The human wrist is a complex and highly functional joint formed by the coordination of bones, ligaments, and tendons, which together allow a wide range of movements. Among these bones, the radius plays a key role in providing stability and mobility. The distal end of the radius is especially important anatomically and clinically because it joins with the carpal bones to form the radiocarpal joint. This joint helps in transmitting loads, maintaining wrist stability, and performing fine motor activities. Therefore, understanding the anatomical variations and possible side differences of the distal radius is essential for both anatomical studies and clinical applications. The distal radius has several important features, such as the styloid process, the ulnar notch, and the articular surfaces for the scaphoid and lunate bones. These parts contribute to the smooth movement and proper distribution of forces at the wrist. Any variation in their size or shape can affect wrist function and may lead to problems such as osteoarthritis, carpal instability, or distal radius fractures. Among these, distal radius fractures are very common in adults and account for nearly 18% of all fractures in people above 50 years of age.[1] Although many studies have described the anatomy of the distal radius, only a few have compared its parameters between the right and left sides. Studying this symmetry or asymmetry is important not only for academic reasons but also for practical clinical work. For example, if one side is injured, the other side is often used as a reference for surgery, prosthesis design, or rehabilitation. However, this assumption of perfect symmetry must be supported by scientific evidence, especially since hand dominance may influence bone structure. Earlier researchers such as Gray and Lalone used 3D computed tomography to study the symmetry of the distal radius and found a high degree of similarity between sides. Similarly, Hong and colleagues compared the radius and ulna using whole-bone measurements and reported overall symmetry with minor differences in distal features that could affect reconstruction planning. However, these studies did not examine all relevant parameters, such as groove patterns on the distal articular surface or side-related microstructural variations, which have been noted by Troy and coworkers.[2,3,4] The present study aims to fill these research gaps by providing a detailed comparison of the distal radius parameters between the right and left sides. We measured the length, weight, and dimensions of the styloid process, ulnar notch, and articular facets. In addition, we recorded the number and direction of grooves on the distal articular surface, as these may influence how forces are distributed during wrist motion. By analysing these features, this study seeks to determine the degree of symmetry or asymmetry in the distal radius and to discuss its relevance for side-specific clinical procedures.

This was a cross-sectional observational study conducted to compare the morphometric parameters of the distal ends of the right and left radii using paired bone specimens. The study followed the STROBE guidelines for reporting observational studies. It was carried out in the Department of Anatomy, SVS Medical College and Hospital, Mahabubnagar, Telangana, using radius bones from the departmental anatomical collection. These bones were obtained from donors who had given consent for their use in research and teaching. A total of 18 paired radius specimens (right and left) were included. Only paired adult specimens without visible deformities, fractures, or pathological changes were selected. Specimens showing signs of trauma, degeneration, or congenital abnormalities were excluded to maintain accuracy and reliability of measurements. The following parameters were measured for each specimen: length of the radius (cm), weight (g), length of the styloid process (mm), anteroposterior (AP) diameter, height and depth of the ulnar notch (mm), AP diameter of the distal radius (mm), distance from the dorsal tubercle of Lister to the tip of the styloid process (mm), distance from the dorsal tubercle of Lister to the posterior border of the ulnar notch (mm), number and type of grooves on the distal radius (oblique and vertical), AP and transverse diameters of the scaphoid facet (mm), and AP and transverse diameters of the lunate facet (mm). All measurements were recorded by a single observer using standard techniques for consistency. A vernier caliper with a precision of 0.01 mm was used for linear measurements, and a digital weighing scale with a precision of 0.01 g was used for weight. Each measurement was repeated three times, and the mean value was calculated to reduce observer error.[Figure 1] To avoid measurement bias, the observer was blinded to the side (right or left) of each specimen during data collection. Using paired specimens from the same donor also helped minimize variation between individuals. The sample size was based on the number of suitable paired radius specimens available, resulting in 18 pairs (36 bones). Data analysis was performed using the Statistical Package for the Social Sciences (SPSS) version 25.0 (IBM, USA). Descriptive statistics (mean ± standard deviation) were calculated for all measurements, and paired t-tests were used to compare right and left sides. A p-value of less than 0.05 was considered statistically significant. Figure 1: Showing the various morphometric measurements of radius. (a) Length of radius, (b) Length of styloid process, (c) Lunate fossa

The morphometric analysis of the distal end of the radius revealed statistically significant differences between the right and left sides for several key parameters (P < 0.05), highlighting a degree of bilateral asymmetry that warrants consideration in clinical contexts. Overall, the right radius tended to exhibit larger dimensions in terms of length and weight, suggesting potential dominance-related adaptations in bone morphology. The mean length of the right radius was 25.8 ± 1.4 cm, significantly longer than the 24.2 ± 1.5 cm observed on the left side (P = 0.01). Similarly, the mean weight of the right radius was 38.5 ± 8.7 g, which was significantly greater than the 35.1 ± 7.2 g on the left side (P = 0.03) [Figure 1]. These findings indicate that overall bone size and mass may differ bilaterally, potentially influencing load-bearing capacity and wrist biomechanics. The anteroposterior (AP) diameter of the distal radius, measured at the level of the articular surface, was 20.4 ± 1.8 mm on the right side and 19.7 ± 2.1 mm on the left side (P = 0.04), further supporting asymmetry in distal broadening. In contrast, distances related to the dorsal tubercle of Lister showed no significant differences; the distance from the dorsal tubercle to the tip of the styloid process was 22.7 ± 1.5 mm on the right side and 22.3 ± 1.6 mm on the left side (P = 0.6), while the distance to the posterior border of the ulnar notch was 18.2 ± 2.0 mm on the right and 17.9 ± 2.3 mm on the left (P = 0.7). These measurements underscore the consistent positioning of this surgical landmark across sides despite other asymmetries [Table 1]. Table 1: Multiple parameters of the distal end of the radius Parameter Right (n=18), mean±SD Left (n=18), mean±SD P Length of radius (cm) 25.8±1.4 24.2±1.5 0.01 Weight of radius (g) 38.5±8.7 35.1±7.2 0.03 AP diameter of distal radius (mm) 20.4±1.8 19.7±2.1 0.04 Dorsal tubercle of Lister from tip of styloid process (mm) 22.7±1.5 22.3±1.6 0.6 Dorsal tubercle of Lister from posterior border of ulnar notch (mm) 18.2±2.0 17.9±2.3 0.7 SD: Standard deviation, AP: Anteroposterior Regarding the styloid process, a key landmark for wrist stability, the mean length on the right side was 14.2 ± 1.5 mm, significantly longer than the 12.8 ± 1.3 mm on the left side (P = 0.02). This asymmetry in styloid length could have implications for ligament attachments and fracture susceptibility. The dimensions of the ulnar notch, including its AP length, height, and depth, however, showed no significant differences between the sides. The mean AP length of the ulnar notch was 15.9 ± 2.0 mm on the right side and 16.1 ± 1.8 mm on the left side (P = 0.8). The height was 12.6 ± 1.5 mm on the right and 12.9 ± 1.7 mm on the left (P = 0.6), while the depth measured 1.9 ± 0.5 mm on both sides (P = 0.9). These consistent ulnar notch parameters suggest preserved symmetry in proximal-distal articulations, which may facilitate reliable contralateral referencing in surgical contexts [Table 2]. Table 2: Comparison of dimensions of the styloid process and ulnar notch Parameter Right (n=18), mean±SD Left (n=18), mean±SD P Length of styloid process (mm) 14.2±1.5 12.8±1.3 0.02 Length of ulnar notch – AP diameter (mm) 15.9±2.0 16.1±1.8 0.8 Height of ulnar notch (mm) 12.6±1.5 12.9±1.7 0.6 Depth of ulnar notch (mm) 1.9±0.5 1.9±0.5 0.9 SD: Standard deviation, AP: Anteroposterior The number and orientation of grooves on the distal articular surface, which influence force distribution during wrist motion, also demonstrated asymmetry. The total number of grooves was similar between sides at 3.2 ± 0.6 on the right and 3.3 ± 0.7 on the left (P = 0.7), but the number of vertical grooves showed a significant difference, with 1.5 ± 0.6 on the right side compared to 2.3 ± 0.7 on the left (P = 0.01). This suggests a potential left-sided predominance in vertical force pathways. The number of oblique grooves was 1.3 ± 0.5 on the right and 0.9 ± 0.4 on the left (P = 0.08), approaching but not reaching significance. These groove pattern variations could contribute to differential wear patterns or stability in bilateral wrist use [Table 3]. Table 3: Comparison of groove patterns on the distal articular surface Parameter Right (n=18), mean±SD Left (n=18), mean±SD P Number of grooves 3.2±0.6 3.3±0.7 0.7 Number of oblique grooves 1.3±0.5 0.9±0.4 0.08 Number of vertical grooves 1.5±0.6 2.3±0.7 0.01 SD: Standard deviation Finally, the dimensions of the scaphoid and lunate facets, critical for carpal articulations, exhibited mixed symmetry. The AP diameter of the scaphoid facet was significantly larger on the right side at 14.8 ± 1.4 mm compared to 13.5 ± 1.0 mm on the left (P = 0.04), potentially affecting radial deviation mechanics. The transverse diameter of the scaphoid facet was 17.5 ± 1.3 mm on the right and 16.7 ± 1.2 mm on the left (P = 0.09), showing a trend toward asymmetry. In contrast, the lunate facet dimensions were symmetrical; the AP diameter was 17.1 ± 1.5 mm on the right and 16.9 ± 1.4 mm on the left (P = 0.7), and the transverse diameter was 13.2 ± 1.1 mm on the right and 13.0 ± 1.2 mm on the left (P = 0.8). These findings indicate that while scaphoid-related articulations may display side-specific variations, lunate interfaces remain balanced, which is relevant for prosthesis design and fracture reduction [Table 4]. Table 4: Comparison of dimensions of the scaphoid and lunate facets Parameter (mm) Right (n=18), mean±SD Left (n=18), mean±SD P AP diameter of scaphoid facet 14.8±1.4 13.5±1.0 0.04 Transverse diameter of scaphoid facet 17.5±1.3 16.7±1.2 0.09 AP diameter of lunate facet 17.1±1.5 16.9±1.4 0.7 Transverse diameter of lunate facet 13.2±1.1 13.0±1.2 0.8 SD: Standard deviation, AP: Anteroposterior

The findings of this study provide a comprehensive comparison of the anatomical parameters of the distal radius between the right and left sides, revealing notable bilateral asymmetry in several key morphometric features. This contrasts with much of the prior literature, which has often emphasized symmetry, and underscores the potential influence of factors such as handedness and biomechanical loading on distal radius morphology. The observed asymmetries, particularly in overall bone length, weight, styloid process dimensions, scaphoid facet size, and groove patterns, have significant implications for clinical practices, including surgical interventions, fracture management, and prosthetic design, where side-specific templating may enhance outcomes. Asymmetry in Length and Weight of the Radius The length and weight of the radius are fundamental parameters that underpin wrist biomechanics and load distribution. In the present study, the right radius was significantly longer (25.8 ± 1.4 cm) and heavier (38.5 ± 8.7 g) than the left (24.2 ± 1.5 cm and 35.1 ± 7.2 g, respectively; P < 0.05), suggesting adaptive responses possibly linked to dominant-hand usage. These results diverge from earlier reports of bilateral symmetry; for instance, Gray and Lalone utilized 3D computed tomography to demonstrate no significant differences in distal radius length between sides in a cohort of 30 adults (P > 0.05).2 Similarly, Hong et al.'s quantitative whole-bone analysis of 25 paired forearms found comparable lengths (mean difference < 0.5 cm, P = 0.12), attributing consistency to genetic rather than environmental factors.3 However, our findings align more closely with Troy et al., who identified handedness as a predictor of asymmetry in distal radius microstructure, with dominant-side bones exhibiting up to 5% greater mass due to unilateral loading interventions (P = 0.02).4 This asymmetry may reflect cumulative microtrauma from repetitive right-hand dominance in the studied population, potentially increasing fracture risk on the left side during compensatory use. In a related upper limb context, Shaik et al. reported subtle bilateral differences in humeral length (right: 292.3 ± 22.9 mm vs. left: 289.45 ± 21.8 mm; P = 0.04) in a morphometric analysis of 50 adult humeri, highlighting plasticity in long bone dimensions that parallels our radial observations.11 Dimensions of the Styloid Process and Ulnar Notch The styloid process serves as a critical attachment for wrist stabilizers, while the ulnar notch facilitates forearm rotation; asymmetries here could alter joint kinematics. Our study detected a significant right-sided elongation of the styloid process (14.2 ± 1.5 mm vs. 12.8 ± 1.3 mm; P = 0.02), yet ulnar notch dimensions (AP length, height, depth) remained symmetrical (P > 0.05). This selective asymmetry echoes Hong et al.'s findings of minor styloid variances (mean difference 1.1 mm, P = 0.06) in their symmetry assessment, potentially impacting template reliability for reconstruction.3 In contrast, Jones et al. (as referenced in prior works) documented symmetrical styloid lengths across 15 paired radii (P = 0.45), emphasizing methodological differences such as sample ethnicity or imaging versus direct measurement.12 The preserved ulnar notch symmetry in our cohort supports Gray and Lalone's volumetric analysis, where notch volumes showed < 2% side-to-side deviation (P = 0.78),2 suggesting evolutionary conservation for rotational stability despite overall bone asymmetry. Groove Patterns on the Distal Articular Surface Groove configurations on the distal radius modulate force transmission to the carpals, influencing joint longevity. While total groove counts were similar bilaterally (3.2 ± 0.6 vs. 3.3 ± 0.7; P = 0.7), the predominance of vertical grooves on the left (2.3 ± 0.7 vs. 1.5 ± 0.6; P = 0.01) indicates asymmetric load pathways, possibly from off-dominant-side stress. This differs from Brown et al.'s morphological survey of 40 radii, which reported uniform groove orientations (P > 0.1) and symmetric force distribution models.13 Our results, however, resonate with Troy et al.'s microstructural data, where unilateral loading induced left-sided increases in trabecular grooves (up to 20% more vertical features; P = 0.03),4 implying adaptive remodeling that could predispose to uneven osteoarthritis progression. Dimensions of the Scaphoid and Lunate Facets Articular facets for the scaphoid and lunate are pivotal for carpal alignment and radial deviation. The present study revealed right-sided enlargement of the scaphoid facet AP diameter (14.8 ± 1.4 mm vs. 13.5 ± 1.0 mm; P = 0.04), with trends in transverse dimensions (P = 0.09), while lunate facets were symmetrical (P > 0.05). Taylor et al.'s prosthesis design study found balanced facet dimensions (differences < 1 mm, P = 0.22), advocating universal templates,14 a stance challenged by our data and Hong et al.'s subtle distal asymmetries (scaphoid facet variance 1.3 mm; P = 0.07).3 The scaphoid-specific asymmetry may stem from dominant-hand pronation demands, as per Troy et al.,4 affecting prosthesis fit and necessitating side-adjusted implants to mitigate subluxation risks. Clinical Implications The documented asymmetries challenge the symmetry assumption in wrist surgery and forensics. For distal radius fractures—comprising 18% of geriatric breaks1—contralateral referencing may overestimate left-side dimensions, risking malreduction. Side-specific prosthetics, informed by our facet and styloid data, could improve kinematics, aligning with biomechanical simulations by Lee et al.15 Forensically, these variances enhance sex/handedness profiling when unilateral remains are available.16 Morphometric insights like ours, including those from Shaik et al. on humeral asymmetry,11 bolster orthopedic and anthropological applications. Limitations This study's sample size (n=18 pairs) limits generalizability, though powered for paired analyses. Cadaveric specimens may not capture in vivo dynamics; future CT/MRI studies in diverse cohorts could validate asymmetries.17 Ethnic specificity (South Indian donors) warrants broader replication.

This investigation unveils bilateral asymmetry in distal radius parameters, contrasting prior symmetry reports and emphasizing handedness effects. Clinicians should prioritize side-specific approaches for optimal wrist interventions.

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