KNEE

Graft size after anterior cruciate ligament reconstruction

Daniel Hensler • Motoko Miyawaki • Kenneth D. Illingworth •

Carola F. van Eck • Freddie H. Fu

Received: 7 January 2013 / Accepted: 24 August 2013 / Published online: 1 September 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract

Purpose The native anterior cruciate ligament (ACL) is

composed of two distinct bundles, the anteromedial (AM)

and posterolateral (PL), and both have been shown to be

reliably measured on magnetic resonance imaging (MRI).

The purpose of this study was to measure the size of the

AM and PL bundles after ACL double-bundle reconstruc-

tions on MRI and compare this to the relative graft size at

the time of surgery.

Methods Between January 2007 and April 2010, 85 knees

were identified after allograft double-bundle ACL recon-

struction with post-operative MRI (1.5 T) and met inclu-

sion criteria. On standard sagittal, coronal and oblique

coronal MRIs, the AM and PL bundles were delineated and

the midsubstance width of the ACL graft was measured.

The images were independently measured in a blinded

fashion by two observers. Linear and curvilinear regression

analysis was used to analyse the relationship between graft

size and time after reconstruction.

Results The mean age of the patients was 24.6 years (SD

10.4). Mean time from surgery to post-operative MRI was

271.5 days (SD 183.4). The mean percentage of the ori-

ginal size of the AM bundle was 86.9 % (SD 9.9) and of

the PL bundle was 88.6 % (SD 9.9). There was no corre-

lation between the relative size of the AM graft and the

time from surgery (r = 0.3, n.s.) and no significant rela-

tionship for the PL graft (r = 0.1, n.s).

Conclusion On average, there was no graft enlargement

of the AM and PL grafts 275.1 days after allograft ACL

double-bundle reconstruction, as the mean relative graft

size was less than 100 % on MRI. This study suggests that

surgeons, who use allografts, should measure the ACL and

replace it with a similar size, as there is a low risk of

hypertrophy of the graft within one year post-operative.

Level of evidence IV.

Keywords Anterior cruciate ligament � Double

bundle � Post-operative graft size � Allograft �Magnetic resonance imaging

Introduction

Recently, more focus has been placed on the anatomy of

the anterior cruciate ligament (ACL) including its two

bundles, the anteromedial (AM) and posterolateral (PL),

named based on their tibial insertions [3, 15, 16]. Recon-

structing these bundles in a double-bundle (DB) ACL

reconstruction has become a widely accepted anatomical

technique. Additionally, individualised anatomical recon-

struction has become more important with placement of the

tunnels in the ACL footprint as well as maximally restoring

the size of the original ACL after intra-operative mea-

surement of its morphology [13, 23, 24, 33, 36, 40].

Although the insertion site size has been a topic of

multiple studies [26], the size of the midsubstance of the

graft has largely been neglected. Anatomical ACL recon-

struction techniques aim to restore the footprint of the ACL

[37]. However, it has not been evaluated if the midsub-

stance dimension of the ACL is also restored.

D. Hensler � M. Miyawaki � K. D. Illingworth �C. F. van Eck � F. H. Fu (&)

Department for Orthopaedic Surgery, University of Pittsburgh

Medical Center, 3471 Fifth Avenue, Suite 1011, Pittsburgh,

PA 15213, USA

e-mail: ffu@upmc.edu

D. Hensler

Department of Trauma Surgery, Trauma Center Murnau,

Murnau, Germany

123

Knee Surg Sports Traumatol Arthrosc (2014) 22:995–1001

DOI 10.1007/s00167-013-2653-2

Measurements on cadavers have identified the AM bundle

to be approximately 7.1 mm in diameter with respect to its

midsubstance. The PL bundle has been measured to be

slightly smaller in diameter (6.7 mm) compared to the AM

bundle. The appearance of the two bundles on magnetic

resonance imaging (MRI) has been described in a cadav-

eric study by Steckel et al. [34]. In their study, MRI was

performed on cadaveric knees, and using oblique sagittal

and oblique coronal planes, they were able to distinguish

the double-bundle structure of the ACL. Using MRI, the

appearance of the AM and PL bundles of the ACL has been

further characterised and their size has been shown to be

reliably measured using standard MRI sequences and

planes [4, 10, 30].

After ACL reconstruction, the graft undergoes a com-

plex remodelling process which is affected by the physi-

ology and biomechanics of the implanted graft [2, 5].

Regardless of the graft used, all intra-articular graft seg-

ments undergo a similar process of remodelling. However,

this graft incorporation and remodelling has been shown to

be slower for allografts compared to autografts [21, 44].

Several studies have investigated the appearance of the

graft over time using MRI with plain and gadolinium-

diethylenetriamine pentaacetic acid (Gd-DTPA) enhanced

imaging [19, 22, 28, 43] and deduced on the phase of

remodelling of the graft. Although histological and bio-

mechanical changes have been thoroughly investigated,

less is known about the influence of hypertrophy of the

graft after surgery. This concept is critical as hypertrophy

of the graft can lead to impingement that not only affects

the range of motion of the knee but also the healing and

remodelling of the graft [18].

The purpose of this study was to investigate the change

in size of the AM and PL bundles after allograft DB ACL

reconstruction on MRI when compared to the original graft

size measured intra-operatively. We hypothesise that dur-

ing the first 12 months after surgery, there is no hypertro-

phy of the graft when compared to the original graft size at

the time of surgery.

Materials and methods

Between 2007 and 2010, 140 knees in 128 subjects after

double-bundle ACL reconstruction with allografts and

clinically available MRIs were identified for this study.

Subjects were excluded if they underwent single-bundle

reconstruction, autograft tissue was used, there was evi-

dence of a re-tear at the time of the MRI, the AM and PL

bundles could not be fully visualised or if concomitant

knee injuries were present. Eighty-five knees met inclusion

criteria. The patient’s age and the time between surgery

and MRI were documented for all subjects.

All patients underwent MRI imaging of the knee using a

1.5-T magnet (GE Signa; GE Healthcare, Waukesha,

Wisconsin) as part of their standard clinical care. Multi-

planar multisequence imaging was obtained through the

knee including: (1) axial proton density fat saturation (FS),

(2) sagittal proton density, (3) sagittal T2 FS, (4) coronal

T1, (5) coronal T2 FS and (6) proton density coronal

oblique (dedicated double-bundle sequence) and axial 3D

gradients through the patellofemoral joint cartilage. T1-

and T2-weighted images were analysed using 3 mm slice

thickness. Sagittal, coronal and coronal oblique sequences

were used for measurement analysis.

The AM and PL bundles were delineated on the sagittal,

coronal and oblique coronal MRI sequences. In our

department, the sagittal images are based on the course of

the bundles, mainly of the AM bundle. Based on the doc-

umented literature, in the sagittal plane, the AM bundle

was defined as the oblique fibres inserting anterior of the

two bundles on the tibia and the proximal aspect of the

femoral insertion on the lateral femoral condyle. Similar to

the AM bundle, the PL bundle was defined as the oblique

fibres inserting posteriorly on the tibial insertion and on the

distal aspect of the femoral insertion on the lateral femoral

condyle.

AM and PL measurements

All measurements were taken on a digital radiology

viewing programme (Stentor; Philips Healthcare, Andover,

Massachusetts) with a measurement accuracy of 0.1 mm. A

single image was selected that best visualised the intra-

articular portion of the AM bundle, and this was repeated

for the PL bundle. A line was drawn along the fibres of the

anterior portion of the graft. In the midsubstance area of the

graft, the width of the graft was measured perpendicular to

the first line. The width of the AM and PL bundles was

measured separately in each plane, sagittal, coronal and

coronal oblique, for a total of 3 mean measurements per

bundle (Fig. 1). This width was then divided by the size of

the allograft used during surgery, to express the size of the

graft as a percentage of the original graft size. The average

of the relative graft size in each of the three planes was

calculated and additionally used for further analysis.

The images were independently measured in a blinded

fashion by two observers: one orthopaedic sports medicine

research fellow and one experienced orthopaedic attending

surgeon. The physicians were blinded to each other’s

measurements and to the intra-operative graft size. The

digital MRI system adjusts accurately to physical data

points, which eliminates inherent system error to the

measurements.

For this study, we obtained IRB approval to use data in

the research registry (University of Pittsburgh, Institutional

996 Knee Surg Sports Traumatol Arthrosc (2014) 22:995–1001

123

Review Board, IRB-Number: 10070199). All patients

presenting to our institution’s sports medicine clinic are

asked to enrol in an Institutional Review Board (IRB)-

approved research registry, which permits use of clinical

data for subsequent clinical research. This allowed us to

prospectively collect patient imaging as part of routine

clinical practice.

Statistical analysis

Data were analysed using SPSS version 16 (SPSS Inc,

Chicago, Illinois). Descriptive statistics including mean,

range and standard deviation were calculated for the col-

lected demographic variables of the included subjects. The

mean, range and standard deviation of the width of the AM

and PL bundles in the sagittal, coronal and coronal oblique

planes were calculated. Subsequently, graft size at the time

of MRI was divided by the original graft size used during

surgery to obtain a percentage. This percentage of graft

hyper/hypotrophy was calculated for the three different

MRI sequences, and the mean of these percentages was

used for further statistical analysis. Linear and curvilinear

regression analysis was used to determine correlation

between post-operative graft size and the time from surgery

with a statistically significance of p \ 0.001. Intra-class

correlation coefficients (ICC) were calculated to determine

inter- and intra-observer reliability for the first 30 subjects

measured. The mean of the measurements of the first and

second observer was used for the statistical analysis of the

data after satisfactory ICC values were obtained. For intra-

observer reliability, the grafts were re-measured after

4 weeks, and for interobserver reliability, this period was

3 weeks.

Results

The mean age was 24.6 years (SD 10.4), and mean time

from surgery to MRI was 271.5 days (SD 183.4). The

mean, range and standard deviation of the relative AM and

PL graft sizes expressed as a percentage of the original size

for the sagittal, coronal and coronal oblique plane as well

as for the overall average of all planes are displayed in

Table 1. The mean graft size after surgery was smaller than

the size of the allograft that was used for both the AM and

PL bundles. Overall, the relative size of the AM bundle

was 86.9 % (SD 9.9) and of the PL bundle was 88.6 % (SD

9.9).

Linear and curvilinear regression analysis was per-

formed between the graft size and the time from the sur-

gery to the MRI. Linear regressions showed the best fit and

demonstrated that there was no significant correlation

between both the mean AM and PL bundle sizes and the

time from surgery in all three planes (Fig. 2).

Inter-observer reliability for measurement of the AM

and PL bundles were 0.88 (95 % CI 0.71–0.95) and 0.84

(95 % CI 0.69–0.92), respectively. Intra-observer reliabil-

ity for measurement of the AM and PL bundles were 0.93

(95 %CI 0.87–0.97) and 0.88 (95 %CI 0.78–0.94),

respectively.

Discussion

The most important finding of the present study was that on

average, there was no hypertrophy of the AM and PL grafts

on MRI after double-bundle ACL reconstruction up to one

year after surgery, which is consistent with the hypothesis

of this study. The averaged relative graft size measured as a

Fig. 1 Measurement methods of the graft size a AM/b PL: T1-weighted image: Measurement of the width (yellow) of the AM and PL bundles

on a coronal oblique image. c T2-weighted image: measurement of AM and PL bundles on one image

Knee Surg Sports Traumatol Arthrosc (2014) 22:995–1001 997

123

percentage of the original allograft size was less than

100 % in both AM and PL bundles, being 86.9 and 88.6 %,

respectively.

Regression analysis showed no influence of time from

surgery on graft size for the AM and PL bundles

(p \ 0.001). Regarding the remodelling of the graft, the

biological response of the grafted collagenous tissue is

directly related to the biomechanical and biochemical

environment into which the graft is placed [20]. Graft

remodelling endures a lengthy process and the transplanted

graft often loses its structural and biomechanical properties

during this biological incorporation [2, 29, 32, 38, 42, 44].

Beynnon et al. [8] showed in a study of 26 dogs that one

year after ACL reconstruction, the graft still had inferior

biomechanical strength in comparison with the native

ACL. Weiler et al. [38] investigated the relationship

between biomechanical and histological properties of a

graft and its appearance on MRI and showed inferior bio-

mechanical properties after 2 years even though the his-

tological appearance of the graft was comparable to the

native ACL. The same study showed a graft hypotrophy

after 6 weeks, with an increase in the cross-sectional area

of up to 107 % of the native ACL after 12 weeks followed

again by a decrease in the midsubstance cross-sectional

area after the 12 weeks time point for the rest of the time

frame. Furthermore, the significant difference in MRI

appearance of the graft after 2 years compared to the native

ACL indicated that even 2 years post-operatively a certain

amount of graft remodelling is still ongoing.

In contrast to Weiler et al. [38], in the present study, an

average diameter of the graft of less than 100 % on MRI of

the original graft diameter in the entire observed time

frame was found and evidence of hypertrophy after

12 weeks was not observed, although a few single mea-

surement showed a increased graft width when compared

to the intra-operative measured graft size. One reason

might be that results of animal studies may not be com-

pletely carried over to human models [7]. Rougraff et al.

[31] reported that between 3 and 8 weeks after transplan-

tation, a substantial portion of the graft remained histo-

logically similar to patellar tendon tissue which might lead

to the suggestion that a large proportion of the original

tendon survives and that ACL graft healing in humans may

not undergo the same complete necrotic stage that has been

reported to occur in animals. However, it needs to be

pointed out that our measurements were conducted on

allografts and our results can unconditionally take over on

measurements of autografts.

In some scenarios, knowledge of the size of the ham-

strings might be helpful, especially if the hamstrings are

too small, the surgeons might have to rely on allografts.

Beyzadeoglu et al. [9] and Wernecke et al. [39] verified

that preoperative measurements of the size of the ham-

strings can be used to predict the possible graft sizes. In

case of potentially small grafts, allografts might be used

during initial surgery and, therefore, our results can add

important information to the knowledge of allograft’s

behaviour over time.

Furthermore, the placement of the grafts is managed in a

different manner, and it is known that graft placement

directly affects the biological reaction and therefore the

remodelling of grafts [11]. The different placement and

orientation of the 2 bundles leads to different mechanical

stresses during rehabilitation and is intimately linked to the

remodelling process [11]. In different studies, the benefi-

cial effect of an additional PL bundle could be shown as it

not only restores the anatomy and kinematics but also

restores the native insertion sites [1, 25, 35, 37, 41]. The PL

and AM bundles act synergistically. The PL bundle is

tensioned in full extension where it helps to restore rota-

tional and anteroposterior stability and it gets loosen when

the knee is flexed where it allows rotation [6, 14]. The AM

bundle is less tight in knee extension and has its main

function in knee flexion where it is more tightened and

mainly stabilizes against anteroposterior translation [3].

Therefore, a different appearance of the 2 grafts on the

MRI can be assumed; however, we could not verify a

significant difference between the two bundles regarding

the graft size over time.

The measurements of the graft sizes are performed using

MRI which can lead to an inaccuracy when measuring a

three-dimensional structure in a two-dimensional fashion.

However, for verification of the measurements, an addi-

tional surgery would have been necessary, and therefore,

non-invasive MRI provides the most logical method for

determining graft size after ACL reconstruction.

In this study, measurements were conducted on different

MRI planes (coronal and sagittal) and in addition on

Table 1 Descriptive analysis of the size of the AM and PL on each image

N = 85 Sagittal width (%) Oblique width (%) Oblique coronal width (%)

AM PL AM PL AM PL

Mean ± SD 87.2 ± 11.3 88.5 ± 11.4 86.9 ± 10.4 88.5 ± 10.5 86.7 ± 11.4 88.8 ± 10.3

Range 53.8–111.4 57.1–116.7 66.3–118.6 63.3–122 55–114.3 57.1–116.7

The size of each bundle on MRI (%)

998 Knee Surg Sports Traumatol Arthrosc (2014) 22:995–1001

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coronal oblique images where the ACL can be outlined in

the best and easiest fashion [12, 17]. Measurements on

sagittal oblique images were taken as the sagittal images,

used in this study, were already based on the course of the

graft. We believe that these additional planes can help

surgeons to evaluate the native ACL as well as the grafts

after ACLR more accurately.

Further limitation of our study is that only intact grafts

were measured and patients with graft tears were excluded.

Therefore, conclusions on the use of the MRI detecting

ACL graft pathologies cannot be made. Hypertrophic grafts

might re-rupture faster due to roof impingement, and this

should be investigated in future studies.

Another limitation of this study was that the cohort

consisted of subjects who underwent MRI of the graft at

different time point, rather than following the same sub-

jects over time with sequential MRI. Therefore, this study

does not take into account the individual changes in graft

size that can occur over time; however, subject-to-subject

variation could be eliminated with this method.

Finally, we only conducted the measurements on

patients who underwent ACL reconstruction with an

allograft. The remodelling process of allografts compared

to autografts takes longer. Jackson et al. [21] investigated

the remodelling behaviour of similar sized auto- and

allograft in 40 goats and showed that autografts demon-

strated a smaller increase in anterior–posterior displace-

ment with better biomechanical properties, a significant

increase in cross-sectional area, a more rapid loss of

large-diameter collagen fibrils and an increased density

and number of small-diameter collagen fibrils compared

to the allografts. This might lead to a different course of

the graft size over time and the size of autografts post-

operatively on MRI; therefore, this should be measured in

future studies.

In addition, the type of graft seems to be important

regarding the remodelling process whether it has a synovial

coverage or not. Mayr et al. [27] showed in an animal study

that load to failure of synovialised grafts is significantly

lower when compared to non-synovialised grafts. How-

ever, autografts were used in this animal study, and

knowledge of the different behaviour of ligamentisation of

the different grafts might be taken over to clinical practice.

A statement on different graft sizes has not been made yet.

To restore anatomy and the biomechanics of the ACL

and to improve clinical outcome, it is not only important to

place the graft in the right position, but also to improve our

understanding of the biology of the graft and the remod-

elling process. Understanding maturation and remodelling

of the graft is critical for surgeons to make appropriate

graft choices for their patients. Nevertheless, to the

authors’ knowledge, a study that investigates the graft size

after ACL reconstruction over time in a clinical setting is

missing and our study can add knowledge of the behaviour

of allografts after ligament reconstruction.

In addition, measurements for both bundles in all 3

planes, which is easy and fast, could be established and

might be performed in the clinical work.

Fig. 2 Correlation between graft size and time from surgery. Each

image has no correlation between size of the AM/PL bundle and time

from surgery

Knee Surg Sports Traumatol Arthrosc (2014) 22:995–1001 999

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Our study suggests that, in cases where an allograft is

used, the ACL size should be measured and replaced with a

similar size as there is a low risk of hypertrophy of the

graft. Further studies will be necessary to investigate post-

operative graft size and remodelling in single-bundle

reconstructions as well as in the use of autografts.

Conclusion

In conclusion, MRI is a valuable tool in ACL surgery.

Accurate visualisation of both bundles of the ACL can be

achieved by the use of special MRI protocols. Measure-

ments of both bundles on MRI are reliable and feasible to

be performed in daily clinical work. The morphology of the

two bundles can be determined on MRI.

Conflict of interest The authors declare that they have no conflict

of interest.

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