Tunnel Expansion with EndoButton in Hamstring ACLs
Abstract & Commentary
Synopsis: In a comparative study, 1-incision hamstring ACL reconstruction with femoral EndoButton fixation was associated with more tunnel expansion than a 2-incision technique fixed proximally with a screw and spiked washer.
Source: Simonian PT, et al. Tunnel expansion after hamstring anterior cruciate ligament reconstruction with 1-incision EndoButton femoral fixation. Arthroscopy 2000;16(7):707-714.
Although we are unclear regarding the clinical significance of tunnel expansion seen on post-operative radiographs, its recognition has raised a great deal of concern among ACL surgeons. In the present study, Simonian and colleagues report that a 1-incision technique using EndoButton fixation on the femoral side resulted in more tunnel expansion (especially on the femoral side) than historical control group in which they used a 2-incision technique with screw and spiked washer fixation. They attribute this to micromotion (sometimes referred to as the "bungee-cord" effect) of the graft within the tunnel. Twenty consecutive patients who underwent hamstring ACL reconstruction with a 1-incision EndoButton technique were compared with 20 consecutive patients who had a 2-incision technique (retrospective control group). Radiographs were obtained at three and 12 months postoperatively, and tunnel measurements were made using calipers on a horizontal light box. The following approximate average tunnel expansion at three months postoperatively was recorded (see Table).
| Table | ||||||
| Tech | AP proxT | LatproxT | AP dist T | Lat dist T | AP Fem | Lat F |
| 1-in* | 3 mm | 7 mm | 3 mm | 2 mm | 6 mm | 6 mm |
| 2-in* | 1 mm | 0.5 mm | 1 mm | 0.3 mm | 1 mm | 1 mm |
| *in = incision | ||||||
These differences did not change significantly at 12 months. All other clinical parameters were similar between the two groups. Simonian et al offer two explanations for this phenomenon. The first issue implicates that EndoButton-creep and micromotion may occur with this device. The second issue involves eccentric drilling required with a 1-incision technique. This basically involves additional drilling of the back wall of the tibial tunnel when passing the drill bit for accurate placement of the femoral tunnel using a 1-incision technique. This of course would explain the increased lateral dimension of the proximal tibia tunnel (the most likely explanation) but not the femoral expansion. Simonian et al do emphasize that although tunnel expansion is significant, the clinical outcome was not affected by these findings.
Comment by Mark D. Miller, MD
Although tunnel expansion is concerning, it does not appear to affect clinical results, at least in the short term. It is unclear whether graft incorporation is adversely affected. Other researchers have noted this phenomenon, and there is still room for additional study on this issue. Tunnel expansion is a concern when revision ACL reconstruction is contemplated. In these cases, it is necessary to consider bone grafting these defects, either in a one- or two-stage reconstruction.
There may be some technical considerations that would reduce the incidence and amount of tunnel expansion associated with the use of the EndoButton and 1-incision technique. Simonian et al typically placed 25 mm of graft into the femoral tunnel. Perhaps if the graft was placed farther into the tunnel, as close to the cortex as possible (a technique that I have adapted with this device), less expansion will occur. Second, and perhaps more important, the second-generation EndoButton is a continuous loop (CL) and does not require a knot to be tied. This will likely result in less creep and micromotion. Unfortunately, the smallest CL device available is 20 mm, and this may affect the first suggestion. Although I have not recognized a problem with tunnel expansion using these two techniques, careful measurement of postoperative radiographs, as was accomplished in the present study, will be necessary to determine if we can reduce the incidence of this concerning finding in the future.
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