Abstract
Background: One of the principal rationales for performing a double-bundle reconstruction of the anterior cruciate ligament is the suggestion that it may be superior to a single-bundle reconstruction in restoring a normal pivot-shift sign. The purpose of this study was to measure the abilities of single-bundle and anatomic double-bundle reconstructions to restore normal knee kinematics and graft forces during a simulated pivot-shift test.
Methods: Graft force and knee kinematics were recorded during a simulated pivot-shift event with and without the anterior cruciate ligament and after graft reconstructions. With a single bundle, the graft was tensioned to restore anterior-posterior laxity at 30° of flexion. With double-bundle reconstructions, the anteromedial graft was first tensioned as above and then the posterolateral graft tension was set with use of one of four protocols: posterolateral tension = anteromedial tension at 10° of flexion (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: A single-bundle reconstruction restored all displacements and rotations during the pivot shift to the intact knee levels. The mean tibial rotations and lateral plateau displacements during the pivot shift with DB2, DB3, and DB4 reconstructions were less than those in the intact knee and also less than those in a single-bundle reconstruction. Before the pivot shift, the mean graft forces with all reconstructions were greater than that of the intact knee; the mean graft forces with the DB3 and DB4 reconstructions were also greater than that of a single-bundle reconstruction. After the pivot shift, the mean graft forces for all reconstructions were less than the levels before the pivot shift with single-bundle forces lower than intact knee levels and DB4 forces higher than intact knee levels.
Conclusions: Reduction or elimination of the pivot-shift sign is an important goal for anterior cruciate ligament reconstruction. In our model, the results show that a single-bundle reconstruction was sufficient to restore intact knee kinematics during a simulated pivot-shift event. The higher graft forces with some double-bundle graft-tensioning protocols reduced the coupled rotations and displacements from an applied valgus moment to less than the intact levels. This overcorrection should theoretically make the knee less likely to pivot but could have unknown clinical consequences.
Clinical Relevance: A double-bundle reconstruction is more technically complex and time-consuming than a single-bundle procedure. The need for a double-bundle reconstruction to restore a normal pivot-shift sign is questioned.
The pivot-shift test is widely accepted as a clinically useful tool in assessing the stability of knees that have sustained an anterior cruciate ligament injury. In this test, a valgus moment is applied to the tibia (sometimes in combination with a slight internal tibial torque) as the knee is flexed passively1-3. Contact force at the posteriorly sloping lateral tibial plateau causes it to subluxate anteriorly, internally rotating the tibia. In the intact knee, this coupled internal rotation is resisted by the anterior cruciate ligament. When the anterior cruciate ligament is absent, the axis of tibial rotation shifts from a point near the tibial spine to a point located medially near the medial collateral ligament, with corresponding increases in internal rotation and anterior subluxation of the lateral tibial plateau1. As the examiner flexes the knee from 0° to 90°, the line of action of the passive iliotibial band tension changes, causing it to become a knee flexor. This tendon force causes the tibia to rotate externally (with a spontaneous reduction of the anteriorly subluxated lateral tibial plateau), producing a classic pivot-shift sign. Reduction or elimination of the pivot shift has been shown to be positively associated with an improved functional outcome following anterior cruciate ligament reconstruction4.
Recently, reconstructions of both anteromedial and posterolateral bundles of the anterior cruciate ligament have been performed with the hope of producing a better clinical outcome. It is claimed that a double-bundle reconstruction may be more effective in controlling rotational stability than a single-bundle reconstruction5, and there is clinical evidence that patients with double-bundle reconstructions demonstrate improved pivot-shift results postoperatively compared with patients receiving a conventional single-bundle reconstruction6-10. The purposes of this study were to determine the tibial loading conditions that produce a simulated pivot-shift event with the anterior cruciate ligament absent and to compare the abilities of single-bundle and anatomic double-bundle reconstructions to restore tibial rotations, tibial displacements, and graft forces to intact knee levels.
Ten fresh-frozen unpaired cadaveric right knee specimens from male donors were used. The mean age (and standard deviation) of the donors at the time of death was 32.0 ± 9.8 years (range, sixteen to forty-three years). The tibial insertion site of the anterior cruciate ligament was mechanically isolated with use of a cylindrical coring cutter, and a cap of bone containing the entire ligament footprint was attached to a load cell that measured resultant force in the ligament11. First, the bone cap was disconnected from the load cell to create an anterior cruciate ligament-deficient condition. The knee was then placed in a testing apparatus that applied a valgus moment to the tibia; this caused the tibia to rotate internally and the lateral tibial plateau to subluxate anteriorly. A weight, applied to the iliotibial tendon through a pulley system, simulated muscle pull; this caused the tibia to rotate externally. Details of this testing apparatus have been described previously12-14 and are presented in the Appendix.
To produce a simulated pivot-shift event, a valgus moment was applied, the knee was flexed between 20° and 40°, and iliotibial force was then applied. With a correct combination of valgus moment and iliotibial force, the anteriorly subluxated lateral tibial plateau reduced spontaneously as the iliotibial force externally rotated the tibia. The combination of valgus moment and iliotibial force necessary to pivot the anterior cruciate ligament-deficient knee and the knee flexion angle at which the pivot occurred were determined by trial and error for each specimen.
Tibial internal-external rotation, varus-valgus rotation, and anterior-posterior displacement of a point on the lateral aspect of the tibia (near Gerdy's tubercle) were recorded with valgus moment alone (before the pivot), and with valgus moment and iliotibial force (after the pivot). Total rotations and displacements during the pivot-shift event were calculated by subtracting the values before the pivot shift from the values after the pivot shift. All rotations and displacements were defined as zero at 0° of flexion (with the knee intact). The bone cap was reattached to the load cell (thereby "restoring" the native anterior cruciate ligament), and the same loading conditions were applied. For these tests, load-cell force was recorded in addition to the kinematic measurements described above.
The anterior cruciate ligament was then cut, and the tibial bone cap was removed from the knee. The remaining fibers of the anterior cruciate ligament were dissected down to their tibial attachment, an outline of the tibial footprint of the anterior cruciate ligament was marked on the bone cap surface, and the centers of the anteromedial and posterolateral bundles were marked. 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 tibial bone cap. The outlines and centers of the anteromedial and posterolateral bundle footprints were marked on the cap replica.
Two patellar tendon allografts (obtained from the Musculoskeletal Transplant Foundation, Edison, New Jersey) were prepared for each knee specimen. The tendon fibers inserting into the tibia were dissected away from the bone surface, leaving a free end. The patellar end of each graft consisted of an appropriately sized patellar bone block. The free end of the graft tissue was trimmed 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 bone-patellar tendon-bone graft. The anteromedial and posterolateral tibial footprints were cored out from the acrylic bone-cap replica with a handheld rotary grinder, and the patellar bone blocks (approximately 8 × 8 × 10 mm long) were potted with polymethylmethacrylate into the recesses.
The knee was flexed to 90°, the tibia was grasped, and an anterior force of approximately 30 N was applied manually. The origins of the slackened posterolateral fiber bundle on the lateral wall of the femoral notch were identified and dissected from the bone, and the center of the posterolateral femoral footprint was marked on the bone surface. This was typically located near the 9:30 o'clock position in the notch (right knee). The remaining anterior cruciate ligament fibers were scraped from the bone, and a 5-mm offset guide was used to drill a 7-mm anteromedial tunnel at the 11 o'clock position (right knee). Then a second 7-mm femoral tunnel was drilled at the center of the posterolateral footprint. This normally left a 1.5 to 3.5-mm bone bridge between tunnel edges.
The cap replica with potted bone blocks was attached to the tibial load cell, for measurement of graft forces. A single low-stretch, high-strength synthetic line (135-lb [61.2-kg] test Spectra Fiber; Izorline, Gardena, California) was sutured into the free end of each graft with use of a whip stitch. The ends of the lines passed through the two femoral tunnels and through separate split clamps cemented into the femoral potting acrylic. The grafts were tensioned by pulling on these lines and were fixed by clamping them.
Pivot-shift testing was then repeated with single-bundle and double-bundle reconstructions. For the single-bundle reconstruction, the anteromedial graft was tensioned to restore anterior-posterior laxity (at ±100 N of applied force) to within 1 mm of the intact knee at 30° of flexion. For double-bundle reconstructions, the anteromedial graft was first tensioned as above and then the posterolateral graft tension was set 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). The DB3 and DB4 conditions were designed to evaluate the effect of increasing tension in the posterolateral graft.
For each graft configuration, the grafts were initially tensioned to the desired levels and three anteroposterior test cycles (with ±100-N tibial force) were applied to precondition the graft. Then the grafts were retensioned for pivot-shift testing. Single-bundle reconstructions were always tested first to provide a baseline reference. The order for testing of double-bundle grafts was randomized. The complete sequence of testing for all experimental conditions is presented in Table I.
A two-factor repeated-measures analysis of variance was used to compare the mean graft forces, tibial rotations, and tibial displacements among test conditions. The factors were graft status (anterior cruciate ligament-intact, anterior cruciate ligament-deficient, single-bundle, DB1, DB2, DB3, and DB4 conditions) and knee flexion angle. Pairwise comparisons were made with use of the Student-Newman-Keuls procedure. The level of significance was set at p < 0.05.
The mean flexion angle (and standard deviation) at which the pivot occurred was 26.7° ± 2.9° (range, 21.0° to 36.0°). The mean valgus moment and the iliotibial force required to produce a pivot shift in the anterior cruciate ligament-deficient knee were 3.4 ± 0.7 N-m (range, 2.5 to 4.3 N-m) and 29.0 ± 5.6 N (range, 19.9 to 37.3 N), respectively.
The mean anteromedial graft tension for a laxity match with the single-bundle anteromedial graft at 30° was 13.6 ± 8.4 N; this increased to 27.6 ± 8.9 N when the knee was extended to 10°. The actual mean graft tensions used for testing were 13.6 ± 8.4 N (anteromedial) for the single-bundle graft, 27.6 ± 8.9 N (anteromedial) and 32.6 ± 18.4 N (posterolateral) for DB1, 13.6 ± 8.4 N (anteromedial) and 18.8 ± 19.4 N (posterolateral) for DB2, 27.6 ± 8.49 N (anteromedial) and 57.8 ± 17.8 N (posterolateral) for DB3, and 13.6 ± 8.4 N (anteromedial) and 43.6 ± 9.8 N (posterolateral) for DB4.
Before the pivot shift (valgus moment alone), the mean coupled internal rotation with the anterior cruciate ligament removed was 8.0° ± 2.5° greater than that for the intact knee (Fig. 1). The mean coupled internal rotation with the single-bundle reconstruction and with the DB1 reconstruction were not significantly different from that of the intact knee. The mean rotations with DB2, DB3, and DB4 reconstructions were significantly less than that of the intact knee and also significantly less than that of a single-bundle reconstruction. After the pivot shift (valgus moment and iliotibial force), the mean external rotations with double-bundle reconstructions were significantly greater than those of the intact knee and of a single-bundle reconstruction (Fig. 1).
The mean total pivot-shift rotations (rotation after pivot minus rotation before pivot) for intact and anterior cruciate ligament-deficient knees were 11.1° ± 3.2° and 19.7° ± 5.5°, respectively (Fig. 2). The mean total pivot-shift rotations with single-bundle and DB1 reconstructions were not significantly different from that of the intact knee, while the mean rotations for the DB2, DB3, and DB4 reconstructions were significantly less than that of the intact knee and also significantly less than that of a single-bundle reconstruction (Fig. 2).
Before the pivot shift, the mean valgus rotation with the anterior cruciate ligament removed was 4.2° ± 2.3° greater than that of the intact knee, and the mean valgus rotations with all reconstructions were not significantly different from that of the intact knee (Fig. 3); the mean valgus rotations for the DB2, DB3, and DB4 reconstructions were significantly less than that of a single-bundle reconstruction. After the pivot shift, the mean valgus rotations with all reconstructions were not significantly different from that of the intact knee (Fig. 3). The mean total pivot-shift varus-valgus rotations (rotation after pivot minus rotation before pivot) for intact and anterior cruciate ligament-deficient knees were 1.4° ± 0.6° and 4.4° ± 1.8°, respectively (calculated from Fig. 3).
Before the pivot shift, the mean anterior displacement of the lateral tibial plateau with the anterior cruciate ligament removed was 11.7 ± 3.8 mm greater than that for the intact knee (Fig. 4); the mean displacements with all reconstructions were significantly less than that of the intact knee, while the mean displacements for the DB3 and DB4 reconstructions were significantly less than that of a single-bundle reconstruction. After the pivot shift, the mean displacements with DB2, DB3, and DB4 reconstructions were significantly less than that of the intact knee (Fig. 4).
The mean total pivot-shift displacements (displacement after pivot minus displacement before pivot) for intact and anterior cruciate ligament-deficient knees were 10.4 ± 2.9 mm and 24.9 ± 5.2 mm, respectively (Fig. 5). The mean total pivot-shift displacement with a single-bundle reconstruction was not significantly different from that of the intact knee, while all mean total pivot-shift displacements for double-bundle reconstructions were significantly less than that of the intact knee and also significantly less than that of a single-bundle reconstruction (Fig. 5).
Before the pivot shift, the mean graft forces were significantly greater than native anterior cruciate ligament forces with all reconstructions (Fig. 6), and the mean graft forces with DB3 and DB4 reconstructions were also significantly greater than that of the single-bundle reconstruction. After the pivot shift, the mean graft force for the single-bundle reconstruction was significantly less than the intact anterior cruciate ligament force, while the mean force for the DB4 reconstruction was significantly greater; the mean forces for the DB3 and DB4 reconstructions were significantly greater than that of the single-bundle reconstruction. The mean graft forces for all reconstructions were reduced significantly (p < 0.05) after the pivot shift had occurred (Fig. 6).
The goal of this study was to determine whether single-bundle and anatomic double-bundle reconstructions could restore normal knee kinematics and graft forces during simulated pivot-shift tests. Although all reconstructions were able to restore knee kinematics to the intact (or less than intact) levels, graft forces were significantly higher than those of the native anterior cruciate ligament for all reconstructions prior to the pivot-shift event.
It is well recognized by clinicians that the ability to consistently elicit a pivot shift requires considerable experience and training. Our simulation of the pivot-shift event provided a consistent methodology for comparing different anterior cruciate ligament reconstructions in the same knee under identical loading conditions. The loading conditions to produce a pivot shift in the anterior cruciate ligament-deficient knee varied among specimens and represented a delicate equilibrium between applied valgus moment and iliotibial force. To produce the pivot-shift event, valgus moment was applied, the knee was flexed to the appropriate angle, and then iliotibial force was applied to rotate the tibia externally. This was a logical sequence for loading since it is the passive iliotibial band tension that produces the pivot in vivo.
The coupled internal rotation from an applied valgus moment alone increased an average of 8.0° ± 2.5° when the anterior cruciate ligament was removed. Coupled internal rotations with DB2, DB3, and DB4 reconstructions were significantly less than that for the intact knee (and within a few degrees of each other). This situation was caused by the higher graft forces that tend to pull the tibia into external rotation. The single-bundle and DB1 reconstructions restored coupled internal rotations from applied valgus moment to intact knee levels. We believe that increased coupled rotation may correspond to the instability associated with the sensation of giving-way that is reported by many patients with an anterior cruciate ligament-deficient knee during twisting, cutting, or pivoting maneuvers that involve changes in direction (presumably with a varus thrust to the knee). It has been reported that the functional outcome of patients undergoing anterior cruciate ligament reconstruction is positively associated with elimination of the pivot-shift sign4, and the absence of giving-way symptoms is an important component of a good functional result.
We found no prior studies in the literature that compared simulated pivot-shift tests with single-bundle and double-bundle reconstructions. Matsumoto and Seedhom1,15, using an applied valgus torque of 12.5 N-m, reported that cutting the anterior cruciate ligament increased internal tibial rotation in some knee specimens between 20° and 40° of flexion. The magnitude of the tibial rotation increase and the mean flexion angle at which it occurred were not reported. Bull et al.16 measured tibial rotations with a mean valgus moment of 7 N-m and a mean iliotibial band tension of 30 N. Cutting the anterior cruciate ligament produced a pivot shift in eight of fifteen knees. The mean increase in tibial rotation was 17° at a mean flexion angle of 56°. In our study, the mean valgus moment and iliotibial force to pivot the anterior cruciate ligament-deficient knee were 3.4 N-m and 29.0 N, respectively, applied at a mean flexion angle of 26.7°. We found that removing the anterior cruciate ligament increased tibial rotation during the pivot-shift event by a mean of 8.6°.
Our test methodology offered a powerful and unique advantage over the previously cited studies. The combination of valgus moment and iliotibial force necessary to reproducibly pivot the anterior cruciate ligament-deficient knee was different for each specimen and was determined through trial and error. We found that it was always possible to find a combination of applied valgus moment and iliotibial force that elicited a pivot shift with the bone cap disconnected (the anterior cruciate ligament-deficient state). Then the bone cap was reattached to the load cell, and the test was repeated to determine the amount of pivot with the anterior cruciate ligament intact. Since the loading conditions required to pivot the anterior cruciate ligament-deficient knee were unknown for each intact specimen, it would have been necessary to test intact knees under a wide variety of loading combinations16 with the hope that one of them would produce a pivot when the anterior cruciate ligament was sectioned. With our methodology, the loading combination to produce a pivot shift was first determined for the anterior cruciate ligament-deficient knee. Then the anterior cruciate ligament was "restored" to measure the pivot shift for an intact knee under the same loading conditions.
We found that the instability created by applying a valgus moment to an anterior cruciate ligament-deficient knee was more complex than has been reported previously. As others have reported1,15, removal of the anterior cruciate ligament increased the coupled internal tibial rotation from an applied valgus moment; this increase averaged 8.0° ± 2.5° in our study. However, removal of the anterior cruciate ligament also increased valgus rotation of the tibia, by an average of 4.2° ± 2.3° in our study. Valgus rotation was restored to normal with all reconstructions, while the coupled internal rotations from an applied valgus moment were overcorrected (less than the intact knee) by the higher graft forces associated with the DB2, DB3, and DB4 reconstructions.
The application of iliotibial force pulled the tibia into external rotation, thereby unwinding the cruciate ligaments from one another; this consistently lowered the force levels in all reconstructions. The higher graft tensions with double-bundle reconstructions left the tibia externally rotated (compared with the intact knee) after the pivot shift had occurred.
There were several limitations to our study. Our tests only simulated the actual pivot-shift event, not the test as performed clinically. In the clinical pivot-shift test, the level of applied valgus moment (possibly combined with a slight internal torque) may change as the examiner flexes the knee, something we could not simulate in our test apparatus. During the clinical test, some measure of applied internal torque may also be required to overcome the passive iliotibial tension in some patients to help to produce the coupled internal tibial rotation. In vivo, the iliotibial force almost certainly varies with flexion angle and is probably much greater than the levels used for our tests.
In the present study, the graft tissue for the single-bundle reconstruction was sized to fit tightly within a 7-mm femoral tunnel. This roughly corresponded to the amount of soft tissue that would be present with a bone-patellar tendon-bone graft that was 8 to 9 mm in width. This was used as a baseline from which a posterolateral graft was added with use of different tensioning strategies. 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 use 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 6-mm posterolateral graft while still leaving an adequate bone bridge between the two tunnels. Clinically, 4.5 to 7-mm grafts are commonly used for double-bundle reconstructions.
Since a primary goal of an anterior cruciate ligament reconstruction is to restore a normal Lachman examination at 30° of flexion, the single-bundle graft was tensioned at this position. Therefore, the laxity with a 10 to 11-mm anteromedial graft would have been the same as with our 7-mm anteromedial graft. However, it is possible that less initial graft tension could have been required to match intact knee laxity at 30° with a larger graft (because of its increased stiffness). This in turn could have increased the internal rotation from the applied valgus moment, and the magnitude of the pivot-shift event. However, relative differences in pivot shift between single and double-bundle grafts would remain unchanged because the same tensioning protocol for the single-bundle graft was used for all reconstructions.
In conclusion, our data show that a conventional single-bundle reconstruction was sufficient to restore intact knee kinematics during a simulated pivot-shift event. Therefore, the need for a more technically complex and time-consuming double-bundle reconstruction to restore a normal pivot-shift sign is questioned.
Details of the test apparatus are available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). 
Note: The human tissues utilized for this study were provided by the Musculoskeletal Transplant Foundation.
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