In-Vivo ACL Behavior and Long-Term Follow-up
In-Vivo ACL Behavior and Long-Term Follow-up
Abstract & Commentary
Synopsis: This study correlates initial ACL graft elongation behavior (stretch) at the time of surgery and long-term ACL biomechanical behavior (A-P laxity). Patients in whom the initial graft matched elongation values of an intact ACL showed no recurrent A-P laxity 5 years later. The other group, which demonstrated increased graft elongation at the time of surgery, stretched out during the 5-year follow-up period.
Source: Beynnon BD, et al. The elongation behavior of the anterior cruciate ligament graft in vivo: A long-term follow-up study. Am J Sports Med. 2001;29:161-166.
This 5-year follow-up study measures the biomechanical behavior of a remodeled ACL graft and correlates these data with the initial elongation behavior of the same graft at the time of surgery. Thirteen patients with an ACL tear and functional instability were treated with an arthroscopic-assisted ACL reconstruction with a bone-patella-bone autograft. Patients with meniscal tears requiring repair, multiligamentous injuries, and medical comorbidities were excluded. The ACL grafts were tested for elongation intraoperatively during passive flexion-extension using a Hall-effect transducer.1 The patients were then divided into 2 separate groups. Patients with grafts that were bounded by a 95% confidence interval of normal ACL elongation values were assigned to group 1. Patients with grafts that stretched excessively during initial passive motion outside a 95% confidence interval were assigned to group 2. Also, initial laxity measurements were performed intraoperatively with the KT 1000 arthrometer which showed no significant difference between the 2 groups. All patients were then followed prospectively.
At 5-year follow-up, laxity measurements (KT 1000 arthrometer) and surgical outcome measures (IKDC, Lysholm, Tegner) were recorded. There was a statistically significant (P = 0.004) increase in A-P laxity in group 2 (4.7 mm side-side difference) compared to group 1 (1.2 mm side-side difference). The grafts in group 2 stretched out over time. Thus, graft elongation at the time of surgery that exceeded the normal limits of an intact ACL (group 2) predicted excessive A-P laxity at 5-year follow-up. Beynnon and colleagues conclude that "not only is restoration of anterior posterior laxity values to within normal limits important, but the biomechanical behavior of the graft produced by flexion-extension of the knee should be appreciated." It is interesting to note, however, that the outcome measures did not show a statistical difference between the 2 groups at 5 years and there was no difference in rates of patient satisfaction.
Comment by Stephen B. Gunther, MD
This is an important addition to the literature on ACL graft behavior. Beynnon et al combine scientific diligence and attention to detail with a wealth of clinical experience. They have shown, for the first time in the orthopedic literature, that initial midsubstance ACL graft elongation behavior correlates with subsequent A-P laxity of the knee.2 The group of patients in whom the initial graft matched elongation values of an intact ACL showed no recurrent A-P laxity 5 years later, and the other group stretched out over time. It is interesting that the A-P laxity measurements in this second group were initially within the "normal" range at the time of surgery. Thus, the ACL grafts that showed increased initial elongation values had normal initial arthrometer measurements and, therefore, could not be deciphered from group 1 patients by current standards of testing (arthrometer testing and physical exam). However, this same group produced abnormally high A-P laxity measurements 5 years later.
It is also interesting to examine why some grafts demonstrated increased elongation values. All ACL reconstructions were performed with autogenous bone-patella-bone autografts using a standard technique. Variable tunnel placement and variable graft tension could explain these differences in initial elongation behavior. Beynnon et al did evaluate tunnel placement radiographically and found no difference between the 2 groups. The graft tension may play an important role when the tunnels are not perfectly anatomic. I suspect that there may be subtle differences in tunnel placement between the 2 groups such as femoral tunnels which are slightly more anterior or slightly more central. It would be interesting to measure tunnel placement in these cases using a 3-dimensional computer model with markers or a 3-D CT scan.
It is also interesting to note that the A-P laxity differences between the 2 groups at 5-year follow-up did not affect patient satisfaction. There were no statistical differences between the outcome scores for these 2 groups, and there was no significant difference in patient satisfaction.
No patients had functional instability or limitations of sporting activities. This begs the question as to what will happen with the patients in group 2 during the next 5 years. Will they stretch out further and become functionally unstable or will they remain functional and stable? We will only know if Beynnon et al publish again at 10-year follow-up.
Dr. Gunther, Assistant Professor of Clinical Orthopaedic Surgery, UCSF Department of Orthopaedic Surgery, San Francisco, CA, is Associate Editor of Sports Medicine Reports.
References
1. Beynnon BD, et al. The measurement of elongation of anterior cruciate ligament grafts in-vivo. J Bone Joint Surg. 1994;76A:520-531.
2. Fleming BC, et al. An in vivo comparison between intraoperative isometric measurement and local elongation of the graft after reconstruction of the anterior cruciate ligament. J Bone Joint Surg. 1994;76A: 511-519.
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