The femur is the longest and most robust bone in the human body. It carries the body's weight from the hip bone to the tibia when a person is standing. The femur consists of a central cylindrical portion called the shaft, as well as two extremities known as the superior and inferior ends. The proximal segment of the femur comprises a head, neck, and two trochanters, specifically the greater and lesser trochanters. The femoral neck has a length of roughly 5 cm. It is thinnest in its middle section and largest towards the sides. The femoral neck articulates with the femoral shaft at an average angle of 135 degrees, known as the inclination angle or neck-shaft angle. In adults, this angle typically ranges from 125 degrees to 140 degrees [1]. The neck-shaft angle (NSA), collodiaphyseal angle (CDA), diaphysio-femoral neck angle, angle of the femur's neck, angle of inclination, cervicodiaphyseal angle, and collum diaphyseal angle are some of the terminology used to describe this angle [2]. This angle is essential to allow the femoral shaft to move freely away from the pelvis during movement [3]. The angle of inclination of the femur, like the humerus, varies between people and sides. The wider pelvis of women results in a smaller angle of inclination compared to men. The femur's angle of inclination plays a crucial role in maintaining stability, controlling

lateral balance, facilitating walking, and enabling hip mobility. The femur's angle of inclination varies throughout a person's life, starting at a significantly higher level during infancy and youth and progressively decreasing to approximately 120 degrees in older adults. Coxa vara refers to a neck shaft angle of less than 120 degrees, and coxa valga refers to an angle of more than 140 degrees. The femur's inclination angle is clinically significant for diagnosing, treating, and monitoring fractures of the neck of the femur, trochanteric fractures, slipping upper femoral epiphysis, developmental dysplasia of the hip, and neuromuscular diseases of the lower extremity. Assessing individuals with known or suspected medical issues and performing corrective surgery for femoral fractures can greatly benefit from understanding the typical angle of the femur's inclination. We can approximate the angle of inclination using a proximal femoral fragment, and establish the necessary neck length to create prostheses that restore the normal angle of inclination. Understanding the angle of inclination is a significant tool for diagnosing and treating fractures in the upper part of the femur. Understanding the neck shaft angle is critical in orthopaedic treatments, such as the repair of femoral neck fractures. Additionally, it aids in the development and bioengineering of orthopaedic implants and hip prostheses. Although several studies have examined the angle of the neck shaft in different populations, there is a lack of precise data regarding the regional demography of Bihar, India. 

Aim and Objectives: This study aims to evaluate the NSA in the population of Bihar using dry bones. The objective of this study is to gather data that can offer insights to engineers and surgeons regarding the design and positioning of implants for total hip replacement (THR) procedures in the population of Bihar.

This present cross-sectional study was conducted on fifty dry adult femurs of unknown gender. Among that, twenty-four femur bones belong to the right side and twenty-six to the left. These human-dried femur bones were obtained from the Department of Anatomy of Nalanda Medical College, Patna Bihar, and also from different medical colleges in the Bihar state of India.

Inclusion criteria: It included the following:

• Femur bone with the intact upper end: Only bones in good enough condition to allow for precise morphometric measurements of the neck of the femur were collected.
• Age and Sex: Adult bones from different age groups were included. Both genders' bones (i.e. male and female) were considered.
• Specimen Quality: Bones with pathological diseases affecting the upper end of the femur or severe abnormalities were removed.

Exclusion criteria: The study excluded bones exhibiting signs of wear and tear, congenital abnormalities, or fractures involving the femur bones.

The neck shaft angle is the angle formed by the alignment of the femoral shaft's long axis and the long axis of the femoral neck. A goniometer is used to measure the angle. The measurement of the neck shaft angle in this investigation involved marking the longitudinal axis of both the shaft and the neck. The longitudinal axis of the neck was delineated by establishing two points: one at the central position of the head and the other at the superior extremity of the midway of the narrowest segment of the neck.  The line symbolizes the axis of the neck, connecting the two spots. The long axis of the shaft was determined by connecting two midpoints, one at the higher end and the other at the lower end of the shaft. This line was then extended at the upper end to intersect the long axis of the neck. The angle was measured with the help of a goniometer and the angle was noted. The mean and standard deviation of the collected data are reported. In this study, an analysis was considered significant if the p-value was less than 0.05. Graph Pad Prism version 9 was used to statistically analyze the results.

Figure 1: The diagram illustrates the process of measuring the neck shaft angle (NSA) on the femur using a goniometer.

We recorded that the neck shaft angle (NSA) was 129.52±3.82 degrees on the right side and 129.96±3.65 degrees on the left. The overall mean value of neck shaft angle (NSA) observed in the study was 129.75±3.70 degrees. The current investigation yielded no statistically significant distinctions between the left and right side characteristics. (Table 1).

Table 1: Showing the observation of the neck shaft angle (NSA)

  All measurements were taken in degrees.

  DISCUSSION:

The measurement of the femoral neck-shaft angle is crucial since it is known to be linked to certain medical problems. Genu varum is commonly linked to Coxa valga, which is characterized by a neck-shaft angle greater than 140 degrees. This condition can result in premature degeneration of the medial compartment of the knee. On the other hand, coxa vara, which is characterized by a neck-shaft angle less than 120 degrees, can have various causes including congenital factors, metabolic conditions such as Paget's disease, untreated trauma, and developmental factors such as osteogenesis imperfecta or postperthes. As shown in Table 02, we compared our findings with those of other scholars worldwide.

Table 2: Demonstrating the comparison of the neck shaft angle with other studies around the globe

All measurements were taken in degrees.

We recorded that the neck shaft angle (NSA) was 129.52±3.82 degrees on the right side and 129.96±3.65 degrees on the left and the overall mean value of neck shaft angle (NSA) observed in the study was 129.75±3.70 degrees, which was much closer to the findings of Roy S et al. [11] among studies on Indian population while other Indian investigators Siwach et al. [8], Rawal et al. [10], Bhattacharya S et al. [12] and Sharma V et al. [15] report less value than our findings. Ziabari et al. [16] conducted a study on an Iranian population to investigate the neck-shaft angle (NSA) in patients with hip fractures. The study involved 80 patients with femoral fractures (40 with neck fractures and 40 with intertrochanteric fractures) and 40 healthy individuals as a control group. Their results showed no significant differences in the NSA between patients with femoral neck fractures (mean NSA 131.04±3.7°), intertrochanteric fractures (mean NSA 132.07±4.1°), and healthy controls (mean NSA 132.8±6.9°). Additionally, they reported that there were no significant differences in NSA between different age groups or between male and female patients. They reported a value of NSA greater than our study.

The analysis of a population's femoral neck-shaft angle is important because it provides information about related diseases and aids in the production of accurate orthopaedic implants. Researchers have previously employed various techniques such as radiography, CT, MRI, and dry bone measurements to quantify the femoral neck-shaft angle [17,18]. Radiography, particularly digital radiography, is more readily accessible at most institutes compared to CT and MRI. Therefore, it can serve as a useful instrument for measuring the neck-shaft angle. CT and MRI, despite their higher accuracy, are not widely accessible at many institutions due to their high cost. Limb positioning is crucial when utilizing digital radiography to assess the neck-shaft angle. Placing the lower limb in internal rotation can be useful for evaluating the actual varus-valgus of the

femoral neck-shaft angle [19]. A significant metric for measuring the femoral neck-shaft axis is the femoral axis. Taking the measurement will reveal a shift towards varus angulations of the neck-shaft angle, indicating an inward deviation of the distal femoral segment. The procedure was conducted using the longitudinal axis of the femoral shaft rather than the axis of the proximal femur [20]. Worldwide, factors such as ethnicity, side, gender, and measurement technique have influenced differences in neck-shaft angle.

This study had limitations in that the age and sex of the femur bones were not examined because the information was not readily available.

The neck shaft angle acquired from this study should be taken into account when performing surgical stabilization of femoral neck fractures or osteotomies in the adult population of North India. It is important to mention that this applies to the design and manufacture of orthopaedic implants and hip prostheses using bioengineering techniques.

1. Standring Susan, Ellis H, Collins P, Johnson D, Shah P, Wigley C, et al: The Anatomical Basis of Clinical Practice. 39th Edition, Elsevier Churchill Livingstone. 2008;1379.
2. Radha P, Ravi SG, Naveen NS, Roopa CR. Evaluation of neck shaft angle of femur on dry bones. J Evol Med Dent Sci 2015;4:5518‑22.
3. Kaur P, Mathew S, George U. A study of neck shaft angle in the North‑West Indian population on radiograph. Int J Basic Appl Med Sci 2013;3:9‑15.
4. Noble PC, Alexander JW, Lindahl LJ, Yew DT, Granberry WM, Tullos HS. The anatomic basis of femoral component design. CORR1988;148‑165.
5. Rubin PJ, Leyvraz PF, Aubaniac JM, et al. The morphology of the proximal femur. A three‑dimensional radiographic analysis. J Bone Joint Surg Br 1992;74:28‑32.
6. Husmann O, Rubin PJ, Leyvraz PF, de Roguin B, Argenson JN. Three‑dimensional morphology of the proximal femur. J Arthroplasty 1997;12:444‑50.
7. Mahaisavariya B, Sitthiseripratip K, Tongdee T, Bohez EL, Vander Sloten J, Oris P. Morphological study of the proximal femur: A new method of geometrical assessment using 3‑dimensional reverse engineering. Med Eng Phys 2002;24:617‑22.
8. Siwach RC, Dahiya S. Anthropometric study of proximal femur geometry and it’s clinical application. Indian J Orthop 2003;37:247‑51.
9. Umer M, Sepah YJ, Khan A, Wazir A, Ahmed M, Jawad MU. Morphology of the proximal femur in a Pakistani population. J Orthop Surg (Hong Kong) 2010;18:279‑81.
10. Rawal B, Ribeiro R, Malhotra R, Bhatnagar N. Anthropometric measurements to design best‑fit femoral stem for the Indian population. Indian J Orthop 2012;46:46‑53.
11. Roy S, Kundu R, Medda S, Gupta A, Nanrah BK. Evaluation of proximal femoral geometry in plain anterior-posterior radiograph in eastern-Indian population. Journal of clinical and diagnostic research: JCDR. 2014 Sep;8(9):AC01.
12. Bhattacharya S, Chakraborty P, Mukherjee AA. Correlation between neck shaft angle of femur with age and anthropometry: A radiographic study. Indian J Basic Appl Med Res 2014;3:100‑107.
13. Adekoya-Cole TO, Akinmokun OI, Soyebi KO, Oguche OE. Femoral neck-shaft angles: A radiological anthropometry study. Nigerian Postgraduate Medical Journal. 2016 Jan 1;23(1):17-20.
14. Adhikari RK, Yadav B, Yadav SK, Singh AC. Comparative study of angle of inclination and neck length of dry femur. Journal of Nobel Medical College. 2017 Aug 22;6(1):44-7.
15. Sharma V, Kumar K, Kalia V, Soni PK. Evaluation of femoral neck-shaft angle in sub-Himalayan population of North West India using digital radiography and dry bone measurements. J Sci Soc 2018;45:3-7.
16. Ziabari SM, Joni SS, Faghani M, Moghaddam AP. Comparative study of the neck shaft angle in femoral neck and intertrochanteric fractures in north part of Iran. International Journal of Burns and Trauma. 2020;10(5):225.
17. Greendale GA, Young JT, Huang MH, Bucur A, Wang Y, Seeman T. Hip axis length in Mid‑Life Japanese and Caucasian U.S. residents: No evidence for an ethnic difference. Osteoporos Int 2003;14:320‑5.
18. Roy S, Kundu R, Medda S, Gupta A, Nanrah BK. Evaluation of proximal femoral geometry in plain anterior‑posterior radiograph in Eastern‑Indian population. J Clin Diagn Res 2014;8:AC01‑3.
19. Miller F, Liang Y, Merlo M, Harcke HT. Measuring anteversion and femoral neck‑shaft angle in cerebral palsy. Dev Med Child Neurol 1997;39:113‑8.
20. Unnanuntana A, Toogood P, Hart D, Cooperman D, Grant RE. Evaluation of proximal femoral geometry using digital photographs. J Orthop Res 2010;28:1399‑404.