By COL Patrick St. Pierre, MD
Although ACL reconstruction is commonly performed for symptomatic patients, graft selection, fixation method, initial graft tension, and speed of rehabilitation remain topics of debate. This update will review the current methods of ligament fixation and discuss the pros and cons of each technique.
Ideal graft fixation allows for anatomic graft placement with sufficient strength to allow accelerated rehabilitation during graft incorporation. Ideally, the fixation would enhance the incorporation of the graft to speed its development into becoming as close to the native ACL as possible. The strength required for activities of daily living is estimated to be 454 N based on the failure load of a normal ACL.1 Fixation techniques have been traditionally measured against this benchmark.
For ACL reconstruction using bone-patellar tendon-bone graft, the gold standard of fixation has been the interference fit screw. This technique is effective with the longest follow-up of current fixation methods. Load to failure has been reported as between 235 ± 124 N and 845.8 ± 188.5 N.2 Most studies have demonstrated values > 500 N and differences in testing techniques probably account for the variability. Screw length, graft-tunnel gapping, and screw divergence have been studied, and generally a 20 mm screw with < 20° divergence and minimal gap size is the acceptable standard.
The development of bioabsorbable materials has led to the use of absorbable screws made of aliphatic polyesters such as PGA and PLA. Using a bioabsorbable material precludes the need to remove the screw in the case of a revision and allows better MR imaging post-operatively. Biomechanical testing has shown similar fixation strength and stiffness when compared with titanium screws in both single load to failure and cyclic loading.3 However, as these screws are absorbed they may fragment or leave behind fibrous tissue rather than bone and this can create problems for revision.
Screws made with calcium phosphates have recently been introduced. Like bioabsorbable screws, they do not hinder revision or MR imaging. The Biocryl screw (Mitek Worldwide, Westwood, Mass) is made with a composite of resorbable L-PLA and the osteoconductive bioceramic, B-tricalcium phosphate (TCP). TCP has been shown to enhance vascular ingress and calcium deposition, without the formation of an intermediary connective tissue layer.4 This suggests that the screws should bond to surrounding bone and be replaced with bone rather than fibrous tissue as they dissolve.
Use of an interference screw technique in the femur can lead to complications such as graft laceration, posterior wall blowout, and screw-graft divergence. Errors in graft preparation may produce small bone plugs, or graft-tunnel mismatch that may leave insufficient bone within the tibial tunnel. Graft protectors and adequate visualization usually prevents inadvertent graft laceration. An Endobutton (Smith & Nephew, Andover, Mass) device may be used for insufficient bone plugs, graft tunnel mismatch, or posterior wall blowout. This suspensory type of fixation provides adequate strength for femoral tunnel fixation. However, because it does not provide aperture fixation it may allow "windshield-wiper" graft motion in the tunnel. The development of tunnel expansion has been shown, however a long-term deleterious effect has not been demonstrated.5
Another method of femoral fixation for bone-tendon-bone constructs is the RigidFix system (Mitek Worldwide, Westwood, Mass). This system uses bioabsorbable cross pins to secure the bone plug within the femoral tunnel. The advantage of this system is that it allows 100% circumferential healing, provides aperture fixation, and does not interfere with revision surgery or postoperative imaging by MRI. McKernan has shown the pullout strength to range between 600-800 N. This technique is also useful in cases of posterior wall blowout because it doesn’t rely on the posterior wall for fixation.
Fixation of soft-tissue grafts such as hamstring tendons or a free quadriceps tendon graft introduces other issues and questions concerning graft fixation. The Endobutton provides adequate fixation strength in an anatomic position, but again does not provide aperture fixation. There is concern that the more distal the fixation from the joint the greater the potential for graft stretch, motion, and tunnel widening. Transfixion pin systems such as the Trans-fix pin (Arthrex, Tampa, Fla) and the Bone-Mulch screw (Arthrotek, Inc, Ontario, Canada) provide for stronger fixation (> 500 N) somewhat closer to the joint. These devices are metal and may interfere with MRI but are usually proximal enough to allow evaluation of the joint. They may require removal for revision surgery. The Mitek RigidFix (Mitek Worldwide, Westwood, Mass) transfixes the graft with 2 absorbable pins much closer to the joint and provides the same advantages as it does for bone-tendon-bone grafts. Finally, soft-tissue interference screws have been produced with less aggressive threads to minimize damage to the grafts. These screws appear to provide adequate fixation strength in the femur, but some studies have questioned their reliability in the tibia.6
Tibial tunnel fixation remains the weakest link for soft-tissue graft reconstructions. Compaction of the tibial tunnel by serial dilation, rather than extraction drilling, may produce denser cancellous bone for stronger interference fit fixation. Another method of interference fixation is provided by the Intra-fix system by Mitek Worldwide. This system uses a sheath that separates the 4 tendon strands and provides uniform compression against the tunnel walls when the interference screw is placed in a moly-bolt fashion. Pullout strength of up to 700 N has been reported in company tests. More traditional methods include the use of sutures around a post (573 ± 109 N), fixation with a low-profile screw and soft tissue washer (821 ± 219 N), and belt-buckle double staples (705 ± 174 N). The strongest ultimate failure load (905 ± 291 N) reported is with the Washer Loc (Arthrotek, Biomet, Inc, Warsaw, Ind) washer plate device that is placed at the distal end of the tibial tunnel. These fixation methods do not provide aperture fixation and leave prominent hardware that may need to be removed if symptomatic.6
ACL reconstruction methods have developed rapidly over the past 20 years. Fixation devices that enhance tendon healing to bone and increase strength of fixation will continue to improve surgical methods to restore ACL stability.
Dr. St. Pierre is Assistant Professor, Uniformed Services University, Orthopedic Co-Director, Primary Care Sports Med Fellowship, DeWitt Army Community Hospital, Ft. Belvoir, Va.
References
1. Noyes FR, et al. J Bone Joint Surg Am. 1984;66-A:344-352.
2. Kurosaka M, et al. Am J Sports Med. 1987;15:225-229.
3. Weiler A, et al. Am J Sports Med. 1998;26:119-128.
4. Block JE, Thorn MR. Tissue International. 2000; 234-238.
5. L’Insalata JC, et al. Knee Surg Sports Traum Arthro. 1997;5:234-238.
6. Magen HE, et al. Am J Sports Med. 1999;27:35-43.
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