Morphometric Comparison of the C1 Vertebra Based on Age and Sex in the Indonesian Population: A CT-Based Study

Sample characteristics

A total of 337 CT scan images of the cervical spine (C1) were analyzed in this study, consisting of 194 male subjects (57.6%) and 143 female subjects (41.4%). The mean age of the sample was 48.64 years, with 16 subjects (4.75%) aged 1–18 years, 147 subjects (43.62%) aged 19–50 years, and 174 subjects (51.63%) aged over 50 years ( Table 1).

C1 morphometry in the axial plane

Six morphometric parameters of the C1 lateral mass were measured in the axial plane, consisting of three linear parameters (length) and three angular parameters (angulation) ( Table 2). The median short lateral mass length in the axial plane (SLa) was 15.50 mm (range 9.43–24.06), while the median long lateral mass length (LLa) was 20.94 mm (range 10.58–34.00). The maximum axial diameter (MDa) was normally distributed with a mean of 7.68 ± 1.59 mm. The average angulation angles in the axial plane were SAa 17.07° ± 4.45°, LAa 29.08° ± 6.06°, and TAa 47.30° ± 8.82°.

C1 morphometry in the sagittal plane

In the sagittal plane, seven morphometric parameters (four length parameters and three angle parameters) were measured, as listed in Table 3 The mean short sagittal length (SLs) for all samples was 17.85 ± 2.20 mm, while the mean long sagittal length (LLs) was 20.09 ± 2.40 mm. The base length (BL) and maximum sagittal diameter (MD) parameters were non-normally distributed, with median values of 10.01 mm (range 3.43–12.66) and 4.86 mm (range 2.20–11.62), respectively. For sagittal angulation, the median SAs was 16.80°, the median LAs was 27.71°, and the mean TAs was 44.91° ± 5.92°.

Morphometry comparison by sex

A comparison of the mean values for each C1 morphometric parameter between male and female subjects is presented in Table 4. The analysis showed that men had significantly higher values for almost all linear dimensions of the C1 lateral mass than women. Specifically, the lateral mass length parameters in the axial plane (SLa and LLa) and sagittal plane (SLs and LLs) were significantly greater in men than in women (all p < 0.001). Similarly, the maximum diameter of the C1 lateral mass in the axial plane (MDa) was on average greater in men (8.16 ± 1.56 mm) than in women (7.12 ± 1.42 mm, p < 0.001). In contrast, the C1 base width (BL) and maximum diameter in the sagittal plane (MDs) did not show significant differences between the sexes (p > 0.05). In the angulation parameters, there were no significant differences between men and women for most angles in the axial plane (SAa, LAa, TAa, all p > 0.05) or lateral angles in the sagittal plane (LAs, p > 0.05). However, sagittal plane angulation angles showed differences between the sexes: men had slightly larger inferior angles (SAs) and total sagittal trajectory angles (TAs) than women. The average TAs, for example, was recorded at 45.78° ± 5.94° in men versus 43.73° ± 5.71° in women, and this difference was statistically significant (p = 0.002).

Morphometry comparison by age group

The comparative analysis revealed statistically significant differences in many C1 morphometric parameters between age groups ( Table 5). There was significant differences between groups in SLa (both right and left sides), SLs (right, left, and mean), LLs (mean), SAs (right and left), LAs (left), and TAs (mean) (p < 0.05). Significant differences were found in SLa (mean aspect), LLa (right, left, and mean), LLs (left side), MDs (right, left, and mean), LAa (right side), SAs (mean aspect), LAs (mean aspect), and TAs (left side) (p < 0.05).

In general, the youngest age group (1–18 years) had smaller C1 morphometric measurements compared to both adults and elderly age groups. For example, the median SLa increased from 12.89 mm in children to 15.36 mm in young adults and 16.08 mm in those aged >50 years (p < 0.05). The mean SLs also increased from 16.41 ± 2.68 mm in group 1 to 18.01 ± 2.11 mm in group 3 (p = 0.020). Conversely, the maximum sagittal diameters of the lateral masses (MDs) tended to decrease with age: the median MDs were 5.15 mm in the 1–18 years group, decreasing significantly to 4.66 mm in the >50 years group (p < 0.001). Post-hoc tests revealed that the differences mainly occurred between the children and the older age groups. Specifically, a comparison between Group 1 and Group 3 revealed significant differences in the SLa, LLa, and MDs parameters ( Table 6), with pediatric subjects having shorter SLa but larger MDs than elderly subjects.

Morphometry comparison by body side

Table 7 shows a comparison of all C1 morphometric parameters between the right and left sides. No significant differences were found between the right and left sides for any of the parameters tested (p > 0.05 for all comparisons), indicating that the C1 lateral mass morphometry is symmetrical between the right and left sides of the body in this population.

Recommended C1 lateral mass screw sizes

Based on the morphometric findings above, this study developed recommendations for C1 lateral mass screw insertion sizes tailored to the subject’s sex and age group. The recommended screw length range was calculated by summing the SLa + BL (as the estimated shallowest length) to LLa + BL (as the estimated deepest length). The safe zone for angulation insertion in the axial plane is determined based on the average TAa parameter, while the safe zone in the sagittal plane is determined from the average TAs parameter. The maximum screw diameter that can be inserted is determined by referring to the average internal diameter of the smallest measured C1 lateral mass. Recommended C1 lateral mass screw sizes by gender are presented in Table 8.

In the male population, the recommended C1 screw length ranges from 25.83 to 31.95 mm, while in the female population it is 24.64 to 30.11 mm. The safe zone for axial insertion angle is estimated at approximately 47.9° in men and 46.5° in women, while the safe zone for the sagittal plane is approximately 45.8° in men and 43.7° in women. The recommended maximum screw diameter is slightly larger in men (approximately 5.09 mm) than in women (approximately 4.89 mm). Viewed by age group, the range of screw lengths tends to increase from childhood to adulthood, then remains relatively stable in old age. In the 1–18 age group, the recommended screw length is approximately 22.8–23.2 mm for men and 28.0–28.9 mm for women. At ages 19–50, the recommended length increases to approximately 25.99–32.53 mm for men and 24.43–29.94 mm for women. The age group >50 years has an optimal length range similar to adults, namely approximately 26.08–31.85 mm (men) and 25.06–30.47 mm (women). In addition to length changes, the maximum screw diameter that can be inserted tends to be smaller in older age: mean ± SD diameter of 4.66 ± 0.73 mm in men >50 years and 4.36 ± 0.74 mm in women >50 years, compared to ±5.7 mm in the younger group ( Table 9). Meanwhile, the range of safe insertion angles is relatively constant across age groups ( Table 10). In the axial plane, the recommended lateral–medial angle ranges from ~17° laterally to ~30° medially for all age categories. In the sagittal plane, the cranial angle of insertion ranges from approximately 28–30°, while the caudal angle ranges from ~13° in children to ~17° in adults, indicating a slight increase in caudal angulation in the older age group.

C1 Morphometric differences by sex

C1 morphometric characteristics in the Indonesian population show differences between men and women. In general, men tend to have larger C1 lateral mass dimensions than women, which impacts the length and depth of screws that can be inserted. In this study, several lateral mass length parameters, such as short lateral anterior (SLa), long lateral anterior (LLa), short lateral superior (SLs), long lateral superior (LLs), and the maximum diameter of the lateral mass (maximum diameter axial, MDa), differed significantly between men and women. This means that male patients can accommodate longer and larger diameter C1 screws than female patients. This finding is consistent with other morphometric studies showing that men generally have larger cervical vertebrae, allowing for the use of larger screw dimensions. For example, a morphometric analysis in an Indian population reported that only 31.8% of female patients could accommodate a 12 mm screw, while 75% of male patients could accommodate a 10 mm screw.^21,22 Similarly, morphometric data from a southern Chinese population showed that the average C1 screw length that could be safely inserted in that population was significantly longer than that found in studies in other populations, supporting the idea that anatomical differences based on sex influence the accommodation capacity of screw length

Biomechanical factors also contribute to this difference. Larger screws (longer or larger diameter) can provide better segment stability by optimally distributing mechanical loads across the posterior screw-rod construct.^21–23 This added stability is particularly beneficial in male patients, whose anatomy allows for the use of larger screws. A study in the United States noted that the average length of the short lateral mass of the atlas (SLa) in a Caucasian population was approximately 17.7–17.9 mm, with safe trajectory angles of approximately 22.2° (minimum medial angle, SAa) and 32.6° (maximum medial angle, LLa). These data indicate that the atlas (C1) in Caucasians can accommodate larger screws than in Asians, and therefore, a larger insertion angle may be required in Caucasian patients to achieve optimal stability.²⁴

C1 Morphometric differences by age

This study also found differences in C1 characteristics across age groups. With age, changes in bone structure occur, including decreased bone density and changes in vertebral shape, which can affect screw insertion techniques and outcomes. Several C1 parameters (including SLa, LLa, LLs, MDa, superior anterior short angle (SAs), superior anterior long angle (LAs), and total trajectory angle (TAs)) showed statistically significant differences between the young (1–18 years), adult (19–50 years), and elderly (>50 years) age groups. Post-hoc tests in this study confirmed that differences were primarily observed between the children/adolescents group compared to the young adults, and between the young adults and the elderly group. Although there is no literature available on the significance of C1 morphometry across age groups in the Indonesian population, these results are clinically important. Elderly patients are at higher risk for postoperative complications, including screw malposition and neurovascular injury, so understanding the differences in C1 morphology across age groups can help plan more appropriate surgical techniques. Growth and development of the cervical spine (including C1) occur throughout childhood and into young adulthood; during adolescence, the bones are still growing, allowing the size and shape of structures such as the lateral mass of C1 to change.³ As adulthood approaches, the bone structure reaches its maximum size, which then determines the maximum screw size that can be accommodated.

Other studies have highlighted that C1 screw placement techniques (both lateral mass screws and C2 pedicle screws) provide good biomechanical and clinical outcomes, but these techniques require a high level of surgical skill.²⁵ With age, bone quality tends to decline, particularly in individuals with osteoporosis, so surgeons may need to consider using larger screws or modifying techniques to ensure stability in more fragile bone. For example, C1 pedicle screws have been found to have biomechanical advantages over C1 lateral mass screws in poor-quality (e.g., osteoporotic) bone. This suggests that in older patients, the option of using larger C1 screws or alternative techniques (such as pedicle screws) may be more frequently necessary to compensate for decreased bone strength.²⁶ Furthermore, various conditions such as trauma or congenital malformations can cause atlantoaxial instability requiring C1–C2 fixation, and the technique used may vary depending on the patient’s age and spinal condition.²⁰ In other words, age can influence not only the screw size selected but also the type of lateral mass screw placement technique most appropriate for each patient.

Comparison with other populations

Comparative studies have shown that the C1 vertebra in the Indonesian population has several distinct morphometric characteristics compared to populations from other regions. In general, the C1 lateral mass screw length obtained in this study (approximately 22–30 mm) is comparable to results from studies in Chinese and Indian subjects, indicating that a standard surgical approach can be applied to various Asian populations. In contrast, European populations tend to have larger atlas (C1) dimensions; studies in European populations report C1 screw lengths ranging from 26–32 mm, with larger trajectory angles reflecting the wider atlas dimensions. In Caucasian patients, lateral mass screws may need to be placed at a more extreme angle (more medial) to achieve the desired stability, while in Asian patients, a more conservative approach (less medial angle) is recommended to avoid injuring surrounding neurovascular structures. These differences between populations emphasize the importance of a surgical approach tailored to patient characteristics and the need for thorough imaging-based preoperative planning. It should be noted that some previous morphometric studies used cadaveric samples, which may not be fully representative of the living population.^3,7

Variations in vascular anatomy in the atlantoaxial region also have implications for safe zone planning. A study by O’Donnell et al. reported a persistent first intersegmental artery with a prevalence of 5–10% and a fenestrated vertebral artery with a prevalence of 1–2% in the United States population.²⁷ The anomalous course of these vertebral arteries generally lies directly above the origin of the lateral atlas mass screw placement. The high prevalence of these anomalies suggests that preoperative CT angiography (CTA) should be routinely performed before upper cervical spine fixation to identify any potentially dangerous arterial variants. In the Caucasian population, taking into account these arterial variations, the safe zone for C1 screw insertion was reported to be approximately 23° minimum medial angle and 32° maximum medial angle, with a mean screw depth of 17.7–17.9 mm.^24,27

Morphometric research in a Turkish population by Simsek et al. found that the average length of the lateral mass C1 screw was ±22 mm, with a minimum length of 14.4 ± 2.0 mm and a maximum of 22.5 ± 2.6 mm.⁶ Data from this Eurasian population indicates that the average length of the lateral mass section in the Indonesian population (predominantly of Southeast Asian ethnicity) is relatively smaller than in the Eurasian population. However, Simsek’s study did not specify the optimal safe zone angle for screw insertion. The study only stated that the ideal screw depth is approximately 19.6 ± 2.2 mm, slightly longer than the length of the lateral mass itself, to ensure that the polyaxial screw head does not become obstructed by the posterior arch of C1, which curves above the lateral mass.⁶

Clinical implications

The above C1 morphometric differences have important clinical implications. There are significant differences between men and women in the capacity for C1 screw placement. Men, with their larger C1 lateral mass, can have longer screws inserted and possibly at a more medial angle, while in women, the safe zone of insertion is more limited. In this study sample, the total safe zone angulation in the axial plane (the combined angle of the lateral-minimum and medial-maximum angles) in men was approximately 48.06° ± 9.05°, while in women it was approximately 46.48° ± 9.14°. Similarly, in the sagittal plane, the safe zone range (from the cranial to the caudal angle) was approximately 45.65° ± 5.99° in men and 43.75° ± 5.73° in women. Interestingly, compared to other Asian populations, the Indonesian population tends to have a wider but relatively shallow safe zone. This means that although the safe trajectory angle range is quite large, the screw length that can be inserted tends to be slightly shorter. Longer screws require a more medial angulation in the axial plane; In this study, the average trajectory angle of the longer screw was 29.65° ± 5.78° medially in men and 28.44° ± 6.45° in women. A lateral (outward) angulation of approximately 18° was considered safe in both men (±5.31°) and women (±5.00°). The cephalad screw placement angle also needs to be considered clinically, especially when using certain techniques such as the Currier technique. No previous morphometric studies have compared variations in C1 lateral mass screw placement methodology or specific screw diameters; therefore, these local findings provide new information for consideration of surgical technique.

Establishing a clear safe zone is crucial for maximizing surgical safety. One study showed that using an appropriate screw trajectory angle can improve implant stability and reduce the risk of complications during surgery.²⁰ Furthermore, the results of this study indicate that the safe zone angle tends to widen from childhood to young adulthood, then narrows again in old age. When the safe zone angle is analyzed by decade of age, statistically significant differences are found in both the axial and sagittal planes, indicating changes in C1 morphology with maturation and aging.

Morphologically, the lateral mass of the C1 vertebra in the Indonesian population is similar to that of other Asian populations (the difference in dimensions is only approximately 1–3 mm). Another study by Ma et al., which compared atlases of Caucasian and Asian populations, reported that Asian populations (including Indonesia, China, India, and Malaysia) generally have a relatively shorter C1 screw insertion length than Caucasian populations (the average in Caucasians is 31 ± 3 mm).²⁸ Several specific dimensions, such as the width and height of the lateral mass, appear to reflect the average body size and posture of the Indonesian population. These distinct anatomical characteristics are important in determining the safe zone for screw insertion and the optimal insertion angle to prevent complications.

The use of imaging technologies such as CT scans is very helpful in understanding C1 morphometry more deeply. With three-dimensional imaging, surgeons can plan procedures more accurately, considering the anatomical variations that exist between individuals.²⁹ A better understanding of the specific morphology of C1 in a population allows surgeons to optimize screw placement techniques to improve outcomes and minimize complications.

Based on the results of this study, the safe zone parameters for C1 lateral mass screw placement in the Indonesian population can be formulated as follows: in the axial plane, the safe trajectory angle ranges from approximately 18° laterally to 29° medially (safe zone range ±46°–48°); in the craniocaudal (sagittal) plane, the safe trajectory angle ranges from approximately 17° cranially (upper) to 29° caudal (lower) (safe zone range ±43°–45°). Knowledge of the boundaries of this safe zone has several clinical implications. First, understanding the safe zone can reduce the risk of neurovascular injury: by knowing the safe zone, surgeons can avoid screw penetration into the spinal canal or the foramen transversarium through which the vertebral artery passes. A study by Lee et al. (2017) showed that violating the safe zone increases the risk of vertebral artery injury by 4.1%.⁷ Second, screw placement within the safe zone maximizes bone purchase and biomechanical stability. Preoperative planning becomes more accurate, and understanding the safe zone allows surgeons to select the optimal screw size and trajectory before surgery. Sonnesen et al. (2013) reported that CT-based preoperative planning that takes the safe zone into account can improve screw placement accuracy by up to 96.7%.³⁰ Finally, intraoperative guidance based on a predetermined safe zone can reduce the risk of screw malposition, especially when the procedure is performed freehand. Gogia et al. noted that the use of safe zone guidance during surgery reduced the rate of lateral mass screw malposition from 7.2% to 2.1%.^28,31,32

Recommendations for C1 lateral mass screw insertion

The morphometric findings in this study can be used to recommend safe C1 screw insertion parameters in the Indonesian population. The smallest maximum diameter of the C1 lateral mass that we found was approximately 4.36 mm (in women aged >50 years), meaning that in such extreme cases, only screws with a diameter of ≤4.0 mm can be safely inserted. This aligns with previous recommendations that in Asian populations, screws with a diameter of 3.5–4.0 mm are generally used, as this size is still tolerated by the bone without compromising the cortical or structural integrity of the vertebra. Screws with a diameter of 3.5–4.0 mm have been reported to provide adequate fixation strength for atlantoaxial stabilization, even when compared with larger-diameter screws, particularly in terms of stability under axial loading.^31,33

Larger-diameter screws do provide slightly higher pullout strength, but this increase in stability is often marginal and outweighed by the increased risk of complications associated with larger diameters, for example, the risk of injury to neurovascular structures surrounding the atlas.²⁵ The use of smaller-diameter screws (e.g., 3.5 mm) is considered preferable, especially in pediatric patients. Although in our study, the 1–18-year-old age group had relatively large mean sagittal lateral mass diameters (MDs) (likely due to the average age of subjects in this group, approximately 13 years), anatomical limitations in younger children may limit the use of larger-diameter screws. A study by Burkhardt et al. demonstrated that a 3.5 mm diameter C1 lateral mass screw could be successfully placed in almost all cases, thus confirming the feasibility and safety of using this small screw in young patients. Furthermore, biomechanically, the 3.5 mm screw proved sufficient for effective fixation in both adult and pediatric cases.³¹