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Coxa valga and antetorta increases differences among different femoral version measurements

Aims To evaluate how abnormal proximal femoral anatomy affects different femoral version measurements in young patients with hip pain. Methods First, femoral version was measured in 50 hips of symptomatic consecutively selected patients with hip pain (mean age 20 years (SD 6), 60% (n = 25) females) on preoperative CT scans using different measurement methods: Lee et al, Reikerås et al, Tomczak et al, and Murphy et al. Neck-shaft angle (NSA) and α angle were measured on coronal and radial CT images. Second, CT scans from three patients with femoral retroversion, normal femoral version, and anteversion were used to create 3D femur models, which were manipulated to generate models with different NSAs and different cam lesions, resulting in eight models per patient. Femoral version measurements were repeated on manipulated femora. Results Comparing the different measurement methods for femoral version resulted in a maximum mean difference of 18° (95% CI 16 to 20) between the most proximal (Lee et al) and most distal (Murphy et al) methods. Higher differences in proximal and distal femoral version measurement techniques were seen in femora with greater femoral version (r > 0.46; p < 0.001) and greater NSA (r > 0.37; p = 0.008) between all measurement methods. In the parametric 3D manipulation analysis, differences in femoral version increased 11° and 9° in patients with high and normal femoral version, respectively, with increasing NSA (110° to 150°). Conclusion Measurement of femoral version angles differ depending on the method used to almost 20°, which is in the range of the aimed surgical correction in derotational femoral osteotomy and thus can be considered clinically relevant. Differences between proximal and distal measurement methods further increase by increasing femoral version and NSA. Measurement methods that take the entire proximal femur into account by using distal landmarks may produce more sensitive measurements of these differences. Cite this article: Bone Jt Open 2022;3(10):759–766.


Introduction
In recent years, there has been an increased focus on femoral version abnormalities, since they have been recognized as a cause of hip pain in young adults and have been integrated into the concept of femoroacetabular impingement (FAI) and hip instability. 1 Femoral version abnormalities can be the cause of FAI, e.g. posterior extra-articular impingement in patients with increased femoral version, or anterior extraarticular subspine impingement in patients with femoral retroversion. [2][3][4] On the other hand, femoral anteversion can contribute to hip instability in patients with developmental hip dysplasia (DDH) and high acetabular version. 5 In addition, these deformities can occur in combination and aggravate or cancel each other out.
Although indications are still evolving, femoral derotational osteotomies are increasingly performed in the setting of FAI and DDH to correct for femoral version abnormalities and improve biomechanics. [5][6][7][8][9][10] Consequently, the exact quantification of femoral version is crucial for diagnosis and surgical planning. Several methods for measuring femoral version have been described. These methods differ regarding the selected anatomical landmarks, which affect resulting femoral version angles. 11 To date, it is unclear how these measurement methods are affected based on common osseous deformities of the femur, and which femoral version measurement method best reflects overall femoral malalignment.
Thus, we aimed firstly to determine method-related differences in femoral version, and secondly to evaluate how varus/valgus deformity (i.e. neck-shaft angle), extent of femoral version, and presence of cam deformity affect femoral version measurements. We hypothesized that femoral version varies significantly based on measurement technique, and that those method-related differences are associated with extent of femoral version, varus/valgus deformity (i.e. neck-shaft angle), and cam deformity (i.e. α angle).

Methods
Study population. This retrospective study was conducted following institutional review board approval, and was based on a two-step approach. First, we screened our institutional database for patients with hip pain and osseous hip deformities who underwent a preoperative CT scan, including the distal femoral condyles, between May 2017 and August 2018. This search yielded 74 hips (61 patients). A total of 24 hips (19 patients) with previous hip surgery were excluded, leading to a total study population of 50 hips (42 patients). Mean age was 20 years (standard deviation (SD) 6) and 60% (25/42) of patients were female. Diagnosis of hip pain was established by a senior paediatric hip surgeon (ENN) based on a history of symptoms for longer than three months, and a positive impingement test result and/or a positive apprehension test result, in the presence of osseous deformities indicative for FAI or DDH. 12,13 Of the included hips, 36% (18/50) of the included hips were diagnosed with hip dysplasia and 64% (32/50) of hips had symptomatic FAI (Table I); 56% (28/50) of hips had subsequent surgery, 14% (4/28) underwent surgical hip dislocation, 46% (13/28) underwent periacetabular osteotomy, and 29% (8/28) underwent hip arthroscopy. CT acquisition. All patients underwent non-contrast CT of the pelvis and the entire femur on a dual-source CT scanner (Somatom Force; Siemens Healthcare, Germany) or a 64-slice multidetector row CT scanner (Sensation; Siemens Healthcare) using automated attenuation-based tube current modulation for reduction of radiation dose. Image analysis was performed on axially reformatted images with 1 mm slice thickness.
Anatomical index measurements in the study population. Femoral version was measured in all patients by a radiology resident (FS) with eight years of experience in diagnostic imaging of the hip using four common methods for femoral version measurement. These methods differ regarding the level of anatomical landmark for the proximal femoral neck axis ( Figure 1). The method of Lee et al 14 uses the most proximal definition at the junction   3D CT models of three patients (retroversion/normal version/anteversion) were manipulated to simulate different α angles (no cam/mild cam/severe cam).

Fig. 4
Mean femoral version measurements of 50 symptomatic patients with hip pain using different methods. Table II. CT-based measurements of femoral version among the four definitions for femoral version of the study population (42 patients (50 hips)).

Mean femoral version, ° (SD; 95% CI)
Lee et al 14    The correlation between method-related differences in femoral version and a) femoral version, b) neck-shaft angle (NSA), and c) α angle of 50 patients are shown.
varus/valgus and cam deformities, each model was manually manipulated to change the NSA (110°, 120°, 130°, 140°, 150°; Figure 2) and α angle (no cam: α angle < 60°, moderate/ severe cam: α angle 70°/ 85°; Figure 3). This resulted in eight models per hip and a total of 24 3D models of the proximal femur. Before the 3D models were used for analysis, we ensured that each of the final models exclusively represented the desired manipulation, i.e. only varus/valgus correction using multiplanar image reformatting. If necessary, further refinements were made. Manipulated models were used to generate 2D masks of the proximal femur, which were then used to quantify femoral version following the aforementioned four femoral version measurements. The model development and manipulation steps are presented in Figures 2 and 3. The model development and manipulation were done by a member of the team with more than ten years of experience in 3D imaging analysis and modelling (AMK). All measurements of femoral version were performed by two expert readers (FS and MKM) to assess interobserver reliability. One expert reader (MKM) repeated all measurements after eight weeks to assess intraobserver reliability. Statistical analysis. Statistical analysis was performed using GraphPad Prism (Version 9.1, GraphPad, USA). Demographic characteristics are presented as percentages or means and SDs. Femoral version measurements were compared between different techniques using repeated measures analysis of variance (ANOVA) with Tukey's post-hoc adjustment for multiple pairwise comparisons. Differences between version measurement techniques were calculated for each hip (e.g. Lee -Murphy) and reported as mean with 95% confidence intervals (CIs). Pearson's correlation was used to determine the associations between method-related differences in femoral version measurements and α angle or NSA. Interobserver reliability of the measurement methods in the 3D manipulation analysis was assessed via ICC. All p-values are two-sided and statistically significant at p < 0.05.

Results
Method-related differences in femoral version. Femoral version was significantly different between all measurement methods (overall and pairwise comparisons, p < 0.001; Tables II and III). Increasing values of femoral version were found using methods with more distal references of the femoral neck axis. Comparing the different measurement methods for femoral version, the lowest values were found for the method of Lee et al 14 Table III).
Association between method-related differences in femoral version and femoral anatomy. Increased femoral version (using the Murphy et al 17 method as reference) was associated with greater differences in femoral version measurement between all methods (r > 0.46 ;p ≤ 0.001; Figure 5a). Similarly, higher degrees of NSA were associated with higher differences of femoral version measurement between all methods (r > 0.37; p = 0.008; Figure 5b). Larger α angles correlated with smaller differences in femoral version only for the differences between the methods of Murphy et al 17 and Reikerås et al 15 (r = -0.28; p = 0.046; Figure 5c). By contrast, there was no significant correlation between larger α angles and smaller differences in femoral version between the methods of Murphy et al 17   The changes in method-related differences ((Diff) Lee et al 14 and Murphy et al 17 ) in femoral version in the 3D parametric analysis depending on a) neckshaft angle (NSA) and b) α angle. In a), the biggest increase in differences of femoral version was seen for femoral anteversion with an increase of 11° (from 16° to 27°) with increasing NSA from 110° to NSA 150°. This effect was less pronounced in b), with an increase in differences of femoral version of 4° (from 16° to 20°) with increasing α angle from 60° to 85° in normal femoral version.
3D parametric manipulation analysis of proximal femur. In the parametric 3D manipulation analysis, the highest differences were seen with femoral anteversion and high NSA. In models femoral anteversion and normal femoral version, an isolated increase in NSA from 110° to 150° resulted in an increase of 11° (from 16° to 27°) and 9° (from 18° to 27°) between most proximal (Lee et al 14 ) and most distal (Murphy et al 17 ) measurement methods of femoral version, respectively (Figure 6a). By contrast, this method-related difference was less pronounced with a 4° increase in the model with femoral retroversion (Figure 6a).
An isolated increase in α angle from 60° to 85° resulted in an increase of 3°, 4°, and 2° in the models with femoral anteversion, normal femoral version, and retroversion, respectively, between most proximal (Lee et al 14 ) and most distal (Murphy et al 17 ) measurement methods (Figure 6b). Detailed results about femoral version can be found in Supplementary Figures a and b. High agreement was found for measurement of femoral version in the 3D manipulation analysis with an ICC ranging from 0.97 for the method of Tomczak et al 16 to 0.99 to the method of Lee et al 14 (Table IV).

Discussion
Our findings indicate substantial differences in femoral version (up to 18°) based on the measurement technique, supporting our first hypothesis. Our findings also show significant associations between method-related differences in femoral version and femoral morphology (i.e. femoral version, NSA, and α angle), supporting our second hypothesis. These findings highlight the importance of measurement technique and proximal femur morphology in treatment planning for surgical correction of femoral version abnormalities.
In general, we observed smaller femoral version in methods using proximal definition of landmarks (i.e. Lee et al 14 ) compared to measurement methods with more distal landmark definition such as the method described by Murphy et al 17 Our findings are in line with previous studies investigating femoral retroversion in slipped capital femoral epiphysis, and in patients with FAI and DDH. 11,21 In these studies, maximum mean differences between proximal and distal measurement methods were as high as 17° (SD 5°) in slipped capital femoral epiphysis and 19° (SD 7°) in FAI and DDH, respectively. 11,21 Considering that surgical correction in femoral derotational osteotomy is usually aimed in the range of 10° to 25°, 7,9,10 these method-related differences are therefore clinically relevant and highlight the importance of reporting femoral version in a standardized manner. In order to facilitate communication among physicians, and to ensure transparency of clinical studies on femoral version, the applied measurement methods should be reported precisely.
A significant association between femoral malalignment and method-related differences in femoral version measurement was observed. Increasing femoral version (using the Murphy et al 17 method as reference) was significantly associated with greater differences in femoral version between the measurement methods. Although indications are still evolving, femoral derotational osteotomies are currently mainly performed in the setting of hip instability and extraarticular impingement in adult patients with excessive abnormalities of femoral version. In this patient cohort, it is crucial to determine the degree of the rotational deformity for planning of the surgical correction supporting the use of more distal measurement methods.
Interestingly, we observed direct associations between NSA and method-related differences in femoral version, which was further confirmed using a 3D parametric analysis. An isolated increase of NSA from 110° to 150° resulted in an 11° and 9° increase in difference of femoral version measurements between the method of Lee et al 14 (Figure 5c). Results of the 3D parametric analysis showed a similar tendency, however the small increase of 3° to 5° might fall within the range of measurement error.
Overall, there is controversy as to which measurement method best reflects femoral malignment. Murphy et al 17 investigated different CT-based measurement methods of femoral version in cadavers, and concluded that more proximal methods which use the greater trochanter (similar to Lee et al 14 ) or the femoral neck axis (similar to Reikerås  17 Accordingly, measurement methods which take the entire proximal femur into account by using distal landmarks such as the Murphy et al method 17 may produce more sensitive measurements of these differences. However, future studies with clinical outcome data are necessary to show the benefit of applying different measurement methods for surgical decision-making. This may be especially important in patients with valgus and femoral anteversion, who are eligible for femoral osteotomies for combined or isolated derotation and angular correction. 3 This study has a number of limitations. First, this is an imaging-based study only without clinical outcome data, precluding any conclusions regarding clinical outcome. Second, only three patients were included in the 3D manipulation study, leading to a small sample size of 24 3D models. However, the 3D manipulation analysis was performed additionally to reproduce findings of the study population with a sufficient sample size (n = 50). Finally, despite being increasingly used to measure femoral version, we did not include MRI scans in this study. 23 This is related to the fact that 3D manipulation of femoral models is more complicated and cumbersome with MRI.
In conclusion, measurement of femoral version angles differs depending on the method used to almost 20°, which is in the range of the aimed surgical correction in derotation femoral osteotomy, and thus can be considered clinically relevant. Differences between proximal and distal measurement methods further increase by increasing femoral version, NSAs, and to a lesser degree decrease by smaller α angles. Accordingly, measurement methods based on distal anatomical methods (i.e. Murphy et al), 17 which take into account the entire proximal femur, may better reflect overall malignment in patients considered for femoral osteotomies.

Take home message
-Measurement of femoral version angles differs depending on the method used to almost 20°, which is in the range of the aimed surgical correction in derotation femoral osteotomy, and thus can be considered clinically relevant.
-Differences between proximal and distal measurement methods further increase by increasing femoral version, increasing neck shaft angles, and to a lesser degree decrease by smaller alpha angles. -Accordingly, measurement methods based on distal anatomical methods, which take into account the entire proximal femur, may better reflect overall malignment in patients considered for femoral osteotomies.

Supplementary material
Figures displaying the neck shaft angle and alpha angle-associated changes in femoral version in the parametric analysis depending on the model with femoral anteversion, normal femoral version, and femoral retroversion.