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The Kinematic Basis of AnteriorCruciate Ligament ReconstructionScott Tashman, PhD, Sebastian Kopf, MD, and Freddie H. Fu, MD

The goals of anterior cruciate ligament (ACL) reconstruction are to restore knee stabilityand function and to preserve joint health. Static tests for anteroposterior laxity (eg,Lachman test or KT-1000 arthrometer) have typically shown restoration of normal ornear-normal laxity with a variety of modern ACL reconstruction techniques. However, ACLreconstruction has failed to prevent the early onset of osteoarthritis, and there is growingevidence that traditional single-bundle ACL reconstruction does not restore normal kneemechanics under functional loading conditions. ACL reconstruction may fail to restorenormal rotational stability during the pivot shift. Abnormal internal-external rotation andab/adduction have been reported after ACL reconstruction during normal daily activitieslike walking and running. Recently, cadaveric studies have shown the potential superiorityof ACL double-bundle (DB) reconstruction for restoring anatomy and mechanical function.However, clinical data demonstrating the clear superiority of DB reconstruction is lackingbecause of the absence of well-controlled clinical studies. Additionally, dynamic kneefunction after anatomic DB ACL has yet to be assessed comprehensively.Oper Tech Sports Med 20:19-22 © 2012 Elsevier Inc. All rights reserved.

KEYWORDS anterior cruciate ligament, knee, kinematics, reconstruction, biomechanics

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Normal tibiofemoral motion is constrained by articularsurfaces, ligaments, capsule, and menisci.1 Damage to

ligaments or menisci may alter these constraints, permittingabnormal motion that alters cartilage loading patterns andincreases risk for osteoarthritis (OA).2-5 Thus, the goal fornterior cruciate ligament (ACL) reconstruction should behe restoration of normal knee anatomy and mechanics, toeturn the joint to normal function, re-establish mechanical/iological homeostasis, and prevent OA. The most commonurgical approach for ACL reconstruction has been a singleraft bundle, with the femoral tunnel drilled through theibial tunnel (trans-tibial). Although this technique has beenenerally perceived to be successful, several recent meta-nalyses have indicated that normal structure and function ofhe knee is restored only 60 to 70% of the time.6,7 Perhaps of

greater concern, 60% to 90% of individuals have radio-graphic evidence of knee OA within 10 to 20 years after ACL

Reprinted with permission from Tashman S, Kopf S, Fu FH: The KinematicBasis of Anterior Cruciate Ligament Reconstruction. Oper Tech SportsMed 16:116-118, 2008 (© 2008 Elsevier Inc.).

Department of Orthopaedic Surgery, University of Pittsburgh, Pitts-burgh, PA.

Address reprint requests to Scott Tashman, PhD, Department of Orthopae-dic Surgery, Biodynamics Laboratory, 3820 South Water Street, Pitts-

sburgh, PA 15203. E-mail: Tashman@pitt.edu

1060-1872/12/$-see front matter © 2012 Elsevier Inc. All rights reserved.doi:10.1053/j.otsm.2012.03.003

reconstruction.8-14 These findings and other similar oneshave reinvigorated interest in improving our understandingof the anatomy and function of the ACL and driven investi-gations into alternative techniques for reconstruction thatmight better replicate function of the native ACL and im-prove long-term outcomes.

Much of the current knowledge concerning ACL anatomyand function has been derived from cadaveric studies evalu-ating normal, ACL-deficient and ACL-reconstructed kneesunder controlled, laboratory conditions. Linear and rota-tional transducers have been used to subject cadaver knees toa variety of dynamically changing flexion and extensiontorques and tendon forces.15 In full knee extension, bothbundles are arranged parallel and, in 90° of knee flexion, theyare twisted. Both bundles resist anterior translation of thetibia relative to the femur, with the anteromedial (AM) bun-dle tauter, from 30° to 90°, and the posterolateral (PL) bundlemore tensioned from 0° to 30° of knee flexion. Furthermore,cadaveric studies have shown that the PL bundle plays a moreimportant role for providing rotational stability in compari-son to the AM bundle.16,17 Failure of conventional ACL re-construction to restore knee kinematics has also been dem-onstrated for cadaveric knees.18 However, it has been difficulto predict clinical outcome based on the results of cadaver

tudies, which cannot replicate functional loading and rep-

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20 S. Tashman, S. Kopf, and F.H. Fu

resent only the “time-zero” condition (immediately after graftfixation).

In vivo functional assessment after ACL reconstruction isessential for evaluating effectiveness of treatment for restor-ing dynamic knee function and relating function to long-termoutcome. Traditionally, static knee stability tests have beenthe standard for evaluating graft function after ACL recon-struction. Typically, the thigh is stabilized with the knee heldat a fixed flexion angle, and an anterior force is applied to thecalf. Most common are the Lachman test and the KT-1000arthrometer (Med-Metric, San Diego, CA), which involve ap-plying an anterior load and assessing the resulting anteriordisplacement of the tibia relative to the femur. However,static stability measures are not well correlated with measuresof functional outcome for ACL-injured or reconstructed sub-jects.19-22

Another functional outcome test commonly performedin clinic is the Pivot shift, which is supposed to evaluatethe rotationally stability of the knee.23,24 However, this testis very subjective and observer dependent, and does notsimulate a physiological knee activity like walking or run-ning.25-27

The limited value of these tests for predicting clinical func-tion is likely related to the difference between measures oflaxity versus measures of dynamic stability. Laxity tests(translational or rotational) measure the maximum displace-ment of the joint in response to an applied external load, inthe absence of muscle forces. These tests attempt to define thelimits of the possible envelope of motion for the knee. How-ever, the knee rarely operates at these limits during normalmovement. It typically stays within a more limited envelopeof function that varies with activity and is controlled by acombination of the passive knee structures and dynamic neu-romuscular control.28 Simple laxity tests cannot simulate theomplex force magnitudes, directions, and rates of applica-ion produced at the knee during most human move-ent.29,30

Neuromuscular factors are important contributors to kneefunction in the ACL injured/reconstructed knee. Muscleforces are known to play a significant role in knee stability,especially in the ACL-deficient and reconstructed individ-ual.31 When the ACL is injured and then reconstructed, the

assive structures may be restored, but motor control deficitsikely remain. Sensory structures have been identified in thentact ACL, which could provide force and length proprio-eptive information to the central nervous system.32 Changes

in reflex response observed after ACL injury suggest thatknee joint control may be adversely affected by ACL loss.33,34

ACL reconstruction technique may influence graft healingand joint proprioception, leading to effects beyond those ofpassive mechanics alone.

Only in vivo, human studies of dynamic joint function andstability can assess the combined effects of surgical methods,tissue healing/remodeling and neuromuscular control onjoint function. Even small instabilities can lead to repetitivemicrotrauma and long-term joint degeneration, if they occurwith high frequency during common activities. Thus, mea-

surement of dynamic knee motion during functional tasks,

and understanding how it is altered by ACL injury/recon-struction, are fundamental prerequisites for optimizing treat-ment.

Most data currently available on dynamic knee functionwere collected with the use of gait analysis techniques usingoptoelectronic or video-based systems to track externalmarkers attached to the skin. These techniques have beenused to investigate the effects of ACL deficiency and ACLreconstruction on knee function during various movementactivities, including walking, jogging, and ascending/de-scending stairs. ACL-deficient knees showed increased in-ternal tibial rotation relative to uninjured knees.35 How-ever, no kinematic differences were found during gaitbetween SB ACL reconstructed and uninjured knees forflexion-extension, abduction-adduction, and internal-ex-ternal rotation.36,37 With more demanding functionalests, such as pivoting after stair descending and jumping,CL reconstruction restored anteroposterior stability butot rotational stability.38 The maximum range of tibial

rotation was reported to be similar between ACL deficientand reconstructed knees.39

There is some concern that studies using skin markers maymiss subtle but important kinematic abnormalities after ACLreconstruction. Depending on skin and underlying tissue,the movement of these markers can be up to 30 mm relativeto underlying bone, especially during high impact activi-ties.40 Radiographic methods such as roentgen stereophoto-grammetric analysis (RSA) enable direct visualization ofbone. Dynamic RSA (D-RSA), which combines small im-planted bone markers with biplane-cine radiography, canachieve accuracy of � 0.1 mm or better at high sample rates(250 frames/s).41 Thus, D-RSA can acquire precise, real-timekinematic data during daily activities like running, inclineand decline walking, and stair climbing. The authors of astudy using D-RSA to evaluate patients after ACL reconstruc-tion found that the reconstructed knees were significantlymore externally rotated (3.8 � 2.3°) and adducted (2.8 �1.6) than the contralateral, uninjured joints. Anterior tibialtranslation did not differ significantly between the recon-structed and the healthy, contralateral knee41 but did in-rease over time after surgery (�0.85 mm from 5 to 12onths).42 Even these are appear to be small changes; their

ffect over time may be responsible for the early onset of OA.5

The studies described above all used ACL reconstructiontechniques representative of the “standard of care” for the last10 to 15 years (single graft bundle, typically trans-tibial dril-ling of femoral tunnel). The cadaver and in vivo studies de-scribed previously (and other similar studies) have high-lighted limitations of this approach for restoring normal kneeanatomy and function and lead to a surge of interest in ana-tomical ACL reconstructions that attempt to better reproducethe 2-bundle anatomy and its insertion sites. It has beenproposed that a more anatomical ACL reconstruction willmore effectively restore native ACL kinematics and dynamicknee function. Cadaveric and in vivo magnetic resonanceimaging studies evaluating knee kinematics after double-bundle ACL reconstruction showed superior results com-

pared with single-bundle ACL reconstruction.43,44 These results

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Kinematic basis of ACL reconstruction 21

were confirmed by a recent meta-analysis, which showed thateven in laxity tests like KT-1000 and pivot shift, double-bundleACL reconstruction achieved superior results in comparison tosingle-bundle ACL reconstruction.45,46 However, these studiesre insufficient to evaluate the efficacy of anatomical ACL recon-truction for restoring normal knee mechanics and preservingong-term joint health. Future studies should include accuratessessment of dynamic knee function, as well as careful assess-ent of clinical and functional outcomes.

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