Fixation Devices for Hamstring ACL Reconstruction

Abstracts & Commentary

Synopsis: Either metal or absorbable cross pins provided the strongest fixation on the femoral side. On the tibial side, the Intrafix screw and sheath was markedly stronger than any other device, while interference screws were the weakest.

Sources: Kousa P, et al. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: Femoral site. Am J Sports Med. 2003;31(2):174-181; Kousa P, et al. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: Tibial Site. Am J Sports Med. 2003;31(2):182-188.

Reconstruction of the ACL commonly employs hamstring autograft, known to be the strongest and stiffest graft in load-to-failure testing. However, the weak link has been fixation. The fixation methods are even more important because it takes at least 3 months before there is much tendon-to-bone healing. In fact, subtle differences in stability between hamstring and patellotendon grafts may have to do with differences in fixation.

Kousa and colleagues performed a series of mechanical tests to examine which fixation devices provided optimal holding ability for the hamstring graft. All of the most commonly used fixation devices were tested. This included on the femoral side the closed looped EndoButton (Acufex Microsurgical, Mansfield, Mass), the Bone Mulch Screw (Arthrotek, Inc., Warsaw, Ind), RigidFix (Mitek Products, Norwood, Mass), and 3 interference fit screws, including the BioScrew (Linvatec Inc., Largo, Fla), SmartScrew (Bionx Implants, Blue Bell, Pa), and the metal RCI screw (Acufex). On the tibial side devices tested included the WasherLoc (Arthrotek), a screw and spiked washer (Acufex), the IntraFix screw and sheath system (Mitek), and 3 interference fit screws—the BioScrew, the SmartScrew, and the metal SoftSilk interference fit screw (Acufex).

Each of the 6 devices were used to fix 10 quadruple human, semitendinosus-gracilis tendon grafts within tunnels drilled in porcine femurs and subsequently tibias. They were tested 10 times with a 1500-cycle loading test between 50-200 N at 1 cycle every 2 seconds. The specimens that survived the cyclic loading were then subjected to a single cycle load-to-failure test at a rate of 50 mm per minute.

On the femoral side, the Bone Mulch Screw (1112 N) was the strongest in the single cycle load-to-failure test, followed by the closed looped EndoButton (1086 N) and RigidFix (868 N). The interference fit screws were markedly worse in the 500-700 N range. On the tibial side, IntraFix was clearly both the strongest and the stiffest fixation device with 1332 N of strength on the load-to-failure test, followed by the WasherLoc (975 N).

Again, the interference fit screws were markedly worse with fixation in the 400-600 range. Another important finding was that the interference fit screws allowed more displacement on cyclic loading, upwards of 4 mm, while the IntraFix had the lowest residual displacement at 1.5 mm. All of the devices did show some displacement over the first 10-50 cycles of loading that seemed to level out thereafter. This prompted Kousa et al’s conclusion that preconditioning of the hamstring graft is important before tibial fixation is applied.

Comment by David R. Diduch, MS, MD

This is an excellent paper that provides much useful clinical information for several reasons. First, this uses all of the currently available and commonly used fixation methods. Furthermore, Kousa et al used very carefully controlled mechanical testing that looked at both single load-to-failure testing and, more importantly, cyclic loading. Their finding that there is a lot of creep in the hamstring graft construct emphasizes the need to preload the graft in tension as the knee is ranged multiple times before final fixation.

Any soft-tissue graft fixation falls into 2 broad categories: suspensory fixation, such as EndoBotton or screw and washer constructs, or aperture fixation, such as interference fit screws. Although aperture fixation may help prevent joint fluid from migrating into the tunnel, which could prevent clot formation and slow healing, it was notable to see the problems with creep and elongation with the interference fit screws. The screw with the best fixation, the SmartScrew, was the one with the largest diameter shank and the shortest threads. One would assume that fixation is primarily being performed by a larger bulk screw to press the graft against the walls of the tunnel. This concept is reinforced by the excellent outcome seen with the IntraFix, which is a screw within a sheath. A big advantage of this device is that the sheath separates each of the 4 hamstring bundles into a separate quadrant to independently tension and compress them against the tunnel margins. Bone plug fixation with interference screws is different in that deeper threads with a narrower diameter shank provide optimal fixation such that creep is not really a problem.

Suspensory fixation with either the EndoButton or the cross pins seems to yield less creep and stronger fixation. Although the Bone Mulch Screw was the strongest construct on the femoral side, one disadvantage is that the tunnel has to be wider than the graft to allow passing the graft up and over the top of the cross pin from below. The RigidFix pins can be used with a more tightly fit graft, which pierces them from the side, once the graft is pulled into place. This may enhance graft-to-bone contact and healing. Suspensory fixation with the EndoButton or a screw and washer construct on the tibia has been implicated in bungee cord effect or windshield wiper tunnel widening. None of these issues were studied by Kousa et al and may be important for a more loosely fitting graft.

In summary, it would appear that interference fit screws are not the best choice for hamstring graft or soft-tissue graft fixation. If one is to use the interference fit screw, one with a large shank diameter rather than deep threads would be best. Cross pins on the femur and IntraFix screw and sheath construct on the tibia provided optimal strength with minimal slippage.

Dr. Diduch is Associate Professor, Department of Othopaedic Surgery, University of Virginia School of Medicine, Charlottesville, VA.