Population-specific reference values for calcaneal angular parameters and their limited utility in sex estimation

Abstract

The calcaneus is the largest tarsal bone located in the lower part of the ankle joint and plays a significant role in walking, running, and maintaining balance. Certain angles of this bone, which is of great clinical and anthropological importance, are used in the diagnosis and treatment of fractures. The aim of this study is to determine the mean values of angular parameters of the calcaneus in a healthy population and to evaluate the accuracy rates of these parameters in predicting sex. In this study, six bilateral angle measurements were performed on radiographs from 540 individuals aged 20–65. Measured angles included Böhler, Gissane, calcaneal inclination, Fowler-Philip, lateral talocalcaneal, and calcaneal compression. The obtained data were compared between sexes, age groups, and right-left sides. Additionally, a receiver operating characteristic analysis was performed for sex prediction. Statistical analyses revealed significant differences between right and left sides in the calcaneal inclination angle and lateral talocalcaneal angle, between sexes in the calcaneal compression angle, and between age groups in the Böhler and Gissane angles. Analyses performed for sex prediction showed that the angles were not suitable for this purpose. The findings and the determination of the mean values of the angles provide useful information for decision-making in clinical diagnosis and treatment processes. It was found that using only calcaneal angles is not appropriate for sex prediction in the Turkish population. The findings may contribute to the establishment of population-specific reference values and provide baseline data for future multivariate or machine learning-based investigations.

Introduction

The calcaneus is a large, irregularly shaped bone located in the lower part of the ankle joint in the body. Together with the talus, the calcaneus plays a crucial role in the subtalar joint and provides the fundamental structure necessary for the human foot to stand upright and move [1, 2]. The calcaneus is involved in activities such as walking and running because it is the bone to which the Achilles tendon attaches [3]. Furthermore, the height and angular positions of the calcaneus are closely related to factors such as athletic performance; therefore, studies on the morphology of the calcaneus in athletes have found significant effects on performance [2]. Calcaneal complications resulting from intraarticular fractures involving the calcaneus in the clinic can have serious effects on the anatomical integrity of the lower extremity. Understanding the morphology of the calcaneus in traditional and modern medicine is instrumental in the diagnosis and treatment of various pathological conditions [1]. The treatment of calcaneal fractures plays a critical role in restoring physical function in the early stages [4].

Furthermore, sex predictions based on the morphological characteristics of the calcaneus can be used in different disciplines. The calcaneus is the largest tarsal bone, has a strong and compact structure, and is generally protected from environmental factors inside socks and shoes, which are reasons why this bone is preferred in sex estimation assessments [5]. Sex determination, one of the fundamental stages of the identification process in forensic medicine, is indispensable for confirming the identities of missing individuals, especially after mass disasters or natural disasters [6]. In archaeological studies, determining the sex of skeletal remains is critical for understanding the demographic structures of past societies [7].

Common angular measurements used in relation to the calcaneus include the Böhler angle (BA) and the Gissane angle. These angles are used to evaluate the morphological shapes of the calcaneus and help determine conditions requiring orthopedic intervention [8].

Studies in the literature have not sufficiently investigated the effectiveness of calcaneus angle parameters for predicting sex. The aim of this study is to reveal the average values of calcaneus angle parameters in the healthy Turkish population and to determine the differences between age groups, right-left sides, and sexes. At the same time, the accuracy rates of the obtained data for sex prediction were evaluated.

Materials and Methods

This study is a retrospective study conducted in accordance with the principles of the Helsinki Declaration, approved by the Necmettin Erbakan University Ethics Committee for Non-Drug and Non-Medical Device Research on February 4, 2022, with decision number 2022/3632. Due to the retrospective nature of this study, the requirement to obtain informed consent was waived by the institutional ethics committee. Bilateral comparative foot radiographs from 8,001 patients who presented to the Department of Radiology between 2018 and 2021 were retrospectively retrieved from the picture archiving and communication system (PACS) database. The scanned images were randomly selected by two researchers according to the criteria. The sample size for this study was determined to be 540 individuals based on a G*Power (3.9.1) analysis conducted using Agoada 2018’s [9] study as a reference. Exclusion criteria were strictly defined to ensure a healthy study population. Individuals with a history of fractures, acute trauma, or chronic bone pathologies (such as osteoarthritis, inflammatory arthritis, or tumors) in the tarsal, metatarsal, and phalangeal regions were excluded. Additionally, those with congenital or acquired structural deformities (e.g., hallux valgus, pes planus/cavis), individuals who had undergone previous surgical interventions in the foot region, and those aged 20 years and below were excluded. Finally, radiographic images of poor quality that did not allow for precise anatomical measurements were also removed from the study. A total of 540 individuals aged 20–65 who met these criteria were included in the study, 137 (25.4%) of whom were male and 403 (74.6%) female. The individuals included in the study were divided into age groups (20–29 years, 30–39 years, 40–49 years, 50–59 years, and 60–65 years) (Table 1). Six angle measurements were performed on the lateral foot X-ray (Table 2, Fig. 1) [10-15].

Table 1.

Participants in the study divided into five different groups based on age

	Value
Sex
Male	137 (25.4)
Female	403 (74.6)
Age group (yr)
20–29	45 (8.3)
30–39	126 (23.3)
40–49	157 (29.1)
50–59	177 (32.8)
60–65	35 (6.5)

Values are presented as number (%).

Table 2.

Definitions of measurements

Measurements	Definitions of measurements
1. Böhler angle	It is the angle between the line connecting the highest point of the posterior articular surface of the talus to the anterior process of the calcaneus and the line extending from the same point on the posterior articular surface of the talus to the highest point of the calcaneal tuber [10].
2. Gissane angle	It is the angle formed between the anterior aspect of the subtalar joint and the posterior aspect of the anterior process of the calcaneus [11].
3. Calcaneal inclination angle	It is the angle between the line passing through the support surface and the anterior and plantar points of the calcaneal tuber and the most plantar point of the calcaneocuboid joint [12].
4. Fowler-Philip angle	It is the angle between a parallel line drawn on the plantar surface of the calcaneus and a tangent drawn on the posterior part of the calcaneal tubercle [13].
5. Lateral talocalcaneal angle	It is the angle formed by a line drawn along the long axis of the talus and another line drawn along the long axis of the calcaneus [14].
6. Calcaneal compression angle	It is the angle between the line drawn tangentially to the plantar surface of the calcaneus and the line extending from the highest point of the posterior talar articular surface to the highest point of the calcaneal tuber [15].

Fig. 1.

Angle measurements performed in the study. (A) Schematic representation. (B) Picture archiving and communication system image. a, Böhler angle; b, Gissane angle; c, calcaneal inclination angle; d, Fowler-Philip angle; e, lateral talocalcaneal angle; f, calcaneal compression angle.

Measurement procedure

All radiographic measurements were conducted using the digital measurement tools integrated into the PACS system. The study involved a retrospective analysis of radiographs obtained in a non-weight-bearing lateral projection, with the ankle in approximately 45° to 55° of plantar flexion. To maintain methodological consistency, only images demonstrating superimposition of the talar domes were included, ensuring a true lateral perspective and minimizing rotational bias. Each angular parameter was measured three separate times by the same investigator, and the average was used for statistical analysis. Measurements were performed based on standardized radiographic landmarks defined in previous studies.

Statistical analysis

Statistical analysis was performed using IBM SPSS Statistics 21.0 software (IBM Co.). The normality of data distribution was assessed with the Kolmogorov–Smirnov test. Intra-observer reliability was assessed using the intraclass correlation coefficient (ICC [3,1]) based on a two-way mixed-effects model. ICC values ranged from 0.750 to 0.907, indicating good to excellent reliability [16]. After confirming adequate reliability, statistical analyses were conducted. For the comparison of morphometric measurements between sexes, the independent samples t-test was employed. Differences between age groups were evaluated using one-way ANOVA, followed by Tukey’s post-hoc test for multiple comparisons. The dependent samples t-test was used to compare the right and left side measurements. Furthermore, to determine the performance of these parameters in sex estimation, receiver operating characteristic (ROC) analysis was conducted. The area under the curve (AUC), sensitivity, and specificity values were calculated to evaluate the predictive accuracy. Categorical variables (such as sex and age groups) were compared using the Pearson chi-square test. Quantitative data were presented as mean±SD, and a significance level of P<0.05 was adopted for all analyses.

Results

The minimum, maximum, mean±SD, and comparison data for all individuals are shown in Table 3. When comparing right and left side values, calcaneal inclination angle (CIA) and lateral talocalcaneal angle (LTCA) values were significantly higher on the right side (P<0.05) (Table 3).

Table 3.

Measurements for all individuals

	Right			Left		P-value
	Minimum–Maximum	Mean±SD		Minimum–Maximum	Mean±SD
BA	12.3–53.0	30.43±5.61		15.9–53.2	30.38±5.39	0.776
GA	46.7–129.6	110.09±6.79		90.1–148.4	110.02±6.34	0.813
CIA	11.3–51.2	26.75±6.25		10.4–46.0	26.13±6.08	0.007*
FPA	23.9–79.3	61.22±6.26		42.2–76.3	61.50±5.82	0.232
LTCA	25.4–65.9	48.03±5.93		23.7–65.5	47.57±5.85	0.040*
CCA	20.0–52.4	32.16±4.31		18.8–45.3	32.17±4.24	0.923

BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle. *P<0.05 significance level.

The P-value resulting from the comparison of the minimum, maximum and mean±SD data for the right and left sides included in the study, along with the parameters between sexes, is presented in Table 4.

Table 4.

Comparison of measurements by sex

	Male (n=137)			Female (n=403)		P-value
	Minimum–Maximum	Mean±SD		Minimum–Maximum	Mean±SD
Age (yr)	20–65	42.29±11.48		20–65	43.81±10.46	0.174
Right
BA	17.6–48.0	31.24±5.86		12.3–53.0	30.16±5.51	0.061
GA	97.6–123.9	110.22±5.99		46.7–129.6	110.04±7.05	0.776
CIA	11.8–43.7	26.54±6.51		11.3–51.2	26.82±6.16	0.656
FPA	34.7–79.3	60.65±6.52		23.9–79.1	61.42±6.17	0.229
LTCA	25.6–65.9	49.31±6.28		25.4–62.9	47.6±5.75	0.005*
CCA	22.1–52.4	33.04±4.36		20.0–44.4	31.86±4.26	0.006*
Left
BA	15.9–53.2	31.01±5.78		16.1–52.0	30.16±5.24	0.127
GA	95.2–148.4	110.69±6.92		90.1–130.0	109.79±6.12	0.177
CIA	12.1–43.3	26.46±6.59		10.4–46.0	26.03±5.90	0.505
FPA	43.9–72.3	60.63±6.08		42.2–76.3	61.8±5.71	0.050
LTCA	28.6–65.1	48.17±6.17		23.7–65.5	47.36±5.73	0.177
CCA	20.0–44.8	32.96±4.44		18.8–45.3	31.91±4.15	0.015*

BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle. *P<0.05 significance level.

When the data were compared across age categories, statistically significant differences were observed in the right side Böhler angle (rBA; P=0.007), right side Gissane angle (rGA; P<0.001), left side Böhler angle (lBA; P=0.001), left side Gissane angle (lGA; P=0.001), and left side calcaneal compression angle (lCCA; P=0.019) values. The Tukey analysis showed which age ranges had statistically significant differences from each other. It was determined that none of the parameter data showed a regular increase or decrease with age (Table 5).

Table 5.

Distribution of data across age groups

	Age range (yr)
	20–29 (n=45)a)	30–39 (n=126)b)	40–49 (n=157)c)	50–59 (n=177)d)	60–65 (n=35)e)	P-value
Age (yr)	21.91±2.7	34.21±3.31	42.91±3.01	52.16±2.66	62.37±2.33	<0.001*
Right
BA	32.54±5.83a,c)	30.65±5.27	29.43±5.68	30.34±5.24	31.93±7.10	0.007*
GA	109.09±5.93	110.73±7.53b,c)	107.83±6.39c,d)	111.89±6.21	110.04±6.80	<0.001*
CIA	26.72±5.55	26.41±6.15	26.72±6.32	26.70±6.72	28.43±4.43	0.574
FPA	61.74±5.88	62.10±6.03	61.23±6.56	60.51±6.45	60.91±4.92	0.272
LTCA	46.60±6.79	47.70±5.90	47.89±5.78	48.60±5.99	48.83±5.14	0.252
CCA	32.55±4.14	31.74±4.35	31.85±4.45	32.36±4.29	33.54±3.75	0.125
Left
BA	33.13±4.82a,b),a,c),a,d)	29.86±4.75	29.62±5.08	30.47±5.69	31.62±6.83	0.001*
GA	108.90±6.60a,b)	112.31±5.53b,c)	108.22±6.27c,d)	110.47±5.85	109.01±8.53	<0.001*
CIA	27.00±6.16	26.15±6.11	26.48±5.96	25.29±6.34	27.76±4.56	0.121
FPA	62.11±4.85	61.62±5.88	61.48±6.25	61.29±5.73	61.41±5.41	0.941
LTCA	46.85±5.87	47.46±6.30	47.49±5.26	47.76±5.78	48.20±7.08	0.855
CCA	32.43±3.64	31.65±4.09b,e)	31.71±3.93	32.53±4.38d,e)	34.01±5.51	0.019*

Values are presented as mean±SD. BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle. The superscript letter combinations indicate statistically significant differences (P<0.05) between the respective age groups based on post-hoc pairwise comparisons: ^a)20–29 yr, ^b)30–39 yr, ^c)40–49 yr, ^d)50–59 yr, ^e)60–65 yr (e.g., ac indicates a significant difference between the 20–29 and 40–49 age groups). *P<0.05 significance level.

When evaluating the correlation between the measurements on the right side, GA showed a very weak positive linear relationship with age (P=0.005, r=0.121). Among the other parameters, some were correlated with each other.

rBA showed moderate positive linear relationship with right side calcaneal compression angle (rCCA; P<0.001, r=0.542), weak positive linear relationship with right side lateral talocalcaneal angle (rLTCA; P<0.001, r=0.211), and right side calcaneal inclination angle (rCIA; P<0.05, r=0.185). The rCIA had moderate positive linear relationship with rCCA (P<0.001, r=0.444) and rLTCA (P<0.001, r=0.620) and weak negative linear relationship with right side Fowler-Philip angle (rFPA; P<0.001, r=–0.316). The rFPA has weak negative linear relationship with rCCA (P<0.05, r=–0.248) and rLTCA (P<0.001, r=–0.317). The rLTCA, on the other hand, shows a moderate positive linear relationship with rCCA (P<0.001, r=0.569) (Table 6).

Table 6.

Correlation of data within right side

		Age	BA	GA	CIA	FPA	LTCA	CCA
Age	r	1	–0.081	0.121	0.045	–0.084	0.065	0.033
	P-value	-	0.061	0.005*	0.293	0.052	0.130	0.441
BA	r		1	–0.046	0.185	0.021	0.211	0.542
	P-value		-	0.282	<0.001*	0.621	<0.001*	<0.001*
GA	r			1	–0.012	0.081	–0.008	0.031
	P-value			-	0.776	0.060	0.846	0.475
CIA	r				1	–0.316	0.620	0.444
	P-value				-	<0.001*	<0.001*	<0.001*
FPA	r					1	–0.317	–0.248
	P-value					-	<0.001*	<0.001*
LTCA	r						1	0.569
	P-value						-	<0.001*
CCA	r							1

BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle; r, correlation coefficient; -, not applicable. *P<0.05 significance level.

On the left side, lBA was found to have moderate positive linear relationship with lCCA (P<0.001, r=0.531) and weak positive linear relationship with left lateral talocalcaneal angle (lLTCA; P<0.05, r=0.234), and left side calcaneal inclination angle (lCIA; P<0.001, r=0.193). The lCIA value had a moderate positive linear relationship with lCCA (P<0.001, r=0.490) and lLTCA (P<0.001, r=0.626) and a moderate negative linear relationship with Left side Fowler-Philip angle (lFPA; P<0.001, r=–0.401). The lFPA has a weak negative linear relationship with lCCA (P<0.001, r=–0.347) and moderate negative linear relationship with lLTCA (P<0.001, r=–0.408). The lLTCA parameter shows moderate positive linear relationship with lCCA (P<0.001, r=0.642) (Table 7).

Table 7.

Correlation of data within left side

		Age	BA	GA	CIA	FPA	LTCA	CCA
Age	r	1	–0.063	0.008	–0.022	–0.044	0.028	0.060
	P-value	-	0.143	0.855	0.613	0.308	0.512	0.165
BA	r		1	–0.051	0.193	–0.030	0.234	0.531
	P-value		-	0.232	<0.001*	0.489	<0.001*	<0.001*
GA	r			1	–0.054	0.004	0.035	–0.038
	P-value			-	0.207	0.933	0.422	0.383
CIA	r				1	–0.401	0.626	0.490
	P-value				-	<0.001*	<0.001*	<0.001*
FPA	r					1	–0.408	–0.347
	P-value					-	<0.001*	<0.001*
LTCA	r						1	0.642
	P-value						-	<0.001*
CCA	r							1

BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle; r, correlation coefficient; -, not applicable. *P<0.05 significance level.

The results of the ROC analysis conducted for the usability of each parameter on the right and left sides for sex prediction are presented in table and graph form (Table 8, Fig. 2).

Table 8.

Receiver operating characteristic analysis results

	AUC	95% CI	Cut-off value	P-value
Right
BA	0.573	0.516–0.631	F<31.05<M	0.029*
GA	0.491	0.435–0.547	F<110.35<M	0.029*
CIA	0.490	0.433–0.547	F<26.2<M	0.029*
FPA	0.463	0.408–0.519	F<61.05<M	0.028*
LTCA	0.589	0.534–0.644	F<48.35<M	0.028*
CCA	0.572	0.517–0.627	F<32.45<M	0.028*
Left
BA	0.555	0.499–0.611	F<30.65<M	0.029*
GA	0.539	0.483–0.595	F<110.55<M	0.029*
CIA	0.515	0.457–0.572	F<26.25<M	0.029*
FPA	0.447	0.390–0.505	F<61.45<M	0.029*
LTCA	0.544	0.487–0.601	F<47.75<M	0.029*
CCA	0.567	0.511–0.624	F<32.35<M	0.029*

BA, Böhler angle; GA, Gissane angle; CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; CCA, calcaneal compression angle; AUC, area under the receiver operating characteristic curve; CI, confidence interval; F, female; M, male. *P<0.05 significance level.

Fig. 2.

Graphical representation of receiver operating characteristic (ROC) analysis results. (A) Right side. (B) Left side. rBA, right side Böhler angle; rGA, right side Gissane angle; rCIA, right side calcaneal inclination angle; rFPA, right side Fowler-Philip angle; rLTCA, right side lateral talocalcaneal angle; rCCA, right side calcaneal compression angle; lBA, left side Böhler angle; lGA, left side Gissane angle; lCIA, left side calcaneal inclination angle; lFPA, left side Fowler-Philip angle; lLTCA, left side lateral talocalcaneal angle; lCCA, left side calcaneal compression angle.

The parameters yielding the two highest accuracy rates were rLTCA (0.589) and rBA (0.573). When examining the results of the left-side parameters for sex prediction, it was observed that the left-side values had higher accuracy rates for the GA and CIA parameters, while the right-side data had higher accuracy rates for the remaining BA, FPA, LTCA, and CCA parameters. The two parameters with the lowest accuracy rates were found to be lFPA (0.447) and lCIA (0.515).

Discussion

The biomechanical functionality of the foot is supported by the connective tissues, muscles, and tendons of the joints located in the foot [17]. The foot transmits the forces generated by body weight and movement to the ground and is one of the three main points of contact between the body and the ground. Due to its biomechanical and surgical importance, various studies related to the calcaneus have been conducted on bone samples, cadavers, and living individuals.

Some morphometric characteristics of the calcaneus are used both to provide clinical benefits and to determine sex from skeletal remains in forensic medicine or archaeological research [18]. Within the scope of this study, six different angle parameters of the calcaneus bone were examined.

Studies in the literature indicate that BA helps increase the success rate of surgical operations for fractures and predict union in the subtalar joint [10]. Studies have shown that BA decreases in intra-articular fractures and that values below 15° are indicative of fracture reduction [19, 20]. Based on these findings in the literature, BA is an important guiding parameter in clinical practice. The results of previous studies evaluating BA are summarized in Table 9. In the present study, the mean rBA and lBA values were 30.43°±5.61° and 30.38°±5.39°, respectively, with no significant bilateral difference (P=0.776). This finding aligns with multiple population-based studies conducted in different geographic regions, including Saudi Arabia, Türkiye, Egypt, Britain, Nepal, and Croatia, all of which reported no significant right–left variation in BA values [15, 21-27]. Although one study reported a unilateral difference in males, this finding was not observed in females [18].

Table 9.

Other studies in the literature evaluating the Böhler and Gissane angles

Authors	Population	n	Age range (yr)	Böhler angle (°)	Gissane angle (°)
Igbigbi and Mutesasira (2003) [46]	Uganda	206	20–40	35.1±7.5	-
Khoshhal et al. (2004) [21]	Saudi Arabia	299	15–72	31.21	116.16
Schepers et al. (2007) [30]	-	33	18–65	32	-
Seyahi et al. (2009) [25]	Türkiye	308	18–79	33.8±4.8	115.0±6.5
Boyle et al. (2011) [29]	New Zealand	100	30–70	39.2	113.8
Shoukry et al. (2012) [15]	Egypt	220	20–40	30.14±4.18	122.9±6.95
Willmott et al. (2012) [22]	Britain	198	16–92	36.4±4.17	-
Kang et al. (2012) [35]	USA	50	18 and over	33.3	-
Isaacs et al. (2013) [23]	Australia	212	-	29.4	-
Arslan et al. (2014) [19]	Türkiye	50	19–88	42.08±8.8	106.04±8.22
Ramachandran and Shetty (2015) [20]	India	184	17–75	31.3±5	108.5
Rokaya et al. (2016) [24]	Nepal	140	15–68	31±5	108.4±10.5
Yıldız and Çetin (2019) [27]	Türkiye	107	6–75	Right 36.77±3.67 Left 33.23±7.20	Right 110.99±10.18 Left 108.96±9.18
Šimunović et al. (2019) [26]	Croatia	130	30–69	34±5	-
Başdelioğlu (2021) [12]	Türkiye	410	20 and over	34.11±3.19	125.180±4.736
Alabi et al. (2020) [28]	Nigeria	110	-	Right 32.73±7.15 Left 28.71±6.65	Right 63.94±20.58 Left 80.23±26.68
Altuntaş (2021) [18]	Türkiye	180	20–64	Male right 29.6±4.9 Female right 30.6±4.5 Male left 29.5±4.7 Female left 30.6±4.5	Male right 117.5±4.5 Female right 118.5±3.8 Male left 117.5±4.5 Female left 118.5±3.8
Present study	Türkiye	540	20–65	Male right 31.2±5.86 Female right 30.1±5.51 Male left 31.0±5.78 Female left 30.1±5.24	Male right 110.2±5.9 Female right 110.0±7.0 Male left 110.6±6.9 Female left 109.7±6.1

Values are presented as number or mean±SD. -, not applicable.

The normal BA range reported in the literature (25°–40°) is consistent with our measurements. Furthermore, no significant sex-based difference was identified in our cohort, which supports the majority of previous investigations indicating limited sexual dimorphism in BA [15, 19, 21-25, 27, 28]. The highest BA values were observed in the 20–29 age group, consistent with earlier reports suggesting relatively higher values in younger adults [15].

The GA is an important parameter in the diagnosis and treatment of calcaneal fractures [20]. Vertical loads can affect subtalar joint anatomy, leading to changes in GA values, and this change is often associated with a decrease in the BA. GA is a measurement that reflects the morphological relationship between the posterior, anterior, and medial talar joint surfaces [24] and restoring it to normal values after trauma is one of the surgical goals [29].

The normal GA range has been reported in the literature as 120°–145° [11]. In our study, rGA and lGA values were measured as 110.09°±6.79° and 110.02°±6.34°, respectively, with no significant difference detected between the two sides (Table 9). This finding is consistent with the bilateral symmetry reported in different populations [21, 24, 25, 27-29]. Furthermore, the absence of a significant difference according to sex is also consistent with previous studies [18, 21, 25, 28].

The literature reports variable findings regarding normal CIA values, particularly in relation to pes planus and pes cavus (Table 10). In the present study, mean CIA values were 26.75°±6.25° on the right and 26.13°±6.08° on the left, with a small but statistically significant bilateral difference, while no sex-based difference was observed. Comparable bilateral comparisons are limited in the literature.

Table 10.

Other studies in the literature evaluating CIA, FPA, and LTCA angles

Authors	Population	n	Age range (yr)	CIA (°)	FPA (°)	LTCA (°)
Mishra et al. (2006) [32]	-	72	16–74	15.31±6.2	58.1±8.28	-
Schepers et al. (2007) [30]	-	33	18–65	23	-	43
Lu et al. (2007) [36]	Taiwan	49	19–73	-	60.14±7.01	-
Akdoğan et al. (2012) [39]	Türkiye	50	4–78	Right 22.98±4.01 Left 21.52±4.32	-	Right 36.44±5.21 Left 36.44±5.21
Kang et al. (2012) [35]	USA	50	18 and over	-	61	-
Dahiru et al. (2013) [14]	Nigeria	63	-	15.08±2.87	-	38.85±8.20
Gutierrez et al. (2013) [34]	Spain	35	9–14.9	17.4±5.4	61.9±6.0	-
Arslan et al. (2014) [19]	Türkiye	69	18–79	21.14±4.85	-	-
Tourné et al. (2018) [13]	-	30	19–67	21.7	59±0.3	-
Yıldız et al. (2018) [31]	-	100	18–57	17.4±3.6	-	-
Başdelioğlu (2021) [12]	Türkiye	210	20 and over	23.56±3.03	-	34.94±3.24
Alabi et al. (2020) [28]	Nigeria	110	-	-	Right 108.54±20.48 Left 100.34±29.72	-
Altuntaş (2021) [18]	Türkiye	180	20–64	Male right 25.8±3 Female right 26.6±2 Male left 25.8±2 Female left 26.6±2	-	Male right 52.7±4.6 Female right 53±4.1 Male left 52.7±4.5 Female left 53±3.9
Present study	Türkiye	540	20–65	Male right 26.5±6.5 Female right 26.8±6.1 Male left 26.4±6.5 Female left 26.0±5.9	Right 61.22±6.26 Left 61.5±5.82	Male right 49.3±6.2 Female right 47.6±5.7 Male left 48.1±6.1 Female left 47.3±5.7

Values are presented as number or mean±SD. CIA, calcaneal inclination angle; FPA, Fowler-Philip angle; LTCA, lateral talocalcaneal angle; –, not applicable.

Previous studies have demonstrated that CIA decreases in extra-articular calcaneal fractures, with lower values reported in fracture groups compared to healthy controls [19, 30]. In non-fracture populations, reported CIA values range from approximately 21° to 25° [19, 30]. In studies addressing Haglund deformity, CIA values around 25° have been described [13], although inconsistent associations have been reported, with some authors identifying higher values in affected individuals [13]. Similarly, variations in CIA have been noted in plantar heel pain and heel spur conditions [12, 31, 32]. Altuntaş (2021) [18] reported no significant bilateral difference in CIA across sexes, consistent with our findings regarding limited sexual dimorphism.

The FPA is a commonly used parameter on lateral radiographs, particularly in the evaluation of Haglund’s deformity. Fowler and Philip (1945) [33] originally defined the normal range as 44°–69°, suggesting that values above 75° may be associated with retrocalcaneal bursitis or Haglund-related pathology. In the present study, mean FPA values were 61.22°±6.26° on the right and 61.5°±5.82° on the left, with no significant bilateral difference (Table 10). These values fall within the traditionally accepted normal range.

Gutierrez et al. (2013) [34] reported comparable mean values and described a negative correlation between FPA and age; however, no age-related association was identified in our cohort. The relationship between FPA and pathological conditions remains inconsistent in the literature. Several studies have reported no significant difference in FPA between individuals with Achilles tendinitis or heel pain and healthy controls [12, 35-37]. In contrast, marked reductions in FPA have been observed following surgical correction of Haglund deformity [14, 38], suggesting that FPA may be responsive to structural modification rather than serving as a reliable primary diagnostic marker.

In our study, no significant differences were observed across age groups or between sexes, supporting the notion that FPA demonstrates structural stability in healthy adults. Taken together, these findings indicate that FPA appears to reflect context-dependent morphological variation in pathological states while remaining relatively stable in physiologically normal calcaneal geometry.

Pes equinovarus is characterized by varus positioning of the calcaneus and adduction of the forefoot, and the LTCA is frequently used in its radiographic assessment [39]. Studies on LTCA in the literature are presented in Table 10. In the present study, mean LTCA values were 48.03°±5.93° on the right and 47.57°±5.85° on the left, with a small but statistically significant bilateral difference. In contrast, Dahiru et al. (2013) [14] reported no significant side-related variation in a Nigerian cohort.

Sex-based analysis revealed a significant difference on the right side but not on the left, whereas most previous studies have reported no meaningful sexual dimorphism in LTCA [18, 39]. Considerable variation in mean LTCA values across populations has been documented, with lower averages reported in Turkish cohorts [12, 39], suggesting possible population-specific morphological differences. Akdoğan et al. (2012) [39] also demonstrated a correlation between LTCA and CIA, supporting the concept of coordinated hindfoot geometry.

In pathological conditions, LTCA has shown sensitivity to structural alteration. Significant side differences have been observed in unilateral calcaneal fractures [30] and postoperative increases following surgical correction have been reported [40]. However, no consistent association has been demonstrated between LTCA and heel spur pathology [12].

Taken together, these findings suggest that LTCA reflects hindfoot alignment and biomechanical adaptation rather than intrinsic sexual dimorphism. While it may demonstrate responsiveness to structural disruption or surgical modification, LTCA appears to maintain relative stability in healthy calcaneal architecture, with inter-population variability potentially contributing to reported differences in mean values.

The CCA is another parameter used in the evaluation of intra-articular calcaneal fractures, particularly in cases involving superior depression and loss of calcaneal height. A reduction in CCA reflects structural collapse or fragmentation of the posterior calcaneal body [15].

In the present study, mean CCA values were 32.16°±4.31° on the right and 32.17°±4.24° on the left, with no significant bilateral difference, consistent with the findings of Alabi et al. (2020) [28]. However, CCA values were significantly higher in males, indicating a potential sex-related difference in calcaneal morphology. Age-related variation was also observed on the left side, suggesting that CCA may partially reflect age-dependent structural remodeling.

Previous studies evaluating surgical correction of calcaneal fractures have demonstrated postoperative increases in CCA values [40], supporting the notion that CCA is sensitive to restoration of calcaneal height following structural disruption.

Overall, these findings suggest that while CCA demonstrates bilateral symmetry in healthy individuals, it may be responsive to sex-related morphological differences and structural changes associated with aging or surgical intervention.

Sex determination from bones has gained importance in various forensic cases or large-scale disasters affecting populations. Sex determination from skeletal remains is an important topic in both forensic medicine and bioarchaeology. In our study, right and left calcaneus measurement parameters were subjected to ROC analysis to determine their accuracy in sex estimation. There are no studies in the literature aiming to estimate sex from these angles. In this study although some parameters reached statistical significance, the AUC values remained below 0.60, indicating low discriminative capacity. Therefore, calcaneal angular measurements alone do not appear to provide reliable accuracy for sex estimation.

This finding is consistent with the broader forensic literature on calcaneal morphometry, which has consistently demonstrated that linear dimensions outperform angular parameters in sex classification. Riepert et al. (1996) [41], in a study of 800 Central Europeans, reported that while all linear distances of the calcaneus were significantly larger in males, the angular measurements did not differ between sexes. Similarly, Zhang et al. (2017) [42], in a Chinese population of 293 individuals, found that direct sex classification using angular measurements yielded accuracy rates of only 52.6%–58.7%, in contrast to 78.2%–89.1% achieved with linear measurements. Zakaria et al. (2010) [43] further reported that among six calcaneal measurements taken from Egyptian radiographs, all linear parameters showed significant sexual differences while angular parameters did not. Taken together, these findings indicate that the limited discriminative capacity of calcaneal angular measurements observed in the present study reflects a consistent pattern across populations rather than a study-specific limitation.

Although statistically significant differences were observed in certain angular parameters between sexes and age groups, the effect sizes were small and the discriminative capacity remained limited. This finding suggests that calcaneal angular measurements alone may not adequately reflect sexual dimorphism. Unlike linear morphometric parameters, angular measurements are more likely influenced by biomechanical loading patterns, lifestyle factors, and foot posture variations rather than intrinsic skeletal dimorphism.

The absence of significant sex-based differences in BA, GA, CIA, and FPA observed in the present study is consistent with the majority of population-based investigations, including those from Türkiye [18, 25], Egypt [15], Brazil [44], Nigeria [14], and Malawi [45]. The significant but modest sex-based differences identified in rLTCA and CCA, while not sufficient for reliable classification, are not without precedent; isolated significant findings in calcaneal angles have been reported in Ugandan [46] and Saudi Arabian [47] populations. These inconsistencies across studies further support the conclusion that any sex-related angular variation is population-dependent and does not translate into a generalizable classification tool. Regarding population-specific differences, the mean angular values obtained in the present study are largely comparable to previously published Turkish data [18, 25], though modest discrepancies exist. In particular, mean GA values in our cohort (right: 110.09°, left: 110.02°) are lower than those reported in Egyptian (122.9°) [15] and Saudi Arabian (116.2°) [21] populations, while mean LTCA values (right: 48.03°, left: 47.57°) exceed those documented in prior Turkish [12, 39] and Nigerian [14] cohorts. Such inter-population variability reinforces the need for locally derived reference values and further explains why angular parameters, which fluctuate across populations, are poorly suited as universal sex estimation tools.

Furthermore, the moderate correlations observed between several angular parameters indicate possible biomechanical interdependence within calcaneal geometry. However, these relationships do not appear to translate into reliable sex classification performance when evaluated independently.

The limited predictive capacity observed in this study may also reflect the inherent constraints of two-dimensional radiographic assessment. Three-dimensional imaging modalities may provide more detailed structural information and improve discriminative modeling in future investigations.

Limitations

This study has several limitations that should be acknowledged. First, the retrospective design may have introduced selection bias. Although weight-bearing status, ankle position, and rotational control were kept as consistent as possible throughout the study, these parameters could not be formally standardized or reported due to the retrospective nature of the study, which may limit the reproducibility of the imaging protocol in future studies. Second, the sample was obtained from a single center and may not fully represent the broader population. Third, the sex distribution was imbalanced, with a higher proportion of female participants, which may have influenced the discriminative performance in sex prediction analyses.

Additionally, all measurements were performed by a single investigator. Although repeated measurements were averaged to reduce intra-observer variability, inter-observer reliability was not assessed. Another limitation is the use of two-dimensional radiographs, which may not fully capture the three-dimensional morphology of the calcaneus.

Future studies incorporating balanced samples, multicenter data, three-dimensional imaging techniques, and multivariate analytical approaches may provide more robust insights.

Conclusion

The present study established population-specific reference values for commonly used calcaneal angular parameters based on a relatively large radiographic dataset. While certain statistically significant differences were detected between sexes and age groups, the overall discriminative performance for sex estimation was limited. These findings suggest that calcaneal angular parameters alone should not be considered reliable indicators for sex determination in Turkish population but may serve as supportive reference data in clinical and anthropological contexts.