Comparing Tibial Fixation Methods for Hamstring Grafts
Abstract & Commentary
Synopsis: The type of fixation device used in an anterior cruciate ligament reconstruction determines the strength and stiffness of a tendon in a bone tunnel following implantation.
Source: Singhatat W, et al. How four weeks of implantation affect the strength and stiffness of a tendon graft in a bone tunnel. Am J Sports Med. 2002;30(4):506-513.
A tendon graft must heal to the bone tunnel in order to function as an anterior cruciate ligament. The present study evaluates the difference in strength and stiffness of a tendon graft fixed with either a bioresorbable interference screw or a WasherLoc device.
In 32 skeletally mature sheep, the long digital extensor tendon was transplanted into a 30-mm hole drilled into the tibial metaphysis. Fixation was performed with a bioabsorbable interference screw (Bio-Interference Screw, Arthrex Inc., Naples, Fla) in 16 sheep, and a WasherLoc (Arthrotek, Inc., Warsaw, Ind), a spiked washer compressed onto the tendon graft by a cortical screw, in the other 16 sheep. After 4 weeks of implantation, the animals were sacrificed and both limbs harvested. A tendon transplantation performed in the contralateral leg at the time of harvest was used as a control for the strength and stiffness of the complex at the time of implantation. For each fixation device, the strength and stiffness of the complex at the tendon-bone interface were determined by incrementally loading the specimens to failure while mounted on a materials testing machine (Instron 5566, Instron Corp., Canton, Mass).
At implantation, all of the fixation complexes failed by pulling the tendon out from the tunnel. After 4 weeks of implantation, all complexes with the bioresorbable screw and 6 of the 8 complexes (75%) fixed with the WasherLoc failed the same way. The remaining 2 WasherLoc complexes failed by rupture of the tendon outside the tunnel.
The strength (down 63%) and stiffness (down 40%) of the complex with the bioresorbable interference screw decreased significantly 4 weeks after implantation. With WasherLoc fixation, 4 weeks of implantation did not significantly change the strength of the complex; however, it significantly increased the stiffness of the complex by 136%. With the bioresorbable interference screw fixation, the strength and stiffness of the tendon-bone interface developed relatively slowly; whereas, development occurred relatively rapidly with WasherLoc fixation.
Comment by Brian J. Cole, MD, MBA, and Nina Shervin, MS
Although numerous studies measuring the strength and stiffness of various fixation methods have been conducted, the present study sought to evaluate the performance of the fixation complex during the first few weeks following implantation. The results of this study indicate that the strength and stiffness of the bioresorbable interference screw deteriorated after 4 weeks of implantation and that the development of the tendon-bone interface was slow. On the other hand, the WasherLoc fixation complex had either maintained or improved its strength and stiffness after implantation and resulted in a more rapid development of the tendon-bone interface.
Given the large discrepancy between 2 commonly used fixation devices, Singhatat and associates suggest that clinical studies be used to determine the effectiveness of fixation devices as opposed to in vitro studies in animal or human cadaveric bone. These findings should also be considered by a clinician when deciding whether to use aggressive rehabilitation after reconstruction of a knee with a torn anterior cruciate ligament. This study confirms the findings of other biomechanical studies that imply that the use of bioabsorbable screws used alone to fix soft tissue grafts in tibial tunnels may be biomechanically insufficient to tolerate aggressive rehabilitation protocols.
Dr. Cole is Assistant Professor, Orthopaedic Surgery, Rush Presbyterian Medical Center, Midwest Orthopaedics, Chicago, IL.