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Commentary & Perspective | ||||||||
Commentary & Perspective on This study was conducted by a highly experienced team of researchers, and their results provide interesting insights and raise a number of questions about the function of reconstructions of the anterior cruciate ligament in resisting abnormal anterior displacements. As the authors pointed out, an effective reconstruction of the anterior cruciate ligament should limit abnormal anterior tibial translation in response to both direct anterior loads and combined rotational loads. Further, experimental in vitro studies have measured anterior tibial translation at the center of the tibia and therefore have underestimated the maximum limit of abnormal anterior tibial translation (subluxation) that occurs with rotational and anterior loads through the lateral compartment1. Unfortunately, many experimental in vitro studies raise more questions than they provide answers. For example, in this study, and in those of Howell et al.2 and those of Markolf et al.3,4, the results showed that after anterior cruciate reconstruction an abnormal tensile behavior and residual abnormal anterior tibial translation occurred in many grafts, even though all of the grafts studied had been placed into the knee joint under seemingly identical conditions. This should be very disconcerting to orthopaedic surgeons. In the present study, both types of grafts (qradruple semitendinosus-gracilis tendon and bone-patellar tendon-bone) used for reconstruction of the anterior cruciate ligament limited anterior tibial translation at high flexion (90°) but provided less stability at lesser degrees of flexion (15° and 30°) where there was unexplained residual anterior tibial translation with anterior loading. For example, at 15° of knee flexion, the residual increase of anterior translation (in response to anterior loading) was 4.6 ± 1.7 mm (mean and standard deviation) in the reconstructed knee (intact, 6.1 ± 2.8 mm; hamstring graft, 10.7 ± 2.7 mm; patellar tendon graft, 8.7 ± 2.7 mm). This amount of residual abnormal tibial translation indicates that reconstructions of the anterior cruciate ligament were only partially effective at lesser degrees of flexion. Two points are important: first, from a clinical standpoint, this amount of residual anterior translation is well beyond that usually reported in the clinical studies in which residual anterior tibial translation of <3 mm has been found in 90% of knees after reconstruction2,5; second, since the anterior cruciate ligament grafts were not entirely functional in the first 4 to 5 mm of anterior tibial translation under anterior loading, one would expect that the grafts would also be less effective in restraining anterior tibial translation in response to combined rotational loads involving internal and valgus tibial torque. In the second loading condition there was also less total anterior tibial translation in the knees with a deficient anterior cruciate ligament; the mean difference was 6.3 mm (at 15° of flexion, 10.8 mm compared with 4.5 mm in the intact knees). Thus, it is not surprising that in-situ forces in the grafts would not approach those in the intact knees under combined rotational loads. In essence, the grafts were partially lax in resisting anterior tibial translation in response to direct anterior loads and they were thus also less effective in reducing coupled anterior tibial translation under combined rotational loads. In examining the results of this study, the most interesting question is, "What explains the inability of the majority of the reconstructions of the anterior cruciate ligament to resist effectively the initial anterior tibial translation and the abnormal tensile behavior that occurred since the grafts exhibited greater tension at greater degrees of knee flexion than at lesser degrees of knee flexion?" Is there a chance that graft slippage occurred under the higher loads imposed in the displacement tests after graft fixation? Would greater pre-tension loads be required prior to fixation? What is the ideal load that would restore normal anterior translation in a reconstruction of the anterior cruciate ligament? The authors utilized a similar magnitude of load on the grafts in all knees, but it is obvious that this loading situation did not restore anterior tibial translation to normal values. It would seem logical to avoid use of symmetrical graft-loading conditions for all knees and rather to choose the load that is required to restore normal anterior tibial translation in each knee (laxity-matched load). It would then be possible to determine if the reconstruction was effective in limiting anterior tibial translation under combined rotational loads. It is important to compare the results of this study with those of a recently published study by Howell et al.2, who measured the total tension in a four-bundle semitendinosus and gracilis tendon graft in fourteen subjects at the time of arthroscopic surgery. An abnormal tensile graft behavior (a graft force greater than 40 N at 80° of knee flexion) was found in ten of fourteen subjects despite what was interpreted by the surgeons as anatomically placed tibial tunnels. The authors found that a perpendicular tibial tunnel in the coronal plane was implicated in certain knees as altering the surgeon’s ability to correctly place and drill the femoral tunnel when using the single-incision arthroscopic technique. An example would be a graft placed too anterior at the femoral site, which would be less effective at low degrees of flexion but would provide increased tension with knee flexion. Is it possible that the reconstructions of the anterior cruciate ligament in this study were not placed into the ideal tibial and femoral positions? The experimental robotic instrumentation that the authors used in their study would allow nicely for determination of these many variables including graft positioning on the tibia and femur in the saggital and coronal planes and graft conditioning and tensioning to first achieve normal tensile behavior in a single bundle graft in response to both anterior and rotational loads. Only then would it be safe to conclude that a two-bundle reconstruction would be a better solution. It is probably true that after reconstruction of the anterior cruciate ligament, the graft serves as a check-rein to anterior tibial translation, but the graft never remodels sufficiently to restore the delicate fine-tuning of joint motions. We are indebted to the authors for devising an interesting and unique experimental methodology, which in future studies may, hopefully, provide some of the answers to these perplexing questions. *The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated. References 1. Howell SM, Wallace MP, Hull ML, Deutsch ML. Evaluation of the single-incision arthroscopic technique for anterior cruciate ligament replacement. A study of tibial tunnel placement, intraoperative graft tension, and stability. Am J Sports Med. 1999;27:284-93. | ||||||||
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