„ REVIEW ARTICLE
The measurement of patellar height
A REVIEW OF THE METHODS OF IMAGING
C. L. Phillips,
D. A. T. Silver,
P. J. Schranz,
V. Mandalia
From Department of
Clinical Radiology
and Department of
Trauma and
Othopaedics, Royal
Devon and Exeter
Hospital, United
Kingdom
„ C. L. Phillips, MRCS, FRCR,
Specialist Registrar in
Radiology
„ D. A. T. Silver, FRCP, FRCR,
Consultant Radiologist
Department of Clinical
Radiology
„ P. J. Schranz, FRCS(Trauma &
Orth), Consultant Orthopaedic
Surgeon
„ V. Mandalia, MS(Orth),
DNB(Orth),
FRCS(Trauma&Orth),
Consultant Orthopaedic
Surgeon
Exeter Knee Reconstruction
Unit, Department of Trauma
and Orthopaedics
Royal Devon & Exeter Hospital,
Barrack Road, Exeter EX2 5DW,
UK.
Correspondence should be sent
to Mr V. Mandalia; e-mail:
vipul.mandalia@rdeft.nhs.uk
©2010 British Editorial Society
of Bone and Joint Surgery
doi:10.1302/0301-620X.92B8.
23794 $2.00
J Bone Joint Surg [Br]
2010;92-B:1045-53.
VOL. 92-B, No. 8, AUGUST 2010
Many radiographic techniques have been described for measuring patellar height. They can
be divided into two groups: those that relate the position of the patella to the femur (direct)
and those that relate it to the tibia (indirect). This article looks at the methods that have
been described, the logic behind their conception and the critical analyses that have been
performed to test them.
In 1938, Blumensaat1 described the first practical radiographic technique for measuring patellar height. Since then, many further attempts
have been made to establish a simple, reliable
and reproducible way of assessing the patella on
standard imaging.
Knowledge of the anatomy and biomechanics
of the patella is fundamental to understanding
the different pathologies of the anterior knee.2-8
Abnormal patellar height is seen in many conditions that affect the patellofemoral joint. Patella
alta, an abnormally high patella, is associated
with anterior knee pain, patellar instability and
Osgood-Schlatter’s disease, whereas patella baja
or infera, an abnormally low patella, may be
observed with anterior knee pain and limitation
of knee flexion, mainly as a complication of surgery or trauma.
Since it was first described, the techniques
devised to measure patellar height and the studies that test their validity have increased in number and complexity. Despite progression from a
simple radiographic measurement to the most
modern biomechanical and imaging technologies, the definition of patellar height and its
causal relationship to the conditions commonly
associated with anterior knee pain remains controversial. We have yet to devise a truly foolproof method that has all the necessary criteria
for a universally accepted system providing
accurate structural information for use in a clinical environment.
Methods of measurement of patellar height.
Many methods have been described with only
two specifically relating to children9,10 and only
one applied to a specific racial group.11 These
are summarised in Tables I and II and can be
divided into two groups, those that relate the
position of the patella to the femur (direct) and
those that relate it to the tibia (indirect). The
terms ‘direct’ and ‘indirect’ have been applied
more latterly, following the realisation that the
quantitative relationship between the patella
and the femur is most important, as the patellofemoral biomechanics hold the key to explaining anterior knee pathology.
Indirect methods. The most widely accepted
radiographic techniques used to measure patellar height are indirect, perhaps influenced by the
pioneering method of Insall and Salvati,12 published in 1971 with the development of a simple
ratio between the length of the patellar tendon
and that of the patella. It proved easy to measure on lateral radiographs, not requiring a
fixed-flexion angle, and the mean normal ratio
of 1.0 (0.8 to 1.2) is easy to remember. It
remains the most popular method, although the
bony landmarks are not always easy to identify,
the size of the patella can vary and pathological
bony overgrowth can distort the native anatomy.13-15 The method also assumes that the tibial tubercle is at a constant distance below the
tibial plateau. These drawbacks can lead to
both intra- and interobserver error.16 However,
modern digital radiographs have such good
soft-tissue definition that the patellar tendon
itself is usually visible, making accurate identification of the tibial tubercle obsolete. Furthermore, a cadaveric study by Schlenzka and
Schwesinger,17 demonstrated that absolute anatomical measurements and the resultant InsallSalvati ratios correlated well with ratios measured on lateral radiographs.
Using a more modern approach, Miller,
Staron and Feldman18 showed that InsallSalvati ratios can also beapplied to MR images
but require a slight change in the normal range
of values in order to be valid. In order to
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C. L. PHILLIPS, D. A. T. SILVER, P. J. SCHRANZ, V. MANDALIA
Table I. Summary of methods of indirect patellar height measurement in chronological order
Authors
Study population*
Imaging method†
Insall and
Salvati12
114:
Lateral XR
‘Normal’ subjects:
20° to 70° flexion
Meniscectomies
No demographics given
Indirect method of
measurement‡
Length of patellar tendon (LT);
Length of patella (LP)
Ratio = LT:LP
Interpretation§
LT:LP ratios:
PA > 1.2;
N: 0.8 to 1.2;
PI: < 0.8
LP
LT
Blackburne and
Peel21
Caton et al22
269:
Lateral XR
171 ‘Normal’ subjects: ≥ 30° flexion to
Meniscal pathology or tension the patellar
contralateral knees
tendon
(121M; 50F);
58 Subluxers (25M; 33F);
40 CMP (10M; 20F)
Horizontal line projected
anteriorly from the tibial plateau.
Height from line to inferior edge
of PAS (A);
Length of PAS (B)
128:
Lateral XR
Clinical patella infera
10° to 80°
(70M; 58F; mean age 36 flexion
years);
141:
Normal subjects asymptomatic (80M;
61F; no age given)
Distance between the inferior
edge of PAS to the anterosuperior
angle of the tibial plateau (AT);
Length of PAS (AP)
Ratio = AT:AP
Lateral XR
deCarvalho et al25 150:
Normal subjects Approximately 30°
asymptomatic
flexion
(20 to 60 yrs; no gender
given)
A:B ratios:
PA: > 1.0;
N: 0.8 to 1.0;
PI: < 0.8
B
A
AP
AT: AP ratios:
PA: > 1.3;
N: 0.6 to 1.3;
PI: < 0.6
AT
Shortest distance between the
inferior edge of PAS and the
anterior tibial plateau (T)
Length of PAS (P)
Ratio = T:P
T:P ratio:
PA: > 1.11
P
T
Micheli et al9
Paediatrics
AP XR
Distance from superior pole of
19:
0° flexion
patella to tibial plateau (A);
Physeal plate fracture; Serial XRs at 6 month Length of patella on AP film (B)
contralateral knee used to
Distance = A-B
(12M; 7F; no ages given) 1 yr intervals over 2 to
10 yrs
A-B distance:
PA: > 0 mm
B A
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THE MEASUREMENT OF PATELLAR HEIGHT
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Table I. Summary of methods of indirect patellar height measurement in chronological order
Authors
26
Egund et al
Koshino and
Sugimoto10
Grelsamer and
Meadows19
(modified InsallSalvati method)
Indirect method of
measurement‡
Study population*
Imaging method†
99:
47 Normal (23M; 24F;
mean age 34 yrs);
left knee;
36 Extraskeletal
malignancy (27M;
9F; mean age 70 yrs);
both knees:
16 Knee pathology
(no demographics
given); both knees
Lateral XR
30° to 40° flexion
Full weight-bearing
with the tibia 15° to
the vertical
Paediatric
36 (59 knees):
Normal subjects
(15M; 21F;
13 to 18 yrs)
Lines drawn through distal
Lateral XRs at
0°, 30°, 60°, 90°, 120° femoral and proximal tibial
physes;
flexion
Midpoint of longest diagonal line
of patella to midpoint of tibia
physis
(P-T) and line connecting
midpoints
of physes (FT)
Ratio = P-T:FT at all angles
Compared with IS and
Blumensaat at 60°
Distance from midpoint PAS (M)
to tibial plateau, perpendicular to
mechanical axis of tibia;
Subject height (H);
Tibial length (TL)
Ratio = M:TL and M:H
Interpretation§
No range of values
given for PA,
N or PI but:
M > F for all absolute
measurements:
M = F for all ratios:
L = R;
Young = Old
M
TL
300:
Lateral XR
100 ‘Normal’ subjects: 20° to 70° flexion
no patellar pathology;
200 subjects with
patellar pathology
No demographics given
The distance between the inferior
PAS and the patellar tendon
insertion (A);
Length of the PAS (B).
Ratio = A:B
FT
P-T
0° to 30°:
Ratio decreased from
1.31 to 0.9; 30° to 90°.
Ratio static between
0.99 to 1.20
3.4% of subjects
had PA with own
method;
45.7% of subjects had
PA with Blumensaat
method;
66.7% had PA with IS
method
A:B ratio:
PA: > 2.0
B
A
Leung et al11
Ethnic (S. Chinese)
Lateral XR
290:
30° to 70° flexion
173 ‘normal’ - included
simple meniscal tears;
54 anterior knee pain;
52 dislocators;
11 OS (15 to 50 yrs;
no gender given)
Patellar tendon length (A1);
Patellar length (A2); Length of
PAS (B)
Patellar alta index = Patellar alta
Index =
(A1 + A2)/B
Compared with IS, MIS, BP and
DeC
A2
B
PAI ratios:
PA: > 3.4;
N: 2.7 to 3.3;
PI < 2.7 in Chinese
population
A1
* CMP, chondromalacia patellae; OS, Osgood-Schlatter
† XR, radiograph; AP, anteroposterior
‡ PAS, patella articular surface; IS, Insall-Salvati; MIS, modified Insall-Salvati; BP, Blackburne-Peel; DeC, deCarvalho
§ PI, patella infera
account for the potential errors arising from morphological
variations in the patella, Grelsamer and Meadows19 developed the modified Insall-Salvati ratio, for which the length of
the articular surface of the patella rather than that of the
patella itself is used. However, the inferior margin of the
articular surface is not easily identifiable and the interVOL. 92-B, No. 8, AUGUST 2010
observer error of this method is higher than for the original
ratio.15,16,20
Blackburne and Peel21 devised a technique that again used
the length of the patellar articular surface for their ratio
denominator, but exchanged the tibial tubercle for the tibial
plateau as a landmark in order to avoid misidentification.
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C. L. PHILLIPS, D. A. T. SILVER, P. J. SCHRANZ, V. MANDALIA
They did not describe how they arrived at the ‘tibial plateau
line’ used. Is it perpendicular to the long axis of the tibia or,
if simply estimated to run parallel to the plateau, which tibial
condyle is used as a reference, bearing in mind that the two
are significantly different in their morphology? It can only be
assumed that the medial plateau is used, as the illustration
implies this, together with the fact that it is concave and thus
has better defined anterior and posterior margins. Seil et al15
and Berg et al16 both found this technique to be the most
reproducible and accurate.
Caton et al22 also modified the numerator by measuring
the distance between the inferior margin of the articular surface of the patella and the anterosuperior angle of the tibial
plateau. However, the problems associated with identifying
the articular margin of the patella would apply similarly to
the tibia.23 Despite this, Berg et al16 found the CatonDeschamp ratio to be very reproducible, closely behind that
of Blackburne and Peel, when looking at the inter-observer
error. This may in part be explained by the fact that the normal range of values is wide (0.6 to 1.3). They did, however,
recognise that it was difficult to use in osteoarthritic knees.16
These four methods are the most popular, owing partly to
their relative simplicity. The lateral radiograph does not need
to be taken with the knee at a fixed angle of flexion as long
as the patellar tendon is under tension at 30° or more.24
These four are the ones most scrutinised in intra- and interobserver variability studies that followed and utilised most
often when comparing anterior knee pathologies. Further,
less widely accepted and therefore relatively unknown, novel
techniques were published in fairly quick succession during
the 1980s and 1990s.
De Carvalho et al25 developed a method in 1985 which is
almost identical to that of Caton and Deschamps, but with a
minor alteration in the tibial landmark. They measured the
shortest distance between the inferior edge of the articular
surface of the patellar and the anterior tibial plateau rather
than its anterosuperior edge. Interestingly, the authors not
only misquoted the methodology of Caton and Deschamps
but also stated that, ‘this assessment never gained wide
acceptance’, despite the close similarity to their own method.
Egund, Lundin and Wallengren26 published a method that
is very similar to the Blackburne-Peel in that the patellar
height is measured perpendicular to the tibial plateau. However, they defined the ‘condylar plane’ as being perpendicular
to the true mechanical axis of the tibial shaft in order to eliminate variations in the inclination of the plateau. The lateral
radiograph must be taken with the subject fully weightbearing with the knee flexed in order to tension the patellar
tendon and the tibia at 15° to the vertical. Although the
authors recognised the problems associated with establishing
a reliable way to measure patellar height, the solutions they
introduced into their method have resulted in a complicated
technique, making it impractical to use routinely; despite
their claims. No range of values is given for the ratios calculated in the paper, which perhaps reflects the complexity of
the method.
Direct methods. Relatively few publications describe and
analyse methods of direct measurement of patellar height.
Even the four most popular methods described above are
indirect, once again raising the point about recognising the
importance of the patellofemoral relationship in the aetiology of anterior knee pathology. Although Blumensaat is considered the pioneer of radiographic measurement of patellar
height, an earlier method was described in 1930 by BoonItt.23,27 This assessed both patellofemoral and patellotibial
relationships, but it required complicated geometrical calculations for each angle of flexion of the knee and proved too
complex for routine use.
Blumensaat1,28 perhaps had early insight into the importance of the patellofemoral relationship, instead using the
roof of the intercondylar notch as a reference line (the ‘Blumensaat line’) on the lateral radiograph of a knee flexed to
30°. The perpendicular height of the inferior pole of the
patella above this line was measured, with normal being
defined as a distance of zero. Any value greater than this was
classified as patella alta. In 1955 Thestrup-Anderson29
applied this to a group of 286 subjects and found that 207
would be classified as having patella alta. He consequently
moved the normal limit to 5 mm above the Blumensaat line.
In 1970, Brattstrom28 pointed out the disadvantages of the
Blumensaat method, indicating that this distance varies with
knee flexion and also that the Blumensaat line does not form
a fixed angle with the femoral axis, with variations up to 30°.
In an attempt to rectify the latter problem, Seyahi et al30
developed the intercondylar shelf angle (ISA)-corrected Blumensaat method, using a fixed ISA value of 147°, which was
the mean angle between the femoral shaft axis and the Blumensaat line on 105 knees. The patellar height was determined using this fixed angle and a comparison was made
with the original Blumensaat technique, but again moving
the normal limit up to 10 mm above the line, and also with
the Insall-Salvati, modified Insall-Salvati and BlackburnePeel ratios. They showed that Blumensaat and the ISA-corrected methods showed similar but poor correlation with the
three other methods, summarising ‘that the intercondylar
notch cannot be a beneficial landmark alone’. This statement
is likely to have prompted the idea of indirect methods that
followed.
A further variation was developed by Hepp23 in 1984,
whereby the perpendicular height of the superior pole of the
patella above the Blumensaat line is measured, rather than
the inferior pole. They also suggested using the ‘patellar
height angle’, which is the angle subtended by the Blumensaat line and a line extending from the superior pole of the
patella to the intersection with the posterior margin of the
femoral condyles. Considering the variation in the position
of the Blumensaat line, these methods do not appear to overcome this problem.
The next attempt at a direct method came from Labelle et
al31 and Laurin.32 However, this appears rather too simplistic and unusually requires a lateral radiograph of the knee
flexed to 90°. If the superior pole of the patella lies above the
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THE MEASUREMENT OF PATELLAR HEIGHT
1049
Table II. Summary of methods of direct patellar height measurement in chronological order
Direct method of
measurement‡
Author/s
Study population* Imaging method†
Blumensaat1
No details available Lateral XR
30° flexion
Perpendicular distance
between the inferior pole of
patella and the ‘Blumensaat’
line projected anteriorly
through the roof of the
femoral trochlea
Bernageau
et al37
44:
Lateral XR
Normal subjects: Full extension
No demographics
given
Distance between inferior
edge of PAS (point R) and
superior edge of femoral
trochlea (point T)
Distance = R to T
Labella et al31
No details
available
Lateral XR
90° flexion
Perpendicular distance from
the distal edge of the PAS to
the femoral condylar
plane (line tangential to
femoral condyles,
perpendicular to anterior
femoral cortex) (VP);
Body height (H);
Ratio = Vertical index of the
patella = VIP = VP:H
Hepp23
2 methods:
1. Perpendicular distance
between superior edge of
PAS and the Blumensaat line;
2. Angle subtended by the
Blumensaat line and the
superior edge of the PAS
to posterior femoral condyle
margin
VOL. 92-B, No. 8, AUGUST 2010
Lateral XRs at 5°
incremental flexion
from
25° to 60° flexion
Distance:
PA > 0 mm
R
T
Height of the superior pole of
patella above the tangent
of the anterior cortical line of
the femur
Lateral XR
Norman et al33 91:
Meniscal pathology 0° flexion
(57 M; 34 F;
10° to 15° external
mean age 33 yrs) rotation
Quads contracted
560 knees
No clinical or
demographic
details given
Interpretation§
Distances R to T:
PA: > +6 mm (i.e. R higher
than T);
PI: < -6 mm (i.e. R lower
than T)
Distance:
PA: > 0 mm
VIP ratio:
0.21 ± 0.02
VP
2
1
Linear decrease in both
distance (58 to 37 mm) and
angle (53° to 32°) with
increasing knee flexion
No range of values given
for PA, N and PI
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C. L. PHILLIPS, D. A. T. SILVER, P. J. SCHRANZ, V. MANDALIA
Table II. Summary of methods of direct patellar height measurement in chronological order
Author/s
Burgess
14
Miller et al18
Seyahi et al30
Biedert and
Albrecht34
Study population* Imaging method†
100:
‘Normal’ subjects:
Medial
meniscectomies;
No demographics
given
44 (46 knees):
Meniscal/ACL
injury
(18M, mean age
40 yrs;
26F, mean age 56
years)
Lateral XR
No specific flexion
angle - enough to
tension the patellar
tendon
Lateral XR
30° to 60° flexion:
MRI
sagittal
0° flexion
Quads relaxed
Direct method of
measurement‡
Distance from midpoint of PAS
to tibial plateau (A);
AP width of femoral condyle
(B) (perpendicular to femoral
axis)
Ratio = A:B
Compared with IS and BP
Length of PAS (AS);
Length of PAS at the level of
anterior aspect of the distal
femoral physeal scar (PH)
Ratio = AS:PH
Compared with IS ratio on XR
and MRI
77 (105 knees):
Lateral XR
‘Normal’
30° flexion
subjects - no
degenerative
change on
plain radiograph
(23M, 54F;
mean age 32 yrs,
range 21 to 62 yrs)
Perpendicular distance
between
the IPP and the ‘intercondylar
shelf angle’ (ISA) corrected
‘Blumensaat’ line projected
anteriorly through the roof of
the femoral trochlea.
ISA = mean angle between
Blumensaat line and
femoral shaft
axis in 105 knees = 147°
Compared with IS, MIS
and BP
66:
‘Normal’ subjects:
Most had meniscal
or ACL
pathologies. No
patellofemoral
complaints;
(44M; 22F; 12 to 36
yrs)
Baseline patella (BLp) = vertical
length of PAS; Baseline
trochlea (BLT)= vertical height
of trochlear articular surface
from most superior femoral
aspect to most inferior aspect
of PAS;
Patellotrochlear index (%) =
BLT/BLP x 100
3 observers at 2 different times
MRI
Sagittal
0° flexion
15° external rotation
Quads relaxed
Interpretation§
A:B ratio:
N: 0.56 to 0.76 (mean 0.66)
B
A
AS:PH ratios:
2.1 ± 0.8 (M)
2.9 ± 2.4 (F)
IS (XT) = 1.0
AS:PH to IS ratio
AS
PH
ISA
BLP
BLT
Distance:
PA: > 10 mm
Poor correlation between
ISA-corrected method and
IS, MIS and BP methods
Patellotrochlear index:
PA: ≤ 12.5%
PI: ≥ 50%
t-test showed 2nd observer
intra-observer error;
Inter-observer correlation
high and significant
* ACL, anterior cruciate ligament
† XR, radiographic
‡ PAS, patella articular surface; IS, Insall-Salvati; BP, Blackburne-Peel
§ PA, patella alta; PI, patella infera; N, normal; ISA, intercondylar shelf angle; MIS, modified Insall-Salvati
tangent of the anterior cortical line of the femur, patella alta
is diagnosed. It is synonymous with the clinical evaluation,
where alta is diagnosed if, with the knee in 90° of flexion, the
patella points to the ceiling, rather than toward the torso.15
The fact the femur is curved makes the anterior cortical line
difficult to position. Patella infera cannot be diagnosed using
this method.
Norman et al33 developed a truly direct method, originally
described in Swedish in 1976 and published in English in
1983, which assessed the height of the patella in its most
proximal position, with the knee in full extension and the
quadriceps contracted. The perpendicular distance between
the inferior margin of the patellar articular surface and the
distal aspect of the femoral condyles, expressed relative to
THE JOURNAL OF BONE AND JOINT SURGERY
THE MEASUREMENT OF PATELLAR HEIGHT
body height rather than absolute measurements, is used
because it was found that this ratio is equal in both males
and females, unlike the absolute distances, thus requiring
only one set of normal values. This gender difference was
also observed by Egund et al.26 The biggest drawback is the
fact that the height is measured parallel to the anterior cortex
of the femur, which is curvilinear, allowing both intra- and
inter-observer errors. No subsequent studies to test this possibility have been performed. Also, the use of a lateral radiograph of the knee in full extension, with the quadriceps fully
contracted, is not part of the standard imaging technique,
making it less practical.
It was not until 1989 that Burgess14 devised a method
using lateral radiographs of the knee at any angle of flexion
which produced tension in the patellar tendon. The direct
ratio between the distance from the midpoint of the articular
surface of the patella to the tibial joint line and the anteroposterior (AP) dimension of the femoral condyles perpendicular to the mechanical axis of the femoral shaft, was
compared with the Insall-Salvati and Blackburne-Peel ratios.
The direct method was deemed better at determining a normal range of values. However, although Burgess applied the
problem of variation in patellar size and the position of the
tibial tubercle to recognise the inaccuracies of indirect
methods, he did not acknowledge that the femoral condyles
could also vary in size. In addition, he did not identify which
condylar width is used in the calculation: the lateral condyle
usually has a greater AP dimension.
The most recent technique was described by Biedert and
Albrecht in 200634 as the ‘patellotrochlear index’. This
involves MRI rather than radiographs and uses chondral
landmarks rather than bony ones. This is logical, because
patellofemoral biomechanics relates to articular surface
interaction. Both Staeubli et al35 and van Huyssteen et al36
have shown a morphological difference between the articular
cartilage and the underlying subchondral osseous anatomy
of the patella and the femoral trochlea. Biedert and
Albrecht34 adapted a radiographic technique, first described
in 1969 by Bernageau et al,37 where the relationship between
the superior line of the trochlea and the inferior edge of the
patella is applied to articular cartilaginous landmarks rather
than the corresponding bony ones. The original technique
of Bernageau resulted in a large range of normal values from
-6 to +6 mm. It was also criticised by Caton38 as not being
useful when planning surgery in patients with patella alta or
infera, and almost impossible to use in patients with severe
trochlear dysplasia. The need to change the imaging modality from plain film radiography to MRI does not make the
technique less practical, as most patients with anterior knee
pain will undergo an MR scan as part of their management.
A similar MRI method was described by Miller et al,18 who
calculated the ratio between the patellar articular surface and
the anterior femoral physeal height on sagittal MR scans,
and tested the accuracy of the standard Insall-Salvati ratio
when using MRI. Unfortunately, the correlation between the
two methods was only fair. Biedert34 deemed the use of the
VOL. 92-B, No. 8, AUGUST 2010
1051
physeal scar as the upper articular landmark to be inaccurate
when developing the patellotrochlear index, subsequently
finding that with the new adjustments, the intra-observer
error was low and the inter-observer correlation was significantly high. It was also acknowledged that the effects of a
tensed quadriceps, a change in the flexion angle and weightbearing on this technique were not addressed. Biedert and
Albrecht’s34 method has recently been tested by Barnett et
al,39 who compared it to the Insall-Salvati, Blackburne-Peel
and Caton-Deschamps ratios in subjects with patellofemoral
dysplasia, and found that the patellotrochlear index does not
correlate well with the other ratios for patella alta, although
all had good intra- and inter-observer reliability. This supported the theory that chondral rather than osseous relationships are more important and clinically relevant.
Paediatric studies. Micheli et al9 described a method of measuring patellar height in children. This study is the only one
identified, where the authors have acknowledged that adult
ratios cannot be applied accurately to the paediatric population (anterior knee pain is common in adolecents) due to lack
of ossification and subsequently developed a new method.
They investigated how the growth spurt can affect the development of patella alta by performing serial radiographs at
six- to 12-month intervals over a period of up to ten years.
Because only a single AP view was used to assess growth in
the lower limb, they adapted the principles of Brattström28 to
fit the radiograph, thus evolving a new technique, but failed
to recognise the significance of skeletal maturity. The length
of the patellar tendon was calculated as the difference
between the inferior pole of the patella and the tibial plateau.
This differed from Brattström, who used the approximate
joint line as his reference point. If this was greater than zero,
then patella alta was diagnosed. However, not surprisingly,
all of the 19 children studied had a positive value, although
they found that patella alta is more likely to develop in girls
than in boys during the adolescent growth spurt. The omission of the ages of the subjects is an important error, as full
ossification may not have occurred and would in part
explain why all subjects had positive values. Walker, Harris
and Leicester40 also showed that young children all have a
high Insall-Salvati ratio, which decreased with age and
became the same as the adult values at approximately ten
years in girls and 12 in boys. Full ossification occurred at
15 and 17 years of age, respectively.
Koshino and Sugimoto10 described a novel technique that
uses the midpoints of the femoral and tibial physes to calculate a ratio between the patellotibial distance and the
tibiofemoral distance, thereby eliminating the need to adjust
for the extent of epiphyseal ossification. Although only a
small series of 59 knees in 36 children, the method produced
stable values for all angles of flexion between 30° and 90°,
making it practical to perform in children, 3.4% of whom
had patella alta using their calculations. The authors also
determined both Blumensaat and Insall-Salvati ratios using
the same radiographs and found that 46% and 67% of the
subjects, respectively, would have been classed as having
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C. L. PHILLIPS, D. A. T. SILVER, P. J. SCHRANZ, V. MANDALIA
patella alta, thus emphasising how adult ratios cannot be
applied to children. They recognised that one drawback with
this method was that cautious line-drawing through the physes is required, owing to their irregular morphology. Aparicio et al41 compared this with the Blackburne-Peel and
Caton-Deschamps methods and found that, although all
three had low inter-observer variability, Koshino’s was not as
reproducible as the Caton-Deschamps ratio, with the latter
also not being affected by skeletal maturity.
Method limitations. A review of all the methods that have
been devised to assess patellar height has demonstrated that,
although the patellofemoral relationship is the most important aspect of understanding the causes of anterior knee pain,
most techniques are indirect. The last to be developed, by
Biedert and Albrecht in 2006,34 is probably the closest to
perfect, to date, as it uses MRI, relates the patella directly to
the femur, and uses cartilaginous landmarks, thereby eliminating osseous variation, which is possibly the greatest cause
of error in measuring patellar height accurately.
Of the studies that resulted in the description of the four
most established methods, Insall-Salvati,12 BlackburnePeel,21 Caton-Deschamp,22 and the modified Insall-Salvati19,
only the Caton-Deschamp included knees that were asymptomatic, in an endeavour to establish a normal range of values. The subjects included in the remaining three studies had
either meniscal or cruciate ligament pathology, which was
deemed acceptable by the authors on the assumption that
these soft-tissue problems have no effect on the
patellofemoral relationship. This has been shown to be
incorrect,42,43 and any proposed method should ideally be
tested using only subjects with clinically and radiologically
normal knees, before applying it to patients with anterior
knee pathology. Of the less well-known methods, only
Egund et al,26 Koshino and Sugimoto,10 Bernageau et al37
and de Carvalho et al25 managed to use normal knees, but
even then, normality was only defined as being ‘subjects with
no previous knee symptoms’. They did not undergo a clinical
examination and none were excluded on the basis of the subsequent radiographic appearance.
Furthermore, little reference is made to the inherent anatomical differences between men and women, and only two
studies look at ethnic diversity,11,44 where both showed that
there are significant differences between Europeans and
Arabs, Africans and Chinese, with the non-European populations having a much higher rate of patella alta. Leung et al11
developed the ‘patella alta index’ as the ratio of the sum of
patellar length and patellar tendon length, to the length of the
patellar articular surface, with a much higher range of normal
values to better suit the Southern Chinese population.
Accuracy also seems to be a problem, with few studies
looking at the inter- and intra-observer errors of any method,
even the four most popular ones. Only three publications can
be identified that compare the Insall-Salvati, the modified
Insall-Salvati, the Blackburne-Peel and Caton-Deshamps
methods, and even then abnormal knees were used.15,16,45
Blackburne-Peel and Caton-Deschamps seem to have better
accuracy and reproducibility, but in the absence of any studies
using normal knees, these cannot be deemed as fully reliable.
As Biedert34 has noted, no study where a new technique
has been described has looked at how quadriceps tension,
the angle of flexion and weight-bearing influences patellar
height. These issues have been looked at separately, but not
in the same study.6,7,10,18,46 The only way to solve this problem is to use dynamic MRI, but at present this is only practical for research purposes to define what is normal.
Yiannakopoulos et al46 have recently shown that in normal
knees the height of the patella changes significantly on
weight-bearing through the effect of quadriceps contraction,
regardless of which of the four most common ratios is used
to calculate this. Although they gave no absolute measurements, there was a significant mean difference between the
ratios calculated for the non-weight-bearing and fully
weight-bearing groups of 0.2 (p < 0.05).46
Having reviewed the extensive literature on this controversial subject, it appears that only the passing of time has led
to acceptance of the most common methods for measuring
patellar height. Not only is there little concrete evidence to
support the accuracy and validity of these methods, but none
has proved to be suitable for universal application, although
both the Blackburne-Peel and Caton-Deschamps methods
may be the most reliable indirect plain-film radiographic
techniques. Direct methods have either proved too complex
or too novel to be adopted regularly. MRI is currently underused in the assessment of patellar height and undoubtedly
has great potential for use in further research. It would
require a very large study with a truly population comprising
all ages, genders and ethnicities to determine a truly normal
range of values for any technique. However, before this vast
undertaking can be considered, a method that accurately
reflects the patellofemoral relationship and can also be used
routinely and repeatedly in a clinical situation needs to be
established. Ideally this should be able to demonstrate the
patellofemoral relationship under physiological loading conditions in the symptomatic knee. Nonetheless, until dynamic
imaging techniques become standardised and routinely
available, we need to identify the best method of imaging and
measuring patellar height from those currently available.
From the limitations described above, there is clearly significant scope for improving the methods designed to date, insofar as to say that it is still very much an ‘evolving’ project,
even after nearly eight decades.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
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