Anatomical Features of the Tarsal Sinus in Patients with Pes Planus: Implications for Clinical Management

Abstract

Background

In the treatment of pes planus, if the implant does not match the anatomical structures of the sinus tarsi, synovitis can develop, causing pain symptoms. In the interest of making clinical recommendations for extra-osseous talotarsal stabilization, the goal of the present study was to characterize the anatomical characteristics of the sinus tarsi in patients with pes planus using magnetic resonance imaging (MRI) scans.

Material/Methods

This was a retrospective study involving 56 pes planus patients and 56 healthy volunteers from January 2014 to May 2022. The sinus tarsi was measured for length and width, for angle with the coronal and sagittal axes, and for length of the subtalar implant.

Results

All examined metrics showed a difference between pes planus patients and healthy participants, with the exception of the subtalar implant’s length. The average sinus tarsi length and width among pes planus patients were 19.23 mm and 2.91 mm, respectively. The angle between the sinus tarsi and the coronal and sagittal axes was 21.418° and 25.077°, while the length of approach was 33.06 mm and 0.76°. The only gender differences that were statistically significant were in the length and width of the sinus tarsi. There were no notable variations between the left and right sides.

Conclusions

Sinus tarsi morphology may be impacted by pes planus. When treating patients with pes planus, clinicians should take these anatomical factors into consideration since they might more completely characterize the anatomical features of the sinus tarsi.

Background

Pes planus is among the most common diseases requiring foot and ankle surgery in clinical practice [1–3]. Flattening of the normal medial longitudinal arch is a complicated foot malformation that causes foot pain [4–7]. The arch curvature during non-weight bearing classifies pes planus as flexible or stiff. Flexible pes planus has a regular arch when not weight bearing, but the arch disappears when weight bearing, and the subtalar joint has more mobility. Severe deformity, low medial longitudinal arch, and ankle injuries might affect pes planus patients’ lifestyles.

Clinicians now prioritize early treatment of flexible pes planus patients. Flexible pes planus can be treated via conservative or surgical treatments [8]. Conservative therapies have low patient compliance, long clinical cycles, and cannot fully rectify the abnormal talar position [9]. Invasive surgery can entail long recovery times and cause problems such as nonunion and infection [10,11]. Flexible pes planus may be treated with extra-osseous talotarsal stabilization (EOTTS). A sinus tarsi implant may stabilize the gap, enabling foot arch repair [12,13]. The sinus tarsi is a slightly tapered space located between the inferior posterior joint and the anterior and middle subtalar joints, running anteriorly, from posteromedially to anteriorly [14]. The sinus tarsi is composed of a long tubular tarsal canal on the medial side, a funnel-shaped sinus tarsi, and a tarsal ostium located below the tail of the sustentaculum, which is filled with neuronal cells, small blood vessels, and fat [7].

When pes planus develops, the sinus tarsi closes, the pressure in the sinus tarsi rises, the nerve endings in the sinus tarsi are compressed, and localized pain occurs around the sinus tarsi. Without treatment, it can result in sinus tarsi syndrome, pes valgus, and metatarsal pain, as well as a poor quality of life for patients. After receiving EOTTS treatment, some patients may experience severe foot pain. Up to 40% of patients may decide to remove the tarsal sinus implant because of pain, primarily because the implant does not match the anatomical structure of the tarsal sinus and is either too deep or too shallow. Implantation of the brake can lead to the development of synovitis and the occurrence of vascular and neurodegenerative problems, causing pain symptoms [15]. The tarsal sinus has not been the subject of enough study, to the authors’ knowledge, to provide trustworthy information. The present study was therefore conducted to collect more detailed anatomical and morphological parameters of the tarsal sinus to confirm that there are differences between healthy persons and patients who have flat feet. Additionally, the objective of this study was to obtain more specific anatomical and morphological parameters of the tarsal sinus. This research offers clinicians new ideas for investigating the anatomical features of the tarsal sinus in patients with flat feet, as well as diagnostic and therapeutic anatomical data in an attempt to hasten patients’ recoveries. Additionally, this study provides clinicians with new ideas for studying the anatomical characteristics of the tarsal sinus in patients with flat feet.

Material and Methods

Ethics

This research was carried out in adherence with the Declaration of Helsinki, following ethical approval from the Ethics Committee of the Affiliated Traditional Chinese Medicine Hospital of Southwest Medical University (BY2022022). All patients gave their informed consent for their clinicopathologic data to be evaluated and published for scientific objectives.

General Data of Patients

From January 2014 to May 2022, 56 healthy volunteers and 56 patients with pes planus were enrolled (57 male and 55 female, 54 left feet and 58 right feet, age range: 10–72). All of the study participants supplied written informed consent before enrollment in the studies. All MRI scans were performed with 1.5-T scanners (Philips Ingenia) with an extremity coil, using the sagittal, axial, and coronal planes with standard pulse sequences and proton density–weighted images with fat saturation (PD-WI FAT SAT), and turbo inversion recovery magnitude; both T2-weighted and T1-weighted. The slice thickness was 3 mm.

Inclusion Criteria

(1) Patients must have met the diagnostic criteria for flexible pes planus. (2) No obvious ankle deformity was observed in the outward appearance. (3) Complete clinical data and MRI imaging data were available for each patient.

Exclusion Criteria

Patient exclusion criteria were: (1) patients with other lower limb injury, previous ankle fractures, or surgery; (2) patients with peripheral neuropathy of the ankle joint.

Confirmation of Pes Planus

Two researchers independently measured lateral ankle X-rays to diagnose pes planus in patients [16–18]. The talo-first metatarsal (Meary angle) is the normal angle of 0±4 degrees between the long axis of the talus and the first metatarsal. The calcaneal inclination angle is the angle between the lower border of the calcaneus and the sagittal axis, which is typically between 10 and 30 degrees [19]. The talocalcaneal angle (Kites angle) is the angle between the long talus and calcaneus, which is typically between 25 and 45 degrees. The talar tilt angle is the angle between the longitudinal axis of the talus and the sagittal axis, which is typically between 0 and 5 degrees. Patients with the aforementioned abnormalities were deemed to have pes planus (Figure 1).

Figure 1.

(A–D) Lateral X-rays of the ankles: A) the axis of the talus; B) the axis of the first metatarsal; C, D, E, F) the sagittal axis of the participants’ body; a) the talo-first metatarsal; b) the calcaneal pitch angle; c) the talocalcaneal angle; d) the talar tilt angle.

Morphological Parameters Measured

MRI scans of 56 patients and 56 healthy volunteers were retrospectively reviewed. The maximal cross section of the sinus tarsi was selected for analysis. In the horizontal plane, the length and diameter of the sinus tarsi and the angle of the sinus tarsi with the coronal axis were measured. The length of the subtalar implant refers to extending the long axis of the sinus tarsi to the skin surface. In the coronal plane, the angle between the sinus tarsi and the sagittal axis was measured. The measurement data are shown in Figures 2 and 3.

Figure 2.

(A–D) Horizontal plane in MRI. A) The length of the sinus tarsi; B) the length of the subtalar implant; C) the diameter of the sinus tarsi; D) the coronal axis; a) the angle of the sinus tarsi in the coronal axis. MRI – magnetic resonance imaging.

Figure 3.

Image illustrating MRI coronal plane measurement. A) The sagittal axis; B) the long axis of sinus tarsi; a) the angle between the sinus tarsi and the sagittal axis. MRI – magnetic resonance imaging. (Image realized by applying Servier Medical ART illustration: http://smart.servier.com).

The parameters measured included the following:

• The length of the sinus tarsi (mm): Distance between the external opening of the tarsal canal and the external opening of the sinus tarsi;

• The length of the subtalar implant (mm): Distance between the outer edge of the talar neck and the medial ankle;

• The diameter of the sinus tarsi (mm): Distance between the middle talocrural joint and the posterior talocrural joint;

• The angle of the sinus tarsi with the coronal axis (°): The angle between the axis of the sinus tarsi and the coronal axis;

• The angle of the sinus tarsi with the sagittal axis (°): The angle between the axis of the sinus tarsi and the sagittal axis.

Statistical Analysis

Data analysis was performed in SPSS (SPSS Statistics 25.0; IBM). The normality assumption for the data was tested using the Shapiro-Wilk test. The data that conformed to a normal distribution were analyzed using t-test and presented as χ̄±s, where “χ̄” represents the mean value and “s” is the standard deviation. Results that did not conform to a normal distribution were expressed as median (range) values and were analyzed using the Mann-Whitney test. Test-retest reliability was determined by calculating an intraclass correlation coefficient (ICC) with a 95% confidence interval, and statistical significance was assigned for P-values less than 0.05. Analysis was performed independently for male and female patients, and left and right feet.

Results

In Table 1, demographic parameters have been summarized. The patient group and controls were comparable with respect to age and body mass index. No differences between patients and healthy volunteers were observed with regard to gender, feet, and age.

Table 1.

Demography of patients and volunteers.

	Patients (n=56)	Healthy volunteers (n=56)	P value
Gender (female), n (%)	23 (41.1)	32 (57.1)	0.09
Feet(the left foot), n(%)	28 (50.0)	26 (46.4)	0.708
Age, mean±SD	36.95±18.980	41.20±16.614	0.21

In Table 2, the length of the sinus tarsi was significantly larger in patients compared with healthy volunteers (19.23±2.91 mm vs 17.51±1.81 mm, P<0.001). The diameter of the sinus tarsi was significantly increased in patients compared with healthy volunteers (2.93±0.66 mm vs 2.43±0.30 mm, P<0.001). Furthermore, the angle of the sinus tarsi in the coronal axis was significantly increased in patients compared with healthy volunteers (21.418±3.973° vs 17.182±2.258°, P<0.001), the angle of the sinus tarsi in the sagittal axis was significantly lower in patients compared with healthy control subjects (25.077±3.253° vs 27.763±3.101°, P<0.001). No statistical difference between the groups with regard to the length of the subtalar implant was found (33.06±3.71 mm vs 32.41±3.53 mm, P<0.001).

Table 2.

Classified measurement on MRI image of the sinus tarsi (mean±SD or mean).

	Patients(n=56)	Healthy volunteers (n=56)	P value
The length of the tarsal arsal sinus, mm	19.23±2.91	17.51±1.81	<0.001*
The diameter of the tarsal sinus, mm	2.93±0.66	2.43±0.30	<0.001*
The length of Hyprocure, mm	33.06±3.71	32.41±3.53	0.352
The angle of the tarsal sinus with the coronal axis, °	21.418±3.973	17.182±2.258	<0.001*
The angle of the tarsal sinus with the sagittal axis, °	25.077±3.253	27.763±3.101	<0.001*

^*P<.05.

MRI – magnetic resonance imaging; SD – standard deviation.

As detailed in Table 3, no statistically significant difference was seen between right and left feet.

Table 3.

Classification and measurement of sinus tarsi from MRI images of the left and right feet.

	The left foot (n=54)	The right foot (n=58)	P value
The length of the tarsal sinus, mm	17.93±2.16	18.78±2.84	0.081
The diameter of the tarsal sinus, mm	2.69±0.57	2.67±5.80	0.791
The length of Hyprocure, mm	32.46±3.96	33.00±3.41	0.439
The angle of the tarsal sinus with the coronal axis, °	19.861±4.414	18.778±3.204	0.138
The angle of the tarsal sinus with the sagittal axis, °	26.380±3.181	26.460±3.689	0.904

^*P<0.05.

MRI – magnetic resonance imaging.

Discussion

The purpose of this study was to measure the structure of the sinus tarsi using MRI images and to investigate the sinus tarsi characteristics in patients with pes planus. MRI has numerous advantages over computed tomography (CT), including no radioactivity in the human body, no biological adverse effects, and high resolution of soft tissue. Therefore, not only is it secure, but it also ensures that the experimental results are scientific [20–22]. In the present study, the length, diameter, angle in the coronal axis, and angle in the sagittal axis of the sinus tarsi were measured using MRI in patients with pes planus to determine the optimal length, diameter, and trajectory of the EOTTS implant. This study is the first to demonstrate through data that the parameters of the sinus tarsi in patients with pes planus differ from those in healthy volunteers, with the aim of providing clinicians with new treatment parameters that are more consistent with the sinus tarsi in patients with pes planus, thereby reducing the incidence of postoperative complications and facilitating the individualized treatment of patients with pes planus. EOTTS can effectively stabilize the distance by inserting a subtalar joint brake in the sinus tarsi to increase subtalar joint stability. It corrects the ectropion of the subtalar joint and reconstructs the arch to preserve the arch’s stability [12,23]. When treating flexible pes planus with a lower distance joint brake, strephenopodia predisposes prolapse of the brake, accounting for surgical failure. Even if the dimension is marginally off from the sinus tarsi, it is necessary to reimplant the matching brake [24]. In addition, the surgeon sequentially positions the trial in the order of the extent of the subtalar implant, until the foot ankle joint is passively extended by 10° as the correction standard. However, the method of placement is subjective, and it is unknown if the placement location and profundity correspond to the anatomical location of the sinus tarsi.

In the present study, patients with pes planus and healthy volunteers exhibited statistically significant differences in sinus tarsi length, diameter, angle in the sagittal axis, and angle in the coronal axis. Therefore, we would contend that the sinus tarsi of pes planus patients differs from the ordinary volunteer. Unquestionably, the use of sinus tarsi data from the general population in the study of subtalar joint stabilization may contribute to complications due to the presence of errors. Our study revealed that the length of the sinus tarsi in patients with pes planus was 19.23±2.91 mm, the diameter was 2.93±0.66 mm, and the angles in the coronal and sagittal axes were 21.418±3.973° and 25.077±3.253°, respectively. Consequently, we recommend using this parameter as a standard for subtalar implant dimensions. We discovered a significant difference in the length of the sinus tarsi between the male and female subjects (19.04±1.96 mm vs 17.67±2.92 mm, P=0.004), which should be considered during surgical procedures. In our study, there were no significant differences between the right and left foot parameters; therefore, we do not believe it is necessary to consider them during surgery.

Several limitations should be acknowledged in this paper. First, the complex structure of the ankle ligament makes it challenging to display the complete length of the ligament at the same level on MRI images, leading to measurement errors. Second, the MRI image resolution of the sinus tarsi in some patients may not be crystal clear. Third, this study’s sample size is relatively modest. In addition, there was a paucity of body mass index (BMI) data, and all the images in this study were collected from a single center in the Southwest region of China, so there was no multi-center research or in-depth analysis.

Conclusions

The structure of the sinus tarsi may be affected by pes planus, and it is hoped that clinicians will refer to this fact during diagnosis and treatment. For subtalar implants in adults, we recommend a length of 33.06±3.71 mm and a diameter of 2.93±0.666 mm. The present research not only enriched the anatomical and morphological database of the sinus tarsi, but also provided foundation and reference information for clinical surgery, such as subtalar joint stabilization. However, the information in this study is only an estimate, and patients should be treated individually.