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Anterior-Posterior and Rotatory Stability of Single and

Double-Bundle Anterior Cruciate Ligament Reconstructions
Keith L. Markolf, Samuel Park, Steven R. Jackson and David R. McAllister
J Bone Joint Surg Am. 2009;91:107-118. doi:10.2106/JBJS.G.01215

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C OPYRIGHT Ó 2009 BY T HE J OURNAL OF B ONE AND J OINT S URGERY, I NCORPORATED

Anterior-Posterior and Rotatory Stability of

Single and Double-Bundle Anterior Cruciate
Ligament Reconstructions
By Keith L. Markolf, PhD, Samuel Park, MD, Steven R. Jackson, and David R. McAllister, MD

Investigation performed at the Biomechanics Research Section, Department of Orthopaedic Surgery, David Geffen School of Medicine at the University
of California at Los Angeles, Los Angeles, California

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 1 30 N) at 10° (DB3), and
posterolateral tension = (anteromedial tension 1 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.

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in
excess of $10,000 from NFL Charities. Neither they nor a member of their immediate families received payments or other benefits or a commitment or
agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any
research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their
immediate families, are affiliated or associated. Human tissues utilized for this study were provided by the Musculoskeletal Transplant Foundation.

J Bone Joint Surg Am. 2009;91:107-18 d doi:10.2106/JBJS.G.01215

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Fig. 1
Mean anteroposterior (AP) laxities (at ±100 N of tibial force) versus knee flexion angle for the intact knee and after single-bundle (SB) and double-bundle (DB)
reconstructions. The error bars indicate the standard deviation. The mean laxities with graft reconstructions were significantly different (p < 0.05) from those
of the intact knee at the following knee flexion angles: 0°, 45°, 70°, and 90° for single bundle; 30°, 45°, 70°, and 90° for DB1; 10°, 30°, 45°, 70°, and 90° for
DB2; and all flexion angles for DB3 and DB4.

T
he treatment of the anterior cruciate ligament-deficient anterior-posterior stability of single-bundle reconstructions with
knee continues to be an area of considerable debate. that of double-bundle reconstructions have shown no subjective
Single-bundle anterior cruciate ligament reconstructions or objective differences between the two procedures13-15, while
have been reported to have success rates ranging from 83% others have found small reductions in laxity values, mea-
to 95%1-3. However, a number of other studies have found less sured on a KT-1000 arthrometer, at 30° of flexion (£1.1 mm)
satisfactory results following anterior cruciate ligament re- with a double-bundle reconstruction16-19. It has also been
construction4-10. claimed that a double-bundle reconstruction is more effec-
One important outcome measure of anterior cruciate tive in controlling rotational stability than is a single-bundle
ligament reconstruction is the ability of the graft to control reconstruction20, and there is clinical evidence that patients
anterior tibial translation. Another important outcome mea- with a double-bundle reconstruction demonstrate improved
sure is the reduction or elimination of the giving-way symptom. pivot-shift results postoperatively compared with patients re-
This is a rotatory instability produced by two important loading ceiving a conventional single-bundle reconstruction14-16,18,19. Rel-
mechanisms. The first mechanism is internal tibial torque, atively few published cadaver studies have directly compared
which causes the cruciate ligaments to wind around each other, anterior-posterior stability of knees with single-bundle and
thereby loading the anterior cruciate ligament. In the anterior double-bundle reconstructions20,21. The isometry of double-
cruciate ligament-deficient knee, resistance to internal rotation bundle grafts has received less study13.
is diminished. The second mechanism is valgus moment, which The purposes of this study were to measure the abilities of
produces a coupled internal rotation of the tibia that increases single and double-bundle anterior cruciate ligament recon-
when the anterior cruciate ligament is removed11. The internal structions to restore anterior-posterior and rotatory stability to
rotations produced by both loading mechanisms are a key the knee and to compare graft forces in reconstructed knees
component of the pivot-shift examination. A reduction or with forces in the intact anterior cruciate ligament for identical
elimination of the pivot shift has been shown to be positively loading conditions. Length changes (isometry) of anterome-
associated with an improved functional outcome following dial and posterolateral grafts in situ were also examined.
anterior cruciate ligament reconstruction12.
The anterior cruciate ligament consists of separate anter- Materials and Methods
omedial and posterolateral bundles, and some surgeons currently
reconstruct both bundles. Several clinical studies comparing the F ourteen fresh-frozen unpaired cadaveric knee specimens
from male donors were used. The mean age (and standard
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Fig. 2
Mean curves of resultant force versus knee flexion angle for passive knee flexion with no applied tibial force. The error bars indicate the standard de-
viation. The mean curves are shown for the intact knee (acl) and after single-bundle (SB) and double-bundle (DB) reconstructions. The mean forces
with graft reconstructions were significantly different (p < 0.05) from native anterior cruciate ligament (acl) forces at all flexion angles, with the
exception of SB at 0°.

deviation) of the donors at the time of death was 36.5 ± 12.7 terolateral bundles were marked on the bone, and the ap-
years (range, seventeen to sixty-seven years). The articular proximate center of each bundle footprint was marked with a
surfaces were inspected visually for evidence of cartilage ero- punch. With use of alginate casting material, a negative mold
sion or other arthritic abnormalities, and none were found. of the tibial bone cap was made and filled with poly-
Ten of the fourteen specimens were also utilized in a related methylmethacrylate, resulting in an acrylic replica (positive) of
study22. The tibia and femur were potted in cylindrical molds the original anterior cruciate ligament bone cap. The marked
of polymethylmethacrylate. The tibial insertion of the ante- outlines (and centers) of the anteromedial and posterolateral
rior cruciate ligament was mechanically isolated with use of a fiber bundles were automatically transferred over to the acrylic
cylindrical coring cutter, and a cap of bone containing the replica during the casting process.
entire footprint was attached to a custom-designed load cell Two patellar tendon allografts (obtained from the
that measured resultant force in the ligament23. Musculoskeletal Transplant Foundation, Edison, New Jersey)
Anterior-posterior laxity testing was performed with were prepared for testing. Each graft consisted of a single,
± 100 N of applied tibial force at six angles of knee flexion. appropriately sized, bone block with attached tissue sized to
Anterior cruciate ligament force was recorded as the knee was fit tightly within a 7-mm femoral tunnel. This roughly cor-
passively extended from 120° to 0° of flexion with no applied responded to the amount of soft tissue that would be present
tibial force, a 100-N anterior tibial force, a 5-N-m internal with an 8 to 9-mm-wide patellar tendon graft. The antero-
tibial torque, and a 5-N-m valgus moment. Tibial rotations medial and posterolateral tibial footprints were cored out
from applied internal torque were recorded with the anterior from the acrylic bone-cap replica, and the patellar bone
cruciate ligament intact, removed, and reconstructed. Specific blocks were potted with polymethylmethacrylate into the
details of our testing apparatus and testing protocols have recesses.
been published previously 24-26 and are presented in the The knee was flexed to 90°, and an anterior force was
Appendix. applied manually to the tibia. The origin of the slackened
The anterior cruciate ligament was cut, and the tibial posterolateral fiber bundle on the lateral wall of the fem-
bone cap was removed from the knee. The remaining an- oral notch was identified and dissected from bone. A 5-mm
teromedial and posterolateral fiber bundles were followed offset guide was used to drill a 7-mm anteromedial tunnel
down to their tibial insertion and were dissected from bone. at the one o’clock position (left knee). Then a second 7-
The outlines of the footprints of the anteromedial and pos- mm tunnel was drilled at the center of the posterolateral
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Fig. 3
Mean curves of resultant force versus knee flexion angle for passive knee flexion with 100 N of applied anterior tibial force. The error bars
indicate the standard deviation. The mean curves are shown for the intact knee (acl) and after single-bundle (SB) and double-bundle (DB)
reconstructions. The mean forces with graft reconstructions were significantly different (p < 0.05) from native anterior cruciate ligament (acl)
forces at the following flexion angles: 70° to 120° for single bundle, 60° to 120° for DB1, 0° and 40° to 100° for DB2, and all flexion angles for
DB3 and DB4.

footprint (approximately the 2:30 position in the notch). This The single-bundle anteromedial graft was tensioned
normally left a 1.5 to 3.5-mm bone bridge between the tunnel to restore anteroposterior laxity to within 1 mm of that of
edges. the intact knee at 30° of flexion, and all tests described above
The cap replica with potted bone blocks was attached to the were repeated with the single-bundle graft. For double-
tibial load cell. Low-stretch, high-strength synthetic lines (135-lb bundle reconstructions, the anteromedial graft was first
[61.2-kg] test Spectra Fiber; Izorline, Gardena, California), whip- tensioned at 30° as above and then the posterolateral graft
stitched into the free end of each graft, were passed through the was tensioned at either 30° or 10° with use of one of four
two femoral tunnels and through separate split-clamps cemented protocols: posterolateral tension = anteromedial tension at
into the acrylic mass used to pot the femur. 10° (DB1); posterolateral tension = anteromedial tension
The knee was placed in the testing apparatus and was at 30° (DB2); posterolateral tension = (anteromedial ten-
extended to 0° of flexion. A dial caliper was used to measure a sion 1 30 N) at 10° (DB3); and posterolateral tension =
baseline length between a forceps clamped to the lines attached (anteromedial tension 1 30 N) at 30° (DB4). All tests were
to the free (femoral) end of the graft and the edge of the femo- repeated with double-bundle grafts. With the double-bun-
ral split-clamp used to fix the graft. The knee was then incre- dle reconstructions, the tibial load cell recorded the com-
mentally flexed to 10°, 30°, 45°, 70°, and 90°. At each flexion bined (total) tension of both grafts.
angle, changes in the distance between the clamped forceps A two-factor repeated-measures analysis of variance was
and femoral split-clamp were recorded relative to the distance used to compare mean anterior-posterior laxities, tibial rota-
at 0°. An increase in graft length with knee flexion corresponded tions, and graft forces among test conditions. The factors were
to slackening of a graft fixed at full extension. The position of reconstruction status (anterior cruciate ligament intact, ante-
the tibia relative to the femur was reproduced at each knee rior cruciate ligament removed, single-bundle graft, and
flexion angle by manually holding the tibia at the midpoint of double-bundle grafts) and knee flexion angle. Pairwise com-
internal-external rotation (for the intact knee) while applying a parisons were made with use of the Student-Neuman-Keuls
slight manual compressive force along the axis of the tibia to procedure. The level of significance was set at p < 0.05. A
maintain tibiofemoral contact at both condyles. A spring scale similar analysis was used to compare mean graft excursions for
was used to apply a 27-N force to the graft lines during the the two grafts; the factors were type of graft (anteromedial or
measurements. posterolateral) and knee flexion angle.
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Fig. 4
Mean curves of tibial rotation versus knee flexion angle for passive knee flexion with 5 N-m of applied internal torque. The error bars indicate the standard
deviation. The mean curves are shown for the intact knee, with the anterior cruciate ligament (acl) cut, and after single-bundle (SB) and double-bundle (DB)
reconstructions. The mean rotations with graft reconstructions were significantly different (p < 0.05) from those of the intact knee at the following flexion
angles: 50° to 120° for DB3 and 60° to 120° for DB4.

Source of Funding reconstructions were 1.2 ± 1.2 mm less than those of the
The funding for this study was provided by NFL Charities. intact knee at 10° and 30°.
For passive knee flexion with no applied tibial force, the
Results mean graft forces with all reconstructions were significantly

T he 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
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
mean graft tensions used for testing were 13.6 ± 8.4 N for the anterior cruciate ligament) were 51.3 ± 43.6 N for DB1, 100.2 ±
anteromedial single bundle; 32.7 ± 19.7 N for the anterome- 56.0 N for DB2, 126 ± 57.2 N for DB3, and 195.2 ± 73.6 N for
dial bundle and 32.6 ± 18.4 N for the posterolateral bundle DB4 reconstructions.
for DB1; 13.6 ± 8.4 N and 18.8 ± 9.4 N, respectively, for DB2; With a 100-N anterior tibial force, the mean single-
32.7 ± 19.7 N and 57.8 ± 17.8 N for DB3; and 13.6 ± 8.4 N and bundle graft forces were not significantly different from anterior
43.6 ± 9.8 N for DB4. cruciate ligament forces with a knee flexion angle between 0°
and 65° (Fig. 3). The mean forces for DB1 reconstructions were
Anteroposterior Stability not significantly different from those of the anterior cruciate
The mean laxities with a single anteromedial graft were not ligament with flexion between 0° and 55°; the mean forces for
significantly different from those of an intact knee at 10° and 30°. DB2 reconstructions were not significantly different with
The mean laxity with a single anteromedial graft was 0.9 ± 0.8 flexion between 5° and 35°. The mean forces for DB3 and DB4
mm greater than that of the intact knee at 0° and 1.1 ± 1.0 mm reconstructions were significantly higher than the mean ante-
less than that of the intact knee at 90° (Fig. 1). The mean rior cruciate ligament forces at all flexion angles (p < 0.05). At
laxities with DB3 and DB4 reconstructions were significantly 0°, the mean increases in graft force (relative to the native ante-
less than those of the intact knee at all flexion angles (p < rior cruciate ligament) were 59.4 ± 75.1 N for DB2, 96.8 ± 76.4
0.05); the mean laxities of DB4 reconstructions were 2.6 ± 1.7 N for DB3, and 137.4 ± 84.9 N for DB4 reconstructions.
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 Rotatory Stability
reconstructions were significantly less than those of the intact For applied internal torque, removal of the anterior cruciate
knee from 30° to 90° (p < 0.05). The mean laxities of the DB2 ligament significantly increased internal rotation with mean in-
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Fig. 5
Mean curves of resultant force versus knee flexion angle for passive knee flexion with 5 N-m of applied internal torque. The error bars indicate the standard
deviation. The mean curves are shown for the intact knee (acl) and after single-bundle (SB) and double-bundle (DB) reconstructions. The mean forces with
graft reconstructions were significantly different (p < 0.05) from the native anterior cruciate ligament (acl) forces at the following flexion angles: 10° to 120° for
DB1 and all flexion angles for DB2, DB3, and DB4.

Fig. 6
Mean curves of tibial rotation versus knee flexion angle for passive knee flexion with 5 N-m of applied valgus moment. The error bars indicate the standard
deviation. The mean curves are shown for the intact knee, with the anterior cruciate ligament (acl) cut, and after single-bundle (SB) and double-bundle (DB)
reconstructions. The mean rotations with graft reconstructions were significantly different (p < 0.05) from those of the intact knee at all flexion angles for DB2,
DB3, and DB4. The mean rotations with the anterior cruciate ligament (acl) cut were significantly different (p < 0.05) from those of the intact knee at 0° to 120°.
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Fig. 7
Mean curves of valgus rotation versus knee flexion angle for passive knee flexion with 5 N-m of applied valgus moment. The error bars indicate the standard
deviation. The mean curves are shown for the intact knee, with the anterior cruciate ligament (acl) cut, and after single-bundle (SB) and double-bundle
(DB) reconstructions. The mean rotations with graft reconstructions were significantly different (p < 0.05) from those of the intact knee at 35° to 120°
for DB4. The mean rotations with the anterior cruciate ligament (acl) cut were significantly different (p < 0.05) from those of the intact knee at all
flexion angles.

creases of 7.3° ± 3.4° at 0° of flexion and 4.0° ± 2.8° at 30° of single-bundle and DB1 reconstructions were not significantly
flexion (p < 0.05) (Fig. 4). The mean rotations with single- different from those of the intact knee. The rotations for DB2,
bundle and DB1 reconstructions remained approximately 2° DB3, and DB4 reconstructions were significantly less than
to 4° greater than those of the intact knee. The mean rotations those of the intact knee (p < 0.05); at 0°, the highly ten-
with DB2 reconstructions were not significantly different from sioned posterolateral graft overcorrected tibial rotation
those of the intact knee. The mean rotations with DB3 and compared with the intact knee.
DB4 reconstructions were not significantly different from The application of valgus moment to the intact knee also
those of the intact knee at flexion angles of <50° and <60°, produced valgus rotation of the tibia. Removal of the anterior
respectively. cruciate ligament significantly increased valgus rotation (p <
Graft forces with applied internal torque were not sig- 0.05); the mean increases at 0° and 30° of flexion were 1.9° ±
nificantly different from anterior cruciate ligament forces for 4.4° and 4.4° ± 2.3°, respectively (Fig. 7). The mean valgus
single-bundle reconstructions (all flexion angles) and DB1 rotations with all reconstructions were not significantly
reconstructions (flexion angles of <10°) (Fig. 5). Graft forces different from those of the intact knee, with the exception
for DB2, DB3, and DB4 reconstructions were significantly of DB4 reconstructions, in which rotations were signifi-
higher than the anterior cruciate ligament forces (p < 0.05). cantly less than those of the intact knee at flexion angles of
The mean anterior cruciate ligament force was 180.3 ± 48.4 N >30° (p < 0.05).
at 0°. The corresponding mean forces for double-bundle The graft forces from applied valgus moment were not
reconstructions were 222.1 ± 55.5 N for DB1, 261.6 ± 48.1 N significantly different from anterior cruciate ligament forces with
for DB2, 280.4 ± 52.7 N for DB3, and 323.6 ± 64.9 N for DB4. a single-bundle reconstruction, whereas the mean forces with all
The application of valgus moment to the intact knee double-bundle reconstructions were significantly higher (p < 0.05)
caused the tibia to rotate internally as the knee was flexed (Fig. (Fig. 8). The mean anterior cruciate ligament force was 75.0 ±
6). Removal of the anterior cruciate ligament significantly in- 37.3 N at 0° of flexion. The corresponding means for double-
creased this coupled internal rotation between 0° and 50° (p < bundle reconstructions were 108.7 ± 45.1 N for DB1, 158.0 ±
0.05); the mean increases at 0° and 30° of flexion were 5.2° ± 56.9 N for DB2, 179.8 ± 62.4 N for DB3, and 246.3 ± 66.5 N
3.2° and 7.8° ± 3.6°, respectively. The mean rotations with for DB4.
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Fig. 8
Mean curves of resultant force versus knee flexion angle for passive knee flexion with 5 N-m of applied valgus moment. The error bars indicate the standard
deviation. The mean curves are shown for the intact knee (acl) and after single-bundle (SB) and double-bundle (DB) reconstructions. The mean forces with
graft reconstructions were significantly different (p < 0.05) from those of the native anterior cruciate ligament (acl) forces at all flexion angles for DB1, DB2,
DB3, and DB4.

Fig. 9
Mean length changes for anteromedial (AM) and posterolateral (PL) grafts; the error bars indicate one standard deviation. All values are referenced to 0° of
flexion. Positive numbers indicate graft-lengthening with increasing knee flexion (corresponding to slackening of a graft fixed at both ends with the knee at 0°
of flexion). The mean values for anteromedial and posterolateral grafts were significantly different beyond 10° of flexion (p < 0.05).
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Graft Isometry Our isometry results indicate that initial tensions in the
The changes in graft length between anteromedial and posterolateral graft (set with the knee in a flexed position) were
posterolateral grafts were significantly different at >10° of magnified as the knee was extended to 0°. This effect was greater
flexion (p < 0.05) (Fig. 9). The mean change in length for when the posterolateral graft was tensioned at 30° because the
the posterolateral graft (4.6 ± 3.0 mm) was significantly posterolateral graft underwent a greater length change. There-
greater than that for the anteromedial graft (3.0 ± 1.0 mm) fore, it may be advisable for the surgeon to avoid high tension in
at 30° of flexion (p < 0.05) (Fig. 9). The mean difference in the posterolateral graft (especially with the knee at 30° of flexion)
the change in length between the anteromedial and the to prevent high forces that could stretch out the graft. Hamada
posterolateral graft at 30° was 1.6 ± 1.0 mm (range, 0.1 to et al.13 also found that both grafts tightened with extension on
3.2 mm). the operating table; the anteromedial graft tightened an average
of 1.6 mm, and the posterolateral graft tightened an average of
Graft Failures 2.1 mm.
Three posterolateral grafts failed during testing; one of them We found only two studies in the literature that directly
failed with applied internal torque and two, with applied compared the anteroposterior stability of single-bundle and
anterior tibial force (during laxity testing). All failures oc- double-bundle reconstructions of the anterior cruciate liga-
curred at 0°, where the force in the posterolateral graft was ment. Mae et al.21 reported that laxity differences between
high because of its nonisometric behavior. Visually, the graft single-bundle and double-bundle reconstructions were 0.7 to
tissue had stretched out in midsubstance. The failed grafts 1.4 mm between 0° and 30° and were not significantly different
were replaced with new ones, and the complete test series was at 60° and 90°. We also found laxity differences between single-
repeated. No failures of the anteromedial graft occurred with bundle and double-bundle reconstructions on the order of 1 to
any test. 2 mm, but there is an important difference in graft-tensioning
protocols between the two studies. We tensioned the single-
Discussion bundle graft to a level that restored normal findings on the

T his study was designed to compare anteroposterior lax-

ities, tibial rotations, and graft forces for single and double-
bundle anterior cruciate ligament reconstructions with those
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
of the intact knee. Overall, a single-bundle reconstruction normal laxity at 30°. With 88-N graft tension, laxity with the
was sufficient to restore coupled tibial rotations, graft forces, single-bundle reconstruction was 2.6 mm less than that of the
and anteroposterior laxities to normal or near-normal levels. intact knee at 30°. All of their double-bundle reconstructions
Because of its nonisometric behavior, the posterolateral graft substantially overconstrained the knees (compared with the
developed relatively high forces near 0° when it was tensioned intact knee) between 0° and 30°. For example, the mean knee
and fixed in a flexed knee position. This tended to reduce laxity with 88 N of tension on each graft was 3.4 mm less than
anteroposterior laxities and tibial rotations produced by that of the intact knee at 30°.
applied internal torque and a valgus moment, and it over- Yagi et al.20 reported a mean difference in anterior tibial
constrained the knee with some double-bundle tensioning translation between single-bundle and double-bundle recon-
protocols. structions of 2.4 mm at 30°, which is comparable with that
A 7-mm single-bundle reconstruction (approximately found in our study. However, since they used the same graft ten-
equivalent to an 8 to 9-mm-wide bone-patellar tendon-bone sions for all knees, neither single-bundle nor double-bundle re-
graft) was used as a baseline from which a posterolateral graft constructions restored a normal result on the Lachman test at 30°.
was added with use of different tensioning strategies. Since a The mean laxities at 30° with single-bundle and double-bundle
primary goal of an anterior cruciate ligament reconstruction is grafts were greater by 3.8 mm and 1.4 mm, respectively, than those
to restore normal findings on the Lachman examination at 30° of the intact knee.
of flexion, the single-bundle graft was tensioned at this po- With our testing protocol, a 5-N-m internal tibial torque
sition. If the single-bundle graft were tensioned at 0°, our data and a 5-N-m valgus moment were applied separately in dif-
show that higher graft tensions would have been required. Be- ferent tests. This allowed evaluation of rotatory stability for each
cause of the repeated-measures study design, our single-bundle mode of loading independently. Both loading modes produced
reconstruction did not have a 10 to 11-mm anteromedial graft, as internal tibial rotations that increased when the anterior cru-
is used clinically. Although it would have been desirable to have ciate ligament was removed. We believe that increased internal
utilized a larger graft, the femoral footprint of the anterior cru- tibial rotation in the anterior cruciate ligament-deficient knee
ciate ligament was too small to accommodate a 10 to 11-mm contributes to the giving-way symptom commonly reported by
anteromedial graft and a 7-mm posterolateral graft while still patients who have sustained an injury of the anterior cruciate
leaving an adequate bone bridge between the two tunnels. ligament. Therefore, the abilities of single-bundle and double-
Clinically, 4.5 to 6-mm grafts are commonly used for double- bundle reconstructions to limit both rotatory instabilities have
bundle reconstructions. We chose 7-mm grafts because they direct bearing on the reduction or elimination of giving-way
could better withstand the repeated forces generated by our symptoms after anterior cruciate ligament reconstructions have
tibial loading tests. been performed.
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There were new and interesting findings related to ap- creases in internal tibial rotation after removal of the anterior
plied valgus moment that have not been reported previously. cruciate ligament were recorded with both modes of loading.
Removal of the anterior cruciate ligament not only increased Coupled tibial rotations from applied valgus moment were
coupled internal rotation of the tibia but also increased valgus restored to normal with a single-bundle graft, but internal
rotation of the tibia as well. This represents a complex com- rotations from applied tibial torque were not. Theoretically,
bined rotatory-valgus instability that may be related to the the posterolateral graft has been claimed to have a slightly
giving-way sensation experienced by patients. All reconstruc- better mechanical advantage in controlling tibial torque than
tions restored valgus rotations to normal between 0° and 30° of the anteromedial graft because the posterolateral graft is acting
flexion. at a greater distance from the axis of rotation20. However, since
Our load cell recorded the combined resultant force the moment arms for both grafts are relatively small, high graft
for both graft bundles. The addition of a posterolateral graft forces are required to generate a resistive torque with both
to an anteromedial graft increased the resultant force as the reconstructions. This explains why tibial rotations from ap-
knee was extended to 0°. Since the force in each graft is plied torque were not restored to normal with the relatively
proportional to its change in length, and the change in length low forces in a single-bundle graft and were restored to normal
for the posterolateral graft with knee extension was greater with the higher graft forces encountered with some double-
than that for the anteromedial graft, we believe that a large bundle reconstructions.
portion of the increase in resultant force from the addition of The key to understanding the forces developed in the
the posterolateral graft was carried by that graft. This is anteromedial and posterolateral grafts is their in situ isometry
further supported by the fact that only posterolateral grafts over a 90° range of motion. The force developed in a fixed
failed during testing, and both grafts were the same size. The graft is directly related to how much it elongates as the knee is
high posterolateral graft forces at 0° were increased even moved. Therefore, the difference in patterns of length change
further by the application of anterior tibial force and internal between anteromedial and posterolateral grafts over the range
torque, modes of loading that produced posterolateral graft of motion determines which bundle develops the highest
failures at 0°. tension. Small differences in graft elongation can lead to large
Yagi et al.20 compared coupled anterior tibial translations differences in graft force because of the inherent stiffness of
and graft forces under a combined 5-N-m internal torque and the graft construct.
10-N-m valgus moment for single-bundle and anatomic double- Length changes of double-bundle grafts can be easily
bundle reconstructions. They found that coupled anterior evaluated on the operating table. With one end of the graft
tibial translations with double-bundle reconstructions were 2.4 fixed and a slight manual tension applied to the free end,
and 2.0 mm less than translations with single-bundle recon- the surgeon moves the knee through a range of motion and
structions at 15° and 30°, respectively; these differences would observes the relative motion of the graft, which provides
most likely represent small changes in tibial rotation (as re- an accurate indication of which graft will be more highly
corded in our tests). These findings have formed the principal loaded. The magnitude of the length change indicates the
basis for the commonly cited claim that a double-bundle re- magnitude of the force that will be developed in a fixed graft
construction provides better control of rotatory knee stability as the knee is extended. By performing this simple test, er-
than a single-bundle reconstruction. rors in graft-tensioning that could produce high graft forces
A direct comparison of our results with those of Yagi may be avoided.
et al.20 is not possible. They applied a combined load of 5 N-m One of the commonly stated justifications for a double-
of internal torque with 10 N-m of valgus moment, while our bundle anterior cruciate ligament reconstruction is better
results are based on applied loadings of 5 N-m of internal restoration of rotatory stability. Our results show that any im-
torque and 5 N-m of valgus moment (applied separately). provements in stability from adding a posterolateral graft came
They found that coupled anterior tibial translations remained with the cost of markedly higher forces in the posterolateral
substantially greater than normal with both single-bundle and graft. These high graft forces reduced the internal rotation from
double-bundle reconstructions. We found that coupled tibial applied tibial torque, overcorrected the internal tibial rotation
rotations from an applied valgus moment were restored to nor- from applied valgus moment, and overconstrained anteropos-
mal with a single-bundle graft and were significantly less than terior laxity of the knee. They also produced occasional ruptures
normal with DB3 and DB4 grafts. We found that internal rotations of the 7-mm posterolateral graft.
from applied internal torque were significantly greater than nor- Some surgeons use 4.5-mm-diameter grafts for double-
mal with single-bundle reconstructions (p < 0.05) and were not bundle reconstructions. The rupture of some 7-mm postero-
significantly different from normal (0° to 45°) with DB3 and DB4 lateral grafts during the course of testing in this study suggests
reconstructions. that 4.5-mm grafts may be clinically at risk. If a 4.5-mm pos-
The question that remains is whether a double-bundle terolateral graft were to rupture or stretch out, the 4.5-mm
reconstruction is necessary to eliminate the rotatory instabil- anteromedial graft would be the only tissue left to carry load.
ities for the two modes of loading that we studied or a single- This could lead to eventual failure of the anteromedial graft
bundle reconstruction is sufficient. Our results provide a as well. Although we know of no anteromedial graft failures
mixed answer to this question. In our tests, significant in- that have been reported in clinical series with double-bundle
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reconstructions, rupture of the posterolateral graft has been tronic version of this article, on our web site at jbjs.org (go to the
reported, with failure rates of 3%27, 7%28, and 11%29. Therefore, article citation and click on Supplementary Material) and on our
it is reasonable to ask whether the relatively small changes in quarterly CD/DVD (call our subscription department, at 781-
anteroposterior and rotatory stability are worth the accom- 449-9780, to order the CD or DVD). n
panying 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 cou- Keith L. Markolf, PhD
pled internal rotations from an applied valgus moment (a Samuel Park, MD
key component of the pivot shift test) to normal levels. On Steven R. Jackson
the basis of our findings, the need for a more technically David R. McAllister, MD
complex and time-consuming double-bundle reconstruction Biomechanics Research Section,
Department of Orthopaedic Surgery,
is questioned.
University of California at Los Angeles Rehabilitation Center,
1000 Veteran Avenue, Room 21-67,
Appendix Los Angeles,
The figures showing the specific details of our testing ap- CA 90095-1759.
paratus and testing protocols are available with the elec- E-mail address for K.L. Markolf: kmarkolf@mednet.ucla.edu

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