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Letters to the Editor |
Taos, New Mexico
Dear Editor:
We read with admiration the articles by Kousa et al. that were published in the March/April 2003 issue, "The Fixation Strength of Six Hamstring Tendon Graft Fixation Devices in Anterior Cruciate Ligament Reconstruction: Parts I and II" (pages 174–188). The discussion that the extraarticular fixation methods "generally provided better strength of fixation than did the interference screws" on the femoral side and the discussion regarding "lower yield loads" when interference screws are used on the tibial side is well supported by the data presented. However, a variable affecting interference screw fixation strength is the ratio of screw diameter to tunnel diameter. In this two-part study, screw diameter is fixed while tunnel diameter apparently varies, based upon the diameter of the tendon graft. Larger screw diameter could result in substantially increased interference screw fixation strength.
In addition, on the tibial side, this study evaluates proximal and "truly anatomic (apertural fixation)" as the method of interference screw fixation. While this method has a theoretical basis, the authors themselves claim to disprove the current theory. An alternative distal method of tibial-side interference screw fixation allows cortical purchase. Cortical purchase could also result in substantially increased interference screw fixation strength. The authors discuss that "no extra benefit" is achieved with increasing tibial interference screw length. However, increased screw length could allow cortical purchase while simultaneously approaching or achieving proximal and "truly anatomic (apertural fixation)."
The authors discuss that "the strength of interference screw fixation has been shown to be affected by many variables." Consideration of ratio of screw diameter relative to tunnel diameter and of tibial-side cortical purchase, or lack thereof, could diminish bias with regard to Methods and Discussion.
Finally, the authors discuss that location of the drill holes (as opposed to fixation strength) "is the most important factor determining success of ACL reconstruction." Discussion with regard to strength of fixation that is adequate to assure a stable knee in vivo could elucidate for readers clinical relevance of the in vitro results.
In summary, the authors must be complimented for a considerable contribution to our literature. Future investigation considering ratio of screw diameter to tunnel diameter or tibial-side cortical purchase, as well as the relationship between fixation strength and clinical outcome, is necessary before one can conclude that any one fixation device is superior.
Tampere, Finland
We appreciate the keen interest Drs. Lubowitz and Guttmann have shown in our articles. They bring up two variables that have previously been proposed as influencing (enhancing) the strength of interference screw fixation of a soft tissue graft, namely the ratio of screw diameter to tunnel diameter and the cortical purchase introduced by a longer interference screw. They also encourage us to enter into discussion with regard to strength of fixation that is adequate to assure a stable knee in vivo. Before proceeding into detailed discussion concerning these particular issues, we believe that a short comment on the overall situation regarding the ACL fixation research is warranted. First, we would like to point out that, traditionally, the orthopaedic research community, particularly researchers in the field of ACL fixation, have been very attracted to the Newton values (ultimate/maximum fixation strength). However, considering on one hand the fact that despite more than 20 years of intensive biomechanical research, our best delineations of the in vivo incident loading (in Newtons) on the ACL graft/fixation are composed of estimations that range between 50 and 500 N and, on the other hand, that an excellent long-term outcome has been reported (at least with the bone-patellar tendon-bone graft) for a fixation composed of sutures tied over a button (a fixation that provides <100 N strength in vitro),5 we venture to suggest that the most important (eligible) property of an ACL fixation might not necessarily be the highest Newton value in the pull-out test.
Further, whether we like it or not, we have to recognize that the field is very much influenced by the implant business, even to the extent that some might consider it to be "commercially driven." Instead of systematically proving many of the novel concepts introduced each year, it is very common that authors (manufacturers) vigorously show their love of the idea and write emotively to convince the rest of the field of the hypothesis, more by the energy and enthusiasm of their conviction (commercial affiliations) than by quiet evidence. As a classic example of such behavior, a concept of bone tunnel wall compaction (to improve the strength of fixation of soft tissue ACL graft) was introduced a few years ago and, accordingly, specific compaction drill bits/serial dilators were widely manufactured and sold to orthopaedic surgeons. However, both the compaction drill bits1,2 and the serial dilators3,4 have since failed under scientific scrutiny.
Now, concerning the ratio of screw diameter to tunnel diameter, there are actually three variables that play a role in this equation: the diameters of the graft, the bone tunnel, and the screw. By coupling two of the three, two concepts commonly used in the ACL literature have been derived: first, the oversizing of the interference screw in relation to tunnel/graft diameter, which is proposed to increase the strength of fixation. Second, the mismatch between the tunnel and graft diameters with the less precise match suggested to result in decreased strength of fixation. Theoretically, both of these concepts make a lot of sense, but unfortunately the proof for their appropriateness in humans is somewhat vague. To our knowledge, the only study supporting the mismatch concept is by Steenlage et al.,7 in which it was indeed shown that the fixation strength of a quadrupled hamstring tendon graft is improved with a more precise match of the bone tunnel diameter to the diameter of the graft. However, the validity of the comparison in this study can be questioned because of an apparent flaw in the study design, as the comparison was performed between two nonpaired groups of knee specimens from cadavers of different mean age.
On the contrary, the results of our two recent ACL fixation studies cast a shadow of uncertainty over the validity of the concept: By comparing the results of a study in which the graft-tunnel diameters perfectly matched (no mismatch)2 to another study using exactly the same drill bits and paired knee specimens, but mismatch between the diameter of the graft and bone tunnel, we observed that the overall average displacement after cyclic loading was actually larger (4.5 mm versus the 3.4 mm) and the failure load was lower (455 ± 115 N versus 480 ± 115 N) in the perfectly matched specimen than in the group with mismatch. In this context, Yamazaki et al.9 recently showed in an in vivo canine study that graft-tunnel diameter mismatch up to 2 mm does not adversely affect the intraosseous healing of the flexor tendon graft (biomechanical strength at 3 and 6 weeks). Accordingly, based on the most recent scientific evidence, we are tempted to question the clinical relevance/validity of the mismatch concept.
As regards the "screw oversizing," we do agree with Drs. Lubowitz and Guttmann that theoretically it is highly plausible that increased screw diameter results in increased strength of fixation. Notwithstanding our reservations about the clinical relevance of such an increase in the strength of fixation above certain limits, there is indeed one study in which an increase in the fixation strength in relation to an increase in screw diameter was shown.8 However, although the authors of this study did observe this positive effect, they actually concluded that increasing screw length improves fixation strength more than oversizing the screw diameter. This finding actually leads us to "cortical purchase," the concept suggesting that a substantially lengthened interference screw could allow the distal part of the screw to tightly grip (purchase) the anterior cortex of the tibia while the proximal part of the screw (head) would simultaneously be approaching or achieving truly anatomic/apertural fixation. Considering that, at present, the most reliable evidence (a cadaveric study) suggests that an increase in screw length from 20 to 40 mm does not improve the strength of fixation of a soft tissue graft,6 the scientific evidence backing up these concepts is scarce, too. Hopefully, the cortical purchase concept will eventually be validated (or refuted) by solid scientific experiment but, at present, it has to be considered merely as a marketing ploy.
Finally, we fully agree with Drs. Lubowitz and Guttmann that apart from, or perhaps rather than, the highest strength of fixation, the superiority of one fixation implant over others depends on a good number of other characteristics. Naturally, an ideal ACL fixation implant provides a fixation that mimics the natural situation as closely as possible. To delineate some of the eligible characteristics, ACL fixation should provide adequate strength in the tibia at or as close to the joint line as possible (contrary to current paradoxical emphasis on the femoral fixation), be bioabsorbable, allow circumferential healing with no artificial substance between the healing tendon and the bone tunnel walls, thus presumably resulting in enhanced incorporation of the graft, have no external hardware, be implanted using a simple, reproducible, and minimally time-consuming surgical technique, and finally, enable equal tensioning of the individual strands of the tendon graft.
REFERENCES
This article has been cited by other articles:
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  T. L. N. Jarvinen, J. T. Nurmi, and H. Sievanen Bone Density and Insertion Torque as Predictors of Anterior Cruciate Ligament Graft Fixation Strength Am. J. Sports Med., September 1, 2004; 32(6): 1421 - 1429. [Abstract] [Full Text] [PDF]
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