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.
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).
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.
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.
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