view

Surgical Technique

ABSTRACT

This study evaluated the stiffness and  yeld load of the “evolgate” (Citieffe, Bologna, Italy), a new device for tibial
fixation of doubled looped semitendinosus and gracilis tendons used in ACL reconstruction. These properties were
determined from load to failure tests of ten paired double looped bovine tendon grafts fixed to porcine tibias. The test
was performed using a servo hydraulic machine (MTS Bionix) at a strain rate of 540 mm/sec.. The mean stiffness of the
evolgate was 125 N/mm (SD 34) as the mean load to failure was 951 (SD 172). Although further studies are needed to
investigate other mechanical properties of this new device, as resistance to slippage and failure load under cyclic
loading, the results of this preliminary test seem to be very encouraging and lead us to continue our researches on this
field.

KEY WORDS: ACL, Biomechanics,  Knee, Semitendinosus, Tendons

INTRODUCTION

Since the first description of Galeazzi (1), ACL reconstruction with semitendinosus and gracilis has became very popular
in the past decade, as laboratory studies demonstrated that a combined four strand hamstring graft, tensioned and
correctly secured, is stronger and stiffer than a ten millimeter patellar tendon graft (2). However, at the time of
reconstruction, the weakest points in an anterior cruciate ligament construct are its points of fixation, especially on the
tibial side. Methods for hamstrings graft fixation to bone should be strong enough to avoid failure, stiff enough to restore
load-displacemnt response and secure enough to resist slippage under cyclic loading during the first 1 to 2 months,
before the conversion from mechanical to biologic fixation.
The purpose of this paper is to describe a new method for tibial fixation of DGST graft which can be used with any
femoral fixation device, and  to present the results of biomechanical tests conducted on animal tissue.

MATERIAL AND METHOD

view
 The «evolgate» is composed by three components all made in a titanium alloy: a spiral (3 cm in length, 9,5
mm in diameter) with a spike positioned at one extremity, a screw of the same length and a washer.
 

view
  Before the passage of tendons, the spiral is inserted into the tibial tunnel with a special impactor which also provides penetration of the spike in the tibial cortex.
 

view
After the tendons are passed into the bone tunnels and secured at the femoral side, the  four
ends of the tendons coming out from the tibial side are properly tensioned; the screw and the washer are then inserted
interfering with the tendons and the spiral, until the washer leans against the tibial cortex. The spike prevents rotation of
the spiral as the screw tightens.

view
 

BIOMECHANICAL TESTS.

The «evolgate» was tested to failure using fresh frozen animal tissue stored at –20°C.  Ten common
digital extensor tendons were harvested from bovine forelimbs. Bovine tendons were used for the graft because the
stiffness and viscoelastic behavior at higher initial stresses are not significantly different from a human double looped
semitendinosus and gracilis graft (3). The bifurcated tendon was divided into two halves. A double looped bovine tendon
graft was prepared by placing the two tendons side-by-side, folding them in half and thinning them until the graft
passed through an 8 mm diameter cylinder. A N°1 suture was used to sew 4 cm of both ends of each tendon using a
criss-crossing stitch. Porcine tibias were used in this study because they are readily available,  inexpensive and have
been used in previous similar studies (4). The porcine tibias were prepared by removing all soft tissues and by  drilling a
tibial tunnel that was 9,5 mm in diameter and 45 mm in length using a commercially available tibial guide for ACL
reconstruction. Structural test of the graft fixation method tibia complexes were administered using a materials testing
machine (MTS Bionix) at a strain rate of 540 mm/sec.
view
The tibia, securely fixed in a metal cylinder, was attached to the base of the testing machine using a custom designed fixture that allowed the tibial tunnel and graft to be loaded in alignment with the motion axis of the actuator. The tendons were wrapped around a rigid bar attached to the upper portion of the materials testing machine, pulled through the tibial tunnel, properly tensioned for 5 minutes with a 2 kg weights and fixed to the tibia using the “evolgate”. The distance from the bar to the arthicular surface of the tibia was kept at 5 cm. to replicate the length of the intraarticular portion of the graft (3 cm.) and the section within the femoral
tunnel. A load to failure test was performed to determine the stiffness and the yield load of the graft fixation
method tibia complex. Mode of failure was also recorded for each test.
The mean stiffness of the evolgate was 125 N/mm (SD 34) as the mean load to failure was 951 (SD 172).

RESULTS

The mean stiffness for the graft-evolgate-tibia complex was 125 N/mm (SD 34 = 17%) as the mean yeld load was   951 (SD 172 = 14%). In six cases failure occurred inside the fixation device; in one case a slippage of the tendon was observed and in one case the fixation device resisted until a fracture of the tibial plateau occurred.

DISCUSSION

Secure graft fixation is important to the success of ACL reconstruction. The goal of the graft fixation is to prevent stretching or failure at graft fixation sites allowing early motion and weight bearing without loss of stability. Any fixation method with poor biomechanical properties has the potential to compromise the clinical outcome, especially if an accelerated rehabilitation protocol is used in the early post operative period. Assuming that during daily activities and accelerated rehabilitation the loads in ACL should be about 20% of its failure capacity, it seems reasonable to consider that a fixation method should be as stiff as the normal ACL and function to loads of at least 500 Newton, if a reconstructed knee is to be intensively rehabilitated (5,6,). Very few tibial fixation devices have been biomechanically proven to be stiff and strong  enough to resist loads produced in the graft before definitive, biological fixation (7). The «evolgate», which is presented for the first time,  seems to respond well to this biomechanical requirements, both for strength and stiffness.
Another advantage of the «evolgate» is that it provides a more anatomic graft fixation near the original ACL insertion
site (aperture fixation) which is preferable as compared with the devices which fix the tendons outside the tibial tunnel
(suspended fixation) (8). Although the «evolgate» provides fixation of the graft deeply into the tibial tunnel, it is not
completely recessed inside the tunnel; therefore it should be considered as a prominent rather than a low profile
fixation device. By fixing the tendons deeply in the tibial tunnel, a secure fixation can be obtained even in cases the
tendons (especially the gracilis) are very short or are accidentally cut during stripping..
Caution should be used in extrapolating the results of our study to clinical estimates as  we cannot assume that the
structural properties of fixation devices determined in animal tissue predict its performance in human knees.
Interference screw fixation, for example, performed significantly worse in human tissue compared with animal tissue,
probably because the interference screw purchases only in cancellous bone, which could vary in density between tissue
sources (9,3). The metal spiral inside the tibial tunnel should avoid the loss of fixation strength related to the low density
of the cancellous bone of the proximal epiphysis of the human tibia. Although further studies are needed to investigate
other mechanical properties of this new device, as resistance to slippage and failure load under cyclic loading, the
results of this preliminary test seem to be very encouraging and lead us to continue our researches on this field.

REFERENCES
1.Galeazzi R. La ricostruzione del legamento crociato anteriore del ginocchio. Atti Accademia Medico Lombarda di Chirurgia. Milano, 1928.
2.Hamner D.L., Brown C.H., Steiner M.E., Hecker A.T. Hayes W. C. Hamstring tendon grafts for reconstruction of anterior cruciate ligament: Biomechanical evaluation of the use of multiple strands and tensioning techniques. Journa l Bone Joint Surgery 81A 549-557, 1999
3. Magen H.E., Hull M.L., Howell S.M., Comparison of Structural and mechanical properties of bovine and humane tendons. Proceedings of the third world congress of biomechancics, Sapporo Japan, August 1998, p.126
4.Liu S.H., Kabo, J.M., Osti L., Biomechanics of two types of bone-tendon-bone graft for ACL reconstruction. Journal Bone Joint Surgery 77B, 232-235, 1995
5.Beynnon B.D., Fleming B.C., Johson R.J. Anterior cruciate ligament strain behaviour during rehabilitation exercises in vivo. Am. J. Sports Med 23, 24-34, 1995
6.Frank C.B., Jackson D.W., The science of reconstruction of the anterior cruciate ligament. Journal Bone Joint Surgery 79A, 1556-1576, 1997
7.Magen H.E., Howell S.M., Hull M.L., Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am. J. Sports Med. 27, 35-43, 1999
8. Ishibashi Y., Rudy T.W., Livesay G.A., The effect of anterior cruciate ligament graft fixationsite at the tibia on knee stability: evaluation using a robotic testing system. Arthroscopy, 13, 177-182, 1997
9. Weller A., Windhagen H.J., Stahelin A.C., Hamstring tendon fixation using interfrence screw. A biomechanical study in calf tibial bone. Arthroscopy 14, 29-37,
1998.

Address for correspondence:

Fabio Conteduca, MD
Via Flaminia 1761
00188 Roma
Italy
e-mail: conte@conteduca.com

Andrea Ferretti, MD
Via Lidia, 73
00179 Rome
Italy
e-mail: aferretti@pronet.it .