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Patellar tendon and hamstring moment-arms andcross-sectional area in patients with anterior cruciateligament reconstruction and controlsEleftherios Kellisa, Evaggelos Karagiannidisa & Glykeria Patsikaa

a Laboratory of Neuromechanics, Department of Physical Education and Sport Science atSerres, Aristotle University of Thessaloniki, TEFAA Serres, Agios Ioannis, Serres, 62110,GreecePublished online: 27 Jan 2014.

To cite this article: Eleftherios Kellis, Evaggelos Karagiannidis & Glykeria Patsika , Computer Methods in Biomechanicsand Biomedical Engineering (2014): Patellar tendon and hamstring moment-arms and cross-sectional area in patients withanterior cruciate ligament reconstruction and controls, Computer Methods in Biomechanics and Biomedical Engineering, DOI:10.1080/10255842.2013.869323

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Patellar tendon and hamstring moment-arms and cross-sectional area in patients with anteriorcruciate ligament reconstruction and controls

Eleftherios Kellis*, Evaggelos Karagiannidis and Glykeria Patsika

Laboratory of Neuromechanics, Department of Physical Education and Sport Science at Serres, Aristotle University of Thessaloniki,TEFAA Serres, Agios Ioannis, Serres 62110, Greece

(Received 6 April 2013; accepted 21 November 2013)

The purpose of this study was to examine the moment-arm and cross-sectional area (CSA) of the patellar tendon (PT) andthe hamstrings after anterior cruciate ligament (ACL) reconstruction. The right knee of five males who underwent ACLreconstruction with a PT graft and five age-matched controls was scanned using magnetic resonance image scans. Based onthree-dimensional (3D) solids of the PT, CSAs and moment-arms of semitendinous (ST), biceps femoris (BF) long head andsemimembranosus (SM) were estimated. Analysis of variance indicated no significant group differences in muscle moment-arms (p . 0.05). 3D moment-arms of PT, STand BF were significantly lower than the corresponding 2D values (p , 0.05).The ACL group displayed a significantly higher maximum BF CSA, a lower ST CSA ( p , 0.05) but similar PT and SMCSAs compared with controls. It is concluded that any alterations in PT properties 1 year after harvesting do not affect kneemuscle moment-arms compared with age-matched controls. Moment-arm estimation differed between 3D and 2D data,although it did not affect comparisons between ACL reconstruction group and controls. Design of rehabilitation programmesshould take into consideration a potential alteration in hamstring morphology following surgery with a PT graft.

Keywords: knee mechanics; patellar tendon graft; hamstrings; anterior cruciate ligament; morphology

Introduction

Knee extensor moment deficits after anterior cruciate

ligament (ACL) reconstruction surgery with a patellar

tendon (PT) graft are common and can continue up to 24

months post surgery (Dauty et al. 2005). Such changes

may be due to reductions in muscle force or moment-arm

or both. Therefore, examination of moment-arms after

ACL reconstruction is worth investigating.

Strength impairments after ACL surgery are

accompanied by changes in volume and cross-sectional

area (CSA) of the involved musculature (Arangio et al.

1997; Snow et al. 2012). An 8.6% decline in quadriceps

CSA after ACL surgery with various grafts has been

reported (Arangio et al. 1997) while ACL reconstruction

with a hamstring graft caused atrophy of the semitendi-

nosus (ST), PT and gracilis and a hypertrophy of the long

head of biceps femoris (BF) muscle (Snow et al. 2012).

This indicates that the effect of ACL surgery may involve

not only the harvested muscle but there may be

morphology changes in other surrounding muscles.

There are inconsistent findings on the progression of PT

morphology and properties after ACL surgery with a PT

graft. Most studies reported an increase in PT thickness at

the donor site 1–2 years after harvesting (Nixon et al. 1995;

Wiley et al. 1997; Kartus et al. 2000; Jarvela et al. 2004;

Svensson et al. 2004; Liden et al. 2006). However, some

studies have shown that the PT became nearly identical to

normal tendon at 2 years (Nixon et al. 1995; Kartus et al.

2000), but others reported that this process is longer (Wiley

et al. 1997; Jarvela et al. 2004; Svensson et al. 2004; Liden

et al. 2006). In addition, there is some evidence that the PT

shortens by 10–20% after ACL reconstruction (Jarvela

et al. 2001), although recent re-examination of the data

indicated that thismay occur only in some patients (Marrale

et al. 2007). A large inter-individual variability in muscle

and tendon morphology responses to ACL surgery (Jarvela

et al. 2004; Marrale et al. 2007) indicates that there is no

single description on PT morphology progression after

ACL reconstruction surgery for all individuals.

The muscle–tendon moment-arm is defined as the

perpendicular distance between themuscle line of action and

the joint rotation centre.A change inmuscle–tendon volume

and CSAmay alter themuscle line of action and therefore its

moment-arm (Sugisaki et al. 2010). Research has shown no

differences in extension and flexion moment-arms (Buford

et al. 1997) and some differences in internal/external tibial

rotation moment-arms (Buford et al. 2001) between ACL-

deficient and normal knees. However, studies have shown

that ACL-deficient patients display altered patellofemoral

kinematics (Van deVelde et al. 2008; Shin et al. 2009)which

may be (Shin et al. 2009; Kothari et al. 2012) restored after

surgery or not (Van de Velde et al. 2008; Carpenter et al.

2009). To our knowledge, no studies have compared muscle

moment-arms after ACL reconstruction surgery in vivo.

q 2014 Taylor & Francis

*Corresponding author. Email: ekellis@phed-sr.auth.gr

Computer Methods in Biomechanics and Biomedical Engineering, 2014

http://dx.doi.org/10.1080/10255842.2013.869323

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Several techniques have been developed to measure

knee joint moment-arms (Tsaopoulos et al. 2006).

Techniques may also be two-dimensional (2D) or three-

dimensional (3D). 2D techniques allow an easier

estimation of internal joint forces (Tsaopoulos et al.

2006) while 3D techniques quantify moment-arms by

taking into consideration that the articular surfaces of the

knee are circular in sagital view (Hollister et al. 1993)

while they are cylindrical in reality (Eckhoff et al. 2003).

One may hypothesise that alterations in muscle–tendon

morphology and knee joint kinematics after ACL surgery

may affect their 3D orientation but they are not reflected in

2D data. Therefore, examination of moment-arms in 2D

versus 3D after ACL reconstruction is worthwhile.

If tendon CSA after ACL surgery reconstruction

increases, then one may suggest that the PT moment-arm

decreases. In this case, an individual who underwent ACL

reconstruction would have a mechanical disadvantage, as

for the same amount of muscle force less moment of force

will be exerted around the knee joint. The purpose of this

study was to examine knee muscle moment-arms and CSA

in people who underwent ACL reconstruction with PT

graft compared with controls. A secondary purpose was

to compare moment-arms obtained in 2D with those

recorded in 3D.

Methods

Subjects

Ten males participated in this project, and they were

divided into an ACL and a control (CON) group. The ACL

group consisted of five males (age 40.21 ^ 7.81 years,

mass 82.6 ^ 6.18 kg, height 179.1 ^ 10.22 cm) who

underwent ACL reconstruction with anterior PT tendon

graft. Inclusion criteria were as follows: (i) age between 30

and 55 years; (ii) an isolated ACL rupture with absence of

any injury of other structures, (iii) unilateral ACL

reconstruction with an autograft PT technique, (iv) surgery

occurred 8–12 months prior to this study, (v) followed the

same postoperative rehabilitation by the same medical

team and (vi) no history of neurological disease, or

vestibular or visual disturbance. Exclusion criteria were (i)

contralateral knee pathology or surgery; (ii) any pathology

or surgery in the hips, ankles or feet; (iii) any neurologic,

cardiovascular, metabolic, rheumatic or vestibular disease

and (iv) any other episode of instability after ACL

reconstruction surgery. All patients had undergone

reconstruction in the right knee by the same orthopaedist,

on average 2.3 months after rupture. Four out of the five

participants received post-surgery physiotherapy treat-

ment by the same physical therapy team. This was based

on an accelerated rehabilitation protocol with early weight

bearing and early range of motion (Beynnon et al. 2005).

The CON group (age 43.20 ^ 7.63 years, mass

77.81 ^ 8.89 kg, height 173.60 ^ 6.81 cm) consisted of

five males who were paired with the reconstructed group

participants with regard to age, body mass and height and

activity profile. All subjects volunteered to participate in

this study after signing informed consent forms. Approval

for the experiment was obtained from the local University

Ethics Committee on Human Research in accordance with

the declaration of Helsinki.

Magnetic resonance imaging

Participants lied in supine position with the hip extended

and the knee flexed at 308 (08 ¼ full extension) inside a

Siemens Expert Plus (1.08T) magnetic resonance imaging

(MRI) unit. A body array coil with scout reception in the

axial, sagital and coronal plane and T1-weighted 3D fast

low-angle shot (F.L.A.S.H.) sequence in lateral plane was

used. The right lower limb was scanned from popliteal

fossa until two-thirds of the thigh (approximately 18 cm

above the proximal end of the patella). The MRI functional

reception parameters were echo time (TE) 10ms, repetition

time (TR) 700ms, flip angle (FA) 358, 200 £ 256 pixel

matrix and a 220-mm field of view, 5mm thickness and

1mm interslice gap. Total scanning time was 6min.

The 3D reconstruction of 2D MRI slices was achieved

using the 3D Doctor software (version 4.0; Able Software,

Lexington, MA, USA). In particular, the boundaries of the

PT, BF, ST, semimembranosus (SM), femur and tibia in

every transversal plane image were outlined manually

using a piecewise linear boundary provided by the

software (Figure 1). The border was drawn as a closed

polygon and, after removal of wrongly traced nodes, the

polygon was smoothed by a B-spline closed curve with

application of the maximum smoothing threshold (Pena

and Foote 2008).

The 3D shape of each region was reconstructed by

forming a 3D surface from the boundary data, using the

respective B-spline curve for each cross section (Figure 1).

This rendering uses a variation on tiling and adaptive

Delaunay triangulation algorithms to create vector-based

boundary lines that outline the 2D image (Watt 2000). The

3D coordinates of all triangle vertices together with an

average unit normal vector for each triangle were then

inserted into the Autocad software (version 2009;

Autodesk, Inc., San Rafael, CA, USA) to measure muscle

moment-arms.

The centroid of the contour of each muscle–tendon

unit was calculated in each slice from the enclosed plane

area, and a second-order polynomial function was fitted

through these centroids to form a centroid curve. This

centroid curve was considered as being representative of

the line of action of the muscle.

The moment-arm was taken as the perpendicular

distance from each muscle–tendon line of action to the

knee joint axis of rotation. To estimate the centre of

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rotation, the medial and lateral condyles were fitted with

circles and the centroids of those circles were taken as

representatives of the posterior portion of the condyles

(Buford et al. 1997; Eckhoff et al. 2003). The axis passing

through the two centres was considered as the joint axis

of rotation. In 2D, the midpoint between the centre of the

medial and lateral condyles, when superimposed onto one

another, was considered as the centre of rotation

(Figure 2).

The CSAs of the muscles and the PT were calculated

by transforming the contours of each tissue from pixel

coordinates to physical coordinates with the use of

parameters from the MRI image header and then running a

trapezoidal integration algorithm. The CSAs were

estimated at two points along each muscle–tendon unit:

first, the CSA at the point where the moment-arm

intersects the line of muscle action (CSAmarm) and, second,

the maximum CSA (CSAmax) obtained from the whole

MRI sequence. In a pilot study, a circular object of known

dimensions was scanned and reconstructed to check the

accuracy of the analysis. The results showed a

measurement error of less than 1.5% and 2.1%, for length

and CSA measurements, respectively. In addition, the

MRI scans were reconstructed three times for four

subjects. The coefficient of variance was 1.82%, and

there was no significant difference between the three

measurements ( p . 0.05).

Statistics

A Shapiro–Wilk test showed that data were normally

distributed. Therefore, a two-way analysis of variance

(ANOVA) was used to examine Group and Method effects

on moment-arms after having checked equality of variance

across groups with the Levene’s test and sphericity

problems with the Mauchly’s test. Independent samples t-

tests were used to examine group differences in CSAs. The

level of significance was set at p , 0.05.

Results

The moment-arm values are presented in Table 1. The

ANOVA showed non-statistically significant two-way

interaction effects and a Group main effect on moment-

arm values ( p . 0.05) while moment-arm values differed

significantly between 2D and 3D techniques for all

muscles ( p , 0.05) except the SM ( p . 0.05).

The t-tests showed that the ACL group showed

significantly lower BF CSAmarm and CSAmax and ST

CSAmarm than controls (Figure 3). In contrast, a higher ST

CSAmax in ACL group was found ( p , 0.05). No

differences in PTand SMCSAs were observed ( p . 0.05).

Discussion

Our hypothesis that any alterations in knee joint

kinematics and muscle morphology after ACL surgery

Medial view

Lateral view

BF

ST

SM

SMPT

PT

BFSMST

PT

A B

C

Figure 1. Schematic illustration of raw magnetic resonance image (MRI) data and analysis. (a) Transversal MRI of the femur and theknee illustrating the outlined contours of the biceps femoris (BF), semimembranosus (SM), semitendinosus (ST) and patellar tendon (PT).(b) Reconstructed 3D image of MRI images. (c) Closer view of the 3D reconstructed knee joint and the surrounding muscles.

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would have an effect on muscle moment-arms was not

confirmed (Table 1). To our knowledge, moment-arm

differences between patients who underwent ACL

reconstruction surgery and controls have not previously

reported. Buford et al. (1997) reported that removal of the

ACL did not have an effect on moment-arms of several

muscles. Provided a similar bone alignment, a change in

moment-arm can take place if there are changes in PT CSA

and its alignment relative to the axis of rotation (Sugisaki

et al. 2010). However, our results indicated that neither

line of action nor CSA of the PT differed between groups

(Figure 1). This is in contrast to Reeves et al. (2009) who

reported a 21% higher PT CSA but no differences in

muscle CSA, stiffness and muscle activation between the

operated and non-operated limbs. The above may be

attributed to variability in PT responses after surgery as

well as the exact time where MRI evaluation took place.

Some studies reported that PT defect is indistinguishable

from normal tendon from 3 months (Linder et al. 1994) or

up to 2 years after surgery (Nixon et al. 1995), while others

found that PT thickness remains higher in the operated

side compared with the non-operated side (Wiley et al.

1997; Kartus et al. 2000) up to 10 years after surgery

(Svensson et al. 2004; Liden et al. 2006). However,

bilateral leg differences in PT morphology do not preclude

that individuals after ACL reconstruction would display

similar PT CSA with typical individuals.

The results indicated minor group differences in

hamstring moment-arms (Table 1). It was interesting that

the BF CSAmax was smaller in the ACL group than in the

Table 1. Moment-arm (mm) of the PT and the hamstrings of the patients who underwent ACL and controls.

3D 2D

ACL Controls ACL Controls

PT 51.33 ^ 4.15 51.72 ^ 5.30 54.11 ^ 5.95* 58.87 ^ 8.30*ST 39.86 ^ 7.14 39.16 ^ 7.32 44.66 ^ 8.40* 49.14 ^ 6.15*SM 29.25 ^ 4.82 33.37 ^ 6.86 33.36 ^ 6.32 32.46 ^ 6.26BF 25.64 ^ 4.51 30.88 ^ 4.10 27.32 ^ 4.22* 34.05 ^ 4.60*

Note: *Significantly differed compared with 3D values.

A B

PT

MedialLateral

Figure 2. Estimation of pattelar tendon (PT) moment-arm. Medial and lateral condyles were fitted with circles and the axis passingthrough the centres of the condyles was considered as the centre of joint rotation in two and three dimensions. (a) Posterior view and (b)sagital view.

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control group while the opposite was observed for the

ST CSAmax (Figure 3). To our knowledge, changes in

hamstring muscle morphology after ACL reconstruction

with a PT bone graft have not been previously

investigated. Some evidence indicates that ACL recon-

struction with a hamstring graft resulted in 50% reduction

in ST CSA and an increase in BF CSA (Snow et al. 2012),

which indicates that there are variable responses in

muscle–tendon morphology between the hamstrings.

However, hamstring muscle strength deficits after PT

surgery are not very high (Dauty et al. 2005). Changes in

hamstring morphology may be linked with the rehabilita-

tion protocol followed after surgery.

Although the ACL group showed a higher ST CSAmax

than controls, the opposite was found for ST CSAmarm

(Figure 3). Previous research indicates that training

adaptations of muscle CSA are negatively correlated

with those of tendon CSA (Seynnes et al. 2009).

Furthermore, adaptations vary along the tendon length

(Seynnes et al. 2009) and this may be related to variations

in local stresses, loading and material properties

(Kongsgaard et al. 2007). As the ST distal tendon is

longest from the remainder hamstrings (Kellis et al. 2012),

it may be possible that the ST tendon shows more variable

responses to post-surgery treatment and ACL surgery

compared with the other hamstrings as well as with the ST

muscle belly. This indicates that adaptations after ACL

surgery and rehabilitation training may differ not only

between the hamstring muscles but also within each

muscle–tendon unit.

Changes in femoral condyles orientation relative to the

muscle–tendon line action may also affect moment-arms.

A posterior shift of the centre of the lateral condyle ACL

surgery has been reported (Logan et al. 2004; Carpenter

et al. 2009). In this case, the joint axis of rotation may

change towards the posterior femoral side which, in

theory, would affect moment-arms. However, our results

did not confirm this suggestion (Table 1). This might

indicate either that the posterior shift of the centre of

medial femoral condyle in the ACL group was not

significant enough to alter moment-arms or that any effects

of this posterior shift have been masked by changes in

orientation and CSA of the muscle–tendon units.

The results of the study indicated that there are no

differences in PT, ST and BF moment-arms between 2D

and 3D techniques (Table 1). Differences between various

techniques of moment-arm measurement have been

discussed (Tsaopoulos et al. 2006) and could mainly be

attributed to the fact that the PT line and the centre of axis

of rotation are not tracked close to the surface of sagital

plane but in the inner area of the joint. Nevertheless, within

the limitations of the study, it appears that 2D data yielded

similar group differences as 3D data. It could be

hypothesised that group differences in 3D orientation of

joint and muscle structures were not significant enough to

differentiate sagital (2D) scans from 3D ones.

This study had several limitations. First, small sample

size precludes generalization of the present findings. For

example, the BF moment-arm was almost 20% smaller in

the ACL group compared with controls (Table 1). Such a

difference indicates that, subject to sufficient sample size,

group differences might be statistically significant. Small

sample size was mainly due to two factors: first, surgeries

with PT graft are not as frequent as those performed with

other grafts and, second, patients had to be supervised by

the same physical therapy team so that they are included in

this project. Second, evaluation of MRI was performed at a

single joint angular position and with the subject at rest

(Tsaopoulos et al. 2006). A change in knee joint angle and

level of effort affects the orientation of the tendon and the

position of the bones and ligaments relative to one another

with a direct effect on moment-arm (Tsaopoulos et al.

2006). In addition, evidence indicates that ACL recon-

struction effects on patellofemoral contact area vary

depending on joint angle (Shin et al. 2009) while

increasing contraction intensity increases moment-arms

by 20–22% (Tsaopoulos et al. 2006; Sheehan 2007).

Based on the above fact, there is a possibility of a different

0

50

100

150

200

250

300

PT SM ST BF

0

500

1000

1500

2000

PT SM ST BF

mm

2m

m2

CSAmarm

CSAmax

* *

*

*

ACLCON

ACLCON

Figure 3. (a) Maximal cross-sectional area (CSAmax) and (b)cross-sectional area at the point where the moment-arm intersectsthe line of muscle action (CSAmarm) of the biceps femoris (BF),semimembranosus (SM), semitendinosus (ST) and patellartendon (PT) in the ACL reconstruction (ACL) and control(CON) group (significantly different compared with controlgroup at p , 0.05).

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orientation of all associated structures at different joint

angles and intensity levels between groups.

Conclusions

This study showed that the PT and hamstrings moment-

arms were not different in a group of individuals who

underwent ACL reconstruction with a PT graft compared

with controls. Patients showed similar CSAs of PT and SM

and different BF and ST CSAs than controls. Finally, 3D

moment-arm values for ST, BF and PT were smaller than

corresponding 2D values.

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