8

winner of the 1983 O’Donoghue award

The biomechanics of anterior cruciateligament rehabilitation and reconstruction*

STEVEN W. ARMS,† MALCOLM H. POPE,†‡ PhD, ROBERT J. JOHNSON,† MD,RICHARD A. FISCHER,† MD, INGA ARVIDSSON,§ RPT, AND EJNAR ERIKSSON,§ MD

From the †Department of Orthopaedics and Rehabilitation, University of Vermont,Burlington, Vermont, and the §Karolinska Institute, Stockholm, Sweden

ABSTRACT

The rehabilitation of knee injuries involving the anteriorcruciate ligament (ACL) is controversial. This paperdescribes strain in the normal and reconstructed ACL

during a series of passive and active tests of kneeflexion with and without varus, valgus, and axial rotationtorques on the tibia. Strain in the human knee ACL wassignificantly different depending on whether the kneeflexion angle was changed passively or via simulatedquadriceps contraction. The knee joint capsule wasfound to be important for strain protection of the ACL.Quadriceps activity did not strain the normal or recon-structed ACL when the knee was flexed beyond 60°,but significantly strained the tissue from 0 to 45° ofknee flexion. Immobilization may not protect the ACL ifisometric quadriceps contractions are allowed to occur.Properly placed reconstructions exhibited strain behav-ior which closely followed the anteromedial band of theACL.

The orthopaedic surgeon faces a dilemma in optimizing thetreatment of ACL injuries.19 The wide variety of treatmentregimens available for both acute and chronic ACL disrup-tions suggest that no single method is uniformly regarded assuperior.19 Very few studies provide long-term clinical evi-dence that the ligament has been successfully repaired orreconstructed.21 ’32 ’34 Data are not available to demonstratethat the ACL reconstructions return to near normal functionfrom a biomechanical standpoint. Are these ligament recon-structions structurally capable of withstanding routine daily

* Presented at the annual meeting of the Amencan Orthopaedic Soaety forSports Medicine, Williamsburg, Virginia, July 1983

t Address correspondence and repnnt requests to Malcolm H Pope, PhD,Department of Orthopaedics and Rehabilitation, University of Vermont Schoolof Mediane, Burlngton, VT 05405

activities, let alone vigorous work or athletics? At the pres-ent time, we can only judge the results by very subjectivemeans, such as the apparent clinical stability and improve-ment in the patient’s functional ability.

Successful reconstructions should have nearly normalstrength to withstand overload and a strain pattern whichclosely mimics the original tissue, thus permitting normal,or nearly normal, joint kinematics. The strain pattern of anACL reconstruction has not been previously established.The rehabilitation of the ACL remains an engima. An

operation as extensive as the reconstruction of knee liga-ments necessitates a dramatic repair response. Classically,the joint has been immobilized for 6 weeks or more. Innu-merable investigations have documented the deleterious ef-fects of knee immobilization on muscles, cartilage, bone,ligaments, and capsule. However, early unprotected motioncould cause excess strain leading to permanent deformationof repaired, reinforced, or replacement ligaments, as well asfailure of the suture line or attachment. 9~ 18, 2’° ’31 ’39 Paulos et

a1.,31 using buckle transducers, have shown that the ACLresists significant load during knee extension from 40° tofull extension. The ACL load is greatly increased if weightis added to the leg. The present workers, 19 22 as well asMacIntosh,28 warned of the potential problem of a compo-nent of quadriceps force on the ACL. Avoidance of earlyquadriceps contractions after repair or reconstuction of theACL has been advised by some workers17 18 20 28 31 whileothers suggest early isometric or isotonic quadriceps andhamstring exercises.8 10 1’3 15 16 Several investigators reportno damage to their repairs or reconstructions using the earlyinstitution of such exercise programs. However, the evidenceis circumstantial. 8 1’3 15 16

Previous researchers have measured elongation or tensionin the ligament using different techniques. Their resultswere often conflicting. Brantigan and Voshell’ applied mo-ments to the knee joint and measured joint rotations. Theyused palpation and excision of the major knee ligaments inan effort to describe the ligaments’ function. Wang et all

9

used pins embedded in the ligament’s attachments andmeasured the pin’s displacement with calipers. Trent et al. 36also used pins, but measured bony motion and calculatedligament lengths. Girgis et al. 14 observed laxity changesunder various loading conditions and after selective section-ing. Lewis et al. 26 and Paulos et a1.31 used buckle transducersto measure force in the ACL. Kennedy et al.24 and Edwardset al.ll used mercury filled rubber tubes, and related chang-ing resistance to strain. Henning et al. 17 used an implantableelongation device to measure strain in vivo. Paulos et a1.31and Henning et a1,l7 are the only authors who describe ACLforce and elongation during simulated or actual quadricepsfunction. However, Paulos tested only one specimen, andHenning tested two live patients with Grade II ACL sprainsbut did not present the actual percentage strain, and did notdescribe strain as a function of flexion angle.

OBJECTIVES

We believe that knowledge of the strain behavior of thehuman ACL is essential for the repair, reconstruction, andrehabilitation of injuries to the ACL. The first goal of ourstudy was to perfect a device and adapt it for strain meas-urement in the ACL during passive knee flexion and activesimulated quadriceps contraction in fresh human cadavers.The most important design criterion was that the deviceshould have an extremely high compliance, so as to followthe natural elongation of the ligament.The study’s objectives were as follows:- To measure the relationship between ACL strain and

knee flexion angle for normal knee range of motion and withthe addition of varus-valgus and axial torques on the tibia.

- To measure strain in the anteromedial fiber bundles ofthe ACL in various loading circumstances which could occurin standard rehabilitation programs. Specifically, this studywas designed to establish the strain pattern in the ACLwhen the knee flexion angle was changed passively, and tocompare this to the ACL strain pattern when a simulated

quadriceps contraction was performed.- To determine the role of the capsule in ACL strain

protection and the effect of a capsulotomy.- To evaluate the strain in ACL reconstructions and to

establish the effect of attachment position on the strain.- To establish the strain in the normal ACL in the anterior

drawer and Lachman tests.- To use the data generated to make clinical recommen-

dations.

MATERIALS AND METHODS

We measured strain with a Hall effect transducer (Fig. 1).3This transducer gives a voltage output proportional to thestrength of a magnetic field.3, 33 The device has an extremelyhigh compliance; therefore, it applies minimal load to itssutured attachment to the ligament. An X-Y recorder wasused, with goniometer output on the X-axis and Hall outputon the Y axis. It can be calibrated to provide a linear output

Figure 1. The strain transducer is shown sutured to the

anteromedial fibers of the ACL.

except in the extremes of its motion.3 The device calibrationwas performed in 0.1 mm increments and is accurate to 0.02mm. The attachment points were 1.0 cm apart; therefore,assuming the attachment technique follows the ligamentperfectly, it is accurate to 0.20% strain. When calibrated

against an optical method for strain percentages rangingfrom 0 to 2%, the optical measurement and the Hall deviceshowed a correlation coefficient of 0.96.When performing our tests on cadaver specimens, it was

necessary to fix the femur so that all applied moments wouldstrain the knee joint and not the hip joint. Each cadaverwas placed on an autopsy table with the knee hanging freelyover one end (Fig. 2). Two skeletal transfixion pins werethreaded through the midfemur and were clamped to a metaljig via Hoffman pin clamps, and this jig was fixed to thetable. A metal stirrup was then attached to the ipsilateralfoot and distal leg. A coupling on the plantar aspect of thisstirrup was aligned with the longitudinal axis of the tibialshaft. Through this coupling a torque (13.6 Nm) was appliedwith a torque wrench.A median parapatellar incision exposed the knee joint,

and the knee extensor mechanism was transected 4 cm

proximal to the patella. Next, the intercondylar notch wasenlarged, if necessary, to avoid impingement on the straindevice, but the attachments of the cruciate ligament wereleft unaltered. As little bone as possible was removed in thisprocess. The ligamentum mucosum was transected and thesynovial sheath overlying the ACL was carefully removed.A goniometer was strapped to the medial side of the knee

and was calibrated at 15° increments through a range of fullextension to 120° of flexion. The Hall effect device wasattached to the midportion of the anteromedial fiber bundlesof the ACL, using two 4-0 Vicryl sutures to anchor two pinson both ends of the device (Fig. 1). The lower attachmentof the device was 4 to 8 mm superior to the tibial attachmentof the ACL. The stability of the transducer attachment wastested by gently tapping on the transducer with needleholders. If device output changed, but did not immediatelyreturn to its original output level, the transducer was sutured

10

Figure 2. The apparatus for rigid femoral fixation, the meas-urement of knee flexion angle, and device for measurementof quadriceps load. The output of the strain device is via wiresthrough the medial capsule.

again, until this tapping did not permanently change thedevice output.This technique provided continuous monitoring of the

ligament, permitting the presentation of a graph of ligamentstrain versus knee flexion angle. Since the knees of thecadaver specimens were initially in extension, the problemof rigor was solved by transecting the quadriceps mechanism.Normal saline was used to keep the soft tissues moist.

Passive ACL strain tests were performed by lowering theleg while grasping the toes, and no other constraints wereplaced on this motion. Care was taken to impart no rota-tional or varus or valgus moments to the knee during thetest. We referred to this strain pattern as the &dquo;passivenormal.&dquo; This test was repeated with the addition of a 15Nm valgus moment (termed passive valgus), and followingthat, a similar varus moment (termed passive varus) result-ing from forces applied at the ankle. Then an external andinternal axial torque (termed passive external and internal)of 13.6 Nm was applied to the coupling on the foot stirrup.

The passive normal ACL strain pattern was obtained fromall other tests to control for changes in technique or nonre-versible strain effects.

Active knee motion strain tests were performed as follows:Three heavy sutures were passed through the quadricepsjust abQve the patella and were braided into a single strand,which was connected to a load cell in series with a handle.The knee flexion angle was changed by forces appliedthrough the quadriceps mechanism. An investigator slowlylowered the leg from full extension by gradually reducingthe force on the braided suture in the quadriceps mechanism.This simulated an eccentric contraction of the quadricepsmuscle (termed eccentric quads). Simulated eccentric con-tractions with the addition of varus, valgus, internal orexternal rotation moments were likewise termed eccentric

quads + varus, eccentric quads + valgus, etc.To investigate the effect of an isometric quadriceps con-

traction, the knee was passively flexed from 0 to 120° atapproximately 10° intervals. The knee flexion angle wasfixed by manually restraining the distal tibia while a 400 Nload was applied to the quadriceps tendon (termed isometricquads). The change in ACL strain with a constant quadri-ceps load was thus measured for different knee flexion

angles. In 12 specimens the anteromedial capsulotomy wasclosed with sutures. These specimens provided the data basefor tests involving applied moments. In five cases, the medialcapsule was not repaired and the lateral capsule was releasedto assure proper patellar tracking. Results were obtained foiboth repaired and unrepaired capsulotomy specimens. Thus,we gained some insight into the role of the capsule. Due totime constraints at autopsy, not all tests could be performedon all knees.

Strain measurement of modified Brostrom6, 7, 12 patellartendon reconstructions were then made in cadaver knees.The ACL was removed and a distally based strip of thepatellar tendon was placed through a drill hole in the prox-imal tibia (through the tibial spine) and sutured into thelateral wall of the femur. With the knee in 20 to 30° flexion,the sutures were tightened as firmly as possible by hand.The strain device was attached in the center of the width ofthe graft, 4 to 8 mm superior to the tibial plateau. Ninespecimens had reconstructions placed in the assumed idealposition (far posteriorly in the lateral surface of the inter-condylar notch). These were termed passive posterior recon-structions. Two reconstructions were placed 1 cm anteriorly.These were termed passive anterior reconstructions. Thelatter were done to assess the variation of placement onstrain patterns. The complete text matrix performed on thenormal ACL was then repeated on the reconstructions.These torques were applied to simulate those potentiallypresent during standard rehabilitation programs. In thismanner we could demonstrate potentially dangerous me-chanical overloads.We also evaluated the strains resulting while performing

a Lachman&dquo; and anterior drawer test on knees with normalACLs. The tests were performed manually with the knee in15 to 20°, and 90° of flexion, respectively.

11

RESULTS

A total of 21 knees from 19 cadavers was studied. The

average cadaver age was 56.9 years, ranging from 33 to 82years, and 76% of the specimens were males. The samplesize of each test is shown in Table 1. The graphs (Figs. 3 to10) demonstrate the mean ACL strained values observed ina variety of test conditions. Knee flexion angle is plotted onthe horizontal axis and the percent strain is plotted on thevertical axis. The percent strain is defined as the change inlength divided by the original length. The original length isthe distance between the attachment points of the trans-ducer when the knee is in passive full extension. All changesin length were measured relative to this original length. Atfull extension we define the ligament as having zero percentstrain. The passive normal strain pattern, devoid of anyapplied torque, is the lower curve in most of the graphs. Atevery 15 degrees, difference in mean strain between selectedtests was evaluated by the Student’s t-test.

Normal ACL strain

The normal pattern of ACL strain showed a minimum strainbetween 30 and 35° of flexion (Fig. 3). Under passive normalmotion as the knee was flexed from 0°, ACL strain rapidlydecreased until 30 to 35°; further flexion to 120° increasedthe strain in the ligament to a maximum of 1.25% at 120°.The maximum range of overall strain was 5.5% for the

passive normals.The passive varus moment increased ACL strain through-

out the range of knee motion (Fig. 3). Passive varus had thegreatest effect during 15 to 45° of knee flexion, where thestrain was increased approximately 4% above the passivenormal strain baseline. Passive valgus also had an effect of

increasing ACL strain, except in full extension. Passive

valgus had the greatest effect between 30 and 45° of flexion,where the strain was increased about 2% above the passivenormal baseline.As shown in Figure 4, the passive internal rotation torque

markedly increased the strain compared to the passive nor-mal curve. The maxima occurred between 10 and 15° andthe minima between 60 and 65°. Maximum strain in this

study was observed with passive internal rotation torque 8%above the passive normal baseline. Compared with the pas-sive normal, passive external rotation resulted in less strainduring the first few degrees of flexion, and between 65 and120°. Between 2° and 65°, the ACL strain decreased toapproximately 2% less than the passive normal curve.

Strains in rehabilitation

Simulated isometric contraction of the quadriceps increasedACL strain significantly above the normal resting level

through the first 45° of knee flexion (P < 0.005, Fig. 5).Isometric contractions at flexion angles greater than 60°actually decreases ACL strain. This decrease was significantat flexion angles of 105° (P < 0.05) and 120° (P < 0.025).Figure 5 also shows ACL strain during simulated eccentricquadriceps contraction. The increase in strain during theeccentric contraction was significant from 0 to 45° of flexion(P < 0.005). During the eccentric quadriceps contraction,the pattern of strain is very similar to the normal pattern atflexion angles greater than 70°. The flexion angle of leaststrain was approximately 40° for passive normal motion, 60°for eccentric quads, and 75° for isometric quads. The in-crease in strain caused by eccentric quads + varus and valguswas evident beyond 0° of flexion (Fig. 6). Internal rotationincreased the strain in the ACL when the leg was lowered

TABLE 1

Sample size of each test performed.

12

Figure 3. Knee flexion angle versus percent strain. The passive normal, varus and valgus moments are indicated. Varus markedlyincreases ACL strain.

Figure 4. Passive normal, internal rotation and external rotation. Internal rotation markedly increases ACL strain.

both eccentrically (Fig. 7) and passively (Fig. 4). Externalrotation of the tibia caused a decrease in ACL strain when

the leg was lowered actively. This can be seen by comparingthe eccentric quads curve with the eccentric quads + externalrotation curve (Fig. 7).

The role of the capsule

All of the preceding tests with applied moments were donein specimens that had a medial capsular closure (repair)

following the implantation of the transducer. In five casesthe capsule was not repaired, and both lateral and medialcapsule were left open. Significant differences between re-paired and unrepaired capsulotomy knees are shown inTable 2. Active or passive varus tests showed no significantdifferences between repaired and unrepaired capsules. Inpassive valgus in full extension, the bilateral capsulotomyknees showed significantly higher ACL strains but at otherflexion angles differences were not significant. Passive ex-ternal rotation moments applied to the bilateral capsulot-

13

Figure 5. Simulated eccentric and isometric quadnceps tests. Quadnceps activity significantly increases ACL strain from 0 to45° of flexion.

Figure 6. Eccentric quadriceps contraction with varus and valgus moments. These are compared to the passive normal andeccentnc quadnceps tests.

omy knees gave higher strains from 0 to 120° (Table 2).Active or passive internal rotation tests showed no signifi-cant differences between unrepaired and repaired capsules.There was no significant difference between repaired andunrepaired capsulotomy specimens under active or passivevarus moments. The eccentric quads and valgus tests showedsignificantly higher strains at some angles (Table 2). Oncomparison of repaired versus capsulotomy specimens forisometric quads and eccentric quads tests devoid of any

applied moments, we found no observed or statistical differ-ences ; therefore, these were grouped to provide a large sam-ple size for the mean passive normal strain, eccentric quads,and isometric quads tests.

ACL reconstructions

Figure 8 demonstrated the first test of flexion against strainfor a modified Brostromh 7 12 patellar tendon reconstruction.

14

Figure 7. Quadriceps activity with internal and external axial torques. Internal rotation increases strain compared to the passivenormal and eccentric quadriceps tests.

TABLE 2The effect on ACL strain of a medial capsular repair versus

bilateral capsulotomy.

a Differences represent the increase in mean ACL strain as aresult of leaving the capsule unrepaired.

This showed a pattern similar to that of the normal ACLwith minimum strains of approximately -3% at 30° of kneeflexion. However, after 10 repetitions of loading, the curvebecame more &dquo;normal.&dquo; The strain minima of approximately-3.5% is between 30 and 45°. These changes were not foundto be statistically significant.The reconstructions responded to the applied varus, val-

gus, and axial torques with strains similar to the anterome-dial fibers of the normal ACL. In some of our tests, however,the internal rotation torque caused the sutures to cut

through the reconstructions. In the reconstructions, therewas a significant difference between the strain in passivemotion and in eccentric quadriceps contraction (at 0° offlexion, P < 0.025; at 15°, 30°, and 45°, P < 0.005). This wassimilar to the pattern demonstrated by the passive normaland eccentric quads in the normal ACL. The reconstruc-

tion’s response to the isometric quads tests were also similarto the normal ACL.

Figure 9 demonstrated the change in strain pattern in twotests when the graft was placed 1 cm anteriorly in the femur.The strain in the anteriorly placed graft was significantlygreater than both the normal ACL and the posteriorly placedgraft in full flexion (P < 0.05). No other tests were performedon the anterior grafts since the sutures cut through thesereconstructions at full flexion.

Clinical laxity tests

Figure 10 demonstrated the strains that may be observed innormal ACL’s under Lachman&dquo; and anterior drawer testing.Both tests increased the strain above the passive normal byapproximately 6%; however, the anterior drawer was per-formed at a flexion angle of 90°, where the ACL was undergreater strain.

DISCUSSION

There is much controversy surrounding the strain in theanterior cruciate for simple knee range of motion and forother applied moments. For example, Brantigan andVoshell5 found that both cruciates remain taut throughoutflexion but more force is placed on them in full flexion. Thisimplies that the ACL would have a constant state of strainup to approximately 80° of flexion. According to Wang etal.38 the ACL does increase in strain with flexion, startingfrom 0% and reaching about 10% strain at 120°. Trent eta1.36 found the anterior fibers of the ACL to be nearlyconstant in length with flexion to 60° and to be at minimumlength at 75°. However, Kennedy et al.24 stated that mini-

15

Figure 8. The passive strain pattern of the normal and reconstructed ACL. The reconstructions’ strain pattern changed as theknee went through a senes of 10 tests. Patellar tendon reconstructions can closely follow the normal anteromedial ACL strainpattern.

Figure 9. Normal anteromedial, posterior femoral attachment reconstructions (N = 9) and anteriorly attached reconstructions (N= 2).

mum ACL length occurs at 30 to 40° flexion, but he did notpresent these data in graphical form, making it impossibleto obtain a quantitative measure of ACL strain. Edwards etal.,&dquo; also using a mercury gauge, describes passive ACLstrain in four knees, showing minimum ACL strain around40° of knee flexion. He did not give the absolute strain, but

presents results as relative elongation from 0% at minimunlength to 100% at maximum length. None of the aboveauthors made measurements in a specific segment of theligament.

Paulos et a1.31 have investigated the effect of muscle

activity on the ACL utilizing a buckle gauge transducer. In

16

Figure 10. The increase in anteromedial ACL strain when Lachman and drawer tests are performed on normal ligaments. Thedrawer test creates more strain in the ligament.

this study of one specimen, the authors did not describeloading in the ACL during passive normal motion. Thebuckle transducer method did not permit computation ofthe stresses on the collagen fibers since the true area ofloading was not known. The buckle device was either appliedover all bundles of fibers or to specific bundles, whichnecessitated a major violation of the ligament. Paulos et a1.31did not state in which fibers they measured load. If the fiberbundles were separated, this would affect the behavior ofthe ligament. Our tests have shown that the buckle intro-duces some prestrain into the ligament. This prestrain willaffect the results.Our measurements of anteromedial fiber strain demon-

strate that during passive flexion the ACL relaxes from fullextension to 30 to 35°, then tightens with further flexion to120°. Since these strains from full extension to 35° are inthe range of zero to -5°, the stress in the ligament is

probably quite low.29 Both our group and Brantigan andVoshell5 have observed that the ACL does not macroscopi-cally wrinkle during these tests.We believe our experiments represent the first nonambi-

guous measurement of strain in the anteromedial fibers ofthe ACL through the normal range of motion. The strainbehavior of the normal ACL needed to be established inorder to compare normal strain behavior with that of areconstruction.

Previous studies have not reported differences in strainbetween the passive and active method of change in theknee flexion angle. We have shown the passive curve is anunrealistic one on which to base therapy. If a mechanismwere available to limit the active anterior displacement ofthe proximal tibia, then the position of least strain on thesurgical repair would appear to be 30 to 35° knee flexion.This is a common position of immobilization after ACL

surgery. It has been assumed that cast braces, casts andsplints can limit this displacement, thus protecting theACL.4 However, it was reported that this is not accomplishedwith a cast.25 Thus, isometric or isotonic quadriceps con-traction performed inside a cast, cast-brace, or moveablesplint may jeopardize the surgical result, especially in thecommonly used flexion angles ranging from 20 to 60°. Ourwork indicated that isotonic or isometric activity of thequadriceps does not produce strains in a properly placedrepair or reconstruction if the knee is flexed to 60° or beyond.This implied that quadriceps activities should be carried outat flexion angles greater than 60° until healing is adequate.

Kapandji2l suggested that external rotation produces de-creased strain; however, others implied increased strain.14,3aWe found that external rotation increases the strain com-

pared to the normal passive strain pattern up to 65°, but atflexion angles above 65° it decreases the strain. However,we have demonstrated that at no time is there a significantpositive strain, as indeed we would expect there to be if theACL is significantly restricting external rotation (Fig. 4).We found that the only time the ACL did significantlyrestrict external rotation was if the capsule was not repaired.This possibly explains our disagreement with other

workers 14 J8 who did not repair the capsule. In internalrotation the ACL strain increases markedly; this is probablybecause the ACL and posterior cruciate ligament interact(or &dquo;wind-up&dquo;).5,2’3 We have demonstrated the importance ofthe medial capsular repair in reducing ACL strain when theknee is subjected to external rotation and valgus moments.The Lachman test (Fig. 10) does not result in as much

positive strain as the drawer test. This is because the Lach-man test is begun with ACL in a position of less initialstrain than the drawer test. Thus, there is less resistance toforce imparted by the examiner and the maneuver termi-

17

nates in a more positive end point (check rein) than duringthe anterior drawer test. This adds to the list of advantagesof the Lachman test presented in our previous work.’9Knowledge of the strain within the normal ACL or within

ACL repairs or reconstructions is vital to their proper pro-tection following surgical procedures. We focused our studyon the anteromedial band because the majority of proceduresattempt to mimic this portion of the ACL. In ligamentreconstruction or repair, it is essentially impossible to totallyreplicate the normal anatomical attachments to the tibiaand femur.&dquo; Thus, in most repairs and reconstructions, anattempt is made to reconstruct those fibers which are most

important for knee stability. Posterolateral structures, forknee flexion angles greater than 15°, do not help controlanterior displacement of the tibia on the femur. In twomeasurements made in the posterolateral fibers of the ACL,we found the strain to decrease immediately from 0° exten-sion with marked laxity between 15 to 70° flexion. Thesefindings agree with those of Girgis et al.,14 but more testingmust be done to elucidate the role of these fibers.The pattern of strain in the ACL reconstructions changed

as the knee underwent multiple tests of flexion-extensionand applied torques; however, the change was not statisti-cally significant. Our work demonstrates, for the first time,that a reconstructed ligament can closely mimic the strainpattern of the anteromedial fibers of the normal ligamentespecially after a few cycles of flexion. The changes weobserved may be due to a change in stiffness of the graftmaterial and/or mechanical behavior of the suture line. Withrepetitive loadings, changes in material stiffness may beaccompanied by graft laxity. Presently, we do not know howthese changes may affect the long-term results. These

changes could be desirable if they slowly permit the graftstrain pattern to become close to normal without any lossof mechanical function. Alternatively, such changes maydestroy the function of the graft.Attempts to reconstruct or repair the ACL require that

the graft or ligament be placed in a position that as closelyas possible conforms to the original anteromedial fibersattachment sites. The increased strain observed in our de-

liberately misplaced ACL reconstructions clearly demon-strates that this will lead to failure. This is supported by thehistological observations of Alm et al.2 in the failures ofseveral misplaced ACL reconstructions.At what flexion angle should the graft or repair be

tightened, and how much tension should be used? Becauseof the multiple variables involved, the answers to thesequestions cannot yet be given. Adequate transplant tensionwas found to be necessary for normal remodeling, but exces-sive strain also caused failures.2,30 A graft material such aspatellar tendon has different stress-strain properties thanthe normal ligament. Viidik ’37 states that a tendon is moreregular in morphology and has different mechanical prop-erties since it has to transmit force in a straight line. Thestrain is probably nonuniform over the graft length andwidth. If the tendon graft exhibits the same strain as thenormal ligament, stresses in the tendon graft may be higherthan stresses in the normal ACL. If the surgeon tightens thegraft so as to produce a prestrain, passive motion could

endanger the reconstruction. However, if the surgeon putsin an excessively lax graft, there may not be enough stimulusfor growth, and the graft will not remodel to the strengthneeded to function as an adequate ACL replacement. Cur-rently, we have no knowledge of how much strain is benefi-cial to the repair and proper healing of the ligament, or howmuch strain is detrimental to this process. However, basedon the results of this study, we recommend placement of thegraft in the attachment sites of the anteromedial fibers ofthe ACL and that the graft be firmly sutured at a flexionangle of 35°. This should be followed by a passive range ofmotion from full extension to 90° to demonstrate that the

repair or graft has not visibly loosened. If motion or quad-riceps activity are utilized, it should be understood that thiswill cause strain in the repair or graft. Our data suggest thatquadriceps activity at knee flexion angles greater than 60°is safe.

Accurate strain measurement and comprehensive descrip-tions of the change of graft stress-strain properties with timeafter implantation are needed. Prospective studies, withfollowup, are needed to establish the longterm effect ofdifferent variables, such as the graft material, choice ofrepair and/or reconstruction, attachment site location, grafttension and position at which the tension is applied. Thestrain history should be compared to changes in morphologyand material properties of ACL repairs or implanted graftsin order to describe the strain limits for optimum remodel-ing.New orthopaedic reconstructive procedures, whether it be

the graft material, fixation position, suture technique orposition of suturing, should be evaluated by techniqueswhich establish the strain patterns. These remarks also

apply to the use of repair techniques or prosthetic ligaments.

CONCLUSIONS

- Strain in the human knee ACL is significantly differentdepending on whether the knee flexion angle is changedpassively or via simulated quadriceps contraction.

- Simulated isometric contraction of the quadriceps sig-nificantly increases anteromedial ACL strain in the rangefrom 0 to 45° of knee flexion.

- Varus and internal rotation moments increase ACLstrain.

- The anteromedial capsule is an important aspect ofstrain protection of the ACL.

- ACL reconstructions that are placed anatomically ex-hibit strain behavior similar to the normal anteromedialACL.

- The Lachman and anterior drawer tests created highstrains in the ACL.

- Those who elect to use motion or quadriceps activityfollowing ACL repair or reconstruction should understandthat strains are produced in the ACL.

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18

2 Alm A, Gillquist J, Stromberg B. The medial third of the patellar ligamentin reconstruction of the anterior cruciate ligament Acta Chir Scan (Suppl)445 5-14,1974

3 Arms S, Boyle J, Johnson R, et al Strain measurement in the medialcollateral ligament of the human knee. An autopsy study. J Biomech 16491-496, 1983

4 Bassette FH, Beck JL, Weiker G A modified cast brace. Its use in

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5 Brantigan OC, Voshell AF The mechanics of the ligaments and menisci ofthe knee joint J Bone Joint Surg 23 44-66, 1941

6 Brostrom L, Enksson E Operative treatment of old ruptures of the antenorcruciate ligament Proceedings of the 9th International Congress on SkiTraumatology and Winter Sports Medicine. Garmich Partenkerchen, WGerm Nevel Verlang, 1970, pp 138-143

7 Brostrom L, Gillquist J, Liljedahl SO, et al Treatment of old ruptures of theantenor cruciate ligament Lakartidmingen 65: 4479-4487, 1968

8 Burn C, Hutzschenreuler P, Passler HH, et al Functional postoperativecare after reconstruction of knee ligaments An experimental study In TheKnee Joint Recent Advances in Basic Research and Clinical Aspects,Amsterdam, Excerpta Medica, 1974, p 108-116

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