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Contributions of the Posterolateral Bundle of the Anterior Cruciate Ligament to Anterior-Posterior Knee Laxity and Ligament Forces
Contributions of the Posterolateral Bundle of the Anterior Cruciate Ligament to Anterior-Posterior Knee Laxity and Ligament Forces Keith L. Markolf, Ph.D., Samuel Park, M.D., Steven R. Jackson, and David R. McAllister, M.D.
Purpose: The purpose of this study was to measure changes in anterior-posterior (AP) laxity and graft forces after cutting the posterolateral (PL) bundle of the anterior cruciate ligament (ACL). Methods: Twelve fresh-frozen cadaveric knees underwent AP laxity testing at ⫾ 100 N of applied tibial force. Resultant forces in the ACL were recorded during passive extension from 120° to 0° with no tibial force, 100 N of anterior tibial force, 100 N of quadriceps force, and 5 Nm of internal tibial torque. The femoral origin of the PL bundle was identified, the ligament fibers were dissected from bone, and tests were repeated. Results: Cutting the PL bundle significantly increased mean laxity by ⫹1.3 mm (at 0°), ⫹1.1 mm (at 10°), and ⫹0.5 mm (at 30°). For the passive knee extension tests, cutting the PL bundle significantly decreased mean ACL force at 0° for all loading modes; the mean decreases were 31 N (with no tibial force), 50 N (with 100 N of anterior force), 33 N (with 100 N of quadriceps force), and 40 N (with 5 Nm of internal torque). Conclusions: The decreases in ACL force at 0° from cutting the PL bundle are consistent with the commonly accepted view that the PL bundle tightens with knee extension. Cutting the taut PL bundle did significantly increase AP laxity between 0° and 30°, but the increases were relatively small. Therefore we conclude that the PL bundle plays a relatively minor role in controlling anterior tibial translation. Clinical Relevance: In view of our findings, the need to reconstruct the PL bundle for better restoration of a normal AP laxity profile is questioned. Key Words: Anterior cruciate ligament—Knee biomechanics—Posterolateral bundle—Double-bundle anterior cruciate ligament.
I
t is generally accepted that there are 2 functioning bundles of the anterior cruciate ligament (ACL). The anteromedial (AM) bundle tightens with knee flexion, and the posterolateral (PL) bundle becomes tight as the knee is extended.1,2 Recently, some sur-
From the Biomechanics Research Section, Department of Orthopaedic Surgery, David Geffen School of Medicine at UCLA, Los Angeles, California, U.S.A. Supported a grant from NFL Charities. Graft tissues used in this study were provided by the Musculoskeletal Transplant Foundation. The authors report no conflict of interest. Received November 29, 2007; accepted February 17, 2008. Address correspondence and reprint requests to Keith L. Markolf, Ph.D., UCLA Department of Orthopaedic Surgery, Biomechanics Research Section, Room 21-67 UCLA Rehabilitation Center, 1000 Veteran Ave, Box 951759, Los Angeles, CA 900951759, U.S.A. E-mail: [email protected] © 2008 by the Arthroscopy Association of North America 0749-8063/08/2407-7600$34.00/0 doi:10.1016/j.arthro.2008.02.012
geons have advocated a double-bundle ACL reconstruction in an attempt to better replicate native ligament anatomy and restore intact knee stability.3-11 However, the contributions of the native PL bundle to AP laxity and to forces developed in the native ACL have received limited study.12,13 Knowledge of forces in the PL bundle of the ACL and its influence on AP laxity is necessary to understand the biomechanical requirements for a PL graft substitute and to provide guidance on possible tensioning protocols for the PL graft of a double-bundle reconstruction. The purpose of this study was to measure changes in anteriorposterior (AP) laxity and graft forces after cutting of the PL bundle of the ACL. METHODS Twelve fresh-frozen cadaveric knee specimens were used. The mean age was 36.6 years (SD, 9.6
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 24, No 7 (July), 2008: pp 805-809
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K. L. MARKOLF ET AL. footprint attachment sites. Graft force measurements during passive knee extension and AP laxity measurements were then repeated. A 1-way repeated-measures analysis of variance was used to determine the statistical significance of mean laxity differences. A 2-way repeated-measures analysis of variance, with pairwise comparisons, was used to compare graft forces before and after cutting of the PL bundle. The level of significance for both analyses was P ⬍ .05. RESULTS
FIGURE 1. AP laxity at ⫾ 100 N of tibial force before and after cutting of PL bundle of ACL. Mean values are shown (with 1-SD error bars). Mean values that are significantly different (sign diff) (P ⬍ .05) from the intact condition are indicated by asterisks.
years; range, 17 to 46 years); all were male. The ACL’s tibial insertion was mechanically isolated by use of a cylindrical coring cutter. The cap of bone containing the entire ligament footprint was incorporated into a cast cylindrical construct of polymethyl methacrylate acrylic, which contained a threaded metal core for attachment to a load cell that recorded resultant force in the ligament. When fixed to the tip of the load cell, the bone cap remained in its precise anatomic position.14 Resultant force in the native ACL was recorded as the knee was passively extended from 120° to 0° flexion with (1) no tibial force, (2) 100 N of anterior tibial force, (3) 100 N of quadriceps force, and (4) 5 Nm of internal tibial torque. Internal-external tibial rotation and varus-valgus rotation were unconstrained during the passive knee extension tests. AP laxity testing at ⫾ 100 N of applied tibial force was performed with the intact ACL at 0°, 10°, 30°, 45°, 70°, and 90° of flexion. At each flexion angle, the tibia was locked at its midrange of internal-external tibial rotation during AP testing. This was done to simulate the straight clinical AP drawer test, where the tibia is firmly grasped to prevent tibial rotation during application of AP force. The specific details of our bone cap isolation technique, test apparatus, and testing protocols have been described in detail in previous publications.14-17 The knee was flexed to 90°, and an anterior force was applied manually to the tibia. Origins of the slackened PL fiber bundle on the lateral wall of the femoral notch were identified and cut at their femoral
Cutting the PL bundle produced small but statistically significant increases in laxity between 0° and 30° of flexion (Fig 1); mean laxity increases were 1.3 mm (0°), 1.1 mm (10°), and 0.5 mm (30°). There were no significant increases in mean laxity at 45°, 70°, and 90° (Fig 1). During passive knee extension with no applied tibial force, cutting the PL bundle significantly decreased mean ACL force at flexion angles of less than 10° (Fig 2); the mean decrease at 0° was 31 N. With 100 N of anterior tibial force, cutting the PL bundle significantly decreased mean ACL force at flexion angles of less than 25°; the mean decrease at 0° was 50 N (Fig 2). Cutting the PL bundle significantly decreased mean ACL force by 33 N at 0° with 100 N of quadriceps force (Fig 3). With 5 Nm of internal tibial torque, cutting the PL bundle significantly decreased mean ACL force by 40
FIGURE 2. Mean curves of ACL force versus knee flexion angle for passive knee flexion with no applied tibial force and 100 N of anterior tibial force. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
AP LAXITY AND GRAFT FORCES IN PL BUNDLE
FIGURE 3. Mean curves of ACL force versus knee flexion angle for passive knee flexion with 100 N of quadriceps tendon force. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
N at 0° (Fig 4) and had no significant effect on internal tibial rotation (Fig 5). DISCUSSION This study was designed to determine the biomechanical function of the ACL’s PL bundle in the intact knee. The modes of loading that we investigated (quadriceps force, anterior tibial force, and internal
FIGURE 4. Mean curves of ACL force versus knee flexion angle for passive knee flexion with 5 Nm of internal tibial torque. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
807
FIGURE 5. Mean curves of tibial rotation versus knee flexion angle for passive knee flexion with 5 Nm of internal tibial torque. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle were not significantly different (ns).
tibial torque) were chosen because the ACL is actively loaded under these conditions.14 Anterior stability and rotatory stability of the knee are particularly important with regard to the Lachman and pivot-shift tests commonly performed during a clinical knee examination. Addition of a PL graft to the traditional AM graft (double-bundle ACL reconstruction) has been suggested to better control anterior and rotatory stability.18,19 However, the biomechanical function of the PL bundle of the native ACL is poorly understood. Our method for identifying and cutting the PL bundle deserves special mention. Ideally, it would be desirable to cut the PL bundle at full extension, where it normally becomes taut. Unfortunately, the AM bundle obscures the PL’s femoral footprint near 0°. A much better view of the footprint is possible at 90°, where the PL bundle remains slackened in response to an applied anterior tibial force. At 90°, these detensioned fibers could be clearly identified and the slackened tissue was carefully cut from the bone surface. This allowed better visualization of the remaining AM bundle fibers, which were then tested with a probe to verify that they were tensioned from the applied anterior tibial force. We tried not to cut any AM bundle fibers during this process. In instances where cutting the PL bundle produced a significant change in resultant ACL force, the force always decreased. It would be reasonable to assume that the original force carried by the PL bundle was roughly equal to the magnitude of force decrease from cutting that bundle. For the internal torque test, cutting
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the PL bundle significantly decreased ACL force at 0° but did not significantly increase internal tibial rotation at 0°. Because the PL bundle is relatively small, the remaining AM bundle was sufficient to limit internal rotation even though the tensioned PL bundle fibers were removed. Our sectioning study was prompted by the current interest in double-bundle ACL reconstruction, where addition of a PL bundle can be viewed as an adjunct to an AM graft. We believe the contribution of the PL bundle to AP stability in the intact knee has direct relevance to the contribution of a PL graft bundle in the ACL-reconstructed knee. However, it is possible that the PL graft of a double-bundle ACL reconstruction could play a more important role in anterior knee stability because graft placements and tensioning protocols could also have significant effects on graft forces and knee kinematics. The data from this study provide a baseline reference for future studies with double-bundle ACL reconstructions. There are 2 prior studies in the literature related to PL bundle function that allow comparisons to our results. Zantop et al.12 tested 5 specimens and found that PL bundle sectioning significantly increased AP laxity only at 30° of flexion, by a mean of 6.7 mm; they found no significant increases at 0°, 60°, and 90°. Our mean measured laxity increase of 0.5 mm at 30° (for 12 specimens) was substantially less than theirs, and we found small but significant laxity increases at 0° and 10° as well. Zantop et al. did not describe how the PL bundle was identified and cut. It is possible that they also sectioned part of the AM bundle, which could explain the relatively large 6.7-mm laxity increase reported. We have no other explanation for the differences between these studies. Our results show that the PL bundle carries force near full extension and are in agreement with findings of Gabriel et al.,13 who showed that forces in the PL bundle from a 134-N anterior tibial load were highest in extension and decreased with increasing flexion. Our results are also in agreement with the commonly held view that the PL bundle becomes taut in extension. Applied internal torque is a mode of loading that generates high ACL forces as the cruciates wind about one another. At full extension, loss of the highly tensioned PL bundle after sectioning would be expected to reduce the resistance to applied internal torque with a corresponding reduction in resultant ACL force at 0°, as we observed. Our internal torque test measured pure torsional stability only. It did not reproduce the pivot-shift instability caused by simultaneous application of a valgus moment and internal
tibial torque, which produces a coupled internal rotation of the tibia with posterior subluxation of the lateral tibial plateau. Because we have shown that the PL bundle is loaded at 0° and plays a relatively small but significant role in providing knee stability at full extension, it would seem reasonable that the PL graft of a double-bundle ACL reconstruction would also be expected to provide stability at 0°. This would suggest that any tensioning protocol for a double-bundle reconstruction should attempt to ensure that the PL bundle remains taut at 0°. A double-bundle ACL graft reconstruction is a more complex technical procedure, involves increased operative time, requires 4 tunnels with more fixation hardware, and necessitates the use of additional graft tissue. In light of our findings, it might be reasonable to question the rationale for reconstructing the PL bundle in an ACL-deficient knee. This was an in vitro study and is subject to all of the experimental limitations of using cadaveric tissues. We made every attempt to identify and section only the PL bundle fibers and are confident that they all were removed. However, it is possible that some AM bundle fibers could have been cut occasionally as well, because direct visibility was sometimes limited in the posterior portion of the notch. If this were the case, our measured changes in laxity and ligament force would be greater than those resulting from PL sectioning alone and would not change our conclusion related to the relatively minor biomechanical function of this ligament bundle. CONCLUSIONS Our measured increases in AP laxity after cutting of the PL bundle at 0° and 10° are consistent with the measured decreases in resultant force of the ACL from anterior tibial loading after the PL bundle has been cut. However, the magnitude of difference in mean laxity after PL bundle sectioning, which is on the order of 1 mm, would suggest that the PL bundle plays a somewhat minor role in providing AP stability to the knee. The relatively minor changes in ligament force profiles after cutting of the PL bundle indicate that the remaining AM bundle fibers continued to be loaded in a near-normal fashion. REFERENCES 1. Girgis FG, Marshall JL, Monajem A. The cruciate ligaments of the knee joint. Anatomical, functional and experimental analysis. Clin Orthop Relat Res 1975:216-231.
AP LAXITY AND GRAFT FORCES IN PL BUNDLE 2. Amis AA, Dawkins GP. Functional anatomy of the anterior cruciate ligament. Fibre bundle actions related to ligament replacements and injuries. J Bone Joint Surg Br 1991;73:260267. 3. Muneta T, Sekiya I, Yagishita K, Ogiuchi T, Yamamoto H, Shinomiya K. Two-bundle reconstruction of the anterior cruciate ligament using semitendinosus tendon with endobuttons: Operative technique and preliminary results. Arthroscopy 1999;15:618-624. 4. Pederzini L, Adriani E, Botticella C, Tosi M. Technical note: Double tibial tunnel using quadriceps tendon in anterior cruciate ligament reconstruction. Arthroscopy 2000;16:E9. 5. Hamada M, Shino K, Horibe S, et al. Single- versus bi-socket anterior cruciate ligament reconstruction using autogenous multiple-stranded hamstring tendons with EndoButton femoral fixation: A prospective study. Arthroscopy 2001;17:801-807. 6. Yasuda K, Kondo E, Ichiyama H, et al. Anatomic reconstruction of the anteromedial and posterolateral bundles of the anterior cruciate ligament using hamstring tendon grafts. Arthroscopy 2004;20:1015-1025. 7. Yasuda K, Kondo E, Ichiyama H, Tanabe Y, Tohyama H. Clinical evaluation of anatomic double-bundle anterior cruciate ligament reconstruction procedure using hamstring tendon grafts: Comparisons among 3 different procedures. Arthroscopy 2006;22:240-251. 8. Aglietti P, Giron F, Cuomo P, Losco M, Mondanelli N. Singleand double-incision double-bundle ACL reconstruction. Clin Orthop Relat Res 2007;454:108-113. 9. Yagi M, Kuroda R, Nagamune K, Yoshiya S, Kurosaka M. Double-bundle ACL reconstruction can improve rotational stability. Clin Orthop Relat Res 2007;454:100-107. 10. Jarvela T. Double-bundle versus single-bundle anterior cruciate ligament reconstruction: A prospective, randomized clinical study. Knee Surg Sports Traumatol Arthrosc 2007;15:500-507.
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11. Muneta T, Koga H, Mochizuki T, et al. A prospective randomized study of 4-strand semitendinosus tendon anterior cruciate ligament reconstruction comparing single-bundle and double-bundle techniques. Arthroscopy 2007;23:618-628. 12. Zantop T, Herbort M, Raschke MJ, Fu FH, Petersen W. The role of the anteromedial and posterolateral bundles of the anterior cruciate ligament in anterior tibial translation and internal rotation. Am J Sports Med 2007;35:223-227. 13. Gabriel MT, Wong EK, Woo SL, Yagi M, Debski RE. Distribution of in situ forces in the anterior cruciate ligament in response to rotatory loads. J Orthop Res 2004;22:85-89. 14. Markolf KL, Gorek JF, Kabo JM, Shapiro MS. Direct measurement of resultant forces in the ACL. An in vitro study performed with a new experimental technique. J Bone Joint Surg Am 1990;72:557-567. 15. Markolf KL, Burchfield DM, Shapiro MM, Shepard MF, Finerman GA, Slauterbeck JL. Combined knee loading states that generate high ACL forces. J Orthop Res 1995;13:930-935. 16. Markolf KL, Burchfield DM, Shapiro MM, Cha CW, Finerman GA, Slauterbeck JL. 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 Joint Surg Am 1996;78: 1728-1734. 17. Markolf KL, Hame S, Hunter DM, et al. Effects of femoral tunnel placement on knee laxity and forces in an anterior cruciate ligament graft. J Orthop Res 2002;20:1016-1024. 18. Mae T, Shino K, Miyama T, et al. Single- versus two-femoral socket anterior cruciate ligament reconstruction technique: Biomechanical analysis using a robotic simulator. Arthroscopy 2001;17:708-716. 19. Yagi M, Wong EK, Kanamori A, Debski RE, Fu FH, Woo SL. Biomechanical analysis of an anatomic anterior cruciate ligament reconstruction. Am J Sports Med 2002;30:660-666.
Purpose: The purpose of this study was to measure changes in anterior-posterior (AP) laxity and graft forces after cutting the posterolateral (PL) bundle of the anterior cruciate ligament (ACL). Methods: Twelve fresh-frozen cadaveric knees underwent AP laxity testing at ⫾ 100 N of applied tibial force. Resultant forces in the ACL were recorded during passive extension from 120° to 0° with no tibial force, 100 N of anterior tibial force, 100 N of quadriceps force, and 5 Nm of internal tibial torque. The femoral origin of the PL bundle was identified, the ligament fibers were dissected from bone, and tests were repeated. Results: Cutting the PL bundle significantly increased mean laxity by ⫹1.3 mm (at 0°), ⫹1.1 mm (at 10°), and ⫹0.5 mm (at 30°). For the passive knee extension tests, cutting the PL bundle significantly decreased mean ACL force at 0° for all loading modes; the mean decreases were 31 N (with no tibial force), 50 N (with 100 N of anterior force), 33 N (with 100 N of quadriceps force), and 40 N (with 5 Nm of internal torque). Conclusions: The decreases in ACL force at 0° from cutting the PL bundle are consistent with the commonly accepted view that the PL bundle tightens with knee extension. Cutting the taut PL bundle did significantly increase AP laxity between 0° and 30°, but the increases were relatively small. Therefore we conclude that the PL bundle plays a relatively minor role in controlling anterior tibial translation. Clinical Relevance: In view of our findings, the need to reconstruct the PL bundle for better restoration of a normal AP laxity profile is questioned. Key Words: Anterior cruciate ligament—Knee biomechanics—Posterolateral bundle—Double-bundle anterior cruciate ligament.
I
t is generally accepted that there are 2 functioning bundles of the anterior cruciate ligament (ACL). The anteromedial (AM) bundle tightens with knee flexion, and the posterolateral (PL) bundle becomes tight as the knee is extended.1,2 Recently, some sur-
From the Biomechanics Research Section, Department of Orthopaedic Surgery, David Geffen School of Medicine at UCLA, Los Angeles, California, U.S.A. Supported a grant from NFL Charities. Graft tissues used in this study were provided by the Musculoskeletal Transplant Foundation. The authors report no conflict of interest. Received November 29, 2007; accepted February 17, 2008. Address correspondence and reprint requests to Keith L. Markolf, Ph.D., UCLA Department of Orthopaedic Surgery, Biomechanics Research Section, Room 21-67 UCLA Rehabilitation Center, 1000 Veteran Ave, Box 951759, Los Angeles, CA 900951759, U.S.A. E-mail: [email protected] © 2008 by the Arthroscopy Association of North America 0749-8063/08/2407-7600$34.00/0 doi:10.1016/j.arthro.2008.02.012
geons have advocated a double-bundle ACL reconstruction in an attempt to better replicate native ligament anatomy and restore intact knee stability.3-11 However, the contributions of the native PL bundle to AP laxity and to forces developed in the native ACL have received limited study.12,13 Knowledge of forces in the PL bundle of the ACL and its influence on AP laxity is necessary to understand the biomechanical requirements for a PL graft substitute and to provide guidance on possible tensioning protocols for the PL graft of a double-bundle reconstruction. The purpose of this study was to measure changes in anteriorposterior (AP) laxity and graft forces after cutting of the PL bundle of the ACL. METHODS Twelve fresh-frozen cadaveric knee specimens were used. The mean age was 36.6 years (SD, 9.6
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 24, No 7 (July), 2008: pp 805-809
805
806
K. L. MARKOLF ET AL. footprint attachment sites. Graft force measurements during passive knee extension and AP laxity measurements were then repeated. A 1-way repeated-measures analysis of variance was used to determine the statistical significance of mean laxity differences. A 2-way repeated-measures analysis of variance, with pairwise comparisons, was used to compare graft forces before and after cutting of the PL bundle. The level of significance for both analyses was P ⬍ .05. RESULTS
FIGURE 1. AP laxity at ⫾ 100 N of tibial force before and after cutting of PL bundle of ACL. Mean values are shown (with 1-SD error bars). Mean values that are significantly different (sign diff) (P ⬍ .05) from the intact condition are indicated by asterisks.
years; range, 17 to 46 years); all were male. The ACL’s tibial insertion was mechanically isolated by use of a cylindrical coring cutter. The cap of bone containing the entire ligament footprint was incorporated into a cast cylindrical construct of polymethyl methacrylate acrylic, which contained a threaded metal core for attachment to a load cell that recorded resultant force in the ligament. When fixed to the tip of the load cell, the bone cap remained in its precise anatomic position.14 Resultant force in the native ACL was recorded as the knee was passively extended from 120° to 0° flexion with (1) no tibial force, (2) 100 N of anterior tibial force, (3) 100 N of quadriceps force, and (4) 5 Nm of internal tibial torque. Internal-external tibial rotation and varus-valgus rotation were unconstrained during the passive knee extension tests. AP laxity testing at ⫾ 100 N of applied tibial force was performed with the intact ACL at 0°, 10°, 30°, 45°, 70°, and 90° of flexion. At each flexion angle, the tibia was locked at its midrange of internal-external tibial rotation during AP testing. This was done to simulate the straight clinical AP drawer test, where the tibia is firmly grasped to prevent tibial rotation during application of AP force. The specific details of our bone cap isolation technique, test apparatus, and testing protocols have been described in detail in previous publications.14-17 The knee was flexed to 90°, and an anterior force was applied manually to the tibia. Origins of the slackened PL fiber bundle on the lateral wall of the femoral notch were identified and cut at their femoral
Cutting the PL bundle produced small but statistically significant increases in laxity between 0° and 30° of flexion (Fig 1); mean laxity increases were 1.3 mm (0°), 1.1 mm (10°), and 0.5 mm (30°). There were no significant increases in mean laxity at 45°, 70°, and 90° (Fig 1). During passive knee extension with no applied tibial force, cutting the PL bundle significantly decreased mean ACL force at flexion angles of less than 10° (Fig 2); the mean decrease at 0° was 31 N. With 100 N of anterior tibial force, cutting the PL bundle significantly decreased mean ACL force at flexion angles of less than 25°; the mean decrease at 0° was 50 N (Fig 2). Cutting the PL bundle significantly decreased mean ACL force by 33 N at 0° with 100 N of quadriceps force (Fig 3). With 5 Nm of internal tibial torque, cutting the PL bundle significantly decreased mean ACL force by 40
FIGURE 2. Mean curves of ACL force versus knee flexion angle for passive knee flexion with no applied tibial force and 100 N of anterior tibial force. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
AP LAXITY AND GRAFT FORCES IN PL BUNDLE
FIGURE 3. Mean curves of ACL force versus knee flexion angle for passive knee flexion with 100 N of quadriceps tendon force. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
N at 0° (Fig 4) and had no significant effect on internal tibial rotation (Fig 5). DISCUSSION This study was designed to determine the biomechanical function of the ACL’s PL bundle in the intact knee. The modes of loading that we investigated (quadriceps force, anterior tibial force, and internal
FIGURE 4. Mean curves of ACL force versus knee flexion angle for passive knee flexion with 5 Nm of internal tibial torque. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle are significantly different (P ⬍ .05) unless otherwise indicated (not significant [ns]).
807
FIGURE 5. Mean curves of tibial rotation versus knee flexion angle for passive knee flexion with 5 Nm of internal tibial torque. Mean curves are shown for the intact knee and after cutting of the PL bundle of the ACL. All comparisons between graph symbols at a given flexion angle were not significantly different (ns).
tibial torque) were chosen because the ACL is actively loaded under these conditions.14 Anterior stability and rotatory stability of the knee are particularly important with regard to the Lachman and pivot-shift tests commonly performed during a clinical knee examination. Addition of a PL graft to the traditional AM graft (double-bundle ACL reconstruction) has been suggested to better control anterior and rotatory stability.18,19 However, the biomechanical function of the PL bundle of the native ACL is poorly understood. Our method for identifying and cutting the PL bundle deserves special mention. Ideally, it would be desirable to cut the PL bundle at full extension, where it normally becomes taut. Unfortunately, the AM bundle obscures the PL’s femoral footprint near 0°. A much better view of the footprint is possible at 90°, where the PL bundle remains slackened in response to an applied anterior tibial force. At 90°, these detensioned fibers could be clearly identified and the slackened tissue was carefully cut from the bone surface. This allowed better visualization of the remaining AM bundle fibers, which were then tested with a probe to verify that they were tensioned from the applied anterior tibial force. We tried not to cut any AM bundle fibers during this process. In instances where cutting the PL bundle produced a significant change in resultant ACL force, the force always decreased. It would be reasonable to assume that the original force carried by the PL bundle was roughly equal to the magnitude of force decrease from cutting that bundle. For the internal torque test, cutting
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K. L. MARKOLF ET AL.
the PL bundle significantly decreased ACL force at 0° but did not significantly increase internal tibial rotation at 0°. Because the PL bundle is relatively small, the remaining AM bundle was sufficient to limit internal rotation even though the tensioned PL bundle fibers were removed. Our sectioning study was prompted by the current interest in double-bundle ACL reconstruction, where addition of a PL bundle can be viewed as an adjunct to an AM graft. We believe the contribution of the PL bundle to AP stability in the intact knee has direct relevance to the contribution of a PL graft bundle in the ACL-reconstructed knee. However, it is possible that the PL graft of a double-bundle ACL reconstruction could play a more important role in anterior knee stability because graft placements and tensioning protocols could also have significant effects on graft forces and knee kinematics. The data from this study provide a baseline reference for future studies with double-bundle ACL reconstructions. There are 2 prior studies in the literature related to PL bundle function that allow comparisons to our results. Zantop et al.12 tested 5 specimens and found that PL bundle sectioning significantly increased AP laxity only at 30° of flexion, by a mean of 6.7 mm; they found no significant increases at 0°, 60°, and 90°. Our mean measured laxity increase of 0.5 mm at 30° (for 12 specimens) was substantially less than theirs, and we found small but significant laxity increases at 0° and 10° as well. Zantop et al. did not describe how the PL bundle was identified and cut. It is possible that they also sectioned part of the AM bundle, which could explain the relatively large 6.7-mm laxity increase reported. We have no other explanation for the differences between these studies. Our results show that the PL bundle carries force near full extension and are in agreement with findings of Gabriel et al.,13 who showed that forces in the PL bundle from a 134-N anterior tibial load were highest in extension and decreased with increasing flexion. Our results are also in agreement with the commonly held view that the PL bundle becomes taut in extension. Applied internal torque is a mode of loading that generates high ACL forces as the cruciates wind about one another. At full extension, loss of the highly tensioned PL bundle after sectioning would be expected to reduce the resistance to applied internal torque with a corresponding reduction in resultant ACL force at 0°, as we observed. Our internal torque test measured pure torsional stability only. It did not reproduce the pivot-shift instability caused by simultaneous application of a valgus moment and internal
tibial torque, which produces a coupled internal rotation of the tibia with posterior subluxation of the lateral tibial plateau. Because we have shown that the PL bundle is loaded at 0° and plays a relatively small but significant role in providing knee stability at full extension, it would seem reasonable that the PL graft of a double-bundle ACL reconstruction would also be expected to provide stability at 0°. This would suggest that any tensioning protocol for a double-bundle reconstruction should attempt to ensure that the PL bundle remains taut at 0°. A double-bundle ACL graft reconstruction is a more complex technical procedure, involves increased operative time, requires 4 tunnels with more fixation hardware, and necessitates the use of additional graft tissue. In light of our findings, it might be reasonable to question the rationale for reconstructing the PL bundle in an ACL-deficient knee. This was an in vitro study and is subject to all of the experimental limitations of using cadaveric tissues. We made every attempt to identify and section only the PL bundle fibers and are confident that they all were removed. However, it is possible that some AM bundle fibers could have been cut occasionally as well, because direct visibility was sometimes limited in the posterior portion of the notch. If this were the case, our measured changes in laxity and ligament force would be greater than those resulting from PL sectioning alone and would not change our conclusion related to the relatively minor biomechanical function of this ligament bundle. CONCLUSIONS Our measured increases in AP laxity after cutting of the PL bundle at 0° and 10° are consistent with the measured decreases in resultant force of the ACL from anterior tibial loading after the PL bundle has been cut. However, the magnitude of difference in mean laxity after PL bundle sectioning, which is on the order of 1 mm, would suggest that the PL bundle plays a somewhat minor role in providing AP stability to the knee. The relatively minor changes in ligament force profiles after cutting of the PL bundle indicate that the remaining AM bundle fibers continued to be loaded in a near-normal fashion. REFERENCES 1. Girgis FG, Marshall JL, Monajem A. The cruciate ligaments of the knee joint. Anatomical, functional and experimental analysis. Clin Orthop Relat Res 1975:216-231.
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