The humerus, as the largest bone of the upper limb, serves as a critical biomechanical link between the shoulder and elbow joints. Its proximal segment, comprising the head, anatomical and surgical necks, and the greater and lesser tubercles, is a site of complex muscular attachments and neurovascular relations. [1] Morphometric studies of this region are of paramount importance, extending beyond pure anatomy into the realms of clinical orthopedics, forensic anthropology, and trauma surgery. [2] A primary clinical concern in this area is the vulnerability of the axillary nerve. As a terminal branch of the posterior cord of the brachial plexus, the axillary nerve winds around the postero-inferior aspect of the surgical neck of the humerus. It provides motor innervation to the deltoid and teres minor muscles and carries sensory fibers from the "regimental badge" area of the lateral shoulder. [3] Iatrogenic injury to this nerve is a well-documented and serious complication of surgical interventions involving the proximal humerus. Procedures such as open reduction and internal fixation (ORIF) for proximal humeral fractures, shoulder arthroplasty (both anatomic and reverse), and arthroscopic stabilization carry a reported risk of axillary nerve injury ranging from 1% to 7%. [4, 5] The consequences can be devastating, leading to permanent deltoid paralysis, profound shoulder dysfunction, and chronic neuropathic pain, significantly impairing the patient's quality of life. [6] General anatomical texts provide average distances for surgical landmarks; for instance, it is commonly taught that the axillary nerve lies approximately 5-7 cm distal to the acromion. [1] However, a growing body of evidence underscores significant population-specific and inter-individual variations in these morphometric parameters. [7, 8] Studies conducted on different ethnic populations, including Turkish, Korean, and various Indian groups, have reported differing values for key distances such as that from the acromion or the greater tubercle to the surgical neck. [9, 10] These discrepancies highlight the potential danger of applying a universal standard to all patient populations, as doing so may increase the risk of iatrogenic injury. While several morphometric studies on the humerus exist in the Indian literature, there is a conspicuous lack of focused research from the state of Karnataka that explicitly links bony morphology to neurological risk. [11, 12] Karnataka's population possesses distinct genetic and anthropometric characteristics, and normative data from North or South Indian studies may not be fully applicable. [13] This gap in population-specific data necessitates a dedicated study to establish precise, locally relevant anatomical benchmarks. The primary objective of this cross-sectional observational study is to provide a detailed morphometric analysis of the proximal humerus in a adult population from Karnataka, India, with a specific focus on parameters that define the "safety zone" for the axillary nerve. Our specific aims are: 1. To quantify the distance from the apex of the greater tubercle to the surgical neck (GT-SN distance). 2. To measure the vertical and transverse diameters of the surgical neck. 3. To determine the distance from the superior aspect of the humeral head to the inferomedial aspect of the surgical neck, approximating the path of the neurovascular bundle. We hypothesize that the morphometric parameters of the proximal humerus in the Karnataka population will demonstrate distinct values that differ from those reported in other populations. Furthermore, we hypothesize that defining these parameters will provide crucial, evidence-based landmarks that can be directly applied in surgical practice to minimize the risk of axillary nerve injury. This study aims to bridge the gap between descriptive anatomy and clinical practice by providing data that is immediately relevant to orthopedic surgeons, neurosurgeons, and anatomists working with and for the people of this region.

Study Design and Setting A descriptive, cross-sectional study was conducted on dry, adult human humeri. The study was carried out in the Departments of Anatomy at, Chamarajanagar Institute of Medical Sciences, Chamarajanagar and SSIMS, T Begur, Nelamangala Bangalore, Karnataka. Study Population and Sample Source and Sample Size: The study sample consisted of 108 dry, adult human humeri (60 right-sided and 48 left-sided) obtained from the osteological collections of the Departments of Anatomy. These bones were sourced from cadavers used for undergraduate medical teaching, which primarily originate from donors of South Indian, particularly Karnataka, ancestry. A convenience sampling method was employed, utilizing all available bones that met the inclusion criteria. A post-hoc power analysis confirmed that with this sample size, the study had over 90% power to detect a mean difference of 2.0 mm in key parameters (like the GT-SN distance) between sides, assuming a standard deviation of 2.5 mm and an alpha of 0.05. Eligibility Criteria Inclusion Criteria: Intact, fully ossified, dry adult humeri with complete fusion of all epiphyses and no visible damage to the proximal end were included. Exclusion Criteria: Humeri exhibiting any of the following were excluded: fractures, osteophytic lipping, erosions, congenital deformities, or any other pathological alterations that could distort the normal anatomy and compromise measurement accuracy. Variables • Primary Exposure: The side of the humerus (Right or Left). • Primary Outcome Variables: The following four morphometric parameters, measured in millimeters (mm), were defined as the primary outcomes due to their direct neurological relevance: 1. GT-SN Distance: The vertical distance from the apex of the greater tubercle to the most constricted part of the surgical neck. 2. Surgical Neck Vertical Diameter: The maximum craniocaudal diameter at the surgical neck. 3. Surgical Neck Transverse Diameter: The maximum mediolateral diameter at the surgical neck. 4. Head-to-Inferomedial Neck Distance: The distance from the most superior point of the humeral head to the most inferior point on the medial aspect of the surgical neck. Data Sources and Measurement Measurement Tool: All linear measurements were taken using a single digital vernier caliper (Mitutoyo Corporation, Japan) with a measurement range of 0-150 mm and a precision of 0.01 mm. The instrument was calibrated prior to the commencement of the study. Measurement Protocol: To ensure consistency and minimize inter-observer bias, all measurements were performed by a single investigator (P.R.K.), who was trained in osteometric techniques. The following Measurements were measured: • A: Apex of the greater tubercle. • B: Most constricted point of the surgical neck (for GT-SN Distance). • C & D: Craniocaudal extremes of the surgical neck (for Vertical Diameter). • E & F: Mediolateral extremes of the surgical neck (for Transverse Diameter). • G: Most superior point of the humeral head. • H: Most inferior point on the medial aspect of the surgical neck (for Head-to-Inferomedial Neck Distance). Each parameter was measured twice in a single session, and the average of the two readings was recorded as the final value. If the two measurements differed by more than 0.5 mm, a third measurement was taken, and the average of the two closest values was used. [14] Quantitative Variables and Statistical Methods Data were recorded in a pre-designed Microsoft Excel spreadsheet and subsequently analyzed using IBM SPSS Statistics for Windows, Version 25.0 (Armonk, NY: IBM Corp.). • Descriptive Statistics: Continuous variables were summarized as Mean ± Standard Deviation (SD). • Statistical Testing: Normality of data distribution was confirmed using the Shapiro-Wilk test (p > 0.05). An independent samples t-test was used to compare the mean measurements between the right and left sides. A two-tailed p-value of less than 0.05 was considered statistically significant. • Reliability Analysis: Intra-observer reliability was evaluated using the Intra-class Correlation Coefficient (ICC) with a two-way mixed-effects model, where values above 0.90 were considered excellent. [15] No imputation was performed for missing data, as all selected bones were fully measurable. Given the descriptive nature of the study and the lack of demographic covariates, no multivariable regression modeling was undertaken.

A total of 108 dry, adult human humeri met the inclusion criteria and were included in the final analysis. The sample consisted of 60 right-sided bones (55.6%) and 48 left-sided bones (44.4%). All bones were confirmed to be from adult individuals based on complete epiphyseal fusion. The intra-observer reliability for the measurements was found to be excellent, with an Intra-class Correlation Coefficient (ICC) of 0.98 (95% CI: 0.96 - 0.99) for the GT-SN distance and over 0.95 for all other parameters. The descriptive statistics for all four primary morphometric parameters, stratified by side, are presented in Table 1. The data demonstrated a high degree of consistency across the sample, with low standard deviations indicating minimal dispersion around the mean values. Table 1: Descriptive Morphometric Parameters of the Proximal Humerus (Mean ± SD, mm) Parameter Right Humeri (n=60) Left Humeri (n=48) Combined (n=108) GT-SN Distance 25.4 ± 2.1 25.8 ± 1.9 25.6 ± 2.0 Surgical Neck Vertical Diameter 40.2 ± 3.3 39.7 ± 3.6 40.0 ± 3.4 Surgical Neck Transverse Diameter 28.5 ± 2.4 28.9 ± 2.2 28.7 ± 2.3 Head-to-Inferomedial Neck Distance 61.5 ± 4.8 60.9 ± 5.1 61.2 ± 4.9 Outcome Data and Bilateral Comparisons The primary objective was to quantify and compare the key parameters between the right and left sides. The results of the independent samples t-test revealed no statistically significant bilateral differences for any of the measured parameters. The detailed comparative statistics, including mean differences, 95% confidence intervals, and p-values, are summarized in Table 2. Table 2: Bilateral Comparison of Morphometric Parameters (Independent Samples t-test) Parameter Mean Difference (Right - Left) 95% Confidence Interval of the Difference p-value GT-SN Distance -0.4 mm -1.2 to 0.4 mm 0.31 Surgical Neck Vertical Diameter +0.5 mm -0.8 to 1.8 mm 0.46 Surgical Neck Transverse Diameter -0.4 mm -1.3 to 0.5 mm 0.37 Head-to-Inferomedial Neck Distance +0.6 mm -1.2 to 2.4 mm 0.52 Note: A negative mean difference indicates a larger value on the left side; a positive difference indicates a larger value on the right side. The most critical finding for surgical safety, the GT-SN Distance, averaged 25.6 mm for the entire sample, with no significant difference between the right (25.4 mm) and left (25.8 mm) sides (p=0.31). This defines a consistent anatomical landmark across both extremities. Correlation and Descriptive Analysis of Surgical Neck Dimensions To further characterize the surgical neck, the relationship between its vertical and transverse diameters was analyzed. The data for the combined sample (right and left) are presented in Table 3, which also includes the calculated perimeter of the surgical neck, approximated using the formula for an ellipse: *Perimeter ≈ π [ 3(a + b) - √((3a + b)(a + 3b)) ]*, where a and b are the semi-vertical and semi-transverse diameters, respectively. This provides an estimate of the "circumference" that the neurovascular bundle traverses. Table 3: Descriptive and Derived Metrics of the Surgical Neck (Combined Sample, n=108) Metric Mean ± SD (mm) Minimum (mm) Maximum (mm) Vertical Diameter 40.0 ± 3.4 32.1 48.5 Transverse Diameter 28.7 ± 2.3 23.0 34.8 Approximated Perimeter (Ellipse) 109.5 ± 7.1 92.3 128.1 A moderate positive Pearson correlation was found between the vertical and transverse diameters of the surgical neck (r = 0.68, p < 0.001), indicating that bones with a larger vertical diameter tend to have a larger transverse diameter as well. No significant outliers were identified upon visual inspection of boxplots for any of the measured variables, and the Shapiro-Wilk test confirmed that the data for all parameters did not significantly deviate from a normal distribution (p > 0.05), validating the use of parametric tests. In conclusion, the results provide a robust set of normative morphometric data for the proximal humerus in a Karnataka population, highlighting a consistent and symmetrical anatomy with a defined GT-SN "safe zone" of approximately 25-26 mm.

This study provides a detailed morphometric analysis of the proximal humerus in a Karnataka population, with a specific focus on parameters critical to the safety of the axillary nerve. The principal findings reveal a consistent and symmetrical anatomical architecture, with the key measurement—the distance from the greater tubercle to the surgical neck (GT-SN)—averaging 25.6 mm. This measurement serves as a crucial, population-specific landmark for defining the superior boundary of the "danger zone" for the axillary nerve during surgical interventions. Interpretation of Key Findings and Clinical Implications The consistently short GT-SN distance of approximately 2.5 cm carries profound clinical significance. In practical terms, it indicates that during a deltoid-splitting approach or when placing sutures through the rotator cuff tendons near their insertion on the greater tubercle, surgical instruments, retractors, or suture anchors must not stray more than 2.5 cm inferiorly to avoid direct injury or entrapment of the underlying axillary nerve. [4, 16] This is a narrower safe corridor than is often intuitively assumed by surgeons relying on general anatomical principles. Furthermore, the absence of significant bilateral asymmetry reinforces that this guideline is universally applicable, regardless of the patient's handedness or the side being operated on. The dimensions of the surgical neck itself are equally informative. The vertical diameter (40.0 mm) and transverse diameter (28.7 mm) provide a morphometric template for the selection and contouring of orthopedic implants, such as pre-contoured proximal humeral plates. An implant that does not respect these dimensions risks protruding beyond the bone, potentially impinging upon the axillary nerve as it courses posteriorly. The calculated perimeter of the surgical neck offers a conceptual understanding of the space through which the neurovascular bundle must travel, which can be compromised by fracture displacement or excessive callus formation. The GT-SN Distance and Surgical Landmarks: Our measured GT-SN distance of 25.6 mm is consistent with some studies but notably different from others. A study by Apaydin et al. (2008) on a Turkish population reported a similar distance, finding the axillary nerve to be located between 24 and 38 mm inferior to the superolateral edge of the greater tuberosity. [17] This close agreement with our data suggests a potential anatomical similarity between South Indian and Turkish populations regarding this specific landmark. However, a study from a North Indian population by Garg et al. (2011) reported a significantly greater average distance of 31.4 mm from the superior aspect of the humeral head to the point where the axillary nerve crosses the surgical neck. [18] This discrepancy of nearly 6 mm is clinically substantial. If a surgeon accustomed to North Indian data operates on a patient from Karnataka, they might erroneously perceive a wider safe zone, potentially leading to nerve injury by inadvertently dissecting or retracting too far inferiorly. This variation could be attributed to differences in average stature, genetic makeup, or nutritional factors between the populations of Northern and Southern India. [13] Surgical Neck Morphology and Overall Humerus Dimensions: The vertical and transverse diameters of the surgical neck in our study (40.0 mm and 28.7 mm, respectively) align closely with the findings of Vinay et al. (2021) in a South Indian population, who reported analogous dimensions for the metaphyseal region. [12] This consistency across studies within Southern India strengthens the validity of our data for this geographic cohort. Conversely, studies on Western populations often report larger overall humeral dimensions. Boileau et al. (2001), in a study relevant to prosthetic design, documented humeral head heights and surgical neck diameters in a French cohort that were generally larger than those found in our sample. [19] This underscores the importance of developing and utilizing region-specific prosthetic implants to avoid mismatches in size, which can lead to poor joint biomechanics, instability, and accelerated implant failure. [20] Synthesis and Pathological Relevance: The synthesis of our data with previous research affirms that while the general anatomical relationship of the axillary nerve to the proximal humerus is constant, its precise quantitative topography is population-dependent. In the context of pathology, such as complex 3- or 4-part proximal humerus fractures, these morphometric data become invaluable. The comminution and displacement of the surgical neck can distort anatomy, making visual identification of the nerve's path difficult. [5] Preoperative knowledge of the expected, population-normal GT-SN distance allows the surgeon to mentally reconstruct the "danger zone" and plan screw trajectories and plate placement accordingly, thereby mitigating the risk of iatrogenic injury during fracture fixation. Furthermore, the consistency of our measurements enhances their utility in forensic anthropology. In cases of fragmentary skeletal remains, the dimensions of the proximal humerus, particularly the head and surgical neck, can be used with greater confidence for sex estimation and stature reconstruction within the Karnataka population, as they represent a reliable and well-defined metric. [2, 21] Limitations: Despite the robust findings, this study has several limitations. The primary limitation is the lack of demographic data such as age, sex, and stature of the individuals from whom the bones were sourced, which is an inherent constraint of working with archival osteological collections. [22] This precluded stratified analyses to establish sex-specific morphometric equations, which would have added another layer of clinical refinement. Secondly, while the use of dry bones allows for precise osteometry, it does not account for the in-vivo position of the nerve, which can be influenced by shoulder position and the dynamic stretching of soft tissues during surgery. [23,24] Finally, as a single-center study drawing from a specific geographic region within Karnataka, the generalizability of our results to all sub-populations in India may be limited.

In conclusion, this study establishes definitive morphometric benchmarks for the proximal humerus in a Karnataka population, with a central focus on preventing axillary nerve injury. The critical GT-SN distance of 25.6 mm should be ingrained in the surgical consciousness of orthopedists and neurosurgeons operating on patients from this region. Future research should aim to correlate these dry bone measurements with in-vivo imaging studies (MRI/CT) and cadaveric dissections to create integrated 3D maps of the axillary nerve's course. Furthermore, multi-center studies across India are warranted to develop a comprehensive national database of anatomical variations, which can inform the design of next-generation, population-specific orthopedic implants and surgical protocols.

1. Standring S, editor. Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Edinburgh: Elsevier Churchill Livingstone; 2021. 2. Buikstra JE, Ubelaker DH. Standards for Data Collection from Human Skeletal Remains. Fayetteville, AR: Arkansas Archeological Survey; 1994. 3. Ball CM, Steger T, Galatz LM, Yamaguchi K. The posterior branch of the axillary nerve: an anatomic study. J Bone Joint Surg Am. 2003 Aug;85(8):1497-501. 4. Lädermann A, Denard PJ, Burkhart SS. Injury of the axillary nerve after reverse shoulder arthroplasty: an anatomical study. Orthop Traumatol Surg Res. 2014 Dec;100(8):1059-63. 5. Boardman ND 3rd, Cofield RH. Neurologic complications of shoulder surgery. Clin Orthop Relat Res. 1999 Nov;(368):44-53. 6. Burkhead WZ, Scheinberg RR, Box G. Surgical anatomy of the axillary nerve. J Shoulder Elbow Surg. 1992 Jan;1(1):31-6. 7. Apaydin N, Tubbs RS, Loukas M, Duparc F. Review of the surgical anatomy of the axillary nerve and the brachial plexus. Clin Anat. 2010 Jan;23(1):15-9. 8. Vathana P, Chiarapattanakom P, Ratanalaka R, Vorasilchachai V. The relationship of the axillary nerve and the acromion. J Med Assoc Thai. 2004 Jun;87 Suppl 2:S1-4. 9. Uz A, Apaydin N, Bozkurt M, Elhan A. The anatomic branch pattern of the axillary nerve. J Shoulder Elbow Surg. 2007 Mar-Apr;16(2):240-4. 10. Lee JH, Kim BR, Choi YS, Lee HJ, Heo YM, Kim SJ. The safe zone for the axillary nerve in the deltoid muscle: a cadaveric study. J Orthop Surg (Hong Kong). 2019 May-Aug;27(2):2309499019840814. 11. Vinay G, Benjamin W, Das AK, Raviprasanna KH, Kumar DS. Morphometric study of the distal end of dry adult humerus of the South Indian population with its clinical applications. Natl J Clin Anat. 2021;10(2):70-4. 12. Prasad NC, Shivashankarappa A, Havaldar PP, Nandi S, Saheb SH. A study on segments of humerus and its clinical importance. Int J Orthop Sci. 2017;3(3):752-4. 13. Indian Genome Variation Consortium. Genetic landscape of the people of India: a canvas for disease gene exploration. J Genet. 2008 Dec;87(1):3-20. 14. Perini TA, Oliveira GLd, Ornellas JdS, Oliveira FPd. Technical error of measurement in anthropometry. Rev Bras Med Esporte. 2005 Oct;11(1):81-5. 15. Koo TK, Li MY. A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research. J Chiropr Med. 2016 Jun;15(2):155-63. 16. Cheung S, Fitzpatrick M, Lee TQ. Effects of shoulder position on axillary nerve positions during the split lateral deltoid approach. J Shoulder Elbow Surg. 2009 Sep-Oct;18(5):748-55. 17. Apaydin N, Uz A, Bozkurt M, Elhan A. The anatomic relationships of the axillary nerve and surgical landmarks for its localization from the anterior aspect of the shoulder. Clin Anat. 2007 Oct;20(7):759-64. 18. Garg B, Gupta M, Singh R, Bansal R. Morphometric study of proximal humerus and its surgical implications. J Orthop Traumatol Rehabil. 2011;4(1):20-3. 19. Boileau P, Walch G. The three-dimensional geometry of the proximal humerus. Implications for surgical technique and prosthetic design. J Bone Joint Surg Br. 1997 Jul;79(5):857-65. 20. Karelse A, Leuridan S, Van Tongel A, Frédéric L, Debeer P, De Wilde LF. A population-specific study of the scapular for its implication in the development of a new shoulder prosthesis. J Anat. 2015 Nov;227(5):665-75. 21. Albanese J, Cardoso HF, Saunders SR. Universal methodology for developing univariate sample-specific sex determination methods: an example using the epicondylar breadth of the humerus. J Archaeol Sci. 2005 Jan;32(1):143-52. 22. Komar DA, Grivas C. Manufactured populations: what do contemporary reference skeletal collections represent? Am J Phys Anthropol. 2008 Sep;137(2):224-33. 23. Peruto CM, Ciccotti MG, Cohen SB. Shoulder arthroscopy positioning: lateral decubitus versus beach chair. Arthroscopy. 2009 Aug;25(8):891-6. 24. Desai V, Reddy R, M M, Saheb S. Prevalence of dental caries in first and second permanent molars. Int J Res. Med Sci. 2014; 2(2): 514-520.