JBJS | A Biomechanical Study of Replacement of the Posterior Cruciate Ligament with a Graft. Part II: Forces in the Graft Compared with Forces in the Intact Ligament*

The Journal of Bone & Joint Surgery, Volume 79, Issue 3

Articles | March 01, 1997

A Biomechanical Study of Replacement of the Posterior Cruciate Ligament with a Graft. Part II: Forces in the Graft Compared with Forces in the Intact Ligament*

KEITH L. MARKOLF, PH.D.†; JAMES R. SLAUTERBECK, M.D.‡; KEVIN L. ARMSTRONG, M.D.†; MATTHEW S. SHAPIRO, M.D.†; GERALD A. M. FINERMAN, M.D.†, LOS ANGELES, CALIFORNIA

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

The Journal of Bone & Joint Surgery. 1997; 79:381-6

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Abstract

A femoral load-cell was installed in twelve fresh-frozen knee specimens from cadavera, to measure the resultant force at the femoral origin of the posterior cruciate ligament during a series of tibial-loading tests. The posterior cruciate ligament was removed, and a ten-millimeter-wide bone-patellar ligament-bone graft was inserted. The knee was flexed to 90 degrees, the graft was pre-tensioned to restore the anterior-posterior laxity to that recorded after installation of the load-cell, and the loading tests were repeated. With the tibia locked in neutral rotation and a 200-newton posterior force applied to the tibia, the mean force generated in the intact posterior cruciate ligament ranged from 220 newtons at 90 degrees of flexion to thirty-six newtons at full extension. When the tibia was locked in external rotation during the posterior drawer test, the force was reduced when the knee was flexed 10 to 70 degrees; when the tibia was locked in internal rotation, the mean force was reduced at only 30 and 45 degrees of flexion. The mean forces in the graft were not significantly different, with the numbers available, from the corresponding values for the intact ligament during application of a straight posterior tibial force (neutral tibial rotation), during application of a fifteen-newton-meter flexion or extension moment (hyperflexion or hyperextension), during application of a ten-newton-meter varus or valgus moment, or during application of a ten-newton-meter internal or external tibial torque. With the numbers available, there were no significant differences between the mean tibial rotations associated with the intact posterior cruciate ligament and those associated with the graft at any angle of flexion, without or with applied tibial torque.CLINICAL RELEVANCE: The amount of force generated in the posterior cruciate ligament during the posterior drawer test depends on the angle of flexion at which the test is performed. When the angle of flexion is near 90 degrees, all of the posterior force applied to the tibia is transmitted to the ligament and the force in the ligament is not affected by the position of tibial rotation. When the test is performed at an angle of flexion near 30 degrees and in neutral tibial rotation, other structures (such as the collateral ligaments and the posterior part of the capsule) help to resist the posterior force applied to the tibia. The position of tibial rotation is important when the test is performed with the knee at an angle of flexion near 30 degrees, as secondary structures pre-tensioned by tibial torque act to reduce the amount of force carried by the posterior cruciate ligament even more. With a few minor exceptions, we found that the forces in a graft used to replace the posterior cruciate ligament were approximately the same as those in the intact ligament. Therefore, there appears to be little justification for restricting low-level rehabilitation activities once the fixation of the graft has healed. However, forces in the graft could be quite high during hyperextension and hypertension, as they are in the intact ligament. Thus, bracing in the early postoperative period may be advisable to prevent these motions.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Increased posterior laxity, which is commonly found after reconstruction of the posterior cruciate ligament, may be due to improper pre-tensioning of the graft or stretching of the graft tissue over time because of high forces in the graft. The graft may elongate if the reconstruction was not protected properly during the initial phase of healing or if long-term physiological demands generated forces that were too high. A knowledge of the forces that develop in the graft is therefore important for considerations related to postoperative bracing, rehabilitation, and permissible activities.

Forces in the posterior and posterolateral fibers of the posterior cruciate ligament have been measured with use of buckle transducers^1,2. We measured the resultant force in the entire posterior cruciate ligament with a load-cell attached to an isolated plug of bone containing the femoral origin⁶. To our knowledge, there currently are no biomechanical data related to forces generated in a graft used to replace the posterior cruciate ligament. The objectives of the present study were (1) to measure the effects of pre-rotation of the tibia on the force generated in the intact posterior cruciate ligament with application of a posterior tibial force, (2) to compare the forces in a graft at laxity-matched pre-tension with those in the intact posterior cruciate ligament during a series of controlled loading tests, and (3) to compare tibial rotations, before and after replacement of the posterior cruciate ligament, as the knee was extended passively without and with applied tibial torque.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were National Institutes of Health Grant RO1 AR40330 and the Dorothy and Leonard Straus Fund.

†Biomechanics Research Section, Department of Orthopaedic Surgery, Rehabilitation Center, University of California at Los Angeles, 1000 Veteran Avenue, Los Angeles, California 90024-1795. E-mail address for Dr. Markolf: kmarkolf@ortho.medsch.ucla.edu.

‡Department of Orthopaedic Surgery, Texas Tech University Health Sciences Center, Lubbock, Texas 79301.

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Fig. 1 Graph of the mean forces in the posterior cruciate ligament (PCL) with application of a 200-newton posterior force to the tibia, according to the angle of flexion of the knee with the tibia locked in three positions of rotation. The error bars indicate one standard deviation of the sample mean. ns = non-significant difference at the p = 0.05 level.

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Figs. 2-A and 2-B: Graphs of the individual curves for force according to the angle of flexion during passive extension of the knee. Fifteen newton-meters of flexion moment (hyperflexion) and fifteen newton-meters of extension moment (hyperextension) were applied at the extremes of motion. Fig. 2-A: Intact posterior cruciate ligament (PCL).

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Fig. 2-B Graft inserted at laxity-matched pre-tension.

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Figs. 3-A through 3-F: Graphs of the mean forces according to the angle of flexion during passive extension of the knee. The mean forces are shown for two conditions: the intact posterior cruciate ligament (PCL) and after insertion of a graft at laxity-matched pre-tension. The error bars indicate one standard deviation of the sample mean. ns = non-significant difference at the p = 0.05 level. Fig. 3-A: With no load applied to the tibia.

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Fig. 3-B With application of a constant 100-newton posterior force to the tibia.

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Fig. 3-C With application of a constant ten-newton-meter internal tibial torque.

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Fig. 3-D With application of a constant ten-newton-meter external tibial torque.

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Fig. 3-E With application of a constant ten-newton-meter varus moment.

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Fig. 3-F With application of a constant ten-newton-meter valgus moment.

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Fig. 4 Graph of the mean tibial rotations according to the angle of flexion during passive extension of the knee. Tibial rotation was defined as 0 degrees at full extension. Three loading conditions are shown: no tibial torque (0 F), ten newton-meters of external tibial torque (ET), and ten newton-meters of internal tibial torque (IT). With the numbers available, the mean tibial rotations after insertion of the graft were not significantly different (ns) from the corresponding mean values for the intact posterior cruciate ligament (PCL) (p > 0.05).

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

For measurements of force in the posterior cruciate ligament or in the graft during a simple flexion-extension cycle, the femur was clamped in the horizontal position with the patella facing down, to eliminate gravitational loading of the posterior cruciate ligament from the weight of the tibia. A loading-handle attached to the potted end of the tibia recorded the applied flexion or extension moment at the extremes of knee motion. The resultant force in the ligament or the graft, the flexion-extension moment, and the angle of flexion of the knee were recorded between the limits of a fifteen-newton-meter flexion moment (hyperflexion) and a fifteen-newton-meter extension moment (hyperextension).

With the ends of the tibia and femur potted in polymethylmethacrylate, the knee was placed in an apparatus⁵ that permitted the knee to be extended manually from 90 degrees of flexion to 5 degrees of hyperextension while a constant posterior force, torque, or varus-valgus moment was applied to the tibia with use of static weights in an unconstrained fashion. All of the weight of the tibial fixture was counterbalanced so that gravitational loading of the posterior cruciate ligament was eliminated. The resultant force in the posterior cruciate ligament and the angle of flexion of the knee were recorded continuously.

Testing Procedures

Twelve knee specimens were prepared for testing, and a load-cell was installed on each posterior cruciate ligament. Before testing, the load-cell was set to zero electrically by applying approximately twenty newtons of anterior force to the tibia at 45 degrees of flexion of the knee. In this state, the fibers of the posterior cruciate ligament were visibly lax and the ligament was unloaded. Anterior-posterior testing was performed with the load-cell in place on the posterior cruciate ligament and the tibia locked in neutral, internal, or external rotation; internal and external rotation were defined as the positions resulting from the application of two newton-meters of internal or external tibial torque. The patellar ligament graft was inserted, the laxity-matched pre-tension was determined, and anterior-posterior testing was repeated with the tibia locked in neutral rotation. For loading tests that were done after insertion of the graft, the load-cell was set to zero electrically before pre-tension was applied³.

Constant tibial-loading testing was performed on the intact knee with no load, with ten newton-meters of internal torque, and with ten newton-meters of external torque to record tibial rotations only. After the load-cell was installed on the posterior cruciate ligament, constant tibial-loading testing was done with no load and with application of 100 newtons of posterior tibial force, with application of ten newton-meters of internal and external torque, and with application of ten newton-meters of varus and valgus moment. This series of testing was repeated after insertion of the graft at the laxity-matched pre-tension.

Statistical Analysis

A two-way analysis-of-variance model with repeated measures was used to determine the significance of differences between the mean forces in the intact posterior cruciate ligament and those in the graft for a given loading test. Multiple pairwise comparisons between the mean values at specific angles of flexion of the knee were made with use of the Student-Newman-Keuls procedure. Similar analysis-of-variance models (with pairwise comparisons) were used to compare the forces in the intact posterior cruciate ligament at three positions of tibial rotation and to compare the forces in the graft with those in the intact posterior cruciate ligament during hyperflexion and hyperextension. A similar model was used to analyze the differences between the mean tibial rotations before and after insertion of the graft. The level of significance was p = 0.05.

Results

Introduction | Materials and Methods | Results | Discussion | References

Forces Generated during Anterior-Posterior Testing

With the tibia locked in neutral rotation and a 200-newton posterior force applied to the tibia, the mean force generated in the posterior cruciate ligament ranged from 220 newtons at 90 degrees of flexion to thirty-six newtons at full extension (Fig. 1). When the tibia was locked in external rotation, the mean force in the posterior cruciate ligament was significantly reduced (p = 0.05) when the knee was flexed 10 to 70 degrees. When the tibia was locked in internal rotation, the mean force in the ligament was significantly reduced (p = 0.05) at only 30 and 45 degrees of flexion. During a straight posterior-drawer test with the tibia in neutral rotation, the mean forces in the intra-articular portion of the graft (as measured by the femoral load-cell) were not significantly different, with the numbers available, from the corresponding values for the intact posterior cruciate ligament at any angle of flexion.

Forces Generated during Hyperflexion and Hyperextension

The forces in the intact posterior cruciate ligament were relatively low throughout the full range of motion, except in some specimens during hyperflexion (Fig. 2-A). The curves for the force in the graft at laxity-matched pre-tension versus the angle of knee flexion were more variable and often erratic beyond 90 degrees of flexion, with some specimens exhibiting relatively high forces in the graft during hyperflexion (Fig. 2-B). With the numbers available, there was no significant difference between the force in the graft during hyperflexion (mean and standard deviation, 87 ± 88.6 newtons) and that during hyperextension (48 ± 62.9 newtons); there also was no significant difference between the corresponding forces in the intact posterior cruciate ligament (70 ± 31.2 newtons during hyperflexion compared with 28 ± 9.3 newtons during hyperextension).

Forces Generated during Constant Tibial Loading

The mean force in the intact posterior cruciate ligament during passive extension of the knee and no load was twelve newtons or less (Fig. 3-A). The mean forces in the graft at laxity-matched pre-tension were significantly greater (p = 0.05) at 5 degrees of hyperextension and at angles of flexion of 70 degrees or more (Fig. 3-A).

With the numbers available, there were no significant differences between the mean forces in the posterior cruciate ligament and those in the graft, at any angle of flexion, during testing with 100 newtons of posterior force applied to the tibia (Fig. 3-B).

At 90 degrees of flexion, the mean force in the posterior cruciate ligament was ninety-one newtons when ten newton-meters of internal tibial torque was applied (Fig. 3-C) and forty-seven newtons when ten newton-meters of external tibial torque was applied (Fig. 3-D). With the numbers available, there were no significant differences between the mean forces in the graft and those in the intact ligament at any angle of flexion for either loading direction (Figs. 3-C and 3-D).

At 90 degrees of flexion, the mean force in the posterior cruciate ligament was sixty-three newtons when a ten-newton-meter varus moment was applied (Fig. 3-E) and forty-nine newtons when a ten-newton-meter valgus moment was applied (Fig. 3-F). There were no significant differences, with the numbers available, between the mean forces in the graft and those in the posterior cruciate ligament at any angle of flexion for either loading direction (Figs. 3-E and 3-F).

Tibial Rotations

With the numbers available, there were no significant differences between the mean tibial rotations for the intact posterior cruciate ligament and those for the graft at any angle of flexion, without or with the application of tibial torque (Fig. 4).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Anterior-Posterior Testing

Our results indicated that the position of tibial rotation and the angle of flexion of the knee are factors that must be considered when a posterior drawer test is done. With the knee in flexion near 90 degrees, the posterior cruciate ligament is the structure that resists posterior force applied to the tibia. As the knee is brought into full extension, the collateral ligaments and the posterior aspect of the capsule also act to resist posterior force on the tibia, and the integrity of the posterior cruciate ligament may be more difficult to assess clinically. When the posterior drawer test is performed in the classic fashion—with the knee at an angle of flexion near 90 degrees—pre-rotation of the tibia should not affect the accuracy of the test with regard to determining if there is an isolated disruption of the posterior cruciate ligament. When the posterior drawer test is performed with the knee at an angle of flexion near 30 to 45 degrees, pre-rotation of the tibia may tighten the capsular and collateral ligaments and lead the examiner to believe that the posterior cruciate ligament is intact and resisting the posterior force applied to the tibia.

Hyperflexion and Hyperextension

The mean forces produced in the posterior cruciate ligament during hyperextension and hyperflexion were consistent with those measured in our previous study⁶, when corrected for the level of applied moment. The forces clearly were quite high in some grafts during hyperflexion and hyperextension. We hypothesized that there was a relationship between a high laxity-matched pre-tension of the graft and high forces in the graft during hyperflexion and hyperextension, but no such association was found. Thus, the considerable scatter that was seen after insertion of the graft may be associated with minute differences in the placement of the graft and, perhaps, in bone geometry.

Another possible explanation for the erratic curves for the forces in the graft at laxity-matched pre-tension versus the angle of knee flexion (Fig. 2-B) is contact of the bone cap with the wall of the tibial tunnel, due to cantilever deflection of the load-cell. If this had occurred at passive knee-flexion angles beyond 90 degrees, one would expect an apparent decline in the resultant force in the graft due to so-called off-loading of the load-cell. Although this pattern was not observed for the intact posterior cruciate ligament during full flexion, the slope of the curves was reduced for angles of flexion beyond 90 degrees after insertion of the graft in some specimens. Four of the six curves that exhibited dips did so at a force level of less than eighty newtons. We believe that it is unlikely that the cap of bone would have come into contact with the wall of the tunnel at these low levels of force.

Constant Tibial Loading

Restoration of proper knee laxity by replacing the posterior cruciate ligament with a graft is not associated with a penalty of high forces in the graft under the loading conditions that we investigated. The results of the present study are very different from those found after replacement of the anterior cruciate ligament with a graft^4,5; in that study, forces in the graft were typically two to three times greater than the corresponding values in the intact anterior cruciate ligament under the same loading conditions.

Tibial Rotation

Torsional rotation was unaffected by replacement of the posterior cruciate ligament with a graft. On the basis of this finding, as well as the fact that the forces in the posterior cruciate ligament also were unchanged when tibial torque was applied, we concluded that either the posterior cruciate ligament plays a relatively unimportant role in controlling tibial rotation or the graft replaced this function effectively.

Clinical Importance

The clinical application of our results should be viewed with a certain degree of caution, as our study involved quasistatic measurements made on muscle-deficient specimens under controlled laboratory conditions. Because there was considerable scatter in the values for pre-tension of the graft, we recommend that the pre-tension be greater than the mean value (forty-three newtons), in order to ensure that proper anterior-posterior laxity is restored in all knees³. This raises the possibility that pre-tension of greater than forty-three newtons may represent an over-tension condition for some patients. It is reasonable to assume that the forces that were generated in the graft during all of the loading tests would increase by approximately the amount of increased pre-tension. However, for most of the loading conditions that we investigated, the forces in the graft were relatively low (approximately 100 newtons or less); thus, even with over-tension, the forces in most grafts would not reach a level that should cause concern. The exceptions to this finding were the high forces that developed in some of the grafts during hyperflexion and hyperextension; therefore, those motions should be avoided during the initial healing period after insertion of a graft and, perhaps, during rehabilitation, at which time the graft is undergoing remodeling and revascularization. Replacement of the posterior cruciate ligament with a graft does not appear to be associated with increased forces in the graft when laxity is restored. Thus, no special restrictions of routine activities postoperatively seem warranted after the bone-block fixation has healed.

NOTE: The authors thank Steve Jackson, for his assistance in the testing and the data analysis, and Stephen Liu, M.D., for his guidance in the initial phases of the work. The tissues used for the patellar ligament grafts were kindly provided by the Musculoskeletal Transplant Foundation.

References

Introduction | Materials and Methods | Results | Discussion | References

Ahmed, A. M.; Hyder, A.; Burke, D. L.; and |and |Chan, K. H.: In vitro ligament tension pattern in the flexed knee in passive loading. J. Orthop. Res.,5: 217-230, 1987.5217 1987 [PubMed]

Lewis, J. L.; Lew, W. D.; Shybut, G. T.; Jasty, M.; and Hill, J. A.: Biomechanical function of knee ligaments. In The American Academy of Orthopaedic Surgeons Symposium on Sports Medicine: the Knee, pp. 152-168. Edited by G. Finerman. St. Louis, C. V. Mosby, 1985.

Markolf, K. L.; Slauterbeck, J. R.; Armstrong, K. L.; Shapiro, M. M.; and |and |Finerman, G. A. M.: A biomechanical study of replacement of the posterior cruciate ligament with a graft. Part I: Isometry, pre-tension of the graft, and anterior-posterior laxity. J. Bone and Joint Surg.,79-A: 375-380, March 1997.79-A375 1997

Markolf, K. L.; Burchfield, D. M.; Shapiro, M. M.; Davis, B. R.; Finerman, G. A. M.; and |and |Slauterbeck, J. L.: Biomechanical consequences of replacement of the anterior cruciate ligament with a patellar ligament allograft. Part I: Insertion of the graft and anterior-posterior testing. J. Bone and Joint Surg.,78-A: 1720-1727, Nov. 1996.78-A1720 1996

Markolf, K. L.; Burchfield, D. M.; Shapiro, M. M.; Cha, C. W.; Finerman, G. A. M.; and |and |Slauterbeck, J. L.: Biomechanical consequences of replacement of the anterior cruciate ligament with a patellar ligament allograft. Part II: Forces in the graft compared with forces in the intact ligament. J. Bone and Joint Surg.,78-A: 1728-1734, Nov. 1996.78-A1728 1996

Wascher, D. C.; Markolf, K. L.; Shapiro, M. S.; and |and |Finerman, G. A. M.: Direct in vitro measurement of forces in the cruciate ligaments. Part I: The effect of multiplane loading in the intact knee. J. Bone and Joint Surg.,75-A: 377-386, March 1993.75-A377 1993

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Fig. 2-B Graft inserted at laxity-matched pre-tension.

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Fig. 3-B With application of a constant 100-newton posterior force to the tibia.

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Fig. 3-C With application of a constant ten-newton-meter internal tibial torque.

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Fig. 3-D With application of a constant ten-newton-meter external tibial torque.

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Fig. 3-E With application of a constant ten-newton-meter varus moment.

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Fig. 3-F With application of a constant ten-newton-meter valgus moment.

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Ahmed, A. M.; Hyder, A.; Burke, D. L.; and |and |Chan, K. H.: In vitro ligament tension pattern in the flexed knee in passive loading. J. Orthop. Res.,5: 217-230, 1987.5217 1987 [PubMed]

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These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.

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KEITH L. MARKOLF, JAMES R. SLAUTERBECK, KEVIN L. ARMSTRONG, MATTHEW S. SHAPIRO, GERALD A. M. FINERMAN; A Biomechanical Study of Replacement of the Posterior Cruciate Ligament with a Graft. Part II: Forces in the Graft Compared with Forces in the Intact Ligament*. The Journal of Bone & Joint Surgery. 1997 Mar;79(3):381-6.

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