Rotational Knee Instability in ACL-Deficient Knees: Role of the Anterolateral Ligament and Iliotibial Band as Defined by Tibiofemoral Compartment Translations and Rotations

Noyes, Frank R. MD; Huser, Lauren E. MEng; Levy, Martin S. PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.16.00199
Scientific Articles
Disclosures
Abstract

Background: The anterolateral ligament (ALL) has been proposed as a primary restraint for knee rotational stability. However, the data remain inconclusive. The purpose of this study was to determine the effect of the ALL and the iliotibial band (ITB) on knee rotational stability.

Methods: A 6-degrees-of-freedom robotic simulator was used to test 14 fresh-frozen cadaveric knee specimens. There were 4 testing conditions: intact, anterior cruciate ligament (ACL)-sectioned, ACL and ALL or ITB-sectioned (determined at random), and ACL and both ALL and ITB-sectioned. Lateral, central, and medial tibiofemoral compartment translations and internal tibial rotations were measured under 100-N anterior drawer (Lachman), 5-Nm internal rotation torque, and 2 pivot-shift simulations (Pivot Shift 1 was 5 Nm of internal rotation torque, and Pivot Shift 2 was 1 Nm of internal rotation torque). Statistical equivalence within 2 mm and 2° was defined as p < 0.05.

Results: Sectioning the ACL alone produced increased pivot shift and Lachman compartment translations (p > 0.05). Further sectioning of either the ALL or the ITB separately produced minor added increases in pivot-shift compartment translations and tibial internal rotations (<2 mm or <3°) in the ACL-deficient knee. Sectioning both the ALL and ITB produced increases not equivalent to the ACL-deficient knee in pivot-shift lateral compartment translations (4.4 mm; 95% confidence interval [CI], 2.7 to 6.1 mm [p = 0.99] for Pivot Shift 1 and 4.3 mm; 95% CI, 2.6 to 6.0 mm [p = 0.99] for Pivot Shift 2), with 10 of 14 knees being converted to a corresponding Grade-3 pivot-shift (>20 mm of lateral translation). Increases in internal rotation after ALL and ITB sectioning occurred at 25°, 60°, and 90° (p = 0.99 for all) and ranged from 1° to 12°, with 21% of the knees having 8° to 12° increases.

Conclusions: With ACL sectioning, a positive pivot-shift anterior subluxation occurred even with intact ALL and ITB structures, which indicates that the latter are not primary restraints but function together as anterolateral secondary restraints. With ACL deficiency, concurrent loss of the ALL and ITB resulted in conversion in a majority of knees (71%) to a Grade-3 pivot-shift subluxation, along with major increases of internal rotation in select knees.

Clinical Relevance: With ACL rupture, major increases in rotational instability are not adequately resisted by native ALL or ITB structures. Therefore, anatomic ALL or ITB surgical reconstruction would not block a positive pivot shift. The potential protective effects of ACL graft-unloading from these structures require further study.

Author Information

1Cincinnati Sports Medicine Research and Education Foundation, Cincinnati, Ohio

2The Noyes Knee Institute, Cincinnati, Ohio

3Department of Operations and Business Analytics, College of Business, University of Cincinnati, Cincinnati, Ohio

E-mail address for F.R. Noyes: cfleckenstein@csmref.org

E-mail address for L.E. Huser: l.huser15@gmail.com

E-mail address for M.S. Levy: martin.levy@uc.edu

Article Outline

The anterolateral ligament (ALL) has been a subject of many recent studies, with descriptions of its anatomy1-5, biomechanical properties4,6-13, and length-change patterns during knee flexion and rotation3,14-18. There is disagreement with regard to the results of kinematic and robotic studies, with some authors suggesting a primary rotational stabilizing role requiring surgical reconstruction6,7,9,19,20. Other authors have reported only minimal increases in internal rotation with ALL disruption, disputing its importance21,22.

The pivot-shift rotational subluxation event is known to be associated with giving-way symptoms and is frequently simulated during in vitro studies on ligament function. Nearly all pivot-shift studies use an internal rotation and valgus loading profile, but fail to include the coupled anterior tibial loading6,9,12,21-24, which has been shown to be necessary to produce maximum tibiofemoral compartment subluxations25,26. In fact, simulated pivot-shift testing with just internal rotation and valgus loading actually reduces the amount of anterior subluxation of the medial and central compartments25. In the present study, the robotic pivot-shift loading profile involved anterior tibial loading, internal tibial rotation, and valgus loading during knee flexion-extension25,26. To our knowledge, this study is the first to investigate the function of the ALL and the iliotibial band (ITB) under conditions of pivot-shift simulation that would tend to produce maximum tibiofemoral subluxation.

The term rotational instability refers to abnormal knee displacements or increased mobility (physical sign), rather than to patients’ subjective symptoms or giving-way episodes. Rotational instability is characterized by the subluxations (translations) of the lateral and medial compartments that define their final position. Changes in the degrees of tibial rotation refer to a motion limit, and although used in many publications to characterize rotational stability, the term does not define the final position of the tibiofemoral compartments, since the latter also requires measurement of the center of rotation relative to the joint.

The purpose of this study was to determine the rotational stabilizing effect of the ALL and ITB under multiple loading conditions tending to induce tibiofemoral compartment subluxations. We hypothesized that the ALL and ITB were secondary restraints for rotational stability, against a background premise that there is considerable variability in this function from one knee to another. This physiologic variability in ligament function assumes clinical importance in demonstrating the different rotational instability patterns between knees with the same anatomic injury.

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Materials and Methods

Fourteen fresh-frozen cadaveric knee specimens were used. The mean age (and standard deviation) of the donors at the time of death was 44 ± 8 years (range, 35 to 55 years), all were male, and 4 specimens were unpaired. Specimens were prepared and potted into custom grips, with the femoral transepicondylar axis positioned to be colinear with the robot’s flexion axis, using a custom-designed alignment stand25,27,28.

Prior to testing, the location of the anatomic center of the tibia relative to the tibial rotation axis of the robot was measured with a 3-dimensional coordinate digitizer (MicroScribe G2; Revware). Digitization established medial and lateral compartment locations and corrected for misalignments between the robot and the specimen due to potting. Medial and lateral tibiofemoral compartment translations were measured at points 25% and 75% of the tibial width26.

A custom 6-degrees-of-freedom robot was used to apply joint motions and loads during testing29 (Fig. 1). Flexion was position-controlled, while medial-lateral translation was passively maintained at zero force by a low-friction linear guide. The remaining motions were operated in either position or load-control depending on the test, with precisions and resolutions previously described25,27,28. The zero-load reference path and intact motion limits were determined also as previously described25,27,28.

Specimens were first tested with all ligaments intact. The anterior cruciate ligament (ACL) was then cut and the testing sequence was repeated, followed by cutting either the ALL (7 specimens) or the ITB (7 specimens), determined at random. Finally, the remaining ligament was sectioned. The ALL was sectioned through a limited anterolateral incision, including capsular tissues anterior to the popliteal tendon. The ITB (deep capsulo-osseous fibers) was sectioned by transecting all ITB fibers that inserted onto the Gerdy tubercle30,31. The specimens were tested in each state under the following loading conditions: 100-N anterior load (at 25°, 60°, and 90° of flexion), 5 Nm of internal rotation torque (at 25°, 60°, and 90° of flexion), and 2 pivot-shift variants that differed only by the internal rotation torque applied (5 Nm in Pivot Shift 1 and 1 Nm in Pivot Shift 2). Both pivot-shift variants incorporated a 100-N anterior load and a 7-Nm valgus torque, in addition to the respective internal rotation torques25,26. Pivot-shift translations and rotations were measured at 25° of flexion25.

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Statistical Methods

Testing for statistical equivalence was conducted in 2 stages. In stage 1, 2 overarching linear mixed models were fit to subsets of the complete data. The models involved the following factors: specimen with 14 levels (random effects); cut with 2 levels (Model 1 was ITB-sectioned compared with ACL-sectioned and ALL-sectioned compared with ACL-sectioned, and Model 2 was ACL-sectioned compared with intact and ACL, ALL, and ITB-sectioned compared with ACL-sectioned; fixed effects); location with 4 levels (lateral, center, medial, and internal rotation; fixed effects); and load condition with 8 levels (fixed effects). Estimates of the specimen means of differences were best summarized by the least squares means (LSMEANS), which are model-based estimates for the corresponding population means. The standard errors of the means were summarized by the corresponding standard errors of the LSMEANS. Stage 1 obtained the best estimates of these means and standard errors, taking into account the interplay of all factors and levels in the experiment. Thus, judging significance of an individual estimate borrows strength from all other model effects, which provides improved power over a methodology limited to consideration of only observations arising from a particular set of levels.

In stage 2, equivalence testing was done using the TOST method (two 1-sided t tests), based on the LSMEANS and standard errors of the LSMEANS32,33. An equivalence limit of 2 mm for difference of translation and 2° for difference of internal rotation was selected by the principal clinician. Rangger et al. reported that 99% of chronic ACL injuries and 95% of acute ACL injuries had a difference of ≥3 mm from the intact state34, so the 2-mm value was chosen to be below that threshold. Additionally, in previous studies25,27,28, we reported a mean increase in internal rotation, with the knee at 25° of flexion with the ACL sectioned, of 1.5°, 1.5°, and 1.6°. The 2° threshold was chosen on the basis of these prior publications. A p value of <0.05 indicates statistical equivalence to the intact or ACL-sectioned state, within the equivalence limit selected. Note that a p value of >0.05 does not demonstrate a statistical difference, but rather indicates uncertainty or an inability to prove equivalence. A post hoc power analysis calculated a power of 0.99 for those conditions when equivalence was determined. All values are reported as mean differences with 95% confidence intervals (CIs) or standard deviations, where appropriate.

As detailed in the study by Harris et al., equivalence testing is the recommended method in situations such as this, rather than reliance on traditional hypothesis testing in which the null hypothesis is that there is no difference between the intact and cut condition on average32. If one failed to reject such a null hypothesis, there is no error protection or confidence associated with the conclusion that the null hypothesis is true. The TOST equivalence testing methodology was developed to provide such confidence.

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Results

Results of ACL Sectioning

The ACL sectioning caused increases in compartment translations during all 100-N anterior tests and pivot-shift tests that were beyond the limits of statistical equivalence to the intact state (p > 0.98 for all; Table I). Internal tibial rotation was also beyond the limits of equivalence to the intact state (i.e., beyond 2°) in both of the pivot-shift test variants and in the anterior translation tests (p > 0.94). All compartment translations and internal rotations for the internal rotation tests (5 Nm) were within the 2 mm, 2° limits of equivalence to the intact state (p < 0.05 for all).

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Results After ALL and/or ITB Sectioning in ACL-Deficient Knees
Anterior Tibial Loading (Lachman 100-N Test)

With the ACL sectioned, cutting the ALL alone produced increases in lateral and medial compartment translations. However, those increases were small enough that specimen behavior failed to reach nonequivalence to the ACL-sectioned state (p < 0.001 for all; Table II). Sectioning the ITB alone also produced increases in lateral compartment translation, for which specimen behavior nonequivalent to the ACL-sectioned state was reached at 25° and 60° of flexion. The largest such increase occurred at 60° of flexion and measured 2.0 mm (95% CI, 0.5 to 3.5 mm; p = 0.48).

With both the ALL and ITB cut, increases in lateral, central, and medial compartment translations caused nonequivalence to the ACL-sectioned state at all flexion angles (Table II). Compartment translations for the simulated Lachman test (25° of flexion) are shown in Figure 2. The largest increase was again at 60° of flexion, with a lateral compartment increase of 5.4 mm (95% CI, 3.3 to 7.5 mm; p = 0.99) and a medial compartment increase of 2.0 mm (95% CI, 0.6 to 3.4 mm; p = 0.48).

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Internal Rotation Test (5 Nm)

With the ACL sectioned, cutting the ALL alone produced increases in internal rotation at all flexion angles, but the changes were small enough that specimen behavior did not reach nonequivalence with the ACL-sectioned state (p < 0.001 for all; Table III). After cutting the ITB alone, the mean increases in internal rotation were 1.4° (95% CI, 0.1° to 2.7°; p = 0.06) at 25° of flexion, 2.5° (95% CI, 0.9° to 4.1°; p = 0.93) at 60° of flexion, and 2.0° (95% CI, 0.5° to 3.5°; p = 0.50) at 90° of flexion, and constituted specimen behavior nonequivalent to the ACL-sectioned state.

After both the ALL and ITB were cut, internal rotation at all degrees of flexion increased further (Fig. 3), with the greatest difference from the ACL-sectioned state being 5.6° (95% CI, 3.6° to 7.6°; p = 0.99), occurring at 60°. However, there was a large variability in increases in internal rotation after cutting both the ALL and ITB. Three of 14 specimens showed increases that were >1 standard deviation above the mean (Fig. 4), and an equal number had internal rotation increases that were >1 standard deviation below the mean, ranging from approximately 1° to 12°.

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Pivot Shift 1 (100-N Anterior Force, 5-Nm Internal Rotation Torque, and 7-Nm Valgus)

Increases in compartment translations with the ALL cut were minimal and constituted specimen behavior statistically equivalent to the ACL-sectioned state (p < 0.001 for all; Table IV). Increases in internal rotation with the ALL (but not the ITB) cut were small enough that specimen behavior remained statistically equivalent to the ACL-sectioned state (p < 0.001). Cutting the ITB (but not the ALL) produced increases in both lateral compartment translation and internal rotation of 1.8 mm (95% CI, 0.6 to 3.0 mm; p = 0.31) and 2.0° (95% CI, 0.4° to 3.8°; p = 0.56), respectively, which sufficed to reach nonequivalence with the ACL-sectioned state.

With both the ALL and ITB cut, the mean increase in lateral compartment translation was 4.4 mm (95% CI, 2.7 to 6.1 mm; p = 0.99), and the mean increase in internal rotation was 5.2° (95% CI, 2.8° to 7.6°; p = 0.99). Compartment translations for a representative specimen are shown in Figure 5. Again, there was a large variability of increases in lateral compartment translation after cutting both the ALL and ITB, ranging from 1 to 9 mm. Three of 14 specimens showed increases that were >1 standard deviation above the mean, and an equal number had increases that were >1 standard deviation below the mean (Fig. 6). The final absolute magnitude of the lateral compartment translations in the simulated pivot-shift test is shown in Figure 7 for each specimen and for each ligament sectioning, to allow comparisons among knee specimens.

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Pivot Shift 2 (100-N Anterior Force, 1-Nm Internal Rotation Torque, and 7-Nm Valgus)

Increases in lateral compartment translation with the ALL cut were minimal, constituting behavior statistically equivalent to the ACL-sectioned state (p < 0.05 for all; Table IV). Increases in internal rotation were small enough that specimen behavior remained statistically equivalent to the ACL-sectioned state when the ALL was cut alone (p < 0.05). However, cutting the ITB alone produced increases in both lateral compartment translation and internal rotation of 1.8 mm (95% CI, 0.5 to 3.1 mm; p = 0.29) and 2.1° (95% CI, 0.2° to 3.8°; p = 0.47), respectively. These increases sufficed to constitute specimen behavior that was not equivalent to the ACL-sectioned state.

After both the ALL and ITB were cut, the mean increase in lateral compartment translation was 4.3 mm (95% CI, 2.6 to 6.0 mm; p = 0.99), and the mean increase in internal rotation was 5.1° (95% CI, 3.2° to 7.2°; p = 0.99). Compartment translations for a representative specimen are shown in Figure 5. Again, there was variability of increases in lateral compartment translation after cutting both the ALL and ITB, as previously described for Pivot Shift 1 (Fig. 6).

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Discussion

Rotational knee stability after ACL sectioning was markedly affected, with increases in medial, central, and lateral compartment translations (Fig. 5) that varied on the basis of the 2 pivot-shift loading variants. The higher internal rotation torque resulted in reduced medial and central anterior subluxation after ACL sectioning, as previously described25,26. Importantly, the abnormal tibiofemoral compartment subluxations occurred with an intact ALL and ITB, indicating that these structures were not primary restraints in the pivot-shift test.

After ACL sectioning, further ALL sectioning resulted in minimal increases in pivot-shift compartment translations, Lachman limits, and internal rotation limits. Specimen behavior in this state was statistically equivalent to that in the ACL-sectioned state, i.e., within 2 mm and 2°, respectively (p < 0.001 for all). Our results agree with those of Spencer et al., who reported, after ALL sectioning in an ACL-sectioned knee, an increase in internal rotation of only 2°, an amount that would be clinically undetectable and consistent with a secondary restraint to internal rotation22. In further agreement, Saiegh et al. measured both translation and rotation in the pivot shift and found increases of only −0.7 mm and 0.3°, respectively21. Our results disagree with those of Monaco et al., who reported a significant increase of 5.5° of internal rotation at 30° of flexion after ALL sectioning, although their loading involved manual (and therefore unquantified) torque conditions20. The results of the present study also show that the ITB acts as a secondary restraint to resist tibial translations and internal rotation in the simulated pivot-shift tests, in agreement with other reports24,35. However, prior studies on the function of the ALL had removed the ITB3,7,12,18,19, possibly overestimating the function of the ALL.

The increases in internal tibial rotation under 5 Nm of torque (Fig. 4) demonstrate variability in abnormal internal rotation limits after ALL and ITB sectioning (range, 1° to 12°). Three of 14 knees were ≥1 standard deviation above the mean and 3 of 14 knees were >1 standard deviation below the mean. Butler et al. reported an average restraining force to anterior drawer (after ACL sectioning) of 20.8% at 90° of flexion for the midlateral capsule (including the ALL), with a range from 1.6% to 36.9%36. The restraining force of the ITB was a mean of 24.8%, but ranged from 9.8% to 44.4%36. Wroble et al. also found 2 subsets of knees that displayed small and large increases in tibial rotation limits, respectively, after cutting the anterolateral structures in an ACL-sectioned knee37. This reflects the inherent variability in the restraining action of the remaining secondary restraints after a primary restraint has been removed. It should be noted that for the 3 knees with increased internal rotation of >1 standard deviation, their 8° to 12° increase in internal rotation resulted in a 5 to 9 mm increase in lateral compartment translation under the same 5 Nm of internal rotation torque conditions. This increased value may have clinical implications, as we have previously published similar increases in lateral compartment translation in the simulated pivot-shift test after ACL sectioning25, and another study found that a positive pivot shift can occur with a mean increase of 6 mm of lateral compartment translation38.

Sectioning both the ALL and ITB was required in order to produce major increases in pivot-shift translations and internal tibial rotations, and the results show variability in the magnitude of rotational instability between knees (Fig. 7). Using the criteria reported by Bedi et al., a clinically positive Grade-2 pivot shift (approximately 16 mm of lateral compartment translation) would be estimated in 10 of 14 knees after ACL sectioning38. It is of interest that lateral compartment translation in 2 knees (specimen numbers 166 and 167) reflected a Grade-3 pivot shift (>20 mm) after ACL sectioning38. This indicates a physiologic laxity of the ALL and ITB, as only minor increases occurred with their subsequent sectioning. With sectioning of the ACL, ALL, and ITB, 10 of the 14 knees had >20 mm of lateral compartment subluxation, consistent with a Grade-3 pivot shift. Applied clinically, a grossly positive Grade-3 pivot-shift test (although highly qualitative) implies concurrent damage or physiologic laxity of secondary anterolateral restraints in order to allow this magnitude of lateral compartment subluxation35,39. This emphasizes the difference in clinical presentation of knees with the same anatomic injury and the importance of a meticulous knee examination, including pivot-shift testing, tibial rotation dial tests, and lateral and medial tibiofemoral gap testing, to determine the function of primary and secondary ligament restraints25,36,40. In an in vitro study with a robotic simulator, Harms et al. showed that a centrally placed bone-patellar tendon-bone graft restores pivot-shift compartment translations to normal under time-zero conditions, even in specimens that had >20 mm of abnormal lateral compartment translation, indicative of diminished anterolateral restraints28. This shows that an extra-articular reconstruction is not necessary at time zero to obviate abnormal compartment pivot-shift subluxations.

Even though the ALL and ITB are secondary restraints on knee rotational stability, there is an argument that an ACL reconstruction would sustain higher graft forces in the absence of secondary restraints resisting some component of loading. A clinical justification has been suggested for concurrent extra-articular reconstruction in select knees with chronic ACL injury or in knees indicated for revision41-50, requiring well-designed future Level-I studies. A Level-II study of chronic ACL bone-patellar tendon-bone allograft reconstructions showed substantial improvements in overall knee score, stability, and return to athletics when an ITB tenodesis was added to the intra-articular reconstruction, with the rationale being to provide protection from deleterious forces and tibial displacement during graft healing46. The extra-articular procedures were more robust ITB tenodeses, different from proposed ALL anatomic reconstructions49-51 and most likely to provide increased anterolateral restraint that may abnormally constrain the joint, raising a concern for future arthritis that requires study. An ALL reconstruction to simulate native ALL function would not be expected to prevent a pivot-shift subluxation, since such subluxation can occur even in knees that have an intact ALL or ITB structure.

There are limitations to this study. It was not possible to simulate in vivo dynamic loading conditions that involve joint compressive forces and/or loading profiles that represent a variety of injury situations. It was not the purpose of this study to reproduce in vivo joint loadings or to study ACL and lateral extra-articular injury mechanisms or the effect of the entire ITB proximal restraint. There may exist anatomic variations in ALL and ITB structures that were not detected2,3,7,21. Additionally, since the anatomic dimensions of the ALL could not be determined without disrupting other tissues, a regional resection of this area was performed. The grading of the pivot shift clinically and the corresponding in vitro laboratory measurements are only an estimation or approximation.

In conclusion, this study shows that the ALL and the ITB are not primary restraints for limiting pivot-shift subluxations, which occurred despite intact native anterolateral restraints. Accordingly, an ALL reconstruction would not limit the abnormal pivot-shift subluxations that occur with ACL disruption. There is considerable variation in rotational stability between knees with the same anatomic injury. The ALL and ITB function in some knees as a primary restraint for internal tibial rotation at high flexion angles. A combined sectioning of the ACL, ALL, and ITB resulted in what would correspond clinically to a Grade-3 pivot shift in three-fourths of knees. This subset of knees with complete loss of all restraints from the anterolateral structures may theoretically have increased forces on an ACL graft.

Investigation performed at The Noyes Knee Institute, Cincinnati, Ohio

Disclosure: The sources of funding for this study include Arthrex, Inc., The Jewish Hospital Foundation, and The Noyes Knee Institute. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJS/A5).

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