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The Journal of Bone and Joint Surgery 78:1720-7 (1996)
© 1996 The Journal of Bone and Joint Surgery, Inc.

Biomechanical Consequences of Replacement of the Anterior Cruciate Ligament with a Patellar Ligament Allograft. Part I: Insertion of the Graft and Anterior-Posterior Testing*

KEITH L. MARKOLF, PH.D.{dagger}, DANIEL M. BURCHFIELD, M.D., PH.D.{dagger}, MATTHEW M. SHAPIRO, M.D.{dagger}, BRENT R. DAVIS, M.D.{dagger}, GERALD A. M. FINERMAN, M.D.{dagger} and JAMES L. SLAUTERBECK, M.D.{dagger}, LOS ANGELES, CALIFORNIA

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


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Nineteen fresh-frozen knee specimens from cadavera were tested for anterior-posterior laxity with 200 newtons of force applied to the tibia. A cylindrical cap of subchondral bone containing the tibial insertion of the anterior cruciate ligament was isolated with a coring cutter and was potted in acrylic. A thin wire was connected to the undersurface of the cap, and relative displacement between the cap and the tibia was measured with an isometer as the knee was extended. The cap of bone was connected to a load-cell that recorded force in the intact ligament during anterior-posterior testing with the tibia locked in neutral, internal rotation, and external rotation. The anterior cruciate ligament was then resected, and a femoral tunnel was drilled at the site where the isometer readings from the wire were the same as those obtained for the intact anterior cruciate ligament. A bone-patellar ligament-bone graft was used to reconstruct the anterior cruciate ligament, and the isometer measurements were repeated with the graft in place. The graft was pre-tensioned at 30 degrees of flexion to restore normal anterior-posterior laxity. Anterior-posterior laxity tests were repeated at this level of pre-tension (laxity-matched pre-tension) as well as at a level that was forty-five newtons greater (over-tension). The moment required to extend the knee was measured before and after insertion of the graft at both levels of pre-tension. When the tibia was locked in positions of internal and external rotation, the anterior-posterior laxities and the forces in the anterior cruciate ligament (generated by an anterior force applied to the tibia) were significantly less than the corresponding values with the tibia in neutral rotation at 20, 30, and 45 degrees of flexion (p <= 0.05). Isometer readings for the intact anterior cruciate ligament indicated that the cap of bone retracted into the joint a mean and standard deviation of 3.1 ± 0.8 millimeters as the knee was extended from 30 degrees of flexion to full extension. For each specimen, the isometer measurements for the trial wire and for the graft were within 1.5 millimeters of those for the intact anterior cruciate ligament. At laxity-matched pre-tension (mean, 28.2 ± 16.8 newtons), the mean anterior-posterior laxities of the reconstructed knees were within 1.0 millimeter of the corresponding means for the intact knees between 0 and 45 degrees of flexion. Over-tensioning of the graft by forty-five newtons decreased the anterior-posterior laxity a mean of 1.2 millimeters at 30 degrees of flexion. Over-tensioning of the graft did not change the moment required to bring the knee to full extension. CLINICAL RELEVANCE: When performing an anterior laxity test with the knee flexed 20 to 30 degrees, the examiner should place the tibia in a position of neutral rotation, as this is the position of greatest laxity at which all anterior force applied to the tibia will be resisted by the anterior cruciate ligament. It is important to recognize that the intact anterior cruciate ligament does not behave in a so-called isometric fashion. Approximately three millimeters of retraction of a trial wire into the joint during the last 30 degrees of extension (as measured with an isometer) is reasonable in order to achieve changes in the length of the graft that approximate those of the intact anterior cruciate ligament.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
At present, the bone-patellar ligament-bone graft is the most frequently used tissue for reconstruction of the anterior cruciate ligament3; however, the details for tensioning of the graft are somewhat subjective and often vary among authors who describe this procedure2,7,12,17. Different amounts of applied initial tension and different positions of flexion have been recommended5,6,10,16.

The isometry of the anterior cruciate ligament has also received considerable study in vitro. In bench studies, separation distances between selected points on the tibia and the femur were measured with an instrumented spatial linkage13 as well as with a computer model20. The authors of those studies agreed that the isometry of the graft is far less sensitive to the site of the tibial tunnel than to that of the femoral tunnel. However, in the operative setting, the surgeon is presented with a knee in which the anterior cruciate ligament is torn or ruptured. Again, the literature has been inconsistent regarding recommendations of isometric insertion sites4,18.

Recent clinical reports have suggested that anterior-posterior laxity is not always restored to normal after substitution of the anterior cruciate ligament with a graft1,9,19. Although an excessive side-to-side difference in anterior-posterior laxity usually does not constitute a clinical failure, the fact remains that in many patients this operative procedure fails to restore normal laxity of the ligaments of the knee. One explanation for these results may be related to the operative technique used to place and tension the graft.

This study had several objectives. First, we wanted to determine the effects of tibial rotation and the angle of flexion of the knee on anterior-posterior laxity and on the force generated in the intact anterior cruciate ligament by an anterior force applied to the tibia. These experiments were performed with an anterior-posterior test apparatus. Second, we wanted to compare isometer measurements for the intact anterior cruciate ligament, the trial wire, and the patellar ligament graft during passive extension of the knee. These experiments were performed with a passive knee-extension apparatus. A third objective was to determine the amount of pre-tension on the graft necessary to restore normal anterior-posterior laxity at 30 degrees of flexion (in other words, the laxity-matched pre-tension) and to measure the effects of repeated cyclical loading (pre-conditioning) on anterior-posterior laxity for the intact anterior cruciate ligament and the graft. These experiments were performed with an anterior-posterior test apparatus. Fourth, we compared anterior-posterior laxity at selected angles of flexion of the knee before and after insertion of the graft at the laxity-matched level of pre-tension. These experiments were performed with an anterior-posterior test apparatus. Finally, the study was performed to determine the effects of over-tensioning of the graft on anterior-posterior laxity and on the moment required to extend the knee fully. These experiments were performed with an anterior-posterior test apparatus and a passive knee-extension apparatus.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 

Preparation of the Specimens
Nineteen fresh-frozen knee specimens were taken from the cadavera of individuals who were fifty to seventy-eight years old at the time of death. The knee joints were inspected visually through a medial parapatellar incision to determine if there was evidence of gross deterioration of the articular cartilage. In all of the specimens, both cruciate ligaments were intact and appeared normal. The tibia and the femur were sectioned at the middle of the shaft and were scraped clean of soft tissue to within ten centimeters of the joint line. The ends of the bones were potted in cylindrical molds of polymethylmethacrylate to allow them to be gripped in the test fixtures. The skin and all soft tissues surrounding the knee capsule were left intact to prevent dehydration of the ligamentous tissues within the knee during testing. The femur was clamped with its shaft parallel to the floor and the patella facing down. A weight was suspended from the tibia to produce an extension moment of 2.5 newton-meters about an axis passing through the joint contact surfaces at full extension. The angle between the potted cylinders containing the tibia and the femur was measured; this angle was defined as 0 degrees of flexion (full extension)14.

Passive Knee-Extension Tests
The knee was placed in a passive knee-extension apparatus. The angle of knee flexion was recorded with an electrogoniometer mounted on a four-bar linkage connected to the tibia. The applied extension force was measured with a loading handle attached to the distal aspect of the tibia. The recorded force was converted to a bending moment by multiplying it by the distance to the joint line. A load-cell was connected to the anterior cruciate ligament or the graft and the output was recorded. The test was performed by manual application of an extension force to the load handle while the tibia was free to seek its own positions of rotation and varus-valgus angulation14.

Anterior-Posterior Force versus Displacement Tests
Tibial force was applied manually to the undercarriage bar by an instrumented force handle attached 7.6 centimeters proximal to the joint line (Fig. 1). The scale factor for the recorded applied force was adjusted to simulate an equivalent force applied to the tibia at the joint line. This was done by performing a simple moment balance about the distal pivot point and calculating the equivalent force (applied at the joint line) required to generate the same moment produced by the instrumented handle. The tibia was locked in positions of neutral, internal rotation, and external rotation during anterior-posterior force-testing. Neutral rotation at each angle of flexion tested was defined as midway between the tibial rotations produced by five newton-meters of internal and external tibial torque. The positions of internal and external rotation were defined as those resulting from application of two newton-meters of internal or external tibial torque. These low levels of tibial torque were meant to simulate torques that might be applied during a clinical examination of the knee. The force applied to the tibia, the resultant force in the anterior cruciate ligament or the graft, and the tibial displacement were recorded. Laxity of the ligaments of the knee was defined as the anterior-posterior displacement of the tibia relative to the femur resulting from the application of a 200-newton anterior-posterior force to the tibia.



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  		Illustration demonstrating how the potted femur was attached to a flexion bar, which was used to flex and extend the knee manually. The tibia always remained level, and the angle of flexion was set by flexing the femur upward. An extension shaft attached to the distal end of the tibia was supported by a spherical rod-end bearing that acted as a pivot about which the tibia could rotate in the sagittal plane. A horizontal undercarriage bar attached to a large bearing housing held a roller that contacted a vertical plate. This constrained anterior-posterior motion of the tibia to the vertical plane as anterior-posterior force was applied to the undercarriage bar. The tibia was locked within the bearing housing to prevent internal-external tibial rotation during the test. A spring-loaded plunger connected to the core of a linear variable differential transformer (not shown) contacted a small plate mounted to the undercarriage bar at the knee joint line. Linear displacement of the undercarriage bar (in the vertical direction) at this point was taken to represent anterior-posterior motion of the tibia at the joint line. Gravitational loading of the tibia from the weight of the bearing housing and the undercarriage bar was counterbalanced by a weight-pulley system (not shown).

 

Test Procedures
The intact specimen was first mounted in the passive knee-extension apparatus, and the extension moment about the knee was recorded as a function of the angle of flexion to as much as 5 degrees of hyperextension.

Next, the effects of preconditioning loading cycles on the laxity of the ligaments of the knee were determined. A 200-newton anterior and posterior force was applied to the tibia of the intact specimen (with the tibia in neutral rotation) at 30 degrees of flexion. The total anterior-posterior laxity was determined for the first, third, and thirtieth loading cycles. After the preconditioning tests, anterior-posterior force-testing was performed at 0, 10, 20, 30, and 45 degrees of flexion of the knee. For these tests, two anterior-posterior loading cycles to 200 newtons of force were completed before the third test was recorded.

A cylindrical block of subchondral bone that incorporated the tibial attachment of the anterior cruciate ligament was mechanically isolated from the tibial plateau with a coring cutter and was potted in polymethylmethacrylate14. The proximal end of an isometer wire was attached to the center of the undersurface of the potted cap of bone. The distal end of the wire was attached to the spring-loaded plunger of an isometer (Acufex Microsurgical, Mansfield, Massachusetts). The barrel of the isometer was fixed to a bracket on the tibia. With the femur clamped horizontally, displacement of the wire relative to the tibia was read (to the nearest 0.5 millimeter) from a scale on the housing of the isometer as the knee was extended from 90 to 0 degrees of flexion. Isometer readings were recorded as positive when the distal end of the wire retracted into the joint. The cap of bone remained roughly centered within the isolation hole because of the spring tension in the wire. This eliminated potential friction between the cap of bone and the inner wall of the isolation hole. The weight of the tibia was manually supported during the test.

The cap of bone containing the tibial insertion of the anterior cruciate ligament was then connected to a three-degrees-of-freedom load-cell fixed to the tibial bracket to record the resultant force in the anterior cruciate ligament14. The passive knee-extension test was repeated, and the resultant force in the anterior cruciate ligament, the extension moment, and the angle of flexion of the knee were recorded. Anterior-posterior testing was repeated with the tibia locked in neutral, internal rotation, and external rotation, and the resultant force in the anterior cruciate ligament, the force applied to the tibia, and the tibial displacement were recorded.

After all tests had been completed with the anterior cruciate ligament intact, the ligament was resected down to femoral bone. The proximal end of the isometer wire was fixed to a point on the inner wall of the lateral femoral condyle such that the relative displacement measurements duplicated as closely as possible those recorded for the intact ligament. A discrepancy of one millimeter over the range of flexion compared with that for the intact anterior cruciate ligament was considered satisfactory. The point of fixation of the wire was often near the central region of the femoral insertion of the ligament, but specific coordinates were not recorded. A drill-guide was used to place a centering pin at this point, and a cannulated ten-millimeter reamer was used to drill a tunnel.

The grafts were prepared from bone-patellar ligament-bone specimens obtained from a tissue bank. The specimens were split longitudinally into two halves, yielding two grafts per specimen. A ten-millimeter-wide section of graft tissue was prepared from the central portion of each half.

The bone at the patellar end was reinforced with wire loops and was potted in a small cylinder of polymethylmethacrylate to make a cap on the graft identical in size to the cap of bone that contained the anterior cruciate ligament. The bone at the tibial end of the graft was interwoven with four strands of high-strength Dacron line. The Dacron lines were passed through the femoral tunnel and clamped proximally. The isometer wire was attached to the cap on the graft, and the relative displacements were measured with the isometer.

The cap on the graft was then attached to the load-cell to record the resultant force in the graft. The Dacron lines exited the femoral tunnel. The graft was oriented such that its wide dimension was aligned in the medial-lateral direction on the tibia. The acrylic surface (at which the graft fibers exited the cap) was flush with the surface of the tibial plateau. This placement simulated as closely as possible the geometrical configuration of a graft located at the center of the intact tibial attachment of the anterior cruciate ligament. With the tibia free to rotate, a calibrated spring-scale was used to apply tension to the Dacron lines at 30 degrees of knee flexion while a posterior force (of approximately twenty-two newtons) was applied to the tibia (as is done commonly at the time of an operation). One end of the spring-scale was connected to the Dacron lines exiting the femoral tunnel and the other end was pulled manually (against the fixed femur) until the desired pre-tension was achieved. The Dacron lines were clamped to maintain the desired pre-tension, and anterior-posterior testing was performed with the tibia in neutral rotation. Through trial and error, a level of pre-tension was found for the graft that produced the same anterior-posterior laxity (within 1.0 millimeter) at 30 degrees of flexion as that for the intact specimen. This pre-tension was termed the laxity-matched pre-tension.

The passive knee-extension test, preconditioning tests, and anterior-posterior force tests (with the tibia in neutral rotation) were repeated at the laxity-matched pre-tension. The graft was then placed in an over-tensioned condition through application of a level of pre-tension that was forty-five newtons greater than the laxity-matched pre-tension. The just described tests were repeated.Of the nineteen specimens tested, the first four had the graft placed at a location in the femur that produced unacceptably high forces in flexion15. As we believe that these specimens were tested before we perfected the use of the isometer, they (and two additional specimens that had a fracture of the cap of bone) were not included in the results for anterior-posterior testing after substitution with the graft.

Statistical Analysis
Two-way analysis of variance with repeated measures was used to determine the significance of differences between the mean anterior-posterior laxities for several conditions of the knee: intact, after resection of the anterior cruciate ligament, after installation of the load-cell, and after insertion of the graft. Multiple pairwise comparisons between the means at specific angles of flexion were made with the Student-Newman-Keuls test. Similar analysis-of-variance models were used to compare forces and the anterior-posterior limits of motion in the intact anterior cruciate ligament at the three positions of tibial rotation. One-way analysis of variance with repeated measures was used to compare the means for the three isometer measurements and the mean laxities for the first, third, and thirtieth anterior-posterior loading-cycles. The level of significance was p <= 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 

Effects of Tibial Rotation on Laxity and Force in the Ligament
Locking of the tibia in internal or external rotation during the anterior-posterior force test significantly reduced the mean anterior-posterior laxity at 20, 30, and 45 degrees of flexion of the knee, compared with the mean anterior-posterior laxity with the tibia in neutral (p <= 0.05) (Fig. 2). The mean force generated in the anterior cruciate ligament by a 200-newton anterior tibial force was also significantly reduced at 20, 30, and 45 degrees of flexion when the tibia was locked in internal or external rotation, compared with the mean force generated when the tibia was in neutral (p <= 0.05) (Fig. 3).



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  		Graph of anterior-posterior laxity versus the angle of flexion with the tibia locked in three positions of rotation. The mean laxities for internal and external rotation were significantly less (p <= 0.05) than those for neutral rotation at 20 degrees of flexion or more. There were no significant differences between the mean laxities for internal and external rotation at any angle of flexion. ns = no significant difference (p > 0.05). The error bars indicate one standard deviation of the sample mean.

 


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  		Graph of the force generated in the anterior cruciate ligament (ACL) from 200 newtons of applied anterior tibial force versus the angle of flexion with the tibia locked in three positions of rotation. The mean levels of force for internal and external tibial rotation were significantly less (p <= 0.05) than those for neutral rotation at 20 degrees of flexion or more. There was no significant difference between the mean levels of force for internal rotation and external rotation at any angle of flexion. ns = no significant difference (p > 0.05). The error bars indicate one standard deviation of the sample mean.

 

Isometry
When the isometer wire was connected to the cap of bone on the intact anterior cruciate ligament, the absolute relative displacement between the wire and the tibia from 90 to 30 degrees of flexion was 1.0 millimeter or less for each specimen. As the knee was extended from 30 to 0 degrees, the mean relative displacement (and standard deviation) between the wire and the tibia was 3.1 ± 0.8 millimeters (range, 2.0 to 4.5 millimeters) (Table I).


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  		TABLE I RELATIVE DISPLACEMENT FROM THIRTY TO ZERO DEGREES OF FLEXION

 
For any given specimen, the relative displacement for the trial wire was within 1.0 millimeter of that recorded with the graft; both relative displacements were within 1.5 millimeters of that recorded for the cap of bone on the intact anterior cruciate ligament (Table I). The mean isometer readings for the intact anterior cruciate ligament, the graft, and the trial wire were not significantly different from one another (Table I).

Pre-Tensioning of the Graft and Anterior-Posterior Preconditioning
The pre-tensioning of the graft required to restore anterior-posterior laxity at 30 degrees of flexion ranged from 11.1 to 66.8 newtons; the mean pre-tension was 28.2 ± 16.8 newtons (Table I). There was no significant difference between the mean laxity for the first loading cycle and that for the third for either the intact anterior cruciate ligament (14.2 ± 2.2 and 14.2 ± 2.4 millimeters, respectively) or the graft (16.1 ± 1.9 and 16.9 ± 2.2 millimeters, respectively). The mean laxity of the intact anterior cruciate ligament for the thirtieth loading cycle was 0.7 millimeter more (14.9 ± 2.4 millimeters) (p <= 0.05) than that for the first loading cycle. The mean laxity of the graft for the thirtieth loading cycle was 1.8 millimeters more (17.9 ± 2.8 millimeters) than that for the first loading cycle (p <= 0.05).

Anterior-Posterior Laxity after Insertion of the Graft
The mean anterior-posterior laxity after installation of the load-cell was not significantly different from that for the intact anterior cruciate ligament at any position of flexion. Sectioning of the anterior cruciate ligament significantly increased the anterior-posterior laxity at all positions of flexion (p <= 0.05) (Fig. 4). With the numbers available, we could detect no significant differences in the mean laxity of the knee at any angle of flexion before or after insertion of the graft (Fig. 4).



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  		Graph of the anterior-posterior laxity versus the angle of flexion. Three conditions are shown: intact, after sectioning of the anterior cruciate ligament (ACL), and after insertion of the graft at laxity-matched pre-tension. The mean laxities after sectioning of the anterior cruciate ligament were significantly greater (p <= 0.05) than those for the intact specimens and the graft at all angles of flexion. The mean laxities for the graft were not significantly different from those for the intact knee at any angle of flexion. ns = no significant difference (p > 0.05). The error bars indicate one standard deviation of the sample mean.

 

Effects of Over-Tensioning of the Graft
Over-tensioning of the graft by forty-five newtons at 30 degrees of flexion resulted in a significant (p <= 0.05) decrease in anterior-posterior laxity ranging from 0.8 to 1.9 millimeters (mean, 1.2 ± 0.5 millimeters). Over-tensioning of the graft did not increase the moment required to extend the knee fully.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 

Measurements of Force
Our group previously estimated errors in the measurements of force in intact anterior cruciate ligaments to be between 3 and 10 per cent14. We believe that errors in the measurements of force in the graft were similar since the same basic load-cell configuration was used for both experiments.

The graft-tibia interface in the current study differed slightly from that seen in the clinical situation. In our configuration, graft tissue exited the acrylic potting flush with the surface of the tibial plateau at the center of the cap. In the operative setting, the graft is passed through a ten-millimeter-wide tunnel hole in the tibia and over its edge. We believe that these geometric dissimilarities were of minor importance in terms of the measurement of forces in the graft.

Another variable that could not be controlled was the structural properties of the graft itself. Force that developed in the graft was affected by the cross section of the graft, the length of the graft, the tissue modulus, and the stiffness of the high-strength Dacron lines that were clamped to fix the free end of the graft. The grafts used in this study were obtained from a tissue bank. As such, they were obtained primarily from younger individuals who had the excellent bone quality that is necessary to sustain the sometimes dramatically high forces in the graft. As tissue properties are also a source of variability in clinically placed grafts, we believe that the scatter in our force and laxity data is representative of that to be expected clinically.

We believe that locking of the tibia in positions of internal or external rotation during the anterior-posterior force test pre-tensioned the collateral ligaments and capsular structures. These tensed structures then acted to resist anterior force applied to the tibia and to reduce the force carried by the anterior cruciate ligament. This effect was observed beyond 10 degrees of flexion of the knee, where collateral ligaments and capsular structures are normally slack with the tibia in neutral rotation. We hypothesize that anterior-posterior laxity was reduced with tibial prerotation by the same mechanism.

Isometry
Retraction of the cap of bone on the intact anterior cruciate ligament into the joint indicated an increase in distance between the insertion sites of the ligament. This corresponds to the tension that developed in the intact ligament. Similarly, when the proximal end of the trial isometer wire or the graft was fixed within the joint, retraction of the distal end of the wire or graft into the joint indicated that tension would develop in the wire or graft if it were fixed at both ends.

Since the isolation hole in the tibia was nearly in line with the fibers of the intact anterior cruciate ligament near extension, measurements of relative displacement for the cap of bone on the intact anterior cruciate ligament reflected gross changes in the length of the ligament. It is well recognized that gross motion of the cap of bone does not represent uniform tightening of all fibers of the anterior cruciate ligament. The relative displacements that we measured during the last 30 degrees of extension of the knee appeared to be caused by tension within the anteromedial fibers. However, one could argue that, since a cord-like graft can never duplicate the fiber-bundle geometry of the intact anterior cruciate ligament, gross motion of the fibers of the intact ligament is a reasonable goal in terms of desired isometer readings. It was expected that isometer measurements with the trial wire would be the same as those for the intact anterior cruciate ligament. This was our stated criterion for acceptance of the site of the trial femoral tunnel. However, it was interesting to find that the mean isometer measurement for the graft was the same as that for the trial wire and the intact anterior cruciate ligament. We believe that this is a result of the care taken to ensure that the caps of bone on the anterior cruciate ligament and on the graft were identical in size and that the insertion of the graft into the acrylic cap was flush with the tibial surface. Both caps remained centered in the tibial isolation tunnel (under isometric spring-tension) as the knee was extended.

Clancy et al. argued that the edge (not the center) of the hole for the femoral tunnel should be located at the trial point determined by the isometer. The rationale for this was that, as tension develops in the graft, its fibers flatten as it passes around the edge of the hole. This would place the bulk of the ligamentous tissue eccentric to the center of the hole, away from the intended central position. Our results do not support this concept.

As the hole for the tibial tunnel was placed at the geometric center of the tibial insertion of the anterior cruciate ligament (to encompass all of the fibers of the ligament during isolation), the center of the graft was also located at this position. In clinical practice, the center of the hole is often located slightly medial and anterior to our selected position. We do not believe that this is a serious shortcoming, since previous studies have shown that the location of the tibial tunnel has a relatively minor effect on the isometry of the anterior cruciate ligament13,20. In preliminary experiments, we verified that locations of the wire as many as three millimeters eccentric from the center of the tibial tunnel resulted in isometer readings that were identical to those for the central location.

Our isometer measurements with the trial wire indicated a relative displacement of 3.5 millimeters during the last 30 degrees of extension of the knee. These measurements are in general agreement with those reported previously8-10,18. Whereas we found that the isometer measurements for the trial wire were an excellent predictor of the behavior of the graft, Fleming et al.11 found no association between the calculated point-to-point changes in length (before drilling of the tunnel) and elongation of the graft. In fact, the spatial linkage measurements predicted that the graft would tighten from 10 to 30 degrees of flexion, while the strain measurements after insertion of the graft indicated the opposite response. There are important differences between the study by Fleming et al. and the present one. Those authors did not use an isometer to determine the position of the femoral tunnel. The center of their tunnel was five to seven millimeters anterior to the posterior aspect of the distal femoral cortex, at the one o'clock or eleven o'clock orientation in the notch. Thus, selection of the femoral drill-point location with the use of anatomical landmarks did not produce the desired pattern of isometry, which is tightening of the graft with extension.

Preconditioning and Pre-Tensioning
The mean 0.7-millimeter increase in the laxity of the ligaments of the knee with the intact anterior cruciate ligament is consistent with known preconditioning effects for ligamentous tissues. This figure represents increases in laxity due to cyclical stretching of both cruciate ligaments. The mean 1.8-millimeter increase in laxity with the graft probably reflects a so-called settling-in phenomenon wherein the fibers of the patellar ligament flatten around the edges of the tunnel holes and the strain distribution in the tissue adjusts to the new boundary conditions. We believe that this amount of increase in cyclical strain is important clinically as the mean increase in laxity after sectioning of the anterior cruciate ligament was 13.2 millimeters. At the time of an operation, preconditioning can be accomplished by maintaining pre-tension during anterior-posterior force cycles and then by tensioning and fixing the graft.

Our data for pre-tensioning of the grafts are most comparable with those of Burks and Leland, who used a KT-1000 arthrometer to determine the pre-tension on the graft required to restore anterior-posterior laxity at 30 degrees of flexion. They reported mean pre-tensions of sixteen newtons for the patellar ligament, thirty-eight newtons for the semimembranosus, and sixty-one newtons for the iliotibial band. These data suggest that stiffness of the tissue may influence the amount of pre-tensioning of the graft necessary to restore anterior-posterior laxity. The discrepancy between their data and ours may be related to their use of the KT-1000 arthrometer to measure the laxity. The dial on this instrument records relative motion between the tibia and the patella, not between the tibia and the femur. A second difference is the magnitude of the load applied to the tibia, which was 200 newtons in the present study compared with eighty-nine newtons in the study by Burks and Leland. One might expect that a greater pre-tensioning of the graft would be required to resist a greater applied anterior tibial force.

Another study that provided data relevant to our measurements is that by Fleming et al.10, who measured anterior-posterior laxity with 150 newtons of force applied to the tibia. They found that no pre-tension was necessary to restore normal anterior-posterior laxity at 30 degrees of flexion when a braided polyethylene prosthesis was used as a substitute for the anterior cruciate ligament. When they applied twenty-seven newtons of pre-tension (which is close to the mean laxity-matched pre-tension of 28.2 newtons in our study), the anterior-posterior laxity was three to four millimeters less than normal at 30 degrees of flexion. The difference between the measured pre-tension in their study and ours may be the result of the increased stiffness of the prosthesis that they used.

We found considerable variability in the amount of pre-tensioning of the graft required to restore normal anterior-posterior laxity. A number of factors could contribute to the scatter. Because the ligaments of some knees are inherently more lax than those of others, we hypothesized that loss of the anterior cruciate ligament in a knee with more laxity could require a greater pre-tension force in the graft because a greater correction of laxity is necessary. However, a regression analysis of pre-tensioning of the graft and laxity of the intact knee demonstrated no significant association between these two variables.

We believe that the greatest source of variability in the amount of pre-tensioning of the graft is related to the location of the hole of the femoral tunnel. In the first four specimens that we tested, we drilled holes that produced excessive force in the graft in flexion15. This indicated that the hole was placed too far anteriorly. Excessive pre-tensioning of the graft was necessary to restore normal anterior-posterior laxity to these specimens. Geometrically, if the femoral tunnel is too far anterior, the graft will lie in a more vertical orientation with respect to the tibia for flexed positions of the knee. This places it at a less favorable angle to resist applied anterior tibial force, thus requiring greater pre-tensioning of the graft.

Laxity
It was encouraging to find that the anterior-posterior laxity was restored at angles of flexion other than that at which the graft was tensioned. We believe that these findings reflect the strict care and attention to detail that was exercised in choosing the location of the hole for the femoral tunnel on the basis of isometer readings. If our test results are to be translated into clinical practice, the surgeon may wish to consider using an isometer for all procedures.

Over-Tensioning of the Graft
The insensitivity of the extension moment about the knee to the pre-tension force in the graft dispels one of the clinical concerns related to attaining full extension if the graft is over-tensioned at the time of the operation. Clinicians recognize the necessity for patients to gain full extension early in the rehabilitation process; our data show that the level of pre-tension in the graft is not a factor in achieving this goal. Although high levels of pre-tension can generate correspondingly large forces at full extension, the graft is in a poor position geometrically to resist extension moment; the moment arm from the ligament to the axis of flexion is small. However, there may be other biological consequences of over-tensioning of the graft, such as increased abrasion at the edges of the tunnel holes. We did observe some fraying of the graft in the portion that passed over the chamfered edge of the femoral tunnel.

Clinical Importance
The performance of an anterior laxity test with the tibia in internal or external rotation has two effects that are of clinical relevance: anterior-posterior laxity is reduced and the force in the anterior cruciate ligament is reduced. The Lachman test was designed to stress the anterior cruciate ligament. Our force data for the intact anterior cruciate ligament confirmed that the force in the ligament is approximately equal to the force applied to the tibia. With internal or external tibial rotation, tightened collateral and capsular structures can also act to resist applied anterior tibial force so that the force in the ligament is reduced. Therefore, the examiner should give special attention to placement of the tibia in neutral rotation during the anterior laxity test. The same care should be observed when the test is performed after insertion of a graft. Clinically, special attention must be paid to controlling tibial rotation during the Lachman test because of the coupled nature of internal-external rotation and anterior-posterior tibial translation.

The intact anterior cruciate ligament is not isometric (meaning that it changes length with extension of the knee). The distance between the attachments of the anterior cruciate ligament on the tibia and the femur increases during the last 30 degrees of extension, thereby developing force in the intact anterior cruciate ligament (and in a properly placed graft). When used correctly, an isometer gives an excellent indication of the changes in length that can be expected in the graft. Approximately 3.0 millimeters of relative displacement of a trial wire is a reasonable goal.

Although we found that a mean of 28.2 newtons of pre-tension was necessary to restore normal anterior-posterior laxity, a force of 66.8 newtons was required in one knee and one of 44.5 newtons, in three. Clearly, the surgeon cannot know which tension is appropriate for a given knee. We believe that it is better to over-tension the graft slightly than to under-tension it. Over-tensioning of the graft will not block extension of the knee. Although over-tensioning will generate greater initial forces in the graft in extension, the level of force could diminish with time as the tissue undergoes stress relaxation. Over-tensioning will reduce anterior-posterior laxity initially, but laxity might be expected to increase with time if the graft elongates permanently with cyclical loading.

NOTE: The authors thank Mike Shepard and Steve Jackson for their assistance in testing and analysis of the data, and Fred Dorey, Ph.D., for his consultation and advice regarding the statistical analysis. The patellar ligament grafts were donated by the Musculoskeletal Transplant Foundation.


   Footnotes
 
*Although none of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but are directed solely to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were Grant RO1 AR40330 from the National Institutes of Health and grants from the Orthopaedic Research and Education Fund and the Dorothy and Leonard Straus Fund. A gift of equipment was also made by Acufex Microsurgical Incorporated, Mansfield, Massachusetts.

{dagger}Department of Orthopaedic Surgery, Biomechanics Research Section, 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.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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