Keywords total hip arthroplasty - femur - canal flare index - dogs - ex vivo study
Introduction
Canine hip dysplasia (HD) is the most diagnosed orthopaedic condition in dogs and
can be associated with a reduced quality of life.[1 ] The management of HD is remarkably challenging and ideally involves a multifactorial
approach, with surgical and conservative components.[2 ]
[3 ]
[4 ]
[5 ]
[6 ]
[7 ]
[8 ] Total hip arthroplasty (THA) is a salvage procedure that can be used to treat dogs
with uncontrolled clinical signs of hip arthrosis. It has been performed successfully
in dogs, enabling maintenance of a functional pelvic limb with effective recovery
of joint mechanisms.[3 ]
[4 ]
[5 ]
[6 ]
[7 ]
Considering the importance of the THA in the management of canine HD and a concerning
rate of complications after this procedure in dogs,[9 ]
[10 ]
[11 ]
[12 ]
[13 ]
[14 ]
[15 ] in particular, femoral fractures and femoral stem subsidence,[16 ]
[17 ]
[18 ]
[19 ]
[20 ] several studies have tried to better understand the morphology of the proximal femur.
These studies assist with THA surgical planning and implant selection, informing the
choice of the implant in both human[8 ]
[21 ]
[22 ]
[23 ]
[24 ] and veterinary medicine.[16 ]
[21 ]
[22 ]
[25 ]
[26 ]
[27 ]
[28 ]
The canal flare index (CFI) was defined by Noble and colleagues[8 ] and it is widely applied in human and veterinary medicine for THA planning and femoral
stem selection. The CFI is the ratio of the intracortical width at the level of the
lesser trochanter to the intracortical width at the level of the isthmus and it is
used as an indirect form of femoral classification.[8 ] In dogs, femurs can be classified into three morphological types: ‘Stovepipe’ (CFI < 1.8),
normal (1.8 < CFI < 2.5) and ‘Champagne Fluted’ (CFI > 2.5) based on CFI.[16 ]
Use of this segmentation improves the selection of patients for THA, choice of surgical
technique, and the design of the ideal femoral implant,[16 ]
[21 ]
[22 ]
[25 ]
[26 ]
[28 ] since accuracy of femoral stem size calculation and femoral canal matching is crucial
to the initial stability of the prosthesis. Maximisation of femoral canal filling
enables appropriate fixation and increases the chance for restoring normal biomechanics
of the hip.[16 ]
[27 ]
[29 ]
[30 ] Additionally, an accurate understanding of each individual femoral characteristic
enables prediction of potential complications, such as femoral stem subsidence and
severe femoral fractures.[16 ]
[17 ]
[18 ]
[19 ]
[20 ] In addition to CFI, there are other factors that need to be considered when selecting
patients for THA and reducing complications. It is important to understand the risk
factors, such as femoral fracture, intraoperative fissures, use of uncemented stem,
osteopenic bones, and others.[27 ]
[31 ] Bone quality can be assessed and may influence rate of potential complications and
the bone mineral density can be measured through dual-energy X-ray absorptiometry.[32 ]
Uncemented systems are currently the most commonly used systems for THA.[3 ]
[4 ]
[5 ]
[15 ]
[33 ]
[34 ] Such systems are totally reliant on biological fixation, so implant stability in
the immediate postoperative period is crucial.[17 ]
[24 ] Geometric fit between the femoral component and the proximal canal is essential
for the success of the technique and reduction in complication rates.[15 ]
[21 ]
The literature describes different methods for CFI calculation with multiple anatomic
regions of the femur used to measure the intracortical width.[16 ]
[26 ]
[35 ]
[36 ]
[37 ]
[38 ] However, due to the anatomy of the proximal metaphysis of the canine femur, there
is a variation in the intracortical width at the three regions normally used to calculate
the CFI (proximal, midpoint and distal aspect of the lesser trochanter), which may
change the final CFI value and even the morphologic femoral classification.[24 ]
[26 ]
[35 ] Few studies have investigated the influence of the CFI calculation method on the
final CFI value.
The aim of this study was to compare the CFI values obtained by different calculation
methods involving measurement of the intracortical widths in the region of the lesser
trochanter (proximal, midpoint and distal end of the lesser trochanter) and to evaluate
the influence of these different measurement points (proximal, midpoint and distal
end of the lesser trochanter) on the final CFI value and femoral classification.
Materials and Methods
Craniocaudal radiographic projections of the femur, obtained from 23 skeletally mature
canine cadavers (mixed breeds, 40–60 kg), were used. The femurs were anatomically
dissected from the specimen and positioned directly on the radiographic cassette,
minimising distortion and magnification of the radiographic image. To achieve accurate
positioning of the femurs, radiotransparent sponges were used for support, such that
the femur rested on the condyles keeping the bone parallel to the cassette ([Fig. 1 ]). Radiographs were taken with digital radiographic equipment (Siemens, São Paulo
State University, Jaboticabal, Via de Acesso Prof. Paulo Donato Castellane s/n, CEP
14884-900, São Paulo, Brazil, RG150/100 gl) and a magnification indicator was positioned
parallel to the femur in all projections. Right and left femurs were considered as
distinct experimental units, resulting in a total sample size of 45 femurs.
Fig. 1 Photographic images of femoral positioning to obtain the craniocaudal radiographic
projection. The images also show the stainless-steel magnification marker, 10 cm long.
The radiographic images were individually analysed by three evaluators. For each radiographic
image, the evaluators measured the intracortical width at the proximal and distal
ends and at the midpoint of the lesser trochanter. The intracortical width was measured
at the level of the isthmus, defined as the narrowest point of the medullary canal
in diaphyseal region, subjectively chosen by each evaluator ([Fig. 2 ]). The ratio of intracortical widths at the different regions of the lesser trochanter
and intracortical width of the isthmus determined the proximal, midpoint and distal
CFI ([Fig. 2 ]).
Fig. 2 Representation of the lines drawn for intracortical width at three different points
in the region of the lesser trochanter: proximal end (green), midpoint (yellow) and
distal end (red) (A ). Measurement of intracortical width at the isthmus (B ).
The power of the test is post-hoc and was calculated using the 'pwr.anova.test' function
of the 'pwr' package. The arguments provided included the number of groups (3), the
sample size in each group (135), the effect size, calculated as the ratio between
the sum of squares between groups (SSB) and the total sum of squares (SST + SSB).
The power of the test was calculated for a significance level of 0.05.
Descriptive analysis of the CFI variable was performed using mean and standard deviation.
The data were submitted to the Shapiro-Wilk normality and Bartlet homoscedasticity
tests. Contrasts within factors were obtained using Bonferroni's multiple comparisons
test. All analyses was performed using R Software (R Core Team, 2020), with a significance
level of 5%.
Results
The repeatability test ensured the integrity of the measurement method and the study
reproducibility. In all three treatments, the power of the test was equal to or close
to 0.80 (80%) ([Table 1 ]) which confers reliability on the results of difference between treatments (Bonferroni)
([Table 2 ]). In other words, the sample size was large enough to ensure that the results are
reliable.
Table 1
Test power for analysis of the three treatments
Treatment
Sample size
Power
Proximal
135
0.83
Midpoint
135
0.83
Distal
135
0.83
Table 2
Descriptive analysis of the canal flare index variable at the three regions for each
of the observers
Treatment
Observer
n
Mean
SD
Min.
Máx.
Bonf.
Proximal
1
45
2.14
0.24
1.03
3.12
2
45
2.15
0.28
1.75
3.06
a
3
Total
45
135
2.17
2.15
0.25
0.29
1.59
1.03
3.18
3.18
Midpoint
1
45
1.94
0.30
0.90
2.83
2
45
1.96
0.31
1.60
2.70
b
3
Total
45
135
1.98
1.96
0.28
0.31
1.45
0.90
2.77
2.83
1
45
1.79
0.33
1.03
2.55
Distal
2
45
1.82
0.31
1.47
2.62
c
3
Total
45
135
1.81
1.81
0.27
0.31
1.41
1.03
2.45
2.62
Abbreviations: Bonf, Bonferroni's multiple comparisons test; max, maximum; min, minimum;
n, number of observations; SD, standard deviation.
There was no meaningful difference between the three observers in measurements at
any level (p < 0.001; [Table 3 ]). The agreement between the evaluators was determined using the intraclass correlation
coefficient with a 95% confidence interval. However, there was a significant difference
in the final CFI value between all levels, in which the mean and standard deviation
were proximal 2.15 ± 0.29, midpoint 1.96 ± 0.31 and distal 1.81 ± 0.31 ([Table 2 ]). In addition, it is worth noting the low standard deviation in all cases, which
indicates that the mean is a good representation of the dataset ([Table 2 ]).
Table 3
Descriptive table of the intraclass correlation coefficients (ICCs) in the interobserver
analyses (reproducibility)
Treatment
ICC (95% CI)
p -Value
Proximal
0.97 (0.94–0.99)
< 0.001
0.97 (0.94–0.99)
< 0.001
Midpoint
0.98 (0.97–0.99)
< 0.001
0.97 (0.93–0.98)
< 0.001
Distal
0.99 (0.98–0.99)
< 0,001
0.97 (0.93–0.99)
< 0,001
Abbreviations: CI, confidence interval.
The boxplot analysis shows that the median measurements of the observers at each region
(proximal, midpoint and distal) are remarkably similar, indicating little variation
in interobserver measurements. However, the median CFI values reduced from the proximal
to midpoint and midpoint to distal regions, with median measurements being highest
proximally and lowest distally ([Fig. 3 ]).
Fig. 3 Boxplot of the canal flare index (CFI) variable for the three different measurement
regions and observers. The black horizontal line inside the box is the median and
the black horizontal line outside the box indicates the mean plus three times the
standard deviation. The ends of the box are the interquartile ranges (25 and 75% of
the data). Different letters indicate statistically significant difference, according
to Bonferroni's multiple comparisons test. The vertical lines indicate all the CFI
values.
[Table 4 ] summarise the results for each region of the lesser trochanter used to calculate
the CFI and the prevalence of each femoral classification depending on the calculation
method.
Table 4
Femoral classification from the calculation of the canal flare index in the proximal,
midpoint and distal region of the lesser trochanter by the three observers
Treatment
Observer
Stovepipe
Normal
Champagne fluted
Proximal
1
6.7
73.3
20.0
2
4.4
77.8
17.8
3
4.4
75.6
20.0
Midpoint
1
22.2
66.7
11.1
2
20.0
64.4
15.6
3
20.0
68.9
11.1
1
51.1
46.7
2.2
Distal
2
46.7
51.1
2.2
3
57.8
42.2
0
As no difference was observed between the three evaluators, results were combined
as follows: for the proximal level, 4% of the femurs were classified as ‘Stovepipe’,
22% as ‘Champagne Fluted’ and 74% as ‘Normal’ ([Fig. 4A ]); for the midpoint level, 20% of the femurs classified as ‘Stovepipe’, 15% as ‘Champagne
Fluted’ and 65% as ‘Normal’ ([Fig. 4B ]); and for the distal level, 46% of femurs classified as ‘Stovepipe’, only 4% as
‘Champagne Fluted’ and 50% as ‘Normal’ ([Fig. 4C ]).
Fig. 4 Femoral classification of the three observers based on the calculation of canal flare
index in the proximal (A ), midpoint (B ) and distal (C ) region of the lesser trochanter.
Discussion
This study demonstrated that the point chosen for endosteal measurement in the region
of the lesser trochanter directly influences the final CFI value. CFI variation is
associated with the different regions of measurement, but it is not associated with
the different observers.
Recently, Sevil-Kilimci and Kara[38 ] investigated how the canine CFI is influenced by the measurement method. However,
that study only investigated the influence of measurements at the proximal and midpoint
regions of the lesser trochanter, while this study evaluates the influence of measurement
at each of three regions of the lesser trochanter on the calculation of the CFI.
In the human literature,[8 ] the optimal region for calculation of CFI is 20 mm above the lesser trochanter.
However, in veterinary medicine there are no anatomical studies that discuss this
issue. In dogs the proximal stem is placed at the metaphyseal region delimited by
endosteal limits at the midpoint of the lesser trochanter. Our study observed that
there was no difference between the evaluators, but that when the proximal and midpoint
of lesser trochanter are used for the calculation of CFI, the populations of the three
femoral types are more homogeneous ([Fig. 4 ]), and the same was observed in the study by Sevil-Kilimci and Kara.[38 ]
In a similar study, Sevil-Kilimci and Kara[38 ] calculated the CFILT-I as the ratio of the endosteal width at the medial aspect
of the lesser trochanter and at the isthmus resulting in 6% stovepipe, 82% normal
and 12% champagne fluted. The CFIPLT-I was calculated as the ratio of the endosteal
width at the proximal aspect of the lesser trochanter and at the isthmus; in this
case no stovepipe femurs were observed, 55% of femurs were normal and 45% were champagne
fluted. Such variation in femoral types according to the region of the lesser trochanter
used to obtain the CFI demonstrates the influence of the calculation method on femoral
classification.
In this study, the mean of proximal, midpoint and distal CFI values were 2.15, 1.96
and 1.81 respectively. In other studies, when the proximal region of the lesser trochanter
is chosen to calculate the CFI, the average is 2.40[35 ] and 2.48.[38 ] The literature also reports a mean of 2.10[39 ] when using the midpoint region of the lesser trochanter.
Substantial differences were observed in the endosteal widths between the regions
of the lesser trochanter. As the endosteal width in the proximal region of the lesser
trochanter is greater than in the midpoint and in the distal region, the calculated
CFI will be highest in this region and lowest in the distal region. In this study,
on average, the endosteal width in the proximal region of the lesser trochanter was
9% greater than in the midpoint region and 17% greater than in the distal region.
Other studies have reported an endosteal width in the proximal region of the lesser
trochanter 14%[35 ] and 12%[38 ] greater than in the midpoint region and 24% greater than in the distal region.[35 ]
Such variation in endosteal width used for the CFI calculation results in variations
in the CFI, which may change the classification of a femur. This has a direct influence
on the planning of surgical procedures such as THA, because the stem is proximally
sited at the diameter of the metaphyseal region, which is used to calculate the CFI.[8 ] It is, therefore, important to be aware, when planning THA procedures, that the
CFI will vary depending on the measurement point.
To reduce potential complications after cementless THA[11 ]
[12 ]
[17 ]
[19 ] such as femoral fissures, fractures and subsidence of the femoral component, CFI
is also important in clinical practice for the selection of the appropriate technique
and implant used in THA, especially femoral systems in some situations such as a stovepipe
femoral morphology. Larger patients with CFI less than 1.8, categorised as stovepipe,
may require the use of cementless collared stems, a cementless stem with a lateral
bolt or a cemented stem to provide protection against implant subsidence in the early
postoperative and implant stabilisation periods.[40 ]
‘Stovepipe’ femurs, which have a lower CFI, are straighter than normal and ‘Champagne-Fluted’
femurs, which have higher CFIs, representing tapered bones with thinner structures.
The use of cemented implants has been recommended in dogs with ‘Stovepipe’ femoral
morphology, since the risks of postoperative femoral subsidence and fracture are higher.[3 ]
[4 ]
[5 ]
In this study, interobserver effects were studied by analysis of results from three
observers, but there was no interobserver difference, demonstrating reliability and
reproducibility of the evaluated data. Previous studies have included measurements
performed by a single observer, eliminating any possibility of interobserver influence
but precluding assessment of interobserver effects.[26 ]
The clinical positioning indicated for the calculation of the CFI is extended leg
ventrodorsal radiographical positioning for standard craniocaudal femoral radiograph.
de Andrade and colleagues demonstrated that the craniocaudal projection with a horizontal
radiographic beam is the best approach to provide true anatomical dimensions of the
canine femur, minimising the effect of the positioning artifact on the CFI values.[37 ] However, this study is anatomical and aimed to provide a more reliable version of
the craniocaudal image of the femur.
The primary limitation of this study is that isolated femurs from cadavers were used,
which allows for ideal radiographic positioning. In clinical practice, it is rarely
possible to achieve optimal positioning for radiography due to pain and other positioning
limitations. Thus, if performed in live animals, the measurements obtained and the
final CFI values could be different from the results of this study.
The radiographic positioning of the femur used in this study is in line with the method
described by Palierne and colleagues[35 ]
[36 ] as well as Sevil-Kilimci's study.[38 ] In this study, the femur was positioned directly on the radiographic cassette, supported
on the condyles and kept parallel to the cassette with the aid of radiotransparent
foam ([Fig. 1 ]). This positioning reduced femoral angulation in relation to the radiographic cassette,
removing the effects of cadaver positioning.
The study of the CFI is highly relevant since the method of measurement can potentially
influence the surgeon's understanding of the geometry of the proximal femur and, consequently,
affect the planning of the procedure.
Conclusion
The CFI decreased when measurements were made at the top, midpoint or bottom of the
lesser trochanter.
The proximal region of the lesser trochanter is the one that relates to the proximal
aspect of the stem. Calculation of CFI in this region allows us to observe a more
uniform population, as used in other studies.[38 ]
Therefore, in THA planning, veterinary surgeons should consider the fact that canal
flare measurements differ between calculation methods. Further morphological studies
of the canine femur are needed to determine and standardise the CFI measurement in
dogs and to better evaluate the CFI measurements influence on the complication rates.
