INTRODUCTION

The purpose of this chapter is to provide a summary of the important biomechanical principles gained from studies from the authors’ laboratory and other investigations regarding the posterior cruciate ligament (PCL) and posterolateral structures (PLS). The primary posterolateral structures of the knee joint are the fibular collateral ligament (FCL) and popliteus muscle-tendon-ligament unit (PMTL), including the popliteofibular ligament (PFL) and posterolateral capsule (PLC). Additional PLS that may be injured and require repair are the iliotibial band femoral and tibial attachments, meniscal tibial and femoral attachments (including fascicles), the fabellofibular ligament, and the lateral and anterior capsule. All of these structures are illustrated in Chapter 2, Lateral, Posterior, and Cruciate Knee Anatomy. Data from these investigations provide the basis for the diagnosis of abnormal knee motion limits in single and combined ligament injuries and allow the surgeon to plan the appropriate ligament reconstructive procedure.

EFFECT OF SECTIONING THE PCL AND PLS ON THE LIMITS OF KNEE MOTION

A series of studies were conducted in cadaveric knees to measure the limits of anteroposterior (AP) translation, internal-external tibial rotation, and varus-valgus rotation (using a six-degrees-of-freedom electrogoniometer) under specific forces and moments with a verified testing apparatus previously described.^{8,24,25,31,64,97}

The PCL and PLS were sectioned in 15 knees first separately and then in combination to measure the resultant abnormal knee motion limits to simulate isolated and combined ligament ruptures.²⁵ The PLS in this investigation included the FCL, PMTL, and PLC. A 100-N force was applied to determine the AP limits, 5 Nm was used for internal-external rotation limits, and 20 Nm was used for adduction-abduction (varus-valgus) limits from 0° to 100° of knee flexion.

In these ligament-cutting experiments, the popliteus tendon was sectioned from the femoral attachment, which effectively removed the entire popliteus muscle tendon and PFL static function. In other experiments to be described,⁶⁷ the PFL was sectioned independently of the popliteus tendon attachment to investigate the individual function of this ligament. The PLC was removed, including all soft tissue structures posterior to the FCL, to ensure that the individual soft tissue components were removed. This included, in addition to the PLC, additional tissue components comprising the fabellofibular ligament and capsular arm of the biceps femoris short head.

Effect on AP Translation Limits

The normal anterior and posterior translation knee limits are shown in Figure 20-1A. The increase in these limits when the PCL is cut are shown in Figure 20-1B, and the further increase in these limits when the PLS are also sectioned are shown in Figure 20-1C. The values for the increase in motion limits are given in Table 20-1.

FIGURE 20-1 The curves show the limits of anterior and posterior translation (vertical axis) when a 100-N anteroposterior (AP) force was applied. A, Intact knees. The curves show the average limits of motion and the standard deviation for nine knees. The range of total AP translation of the intact knee is shown in shaded green in A, B, and C. B, Posterior cruciate ligament (PCL) is cut first. The increase in posterior translation after cutting the PCL is shown in the orange-shaded area (PCL deficit). The limit of posterior translation, and therefore the amount of increase, is controlled by the remaining intact structures. The unshaded portion (+ PLS deficit) shows the added increase when the posterolateral structures (fibular collateral ligament [FCL], capsule, popliteus muscle-tendon-ligament [PMTL]) were cut after the PCL had first been removed. A concurrent external rotation took place with this cut. C, Posterolateral structures are cut first. There was only a small increase (PLS deficit) in the posterior limit near full extension when the posterolateral structural elements were cut first. A concurrent external rotation was also present.

(A–C, From Grood, E. S.; Stowers, S. F.; Noyes, F. R.: Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures. J Bone Joint Surg Am 70:88–97, 1988.)

TABLE 20-1 Increased Motion Compared With Normal Motion that Occurred when the Indicated Structures Were Sectioned*

The data show that the PCL is a primary restraint to posterior tibial translation throughout knee flexion, with the exception of a small increase in posterior translation at full extension when the PLS are cut. The clinical finding of a knee with increased posterior translation at 30° to 45° of knee flexion, similar to the posterior translation limit at 90° (Fig. 20-2), indicates associated injury to the PLS and the medial structures.

FIGURE 20-2 Severe posterior tibial subluxation is shown in a chronic PCL-deficient knee, which is either due to physiologic laxity of the remaining secondary restraints or the result of a traumatic injury.

It should be noted that a complete description of the translation limits requires an anatomic measurable point on the tibia, which is selected at the midcoronal point of the tibia. The limits of posterior translation to the medial and lateral tibiofemoral joints are described in a subsequent study.⁶⁴

In a knee with a combined deficiency of the PCL and the PLS, the abnormal posterior tibial translation is at least four to five times the normal limit throughout knee flexion.

Effect on Internal-External Tibial Rotation Limits

The normal knee motion limits to internal and external tibial rotation are shown in Figure 20-3A. There is no increase in external tibial rotation when the PCL is cut alone (see Fig. 20-3B). This finding demonstrates that the PLS are the primary restraint for external tibial rotation throughout knee flexion. When the PLS only are sectioned, an increase in external tibial rotation occurs (see Fig. 20-3C), which is greatest at 30° of knee flexion. The amount of external tibial rotation decreases as the knee is flexed, showing the influence of the PCL in limiting external tibial rotation after the PLS are sectioned.

FIGURE 20-3 The limits of internal and external rotation of the tibia when a 5-N torque was applied. The fully extended position, measured in the intact knee, was used as the zero-rotation reference. A, Intact knees. The upper curve shows the limit of external rotation. The broken line shows the average position of the knee during passive flexion with the tibia hanging freely. The range of tibial rotation in the intact knee is shaded in A, B, and C. B, PCL is cut first. No change was found in external tibial rotation. C, Posterolateral structures (FCL, capsule, PMTL) are cut first. Increases in external tibial rotation occurred at low flexion angles. With added PCL sectioning, the increase in external tibial rotation occurred at high flexion angles.

(A–C, From Grood, E. S.; Stowers, S. F.; Noyes, F. R.: Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures. J Bone Joint Surg Am 70:88–97, 1988.)

These data indicate that the PLS provide the primary restraint to external tibial rotation at low knee flexion positions and, therefore, should be tested in this range (20°–40° flexion). Increases in external tibial rotation at 90° indicate injury to both the PLS and the PCL, consistent with the dial test (see Chapter 22, Posterolateral Ligament Injuries: Diagnosis, Operative Techniques, and Clinical Outcomes). The data are in disagreement with the classic interpretation of the posterolateral drawer test at 90° flexion, indicating injury to only the PLS. The resisting function of the FCL, popliteus tendon, and PFL is described in separate studies later in this chapter.

Gollehon and coworkers²² reported an increase in external tibial rotation of 20° flexion after sectioning the PLS at 30° of flexion. Sectioning of the PCL along with the PLS did not produce further significant increases in external tibial rotation (Fig. 20-4).

FIGURE 20-4 Primary internal and external tibial rotations resulting from 4.5 Nm of internal and external tibial torque in intact knees and after ligament sectioning. A, Increased internal rotation occurred only with combined section of the lateral collateral ligament (LCL [FCL]), deep structures (PMTL and capsule), and anterior cruciate ligament (ACL). B, Increased external rotation occurred at all angles of flexion with combined section of the LCL and deep structures, with additional increases at 60° and 90° when the PCL was sectioned. The broken lines indicate a lack of statistical difference from the adjacent curve unless otherwise stated.

(A and B, From Gollehon, D. L.; Torzilli, P. A.; Warren, R. F.: The role of the posterolateral and cruciate ligaments in the stability of the human knee. A biomechanical study. J Bone Joint Surg Am 69:233–242, 1987.)

Effect on Adduction-Abduction Rotation Limits

The normal limits of adduction and abduction rotation are shown in Figure 20-5A. Sectioning the PCL produces very small changes in these limits (see Fig. 20-5B). When the PLS are sectioned alone, large increases in adduction occur (see Fig. 20-5C), indicating that these structures are the primary restraint to this knee motion. Once the PLS are removed, the PCL becomes a primary restraint and large increases in the adduction limit occur with knee flexion.

FIGURE 20-5 The limits of adduction and abduction rotation when a 20 Nm moment was applied. The fully extended position, measured in the intact knee, was used as the zero-rotation reference. A, Intact knees. The upper curve shows the limit of adduction rotation, and the lower curve, the limit of abduction rotation. The broken line shows the amount of adduction when the knee is passively flexed, with the tibia upside down to ensure tibiofemoral contact in both the medial and the lateral compartment. B, PCL is cut first. C, Posterolateral structures (FCL, capsule, PMTL) are cut first.

(A–C, From Grood, E. S.; Stowers, S. F.; Noyes, F. R.: Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures. J Bone Joint Surg Am 70:88–97, 1988.)

The limits to adduction (varus angulation) are graphed in Figure 20-6²⁵ after sectioning the FCL, the PLS (PMTL and PLC), and all structures (PCL, FCL, PMTL, and PLC). The data demonstrate that the FCL is the primary restraint for lateral joint opening, with further increases after the remaining PLS are sectioned.

FIGURE 20-6 Increases in the limits to adduction (varus angulation) are shown after sectioning the FCL, the posterolateral capsule (PLC; PMTL and PLC), and all structures (PCL, FCL, PMTL, and PLC). The data demonstrate that the FCL is the primary restraint for lateral joint opening, with further increases after the remaining posterolateral structures are sectioned.

(From Grood, E. S.; Stowers, S. F.; Noyes, F. R.: Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures. J Bone Joint Surg Am 70:88–97, 1988.)

Markolf and associates⁵² developed an in vitro system in cadaveric knees in which load transducers were used to measure ACL and PCL forces under different loading situations. After sectioning the FCL and the other PLS, large increases were noted in resisting tensile forces in both cruciate ligaments under varus loading (Fig. 20-7) and in the PCL under posterior tibial loads and external tibial torques (Fig. 20-8). These findings are in agreement with prior studies, because the loss of the PLS converts both cruciate ligaments into primary restraints for varus loading, in which the ligaments are poorly positioned with small lever arms to resist this motion.

FIGURE 20-7 Graph shows the forces generated in the ACL and the PCL by 10.0 Nm of applied varus bending moment before (INTACT) and after section of the LCL (FCL) and the posterolateral structures (LCL). The increases in the mean force in the ACL after ligamentous section were significant at all angles of flexion; the increases in the mean force in the PCL were significant from 45° to 90° of flexion. The mean values for eight specimens are shown; the error bars indicate the standard error of the mean (SEM).

(From Markolf, K. L.; Wascher, D. C.; Finerman, G. A.: Direct in vitro measurement of forces in the cruciate ligaments. Part II: the effect of section of the posterolateral structures. J Bone Joint Surg Am 75:387–394, 1993.)

FIGURE 20-8 Graph shows the forces generated in the PCL by 50 N of posterior tibial force, with the tibia rotated externally by 7 Nm of applied tibial torque. Application of the 50-N posterior tibial force had no significant effect on the mean force in the PCL in the intact specimens (INTACT). After ligamentous section (LCL [FCL]), the posterior tibial force increased the mean force in the PCL at all angles except full extension. The mean values for eight specimens are shown; the error bars indicate the SEM.

(From Markolf, K. L.; Wascher, D. C.; Finerman, G. A.: Direct in vitro measurement of forces in the cruciate ligaments. Part II: the effect of section of the posterolateral structures. J Bone Joint Surg Am 75:387–394, 1993.)

The importance of load-sharing of the PLS with the PCL is demonstrated in many studies, which reinforces the need in knees with combined PCL-PLS ruptures to also perform posterolateral reconstructive surgery. An abnormal medial or lateral joint opening (gap test) to varus or valgus loading at arthroscopy indicates the necessity for concurrent medial or posterolateral ligament reconstruction. Otherwise, the majority of forces are placed on the ACL or PCL grafts, which may stretch out or fail postoperatively. The goal is to restore normal load-sharing in which the majority of varus-valgus loads and internal-external rotation torques are placed upon the medial and lateral structures and not the cruciate ligaments.

One problem with cadaveric studies that measure knee motion limits after ligament sectioning is that the true resulting millimeters of anterior or posterior translation (subluxation) of the medial and lateral tibiofemoral compartments are not measured or described. For example, when 15° of increased external tibial rotation is measured after cutting the PLS, the resultant abnormal millimeters of posterior translation of the lateral tibial condyle are unknown. This is because it is also necessary to measure and compute the center of tibial rotation to determine the effect of the increase in degrees of external tibial rotation on the lateral and medial tibiofemoral compartments. There are few published data on the true millimeters of tibial subluxation of the medial and lateral compartments (rotatory subluxations) that occur after knee ligament injuries, which are discussed in further detail later in this chapter.⁶⁴

ROLE OF THE POSTERIOR OBLIQUE LIGAMENT

Petersen and colleagues,⁶⁹ in a cadaveric study using a robotic testing system, examined the restraint of the superficial medial collateral ligament (SMCL), the deep medial collateral ligament (DMCL), the posterior oblique ligament (POL), and the posteromedial capsule (PMC) in resisting posterior tibial translation after PCL sectioning. The data shown in Figure 20-9 demonstrate the increase in posterior tibial translation after sequential ligament sectioning of the medial and posteromedial structures. The authors concluded that the POL has a much larger role than the SMCL and DMCL in resisting posterior tibial translation and internal tibial rotation. It should be noted that these two structures were first sectioned and no studies were done when the POL was sectioned first. This indicates that there could be a sectioning artifact introduced in the study. Even so, there are posteromedial oblique capsular fibers from the medial femoral condyle to the tibia, just posterior to the SMCL, that resist internal tibial rotation and posterior tibial translation (after PCL sectioning). The study concluded discrete oblique fibers form the middle arm of the POL described by Hughston and Eilers.³⁶ Of interest in this cadaveric study was the finding that a valgus loading of the knee joint close to knee extension (with PCL deficiency) produced an increased posterior tibial translation of the medial compartment with absence of the POL and PMC.

FIGURE 20-9 Posterior tibial translation (PTT) in response to a posterior load of 134 N (mean ± standard deviation [SD]). The effect of sectioning the secondary medial restraints resulted in a marked increase in posterior tibial translation.

(From Petersen, W. J.; Loerch, S.; Schanz, S.; et al.: The role of the posterior oblique ligament in controlling posterior tibial translation in the posterior cruciate ligament–deficient knee. Am J Sports Med 36:495–501, 2008.)

Robinson and coworkers⁷⁴ conducted a cadaveric study of the medial and posteromedial structures and could not identify a distinct separate POL structure, but did identify an oblique portion of the PMC at which fibers could be tensioned under internal tibial rotation loading. Robinson and associates⁷³ and Haimes and colleagues³¹ studied the contribution of the PMC, which included the POL. Robinson and associates⁷³ reported that the PMC resisted 28% of the posterior tibial load when the tibia rotated freely in the extended knee, which rose to 42% when the tibial was subjected to internal rotation. These authors concluded the PMC resisted posterior tibial translation close to full extension and less so with knee flexion, which relaxes the PMC. With knee flexion, the SMCL resisted posterior tibial translation.

In knees with chronic PCL instability and associated medial and posteromedial ligament and capsular injury, the integrity of all the structures should be restored. Specific tests for increased internal tibial rotation using the dial test are performed. The problem is with PCL deficiency, the tibia drops posteriorly and it is difficult to judge the added posterior subluxation on internal tibial rotation that tests the role of the PMC (including the POL).

FUNCTION AND ROLE OF THE PMTL

The PMTL includes (1) the muscular tibial origin, (2) the fibular origin (PFL), (3) the merging of both of these components to the popliteal tendon inserting on the lateral femoral condyle, (4) the inferior and superior popliteomeniscal fascicles forming the popliteal hiatus, and (5) the soft tissue attachments to the lateral meniscus and posterior tibia. The static stabilizing function of the PMTL that requires surgical reconstruction has been debated.^40,61,67 The authors advanced the hypothesis that the PFL has a secondary role in that it provides a functional attachment of the popliteus tendon to the fibula. However, removal of this fibular attachment has little effect on the limits of motion as long as the popliteal tendon femoral and tibial attachments remain intact.⁶⁷ This hypothesis questions operations in which only the PFL is reconstructed, without attention to the FCL or popliteus tendon femoral and tibial attachments.^54,78,86

As described previously, Grood and coworkers²⁵ reported that increases in external tibial rotation (posterior subluxation lateral tibial plateau) is highly dependent on the order of sectioning of the PLS. The last remaining PLS sectioned will, in general, provide the largest effect in terms of increased knee motion limits after the other primary and secondary restraints have been sectioned. Therefore, the experimental design of the order of ligament sectioning may induce an artifact and an incorrect assignment of a primary restraining role to a soft tissue structure. Shahane and associates⁷⁸ in a cadaveric study concluded that the PMTL was the primary restraint to external tibial rotation and the FCL was a secondary restraint. However, this study did not include sectioning the FCL first to determine whether increases in external tibial rotation occurred prior to sectioning the PMTL. In contrast, Veltri and colleagues^92,93 reported that sectioning the PFL (after first sectioning the FCL, popliteus tendon intact) resulted in only small and minor increases in external tibial rotation.

The cadaveric study by Pasque and coworkers⁶⁷ in the authors’ laboratory implemented an experimental system similar to that already described,^24,25 which used an instrumented spatial linkage and loading of the knee joint with a 100-N posterior force, a 10-Nm varus force, and a 5-Nm external moment from 0° to 120° of knee flexion. The ligament sectioning of the PLS was varied in order to more precisely define the restraining function of the individual structures. The results are shown in Figures 20-10 and 20-11. Sectioning the PFL in the intact knee produced no statistically significant changes in motion limits, whereas sectioning both the popliteus tendon and the PFL together (removing the function of the PMTL) produced a small increase in external rotation from 3° to 5°.

FIGURE 20-10 Graph shows the increase in external rotation (A), varus rotation (B), and posterior translation (C) from the intact knee when the PFL and popliteus tendon were sectioned. The data are given as the mean ± SEM. (1-tailed test, *P < .05; ***P < .001 after correction of multiple comparisons). POP, popliteus tendon.

(A–C, From Pasque, C.; Noyes, F. R.; Gibbons, M.; et al.: The role of the popliteofibular ligament and the tendon of popliteus in providing stability in the human knee. J Bone Joint Surg Br 85:292–298, 2003.)

FIGURE 20-11 Graph shows the increase in motion limits from the intact knee after external rotation (A), varus rotation (B), and posterior translation (C). The open circles represent the intact knee. The dark circles represent sectioning of the PCL, FCL, and posterolateral capsule (CAP). This denotes that multiple ligaments have been sectioned, leaving only the PFL and popliteus tendon (POP) structures remaining. Only small increases were noted after the PFL had been cut (inverted triangle). Subsequent sectioning of the POP (dark square) resulted in large increases in movement limits. Data are given as the mean ± SEM.

(A–C, From Pasque, C.; Noyes, F. R.; Gibbons, M.; et al.: The role of the popliteofibular ligament and the tendon of popliteus in providing stability in the human knee. J Bone Joint Surg Br 85:292–298, 2003.)

In another group of knees, the PCL, FCL and PLC were first sectioned, followed by the PFL and then the popliteus tendon. Only small increases were noted after sectioning the PFL as long as the popliteus tendon attachments remained intact.

In a separate study in the authors’ laboratory, Wroble and associates⁹⁷ reported the effect on external tibial rotation when only the popliteus tendon was sectioned in ACL-deficient knees (Fig. 20-12), and when only the FCL was sectioned along with the iliotibial band and lateral capsule (Fig. 20-13). Consistent with the studies previously discussed, only small increases in external tibial rotation were found when individual PLS were sectioned.

FIGURE 20-12 Limits of external rotation with a 5-Nm moment for intact specimens and with structures cut. ALL = ACL, POP (popliteus tendon), PMTL, FCL, ALS (iliotibial band and anterolateral capsule). Statistically significant increases in external rotation were found in the ACL + POP + PMTL cut state at 60° of flexion and higher. Statistically significant increases in external rotation were found at all flexion angles for the ACL + POP + PMTL + FCL and the ALL cut states. The intact FCL limits external rotation at all flexion angles after the ACL + POP + PMTL are sectioned.

(From Wroble, R. R.; Grood, E. S.; Cummings, J. S.; et al.: The role of the lateral extra-articular restraints in the anterior cruciate ligament–deficient knee. Am J Sports Med 21:257–262; discussion 263, 1993.)

FIGURE 20-13 Limits of external rotation with a 5-Nm moment for intact specimens and with ACL + ALS (iliotibial band and anterolateral capsule) + FCL and ALL structures (ACL + ALS + FCL + PMTL + POP) cut. Statistically significant increases were found at all flexion angles for both cut states. The difference between the ACL + ALS + FCL and the ALL curves represents the restraining function of the PMTL and POP toward external rotation. Note that increases occur at all flexion angles.

(From Wroble, R. R.; Grood, E. S.; Cummings, J. S.; et al.: The role of the lateral extra-articular restraints in the anterior cruciate ligament–deficient knee. Am J Sports Med 21:257–262; discussion 263, 1993.)

All of the PLS provide important restraints to external tibial rotation, whereas the PFL serves as a secondary restraint in providing a fibular origin to the PMTL. In acute knee injuries, this fibular attachment should be repaired if possible; however, graft reconstruction for the FCL and popliteus tendon tibial-femoral attachments is advocated (see Chapter 22, Posterolateral Ligament Injuries: Diagnosis, Operative Techniques, and Clinical Outcomes). The FCL functions to limit external tibial rotation, particularly at low knee flexion angles, and is an important structure to reconstruct in posterolateral injuries. Sugita and Amis⁸⁷ measured the orientation of the FCL and PFL in cadaveric knees and reported that the FCL slackened with increasing knee flexion, achieving a vertical position at 70° flexion, which decreased its ability to resist external tibial rotation. The PTML maintained an oblique orientation, which allowed effective resistance of external tibial rotation with increasing knee flexion.

LaPrade and colleagues⁴¹ conducted an in vitro cadaveric knee study in which a specially designed buckle transducer was attached to the FCL, popliteus tendon, and PFL. The knee joints were subjected to AP forces (67 N), varus-valgus torques (12 Nm) and internal-external rotation torques (6 Nm). A reciprocal relationship of load-sharing with knee flexion was found, because the FCL had higher loads resisting the external rotation torque at low flexion positions and the popliteus complex had higher loads at 60° and 90° of knee flexion (Fig. 20-14). Even though buckle transducers have inherent problems in truly measuring ligament tensile-resisting loads, the results show that all three structures are important in resisting external tibial rotation loads and the presumed importance of anatomic reconstruction of these structures.

FIGURE 20-14 Forces in the FCL, popliteus tendon (PLT), and popliteofibular ligament (PFL) during application of an external rotation torque of 6 Nm.

(From LaPrade, R. F.; Tso, A.; Wentorf, F. A.: Force measurements on the fibular collateral ligament, popliteofibular ligament, and popliteus tendon to applied loads. Am J Sports Med 32:1695–1701, 2004.)

POSTERIOR SUBLUXATIONS OF THE MEDIAL AND LATERAL TIBIOFEMORAL COMPARTMENTS

The millimeters of posterior subluxation of the medial and lateral tibiofemoral compartments after sectioning the PCL and PLS were measured in whole cadaver lower limbs in a testing apparatus previously described.⁶⁴ The position of the lateral and medial tibial plateaus was determined by three reference points: the AP translation of a point located at the center of each plateau, and the medial and lateral location of points located midway between the tibial edges and the joint centers.

The purposes of this investigation were to, first, determine the AP position of the medial and lateral tibial plateaus when a known external rotation torque was applied at 30° and 90° of knee flexion. Second, to measure how the positions of the lateral and medial tibial plateaus change and the magnitude of this change (subluxation) after sectioning the PLS alone and in combination with the PCL. The anterior and posterior meniscofemoral ligaments were also sectioned when present. Studies have shown that these structures are not the primary restraints for posterior tibial translation or external rotation and provide a restraint only after the PCL has been removed.^29,34,42 The study also determined the effect of physiologic joint laxity, or preexisting ligament laxity, on the magnitude of rotatory subluxation of the tibia after sectioning the PLS alone and in combination with the PCL.

Posterior Tibial Translation

The mean values of AP translation and external tibial rotation are shown in Figure 20-15 for intact knees and after sectioning the PLS and PLS/PCL. Sectioning only the PLS resulted in a mean increase in posterior translation of the lateral tibial plateau of 8.0 mm at 30° of flexion over the intact state (P < .01), and a smaller increase of 2.7 mm at 90° of flexion. There was no significant increase in posterior translation of the medial tibial plateau at either flexion angle. There was a large range of posterior tibial subluxations at both 30° (Fig. 20-16) and 90° of knee flexion (Fig. 20-17).

FIGURE 20-15 The mean values of external tibial rotation and the millimeters of AP translation for medial, central, and lateral tibial reference points are shown for 30° and 90° of knee flexion under the predetermined loads applied to the knee joint (100 N posterior load, 5 Nm external rotation torque). The neutral position (0° rotation, 0 mm AP translation) was determined in the intact knee by allowing the tibia to hang vertically by its own weight.

(From Noyes, F. R.; Stowers, S. F.; Grood, E. S.; et al.: Posterior subluxations of the medial and lateral tibiofemoral compartments. An in vitro ligament sectioning study in cadaveric knees. Am J Sports Med 21:407–414, 1993.)

FIGURE 20-16 Tibiofemoral position at 30° of knee flexion. The degrees of external rotation and the millimeters of AP translation for the medial, central, and lateral points are shown for two specimens, one with the least posterior tibial subluxation and one with the greatest posterior tibial subluxation (100 N posterior load, 5 Nm external rotation torque).

(From Noyes, F. R.; Stowers, S. F.; Grood, E. S.; et al.: Posterior subluxations of the medial and lateral tibiofemoral compartments. An in vitro ligament sectioning study in cadaveric knees. Am J Sports Med 21:407–414, 1993.)

FIGURE 20-17 Tibiofemoral position at 90° of flexion. The degrees of external rotation and the millimeters of AP translation for medial, central, and lateral points are shown for two specimens, one with the least posterior tibial subluxation and one with the greatest posterior tibial subluxation.

(From Noyes, F. R.; Stowers, S. F.; Grood, E. S.; et al.: Posterior subluxations of the medial and lateral tibiofemoral compartments. An in vitro ligament sectioning study in cadaveric knees. Am J Sports Med 21:407–414, 1993.)

Sectioning both the PCL and the PLS caused significant increases in posterior translation of both the medial and the lateral tibial plateaus at 30° and 90° of flexion (P < .01). The increase in posterior translation of the lateral tibial plateau compared with the intact state averaged 17.8 mm and 23.5 mm at 30° and 90° of flexion, respectively. The increase in posterior translation of the medial tibial plateau over the intact state averaged 7.6 mm and 12.3 mm at 30° and 90° of flexion, respectively.

External Tibial Rotation

The mean values of the external tibial rotation limits for the intact knees and after sectioning the PLS and PLS/PCL are shown in Table 20-2. Cutting the PLS caused a significant increase (P < .01) in external tibial rotation at both 30° (mean increase, 13.0°) and 90° (mean increase, 5.4°) of flexion. Sectioning the PCL along with the PLS caused a further increase in external tibial rotation at 30° (mean increase, 5.0°; not significant) and at 90° of flexion (mean increase 15.1°, P < .01). Considerable variability was present between specimens in the amount of measured external tibial rotation at 30° (see Fig. 20-13) and 90° (see Fig. 20-14) of flexion.

TABLE 20-2 External Rotation Limits (100 N Posterior Force, 5 Nm External Moment)

Flexion	External Rotation (°; Mean ± SD)
30°
Intact	18.2 ± 3.9
PLS* cut	31.2 ± 5.0
PLS, PCL cut†	36.2 ± 4.7
90°
Intact	17.4 ± 3.5
PLS cut	22.8 ± 5.0
PLS, PCL cut†	37.9 ± 3.0

SD, standard deviation.

* PLS, posterolateral structures including fibular collateral ligament (FCL).

† PCL, posterior cruciate ligament and ligaments of Humphry and Wrisberg cut in addition to the PLS and FCL.

From these data, a classification system of rotatory subluxations was devised based on two concepts: the final position of the medial and lateral tibial plateaus under defined loading conditions (such as with either internal or external tibial rotation at a defined knee flexion angle) and the position of each plateau. There are three possible positions for each plateau: anterior subluxation, normal position, or posterior subluxation. For each of these positions of the lateral tibial plateau, there exist three corresponding positions for the medial tibial plateau (Fig. 20-18).

FIGURE 20-18 Three types of tibiofemoral situations observed with increased external tibial rotation. A, Abnormal posterior translation of the lateral tibial plateau (at 30° of knee flexion under the loading conditions described in the study). B, Abnormal anterior translation of the medial tibial plateau under loading conditions of 5 Nm (at 30° of knee flexion). C, Abnormal posterior translation of the lateral tibial plateau plus abnormal anterior translation of the medial tibial plateau after sectioning both the medial and the lateral ligament structures. AS, anterior subluxation; FCL, fibular collateral ligament; LTP, lateral tibial plateau; MCL, medial collateral ligament; MTP, medial tibial plateau; N, normal position; PLS, posterolateral structures (FCL, PMTL); PMC, posterior medial capsule; PS, posterior subluxation.

(A–C, From Noyes, F. R.; Stowers, S. F.; Grood, E. S.; et al.: Posterior subluxations of the medial and lateral tibiofemoral compartments. An in vitro ligament sectioning study in cadaveric knees. Am J Sports Med 21:407–414, 1993.)

This study showed that in normal knees under the conditions of external tibial rotation (5 Nm) and a posterior force (100 N) at 30° of flexion, the lateral tibial plateau displaced a mean of 7.5 ± 2.9 mm. After sectioning the PLS, a further increase to 15.5 ± 3.5 mm was noted, representing an increase of 8 mm of posterior tibial subluxation of the lateral tibia plateau from normal. This is the amount that the clinician may palpate on the lateral tibiofemoral step-off with external tibial rotation in knees with ruptures to the PLS (compared with the opposite knee). After sectioning the PCL (in addition to the PLS), a mean posterior translation of 25.3 ± 5.2 mm was demonstrated, representing a 17.8-mm increase from normal.

Under the same loading conditions at 90° of flexion, sectioning only the PLS resulted in a small (2.7-mm) increase in posterior translation of the lateral tibial plateau. These data agree with the external tibial knee motion limits previously reported,²⁵ in which small and insignificant increases in external tibial rotation occurred at 90° after PLS sectioning. When the PCL was also sectioned, the lateral tibial plateau demonstrated a gross posterior subluxation, with a mean posterior translation of 34.2 ± 3.5 mm (23.5 mm from normal).