Abstract
Background: Some surgeons presently reconstruct both the anteromedial and posterolateral bundles of the anterior cruciate ligament. The purposes of this study were to measure the abilities of single-bundle and anatomic double-bundle reconstructions to restore anteroposterior laxities and rotational kinematics to intact knee levels and to compare graft forces in reconstructed knees with forces in the native anterior cruciate ligament for the same loading conditions.
Methods: Native anterior cruciate ligament force and tibial rotations were recorded during passive knee extension tests with and without applied tibial loads. The anteromedial and posterolateral bundles were reconstructed with patellar tendon tissue sized to fit tightly within 7-mm femoral tunnels. Testing was repeated with the anteromedial graft alone (single bundle), tensioned to restore anteroposterior laxity at 30° of flexion, and with double-bundle grafts. For double-bundle reconstructions, the anteromedial graft was first tensioned as above and then the posterolateral graft was tensioned with use of one of four protocols: posterolateral tension = anteromedial tension at 10° (DB1), posterolateral tension = anteromedial tension at 30° (DB2), posterolateral tension = (anteromedial tension + 30 N) at 10° (DB3), and posterolateral tension = (anteromedial tension + 30 N) at 30° (DB4).
Results: The posterolateral graft underwent a greater length change than the anteromedial graft between 0° and 90°. This difference in elongation patterns produced high forces in the posterolateral graft at 0° when both grafts were tensioned and fixed at 30°. The mean laxities for single-bundle reconstructions were within 1.1 mm of those of the intact knee between 0° and 90°; the mean graft force at 0° was 76 N. The mean laxities for DB4 reconstructions were from 0.9 to 2.8 mm less than those of the intact knee, and the mean graft force at 0° was 264 N. Coupled internal tibial rotations from valgus moment were normal with the single-bundle graft. Internal rotations from tibial torque were approximately 2° to 4° greater than normal with a single-bundle graft. DB3 and DB4 reconstructions overcorrected the coupled tibial rotations from valgus moment and restored tibial rotations from internal torque to normal from 0° to 45°. The graft forces from tibial torque and valgus moment were normal with the single-bundle graft. The mean double-bundle graft forces at 0° were 57 N to 143 N and 34 N to 171 N greater than normal for internal torque and valgus moment, respectively.
Conclusions: The single-bundle reconstruction produced graft forces, knee laxities, and coupled tibial rotations that were closest to normal. Adding a posterolateral graft to an anteromedial graft tended to reduce laxities and tibial rotations, but the reductions were accompanied by markedly higher forces in the posterolateral graft near 0° that occasionally caused it to fail during tests with internal torque or anterior tibial force.
Clinical Relevance: The relatively small improvements in anteroposterior laxities and tibial rotations from adding a posterolateral graft may not be worth the high graft forces necessary to achieve them. The high forces in the posterolateral graft recorded during our tests present cause for concern and may help to explain the posterolateral graft ruptures that have been reported clinically. The need for a double-bundle reconstruction to restore anteroposterior laxity and rotatory stability is questioned.
The treatment of the anterior cruciate ligament-deficient knee continues to be an area of considerable debate. Single-bundle anterior cruciate ligament reconstructions have been reported to have success rates ranging from 83% to 95%1-3. However, a number of other studies have found less satisfactory results following anterior cruciate ligament reconstruction4-10.
One important outcome measure of anterior cruciate ligament reconstruction is the ability of the graft to control anterior tibial translation. Another important outcome measure is the reduction or elimination of the giving-way symptom. This is a rotatory instability produced by two important loading mechanisms. The first mechanism is internal tibial torque, which causes the cruciate ligaments to wind around each other, thereby loading the anterior cruciate ligament. In the anterior cruciate ligament-deficient knee, resistance to internal rotation is diminished. The second mechanism is valgus moment, which produces a coupled internal rotation of the tibia that increases when the anterior cruciate ligament is removed11. The internal rotations produced by both loading mechanisms are a key component of the pivot-shift examination. A reduction or elimination of the pivot shift has been shown to be positively associated with an improved functional outcome following anterior cruciate ligament reconstruction12.
The anterior cruciate ligament consists of separate anteromedial and posterolateral bundles, and some surgeons currently reconstruct both bundles. Several clinical studies comparing the anterior-posterior stability of single-bundle reconstructions with that of double-bundle reconstructions have shown no subjective or objective differences between the two procedures13-15, while others have found small reductions in laxity values, measured on a KT-1000 arthrometer, at 30° of flexion (=1.1 mm) with a double-bundle reconstruction16-19. It has also been claimed that a double-bundle reconstruction is more effective in controlling rotational stability than is a single-bundle reconstruction20, and there is clinical evidence that patients with a double-bundle reconstruction demonstrate improved pivot-shift results postoperatively compared with patients receiving a conventional single-bundle reconstruction14-16,18,19. Relatively few published cadaver studies have directly compared anterior-posterior stability of knees with single-bundle and double-bundle reconstructions20,21. The isometry of double-bundle grafts has received less study13.
The purposes of this study were to measure the abilities of single and double-bundle anterior cruciate ligament reconstructions to restore anterior-posterior and rotatory stability to the knee and to compare graft forces in reconstructed knees with forces in the intact anterior cruciate ligament for identical loading conditions. Length changes (isometry) of anteromedial and posterolateral grafts in situ were also examined.
Fourteen fresh-frozen unpaired cadaveric knee specimens from male donors were used. The mean age (and standard deviation) of the donors at the time of death was 36.5 ± 12.7 years (range, seventeen to sixty-seven years). The articular surfaces were inspected visually for evidence of cartilage erosion or other arthritic abnormalities, and none were found. Ten of the fourteen specimens were also utilized in a related study22. The tibia and femur were potted in cylindrical molds of polymethylmethacrylate. The tibial insertion of the anterior cruciate ligament was mechanically isolated with use of a cylindrical coring cutter, and a cap of bone containing the entire footprint was attached to a custom-designed load cell that measured resultant force in the ligament23.
Anterior-posterior laxity testing was performed with ± 100 N of applied tibial force at six angles of knee flexion. Anterior cruciate ligament force was recorded as the knee was passively extended from 120° to 0° of flexion with no applied tibial force, a 100-N anterior tibial force, a 5-N-m internal tibial torque, and a 5-N-m valgus moment. Tibial rotations from applied internal torque were recorded with the anterior cruciate ligament intact, removed, and reconstructed. Specific details of our testing apparatus and testing protocols have been published previously24-26 and are presented in the Appendix.
The anterior cruciate ligament was cut, and the tibial bone cap was removed from the knee. The remaining anteromedial and posterolateral fiber bundles were followed down to their tibial insertion and were dissected from bone. The outlines of the footprints of the anteromedial and posterolateral bundles were marked on the bone, and the approximate center of each bundle footprint was marked with a punch. With use of alginate casting material, a negative mold of the tibial bone cap was made and filled with polymethylmethacrylate, resulting in an acrylic replica (positive) of the original anterior cruciate ligament bone cap. The marked outlines (and centers) of the anteromedial and posterolateral fiber bundles were automatically transferred over to the acrylic replica during the casting process.
Two patellar tendon allografts (obtained from the Musculoskeletal Transplant Foundation, Edison, New Jersey) were prepared for testing. Each graft consisted of a single, appropriately sized, bone block with attached tissue sized to fit tightly within a 7-mm femoral tunnel. This roughly corresponded to the amount of soft tissue that would be present with an 8 to 9-mm-wide patellar tendon graft. The anteromedial and posterolateral tibial footprints were cored out from the acrylic bone-cap replica, and the patellar bone blocks were potted with polymethylmethacrylate into the recesses.
The knee was flexed to 90°, and an anterior force was applied manually to the tibia. The origin of the slackened posterolateral fiber bundle on the lateral wall of the femoral notch was identified and dissected from bone. A 5-mm offset guide was used to drill a 7-mm anteromedial tunnel at the one o'clock position (left knee). Then a second 7-mm tunnel was drilled at the center of the posterolateral footprint (approximately the 2:30 position in the notch). This normally left a 1.5 to 3.5-mm bone bridge between the tunnel edges.
The cap replica with potted bone blocks was attached to the tibial load cell. Low-stretch, high-strength synthetic lines (135-lb [61.2-kg] test Spectra Fiber; Izorline, Gardena, California), whip-stitched into the free end of each graft, were passed through the two femoral tunnels and through separate split-clamps cemented into the acrylic mass used to pot the femur.
The knee was placed in the testing apparatus and was extended to 0° of flexion. A dial caliper was used to measure a baseline length between a forceps clamped to the lines attached to the free (femoral) end of the graft and the edge of the femoral split-clamp used to fix the graft. The knee was then incrementally flexed to 10°, 30°, 45°, 70°, and 90°. At each flexion angle, changes in the distance between the clamped forceps and femoral split-clamp were recorded relative to the distance at 0°. An increase in graft length with knee flexion corresponded to slackening of a graft fixed at full extension. The position of the tibia relative to the femur was reproduced at each knee flexion angle by manually holding the tibia at the midpoint of internal-external rotation (for the intact knee) while applying a slight manual compressive force along the axis of the tibia to maintain tibiofemoral contact at both condyles. A spring scale was used to apply a 27-N force to the graft lines during the measurements.
The single-bundle anteromedial graft was tensioned to restore anteroposterior laxity to within 1 mm of that of the intact knee at 30° of flexion, and all tests described above were repeated with the single-bundle graft. For double-bundle reconstructions, the anteromedial graft was first tensioned at 30° as above and then the posterolateral graft was tensioned at either 30° or 10° with use of one of four protocols: posterolateral tension = anteromedial tension at 10° (DB1); posterolateral tension = anteromedial tension at 30° (DB2); posterolateral tension = (anteromedial tension + 30 N) at 10° (DB3); and posterolateral tension = (anteromedial tension + 30 N) at 30° (DB4). All tests were repeated with double-bundle grafts. With the double-bundle reconstructions, the tibial load cell recorded the combined (total) tension of both grafts.
A two-factor repeated-measures analysis of variance was used to compare mean anterior-posterior laxities, tibial rotations, and graft forces among test conditions. The factors were reconstruction status (anterior cruciate ligament intact, anterior cruciate ligament removed, single-bundle graft, and double-bundle grafts) and knee flexion angle. Pairwise comparisons were made with use of the Student-Neuman-Keuls procedure. The level of significance was set at p < 0.05. A similar analysis was used to compare mean graft excursions for the two grafts; the factors were type of graft (anteromedial or posterolateral) and knee flexion angle.
Source of Funding
The funding for this study was provided by NFL Charities.
The mean graft tension (and standard deviation) for a laxity match with the single-bundle graft was 13.6 ± 8.4 N at 30° and 32.7 ± 19.7 N when the knee was extended to 10°. The actual mean graft tensions used for testing were 13.6 ± 8.4 N for the anteromedial single bundle; 32.7 ± 19.7 N for the anteromedial bundle and 32.6 ± 18.4 N for the posterolateral bundle for DB1; 13.6 ± 8.4 N and 18.8 ± 9.4 N, respectively, for DB2; 32.7 ± 19.7 N and 57.8 ± 17.8 N for DB3; and 13.6 ± 8.4 N and 43.6 ± 9.8 N for DB4.
Anteroposterior Stability
The mean laxities with a single anteromedial graft were not significantly different from those of an intact knee at 10° and 30°. The mean laxity with a single anteromedial graft was 0.9 ± 0.8 mm greater than that of the intact knee at 0° and 1.1 ± 1.0 mm less than that of the intact knee at 90° (Fig. 1). The mean laxities with DB3 and DB4 reconstructions were significantly less than those of the intact knee at all flexion angles (p < 0.05); the mean laxities of DB4 reconstructions were 2.6 ± 1.7 mm and 2.8 ± 1.4 mm less than those of the intact knee at 10° and 30°, respectively. The mean laxities of DB1 and DB2 reconstructions were significantly less than those of the intact knee from 30° to 90° (p < 0.05). The mean laxities of the DB2 reconstructions were 1.2 ± 1.2 mm less than those of the intact knee at 10° and 30°.
For passive knee flexion with no applied tibial force, the mean graft forces with all reconstructions were significantly higher than the native anterior cruciate ligament forces (p < 0.05), with the exception of the single-bundle reconstruction at 0° (Fig. 2). At 0°, the mean increases in graft force (relative to the native anterior cruciate ligament) were 51.3 ± 43.6 N for DB1, 100.2 ± 56.0 N for DB2, 126 ± 57.2 N for DB3, and 195.2 ± 73.6 N for DB4 reconstructions.
With a 100-N anterior tibial force, the mean single-bundle graft forces were not significantly different from anterior cruciate ligament forces with a knee flexion angle between 0° and 65° (Fig. 3). The mean forces for DB1 reconstructions were not significantly different from those of the anterior cruciate ligament with flexion between 0° and 55°; the mean forces for DB2 reconstructions were not significantly different with flexion between 5° and 35°. The mean forces for DB3 and DB4 reconstructions were significantly higher than the mean anterior cruciate ligament forces at all flexion angles (p < 0.05). At 0°, the mean increases in graft force (relative to the native anterior cruciate ligament) were 59.4 ± 75.1 N for DB2, 96.8 ± 76.4 N for DB3, and 137.4 ± 84.9 N for DB4 reconstructions.
Rotatory Stability
For applied internal torque, removal of the anterior cruciate ligament significantly increased internal rotation with mean increases of 7.3° ± 3.4° at 0° of flexion and 4.0° ± 2.8° at 30° of flexion (p < 0.05) (Fig. 4). The mean rotations with single-bundle and DB1 reconstructions remained approximately 2° to 4° greater than those of the intact knee. The mean rotations with DB2 reconstructions were not significantly different from those of the intact knee. The mean rotations with DB3 and DB4 reconstructions were not significantly different from those of the intact knee at flexion angles of <50° and <60°, respectively.
Graft forces with applied internal torque were not significantly different from anterior cruciate ligament forces for single-bundle reconstructions (all flexion angles) and DB1 reconstructions (flexion angles of <10°) (Fig. 5). Graft forces for DB2, DB3, and DB4 reconstructions were significantly higher than the anterior cruciate ligament forces (p < 0.05). The mean anterior cruciate ligament force was 180.3 ± 48.4 N at 0°. The corresponding mean forces for double-bundle reconstructions were 222.1 ± 55.5 N for DB1, 261.6 ± 48.1 N for DB2, 280.4 ± 52.7 N for DB3, and 323.6 ± 64.9 N for DB4.
The application of valgus moment to the intact knee caused the tibia to rotate internally as the knee was flexed (Fig. 6). Removal of the anterior cruciate ligament significantly increased this coupled internal rotation between 0° and 50° (p < 0.05); the mean increases at 0° and 30° of flexion were 5.2° ± 3.2° and 7.8° ± 3.6°, respectively. The mean rotations with single-bundle and DB1 reconstructions were not significantly different from those of the intact knee. The rotations for DB2, DB3, and DB4 reconstructions were significantly less than those of the intact knee (p < 0.05); at 0°, the highly tensioned posterolateral graft overcorrected tibial rotation compared with the intact knee.
The application of valgus moment to the intact knee also produced valgus rotation of the tibia. Removal of the anterior cruciate ligament significantly increased valgus rotation (p < 0.05); the mean increases at 0° and 30° of flexion were 1.9° ± 4.4° and 4.4° ± 2.3°, respectively (Fig. 7). The mean valgus rotations with all reconstructions were not significantly different from those of the intact knee, with the exception of DB4 reconstructions, in which rotations were significantly less than those of the intact knee at flexion angles of >30° (p < 0.05).
The graft forces from applied valgus moment were not significantly different from anterior cruciate ligament forces with a single-bundle reconstruction, whereas the mean forces with all double-bundle reconstructions were significantly higher (p < 0.05) (Fig. 8). The mean anterior cruciate ligament force was 75.0 ± 37.3 N at 0° of flexion. The corresponding means for double-bundle reconstructions were 108.7 ± 45.1 N for DB1, 158.0 ± 56.9 N for DB2, 179.8 ± 62.4 N for DB3, and 246.3 ± 66.5 N for DB4.
Graft Isometry
The changes in graft length between anteromedial and posterolateral grafts were significantly different at >10° of flexion (p < 0.05) (Fig. 9). The mean change in length for the posterolateral graft (4.6 ± 3.0 mm) was significantly greater than that for the anteromedial graft (3.0 ± 1.0 mm) at 30° of flexion (p < 0.05) (Fig. 9). The mean difference in the change in length between the anteromedial and the posterolateral graft at 30° was 1.6 ± 1.0 mm (range, 0.1 to 3.2 mm).
Graft Failures
Three posterolateral grafts failed during testing; one of them failed with applied internal torque and two, with applied anterior tibial force (during laxity testing). All failures occurred at 0°, where the force in the posterolateral graft was high because of its nonisometric behavior. Visually, the graft tissue had stretched out in midsubstance. The failed grafts were replaced with new ones, and the complete test series was repeated. No failures of the anteromedial graft occurred with any test.
This study was designed to compare anteroposterior laxities, tibial rotations, and graft forces for single and double-bundle anterior cruciate ligament reconstructions with those of the intact knee. Overall, a single-bundle reconstruction was sufficient to restore coupled tibial rotations, graft forces, and anteroposterior laxities to normal or near-normal levels. Because of its nonisometric behavior, the posterolateral graft developed relatively high forces near 0° when it was tensioned and fixed in a flexed knee position. This tended to reduce anteroposterior laxities and tibial rotations produced by applied internal torque and a valgus moment, and it overconstrained the knee with some double-bundle tensioning protocols.
A 7-mm single-bundle reconstruction (approximately equivalent to an 8 to 9-mm-wide bone-patellar tendon-bone graft) was used as a baseline from which a posterolateral graft was added with use of different tensioning strategies. Since a primary goal of an anterior cruciate ligament reconstruction is to restore normal findings on the Lachman examination at 30° of flexion, the single-bundle graft was tensioned at this position. If the single-bundle graft were tensioned at 0°, our data show that higher graft tensions would have been required. Because of the repeated-measures study design, our single-bundle reconstruction did not have a 10 to 11-mm anteromedial graft, as is used clinically. Although it would have been desirable to have utilized a larger graft, the femoral footprint of the anterior cruciate ligament was too small to accommodate a 10 to 11-mm anteromedial graft and a 7-mm posterolateral graft while still leaving an adequate bone bridge between the two tunnels. Clinically, 4.5 to 6-mm grafts are commonly used for double-bundle reconstructions. We chose 7-mm grafts because they could better withstand the repeated forces generated by our tibial loading tests.
Our isometry results indicate that initial tensions in the posterolateral graft (set with the knee in a flexed position) were magnified as the knee was extended to 0°. This effect was greater when the posterolateral graft was tensioned at 30° because the posterolateral graft underwent a greater length change. Therefore, it may be advisable for the surgeon to avoid high tension in the posterolateral graft (especially with the knee at 30° of flexion) to prevent high forces that could stretch out the graft. Hamada et al.13 also found that both grafts tightened with extension on the operating table; the anteromedial graft tightened an average of 1.6 mm, and the posterolateral graft tightened an average of 2.1 mm.
We found only two studies in the literature that directly compared the anteroposterior stability of single-bundle and double-bundle reconstructions of the anterior cruciate ligament. Mae et al.21 reported that laxity differences between single-bundle and double-bundle reconstructions were 0.7 to 1.4 mm between 0° and 30° and were not significantly different at 60° and 90°. We also found laxity differences between single-bundle and double-bundle reconstructions on the order of 1 to 2 mm, but there is an important difference in graft-tensioning protocols between the two studies. We tensioned the single-bundle graft to a level that restored normal findings on the Lachman test at 30° for each specimen. In the study by Mae et al., all grafts were tensioned to the same level. With 44-N graft tension, their single-bundle reconstruction restored normal laxity at 30°. With 88-N graft tension, laxity with the single-bundle reconstruction was 2.6 mm less than that of the intact knee at 30°. All of their double-bundle reconstructions substantially overconstrained the knees (compared with the intact knee) between 0° and 30°. For example, the mean knee laxity with 88 N of tension on each graft was 3.4 mm less than that of the intact knee at 30°.
Yagi et al.20 reported a mean difference in anterior tibial translation between single-bundle and double-bundle reconstructions of 2.4 mm at 30°, which is comparable with that found in our study. However, since they used the same graft tensions for all knees, neither single-bundle nor double-bundle reconstructions restored a normal result on the Lachman test at 30°. The mean laxities at 30° with single-bundle and double-bundle grafts were greater by 3.8 mm and 1.4 mm, respectively, than those of the intact knee.
With our testing protocol, a 5-N-m internal tibial torque and a 5-N-m valgus moment were applied separately in different tests. This allowed evaluation of rotatory stability for each mode of loading independently. Both loading modes produced internal tibial rotations that increased when the anterior cruciate ligament was removed. We believe that increased internal tibial rotation in the anterior cruciate ligament-deficient knee contributes to the giving-way symptom commonly reported by patients who have sustained an injury of the anterior cruciate ligament. Therefore, the abilities of single-bundle and double-bundle reconstructions to limit both rotatory instabilities have direct bearing on the reduction or elimination of giving-way symptoms after anterior cruciate ligament reconstructions have been performed.
There were new and interesting findings related to applied valgus moment that have not been reported previously. Removal of the anterior cruciate ligament not only increased coupled internal rotation of the tibia but also increased valgus rotation of the tibia as well. This represents a complex combined rotatory-valgus instability that may be related to the giving-way sensation experienced by patients. All reconstructions restored valgus rotations to normal between 0° and 30° of flexion.
Our load cell recorded the combined resultant force for both graft bundles. The addition of a posterolateral graft to an anteromedial graft increased the resultant force as the knee was extended to 0°. Since the force in each graft is proportional to its change in length, and the change in length for the posterolateral graft with knee extension was greater than that for the anteromedial graft, we believe that a large portion of the increase in resultant force from the addition of the posterolateral graft was carried by that graft. This is further supported by the fact that only posterolateral grafts failed during testing, and both grafts were the same size. The high posterolateral graft forces at 0° were increased even further by the application of anterior tibial force and internal torque, modes of loading that produced posterolateral graft failures at 0°.
Yagi et al.20 compared coupled anterior tibial translations and graft forces under a combined 5-N-m internal torque and 10-N-m valgus moment for single-bundle and anatomic double-bundle reconstructions. They found that coupled anterior tibial translations with double-bundle reconstructions were 2.4 and 2.0 mm less than translations with single-bundle reconstructions at 15° and 30°, respectively; these differences would most likely represent small changes in tibial rotation (as recorded in our tests). These findings have formed the principal basis for the commonly cited claim that a double-bundle reconstruction provides better control of rotatory knee stability than a single-bundle reconstruction.
A direct comparison of our results with those of Yagi et al.20 is not possible. They applied a combined load of 5 N-m of internal torque with 10 N-m of valgus moment, while our results are based on applied loadings of 5 N-m of internal torque and 5 N-m of valgus moment (applied separately). They found that coupled anterior tibial translations remained substantially greater than normal with both single-bundle and double-bundle reconstructions. We found that coupled tibial rotations from an applied valgus moment were restored to normal with a single-bundle graft and were significantly less than normal with DB3 and DB4 grafts. We found that internal rotations from applied internal torque were significantly greater than normal with single-bundle reconstructions (p < 0.05) and were not significantly different from normal (0° to 45°) with DB3 and DB4 reconstructions.
The question that remains is whether a double-bundle reconstruction is necessary to eliminate the rotatory instabilities for the two modes of loading that we studied or a single-bundle reconstruction is sufficient. Our results provide a mixed answer to this question. In our tests, significant increases in internal tibial rotation after removal of the anterior cruciate ligament were recorded with both modes of loading. Coupled tibial rotations from applied valgus moment were restored to normal with a single-bundle graft, but internal rotations from applied tibial torque were not. Theoretically, the posterolateral graft has been claimed to have a slightly better mechanical advantage in controlling tibial torque than the anteromedial graft because the posterolateral graft is acting at a greater distance from the axis of rotation20. However, since the moment arms for both grafts are relatively small, high graft forces are required to generate a resistive torque with both reconstructions. This explains why tibial rotations from applied torque were not restored to normal with the relatively low forces in a single-bundle graft and were restored to normal with the higher graft forces encountered with some double-bundle reconstructions.
The key to understanding the forces developed in the anteromedial and posterolateral grafts is their in situ isometry over a 90° range of motion. The force developed in a fixed graft is directly related to how much it elongates as the knee is moved. Therefore, the difference in patterns of length change between anteromedial and posterolateral grafts over the range of motion determines which bundle develops the highest tension. Small differences in graft elongation can lead to large differences in graft force because of the inherent stiffness of the graft construct.
Length changes of double-bundle grafts can be easily evaluated on the operating table. With one end of the graft fixed and a slight manual tension applied to the free end, the surgeon moves the knee through a range of motion and observes the relative motion of the graft, which provides an accurate indication of which graft will be more highly loaded. The magnitude of the length change indicates the magnitude of the force that will be developed in a fixed graft as the knee is extended. By performing this simple test, errors in graft-tensioning that could produce high graft forces may be avoided.
One of the commonly stated justifications for a double-bundle anterior cruciate ligament reconstruction is better restoration of rotatory stability. Our results show that any improvements in stability from adding a posterolateral graft came with the cost of markedly higher forces in the posterolateral graft. These high graft forces reduced the internal rotation from applied tibial torque, overcorrected the internal tibial rotation from applied valgus moment, and overconstrained anteroposterior laxity of the knee. They also produced occasional ruptures of the 7-mm posterolateral graft.
Some surgeons use 4.5-mm-diameter grafts for double-bundle reconstructions. The rupture of some 7-mm posterolateral grafts during the course of testing in this study suggests that 4.5-mm grafts may be clinically at risk. If a 4.5-mm posterolateral graft were to rupture or stretch out, the 4.5-mm anteromedial graft would be the only tissue left to carry load. This could lead to eventual failure of the anteromedial graft as well. Although we know of no anteromedial graft failures that have been reported in clinical series with double-bundle reconstructions, rupture of the posterolateral graft has been reported, with failure rates of 3%27, 7%28, and 11%29. Therefore, it is reasonable to ask whether the relatively small changes in anteroposterior and rotatory stability are worth the accompanying high graft forces necessary to achieve them. This question is especially relevant since we found that a single-bundle reconstruction provided laxities within 1.1 mm of those of the intact knee from 0° to 90° and graft forces that were closer to normal than any double-bundle reconstruction tested. A single-bundle reconstruction also restored the coupled internal rotations from an applied valgus moment (a key component of the pivot shift test) to normal levels. On the basis of our findings, the need for a more technically complex and time-consuming double-bundle reconstruction is questioned.
The figures showing the specific details of our testing apparatus and testing protocols are available with the electronic version of this article, on our web site at (go to the article citation and click on Supplementary Material) and on our quarterly CD/DVD (call our subscription department, at 781-449-9780, to order the CD or DVD). 
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