Force Measurements on the Fibular Collateral Ligament, Popliteofibular Ligament, and Popliteus Tendon to Applied Loads

Transcription

1 DOI: / Force Measurements on the Fibular Collateral Ligament, Popliteofibular Ligament, and Popliteus Tendon to Applied Loads Robert F. LaPrade,* MD, PhD, Andy Tso, MS, and Fred A. Wentorf, MS From the Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, Minnesota Background: Little information is known about the forces seen on the main individual structures of the posterolateral knee to applied loads. This information is needed to determine which structures should be reconstructed and also the relative strengths needed for reconstruction grafts. Purpose: To determine in vitro forces in the fibular collateral ligament, popliteofibular ligament, and popliteus tendon for various posterolateral knee loading conditions. Study Design: Cadaveric study. Results: The fibular collateral ligament was loaded in varus, internal rotation, and external rotation. The highest amount of force seen on the fibular collateral ligament was at 0 of knee flexion with external rotation, with the mean load response to external rotation significantly less at 90. Fibular collateral ligament varus load response at 0, 30, and 60 was fairly constant, with a significant decrease at 90 compared to 30 of knee flexion. The popliteus tendon and popliteofibular ligament were loaded with an external rotation moment and were noted to have similar loading patterns. The mean load response on both the popliteus tendon and the popliteofibular ligament peaked at 60 of knee flexion. The mean popliteus tendon and popliteofibular ligament load response at 0 was significantly less than the mean load response at 30, 60, and 90 of knee flexion. Conclusions: High relative loads were seen on the fibular collateral ligament with varus and external rotation and on the popliteus tendon and popliteofibular ligament, with external rotation. A reciprocal relationship of load sharing in external rotation depending on knee flexion angle was revealed that has not been previously reported. The force on the fibular collateral ligament with external rotation loads was higher than the load on the popliteus complex at lower flexion angles, with the popliteus complex having higher load sharing at 60 and 90 of knee flexion. These results provide a measure of the potential for failure of these structures with joint loading and guidelines for both graft strength requirements for surgical reconstructions and postoperative rehabilitation protocols. Keywords: posterolateral knee; fibular collateral ligament; force measurement; popliteofibular ligament, popliteus tendon Most of our knowledge about the relative importance of individual anatomical structures of the posterolateral knee in providing static stability is derived from sequential cutting of these structures during motion testing. 5,6,14,20 However, to our knowledge, there have been no studies that directly measure the force on these structures during joint loading. To determine an anatomical surgical *Address correspondence to Robert F. LaPrade, MD, PhD, Department of Orthopaedic Surgery, University of Minnesota, 2450 Riverside Avenue, R200, Minneapolis, MN ( lapra001@umn.edu). No author or related institution has received financial benefit from research in this study. See Acknowledgment for funding information. The American Journal of Sports Medicine, Vol. 32, No. 7 DOI: / American Orthopaedic Society for Sports Medicine reconstruction for grade III posterolateral injuries 3,4 that closely replicates the function of the native structures, it is necessary to determine the relative loading of these native structures in the uninjured state. In addition, we wanted to test whether the structures that were noted to cause the most joint motion changes in static biomechanical studies were functionally loaded in other joint loading conditions, where they may function as primary or important secondary stabilizers to joint motion testing, which may not have been detected in sequential cutting studies. 5,6,14,20 It is important to recognize that although the selectivesectioning method of assessing the importance of a specific, or group of, structure(s) to provide static stability to the knee is useful, cutting these structures changes the intricate interactions and relationships of the remaining knee structures and cancels the effect of the specific sectioned 1695

2 1696 LaPrade et al The American Journal of Sports Medicine structure(s). This means that the results obtained from the selective-sectioning methods are dependent on the sequence in which the structures were cut. Therefore, the conclusions about the contribution of a particular structure to knee stability are obtained indirectly because they are based on comparisons between intact and cut states rather than actual direct measurements. Measurement of the relative direct force in structures during joint loading is important to determine if surgical reconstructions can replicate the relative function of structures that they replace. It is desired that a surgical reconstruction have relative forces for an applied load for the reconstructed ligament(s) that would closely approximate those values found between the native intact ligaments under similar loading conditions. In addition, the direct method to measure the forces in the fibular collateral ligament, popliteus tendon, and popliteofibular ligament could potentially yield new information about the relative importance of these structures in providing stability to the posterolateral corner of the knee. This is important because previous biomechanical studies have found the main primary restraint to varus translation to be the fibular collateral ligament and the primary restraint to external rotation to be the popliteus complex. 5,6,14,20 Reconstructions based solely on these findings may not accurately represent the amount of force seen on these important posterolateral knee stabilizers during in vivo loading conditions, which could lead to increased laxity or reconstruction graft failure over time. With this in mind, the purpose of our study was to measure the force in the intact fibular collateral ligament, popliteus tendon, and popliteofibular ligament during in vitro loading for posterolateral knee loading conditions. Biochemical testing studies have shown these 3 structures to be the most important structures that stabilize the posterolateral knee. 5,6,20 We believe this information is necessary to determine the clinical relevance of these structures when static loading is performed and if surgical reconstructions of these structures can restore normal force relationships under similar loading conditions. MATERIALS AND METHODS Preparation Six nonpaired fresh-frozen cadaveric knees were used in this study. Knees were taken out of storage at 20 C and thawed to room temperature before testing. The bony ends of the specimens were stripped of all soft tissues and potted in polymethylmethacrylate. After the knee was potted, the posterolateral corner was dissected out to expose the 3 structures tested in this study: the fibular collateral ligament, the popliteus tendon, and the popliteofibular ligament. With the specimen aligned in the knee testing machine, 9-12 a buckle transducer was installed onto the structure to be tested (Figure 1). The details of these testing devices have been reported in detail previously Individual buckle transducers were designed for each Figure 1. Posterolateral view of a right knee with a buckle transducer attached to the popliteus tendon (PLT). PFL, popliteofibular ligament. structure in this study. They consisted of a stainless steel frame with semiconductor strain gauges mounted on a removable crossbar. The crossbar of the buckle transducer was inserted under the ligament with a hemostat and then pulled through the frame held on top of the ligament. Crossbar deflection was caused by changes in ligament load, which was quantified by strain gages attached to the crossbar. The strain gages were then connected to a Wheatstone bridge. The Wheatstone bridge was then connected to a data acquisition system. The buckle transducers were applied to 1 structure at a time to make sure contact did not occur between the frames during biomechanical testing, which could cause interference with data acquisition. The buckle transducer was subjected to pretest calibration to determine the force measured by the buckle transducer compared to a known applied force, according to our previously reported technique A short strip of Mersilene surgical tape (Ethicon, Somerville, NJ) was tied into a loop around the ligament at the flexion angle to be tested. After zeroing the buckle transducer output and starting the data acquisition system, the loop was pulled with a load cell in a stepwise manner along the ligament fibers, without contact between the loop and the crossbar of the buckle transducer. Data were acquired for 3 such trials for each pretest calibration, with each trial lasting about 8 seconds and the output zeroed before each trial. The loop was removed on completion of pretest calibration of the buckle transducer. After testing was completed on each cadaveric knee, posttest calibration was performed in the same manner as pretest calibration to determine if the mechanical integrity of the tested ligament changed over the course of biomechanical testing. Biomechanical Testing Testing consisted of conducting a sequence of loading tests at each of 4 knee flexion angles (0,30,60, and 90 ). Data

3 Vol. 32, No. 7, 2004 Force Measurements to Applied Loads 1697 TABLE 1 Mean Load Response of Force (N) Per Applied Moment (J) (mean ± SD, N/J) for the Fibular Collateral Ligament (FCL), Popliteus Tendon (PLT), and Popliteofibular Ligament (PFL) for an Applied Loading Test Ligament and Loading Condition 0 of Flexion 30 of Flexion 60 of Flexion 90 of Flexion FCL anterior drawer 0.03 ± ± ± ± 0.03 FCL posterior drawer 0.37 ± ± ± ± 0.05 FCL varus rotation 9.5 ± ± ± ± 2.6 FCL valgus rotation 0.39 ± ± ± ± 0.07 FCL internal rotation 2.5 ± ± ± ± 5.7 FCL external rotation 17.0 ± ± ± ± 1.7 PFL anterior drawer 0.00 ± ± ± ± 0.00 PFL posterior drawer 0.04 ± ± ± ± 0.01 PFL varus rotation 0.07 ± ± ± ± 0.02 PFL valgus rotation 0.01 ± ± ± ± 0.02 PFL internal rotation 0.48 ± ± ± ± 0.90 PFL external rotation 2.9 ± ± ± ± 6.8 PLT anterior drawer 0.01 ± ± ± ± 0.01 PLT posterior drawer 0.09 ± ± ± ± 0.01 PLT varus rotation 0.44 ± ± ± ± 0.29 PLT valgus rotation 0.02 ± ± ± ± 0.23 PLT internal rotation 0.02 ± ± ± ± 1.1 PLT external rotation 4.1 ± ± ± ± 6.2 were obtained for 3 trials of each loading test at each flexion angle, and the results were averaged. The loading sequence for all ligaments was an anterior force (67 N), a posterior force (67 N), a varus torque (12 N. m), a valgus torque (12 N. m), an internal rotation torque (6 N. m), and an external rotation torque (6 N. m). The ligaments were moistened with a 0.9% saline spray regularly, and the buckle transducer output was zeroed before each trial Data Analysis The force in the ligament measured by the buckle transducer was plotted against the load cell force, and a linear regression was performed for the aggregate data of the 3 trials. The force through the ligament was then plotted against the applied force or moment, and a linear regression was applied to the combined data of the 3 trials. The slope of the resulting regression line was recorded as the ligament s load response to that loading test at that flexion angle. Paired sample 2-tailed Student t tests were calculated to compare the pretest to the posttest buckle transducer calibration constants at each flexion angle for each ligament tested. One-way analysis of variance (ANOVA) testing with repeated measures was used to determine whether there were significant differences among the load responses for both the varus and internal rotation loading tests. For the external rotation load responses, a 2-way ANOVA with repeated measures of 1 factor (knee flexion angle) was conducted to assess whether there was a significant interaction effect between the ligament and flexion angle variables, that is, whether there was a significant difference among the loading patterns of the 3 ligaments tested across all flexion angles. One-way repeated-measures ANOVAs were used to compare the load responses across the flexion angles for each particular ligament, and 1-way ANOVAs with independent samples were executed to compare the load responses across the ligaments for each particular flexion angle. For a given ANOVA, a significant difference was determined to be present if the P value was less than.05 (P <.05). The Tukey honest significant difference (HSD) test was then used for post hoc comparisons to detect significant differences between particular pairs of the load responses considered in the given ANOVA. Two load responses were significantly different if the difference between them was greater than the HSD at the.05 level of significance, the value of which depended on the mean square and degrees of freedom associated with the random variability calculated in the given ANOVA of ligament load responses after significance was determined on ANOVA testing. RESULTS Loading Tests For all 3 ligaments, no significant differences were found between the pretest and posttest buckle transducer calibration constants at any flexion angle. This comparison indicates that, over the course of testing a given ligament, the mechanical integrity of the tissue did not degrade significantly to compromise the validity of the testing method. The load responses of the ligaments for particular loading tests at a particular flexion angle are reported in Table 1. Only mean load responses (ligament/tendon force, N, per applied moment, J) greater than 1.0 are reported in the following paragraphs because loading responses below

4 1698 LaPrade et al The American Journal of Sports Medicine. load response (N/N m) N m external rotation moment 6 N. m internal rotation moment 12 N. m varus moment load response (N/ N-m) FCL PFL PLT knee flexion angle Figure 2. Forces in the fibular collateral ligament (FCL) during applied loads. 1.0 signify a small load for that ligament for that particular load application and were considered not to be clinically or functionally significant. Force on the Fibular Collateral Ligament knee flexion angle Figure 3. Forces in the fibular collateral ligament (FCL), popliteus tendon (PLT), and popliteofibular ligament (PFL) during application of an external rotation torque of 6 N m. The fibular collateral ligament was observed to be loaded in 3 different loading tests: varus moments and external rotation and internal rotation torques (Table 1). Anterior and posterior forces and valgus moments did not load the fibular collateral ligament at any flexion angle for these applied loads. The varus moment responses for the fibular collateral ligament were fairly constant over the first 3 knee flexion angles, averaging approximately 10 N/J from 0 to 60 before falling to 8.1 N/J at 90 (Figure 2). This mean load response at 90 was significantly less than the mean load response at 30 of knee flexion (P <.01). The mean load response for an external rotation torque on the fibular collateral ligament peaked at 30 of knee flexion and decreased with increasing knee flexion angle (Figure 3). As in varus loading, the mean load response at 90 was significantly less than the mean load response at any other knee flexion angle (P <.01 at 0 ; P <.01 at 30 ; P <.05 at 60 ). The fibular collateral ligament load response at 60 was also significantly less than at 0 (P <.05) and 30 (P <.01) of knee flexion. The load responses to an internal rotation torque on the fibular collateral ligament averaged 5.1 N/J through all 4 flexion angles, with no significant differences detected among them. As a group, the results for the internal rotation torque on the fibular collateral ligament had the highest relative variability of all the loading tests analyzed, in comparing the magnitude of the standard deviation and the magnitude of the mean at all 4 flexion angles (Table 1). Force on the Popliteus Tendon and Popliteofibular Ligament External rotation torque was the only applied load that substantially loaded the popliteus tendon and the popliteofibular ligament. Both of these ligaments had similar loading patterns to an external rotation torque; their mean load responses generally increased with increasing knee flexion angles, peaking at 60 of knee flexion before slightly decreasing at 90 (Figure 3). Tukey HSD tests, executed after 1-way independentsample ANOVAs were conducted to compare the load responses across the ligaments at each flexion angle, did not detect a significant difference between the mean popliteus tendon load response and the mean popliteofibular ligament load response to an external rotation torque at any flexion angle. When comparing the load responses of the respective ligaments across the flexion angles, the 1-way repeatedmeasures ANOVA showed that a significant difference occurred among the external rotation torque responses for the popliteus tendon (P <.01) and the popliteofibular ligament (P <.0001) among the different knee flexion angles tested. The mean popliteus tendon load response to an external rotation torque at 0 was significantly less than the mean load response at 30 (P <.05), 60 (P <.01), and 90 (P <.01) of knee flexion. The mean popliteofibular ligament load response to an external rotation torque at 0 was also

5 Vol. 32, No. 7, 2004 Force Measurements to Applied Loads 1699 significantly less than the mean load response at 30 (P <.05), 60 (P <.01), and 90 (P <.01) of knee flexion. Comparison of Force on the Fibular Collateral Ligament, Popliteus Tendon, and Popliteofibular Ligament in External Rotation The 2-way ANOVA with repeated measures of 1 factor (knee flexion angle) detected a significant interaction (P <.0001) between the ligament and flexion angle variables, indicating that there was a significant difference among the loading patterns of the 3 ligaments tested across all knee flexion angles. One-way independent-sample ANOVAs comparing the load responses across the ligaments at each flexion angle detected significant differences among the external rotation torque responses at 0 (P <.0001), 30 (P <.01), and 90 (P <.01) but not at 60 of knee flexion. The mean load response of the fibular collateral ligament was significantly higher, higher, and lower than those of the popliteus tendon and popliteofibular ligament at 0, 30, and 90 of knee flexion, respectively. As mentioned earlier, there were no significant differences between the mean load responses of the popliteus tendon and popliteofibular ligament at any knee flexion angle. DISCUSSION The posterolateral corner of the knee has many structures that contribute to its static stability. It has been determined in several anatomical and experimental studies that the fibular collateral ligament, popliteus tendon, and popliteofibular ligament are consistently present anatomically and are the most important individual structures to provide primary static stability to the posterolateral knee. 5,6,10,18-20 Although the primary and secondary static importance of these structures has been the focus of several experimental studies, we felt that a major piece of the puzzle in understanding the function of individual posterolateral knee structures was the relative forces seen on the fibular collateral ligament, popliteus tendon, and popliteofibular ligament during functional loading. This information, combined with the information gained from sequential cutting studies, 5,6,9,10,14,20 would help to analyze the total effect and importance of these individual structures on knee stability during functional loads. The direct force measurements that we obtained in this study provide important information that supplements the indirect information from sequential ligament cutting studies about the relative importance these posterolateral knee structures contribute to normal knee function. 5,6,10,14,20 In previous cutting studies, it was determined that the fibular collateral ligament is an important primary restraint to varus instability. 5,6,10 In our study, we also found the fibular collateral ligament to be loaded with application of a varus moment. In addition, we found the fibular collateral ligament to be highly loaded with an external rotation torque in the early ranges of knee motion (0,30 ) and to be significantly more loaded than the popliteus complex. This would imply that the fibular collateral ligament has an important role in preventing external rotation in early knee flexion that has not been emphasized or recognized in previous posterolateral knee cutting studies. 5,6,9,20 We theorize that possibly some of the difficulty in obtaining good outcomes in fibular collateral ligament reconstruction procedures, which commonly have resulted in a high amount of recurrent laxity, 7,8,15 could be a lack of understanding of this role. The high variability found among the fibular collateral ligament load response to internal rotation (Table 1, Figure 2) corroborates the results of previous posterolateral knee biomechanical studies. These studies have noted a large variability between knees in selective-sectioning studies assessing the role of the posterolateral structures in preventing internal rotation. 1,21 The large standard deviation found in our study among the fibular collateral ligament load response to an internal rotation force complements the finding of previous biomechanical cutting studies that there is a large variation between knees of the role of the fibular collateral ligament in preventing internal rotation. 1,21 The clinical implications of this high variability of the fibular collateral ligament in providing stability to internal rotation of the knee are unknown. To our knowledge, there currently does not appear to be a means to clinically assess internal rotation stability, or laxity, of the knee. Although the fibular collateral ligament was highly loaded with an external rotation torque in lesser amounts of knee flexion, the popliteus tendon and popliteofibular ligament became more highly loaded with higher amounts of knee flexion (Figure 3). The role of the popliteus tendon and popliteofibular ligament in preventing increased primary external rotation has been recognized previously through selective cutting studies. 10,20 It would thus appear that the fibular collateral ligament and popliteus complex (popliteus tendon and popliteofibular ligament) have complementary roles as stabilizers to external rotation of the knee, which has not previously been appreciated. The fibular collateral ligament has a more important role in primary restraint to external rotation near extension, whereas the popliteus complex assumes a more important primary role with increasing knee flexion. To compare the load response in this study to the loads potentially seen in vivo, it is necessary to convert these load responses into forces and compare them to the moments seen on the knee during ambulation. Prodromos et al 16 reported that high varus moments in gait start at 4% body weight (BW) height (HT). Assuming a typical body weight of 690 N (155 lb) and height of 1.8 m (5 ft, 10 in), 4% BW HT yields 50 N m. Extrapolation of this varus moment to the force seen on the fibular collateral ligament in extension in this study demonstrates that the varus force on the fibular collateral ligament would theoretically be 471 N with level gait in patients with high varus moments in gait. In this instance, the patients would have a varus thrust gait pattern. 15 To our knowledge, only 1 study on the pullout strengths of posterolateral knee structures and/or grafts has been performed. Maynard et al 14 found that the native fibular collateral ligament fails at an average force to failure of 747 N, and the popliteofibular

6 1700 LaPrade et al The American Journal of Sports Medicine ligament then fails at 425 N when both are pulled concurrently to failure. These results would imply that patients with high varus moments would place a high load on a fibular collateral ligament reconstruction graft after surgery. In any event, it is clear that the initial forces on a fibular collateral ligament repair or reconstruction with ambulation can be quite high for the native repaired or reconstructed structures, which lends credence to the anecdotal recommendations of maintaining patients in a nonweightbearing protocol for 6 to 12 weeks after a repair or reconstruction of the posterolateral knee to prevent stretching out of the injured structures. 3,4,8 In addition, with the pullout strength of a bone patellar tendon bone ACL graft measured to be approximately 416 N 17 and the force measured on the native ACL with a simulated 200 N Lachman test at 20 to be 200 N, 13 the forces seen on these posterolateral knee structures compared to other structures in the knee are potentially quite high and functionally important. In addition, there appears to be a high external rotation moment load on the popliteus tendon/popliteofibular ligament complex and fibular collateral ligament with gait. Andriacchi et al 2 reported that the peak external rotation moment in the knee was approximately 8.5 N m with gait. In extrapolating the peak external rotation moment seen on the popliteus tendon and popliteofibular ligament in this study based on the study by Andriacchi et al, the approximate total force seen on the popliteus tendon and popliteofibular ligament from an external rotation moment was 105 N for each structure, whereas the force seen on the fibular collateral ligament for this external rotation moment was 168 N. One of the limitations of this study is that it is a static in vitro study of ligaments that interact dynamically in vivo. However, direct measurement of force provides new useful information compared to other indirect means of the primary and secondary restraint roles of these structures to specific loads. This study does not replicate the dynamic stability provided by the biceps femoris and popliteus muscle complexes. Because this study did not fully account for the dynamic stability provided by these muscle complexes, it is possible that the magnitude of the in vitro results may be different than in the in vivo condition. Although buckle transducers have been demonstrated to be efficient for the measurement of direct forces, 11 impingement of the buckle transducers can cause fluctuations in the measured force of ligaments. Although we attempted to avoid buckle impingement in this study, the higher standard deviations seen on structures with rotational loading may in fact be due to this unrecognized limitation. In summary, to our knowledge, our study is the first to measure directly the actual force present on posterolateral knee structures with applied loads. It has yielded information about the primary stabilizing function of these structures, which was previously not well recognized. Varus moments caused a high amount of force on the fibular collateral ligament at all tested flexion angles. During external rotation torques, the fibular collateral ligament showed high loads early in flexion, with the popliteus tendon and popliteofibular ligament being loaded more in higher flexion angles. An anterior, posterior, or valgus applied force was not seen to load the fibular collateral ligament, popliteus tendon, or popliteofibular ligament for the loads applied in this study. We believe that these results provide a measure of the potential for failure of these structures during joint loading and serve to both guide graft strength requirements for posterolateral knee reconstructions and develop appropriate postoperative rehabilitation protocols. ACKNOWLEDGMENT This study was supported by the Sports Medicine Research Fund of the Minnesota Medical Foundation and a Department of Orthopaedic Surgery Faculty Development grant. REFERENCES 1. Ahmed AM, Hyder A, Burke DL, et al. In-vitro ligament tension pattern in the flexed knee in pressure loading. 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