1

The foot is a dynamic structure that supports human weight, ensures ground contact, and facilitates mobility . Describing fetal anatomy is crucial for understanding congenital deformities (such as pes equinovarus), performing peripheral nerve blocks in newborns, and planning surgical procedures. This study documents anatomical structures during early development, allowing clinicians to better predict potential variations ^,,, .

Furthermore, because the foot structure undergoes significant changes with postnatal loading, the onset of walking, and biomechanical adaptation, examining nerve variations during the pre-weight bearing period provides valuable information about the developmental origins of adult anatomy. A detailed characterization of the course, number, and variations of the terminal branches of the tibial nerve during fetal development may also help reduce the risk of iatrogenic nerve injury during surgical interventions ^,,, .

With its relatively large sample size, the study provides normative reference data for the fetal period and establishes an anatomical foundation for future pediatric, biomechanical, and surgical research .

The foot is at high risk of developing various deformities due to trauma and loading ^, . Therefore, it is important to evaluate foot anatomy from the embryological period to develop more effective treatment strategies for potential trauma and deformities. Furthermore, a detailed understanding of regional anatomy is critical for numerous clinical interventions, such as flexor tendon surgery, tarsal tunnel release, mobilization of neurovascular island flaps, hallux valgus treatment, Morton neuroma injections, and heel pain management ^,,, .

The tibial nerve (TN) is of interest to both anatomists and clinicians due to its course and variations. Horwitz (1938) defined the terminal branches of the TN, consisting of the medial plantar nerve (MPN), lateral plantar nerve (LPN), and medial calcaneal nerve (MCN), as a trifurcation . Subsequent studies have demonstrated that these terminal branches can exhibit significant variations ^, . In the literature, the TN bifurcation level is classified based on whether it is inside or outside the tarsal tunnel and its relationship to the medial malleolus–calcaneal axis (MCA) ^,,,,, . Dellon and Mackinnon (1984) defined the MCA—the line between the medial malleolus and the medial prominence of the calcaneal tuberosity—to define the TN bifurcation level . This study evaluated the TN bifurcation level, its relationship with the MCA, and its relationship with the tarsal tunnel. A detailed understanding of the TN’s relationship with the tarsal tunnel is important to reduce iatrogenic injuries during surgical and invasive ankle procedures ^,, .

Studies have demonstrated the presence of connections between the common digital plantar nerves, which are superficial branches of the MPN and LPN ^, . (Some studies have also reported connections between their deep branches ^, .

In this study, we aim to examine in detail the MCN, MPL and LPN branches, which are the terminal branches of TN, in fetal cadavers, to reveal the existing variations and to classify them in accordance with the literature. The scarcity of studies conducted in this area, especially on fetuses and children, and the methodological differences of existing studies constitute a significant obstacle in determining the risk of complications of interventional processes in the region. The results of the present study are going to fill the knowledge deficiency in this field by providing basic data for region-specific interventional clinical trials.

2

In current study, fetuses obtained from Suleyman Demirel University, Faculty of Medicine, Department of Anatomy, and Isparta Maternity and Children’s Hospital were used. A total of 116 feet from 58 fetuses (17–40 weeks of gestation) with no external anomalies or pathologies, who had died due to unknown causes through abortion or during the perinatal period, were included in the study. Dissections were performed by 2 anatomists in the anatomy laboratory.

To determine the bifurcation point of the terminal branches of the TN (the MCN, the MPN, and the LPN), the medial malleolus and the calcaneal tuberosity were palpated. Thereafter, the medial side of approximately 1/3 distal part of the leg was dissected. After removing the skin, the flexor retinaculum was exposed. The TN, located within one of the four channels formed between the flexor retinaculum and the bony structures, were identified. The skin of the plantar region was dissected and the plantar aponeurosis was exposed. The TN was followed and MPN and LPN were revealed. The bifurcation of the TN was classified based on whether it occurred proximal to, at the level of, or distal to the MCA ( Fig. 1 ).

Malleolar-calcaneal axis (MCA: Malleolar-calcaneal axis, MM: Medial malleolus, MP: Medial process of the calcaneal tuberosity).

The distance between the origin of the MCN and both the medial malleolus and the tuberosity of the navicular bone was measured using a digital caliper. The superficial branches forming anastomoses between the MPN and the LPN were classified in accordance with the literature ( Fig. 2 ). If an anastomosis was present, its distance from the heel was measured using a digital caliper. The motor branches innervating the lumbrical muscles were identified, which nerve innervated which muscle was revealed.

The medial and lateral plantar nerve classification (MPN: Medial Plantar Nerve, LPN: Lateral Plantar Nerve).

The course of the TN’s terminal branches and their motor branches was examined under a “EUROMEX Edublue 1805-S binocular digital stereo microscope.” Length measurements were performed using a digital caliper. Illustrations were drawn using an Apple iPad. Additionally, ethical approval for this study was obtained from the Ethics Committee of Suleyman Demirel University Faculty of Medicine.

Statistical analyses were conducted using SPSS version 26.0. Data were expressed as count (n), percentage (%), and mean ± standard deviation (SD). A p-value of < 0.05 was considered statistically significant. Continuous variables that did not follow a normal distribution in both sexes were identified as fetal age (weeks), foot length, and the distance of the MPN-LPN anastomosis from the heel. For these parameters, the Mann-Whitney U test was used, whereas the Student’s t -test was applied to normally distributed variables, such as the MCN-tuberosity distance, the MCN classification and MCN-medial malleolus (MM) distance.

In this study, the MCN was classified into six types ( Fig. 3 ) according to the classification made by Zhang et al. (2021) :

• •

  Type I – A single MCN originating from the TN

• •

  Type II – A single MCN originating from the LPN

• •

  Type III – Two MCNs, one originating from the TN and the other from the LPN

• •

  Type IV – Two MCNs, both originating from the TN

• •

  Type V – Two MCNs originating from the TN with a common origin

• •

  Type VI – Three MCNs, one originating from the TN and two from the LPN

The MCN Classification (TN: Tibial nerve, MPN: Medial plantar nerve, LPN: Lateral plantar nerve, MCN: Medial calcaneal nerve).

3

In our study, the gestational age of female fetal cadavers was 28.51 ± 7.09 weeks, while that of male fetal cadavers was 29.31 ± 6.83 weeks (z =-0.630, p = 0.529). Based on trimester classification, 20 fetal cadavers were in the second trimester (female = 12, male = 8), 30 fetal cadavers were in the third trimester (female = 13, male = 17), and 8 fetal cadavers were in the fourth trimester (female = 4, male = 4) (χ² = 2.667, p = 0.264).

Comparative analyses of parameters according to sex and sides are shown in Table 1 and Table 2 .

Table 2

Comparison of relevant parameters by side.

		Right N (%) or mean±SD			Left N (%) or mean±SD			p
MCN Typing	1	11 (37.9)	8 (27.6)	19 (32.8)	9 (31.0)	7 (24.1)	16 (27.6)	0.555
	2	10 (34.5)	14 (48.3)	24 (41.4)	10 (34.5)	12 (41.4)	22 (37.9)
	3	6 (20.7)	5 (17.2)	11 (19)	5 (17.2)	4 (13.8)	9 (15.5)
	4	0 (0)	1 (3.4)	1 (1.7)	2 (6.9)	2 (6.9)	4 (6.9)
	5	2 (6.9)	0 (0)	2 (3.4)	3 (10.3)	2 (6.9)	5 (8.6)
	6	0 (0)	1 (3.4)	1 (1.7)	0 (0)	2 (6.9)	2 (3.4)
MCN- Tuberosity of the navicular bone distance	1	8.79 ± 2.77	13.74 ± 4.20	10.87 ± 4.17	10.23 ± 4.44	11.82 ± 5.27	10.93 ± 4.72	0.971
	2	11.51 ± 5.78	11.44 ± 3.61	11.47 ± 4.52	11.99 ± 4.75	11.76 ± 3.84	11.86 ± 4.17	0.761
	3	14.15 ± 2.81	11.45 ± 2.32	12.93 ± 2.84	11.16 ± 3.09	13.92 ± 6.58	12.39 ± 4.81	0.759
	4	0	10.80	10.80	10.75 ± 4.46	11.33 ± 2.65	11.04 ± 3.01	0.947
	5	11.38 ± 3.23	0	11.38 ± 3.23	15.48 ± 7.69	8.92 ± 1.09	12.86 ± 6.54	0.781
	6	0	8.77	8.77	0	9.71 ± 1.50	9.71 ± 1.50	0.699
MCN-MM distance	1	6.46 ± 2.13	9.04 ± 2.91	7.54 ± 2.74	7.45 ± 2.76	7.41 ± 3.47	7.43 ± 2.98	0.909
	2	8.43 ± 3.60	7.26 ± 3.12	7.75 ± 3.30	9.29 ± 3.59	8.52 ± 2.43	8.87 ± 2.96	0.233
	3	10.20 ± 2.05	8.01 ± 4.45	9.20 ± 3.37	6.98 ± 2.00	6.67 ± 3.05	6.84 ± 2.35	0.093
	4	0	8.03	8.03	6.15 ± 1.44	12.32 ± 3.62	9.23 ± 4.21	0.814
	5	9.97 ± 4.17	0	9.97 ± 4.17	7.36 ± 3.05	4.48 ± 0.99	6.21 ± 2.72	0.203
	6	0	7.54	7.54	0	7.67 ± 1.88	7.67 ± 1.88	0.964
MPL-LPN Typing	1	26 (89.7)	22 (75.9)	48 (82.8)	23 (79.3)	26 (89.7)	49 (84.5)	0.894
	2	1 (3.4)	1 (3.4)	2 (3.4)	2 (6.9)	1 (3.4)	3 (5.2)
	3	1 (3.4)	5 (17.2)	6 (10.3)	3 (10.3)	1 (3.4)	4 (6.9)
	4	1 (3.4)	1 (3.4)	2 (3.4)	1 (3.4)	1 (3.4)	2 (3.4)
MPL-LPN heel length	1	–	–	–	–	–	–	–
	2	26.66	33.46	30.07 ± 4.82	31.81 ± 6.94	30.79	31.47 ± 4.94	0.776
	3	22.19	28.25 ± 2.64	27.24 ± 3.42	31.56 ± 4.47	33.40	32.02 ± 3.76	0.071
	4	14.25	23.75	19.00 ± 6.71	39.02	34.93	36.97 ± 2.89	0.074
TN bifurcation	1	9 (31.0)	14 (48.3)	23 (39.7)	11 (37.9)	12 (41.4)	23 (39.7)	0.969
	2	12 (41.4)	11 (37.9)	23 (39.7)	11 (37.9)	11 (37.9)	22 (38.8)
	3	8 (27.6)	4 (13.8)	12 (20.7)	7 (24.1)	6 (20.7)	13 (21.6)

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