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The Knee 13 (200

The tibial tuberosity–trochlear groove distance; a comparative

study between CT and MRI scanning

Philip B. Schoettle a,T, Marco Zanetti b, Burkart Seifert c, Christian W.A. Pfirrmann b,

Sandro F. Fucentese a, Jose Romero a

aOrthopedic Department, University of Zurich, Uniklinik Balgrist, Forchstr. 340, 8008 Zurich, SwitzerlandbRadiological Department, University of Zurich, Uniklinik Balgrist, SwitzerlandcInstitute for social and preventive medicine, University of Zurich, Switzerland

Received 9 May 2005; accepted 6 June 2005

Abstract

CT scan is the gold standard for the measurement of the tibial tuberosity–trochlear groove distance (TTTG). The aim of this study was to

evaluate the reliability of the TTTG on MRI compared to CT scan. Twelve knees in 11 patients underwent CT and MRI examination for

patellofemoral instability or anterior knee pain. Both the bony and the cartilaginous landmarks of the trochlear groove were used for the

measurement of the TTTG. The measurements were performed by two experienced musculoskeletal radiologists. The interrater, intermethod

and interperiod reliability was calculated using a restricted maximum likelihood estimation and a Bland–Altman analysis. The mean TTTG

referenced on bony landmarks was 14.4T5.4 mm on CT scans, and 13.9T4.5 mm on MR images. The mean TTTG referenced on

cartilaginous landmarks was 15.3T4.1 mm on CT scans, and 13.5T4.6 mm on MR images. An excellent interrater (82%), intermethods

(86%), and interperiod (91%) quantitative reliability was found. TTTG can be determined reliably on MRI using either cartilage or bony

landmarks. Additional CT scans are not necessary.

D 2005 Published by Elsevier B.V.

Keywords: Knee; Patellofemoral; Instability; TTTG; Trochlear dysplasia; MRI; CT

1. Introduction

Patellar instability is a common orthopaedic problem

and precise diagnosis is mandatory for optimal treatment.

Patellofemoral disorders can be categorized into five

types;

(a) Patellar instability with dislocation or subluxation

associated with trochlear dysplasia

(b) Patellar instability with dislocation or subluxation

associated without trochlear dysplasia

(c) Patellofemoral pain syndrome without dislocation,

associated with trochlear dysplasia

0968-0160/$ - see front matter D 2005 Published by Elsevier B.V.

doi:10.1016/j.knee.2005.06.003

T Corresponding author.

E-mail address: Philip.schoettle@charite.de (P.B. Schoettle).

(d) Patellofemoral pain syndrome without anatomical

abnormalities

(e) Isolated patellofemoral arthritis.

One of the anatomically relevant factors for these

disorders is a lateralised tibial tuberosity, or a medialised

trochlear groove in the case of trochlear dysplasia [1,2].

Bland [8] first described the measurement of the distance

between tibial tuberosity and the trochlear groove (TTTG).

This measurement is obtained from CT scanning and is

abnormal in 56–93% of cases with patellar instability [3]

and nearly all cases with trochlear dysplasia [1,2]. An

excessive TTTG can be used as the indication for a distal

realignment procedure such as medialising the tibial

tubercle [2].

Patellofemoral pain syndromes may require assessment

of the cartilage and the soft tissue and ligamentous

6) 26 – 31

Fig. 1. a and b. A line through the deepest point of the trochlea perpendicular to the posterior epicondyle tangent is drawn. (a: CT scan, b: MRI).

P.B. Schoettle et al. / The Knee 13 (2006) 26–31 27

structures of the patellofemoral joint [1,4]. This is most

suitably done by MRI scanning.

If the TTTG distance could be reliably measured on MR

images, CT scans could be avoided. However, measurement

methods for the TTTG distance have not been established

on MR images. On CT scans, only the bony architecture of

the trochlear groove is considered. The architecture of the

cartilaginous surface of the trochlea may however have a

more important impact on the maltracking of the patella [5].

The aim of this study was to evaluate not only the bony [6],

but also a ‘‘cartilagino-tendinous’’ TTTG distance on both

CT and MRI scanning, using the patellofemoral gliding

surface and the centre of the patellar tendon as landmarks.

2. Patients and methods

2.1. Patients

CT and MRI scans were performed with axial sections from the

bony-cartilaginous border at the femoral site down to and with the

Fig. 2. a and b. Tibial sequence: a line parallel to the trochlea line through the m

parallels (arrow) describes the TTTG.

tibial tuberosity on 12 consecutive knees (10 female, 2 male) in 11

patients. All patients had a history of patellar instability (n =10) or

anterior patellofemoral pain syndrome (n =2). The prospective

evaluation of patient imaging data and records was approved by the

local ethics committee. Patient informed consent was not required

for the retrospective review.

2.2. Imaging

All examinations were performed with the same CT scanner

(Somatom Plus 4, Siemens Medical Solutions, Erlangen, Germany)

and the same MRI scanner (Siemens 1.5 T Symphony, Siemens

Medical Solution, Germany). The CT scans were performed first

on a single detector CT system using a bone algorithm (section

thickness of 3 mm, 120 kV, 146 mA).

The standard knee MRI examination protocol (sagittal inter-

mediate and T2 weighted, coronal T1-weighted, coronal STIR

sequence, and axial gradient echo sequence covering the patellar

cartilage) was adapted by adding a T1 weighted turbo-spinecho

axial sequences covering the entire trochlea femoris and tibial

tuberosity. Section thickness was 3 mm, distance factor of 50%,

TR/TE 625/20 ms, matrix 512�512, and FOV 160 mm).

ost anterior portion of the tibial tuberosity. The distance between the two

Fig. 3. a and b. On the most cranial femoral sequence with complete cartilaginous coverage of the trochlea a perpendicular line through the deepest point of the

cartilaginous trochlea to the tangent to the cartilaginous posterior epicondyles is drawn.

P.B. Schoettle et al. / The Knee 13 (2006) 26–3128

Bony TTTG was assessed between the most anterior point of

the tibial tuberosity and the deepest bony point of the trochlear

groove, perpendicular to the tangent to the bony borders of the

posterior condyles on axial CT scans as described by Goutallier

and Bernageau [6]. The lines first drawn on the axial section

tangential to the posterior femoral condyles (bl) and a perpendic-

ular line through the deepest point of the trochlea (Fig. 1a) were

transferred to the axial section with the most anterior point of the

tibial tuberosity, and a perpendicular line to the baseline (bl)

through the most anterior point of the tibial tuberosity was drawn.

Then the distance between the two perpendicular lines (bony

TTTG) was measured in millimetres (Fig. 2a). The same measure-

ments were performed on the MR images (Figs. 1 and 2b).

Cartilaginous-tendon TTTG was assessed on MR images

between the deepest cartilaginous point of the trochlear groove

and the centre of patellar tendon insertion on the tibial tuberosity.

The first craniocaudal image that depicted complete cartilaginous

trochlea was used to determine the deepest point within the

trochlear groove and the insertion centre of the patellar tendon to

the tibial tuberosity was determined on the image with a complete

attachment of the patellar tendon to the tuberosity. Therefore the

Fig. 4. a and b. A middle vertical line between the two external points of the patell

the patellar tendon.

axial image through the femur was evaluated and a tangent to the

cartilaginous border of the posterior condyles was set and a

perpendicular line to the deepest point of the cartilaginous trochlea

was drawn (Fig. 3a). These lines were transferred to all axial

images. The first craniocaudal sequence with complete attachment

of the patellar tendon was evaluated. Between the two most

external insertion points of the patellar tendon, a line was drawn

and a mid-vertical line was set through the tuberosity to determine

the centre of the patellar tendon insertion (Fig. 4a). Then a

perpendicular line was drawn to the epicondylar tangent through

this point and the TTTG was measured in the way as mentioned

above (Fig. 5a). The same measurements were performed on CT

scans (Figs. 3b–5b).

All pictures measurements were performed by two experienced

musculoskeletal radiologists (M.Z., 10 years and C.P., 6 years)

using the PACS* system.

During all analysis procedures, M.Z. and C.P. were blinded to

each other. All data were compared to each other, using a paired t-

test and an interrater, intermethods (CT vs. MRI) and interperiod

(bony vs. cartilaginous TTTG) reliability was evaluated, using

SPSS 11.0.

ar tendon insertion to the tuberosity is drawn to mark the insertion centre of

Fig. 5. a and b. A line parallel to the trochlear line through the patellar tendon centre is drawn and distance between the two lines (arrow) is measured (TTTG).

P.B. Schoettle et al. / The Knee 13 (2006) 26–31 29

2.3. Statistical analysis

Mean bony and cartilaginous TTTG values and SDs were

calculated separately for all knees; a paired t-test was used to

determine the significance of the differences ( p <0.05).

The interrater, interperiod (bony vs. cartilaginous TTTG), and

intermethods (CT vs. MRI measurement) reliabilities were

computed using restricted maximum likelihood estimation in a 4-

way random analysis of variance using SPSS (SAS; SPSS; Cary,

NC). An interrater reliability is the ratio of variance components

not influenced by the rater to the sum of all variance components.

An interperiod reliability is the ratio of variance components not

influenced by the period to the sum of all variance components. An

intermethods reliability is the ratio of variance components not

influenced by the method to the sum of all variance components.

The reliability is rated as excellent/acceptable, if the result is equal

to or more than 80% [7]. Finally to determine whether the TTTG

predicted with CT scans is exchangeable with the observed TTTG

in the MRI for the bony as well as for the cartilaginous (functional)

TTTG we used the Bland–Altmann plot [8].

Bland-Altmann: bony CT vs. bony MRI

bo

ny:

CT-

MR

I TT

TG

5

4

3

2

1

08 12 16 20 24

-1

-2

3. Results

The mean bony TTTGTSD was 14.4T5.4 mm as determined

on CT scans, and 13.9T4.5 mm, as determined on MRI. Mean

cartilaginous TTTGTSD was 15.3T4.6 mm as determined on CT

scans, and 13.5T4.1 mm, as determined on MRI (Table 1). There

was a significant difference in mean cartilaginous TTTG,

obtained on CT scan and on MRI ( p =0.006) only. There was

an excellent interrater reliability of 82%, interperiod reliability of

91% and intermethods reliability of 86%. The Bland–Altman

plots are presented in Figs. 6–9. The differences between the

observed and the predicted TTTG showed results inside the limit

of agreement (biasT2SD) for all cases (12 of 12) in all matched

groups (Figs. 6–9).

Table 1

Mean bony and cartilaginous TTTG in mm, obtained by CT and MRI

TTTG: Bony CT Bony

MRI

Cartilaginous

CT

Cartilaginous

MRI

mean valueTSD: 14.4T5.4 13.9T4.5 15.3T4.6 13.5T4.1

The limits of agreement were 0.5T3.9 mm for the TTTG

measurement, obtained by the bony CT in comparison to those

obtained by the bony MRI (Fig. 6). The limits of agreement were

1.8T3.7 mm for the data obtained by the cartilaginous CT in

comparison to those, obtained by the cartilaginous MRI (Fig. 7).

The limits of agreement were �0.9T3.0 mm for the bony CT

measurements in comparison to those, obtained by the functional

CT (Fig. 8). The limits of agreement were 0.4T1.6 mm for the bony

MRI results in comparison to those, obtained by the functional MRI

(Fig. 9). Thus all differences in all measurements were within 3.4

mm, except in one case, where the difference was 5.6 mm, when

comparing the CT and MRI functional TTTGs (Fig. 7).

4. Discussion

Several anatomical factors such as a trochlear dysplasia

are associated with an increased TTTG and predispose for

patellofemoral instability [2,3,9,10]. The gold standard

bony: mean CT, MRI TTTG

-3

-4

Bias +2SD Bias -2SD diff. bony CT vs bony MRIBias

Fig. 6. Bias: +0.48, Bias +2SD: +4.34, Bias �2SD: �3.4.

Bland-Altmann: funct. CT vs. funct. MRI

functional: mean CT, MRI TTTG

fun

ctio

nal

: CT-

MR

I TT

TG

5

4

3

2

1

08 12 16 20 24

-1

-2

-3

6

Bias +2SD Bias -2SD diff. funct. CT vs funct. MRIBias

Fig. 7. Bias: 1.82 mm, Bias +2SD: 5.56 mm, Bias �2SD: �1.93 mm.

Bland-Altmann: bony MRI vs. funct. MRI

MRI: mean bony, funct. MRI

MR

I: b

ony

-fu

nct

. MR

I

-1,5

2,5

2

1,5

1

-1

8 12 16 20

0,5

0

-0,5

Bias +2SD Bias -2SD diff. bony MRI vs funct. MRIBias

Fig. 9. Bias: 0.44 mm, Bias +2SD: 2.05 mm, Bias �2SD: �1.17 mm.

P.B. Schoettle et al. / The Knee 13 (2006) 26–3130

method for measuring TTTG distance is by CT scan [6].

However Saudan and Fritschy [11] showed an interrater

reliability of less than 60% in TTTG determination by this

method. As determination of the deepest point within the

trochlear groove is difficult in severe trochlear dysplasia,

Julliard [12] suggested another technique to determine

TTTG in those cases. They used the centre of the patellar

tendon insertion and estimated the cranial trochlear groove

point by overlying images of the trochlea.

Severe trochlear dysplasia can be treated by trochleo-

plasty [1,4,13]. As osteochondral degeneration of the

Bland-Altmann: bony CT vs. funct. CT

CT: mean bony, funct. TTTG

CT:

bo

ny-f

un

ct. T

TT

G

-4

3

2

1

08 12 16 20 24

-1

-2

-3

Bias +2SD Bias -2SD diff. bony CT vs funct. CTBias

Fig. 8. Bias: �0.90 mm, Bias +2SD: 2.06 mm, Bias �2SD: �3.87 mm.

trochlea may be a contraindication for trochleoplasty,

such defects have to be assessed preoperatively, e.g. by

MRI [4].

It is therefore, desirable to have an one-step imaging

technique which addresses the TTTG and the state of the

articular cartilage at the same time. Since the latter cannot be

determined sufficiently by CT scans, we have compared the

TTTG measured on MRI with the CT scan measurement.

Moreover, MRI can also address the ‘‘functional’’ TTTG

between the deepest point of the cartilaginous trochlear

surface and the insertion-centre of the patellar tendon,

instead of the bony trochlear groove, which differs from the

cartilaginous surface on the medial as well as the lateral side

[14]. Using the centre of the patellar tendon instead of the

most anterior point of the tibial tuberosity seems more

reasonable since the centre of the patellar tendon is the

direction where the patella is pulled proximally. It is a more

physiological measurement.

Our results demonstrate a high correlation between the

CT scan and MRI scan measurements (intermethods

reliability of 86%) [7]. There was no significant difference

between the two methods. Furthermore, we found an

interrater reliability of 82%, which was nearly as good as

the intermethods reliability proving the independence on the

measurer. The interperiod reliability of 91% proved, that the

results, obtained with the new method of TTTG measure-

ment using the cartilaginous trochlea and the centre of the

patellar tendon are highly correlated with the data obtained

with the original method [6].

As we found a high reliability of results and very tight

limits of agreement, it is possible to replace the CT scans for

TTTG measurements by MRI. We found only one case with

a difference of measurement for the functional TTTG of 5.6

P.B. Schoettle et al. / The Knee 13 (2006) 26–31 31

mm between the data obtained by the CT with that obtained

by the MRI. But in this case, both measurements (obtained

by CT and MRI) were pathological (>15 mm) [9]. The

indication for eventual surgical intervention would have

been the same [15].

5. Conclusion

The present study demonstrates that either CT or MRI can

be used for measurement of the TTTG. MRI has the

advantage that it can assess the state of the articular cartilage.

The use soft tissue landmarks (patellar tendon, deepest site of

the cartilage in the trochlea) for TTTG measurement seems

more reasonable, since they represent the true points of

interest, where the forces act on the patellofemoral joint

during flexion–extension movement.

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