The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 6

Scientific Articles | June 01, 2004

Two-Bundle Posterior Cruciate Ligament Reconstruction: How Bundle Tension Depends on Femoral Placement

Jason T. Shearn, PhD¹; Edward S. Grood, PhD¹; Frank R. Noyes, MD²; Martin S. Levy, PhD¹

¹ Noyes Tissue Engineering and Biomechanics Laboratories, Department of Biomedical Engineering (J.T.S., E.S.G.), and Department of Quantitative Analysis (M.S.L.), University of Cincinnati, Cincinnati, OH 45221. E-mail address for J.T. Shearn: shearnj@email.uc.edu
² Cincinnati Sportsmedicine and Orthopaedic Center, 311 Straight Street, Cincinnati, OH 45219

View Disclosures and Other Information

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Cincinnati Sportsmedicine Research and Education Foundation. None of the authors 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, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated

Investigation performed at the Noyes Tissue Engineering and Biomechanics Laboratories, Department of Biomedical Engineering, University of Cincinnati, Cincinnati; Department of Quantitative Analysis, University of Cincinnati, Cincinnati; and Cincinnati Sportsmedicine and Orthopaedic Center, Cincinnati, Ohio

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J Bone Joint Surg Am, 2004 Jun 01;86(6):1262-1270

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Abstract

Background: Clinically, one-bundle posterior cruciate ligament reconstructions frequently result in the return of abnormal posterior translation. We hypothesized that the return of posterior translation is caused by a nonuniform distribution of load among the graft fibers. The purpose of the present study was to determine how the femoral attachment location of the second bundle of a two-bundle posterior cruciate ligament reconstruction affects the anterior bundle tension and the load distribution between the graft bundles.

Methods: One and two-bundle posterior cruciate ligament reconstructions (one one-bundle type and three two-bundle types) were performed in nineteen cadaveric knees. The grafts were tensioned to restore posterior translation to within ±1 mm of that of the intact knee at 90° of flexion while a 100-N posterior force was applied to the proximal part of the tibia. For each reconstruction, the total graft tension was a minimum of 2.3 times larger than the applied posterior force. Bundle tension and knee motions were measured as the knee was cycled from 5° to 120° of flexion while a 100-N posterior force was applied. Analysis of variance was used to compare the four reconstructions, and post hoc testing was performed with use of Fischer's protected least significant difference method.

Results: Two-bundle reconstructions involving a middle-distal or middle-middle second bundle significantly reduced the tension in the anterior bundle in comparison with the tension in the one-bundle (anterior-distal) reconstruction. The peak anterior-bundle tensions with the middle-distal and middle-middle second bundles were 43% and 37% less than the peak bundle tension for the one-bundle reconstruction (p < 0.001 and p = 0.002, respectively). With the exception of the average bundle tension, the tension parameters calculated for the middle bundle decreased as the distance from the articular cartilage increased. The peak tensions for the middle-middle and middle-proximal bundles were 32% and 61% less than that for the middle-distal bundle (p = 0.028 and p = 0.001, respectively).

Conclusions: The femoral position of the second bundle significantly affected the tension in the anterior bundle and the load distribution. A second bundle placed in a middle or distal position resulted in a significant reduction in anterior bundle tension and in cooperative load-sharing (with the bundles functioning together). A proximal second bundle resulted in reciprocal loading (with one bundle functioning in flexion and one in extension), but the tension in the anterior bundle was not different from the tension in the one-bundle reconstruction.

Clinical Relevance: The present study demonstrated that a proximal-to-distal change in the femoral position markedly affects bundle tension and function. A middle placement of the second bundle appears to be the most ideal because the bundles exhibit cooperative load-sharing and the peak loads are significantly less when compared with other reconstructions.

Figures in this Article

Posterior cruciate ligament reconstructions originally were based on the operative rationale used for anterior cruciate ligament reconstructions, which placed the femoral tunnel in the isometric region of the native anterior cruciate ligament¹. However, femoral isometric placement of a posterior cruciate ligament graft subsequently was shown to produce abnormal knee kinematics, particularly with the knee in >45° of flexion^2,3. Since the majority of posterior cruciate ligament fibers change in length during flexion-extension and only 5% to 15% of the femoral footprint is truly isometric^4-6, researchers and clinicians began to study nonisometric reconstructions.

Typically, the nonisometric femoral tunnel was placed in the anterior aspect of the distal region of the femoral footprint of the posterior cruciate ligament. This region, which tightens with flexion, was chosen because the bulk of the native posterior cruciate ligament has been shown to tighten with flexion^7,8. The nonisometric reconstruction initially restores normal knee kinematics^2,9. However, with time, abnormal posterior translation frequently returns^10-12, possibly because of poor graft placement^2,13, inadequate graft pretension, and/or graft elongation^2,11,13. The excessive graft elongation, which occurs in response to a highly nonuniform distribution of force among graft fibers, is believed to be the primary mechanism of failure of posterior cruciate ligament reconstruction^2,11,13.

In an attempt to improve the success of nonisometric reconstructions, a second graft bundle was added^10,13,14. In theory, a second bundle should provide a more uniform load distribution between the graft bundles and decrease the amount of graft elongation because the orientation of the native posterior cruciate ligament is better replicated^1,8. Biomechanical studies have demonstrated that a two-bundle reconstruction can restore the normal posterior translation limit over the entire range of knee motion^10,13,14. However, the tension in the anterior bundle also is considerably influenced by the position of the second bundle^10,13,14. Mannor et al.¹³ reported that the second bundle can generate high tension at the extremes of motion and that the load-sharing for the two-bundle construct is dependent on the femoral position of the second bundle. However, the authors of previous studies did not examine the effect of proximal-distal changes in the position of the second bundle on bundle tension^10,14 or compare the change in anterior bundle tension caused by the addition of a second bundle to a one-bundle posterior cruciate ligament reconstruction¹³. The purpose of the present study was to determine how the location of the femoral attachment of the second bundle of a two-bundle posterior cruciate ligament reconstruction affects the tension in the anterior bundle and the distribution of load between the graft bundles.

Materials and Methods

Materials and Methods | Results | Discussion | Acknowledgements | References

Study Design

The relationship between bundle tension and femoral attachment site was investigated with use of both one and two-bundle patellar tendon grafts. Posterior cruciate ligament reconstructions were performed on nineteen unembalmed cadaveric lower limbs from fourteen donors (four male donors and ten female donors) who had had a mean age of 70.4 years (range, fifty to eighty-eight years) at the time of death. The posterior motion limit for the intact and reconstructed knees was determined by cycling the knee from 5° to 120° of flexion while a 100-N posterior force was applied. The knee configurations that were tested included the intact knee, the posterior cruciate ligament-deficient knee (with removal of the ligaments of Humphrey and Wrisberg when present), a one-bundle reconstruction, and three different two-bundle reconstructions (Fig. 1). The femoral attachment site for the one-bundle grafts and the first (anterior) bundle of the two-bundle grafts was located within the anterior femoral region (Figs. 2 and 3). The femoral attachment site for the second (middle) bundle of the two-bundle grafts was located in the middle region of the femoral footprint in one of three sites (middle-distal, middle-middle, and middle-proximal) that varied in depth within the notch (Figs. 2 and 3). The tibial ends of the one and two-bundle grafts were placed at the center of the tibial attachment of the posterior cruciate ligament. The reconstructions were tensioned to restore posterior translation, which was measured with a custom-designed instrumented spatial linkage, to within ±1 mm of the intact knee at 90° of flexion while a 100-N posterior force was applied to the proximal part of the tibia. The anterior-distal, middle-distal reconstruction and the anterior-distal, middle-middle reconstruction were tensioned to share load, whereas the anterior-distal, middle-proximal reconstruction was tensioned so that the anterior bundle carried 80% of the resistive force¹³. Bundle tension was measured at the femoral end of the graft during tensioning and subsequent testing of the limits of knee motion with use of strain-gauge load-cells (ELW-B1-200L; Entran Devices, Fairfield, New Jersey) with a sensitivity of 0.9 mV/lb.

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Fig. 1: Knee configurations tested and measurements made. A/P = anterior-posterior; PCL = posterior cruciate ligament; MFL = meniscofemoral ligaments; AD1 = one-bundle anterior-distal reconstruction; AD2-MD = two-bundle anterior-distal, middle-distal reconstruction; AD2-MM = two-bundle anterior-distal, middle-middle reconstruction; and AD2-MP = two-bundle anterior-distal, middle-proximal reconstruction.

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Fig. 2: Sagittal view of the femur in 90° of flexion, illustrating the center for each tunnel. As a 10-mm patellar tendon graft was used for the one-bundle anterior-distal reconstruction, the graft centroid was 2.5 mm closer to the articular cartilage margin than the tunnel center. The centroid for the other bundles is the same as the center of the tunnel. AD1 = anterior-distal attachment for the one-bundle reconstruction, AD2 = anterior-distal attachment for the two-bundle reconstruction, MD = middle-distal attachment, MM = middle-middle attachment, and MP = middle-proximal attachment.

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Fig. 3: Sagittal views of the femur in 90° of flexion. A, One-bundle anterior-distal reconstruction. The anterior-distal attachment (AD1) is 6.7 mm from the trochlear groove and 10.6 mm from the cartilage edge. B, Two-bundle anterior-distal, middle-distal reconstruction. The anterior-distal attachment (AD2) is 5.2 mm from the trochlear groove and 6.3 mm from the cartilage edge, and the middle-distal attachment (MD) is 14.5 mm from the trochlear groove and 6.3 mm from the cartilage edge. C, Two-bundle anterior-distal, middle-middle reconstruction. The middle-middle attachment (MM) is 14.8 mm from the trochlear groove and 9.5 mm from the cartilage edge. D, Two-bundle anterior-distal, middle-proximal reconstruction. The middle-proximal attachment (MP) is 12.1 mm from the trochlear groove and 13.3 mm from the cartilage edge.

Methods

Specimen Preparation

The specimens were prepared according to a previously described protocol¹³. Briefly, the proximal one-half of the soft tissue was removed from the femur, and the distal one-third of the leg was removed. The patellar tendon was removed through an anterior midline incision, and the posterior cruciate ligament was exposed through a posterior midline incision.

Reconstructions

Bone-patellar tendon-bone grafts were used because the bone ends facilitated graft gripping and prevented graft slippage. One-bundle grafts were composed of the central 10-mm section of the patellar tendon along with bone blocks measuring 10 × 20 mm. Two-bundle Y-shaped grafts were formed by cutting the one-bundle grafts into medial and lateral halves. The tibial bone block was not divided, thereby leaving the two bundles attached. The 5-mm-wide bundles were used to minimize tunnel size. With larger tunnels, the accuracy of graft placement would be compromised and the removal of large quantities of bone stock could lead to fractures. The bone blocks were fixed within cylindrical stainless-steel grips with use of polymethylmethacrylate. Two grip sizes were used: a grip with a 10-mm inside diameter was used for the large bone blocks, and a grip with a 5-mm inside diameter was used for the small bone blocks.

The graft ends were attached to the tibia and femur with use of cylindrical fixtures that were placed within holes that had been drilled through the bone. One fixture was placed in the femur for a one-bundle reconstruction, and two fixtures were placed in the femur for a two-bundle reconstruction. The fixture design and the attachment to bone prevented rotation of the bone blocks while allowing for the adjustment of bundle tension.

The tibial fixture (inside diameter, 12 mm) was placed within a 16-mm drill-hole that started just lateral to the tibial tubercle and exited at the center of the tibial attachment of the posterior cruciate ligament. The bone grip was oriented to place the collagenous material in the proximal region of the tibial fixture and was positioned so that the end of the tibial bone block was flush with the tunnel exit.

The femoral fixture for the one-bundle grafts had an inside diameter of 12 mm. This fixture was placed within a 16-mm drill-hole that started medial to the trochlear groove and exited within the anterior region of the posterior cruciate ligament. The center of the femoral hole was chosen so that the distal edge of the graft was 3 mm proximal to the articular cartilage margin. The bone grip was positioned to place the graft fibers in the distal region of the femoral fixture. As a patellar tendon was used, the centroid of the graft fibers was 5.5 mm from the articular cartilage margin. The femoral fixtures for the two-bundle grafts had an inside diameter of 8 mm. These fixtures were placed within 10-mm drill-holes that started on the medial femoral condyle and ended within the substance of the posterior cruciate ligament. The centers for the anterior-distal and middle-distal holes were chosen so that the distal edge of the graft was 2 mm proximal to the articular cartilage margin. The graft fibers filled the entire diameter of the smaller bone grips. Therefore, the centroid for the anterior bundle was 4.5 mm proximal to the articular cartilage margin. The center for the middle-middle hole was chosen so that the distal edge of the graft was 5 mm proximal to the articular cartilage margin, and the center for the middle-proximal hole was chosen so that the center was on or just distal to the proximal edge of the native posterior cruciate ligament. For all of the bundles, the graft fibers were located along the distal edge of the grip so that the transverse axis of the graft was parallel to the anterior-posterior direction of the femur.

Graft Tensioning

The grafts were tensioned to restore posterior translation to within ±1 mm of that of the intact knee at 90° flexion while a 100-N posterior force was applied to the proximal part of the tibia. The grafts were implanted, and each bundle was tensioned to 45 N. The knee was cycled five times from 5° to 120° of flexion to precondition the graft and then was returned to 90° of flexion. The bundles were then tensioned to return the posterior translation to normal. The average tension needed to restore the posterior translation limit for the one and two-bundle reconstructions is shown in Table I. The total bundle tension was not significantly different between the reconstructions. The anterior-distal, middle-distal reconstruction and the anterior-distal, middle-middle reconstruction were tensioned to share the load equally at 90°. The anterior-distal, middle-proximal reconstruction was tensioned so that the anterior bundle would carry approximately 80% of the load at 90° flexion. This load distribution was chosen because the proximal fibers in the native posterior cruciate ligament shorten with flexion and can only support minimal tension. If tension was shared between the anterior-distal and middle-proximal bundles at 90° flexion, the middle-proximal bundle tension would still peak in extension because femoral graft position controls the shape of the tension curve, not the magnitude¹⁵.

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TABLE I Posterior Translation After Graft Tensioning*

Reconstruction	Number of Knees	Change in Posterior Translation*†(mm)	Anterior-Distal Bundle Tension*(N)	Second Bundle Tension*(N)	Total Bundle Tension*(N)
One-bundle (anterior-distal)	5	-0.7 ± 0.3	230.3 ± 23	NA	230.3 ± 23
Two-bundle (anterior-distal, middle-distal)	5	-0.1 ± 0.3	123.3 ± 6	152.3 ± 14	275.6 ± 19
Two-bundle (anterior-distal, middle-middle)	5	-0.1 ± 0.4	148.0 ± 12	155.1 ± 15	303.1 ± 22
Two-bundle (anterior-distal, middle-proximal)	4	0.6 ± 0.5	246.9 ± 20	40.8 ± 16	287.7 ± 25

The data are for 90° of flexion with a 100-N posterior force applied to the proximal part of the tibia. The values are given as the mean and the standard error of the mean. †Compared with the intact knee.

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TABLE I Posterior Translation After Graft Tensioning*

Reconstruction	Number of Knees	Change in Posterior Translation*†(mm)	Anterior-Distal Bundle Tension*(N)	Second Bundle Tension*(N)	Total Bundle Tension*(N)
One-bundle (anterior-distal)	5	-0.7 ± 0.3	230.3 ± 23	NA	230.3 ± 23
Two-bundle (anterior-distal, middle-distal)	5	-0.1 ± 0.3	123.3 ± 6	152.3 ± 14	275.6 ± 19
Two-bundle (anterior-distal, middle-middle)	5	-0.1 ± 0.4	148.0 ± 12	155.1 ± 15	303.1 ± 22
Two-bundle (anterior-distal, middle-proximal)	4	0.6 ± 0.5	246.9 ± 20	40.8 ± 16	287.7 ± 25

Testing

The posterior motion limit was determined while a 100-N posterior force was applied to the proximal part of the tibia with use of two pneumatic actuators. To avoid constraining tibial rotation, a linear bearing that was oriented in the medial-lateral direction was placed at the effector end of the actuators, and the actuator force was applied to a 73-mm polyvinyl chloride hemicylinder that was attached anteriorly to the proximal part of the tibia. The hemicylinder was attached to the proximal part of the tibia with two screws and polymethylmethacrylate. During each motion-limit test, the knee was cycled from 5° to 120° of flexion while the knee motions described by Grood and Suntay¹⁶ were recorded with use of an instrumented spatial linkage that was accurate to 0.5 mm and 0.5°¹⁷. The posterior cruciate ligament and the meniscofemoral ligaments were then severed approximately 5 mm from their femoral attachment sites through an anterior midline incision. The portion of the fibers that remained attached to the femur were used as a guide for graft-bundle placement. The tibial attachment of the posterior cruciate ligament was resected through a posterior midline incision. The knee was then reconstructed with either a one or two-bundle technique, and the posterior motion limit was determined for the reconstructed knee while the tension in each bundle was measured.

Statistical Analysis

The specimens were assigned to groups on the basis of the type of reconstruction that was performed. Three of the four groups had five specimens, and one group (the anterior-distal, middle-proximal group) had four specimens. The bundle tension residuals were normal and homoscedastic (that is, there was equal variance between and within groups). Analysis of variance was performed for the average bundle tension with use of the reconstruction method as a fixed factor and the flexion angles (5°, 10°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, and 120°) as repeated measures. A one-way between-subjects analysis of variance was performed for peak tension. Regression analysis of the relationship between bundle tension and flexion angle was then performed for each bundle, and a one-way between-subjects analysis of variance was performed to determine if the slopes calculated with regression analysis were significantly different. A one-way within-subjects analysis of variance was performed for the two-bundle reconstructions to determine if the average bundle tension, the peak bundle tension, and the slope calculated with regression analysis were significantly different between the two bundles. If analysis of variance demonstrated a difference, post hoc testing was performed with use of Fischer's protected least significant difference method. Power was calculated for the tests that were found to be insignificant even though the data would suggest that a difference existed. The level of significance was set at p < 0.05, and the analyses were performed with use of SPSS software (version 10.1; SPSS, Chicago, Illinois).

Results

Materials and Methods | Results | Discussion | Acknowledgements | References

Anterior Bundle

The two-bundle reconstructions involving the middle-distal and middle-middle second bundles significantly reduced the tension in the anterior bundle in comparison with the tension in the one-bundle reconstruction (Table II and Figs. 4, 5, and 6). The average anterior-bundle tensions with the middle-distal and middle-middle second bundles were 50% and 41% less than that for the one-bundle reconstruction (p < 0.001 and p = 0.002, respectively). The peak anterior-bundle tensions with the middle-distal and middle-middle second bundles were 43% and 37% less than that for the one-bundle reconstruction (p < 0.001 and p = 0.002, respectively). In addition, the anterior-bundle-tension slope with the middle-distal and middle-middle second bundles was significantly less than that for the one-bundle reconstruction (p = 0.006 and p = 0.026, respectively).

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TABLE II Anterior-Bundle Tension

Reconstruction	Average Tension*(N)	Peak Tension*(N)	Angle at Peak Tension	Slope*(N/30° flexion)
One-bundle (anterior-distal)	149.3 ± 11	236.2 ± 23	105°	49.9 ± 4
Two-bundle (anterior-distal, middle-distal)	74.7 ± 7	133.9 ± 12	75°	18.9 ± 4
Two-bundle (anterior-distal, middle-middle)	88.6 ± 9	148.0 ± 12	90°	25.7 ± 5
Two-bundle (anterior-distal, middle-proximal)	133.2 ± 14	246.9 ± 20	90°	60.0 ± 6

The values are given as the mean and the standard error of the mean.

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TABLE II Anterior-Bundle Tension

Reconstruction	Average Tension*(N)	Peak Tension*(N)	Angle at Peak Tension	Slope*(N/30° flexion)
One-bundle (anterior-distal)	149.3 ± 11	236.2 ± 23	105°	49.9 ± 4
Two-bundle (anterior-distal, middle-distal)	74.7 ± 7	133.9 ± 12	75°	18.9 ± 4
Two-bundle (anterior-distal, middle-middle)	88.6 ± 9	148.0 ± 12	90°	25.7 ± 5
Two-bundle (anterior-distal, middle-proximal)	133.2 ± 14	246.9 ± 20	90°	60.0 ± 6

The values are given as the mean and the standard error of the mean.

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Fig. 4: Bundle tension for the one-bundle reconstruction. SEM = standard error of the mean.

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Fig. 5: Bundle tension for the anterior-distal, middle-distal reconstruction. AD2 = anterior bundle, MD = middle-distal bundle, and SEM = standard error of the mean

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Fig. 6: Bundle tension for the anterior-distal, middle-middle reconstruction. AD2 = anterior bundle, MM = middle-middle bundle, and SEM = standard error of the mean.

The two-bundle reconstruction involving the middle-proximal second bundle did not change the tension in the anterior bundle in comparison with the tension in the one-bundle reconstruction (Table II and Figs. 4 and 7).

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Fig. 7: Bundle tension for the anterior-distal, middle-proximal reconstruction. AD2 = anterior bundle, MP = middle-proximal bundle, and SEM = standard error of the mean.

The average anterior-bundle tension for the two-bundle reconstruction involving the middle-proximal second bundle was significantly larger than the anterior-bundle tension for the other two-bundle reconstructions (Table II and Figs. 5, 6, and 7). The average anterior-bundle tension for the anterior-distal, middle-proximal reconstruction was 78% and 50% greater than those for the other two-bundle reconstructions (p < 0.018); the peak anterior-bundle tension for the anterior-distal, middle-proximal reconstruction was 84% and 67% greater than those for the other two-bundle reconstructions (p < 0.018); and the anterior-bundle-tension slope for the anterior-distal, middle-proximal reconstruction was 217% and 133% greater than those for the other two-bundle reconstructions (p < 0.06).

The anterior-bundle tension for the two-bundle reconstruction involving the middle-distal second bundle was not significantly different from that for the two-bundle reconstruction involving the middle-middle second bundle (Table II and Figs. 5 and 6).

Middle Bundle

With the exception of the average bundle tension, the tension parameters calculated for the middle bundle decreased as the distance from the articular cartilage increased (Table III and Figs. 5, 6, and 7). The peak bundle tensions for the middle-middle and middle-proximal bundles were 32% and 61% less than that for the middle-distal bundle (p = 0.028 and p = 0.001, respectively). The peak bundle tension for the middle-proximal bundle was also 43% less than that for the middle-middle bundle, but the difference was not significant. With the level of alpha set at 0.05, the power to determine the difference of 90.3 N was approximately 77%. The tension slope for the middle-distal bundle was significantly greater than that for the other middle bundles (p < 0.021), and the tension slope for the middle-middle bundle was significantly greater than that for the middle-proximal bundle (p = 0.005).

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TABLE III Middle-Bundle Tension

Reconstruction	Average Tension*(N)	Peak Tension*(N)	Angle at Peak Tension	Slope*(N/30° flexion)
Two-bundle (anterior-distal, middle-distal)	93.2 ± 15	309.1 ± 27	120°	68.4 ± 6
Two-bundle (anterior-distal, middle-middle)	124.5 ± 8	210.1 ± 29	120°	31.8 ± 4
Two-bundle (anterior-distal, middle-proximal)	74.2 ± 8	119.8 ± 29	5°	-19.0 ± 5

The values are given as the mean and the standard error of the mean.

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TABLE III Middle-Bundle Tension

Reconstruction	Average Tension*(N)	Peak Tension*(N)	Angle at Peak Tension	Slope*(N/30° flexion)
Two-bundle (anterior-distal, middle-distal)	93.2 ± 15	309.1 ± 27	120°	68.4 ± 6
Two-bundle (anterior-distal, middle-middle)	124.5 ± 8	210.1 ± 29	120°	31.8 ± 4
Two-bundle (anterior-distal, middle-proximal)	74.2 ± 8	119.8 ± 29	5°	-19.0 ± 5

The values are given as the mean and the standard error of the mean.

The average tension for the middle-middle bundle was significantly larger than those for the other two middle bundles. The average tensions for the middle-distal and middle-proximal bundles were 25% and 40% less than that for the middle-middle bundle (p = 0.028 and p = 0.003, respectively). The average tension for the middle-distal bundle was not significantly different from that for the middle-proximal bundle.

Discussion

Materials and Methods | Results | Discussion | Acknowledgements | References

In the present study, we found that the femoral position of the second bundle significantly affected bundle tension but that the presence and the femoral position of the second bundle did not affect the posterior translation limit (Fig. 8). The changes that were noted for the anterior and middle bundles resulted from a 7 to 8-mm proximal-distal difference in the location of the femoral attachment of the middle bundle. The anterior-distal, middle-distal reconstruction and the anterior-distal, middle-middle reconstruction distributed load evenly between the bundles (Figs. 5 and 6) while the anterior-distal, middle-proximal reconstruction exhibited a reciprocal load distribution in which the anterior bundle functioned in flexion and the middle bundle functioned in extension (Fig. 7). However, for the anterior-distal, middle-distal reconstruction, the tension in the middle-distal bundle increased to >300 N whereas the tension in the anterior bundle declined to approximately 50 N. The increase in tension in the middle-distal bundle, which inhibits load-sharing in high flexion, is attributable to a larger tibiofemoral separation distance with high degrees of knee flexion. A load-sharing two-bundle reconstruction should provide better graft longevity in comparison with a one-bundle reconstruction because the load distribution is more uniform, which should reduce or stop graft elongation.

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Fig. 8: Change in posterior translation compared with the intact knee. The gray-shaded area represents translation within ±1 mm of that in the intact knee. AD1 = single-bundle anterior-distal reconstruction; AD2-MD = two-bundle anterior-distal, middle-distal reconstruction; AD2-MM = two-bundle anterior-distal, middle-middle reconstruction; and AD2-MP = two-bundle anterior-distal, middle-proximal reconstruction.

In the past decade, multiple investigators have examined the biomechanical properties of posterior cruciate ligament reconstructions. Harner et al.¹⁰, Markolf et al.¹¹, and Stone et al.¹⁸ also showed that a one-bundle anterior and distal reconstruction is capable of controlling posterior translation. However, the graft tensions that were recorded in the studies by Harner et al.¹⁰ and Stone et al.¹⁸ were two times less than the graft tension for the one-bundle reconstruction in the present study (Fig. 4). The lower forces could be attributed to the tensioning techniques employed in those studies, which resulted in residual posterior translation. Harner et al.¹⁰ and Stone et al.¹⁸ tensioned the grafts to a predetermined force level, but we tensioned the grafts to restore posterior translation to within ±1 mm of that of the intact knee at 90° of flexion. Markolf et al.¹¹ also recorded graft tensions that were approximately half the tension for our one-bundle reconstruction, but those authors tensioned the grafts to restore normal posterior translation at 90° of flexion, as was done in the current study. The graft tension recorded by Markolf et al.¹¹ (approximately 125 N) should not be able to restore normal posterior translation at 90° of flexion because of the oblique orientation and role of the posterior cruciate ligament in the knee. The posterior cruciate ligament is elevated approximately 60° from the anterior-posterior tibial axis with the knee in 90° of flexion, which would result in half of the posterior cruciate ligament tension resisting a posteriorly applied load, and the posterior cruciate ligament has been shown to resist 95% of the applied posterior force at 90° of flexion¹⁹. Therefore, to resist a 100-N applied posterior force, a graft tension of approximately 190 N would be required to restore normal posterior translation at 90°. For the current study, the graft tension required to restore the normal posterior translation limit was between 230 and 300 N (Table I). The larger graft forces can be explained by the fact that the bundles also deviate medially from the anterior-posterior tibial axis.

Several investigators have examined the biomechanical behavior of two-bundle posterior cruciate ligament reconstructions^10,13,14. In those studies, the posterior translation was restored to normal by the two-bundle reconstructions even though the second bundles had different femoral attachment locations (including distal^13,14, middle¹⁰, and proximal¹³ locations). Race and Amis¹⁴ recorded loading patterns that were similar to the ones that we observed in association with the two-bundle reconstruction involving a middle-distal second bundle, but the magnitude of the bundle tensions could not be compared with those in the current study because those authors performed the testing without an applied posterior load. Harner et al.¹⁰ placed their bundles in the distal and middle thirds of the femoral footprint of the posterior cruciate ligament. Harner et al.¹⁰ reported an overall graft tension that was 80% of the applied posterior force at 90°. On the basis of the geometry and function of the posterior cruciate ligament graft described in the previous paragraph, the graft tension reported by Harner et al.¹⁰ appears to be lower than what would be required to restore the normal posterior translation limit. Mannor et al.¹³ studied both a distal and proximal position for the second bundle, and their findings regarding bundle tension agree with the results of the present study.

One potential problem with the two-bundle reconstructions that were tested in the present study is that the total graft tension (the sum of the individual bundle tensions) was larger than the bundle tension for the one-bundle reconstruction, which has been shown to fail over time^10-12. The increased total graft tension can be attributed to the orientation of the second bundle. In the one-bundle reconstruction, the graft has a minimal deviation from the sagittal plane. In the two-bundle reconstructions, the anterior bundle has the same orientation as it does in the one-bundle reconstruction, but the middle bundle has a greater deviation from the sagittal plane. This increased deviation results in a smaller proportion of the middle-bundle tension resisting the posterior translation. Therefore, to restore the normal posterior translation limit and to maintain load-sharing, the total graft tension has to increase.

The selection of graft material and graft size in the present study should not be viewed as a recommendation for clinical use. The selection of the patellar tendon was necessitated by our method of holding and tensioning the graft and measuring bundle tension. The patellar tendon allowed us to grip bone at both ends in order to ensure adequate fixation. The 5-mm bundle width was needed for several reasons. First, the size of the graft bundle determined the size of the tunnel to be drilled in the femur. The tunnel not only had to hold the bone grip but also had to accommodate the fixtures that allowed the bundles to be tensioned and the bundle tension to be measured. A larger tunnel would have inhibited our ability to place these tunnels accurately, and two large femoral tunnels could have weakened the femoral condyle and caused a fracture of the condyle. Second, a 10-mm-wide patellar tendon graft has been used clinically as a single bundle to reconstruct the posterior cruciate ligament. We wanted to show that any improvements in the reconstruction were caused by the position of the second bundle and not by the addition of collagenous material. The goal of the present study was to determine the effects of altering femoral tunnel placement and not to evaluate graft selection. Even though our methods required smaller bundles than would be used clinically, we believe that the results of this study are applicable to the clinical environment, regardless of our graft choice, because Galloway et al.² found that a thin wire cable and a 10-mm Achilles tendon graft demonstrated the same behavior as long as the center of the tunnel holding the construct was the same.

Several conclusions can be drawn from the present study. First, the femoral attachment location of the middle bundle significantly affects the tension in the anterior and middle bundles. Second, the change in the loading characteristics of the middle bundle was seen with only a 7 to 8-mm change in the proximal-distal dimension of the location of the femoral attachment. The two-bundle reconstructions involving a middle-distal or a middle-middle second bundle exhibited cooperative load-sharing, whereas the two-bundle reconstruction involving a middle-proximal second bundle exhibited reciprocal load-sharing. Third, the high tensile loads that are required to control posterior translation following posterior cruciate ligament reconstruction can be attributed to the oblique orientation of the graft with respect to the anterior-posterior tibial axis. Finally, on the basis of the results of the present study, we recommend that the second bundle should be placed in a middle-middle location. This configuration significantly reduced tension in the anterior bundle and avoided the deleterious peak forces that were seen in association with the other two-bundle reconstructions that were examined in the present study.

Acknowledgements

Materials and Methods | Results | Discussion | Acknowledgements | References

Note: The authors thank the Musculoskeletal Tissue Foundation for the gift of research tissue.

References

Materials and Methods | Results | Discussion | Acknowledgements | References

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Topics

femur ; knee joint ; posterior cruciate ligament ; reconstructive surgical procedures ; tibia ; tissue transplants ; transplantation ; posterior cruciate ligament reconstruction

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Jason T. Shearn, Edward S. Grood, Frank R. Noyes, Martin S. Levy; Two-Bundle Posterior Cruciate Ligament Reconstruction: How Bundle Tension Depends on Femoral Placement. The Journal of Bone & Joint Surgery. 2004 Jun;86(6):1262-1270.

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