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Knee Surgery, Sports Traumatology,
Arthroscopy
ISSN 0942-2056
Knee Surg Sports Traumatol Arthrosc
DOI 10.1007/s00167-014-3137-8
The sulcus line of the trochlear groove is
more accurate than Whiteside’s Line in
determining femoral component rotation
Simon Talbot, Pandelis Dimitriou, Ross
Radic, Rachel Zordan & John Bartlett
1 23
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KNEE
The sulcus line of the trochlear groove is more accurate
than Whiteside’s Line in determining femoral component rotation
Simon Talbot •Pandelis Dimitriou •
Ross Radic •Rachel Zordan •John Bartlett
Received: 15 January 2014 / Accepted: 5 June 2014
ÓSpringer-Verlag Berlin Heidelberg 2014
Abstract
Purpose The sulcus line (SL) is a three-dimensional
curve produced from multiple points along the trochlear
groove. Whiteside’s Line, also known as the anteroposte-
rior axis (APA), is derived from single anterior and pos-
terior points. The purposes of the two studies presented in
this paper are to (1) assess the results from the clinical use
of the SL in a large clinical series, (2) measure the SL and
the APA on three-dimensional CT reconstructions, (3)
demonstrate the effect of parallax error on the use of the
APA and (4) determine the accuracy of an axis derived by
combining the SL and the posterior condylar axis (PCA).
Methods In the first study, we assessed the SL using a
large, single surgeon series of consecutive patients under-
going primary total knee arthroplasties. The post-operative
CT scans of patients (n=200) were examined to deter-
mine the final rotational alignment of the femoral compo-
nent. In the second study, measurements were taken in a
series of 3DCT reconstructions of osteoarthritic knees
(n=44).
Results The mean position of the femoral component in
the clinical series was 0.6°externally rotated to the surgical
epicondylar axis, with a standard deviation of 2.9°(ranges
from -7.2°to 6.7°). On the 3DCT reconstructions, the
APA (88.2°±4.2°) had significantly higher variance than
the SL (90.3°±2.7°)(F=5.82 and p=0.017). An axis
derived by averaging the SL and the PCA?3°produced a
significant decrease in both the number of outliers
(p=0.03 vs. PCA and p=0.007 vs. SL) and the variance
(F=6.15 and p=0.015 vs. SL). The coronal alignment
of the SL varied widely relative to the mechanical axis
(0.4°±3.8°) and the distal condylar surface (2.6°±4.3°).
Conclusions The multiple points used to determine the
SL confer anatomical and geometrical advantages, and
therefore, it should be considered a separate rotational
landmark to the APA. These findings may explain the high
degree of variability in the measurement of the APA which
is documented in the literature. Combining a geometrically
correct SL and the PCA is likely to further improve
accuracy.
Keywords Knee Arthroplasty Rotation Femoral
component rotation Total knee arthroplasty Whiteside’s
Line Sulcus line Epicondylar axis
Introduction
Successful total knee arthroplasty (TKA) is dependent on
the accurate alignment of the components. Inaccurate
femoral component rotation is associated with poor out-
comes due to patellofemoral maltracking, flexion instabil-
ity, and soft tissue imbalance [1,2,5,7,15,34]. Obtaining
correct femoral component alignment is difficult as the
definitive landmarks are not readily accessible during
S. Talbot (&)P. Dimitriou R. Radic
Western Health, Melbourne, VIC, Australia
e-mail: info@simontalbot.com.au
P. Dimitriou
e-mail: pdimitriou@me.com
R. Radic
e-mail: rossradic@gmail.com
S. Talbot R. Zordan J. Bartlett
Warringal Private Hospital, Melbourne, VIC, Australia
e-mail: rachelzordan@gmail.com
J. Bartlett
e-mail: r.j.bartlett@bigpond.com.au
123
Knee Surg Sports Traumatol Arthrosc
DOI 10.1007/s00167-014-3137-8
Author's personal copy
surgery, and therefore surrogate landmarks and techniques
are adopted [21–23,37].
Several techniques are commonly used to determine the
rotational alignment of the femoral component. Measured
resection techniques use surface-derived landmarks to
direct the rotational alignment of the implant. The most
frequently referenced bony landmarks are the posterior
condylar axis (PCA) [16,19,24], the surgical epicondylar
axis (SEA) [4,17,42], the anatomical epicondylar axis
(AEA) [33,46], and the anteroposterior axis (APA) [3,27,
43]. The gap-balancing technique [13] attempts to achieve
a balanced knee by beginning with the tibial resection
perpendicular to the tibial axis, and subsequent femoral
resection based on ligament tensioning from this cut sur-
face. The use of these techniques has continued to produce
high rates of unacceptable femoral component malrotation
[25,36,38,40]. As a result, alternative techniques
employing preoperative CT scans, computer navigation,
patient-specific instrumentation (PSI), or the combination
of landmarks are being considered [38,40].
Based on anatomical and kinematic data, the accepted
gold standard for correct femoral component rotation is the
SEA measured on a CT scan [2,4,11,28,30]. Unfortu-
nately, both epicondyles are relatively broad structures
covered in dense soft tissue in vivo making them difficult
to identify intraoperatively [3,22,39]. Many studies
comparing landmarks are difficult to interpret as they fail to
use post-operative CT scans referencing the SEA [14,24].
The sulcus line (SL) is a curve derived from joining
multiple points in the trochlear groove. In practice, many
surgeons use a version of the SL by marking out a line
along the depth of the trochlear groove. In comparison, the
APA, also known as Whiteside’s Line, is defined as the
axis taken from two points—the deepest part of the patella
groove anteriorly to the centre of the intercondylar notch
posteriorly [3]. Both anatomical and clinical studies sug-
gest that the APA is an inaccurate axis and that it is dif-
ficult to consistently reproduce [27,33,37,40,44].
It is hypothesised that there are two reasons why the SL
is more accurate than the APA. Firstly, the APA relies on
the accuracy of the anterior point which is in the proximal
section of the trochlear. This section is often affected by
osteoarthritis and dysplasia. The SL references the vertical
section extending anteriorly from the intercondylar notch
into the anterior trochlear groove, but does not rely on the
proximal section of the trochlear. Secondly, even though
both axes reference the trochlear groove, the SL has geo-
metrical advantages which make it a more accurate land-
mark. The rotational component of the trochlear groove
can be isolated by viewing the SL along the coronal axis of
the trochlear groove. This axis can be identified intraop-
eratively, and on three-dimensional CT reconstructions, as
the viewpoint at which the curved SL becomes a straight,
vertical line. Conversely, as the APA is only derived from
two points, there is no way in which the rotational com-
ponent of the trochlear groove can be isolated.
The following investigation comprises of two studies.
The first study is a retrospective review of post-operative
CT scans in 200 total knee replacements in which we aim
to (1) determine the femoral component rotational align-
ment achieved with the use of the SL and (2) compare these
results with other studies which have used post-operative
CT scans to assess rotational axes. The second study
examines the anatomical landmarks in preoperative three-
dimensional CT reconstructions of 44 osteoarthritic knees.
It aims to (1) compare a geometrically correct SL measured
along the coronal axis of the trochlear groove to other
anatomical landmarks, including the APA, (2) demonstrate
the effect of parallax error on the measurement of the APA,
and (3) measure the accuracy and variability of an axis
derived by combining the PCA and a geometrically correct
SL.
Materials and methods
Study 1
A retrospective review of a consecutive case series of
patients undergoing primary TKA by one surgeon from
July 2008 to June 2009 was conducted to document the
femoral rotation of the prosthesis on post-operative CT
scans (n=228). Excluded from the study were CT scans
which were determined by either of the assessors as not
providing adequate visualisation of the landmarks due to
metallic artefact (n=28), leaving 200 cases.
Surgical technique
All TKAs were performed by one surgeon, very experi-
enced in TKA, and who has used the SL extensively for
many years. The same surgical approach, consisting of a
midline skin incision and a medial parapatellar approach,
was applied to all patients. Conventional instruments were
used, with intramedullary (IM) femoral alignment and
extramedullary tibial alignment jigs. Prostheses inserted
were the Triathlon (Stryker Howmedica Osteonics, Allen-
dale, NJ) or Active (Advanced Surgical Design and Man-
ufacture, ASDM, Sydney, Australia) knee. All components
were cruciate retaining, and all femoral components were
uncemented.
The femoral rotational alignment was based only on the
SL. The line was meticulously drawn by palpating the
trochlear groove. Multiple diathermy marks were made in
the deepest part of the groove at the tip of the thumb. The
marks were extended from the anterior edge of the
Knee Surg Sports Traumatol Arthrosc
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intercondylar notch to the proximal trochlear groove. The
most proximal aspect of the trochlear groove was disre-
garded if it was found to deviate from the rest of the
groove.
The SL was then viewed along the coronal plane ori-
entation that produced a straight, vertical line (Fig. 1).
When the SL is viewed along a different coronal orienta-
tion, it becomes a curve (Fig. 2). This vertical line was then
transferred onto the surface of the distal condyles with a
perpendicular T-piece. The horizontal limb was marked
with a further diathermy line (Fig. 3).
An IM rod was inserted through the centre of the knee.
The anterior femoral cut was made first with a cutting
block attached to the IM rod which was rotated to match
the horizontal diathermy line. A stylus was used to avoid
notching. The distal cut was made by pinning a flat distal
cutting block onto the anterior surface. The appropriately
sized 4-in-1 cutting block was then placed on the distal cut
surface with an anterior ledge which sat on the anterior
femoral cut. This was pinned, and the posterior and
chamfer cuts were completed.
Post-operative CT evaluation
An initial scanogram was produced, and 1.25-mm slices
were performed from approximately 5 cm above the
anterior flange of the femoral component to immediately
below the tibial stem.
The medial and lateral epicondyles were identified. The
AEA was measured from the most prominent point on the
lateral epicondyle to the most prominent point on the
medial epicondyle. The medial sulcus was identified where
possible. If the sulcus was not visible, the absence of a
sulcus was documented. The SEA was measured from the
most prominent point on the lateral epicondyle to the
centre of the sulcus of the medial epicondyle as outlined by
Berger [8]. The rotational alignment of the femoral com-
ponent was measured from the under surface of the anterior
flange, in the manner described by Moon et al. [26], as this
surface was less affected by the artefact than the posterior
condyles. All measurements were taken and reported to one
decimal place.
Two assessors independently reviewed the CT scans and
calculated the measurements. Intraobserver and interob-
server reliability was both very good (intraobserver
r=0.95 and 0.97 and interobserver r=0.93).
Statistical analysis
Analyses to determine intraclass correlation coefficients
(ICC), means, standard deviation (SD), and ranges were
Fig. 1 The SL is a straight line when it is viewed along the coronal
alignment of the trochlear groove
Fig. 2 The SL appears as a curve when it is not viewed along the
coronal alignment of the trochlear groove
Fig. 3 A T-piece was used to translate the vertical SL into a
horizontal line
Knee Surg Sports Traumatol Arthrosc
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conducted. Pearson’s correlation and independent samples
ttests were conducted to ascertain the relationships
between age, gender, and side with the rotation of the
femoral component. All analyses used the Statistical
Package for the Social Sciences (SPSS) v.16.0.
Study 2
This was a retrospective review of a consecutive series of
preoperative CT scans of patients who were undergoing
TKA for severe osteoarthritis.
In total, 44 CT scans were assessed. All scans were
performed using the Perth protocol [10], and 1.25-mm
slices were performed from hip to ankle (GE Optima 660
Brightspeed, 128 slice scanner). The data were imported
into the Osirix (Osirix v.5.6 64-bit, Pixmeo Sarl, Switzer-
land) proprietary software program.
A volume rendering three-dimensional (3D) reconstruc-
tion was performed. Bone subtraction techniques were used
to remove the patella and tibia. A 3D surface rendering was
then performed. The medial and lateral epicondyles and the
medial epicondylar sulcus were viewed in multiple planes
and marked with region of interest (ROI) points. These
points were then used as the reference points for both the
two-dimensional and three-dimensional measurements.
The first measurements were taken from two-dimen-
sional axial slices in order to recreate the techniques used
in previous studies. The first technique for measuring the
APA (APA-1) was determined by marking the deepest
point in the groove anteriorly and the midpoint in the notch
posteriorly as per the technique used by Nagamine et al.
[27] (Figs. 4,5). Measurements were taken on the axial
slice that most clearly showed the epicondyles and medial
epicondylar sulcus. The AEA, SEA, and PCA were mea-
sured as per the technique described by Berger et al. [8].
The deepest points in the trochlear groove were marked
as ROIs on the surface rendering and confirmed using the
orthogonal views in the MPR (multi-planar reconstruction)
format as per the technique used by Iranpour et al. [20].
Following confirmation of 8–12 ROI points, the groove
was viewed as a 3D surface rendering which was rotated
along the axis of the SL in order further assess the accuracy
of the points (Figs. 6,7).
The APA and SL were then measured with the femur
orientated along the mechanical axis (MAx). The 3D sur-
face rendering was rotated in each plane until the femoral
head was in line with the centre of the knee. The angles
were measured relative to the ROIs placed on the epicon-
dyles (SEA). The second technique for measuring the APA
(APA-2) was measured from the deepest point on the notch
anteriorly to the closest point to the intercondylar notch on
the articular surface posteriorly as per technique described
by Victor et al. [42] (Fig. 5). The SL was also measured on
the same image. When the SL was noted to be curved, due
to variation between the coronal alignment of the SL and
the MAx, a best fit was measured concentrating on the
vertical section of the line.
The 3D surface rendering was then aligned along the
coronal axis of the SL (CAxSL) by rotating the coronal
alignment of the reconstruction until the straightest
Fig. 5 The SL is compared to the two-dimensional (APA-1) and
three-dimensional (APA-2) techniques for determining the APA
Fig. 4 APA-1 was measured on a single CT slice
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possible version of the SL was visible. This technique
most closely matches the intraoperative technique of
altering the viewpoint of the surgeon until the SL appears
to be a straight line rather than the shallow curve that is
apparent when it is viewed in an oblique plane (Figs. 1,
2).
The CAxSL was then measured on the MPR screen in
comparison with the MAx (Fig. 6). The femur was orien-
tated to the MAx in the sagittal plane and to the SL in the
axial plane. The SL was then viewed as a straight line in
the coronal plane. A line was extended along the CAxSL
towards the hip and was measured relative to the MAx. A
varus deviation (towards the anatomical axis) was assigned
a positive value, and valgus deviation was assigned a
negative value. The distal condylar axis (DCA) was mea-
sured as the angle between the MAx and a line across the
distal surface of the condyles.
The ROI points were inserted by a single observer.
To assess intraobserver reliability, insertion of the
points was repeated for a second time in ten femurs at
least a week later. The intraobserver reliability was
found to be very good (r=0.93). All measurements
were taken by two assessors (an orthopaedic surgeon
and a registrar). Interobserver reliability was also found
to be very good, with r=0.93.
Statistical analysis
Analyses to determine ICC, means, SD, and ranges were
conducted. Ttests and one-way ANOVA were applied to
compare groups. Outliers, beyond 3 °from the SEA, were
determined for each group and comparative analysis
between each groups conducted using chi-square test.
Significance was set at the pB0.05 level. SPSS v.16.0 was
used for all analyses.
Ethics approval was sought and received for each study
from the Western Centre for Health Research Education
(QA2013.45 and QA2013.46).
Results
Study 1
The average age of participants was 68 years (SD =8.9).
See Table 1for demographic data.
Fig. 6 The trochlear groove and epicondyles were marked on multi-planar reconstructions
Fig. 7 The landmarks were confirmed by rotating 3-D reconstructions
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The sulcus was identified in 181 (91 %) scans. Thus, the
AEA and the SEA were calculated on 200 and 181 scans,
respectively. The results of this are summarised in Table 2.
Of the 181 knees with an identified sulcus, 64 (35 %)
were deemed outliers, with deviation greater than 3°from
the SEA. Of these outliers, 16 deviated greater than 5°from
the SEA.
Male patients had a significantly externally rotated
femoral component position (mean =1.04°) compared
with female patients (mean =0.17°,t=2.04, and
p=0.043). All remaining analyses were non-significant.
Study 2
There was a significant difference between the SEA and the
APA-1 and APA-2 (p\0.05). There was no significant
difference between the SEA and the SL measured along the
MAx, the SL measured along the CAxSL, the PCA?3°,or
the average of PCA?3 and SL. The axial rotational mea-
surements and results of the comparative analysis are
outlined in Table 3and summarised in Fig. 8.
The variability of the SL was significantly less than the
variability of the APA-2 on one-way ANOVA (F=5.82
and p=0.017). The axis derived from the average of the
PCA?3 and the SL was significantly less variable than the
SL (F=6.15 and p=0.015). The SL on 3D reconstruc-
tion measured along the MAx had a higher SD than the SL
measured along the CAxSL (3.2°vs. 2.7°); however, the
difference in variance did not reach statistical significance
(F=3.71, p=0.052).
The number of outliers, greater than 3°from the SEA,
was calculated for the PCA?3°and the SL along the
CAxSL. A combined axis was calculated by averaging the
PCA?3°and the SL along the CAxSL for each knee
(PCA–SL). This derived axis produced a mean of 0.5°(SD
2.2°, range from -4.8°to 5.2°). The PCA–SL was found
have significantly less outliers when compared with either
the PCA?3°or the SL along CAxSL individually, with
pvalues of 0.03 and 0.007, respectively. For all outlier
data, see Table 4.
The coronal axis measurements are summarised in
Table 5.
Discussion
There are several important findings from these studies.
Firstly, the SL should be considered a separate, and more
accurate, rotational landmark than the APA. Secondly, the
Fig. 8 Mean and standard deviation of each landmark relative to the
SEA
Table 1 Demographic data
Variable n%
Gender
Male 101 51
Female 99 49
Side
Left 95 48
Right 105 52
Prosthesis
Active 117 59
Triathlon 83 41
Table 2 Axial alignment of femoral components relative to epic-
ondylar axes
Mean SD Range
Component to AEA -3.2°2.9°-10.8°to 3.2°
Component to SEA 0.6°2.9°-7.2°to 6.7°
Table 3 Rotational variations between axes and variations in coronal
viewpoint
Mean SD Range Difference
from SEA
AEA to SEA 3.7°0.6°2.4°–4.8°N/A
APA-1 on 2D axial -1.5°3.6°5°to -8.2°p=0.008
PCA?3°0.7°2.5°7.1°to -5.7°n.s.
APA-2 on 3D
reconstruction
-1.8°4.2°7.8°to -11.8°p=0.005
SL along MAx 0.0°3.2°5.1°to -7.4°n.s.
SL along CAxSL 0.3°2.7°4.7°to -4.9°n.s.
Mean PCA?3°and
SL along CAxSL
0.5°2.2°5.2°to -4.8°n.s.
Results are all relative to the SEA. 90°added to vertical axes. Neg-
ative results are internally rotated. (For illustration of APA-1, APA-2,
and SL, see Fig. 5)
Knee Surg Sports Traumatol Arthrosc
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coronal alignment of the trochlear groove must be con-
sidered whenever it is referenced to guide femoral com-
ponent rotation. Finally, the combination of the SL and the
PCA is likely to improve rotational accuracy over either
one used individually.
The exclusive use of SL to determine femoral compo-
nent rotation led to an accurate and reproducible result in a
large clinical series. To our knowledge, there are no similar
studies assessing the post-operative results achieved using
the SL to determine femoral component rotation, thus
making comparison difficult.
The only established, valid, and reproducible technique
for measuring femoral component rotation is the use of CT
scans [6,8]. There are few studies (Table 6) that have used
CT scans to assess the post-operative results achieved with
the use of any landmarks or techniques [25,36,38].
Several studies have compared the use of the measured
resection to the gap-balancing techniques using computer
navigation or intraoperative measurements to assess the
differences [12,18,29,35,39,45]. Whilst they have often
found large differences between techniques, without com-
parison with the gold standard of a CT scan of the ep-
icondyles, it is difficult to comment on the relevance of
their results.
The second study confirms that when the SL is measured
along its own coronal axis (the CAxSL), it is a more
accurate landmark for determining the SEA of the femur
than the APA. This explains the reasons for the accurate
femoral component positioning in the first study despite a
large amount of the literature which predicts that refer-
encing the trochlear groove should lead to poor results. The
technique of measuring the SL on a 3DCT scan, which is
orientated until the SL becomes a straight, vertical line,
reflects the clinical use of the SL during surgery (Figs. 1,2,
3). The trochlear groove is a three-dimensional arc which
has a rotational component and a coronal orientation. In
comparison, referencing the groove with two points such as
the APA [3,43] is likely to lead to additional variability
due to both anatomical and geometrical reasons. This is
shown in Fig. 9which show the APA marked on the same
femur viewed from two different coronal viewpoints. A
parallax error causes the angle between the APA and the
fixed epicondylar axis to vary between the two viewpoints.
This error is extremely difficult to detect intraoperatively
and has not been accounted for in previous studies mea-
suring the APA.
There are two anatomical interpretations that have been
used in studies of the APA. Nagamine et al. [27] deter-
mined the APA on a single 2D axial CT slice (Fig. 4). They
isolated the slice that showed the epicondyles most prom-
inently and then measured the low point in the groove
anteriorly to the high point in the notch posteriorly. Ana-
tomically, the posterior point localised with this technique
is deep within the intercondylar notch and is 1–2 cm pos-
terior to the articular surface of the trochlear groove
(Fig. 5, APA-1). This posterior point is not readily acces-
sible during surgery, which makes this measurement less
clinically relevant. We recreated this technique and
obtained a mean of -1.5°and a SD of 3.6°. These results
are very similar to those obtained by Nagamine et al. [27]
who reported a mean of -1.4°and a SD of 3.3°.
Most other studies have used a posterior point on the
articular surface in the floor of the trochlear groove
immediately anterior to the intercondylar notch. This is the
same starting point as the SL. However, the anterior point
used to measure the APA is the deepest point of the
trochlear groove anteriorly (Fig. 5, APA-2). This point
references the most proximal section of the trochlear
groove that is more variable in normal knees and more
prone to bone erosion and osteophyte formation in osteo-
arthritic knees. Our results confirm the inconsistency of the
Table 4 Outliers [3°from SEA
Outliers [3°
from SEA
Percentage
(n=44) (%)
APA-2 on 3D reconstruction 19 43
PCA?3°13 30
SL along CAxSL 14 32
Average PCA?3°and SL 7* 16
* The average of the PCA?3°and SL (PCA–SL) produced signifi-
cantly less outliers than the PCA?3°(p=0.03) or the SL along
CAxSL (p=0.007)
Table 5 Coronal axis measurements
Mean SD Range
CAxSL to MAx 0.4°3.8°9.4°to -7.3°
DCA to MAx -2.2°3.1°4.7°to -8.1°
CAxSL to DCA 2.6°4.3°13.7°to -4.6°
Negative values are valgus
Table 6 Comparable research assessing femoral component rotation
relative to SEA with post-operative CT scans
References Axis NMean SD Range
Luyckx [25] Preoperative CT 48 2.4°2.5°-2.8°
to 6.9°
Gap balancing 48 1.7°2.1°-2.5°
to 6.5°
Sto
¨ckl et al. [38] PCA?3 32 1.1°2.8°-2°to
12°
APA and
epicondylar
32 -0.4°2.4°-7°to
4°
Seo et al. [36] Mechanical axis
derived
120 1.6°2.2°-4.8°
to 7.9°
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anterior point of the APA. We measured the APA on 3D
reconstructions, aligned to the MAx, from the posterior
point in the trochlear groove to the deepest point in the
trochlear groove anteriorly. This produced a mean of -1.8°
and a SD of 4.2°compared with the SL measured along the
same axis which produced a mean of 0.0°(SD 3.2°). It was
frequently noted that the most proximal section deviated
noticeably from the rest of the groove and that it was often
affected by osteophytes and bone erosion.
The inconsistency of the anterior point in the APA is
supported by Victor et al. [41] who found considerably
poorer intraobserver and interobserver reliability for
detecting the deepest point of the trochlear groove anteri-
orly than for detecting the knee centre in the groove dis-
tally. Cerveri et al. [9] also reported that the interobserver
reliability for detecting the point in the anterior trochlear
groove was significantly worse than for the distal point.
Cerveri et al. [9] also demonstrated improved repeatability
of a computer-derived technique for marking multiple
points along the trochlear groove in comparison with the
APA as measured by surgeons marking only two points.
Piriou et al. [32] recently obtained consistent results by
referencing several points along the trochlear groove using
computer navigation instead of relying on a single anterior
point.
The key anatomical differences between the SL and the
APA are highlighted by research that measured the APA after
the distal femoral resection [37,44]. By removing the distal
femur, the majority of the vertical component of the trochlear
groove, that is essential in producing the SL, is removed.
The geometrical advantage of the SL occurs due to the
use of multiple points along the floor of the trochlear
groove. This allows the orientation of the trochlear groove
to its own coronal axis, which is impossible to achieve with
the APA. The variations between the APA and the epic-
ondylar axes caused by the alteration in the coronal view-
point can be illustrated by comparing oblique views of the
reconstructions (Figs. 1,2,9). This concept may explain the
findings of Iranpour et al. [20] when they noted that the
coronal orientation that leads to the least medial and lateral
variability of the multiple points measured along the
trochlear groove was the orientation along the groove itself.
Crucial to the concept of isolating the rotational compo-
nent of the trochlear groove from the coronal component is
the variability in the coronal alignment of the SL. This was
discussed by Iranpour et al. [20] who used a similar technique
for identifying multiple points in the sulcus. They compared
the coronal alignment of the SL to the coronal alignment of a
DCA derived from sphere matching to the condyles. They
found a mean of 0°but a large SD of 5°. In the current study,
we compared the CAxSL to the MAx and found a mean of
0.4°(SD 3.8°, range from -7.3°to 9.4°). We also compared
the CAxSL to a line perpendicular to the DCA, mean 2.6°
(SD 4.3°, range from -13.7°to 4.6°). These very wide ranges
are consistent with the work of Iranpour et al. [20]. The
significance of this variation is that it effects both the mea-
surement of the APA or SL in anatomical studies and also the
intraoperative use of the SL.
When using the SL during surgery, the line is usually
translated into a horizontal line on the distal condylar
surface, as per the technique described previously (Fig. 3)
[39]. Since identifying the variability in the CAxSL rela-
tive to the DCA, it has become apparent that there is a
further geometrical flaw in this technique. This error is
separate to the geometrical error caused when observing
the SL from an orientation other than along the CAxSL that
we have described above. This translating error will occur
during surgery even though we are orientated along the
coronal alignment of the SL. Whenever transferring the
rotational line from one coronal plane (the CAxSL) on to
Fig. 9 Due to parallax error the rotational angle of the APA relative
to a fixed landmark such as the epicondylar axis changes as the
coronal viewpoint changes
Knee Surg Sports Traumatol Arthrosc
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another (the DCA), there is a risk of changing the rotational
angle. This will occur whenever there is also a deviation in
the sagittal plane.
In practice, this occurs when using a device such as the
T-piece to transfer the vertical SL into a horizontal line on
the surface of the condyles [39]. If there is any flexion or
extension in the sagittal plane of the T-piece, relative to the
planned distal femoral cut, in combination with any dif-
ference between the coronal alignment of the SL and the
DCA, then a rotational error will occur. This error can be
calculated from Euler’s rotational theorem using the for-
mula, tanh
3
=sinh
1
sinh
2
/cosh
1
where h
1
=coronal plane
variation (CAxSL–DCA), h
2
=sagittal plane variation of
T-piece to planned distal femoral cut, and h
3
=resultant
rotational variation in axial plane.
1
In practice, the error
produced with the intraoperative use of a T-piece is likely
to be several degrees (Fig. 10). It becomes more likely in
patients with a large divergence between the DCA and the
CAxSL (Fig. 11).
A trochlear alignment guide (TAG) has been developed
that corrects for the geometrical errors inherent in the use
of the SL during surgery and transfers that angle onto the
cut distal surface of the femur. This allows direct com-
parison of the geometrically accurate SL with the PCA.
From the results of the 3D CT study, we anticipate that the
averaging of these axes will decrease the likelihood of
femoral component malrotation. Similar results have
recently been shown by combining the APA and PCA
using PSI [31]. We predict that even greater accuracy may
be achieved by using a geometrically correct SL instead of
the APA. The use of the TAG allows the SL and PCA to be
combined using conventional instruments rather than only
with PSI.
These concepts have implications for current techniques
of measuring the APA or SL on CT scans and MRI scans
and for their use in computer navigation systems. The
reference of the APA during landmark identification
associated with computer navigation could be improved by
compensating for the variation between the coronal align-
ment of the groove and the MAx. With the increasing
popularity of patient-specific instruments based on preop-
erative CT and MRI scans, we believe that this geometrical
concept must be included in the measurement algorithms if
the trochlear groove is being used to assist in rotational
alignment.
These studies have a number of limitations. In the first
study, our use of two-dimensional axial CT slices to
measure the rotational alignment of the femoral component
is likely to lead to a measurement error that could be
reduced with 3D reconstructions. In addition, due to the
presence of metal artefact, 28 scans were excluded. There
is no comparison group in the first study, and the surgeon is
very experienced in this technique; therefore, the findings
may not extend to other surgeons. However, the study has a
number of strengths. To our knowledge, this is the largest
study to use post-operative CT scans to assess femoral
component rotation and it is also the only study assessing
the radiological outcomes from the use of the SL, making
the findings novel.
The 3D CT study relies on meticulous positioning of
the points along the trochlear groove, which are then
Fig. 10 The rotational angle of the horizontal limb of the T-piece
will change with flexion or extension of the T-piece even though the
vertical limb stays aligned with the SL
1
Euler’s theorem and its proof are contained in paragraphs 24–26 of
the appendix (Additamentum. pp. 201–203) of L. Eulero (Leonhard
Euler), Formulae generales pro translatione quacunque corporum
rigidorum (General formulas for the translation of arbitrary rigid
bodies), presented to the St. Petersburg Academy on October 9, 1775.
Knee Surg Sports Traumatol Arthrosc
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checked on each axis view and on the reconstructions.
The use of CT scans disregards the contribution of
articular cartilage to the shape of the trochlear groove.
However, with MRI scans, it is very difficult to accurately
compare the coronal alignment of the trochlear groove to
the MAx of the femur.
Conclusion
These findings demonstrate the importance of understand-
ing the three-dimensional nature of the trochlear groove.
When this concept is appreciated, the groove is a reliable
landmark for determining femoral component rotation.
Additional accuracy can be obtained by combining the
PCA with a geometrically correct SL.
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