1biomec Esquina Posterolateral

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Arch Orthop Trauma Surg (2007) 127:743–752

DOI 10.1007/s00402-006-0241-3

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ARTHROSCOPY AND SPORTS MEDICINE

Anterolateral rotational knee instability: role of posterolateralstructures

Thore Zantop · Tobias Schumacher ·

Nadine Diermann · SteV en Schanz ·

Michael J. Raschke · Wolf Petersen

Received: 2 October 2006 / Published online: 28 October 2006 Springer-Verlag 2006

Abstract

Introduction The aim of this study was to determinethe anterolateral rotational instability (ALRI) of the

human knee after rupture of the anterior cruciate liga-

ment (ACL) and after additional injury of the diV erent

components of the posterolateral structures (PLS). It

was hypothesized that a transsection of the ACL will

signiWcantly increase the ALRI of the knee and

furthermore that sectioning the PLS [lateral collateral

ligament (LCL), popliteus complex (PC)] will addi-

tionally signiWcantly increase the ALRI.

Materials and methods Five human cadaveric knees

were used for dissection to study the appearance and

behaviour of the structures of the posterolateral corner

under anterior tibial load. Ten fresh-frozen human

cadaver knees were subjected to anterior tibial load of

134 N and combined rotatory load of 10 Nm valgus and

4 Nm internal tibial torque using a robotic/universal

force moment sensor (UFS) testing system and the

resulting knee kinematics were determined for intact,

ACL-, LCL- and PC-deWcient (popliteus tendon and

popliteoWbular ligament) knee. Statistical analyses

were performed using a two-way ANOVA test with

the level of signiWcance set at P < 0.05.

Results Sectioning the ACL signiWcantly increased

the anterior tibial translation (ATT) and internal tibial

rotation under a combined rotatory load at 0 and 30°

Xexion (P < 0.05). Sectioning the LCL furtherincreased the ALRI signiWcantly at 0°, 30° and 60° of

Xexion (P < 0.05). Subsequent cutting of the PC

increased the ATT under anterior tibial load

(P < 0.05), but did not increase the ALRI (P > 0.05).

Conclusion The results of the current study conWrm

the concept that the rupture of the ACL is associated

with ALRI. Current reconstruction techniques should

focus on restoring the anterolateral rotational knee

instability to the intact knee. Additional injury to the

LCL further increases the anterior rotational instabil-

ity signiWcantly, while the PC is less important. Cau-

tions should be taken when examining a patient with

ACL rupture to diagnose injuries to the primary

restraints of tibial rotation such as the LCL. If an addi-

tional extraarticular stabilisation technique is needed

for severe ALRI, the technique should be able to

restore the function of the LCL and not the PC.

Keywords ACL reconstruction · Revision ·

Rotational instability · Non-coopers ·

Robotic/UFS testing system

Introduction

The surgical reconstruction of the ACL is a common

procedure to restore knee stability, and good to excel-

lent clinical results have been reported. However, a crit-

ical review of the literature reveals that the success rates

reported for ACL reconstruction after relatively short-

term follow-up are between 69 and 95% [10]. The

causes for the failure after ACL reconstruction are mul-

tifactorial (tunnel malplacement, infection, insuYcient

This study is a winner of the AGA DonJoy Award 2006.

T. Zantop (&) · T. Schumacher · N. Diermann ·S. Schanz · M. J. Raschke · W. PetersenDepartment of Trauma, Hand and Reconstructive Surgery,Westfalian Wilhelms University Muenster,Waldeyer Strasse 1, 48149 Muenster, Germanye-mail: [email protected]

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744 Arch Orthop Trauma Surg (2007) 127:743–752

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Wxation, bone tunnel enlargement, postoperative stiV-

ness) [10, 15, 18, 20]. One important factor for the fail-

ure of an ACL reconstruction might be an insuYcient

treatment of associated injuries such as the menisci or

injuries of the posteromedial or posterolateral corner

[2, 16, 26, 27, 41]. Especially, associated injuries to the

posterolateral corner may be underestimated and may

play an important role for the surgical outcome [1, 16].It is well known that the anterior cruciate ligament

(ACL) is the primary restraint to tibial anterior trans-

lation. However, in addition to Xexion and extension,

the human knee also allows tibial internal–external

rotations due to the lack of congruency of the femoral

condyles and tibial plateau [1, 37, 47, 48]. In general,

the lateral compartment is more mobile than the

medial because of the attachment of the medial menis-

cus to the joint capsule [32, 37]. The mobility of the lat-

eral compartment increases in the weight-bearing

situation because of the anatomical characteristics of

the medial and the lateral tibial plateau. When bearingweight, the concavity of the medial plateau stabilizes

the medial femoral condyle, whereas the convexity of

the lateral plateau cannot stabilize the lateral femoral

condyle [1, 37]. Thereby, under an anterior tibial load,

anterior tibial translation (ATT) is accompanied by a

tibial internal rotation (“coupled tibial rotation”,

Fig. 1). In the ACL-injured knee, this coupled tibial

rotation may lead to anterolateral rotatory instability

(ALRI) as determined by Hughston et al. [14]. For a

patient, these altered kinematics may cause symptoms

of instability such as giving way phenomenon and can

clinically be assessed by the pivot shift test [37].

While the role of the ACL in controlling ATT is well

understood, there is much more controversy about its

role in controlling tibial internal rotation. Lipke et al.

[25] for example could show that the injury of the ACLled to a signiWcant increase in internal tibial rotation.

Similar results are reported by Amis and Scammel [2].

In their study, coupled internal rotation increased sig-

niWcantly after ACL rupture. In contrast, biomechani-

cal investigations of Fukubashi et al. [11] showed that

coupled tibial rotation decreased after transsection of

the ACL.

The converse results reported in these studies may

reXect the consequences of isolated ACL injury after

trauma versus isolated transsection of the ACL in a bio-

mechanical experiment. A clinical study reported a high

incidence of associated injuries of the posterolateralstructures (PLS) in patients with an ACL rupture after

rotational trauma [38]. As the axis of rotation of the

tibia plateau is close to the line of action of the ACL,

this ligament seems to be only a secondary restraint

against rotatory loads while the peripheral PLS are bet-

ter placed for controlling tibial rotation [1, 26].

The PLS of the knee can be divided into two pri-

mary components, the lateral collateral ligament

(LCL) and the popliteus complex (PC) (Fig. 2). The

PC consists of the popliteus muscle–tendon unit and

ligamentous connections between the popliteal tendon

and the Wbula, tibia and meniscus, known as the pop-

liteoWbular ligament and popliteotibial and popliteo-

meniscal fascicles, respectively [7, 16, 19]. With its

tendinous and ligamentous components, the PC

imparts both static and dynamic restraint to the knee

[7, 16, 40]. The arcuate ligament complex and fabelloW-

bular ligament are also considered part of the PLS, but

the importance of these structures is believed to be rel-

atively minor [7, 16, 19, 36, 40] (Fig. 2).

The role of the posterolateral corner in rotational

posterolateral instability has been extensively studied

[7, 16, 40]. However, when compared with posterolat-

eral rotational instability, there has been less work on

the role of the structures of the posterolateral corner in

ALRI. Because of this lack of work, present under-

standing of the functions of these structures in ALRI

remains incomplete, particularly relating to the control

of tibial rotational laxity. Therefore, clinically contro-

versy exists about the management of the resulting

complex instability.

The aim of the present study is to determine the rota-

tional instability of the human knee after the rupture of

Fig. 1 Due to the attachment of the medial meniscus to the jointcapsule and the convexity of the lateral tibial plateau, the lateralcompartment of the knee joint is more mobile than the medialcompartment. An anterior tibial translation is therefore associ-ated with an internal tibial rotation. This coupled motion is re-garded as coupled tibial rotation

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Arch Orthop Trauma Surg (2007) 127:743–752 745

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the ACL and after additional injury to the extraarticu-

lar primary restraints such as LCL and PC (popliteus

tendon, popliteoWbular ligament) and to elucidate

which structures of the posterolateral corner play an

essential role in controlling the ALRI. To accomplish

this, a robotic/universal force moment sensor (UFS)

testing system will be used as introduced by Woo et al.

and described earlier [24, 31, 33, 45] (Fig. 3). This sys-

tem allows obtaining the knee kinematics in response to

diV erent external loading conditions testing the same

specimen in diV erent conditions: intact, ACL deWcient,

LCL deWcient and with sectioned PLS. The application

of a combined rotatory and valgus load is a biomechan-

ical method to simulate the pivot shift test and to assess

rotational stability in an in vitro setting [17, 42, 46].It was hypothesized that a transsection of the ACL

will signiWcantly increase the ALRI of the knee and

furthermore that sectioning the PLS (LCL, PC with

popliteus tendon and popliteoWbular ligament) will

additionally signiWcantly increase the ALRI.

Materials and methods

Specimens

In this study 15 fresh-frozen human cadaveric kneeswere used (age range: 59–78 years). Ten knees were

used for the kinematic study; Wve knees were used for

dissection to study the appearance and behaviour of

the structures of the posterolateral corner. No speci-

mens did show any signs of surgical intervention and

the knees were radiographed and clinically examined

to exclude specimens with bony abnormalities and

osteoarthritis or ligamentous injury.

Prior to testing, the knees were stored at ¡20° and

thawed for 24 h at room temperature [35]. Femur and

tibia were cut 20 cm from the joint line and the sur-

rounding skin and muscles more than 10 cm away from

the joint line were removed. To maintain the anatomic

position in the proximal tibioWbular joint, the Wbula

was rigidly Wxed to the tibia with a cortical screw.

Fig. 2 Anatomical preparation of the posterolateral structures,view from posterolateral. The posterolateral structures of theknee can be divided into two primary components, the lateral col-lateral ligament (LCL, 1) and the popliteus complex consistingout of popliteus tendon ( 2) and the popliteoWbular ligament ( 3)

Fig. 3 Robotic/UFS testingsystem (a). The roboticmanipulator can move theknee in six degrees of free-dom, while the universal forcemoment sensor (UFS) canmeasure three orthogonalforces and moments (b). Theknee is mounted to the systemwith the tibia attached to theXunch of the robot via theUFS while the femur ismounted to the base

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Femur and tibia were then securely Wxed within thick-

walled aluminium cylinders with polymethylmethacry-

late bone cement (Palacos, Merk, Darmstadt, Ger-

many). The femoral cylinder was mounted to the base

of the robot (KR 125, KUKA Robots, Augsburg, Ger-

many) with a custom-made clamp while the tibial cylin-

der was connected through a UFS (FTI Theta 1500-

240, Schunk, LauV en, Germany) (Fig. 3). The UFS wasWrmly Wxed to the end eV ector of a six degree of free-

dom robotic manipulator. To prevent exsiccation, spec-

imens were kept moist using saline solution (0.9%).

Anatomical study

To study the anatomy of the posterolateral corner the

skin and subcutaneous fat, tissue was removed in Wve

knees leaving the ligamentous and tendinous structures

intact. The appearance of the PLS was recorded as the

specimens were extended, Xexed and rotated using dig-

ital photography. We studied how the ligamentsappeared to act and in what positions diV erent parts of

the complex either tightened or slackened.

Robotic/UFS testing system

To determine knee kinematics, a testing system for

knee kinematics, which combines robotic technology

with a UFS was used. The robot (KR 125, KUKA

Robots, Augsburg, Germany, Fig. 3) is a six-joint, seri-

ally articulated manipulator, which allows six degree of

freedom movement of the knee. The system is capable

of highly accurate kinematic measurements, such as

anteroposterior translation, medial–lateral translation,

proximal–distal translation, varus–valgus rotation and

internal–external rotation of joint motion [24, 31, 33,

45, 46]. The repeatability of this system is 0.2 mm and

0.02° for orientation and position of the end eV ector,

respectively [24, 31, 33, 45, 46]. The UFS can measure

three forces and three moments along a Cartesian axis

system with repeatability of 0.2 N for forces and

0.01 Nm for moments [24, 31, 33, 45, 46] (Fig. 3).

The robotic manipulator is capable of achieving

positional control of the knee in six degrees of free-

dom, while the UFS can measure three orthogonal

forces and moments. Simultaneously, this system is

capable of operating in a force-controlled mode via the

force feedback from the UFS to the robot.

Testing protocol

The experimental protocol and the data acquired are

displayed in Table 1. The path of passive Xexion–

extension of the intact knee joint was determined by

the robotic/UFS testing system by targeting force and

moment of zero in all remaining degrees of freedom.

The system found the positions of the knee that mini-

mized all external forces and moments applied to the

joint throughout the range of Xexion from 0° to 90° in

increments of 1°. The positions determined by this pro-

cedure served as the starting point for application of

external loads. To imitate clinical evaluation for theknee, an anterior tibial load of 134 N and a combined

rotatory load of 10 Nm valgus and 4 Nm internal tibial

torque was applied at 0°, 30°, 60° and 90° of knee Xex-

ion. The anterior tibial load was chosen because the

ACL is the primary restraint to ATT and to simulate

clinical tests such as the anterior drawer or Lachman

tests. The force of 134 N was chosen because this is the

force used for instrumented knee laxity measurements

in the KT 1000 [8]. To evaluate the kinematics in

response to a pivot shift test [23, 37], a combined rota-

tory load of 10 Nm valgus and 4 Nm internal tibial

torque was chosen. The forces have been used previ-ously [33, 46] to perform a simulated pivot shift test

[17]. Using this approach, the same specimen can be

tested in diV erent conditions: intact, ACL-, LCL- and

PC-deWcient (popliteus tendon and popliteoWbular lig-

ament) knee thereby increasing the statistical power.

Next, the ACL was cut through a small lateral para-

patellar incision. The external loading conditions were

then reapplied to the knee and the new kinematics

were recorded. Subsequently, the LCL and the PC

(popliteus tendon and popliteoWbular ligament) were

cut (Fig. 2) through a small posterolateral incision and

the resulting kinematics was recorded by the testing

system (Table 1). The order of sectioning the LCL and

the PC was alternated (Table 1).

Statistics

The kinematic data for the intact, ACL-, LCL- and PC-

deWcient knees were analysed by using a two-factor

repeated-measures analysis of variance (ANOVA).

Since all tests were performed in the same specimen,

multiple contrasts were performed. The two factors eval-

uated were the condition of the knee and the knee Xex-

ion angle. The dependent variables evaluated were knee

kinematics. The level of signiWcance was set at P < 0.05.

Results

Macroscopic observations

In all specimens careful dissections revealed a strong

LCL, popliteus tendon and popliteoWbular ligament

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(Fig. 4). The LCL was tight in full extension and slack-ened as the knee Xexed beyond 45° of Xexion. ATT in

neutral rotation position tensioned the LCL (Fig. 4).

Internal tibial rotation of the tibia moved the tibial

insertion of the LCL anteriorly thereby tensioning this

structure to a higher degree of Xexion. ATT in tibial

internal rotation did additionally tension the LCL.

When the tibial insertion of the LCL at the Wbular head

was anterior to the origin of the LCL at the lateral

epicondyle, the LCL seemed to be aligned with the

ACL (Fig. 4). In internal rotation, the LCL slackened

after more than 60° of knee Xexion.

The PC with the popliteoWbular ligament, however,seemed to be tight over the whole range of passive Xex-

ion and extension.

ATT under anterior tibial load

Under the 134 N anterior tibial load, ATT of the intact

knee was a mean of (§SD) 2.9 (§1.2), 8.4 (§1.7), 10.2

(§2.2) and 7.8 mm (§1.9) at full extension, 30°, 60° and

90° of knee Xexion, respectively (Fig. 5). After the

ACL was sectioned, the translations increased signiW-

cantly at all Xexion angles tested (P < 0.05). The result-

ing ATT under 134 N anterior tibial load was a mean

of 8.2 (§1.5), 14.2 (§1.3), 16.7 (§2.5) and 13.4 mm

(§1.6). After sectioning the LCL, the ATT was a mean

of 12.2 mm (§3.1) at full extension, 23.7 mm (§1.9) at

30°, 18.6 mm (§3.6) at 60° and 13.8 mm (§2.7) at 90° of

knee Xexion (Fig. 5). This diV erence was statistically

signiWcant when compared to the ACL-deWcient knee

at full extension and 30° of Xexion (P < 0.05). Com-

pared to the LCL-deWcient knee, sectioning the pop-

liteoWbular ligament and the popliteus muscle did

further increase the ATT in response to an anterior tib-ial load signiWcantly at 90° of knee Xexion up to

18.3 mm (§2.9) (P < 0.05).

ATT under combined rotatory load

Anterior tibial translation in response to a combined

rotatory load of 10 Nm valgus and 4 Nm internal rota-

tion was comparable to the ATT under an anterior tib-

ial load (Fig. 6). In response to a combined rotatory

load, the ATT for the intact knee was 2.9 (§1.9), 9.9

(§3.1), 8.5 (§4.1) and 7.8 mm (§3.3) for 0°, 30°, 60°

and 90° of knee Xexion, respectively. The valuesincreased after the sectioning of the ACL up to 7.9 mm

(§2.4) at 0°, 14.9 mm (§2.7) at 30°, 9.3 mm (§4.2) at

60° and 8.7 mm (§3.5) at 90° (Fig. 6). The increase in

ATT at full extension and 30° of knee Xexion was sta-

tistically signiWcant (P < 0.05). The LCL was found to

be the primary stabilizer to ATT under combined rota-

tory load. Sectioning the LCL increased the ATT sig-

niWcantly at full extension, 30° and 60° knee Xexion

(P < 0.05) up to 11.7 (§2.1), 20.7 (§3.0) and 14.2 mm

(§2.5), respectively (Fig. 6). Sectioning of popliteus

tendon and popliteoWbular ligament did not increase

the ATT in response to a combined rotatory load sig-

niWcantly (P > 0.05).

Coupled tibial internal rotation under combined

rotatory load

The internal tibial rotation in response to a combined

rotatory load of 10 Nm valgus and 4 Nm internal

rotation for the intact knee revealed a maximum inter-

nal rotation at 30° of knee Xexion (Fig. 7). The mean

Table 1 The experimentalprotocol and the data ac-quired

Loading condition Data obtained

Intact knee134 N anterior tibial load (ATL) Intact knee kinematics in response to ATL10 Nm valgus and 4 Nm internaltibial torque at 0°, 30°, 60° and 90°

Intact knee kinematics in responseto combined rotatory load

Transsection of the ACL134 N anterior tibial load ACL-deWcient knee kinematics in response to ATL

10 Nm valgus and 4 Nm internaltibial torque at 0°, 30°, 60° and 90°

ACL-deWcient knee kinematics in responseto combined rotatory load

Transsection of the LCL134 N anterior tibial load LCL-deWcient knee kinematics in response to ATL10 Nm valgus and 4 Nm internaltibial torque at 0°, 30°, 60° and 90°

LCL-deWcient knee kinematics in responseto combined rotatory load

Transsection of the posterolateral complex(popliteal tendon, popliteoWbular ligament)

134 N anterior tibial load Posterolateral-deWcient knee kinematicsin response to ATL

10 Nm valgus and 4 Nm internaltibial torque at 0°, 30°, 60° and 90°

Posterolateral-deWcient knee kinematics inresponse to combined rotatory load

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tibial internal rotation for the intact knee was 6.6°

(§1.0), 21.1° (§1.7), 17.1° (§1.8) and 23.6° (§0.6) at

full extension, 30°, 60° and 90° of knee Xexion, respec-

tively. Subsequent sectioning of the ACL increased the

internal tibial rotation at all Xexion angles; however,

most pronounced was this increase at full extension

and 30° of knee Xexion up to 9.5° (§1.3) and 24.6°

(§1.5), respectively (Fig. 7). At these two Xexion

angles, the diV erence between intact and ACL-deW-

cient knee was statistically signiWcant (P < 0.05). Sec-

tioning the LCL had an additional eV ect on the

increase of the internal tibial rotation. The internal

rotation increased up to 12.0 (§1.9) at full extension,

27.2 (§1.5) at 30°, 22.6 (§2.1) at 60° and 25.9 (§2.2) at

90° of knee Xexion (Fig. 7). At full extension, 30° and

60° of knee Xexion this increase was statistically signiW-

cant (P < 0.05). Sectioning of the popliteus tendon and

the popliteoWbular ligament had no increasing eV ect on

the internal tibial rotation of the knee in response to a

combined rotatory load.

Fig. 4 The role of the LCL in limiting the anterolateral kneeinstability. Under combined rotatory load of valgus torque andtibial internal rotation the tibial insertion of the LCL is displacedanteriorly (a). This causes the LCL to tension and thereby limit-ing the anterior tibial translation (b). In ACL deWciency, thismechanism is even more pronounced (c). The course of the LCLis aligned with the ACL and the LCL is the primary restraint toanterior tibial translation (d)

Fig. 5 Anterior tibial translation in mm in response to anteriortibial load of 134 N (mean§ SD). Asterisks indicate statisticallysigniWcant diV erences (P < 0.05)

0

5

10

15

20

25

30

0 30 60 90

Knee flexion (degree)

) m m ( n o i t a l s n a r t l a i b i t r o i r e t n A

intact ACL def LCL def PC def

*

*

*

*

*

*

*

Fig. 6 Coupled anterior tibial translation in mm under combined10 Nm valgus and 4 Nm internal tibial torque (mean§ SD).

Asterisks indicate statistically signiWcant diV erences (P < 0.05)

0

5

10

15

20

25

30

0 30 60 90


n o i t a l s n a r t l a i b i t r o i r e t n A

) m m (


*

*

*

*

*

Fig. 7 Internal tibial rotation in degree under combined 10 Nm

valgus and 4 Nm internal tibial torque (mean§

SD). Single aster-isk indicates statistically signiWcant diV erent when compared tothe intact knee (P < 0.05). Double asterisks indicate statisticallysigniWcant diV erent when compared to the intact and the ACL-deWcient knee

0

5

10

15

20

25

30

0 30 60 90


) e e r g e d ( n o i t a t o r l a i b i t l a n r e t n I


*

***

**

**

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Discussion

The aim of the current study was to evaluate the rota-

tional instability of the ACL-deWcient and the postero-

lateral-deWcient knee under combined rotatory load.

The result supports our hypothesis that the transsec-

tion of the PLS results in increased ATT under com-

bined rotatory load. Furthermore, the results suggestthat the LCL is the primary restraint to limit ATT

under a combined rotatory load of valgus and internal

tibial rotation. Injury to the LCL statistically increases

the rotational instability of the ACL-deWcient knee.

Subsequent sectioning of the popliteus tendon and the

popliteoWbular ligament had no signiWcant eV ect on the

anterior tibial rotation or the internal tibial rotation

under combined rotatory load. However, sectioning

the PC increased ATT under an anterior load.

The role of the ACL for knee joint stability has been

described as the primary restraint to tibial anterior

translation is well understood [5, 8, 11]. However, thereis some controversy about the role of the ACL to con-

trol tibial internal rotation [1–4, 12, 46]. Some biome-

chanical studies showed that the ACL does not

contribute in controlling tibial internal rotation [11,

22]. Others could demonstrate that the cutting of the

ACL leads to an increase in internal tibial rotation [2,

3, 29, 30, 46]. These diV erences may be caused by meth-

ods used in these studies. With the UFS/robotic system

the present study shows that transsection of the ACL

leads to increased ATT under a combined load with

4 Nm internal tibial and 10 Nm valgus torque.

A study investigating three-dimensional knee kine-

matics in patients after ACL rupture showed that

patients with ACL injury present with signiWcant rota-

tional instability at higher demanding activities [13].

Interestingly, this altered knee kinematics could not be

restored with a single bundle ACL reconstruction [13,

34]. It has been hypothesised that the single bundle

reconstruction cannot restore rotational instability

because the posterolateral bundle is not restored with

this technique [34, 42]. This data are in accordance with

the results presented by Tashman et al. [39]. Using a

250 frames/s stereoradiographic system, these authors

investigated the three-dimensional kinematics of

patients after ACL reconstruction during simulated

downhill running on a treadmill. Even though the ATT

was similar for the reconstructed and uninjured limbs,

the tibial rotation of the reconstructed knees was not

restored to normal [39].

Several biomechanical studies have shown that the

posterolateral bundle plays a role in controlling rota-

tional stability [12, 43, 46]. Therefore, several authors

recommend double bundle ACL reconstruction to

achieve better rotational stability [6, 12, 42, 44, 46–48].

The results of the present study underline the role of

the ACL in controlling rotational stability, but they

also show the important role of the LCL in rotational

instability. At 60° of Xexion, the LCL was the primary

restraint in limiting ATT against a combined rotatory

and valgus load (simulated pivot shift test). We

hypothesize that not only the technique of ACL recon-struction should be considered to be a cause for resid-

ual rotational instability after single bundle ACL

reconstruction. It seems likely that untreated injuries

of the PLS may also responsible for residual rotational

instability. It has been shown that the mechanism of

many ACL injuries has a rotational component [30,

36]. It is hard to believe that rotational forces are able

to stretch the ACL to failure, leaving the PLS intact.

This theory is underlined by the Wndings of Stäubli and

Birrer [38], which show a high incidence of associated

injuries of the PLS in patients with ACL rupture.

To assess ALRI clinically, the pivot shift test can beused [23, 37]. This test has been reported to correlate

with instability symptoms, reduced sports activity and

meniscal damage [18, 21]. Recent clinical studies have

documented a correlation between surgical outcome

after ACL reconstruction and the presence of pivot

shift test [15, 20]. In the current study a simulated pivot

shift was applied by the robotic/UFS testing system. A

load application of 10 Nm valgus and 4 Nm internal

tibial torque has been used to simulate a pivot shift

test. This load has been used previously for the evalua-

tion of the knee kinematics in response to a pivot shift

test [18, 33, 42, 46].

It has been shown that the pivot shift test is extremely

variable, both between examiners and between patients

[1, 4, 30]. The results of the present study suggest that

additional injuries to the LCL might be a factor, which

explains the diV erences in pivot shift between patients. It

seems to be necessary to develop a standardized method

to assess rotational instability of the ACL-injured knee.

This method would help to detect possible associated

injuries of the peripheral ligamentous structures. An

objective method for grading or to classify the ALRI

could be of clinical relevance. A rough classiWcation

would be ALRI with or without involvement of the PLS.

The observations of the dissection study explain the

results of the biomechanical experiments. In internal

tibial rotation, the tibial insertion of the LCL is moved

anteriorly and thereby tightened. The course of the

LCL is aligned with the ACL and primary restraint to

withstand ATT under combined rotatory load. With an

injury to the LCL this mechanism may be insuYcient

thereby further increasing the rotational tibial laxity

(Fig. 4).

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In the current study, we did also evaluate the eV ect

of cutting the PC (popliteus tendon and popliteoWbular

ligament) on the resulting knee kinematics. The unex-

pected results show a signiWcant increase in ATT at 90°

under anterior tibial load (P < 0.05). In the posterior

aspect of the knee, the popliteus tendon wraps around

the lateral femoral condyle. At a Xexion angle of 90°,

the femoral origin of the popliteus tendon is rolledanterior and proximal to the lateral epicondyle,

thereby tensioning the Wbres of the popliteus tendon.

In an ACL-deWcient knee, this tensioning of the ten-

don wrapped around the femoral condyle may contrib-

ute to restrain ATT under anterior tibial load.

Using a robotic/UFS testing system, Kanamori et al.

[17] investigated the forces in the MCL and PLS in the

intact and the ACL-deWcient knee. In the ACL-deW-

cient knee, the in situ forces of the PLS under 134 N

anterior tibial load were reported to be Wve times as

high as in the intact knee condition. The authors con-

cluded that, although both the MCL and PLS play onlya minor role in resisting anterior tibial loads in the

intact knee, they become signiWcant after ACL injury

[17]. The results of the current study strongly resemble

these Wndings and provide the kinematic data for the

previously published in situ forces [17].

During the last decades there has been a tremen-

dous eV ort to improve arthroscopic techniques for

ACL reconstruction. The latest development is the

double bundle ACL reconstruction technique, which

aims to restore rotational knee stability. The results of

the present study suggest that this technique might be

able to restore knee kinematics only in knees with iso-

lated ACL injury or in knees with minor injury of the

PLS. We hypothesize that in patients with higher

degree of ALRI an additional extraarticular procedure

might be necessary. In the literature diV erent recon-

struction procedures for the PLS can be found such as

the Larson sling (reconstruction of the LCL and the

popliteoWbular ligament) [7], augmentation of the LCL

with a strip of the biceps tendon [9] or popliteus

bypasses (graft between tibia and femur) [28]. Based

on the results of the present study, the aim of these

reconstruction techniques should be a technique that

primarily restores the LCL. However, more biome-

chanical research is needed to evaluate the best periph-

eral reconstruction technique for ALRI.

Some limitations apply to the current study. First,

we investigated the resulting knee kinematics under

anterior tibial load and coupled rotatory load. Theoret-

ically, the application of an axial compression force

may have additional eV ects on the ATT. However,

adding another testing condition would have signiW-

cantly increased the duration of testing thereby causing

the tissue to exsiccate. Secondly, the age of the human

cadaver knees as used in this study may not represent

the typical age for patients suV ering ACL rupture. The

scarcity of human donors makes it impossible to test

young human knees in numbers to provide statistically

signiWcant conclusions. Additionally, the test set-up did

not incorporate muscle activity. Theoretically, contrac-

tion of muscles such as the hamstring tendons or thepopliteus would have an important eV ect on the result-

ing knee kinematics. However, a study evaluating

EMG signals of intact and ACL-deWcient knees

observed only minor popliteus EMG signal diV erences

after ACL rupture [41]. The authors concluded that

the popliteus muscle does not actively contribute to

rotational instability such as the pivot shift.

In conclusion, the current study shows the impor-

tance of the LCL for the resulting knee kinematics

under combined rotatory load. The LCL was found to

be the primary stabilizer at 60° under a combined rota-

tory load in limiting the ATT. When the results of thecurrent in vitro study are transferred to the clinical set-

ting, they may suggest that ACL rupture results in an

anterior tibial instability as well as an ALRI. However,

injury to the LCL signiWcantly increases ALRI. It

would be of clinical relevance to distinguish ALRI with

or without the involvement of the PLS. In patients with

severe rotational instability and rupture of the ACL

and LCL, the necessity of additional extraarticular sta-

bilization procedures needs to be discussed. Further

research is needed to elucidate the eV ect of combined

ACL and posterolateral reconstruction when the pri-

mary restraints to internal tibial rotation are torn.

Acknowledgments Funding of the robotic/UFS testing systemwas received by a grant from the German Speaking Associationfor Arthroscopy (AGA) and is deeply appreciated by the authors.

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