AGE AND GENDER DIFFERENCES IN MODIFIED FEMORAL NECK-SHAFT ANGLE: AN MRI BASED OBSERVATIONAL STUDY

Background: Femoral neck-shaft angle has its importance in describing biomechanics of hip. Measurement methods like plain radiography and CT scans have its limitations. The modied neck-shaft angle (mNSA) drawn on MRI scans is less susceptible to rotational effects and is more reliable for getting normal values. The present study was undertaken to assess the neck shaft angle on MRI and establish differences according to age and gender. Total 200 adults were recruited and equally distributed under four study cohorts according to age and gender. MRI of hip Methods: joint was analysed and mNSA was measured using pre-dened axial lines and landmarks. The mean age of the participants was Results: o o 49.76±18.184 years (18 to 87 years). The mean mNSA was 147.855 . Males had signicantly higher mean mNSA (149.93±6.61) than females o (145.78±5.71) . Though the younger age group (<60 years) had higher mNSA values (148.32±6.65) than older (> 60years) age group (147.3900±6.35), the difference was not statistically signicant. We conclude that gender-based variation exists in the population Conclusions: with the higher values of mNSA in males as compared to females in any age group. The age based difference is also present, but it is not signicant. Neck shaft angle calculated on the MRI has the advantage that it is unaffected by rotation of the hip and is radiation free. The orthopaedic surgeons can use the mNSA for the diagnosis and planning surgeries around hip, for designing implants and prosthesis. It can also be of help for the forensic anthropologists and to the anatomists. Multicentric studies may be undertaken to include a larger population.


INTRODUCTION
The Hip joint is weight-bearing joint which is supported through 1 robust muscle groups and ligaments . Femoral head inclination, measured as neck-shaft angle, is essential in describing biomechanics of the hip joint. The neck-shaft angle (NSA), also known as the Caput -Collum-Diaphyseal angle (CCD angle) is the angle subtended by the 2 femoral head with the shaft of femur in the coronal plane .
NSA plays a pivotal role in the biomechanics of the hip joint. Gait changes occurring in hip pathologies like congenital dysplasia of the hip, cerebral palsy (CP), congenital coxa vara, etc., have relation to 3 change in normal neck-shaft angle . Femoral neck-shaft angle is also an important predictor for femur neck fractures in osteoporotic 4 postmenopausal women .Surgeries on the proximal femur are aimed to remove pathology and restore anatomy and biomechanics as far as possible. Precise measurement of neck-shaft angle is valuable not only for pre-operative templating in complex surgeries like total hip arthroplasty (THA) but also for calculating the angle for osteotomies and for fracture xations. This can help in reducing the number of 5 complications .
The value of neck-shaft angle varies with age, being very high during the neonatal period. It becomes more varus with increasing weightbearing and activity level during childhood, becomes stable in midadolescence to adulthood period and may decrease gradually in more 6 advanced age . Studies have also shown gender-specic differences in neck-shaft angle, stating that females have smaller neck-shaft angle which relates to their wider pelvis and shorter length of femur while 7,8 some studies have also shown higher values for females . Nelson and Megyesi (2004) researched ethnic and sex variations in bone structure and consequently established the want for creating gender-specic 9 implants .
Several methods have been proposed in the past for the measurement of the neck-shaft angle. Methods such as radiography, dry bone measurement and Computerized Tomography scan (CT) are there in [10][11][12] literature, but none of the methods is reliable .
Digital radiography was most acceptable for this purpose due to its easy availability and affordability. But there is a possibility of error without its standardization that can appear due to rotation of the hip. The rotation of hip may lead to overestimation or valgization of NSA and in a similar way exion of the hip can represent as underestimation 13 or varization of the angle . Thus, it requires positioning the hip in approximately 15-20 degrees of internal rotation to standardize the Antero-posterior (AP) radiograph. However, correct positioning is difcult to achieve in cases of trauma due to pain and also in cases of contractures, like in patients of cerebral palsy, osteoarthritis, etc. Due to these drawbacks of radiography, pre-operative measurements (size and shape) for a femoral component in custom-made prosthesis cannot be accepted as accurate. The Femoral neck-shaft angle can be accurately measured by using CT scan and MRI images. But their use was limited until now because of non-accessibility in many institutes 14 and high-cost factor .
CT scan is superior to all other methods for assessment of proximal femur structure but the high dose of radiations is questionable and has ethical issues for use in healthy people or fertile aged women. MRI, on the other hand, does not have any radiation hazard and for this reason, it 15 is best for analysing Neck shaft angle .
The modied neck-shaft angle (mNSA) is barely sensitive to the effects of rotation like the traditional NSA. This new method for measuring NSA was described by Christoph Kolja Boese et al in 2016 16 and they found it to be better and reliable than old methods .
Currently surgeries for proximal femur fractures and other diseases of hip have become very common and they use a variety of implants, like Dynamic Hip Screw, Dynamic condylar screw, the proximal femoral nail (PFN), angled blade plates and, all types of prosthesis that are used in hip arthroplasty. All these are manufactured according to the biomechanical consideration and the anthropometric data available for the western world. Furthermore, the appropriate use of the Neck-Shaft Angle (NSA) in the development of orthopaedic implants and prosthesis may have impact on the durability of implants and in getting [17][18][19][20] good xations . The objective of the present research was to evaluate the modied femoral neck-shaft (mNSA) angle of the proximal Femur measured by MRI scan, and to determine any differences of these We conclude that gender-based variation exists in the population Conclusions: with the higher values of mNSA in males as compared to females in any age group. The age based difference is also present, but it is not signicant. Neck shaft angle calculated on the MRI has the advantage that it is unaffected by rotation of the hip and is radiation free. The orthopaedic surgeons can use the mNSA for the diagnosis and planning surgeries around hip, for designing implants and prosthesis. It can also be of help for the forensic anthropologists and to the anatomists. Multicentric studies may be undertaken to include a larger population.

KEYWORDS
MRI hip, modied neck-shaft angle, proximal femur geometry. measurements according to the age and gender.

This observational study was conducted at Teerthanker Mahaveer
Medical College and Research Centre, Moradabad, India after getting approval from ethical committee, between 2018 to 2019. All participants above the age of 18years were included in the study while participants with any pathology of hip joint, any history of previous surgery around hip joint, deformity in hip, presence of orthopaedic implant around the hip joint or having any contraindications to MRI were excluded from the study. Participants were further grouped under 4 cohorts for analysis consisting of 50 scans (50 hips) in each : (I) males 18-60 years age, (II) males above 60years age, (III) females18-60years age, (IV) females above 60years age. All these participants underwent MRI of hip. Images were observed and evaluated by a single investigator on console. Modied Femoral Neck -shaft angle was measured as shown in Figure 1. A perpendicular to the FLA was drawn, cutting the apex of the Lesser Trochanter. The Modified Femoral NSA (mNSA) was dened as the angle between the FLA and the modied FNA (mFNA).

Statistical Analysis:
Data was analysed statistically by nding the mean, standard deviation and percentage of the parameters studied. The statistical software used for descriptive statistics was SPSS (version 20.0). The results of these parameters were compared between the cohorts studied and the results were correlated using appropriate statistical test -unpaired student ttest, one way and two way ANOVA tests. A value of p ≤ 0.05 was considered signicant.

RESULTS
Total of 200 participants (or 200 hips) were equally distributed among 4 age and gender wise cohort groups. The mean age of overall studied participants was 49.76 years (18 to 87years, SD 18.184years). Mean age of females and males were 50.45 years (18 to 83 years) and 49.07 years (18 to 89 years) respectively as shown in Table 1.

Gender based analysis:
The data was grouped by gender as shown in Table 2 and we observed that males (n= 100) had higher values of mean mNSA (149.93±6.61 degrees) than that of females (n=100) (145.78±5.71 degrees). This difference was found to be statistically signicant (t = 4.747, p<0.0001).

*independent student t-test
Age based analysis: All the measured values were distributed among 2 groups with cut-off age of 60years as shown in Table 3. Out of the total 200 participants, the group of younger age participants (n= 100) (<60 years) had higher neck-shaft angle (148.32±6.65 degrees) than older (> 60years) ones (n = 100) (147.3900±6.35). But this difference was not statistically signicant (t=1.011; p>0.05). The measurement data frequency histogram is shown in Figure ( Table 5 shows signicant differences in between the paired males and females of age wise groups ( < 0.0001) using two-way ANOVA test. But no statistically signicant difference was found in mean neck-shaft angle according to the age variation (p>0.05).  The trend of the mNSA in the studied 200 participants is shown in gure 3. The trend for both the genders was found to be same that is sloping downward from younger to older age. But the means of mNSA was higher for males compared to females for both younger and older age groups.

DISCUSSION
Many studies conducted on the measurement of femoral neck-shaft angle have shown variation in neck-shaft angle which is attributable to 17 varying levels of activity, morphology, race, and lifestyle . Since the neck-shaft angle affects the biomechanics of the hip, therefore, the goal of all hip-related surgeries is to restore the normal neck-shaft angle. This is also important in cases of hip arthroplasty where femoral 45 components are designed with the proper knowledge of NSA . In the present study, we made our focus to get the normal values of NSA with the help of a standardized technique using MRI and observing the differences according to age and gender since the data for Indian subpopulation was lacking.
In this study, the mean modied neck-shaft angle for males was 149.93±6.61 degrees and for females, it was 145.78±5.71 degrees. The mean NSA for the 200 studied participants was 147.85±6.50 degrees. We found that the mean mNSA of males was signicantly higher (p< 0.0001) than females. The average mean of mNSA for participants less than 60 years of age group was higher (148.32±6.65 degrees) than for participants of more than 60 years of age group ( The present study used a clearly dened method for drawing the landmarks and making axis for measurement of a modied form of NSA using a non-invasive modality (MRI) which was highly reproducible. We used a large cohort of 200 adult Indian populations that covered both the genders and included a wide range of ages (18-87 years).

CONCLUSION
From our study it can be concluded that gender based variation does exist in the studied population with the higher values of neck shaft angle in males as compared to females in any age group. However, this study did not establish any signicant difference in the angles among the two age groups i.e. participants of less than 60yrs and more than 60yrs. The present study can be of use to orthopaedic surgeons for planning surgeries like osteotomies, to the biomedical engineers for designing implants and prosthesis corresponding to this ethnic group, to the forensic anthropologists to determine racial variation of the angle and also to the anatomists. It can be recommended from this study that MRI can be used as a standard method to measure NSA for the investigation of proximal femoral geometry. We also recommend for a multicentre study using this method for establishing normal values for a larger subset of population.

STRENGTH AND LIMITATIONS OF THE STUDY
The strength of our study was that the study included a large cohort of participants in the study (200 adults) of a variety of ages (18-87 years). Also, the modality used for study purposes was MRI which is least affected by anteversion and joint contractures and is also non-invasive for study in a healthy population.
The limitations of the study are that it did not include the paediatric population, the subset where hip-related surgeries for conditions like developmental dysplasia of the hip (DDH) are required for correction of hip biomechanics. Also MRI is an expensive technique that is not easily available. The present study did not take into account the side laterality, anteversion angle of femur, physical activity, height and weight of the participants which might affect the neck-shaft angles.