| Objective:Most joint surgeons have reached a consensus that the position of bone tunnel is one of the most important factor in the anterior cruciate ligament (ACL) reconstruction. With the long term follow-up and the good clinical outcome, single-bundle ACL reconstruction was still the choice of many researchers.There are two single-bundle surgical types called isometric and anatomic reconstructions.The graft fixations in ACL reconstruction include absorbable interference screw, Endobutton, Rigidfix, etc. In case of Endobutton fixation, graft swings in the bone tunnel when the joint flex and extend, which affects the graft stability and increases graft wearing. Rigidfix fix the tendon with two transverse screws. The fixation is secure but the technique is relatively difficult to master. The interference screws are widely used since there secure fixation, however, the graft is crushed away from center of bone tunnel. With computer assisted navigation system, some researchers could precisely locate bone tunnel position, however, since the graft is off from the bone tunnel center because of the interference screws , The tendon's actual position is not very satisfactory.The purpose of this study is to study the offset of the graft induced by absorbable interference screw fixation and how much the offset will affect the anterior cruciate ligament reconstruction.Method:Totally 19 fresh-frozen cadaveric knees were used in this study, The knee specimen were provided by the Third Hospital of Hebei Medical University. In the 19 knees, 5 were randomly selected and used for the study of graft position shift,7 knees were randomly assigned into the isometric group,7 knees were randomly assigned into the anatomic group.Each knee of isometric group and anatomic group,the fibula was fixed on the tibia with a Kirschner wire. The Achilles tendon was harvested from each specimen and prepared as autograft.The ACL were carefully removed from the 5 knees used for the study of graft position shift. The lateral femoral condyles were dissected. In each condyle three bone tunnels were drilled at the diameter of 7, 8, 9mm, respectively. The distance between the every two tunnels was at least 3mm, so that each bone tunnel would not interact others.The cartilage at the posterior margin of proximal femoral condyle was taken as the reference point. Autografts and interference screws of the same diameter as the bone tunnels were fixed into the bone tunnel. The graft morphology was studies right after fixation. It is found that the grafts were crushed into a cresentic-like shape by the screw. On a transverse section of the graft, the intersection of long and short axis is off from center of the tunnel. The distance between these two points (center of the graft to center of the bone tunnel) were measured.Knees of the isometric group: Place the femoral tunnel drill guide at femoral end of anteromedial bundle of ACL, close to the over-the-top position.The tibial drill guide was placed in the central fibers of footprint. Bone tunnels of 7mm in diameter were drilled. A Retrobutton system was used as the fixation of the femoral tunnel.The knee joint was extended to 0°, mark on the graft at tibial articular surface level. The knee joint was flexed to 120°and mark on the graft again. The distance between these two marks is the graft length change when the knee is flexed from 0°to 120°. The knee joint was flexed at 90°. The graft was tensioned and fixed in the femoral tunnel with a 7mm interference screw. Measure the graft length change when the knee is flexed from 0°to 120°.Remove screw and femoral fixation device. The femoral bone tunnel was filled with bone cement and bone impaction. A new tendon was prepared and looped into the Retrobutton device. Insert the femoral tunnel aimer again, place the tip at 2.5mm anteroinferior to the original point and drill a 7mm femoral tunnel. Lead the graft into the femoral tunnel and fix it with interference screw. Measure the graft length change when the knee is flexed.The same procedure was repeated on the other six knees.Knees of the anatomic group:a knee joint was fixed on a Biomechanical Knee Joint Test Equipment. The tibia was fixed on a CSS-44020 biomechanical test machine. The anterior-posterior shift was measured when knee joint was bend at 0°, 15°, 30°, 60°and 90°. A constant load of 134N was applied to the tibia and the shift speed was 50mm/min. The shift distance was measured and recorded as data of intact group. Then remove the ACL from these joints and the above anterior-posterior shift measurement was repeated on the same knee joint. The data was recorded as of group of deficient group.Place the femoral and tibial tunnel aimer in the center of ACL femoral and tibial footprint. Bone tunnels of 7mm in diameter were drilled. Lead the graft into the bone tunnel. A femoral and tibial interference screws in the same diameter with the bone tunnel were screwed in to a desired depth. The anterior-posterior shift was measured again when knee joint was flexed at 0°, 15°, 30°, 60°and 90°. The data was recorded as of the group of offset group.Remove the screws. The femoral bone tunnel was filled with bone cement and bone impaction. A new tendon was prepared. Insert the femoral tunnel aimer again, place the tip at 2.5mm anteroinferior to the original point and drill a 7mm femoral tunnel. Lead the graft into the tunnels and fix it with interference screws as before. The anterior-posterior shift was measured again when knee joint was flexed at 0°, 15°, 30°, 60°and 90°. The data was recorded as of the group of corrected group.The same procedure was repeated on the other six knees.The data of isometric characteristics was compared between groups with two-way Student's t-test. The Data of tibia forward translation was compared between groups with two-way analysis of variance. P<0.05 was accepted as significant.Results:The graft position shift induced by 7mm, 8mm and 9mm interference screw were (2.36±0.11)mm, (2.72±0.06)mm and (3.00±0.06)mm, respectively. The graft isometry in ACL reconstruction: intact knee, ACL reconstruction with and without location corrected bone tunnel. The graft length changes in these 3 groups are (2.33±0.37)mm, (3.71±0.48)mm and (2.51±0.32)mm, respectively.There are significant differences of the graft length change between the three groups and the length of 3mm,which indicates that non-corrected bone tunnel ACL reconstruction is not isometric. However, the average graft length change decreased to less than 3mm after the bone tunnel location is corrected.The tibial anterior translation: At 0°and 15°, There is no significant difference between the Intact group the offset group and the corrected group(P> 0.05); significant difference between the Intact group and the lack group (P < 0.05). At 30°, 60°and 90°, there are significant differences between the Intact group and the the other 3 groups (P> 0.05). At 0°, 15°, 30°, 60°and 90°, significant differences are found between the deficient group and the other 3 groups (P < 0.05). At 30°and 60°, the corrected bone tunnel knee group shows less anterior tibial translation(P<0.05), And there is no significant difference at 90°(P>0.05).Conclusions:In ACL reconstruction, the early outcome with interference screw fixation is satisfactory because the total diameter of screw and graft is more than that of bone tunnel. But in the bone tunnel, the graft is pushed tightly toward the tunnel wall, which shift the graft away from the ideal position. Both isometric and anatomic reconstructions ,there are considerable graft position shift induced by screw, which may be one of the reasons of ligament laxity after surgery. In this study, we measured the precise graft position shift induced by 7mm, 8mm and 9mm interference screw, which may be of help to clinical doctors when they choose the bone tunnel location. The position shift should be taken into consideration in advance of start drilling.With pushing by interference screw, the graft will finally be set to an ideal location. The problem of position shift can be partly resolved by adjusting the bone tunnel to a corrected point,Which helps in preventing joint laxity after the ACL reconstruction surgery.
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