Screw Divergence in Femoral Tunnel Fixation
Abstract & Commentary
Synopsis: Interference screws used for ACL graft fixation have less divergence from the graft tunnel if inserted with the screwdriver placed up through the tibial tunnel rather than through the anterior medial portal. However, such divergence does not necessarily negatively affect graft pullout.
Source: Miller CM, et al. Arthroscopy. 2002;18(5):510-514.
Using fresh frozen human cadaveric knees, Miller and colleagues compared the angle of divergence between bioabsorable interference screws inserted into the femoral tunnel by a screwdriver placed in the anterior medial portal to those inserted with the screwdriver placed up through the tibial tunnel. More importantly, they also examined the effect of the degree of divergence on fixation strength of hamstring grafts.
The cadaveric knees used for the study were matched pairs so that variation of bone density from one cadaver to another was diminished. However, no mention was made as to whether the dominant vs. nondominant leg was distributed equally between the 2 fixation methods, rather Miller et al state "each knee was randomly assigned." Divergent angles were measured on radiographs with the shaft of the screwdriver reinserted after knee disarticulation and the path of the screwdriver shaft was compared with the tunnel path. To evaluate graft fixation, the mounted specimens were cycled at 2 Hz with an applied load between 50 and 250 newtons (which Miller et al state are forces similar to those seen by the ACL somewhere between walking and jogging) for a thousand cycles.
In both AP and lateral planes, screws inserted by a screwdriver placed through the anterior medial portal had significantly more divergence than screws inserted by screwdrivers placed up through the tibial tunnels. The greatest angle of divergence was seen in the lateral plane. Despite the variation in divergence, no significant difference was found between the 2 groups for 3 mm and 5 mm graft pullout.
Miller et al conclude that given the increased technical difficulties encountered with screw insertion through the tibial tunnel and the lack of significant differences in pullout strength, insertion through the anterior medial portal is reasonable.
Comment by Letha Y. Griffin, MD, PhD
It is reassuring to know that slight divergence in screw-tunnel angle (up to at least 19° on lateral view in this article) does not result in significantly greater differences with regards to 3 mm and 5 mm of pullout of hamstring grafts when cyclically loaded. Moreover, Miller et al point out that the angle of divergence in their study may be greater than one would expect in normal knees since in cadaveric bone with decreased bone mineral density, interference screws may be more likely to penetrate bone (diverge) rather than find their way up the femoral tunnel.
Unfortunately, Miller et al do not provide landmarks for the anterior medial portal they selected, nor do they define how they measured the knee angle (110°) at the time of screw insertion. In this reviewer’s experience, inferiorly enlarging the anterior medial portal and maximally flexing the knee provides for easier screw insertion into the femoral tunnel. Lastly, one has to emphasize that only one manufacturer’ s screw was used in this present study (Arthrex bioabsorbable interference screw). One wonders if all bioabsorbable screws act similarly.
Dr. Griffin, Adjunct and Clinical Faculty, Department of Kinesiology and Health, Georgia State University, Atlanta, GA, is Associate Editor of Sports Medicine Reports.
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