005), Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees, The Knee 12135-138

[12.] Toshiharu Kudo, Harukazu Tohyama et al., (2005), The effect of cyclic loading of the biomechanical characteristics of the FGT complex after ACL reconstruction using bone mulch screw/WasherLoc fixation, Clinical Biomechanics 20414-420.

D. Davidson Jebaseelan * ($), Shirish S. Pathak (#), A.R .Acharya *, Clement Joseph (#), David V Rajan (#)

* School of Mechanical Sciences, Karunya University, Karunya Nagar, Coimbatore 641 114

(#) Department of Orthopaedics, Sports Injury & Arthroscopy Clinic, GKNM Hospital, P.N. Palayam, Coimbatore 641 037

($) Corresponding author e-mail: davie@rediffmail.com
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Author:Jebaseelan, Davidson D.; Pathak, Shirish S.; Acharya, A.R.; Joseph, Clement; Rajan, David V.
Publication:Trends in Biomaterials and Artificial Organs
Geographic Code:9INDI
Date:Jan 1, 2008
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