Proximal Femoral Anatomy in the Normal Human Population
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- Toogood, P.A., Skalak, A. & Cooperman, D.R. Clin Orthop Relat Res (2009) 467: 876. doi:10.1007/s11999-008-0473-3
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In this study, we developed a complete description of the morphology of the proximal femur. Then, using this framework, we (1) determined normal population means, standard deviations, and ranges; (2) established differences among subpopulations; and (3) showed correlations among the various measurements. To accomplish these objectives, we analyzed 375 adult femurs. Specimens were digitally photographed in standardized positions, measurements being obtained using ImageJ software. Three parameters of the head-neck relationship were assessed. Translation was examined through four raw offset measurements (anterior, posterior, superior, inferior) used to calculate anterior-posterior and superior-inferior ratios. Rotation was investigated through anteroposterior (AP) and lateral physeal angles. Concavity was examined using alpha, beta, gamma, and delta angles. Two parameters of the neck-shaft relationship were assessed, neck version and angle of inclination. Average anterior-posterior and superior-inferior ratios were 1.14 and 0.90. Average AP and lateral physeal angles were 74.33° and 81.83°, respectively. Averages for alpha, beta, gamma, and delta angles were 45.61°, 41.85°, 53.46°, and 42.95°, respectively. Average neck version and angle of inclination were 9.73° and 129.23°, respectively. Differences existed between males and females and between those younger and older than 50 years. Correlations were observed between translation and concavity, and translation and the neck-shaft relationships.
Level of Evidence: Level II, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.
The morphology of the proximal femur, specifically the relationships among the head, neck, and proximal shaft, has been a subject of interest and debate in orthopaedic literature dating back to at least the middle of the 19th century . As an area susceptible to numerous pediatric and adult disorders, many of which may correlate with variations in this morphology or whose treatment might benefit from a detailed understanding of this area’s anatomy, a substantial body of research aimed at academically defining and pragmatically measuring the proximal femur’s dimensions has developed. These efforts have led to a robust vocabulary for discussing proximal femoral anatomy and abundant methods for its quantification through various linear and angular measures.
Perhaps as a result of their ease of appreciation, the earliest efforts to quantify the proximal femur beyond mere description of its components began with examining the relationship between the femoral shaft and neck. Two well-known parameters have long defined this relationship, angle of inclination (also referred to as neck-shaft angle) and neck version. Although the former of these two measurements has a widely accepted theoretical definition, average value (135°), and standard radiographic method of determination , the latter has produced more than a century’s worth of investigation regarding its true definition, normal values, and preferred method of measurement [1, 3–5, 8, 11, 16, 20–23, 26, 29]. Regardless of the volume of research each has generated, however, both are firmly grounded statutes of modern orthopaedics.
Although the relationship between the shaft and neck of the femur has been quantitatively scrutinized by numerous authors for more than a century, critical evaluation of the head-neck relationship is still in relative infancy. Although an abnormal relationship between the head and neck had been a speculated cause of impingement, joint destruction, and osteoarthritis [6, 7, 15, 17, 18, 25, 27, 28] since the mid-20th century, only more recently have efforts been made to quantify this relationship beyond the qualitative descriptions provided by Murray  and Stulberg et al. . Specifically, from the desire to further investigate femoroacetabular impingement (FAI), two new classes of measurements have been devised during the last decade to evaluate the relationship between the femoral head and neck.
The first measurement, called the alpha angle, was devised by Nötzli et al.  and attempted to quantify the concavity of the anterior head-neck junction. Although only tested against a small sample, this measurement was successful in differentiating individuals with FAI resulting from deficient concavity of the anterior head-neck junction from matched controls. Such results tentatively support the use of the alpha angle for continued quantification of the head-neck relationship.
The second measurement, head-neck offset, recently was quantified [10, 24]. This linear measurement can be used to determine the position of the head relative to the borders of the femoral neck in a plane perpendicular to the femoral neck axis. Like the alpha angle, it has been successful in differentiating individuals with FAI resulting from deficient offset of the anterior head-neck junction from matched controls and is a promising method for quantification of an aspect of the head-neck relationship.
Despite the consideration given to describing and defining proximal femoral anatomy by previous authors, there are at least two essential informational deficiencies in the available literature. First, with perhaps the exception of neck version, there has yet to be a published study that rigorously determined the normal values and degrees of variability in a healthy (normal) population for the defining characteristics of the proximal femur. Second, to the knowledge of the senior authors (AS, DRC), no description exists regarding the angular relationship between the capital physeal scar and the femoral neck axis, a measure that would quantify rotation movements of the head on the neck. The purpose of our study, therefore, was to combine the existing measurements of the proximal femur (or modifications and expansions of these measures as necessary) with an original measure of femoral head rotation to create a complete description of the proximal femur’s anatomy and use this devised framework to produce a global assessment of this area’s morphology in healthy individuals. Specifically, our objectives were to (1) assess the relationship between the head and neck in terms of translation, rotation, and head-neck junction concavity; (2) assess the relationships between the neck and proximal shaft in terms of neck version and angle of inclination; (3) show what differences exist between various subpopulations based on gender and age; and (4) determine if major correlations exist between any of the various measurements.
Materials and Methods
Sample selection and demographics
Original number of femora
Number of abnormal femora
Final number of femora
Mean age (years)
Age range (years)
Each of the specimens was digitally photographed in two standardized positions, termed AP and lateral. For the AP photographs, we first placed each pair of femora in a supine position on a flat laboratory bench with anterior surfaces directed toward the ceiling and femoral shafts parallel to one another. In this position, specimens rested distally on the convex surfaces of the medial and lateral condyles and proximally on the greater trochanter. The femoral neck then was made parallel to the superior surface of the laboratory bench by either rotating the femoral shaft internally and supporting the lateral condyle if the neck axis was anteverted or rotating the femoral shaft externally and supporting the medial condyle if the neck axis was retroverted. Parallelism between the femoral neck and laboratory bench was determined through visual inspection. The investigator taking the photographs (PAT) used square cards, approximately 1 mm in thickness, to increasingly support the medial or lateral condyle until the axis of the neck appeared parallel to the laboratory bench surface. By taking a photograph from directly overhead (camera lens parallel to the laboratory bench and femoral neck axis as confirmed by a level), we obtained accurate AP pictures; any potential distortion resulting from neck version was eliminated by making all components of the setup parallel. For the lateral photographs, we again placed each pair of femurs on the flat laboratory bench surface with anterior surfaces facing up. The femora then were abducted until the femoral necks were parallel with the plane produced by the edge of the laboratory bench. Parallelism again was determined through visual inspection. The investigator taking the photographs (PAT) increasingly abducted the femoral shafts until the axis of the neck appeared parallel to the laboratory bench edge from overhead. Additionally, each femur was checked to ensure the medial and lateral condyles rested on the surface of the laboratory bench distally, allowing the table surface to represent the transcondylar axis. By taking pictures with the lens of the camera parallel to the edge of the laboratory bench (as confirmed using a T-square ruler) and even with its surface (as confirmed through the camera’s view finder), we obtained accurate lateral images. Any distortion produced by the angle of inclination was eliminated by making the neck axis and camera parallel.
Using ImageJ software (nih.gov), we obtained 12 raw measurements from each specimen, six from each of the two views. These measurements were used to define three parameters of the head-neck relationship (translation, rotation, concavity) and two parameters of the neck-shaft relationship (neck version, angle of inclination).
Ranges, means, and standard deviations were determined for each of the measurements made for the population as a whole and for the various subpopulations based on gender and age (younger or older than 50 years at the time of death). We used paired Student’s t tests to establish the significance of any noted differences and post hoc power analysis, which revealed a power greater than 0.90 for all examined differences, was used to confirm the adequacy of the sample size. Pearson’s coefficients were calculated to examine correlations between variables.
Summary of measurements for the entire sample
AP physeal angle
Lateral physeal angle
Angle of inclination
Examining the two parameters defining the relationship between the femoral neck and proximal shaft showed, while neck version and angle of inclination had average results similar to those typically seen clinically , both also had ranges including values well beyond these dogmatic means (Table 2). Specifically, although the population’s average neck version (mean, 9.73°) and angle of inclination (mean, 129.23°) were similar to the 10° and 130° listed in Koval and Zuckerman’s Handbook of Fractures , both also had ranges greater than 40°.
Differences based on gender or age
AP physeal angle
Lateral physeal angle
< 50 years
> 50 years
Correlations between measurements of translation and concavity/neck-shaft
Pearson’s coefficient (R)
Angle of inclination
Despite the existence of a substantial body of literature dedicated to describing and defining the anatomy of the proximal femur, a quantitative, global examination of the relationships among the shaft, neck, and head had yet to be produced. With this informational deficiency as an impetus, we attempted to (1) assess the relationship between the head and neck in terms of translation, rotation, and head-neck junction concavity; (2) assess the relationships between the neck and proximal shaft in terms of neck version and angle of inclination; (3) show what major differences exist between various subpopulations based on gender and age; and (4) determine if major correlations exist between any of the various measurements.
This study had certain limitations. With regard to data collection, the available specimens were approximately 100 years old, and thus these individuals lived during a period when rickets and other bony abnormalities were more likely to develop and go untreated. In addition, it is unknown how differences in lifestyle and general nutrition might affect the generalizability of these results to contemporary populations. Also, regarding the clinical applicability of these results, the methods of measurement for a cadaveric sample, which allow observation of an entire, cleaned bone, obviously cannot be used in living patients. Although the necessary imaging technology currently exists to obtain these measurements from patients, before these data can be translated into clinical practice, such images must become routine. Specifically, although previous authors have used study-specific MRI sequences to determine measurements of translation and concavity [10, 19, 24], such imaging sequences are not used routinely in the contemporary evaluation of patients. Many institutions do not insist on absolutely eliminating version when obtaining plain AP films and thus do not collect accurate AP views as we used in this study. Until collection of derotated plain films becomes routine, the clinical applicability of the data from this study is somewhat limited.
Although comparative data were not available for the unique measures in this study (AOS/POS, SOS/IOS, AP physeal angle, lateral physeal angle, beta angle, gamma angle, delta angle), the similarity between our results and those of others was very good when such data did exist. The average alpha angle reported by Nötzli et al.  for healthy controls (without FAI) was 42°, similar to our 45.61°. The average neck version as reported by Kinsley and Olmsted  was 8.021°, similar to our 9.73°. The average angle of inclination as reported by Hoaglund and Low  was 135°, similar to our 129.23°. These results suggest the validity of our data in general and substantiate the novel method by which it was collected.
In addition to providing benchmark data, the previously unestablished measures of femoral head translation and rotation contest certain assumptions of the head-neck junction. Although the femoral head often is depicted as centered on the neck, our data suggest its true position is more often slightly anterior and inferior. Similarly, although the physeal scar might be assumed to be perpendicular to the axis of the neck, our results show it more often is substantially anteverted and abducted. Such findings question the validity of modern surgical techniques not taking such head-neck variables into account. As an example, femoral head resurfacing, a bone-conserving alternative to traditional THA, relies on the assumptions of negligible translation and rotation (AOS/POS and SOS/IOS ratios of 1, AP and lateral physeal angles of 90°) for positioning of the resurfacing prosthesis. Data from this study suggest, however, such idealized positioning may not reproduce an individual’s native anatomy, potentially reducing the lifespan of the prosthesis and altering biomechanical and patient function.
Also of concern to modern surgical technique were the large standard deviations and ranges recorded for the various measures, exemplified by the values obtained for neck version and angle of inclination (Table 2). Although the average values for these parameters were similar to those historically accepted (10° and 130°, respectively), their standard deviations were substantial (9.28° and 6.24°, respectively) and their ranges included values well beyond what modern prostheses currently are able to reproduce. Such results highlight the degree of variability likely to be encountered in a surgical population and challenge surgeons to be mindful of the impact that individual anatomic variation might have on outcomes for procedures not taking this variability into consideration.
The generated data also showed previously undescribed major differences between genders and ages in all three parameters of the head-neck junction. Regarding translation, males more often had inferior positioning than females. Similarly, with respect to rotation, males had, on average, more abduction and anteversion. Finally, examining concavity, males and those older than 50 years had, on average, larger alpha angles (and so less concavity) than females and those younger than 50 years, respectively. Given the higher incidence of osteoarthritis of the hip in elderly men , these results reinforce the results of numerous studies, which implicate abnormalities of the anterior head-neck junction in this condition [6, 7, 10, 15, 17–19, 24, 25, 27, 28]. And more broadly, observation of these anatomic differences raises questions of the need for recognition of these differences during THA and whether such differences could be correlated to the incidence of various disorders among genders and ages. Although disorders were consciously excluded from our study sample, their correlation to these gender- and age-dependent variations may be grounds for further research.
Second, both measures of translation correlated with a defining parameter of the neck-shaft relationship. Specifically, as neck version and angle of inclination increased, the AOS/POS and SOS/IOS ratios decreased, respectively; the position of the femoral head relative to the neck thus somewhat compensating for increasingly severe angles between the neck and shaft. Although speculations regarding the etiology of these correlations are provisional, the neutralizing nature of the translations and lack of major differences in translation, neck version, and angle of inclination between populations younger and older than 50 years perhaps suggests an embryologic or developmental origin. Also, although previous research has not shown a correlation between angle of inclination and slipped capital femoral epiphysis , our results suggest the relationship between angle of inclination and the development of a translational disorder such as slipped capital femoral epiphysis might be worth revisiting.
We thank Yohannes Haile-Selassie, PhD, Director of Physical Anthropology, and Lyman Jellema, MS, Collections Manager, at the Cleveland Museum of Natural History for help with this study.