Abstract
Purpose
Tarsal tunnel neurovascular vulnerability lacks robust anatomical classification. We established an MRI-based retromalleolar canal classification to assess its clinical utility for surgical approach guidance and pathology diagnosis.
Methods
This retrospective study analyzed normal ankle 3 T MRIs from 250 consecutive patients (2018–2023). Three musculoskeletal radiologists independently classified retromalleolar canal morphology using axial T2-weighted turbo spin-echo sequences (Siemens Skyra 3 T) acquired at a standardized plane 4 mm superior to the talar dome.
Results
Three radiologists demonstrated excellent interobserver reliability in all measurements (ICC >0.85, P < 0.001). Based on morphological parameters including shape, dimensions, and angular relationships, we classified retromalleolar canals into four distinct types: R (predominant, 60.4 %), V (18 %), K (16.8 %), and L (4.8 %). Morphometric analysis revealed significant differences across all classification groups (P < 0.001), particularly in groove depth and medial malleolar angle measurements.
Conclusions
The novel morphological classification may complement existing methods, potentially improving posterior ankle arthroscopy safety and aiding in the understanding of pathologies such as posterior tibial tendon dislocation and tarsal tunnel syndrome.
1
Introduction
The retromalleolar canal is a tubular structure formed by the medial malleolus and the flexor retinaculum. There are important tendons, blood vessels and nerves in the canal, including tendon and sheath of posterior tibial muscle, tendon and sheath of flexor digitorum longus, posterior tibial artery and vein and tibial nerve, tendon of flexor hallucis longus and tendon sheath . A correct understanding of the retromalleolar canal’s structure plays a crucial role in diagnosing foot diseases, planning the posterior medial surgical approach, and exploring the etiology, diagnosis, and treatment of tarsal tunnel syndrome.
The retromalleolar canal’s wall is composed of a hard and flexible structure. The inelastic fibre tunnel within this canal is susceptible to compression, which can result in compressive neuropathy of the tibial nerve or its branches . Tarsal tunnel syndrome (TTS) is characterised by a range of symptoms, including pain, numbness and sensory abnormalities. These symptoms can have a significant impact on the quality of life of patients , . Although its prevalence and incidence are unclear, 60–80 % of patients can determine the specific cause of TTS ,, . The aetiology of the condition may be categorised as follows: trauma, space-occupying lesions and foot deformities . The utilisation of a combination of symptoms, physical examination, colour ultrasound and MRI facilitates the diagnosis of the disease. However, in clinical settings, ankle tunnel syndrome is frequently misdiagnosed as other podiatric diseases, potentially attributable to a paucity of knowledge regarding the anatomical characteristics and morphology of the retromalleolar canal. The retromalleolar canal is diminutive in size, the tubular architecture demonstrates a heightened susceptibility to iatrogenic injury, and the anatomical structure is relatively complex. To date, there is a paucity of literature concerning the overall morphological classification of the retromalleolar canal.
The posteromedial approach carries risk due to its proximity to posterior neurovascular structures and is therefore not recommended . The anterior arthroscopic entrance is difficult to access the complete posterior part of the ankle, and the posterior medial approach is of great significance for treating the disease . In posterior ankle arthroscopy, the flexor hallucis longus (FHL) tendon serves as a critical anatomical guide. Correct portal placement is crucial for optimal arthroscopic diagnosis and treatment , . An improper portal placement risks damaging surrounding tendons and neurovascular structures, while complicating the procedure. Comprehensive knowledge of the retromalleolar canal anatomy and posterior neurovascular bundle is therefore crucial for determining the optimal incision site in both arthroscopic and open tarsal tunnel release.
The purpose of this study is to classify the retromalleolar canal based on MRI, investigate the relationship between its morphology and the bone structure of the distal tibia, and assess the clinical significance of these findings for managing TTS and guiding surgical interventions.
2
Materials and methods
2.1
Patients
A retrospective analysis was performed of imaging data collected between 2018 and 2023 from 250 patients who underwent MRI of the ankle joint at the hospital. Of the patients, 137 were men and 113 were women, aged 18–60 years. Of the image data, 132 were left-sided and 168 were right-sided, and they were in patients with an average age of 42.26 ± 12.40 years old. Because of limited availability of data, information on height and weight was not included.
The present study was approved by the Ethics Committee of the hospital in batch number BY2023027.
Inclusion criteria were:
1) age 18–60 years old; 2) no foot disease or other specific medical conditions; 3) Normally developed retromalleolar canal and distal tibiofibula; 4) no history of retromalleolar canal and distal tibiofibula injury
Exclusion criteria were:
1) developmental deformities of retromalleolar canal and distal tibiofibula; 2) fractures around the scapula; 3) infectious or chronic wasting disease.
2.2
Methods
MRI examinations were performed on a Siemens Magnetom Skyra 3 T scanner with patients positioned in a supine position. Scans using axial T2-weighted sequences covered the region from the mid-tibial shaft to the calcaneus. Three board-certified musculoskeletal radiologists independently assessed the morphology of the retromalleolar canal. In cases of classification disagreements, a senior radiologist with 15 years of experience conducted a consensus review. Final measurements represent the average of the three radiologists’ assessments.
To standardize the measurement plane and eliminate potential bias from varying sectional heights, all morphological evaluations and measurements were performed at the same axial slice positioned 4 mm above the tibiotalar joint. Parameters, including the malleolar groove opening angle (∠α), distances between key anatomical landmarks, and canal dimensions, were consistently measured at this standardized level.
The measurements were performed 4 mm above the tibiotalar joint in the coronal plane as follows ( Fig. 1 , Fig. 2 ):
4 mm above the tibiotalar joint in the coronal plane.
Method for measuring the retromalleolar canal.
∠α: medial malleolar groove opening angle：The angles between the groove’s two ends (A, B) and the deepest point (C) in the trench on a plane;
∠β 1: The angle between end A of the groove and the DE line.
∠β 2: The angle between end B of the groove and the DE line.
∠γ: The angles between the groove’s two ends (A, B) and the fibula incision site at the posterior malleolus (F).
AB: Length of medial malleolus groove: Distance between the two ends of the groove (A, B).
CP: Distance between the deepest point (C) to the AB line.
CI: retromalleolar canal width(CI): The farthest distance between the both edges of the retromalleolar canal was defined as the width
GH: retromalleolar canal length (GH): The longest distance between the upper and lower edges of the retromalleolar canal was defined as the length.
2.3
Statistical analysis
Statistical analyses were conducted using SPSS version 20.0. Data were presented as means ± standard deviations. Correlation analysis assessed the agreement among the three observers. Morphological differences between the retromalleolar canal types were evaluated using one-way ANOVA (LSD-t), with p < 0.05 considered statistically significant.
3
Results
All the data measured by 3 radiologists have obvious correlation (ICC＞0.85, P < 0.001; Table 2 ). According to the results of 250 cases, the retromalleolar canal can be categorized into four distinct types.: R type (60.4 %), V type (18 %), K type(16.8 %), and L -type (4.8 %) ( Fig. 3 a-d, Fig. 4 ). The measurement results of morphological parameters of Retromalleolar canal corresponding to each classification are shown in Table 1 . There are significant differences in the measured values of γ (P < 0.001), AB (P < 0.001) and CI(P = 0.002) between the K-and L -type. There are significant differences between the measured values of α (P < 0.001), AB(P = 0.001), CP (P < 0.001) and CI(P = 0.002) of the K-and R-type. There are significant differences in β1 (P = 0.003), γ (P < 0.001) and AB (P < 0.001) between the K- and V-type. There are significant differences in the measured values of α (P < 0.001), γ (P < 0.001), AB(P = 0.029) and CP (P < 0.001) between the L- and R-type. There is a significant difference in CI(P = 0.033) between L type-and V-type. The α (P < 0.001), β1((P = 0.027), γ (P < 0.001), AB (P < 0.001), CP (P < 0.001) GH (P = 0.034) of the R-and V-type.
Morphological classification of the retromalleolar canal: (A) R type, (B) V type, (C) K type, (D) L type.
Cross section of the Retromalleolar Canal: a. Flexor hallucis longus; b. Posterior tibial artery and vein(s), Tibial nerve; c. Flexor digitorum longus tendon; d. Tibialis posterior tendon; e. Deep peroneal nerve; f. Anterior tibial artery and vein; g. Tibialis anterior tendon; h. Extensor hallucis longus muscle; i. Extensor digitorum longus tendon; j. Peroneus longus tendon; k. Peroneus brevis tendon; l. Peroneal artery and vein.
Table 1
Anatomical morphological measurements of Retromalleolar canal and distal tibia (x̄±S).
| Retromalleolar canal type | ||||
|---|---|---|---|---|
| Distribution | “K”type | “L”type | “R”type | “V”type |
| 42(16.8 %) | 12 (4.8 %) | 151(60.4 %) | 45(18 %) | |
| α | 173.58 ± 6.03 | 175.74 ± 4.20 | 143.75 ± 9.62 | 172.74 ± 6.22 |
| β1 | 12.96 ± 3.63 | 14.53 ± 4.41 | 15.30 ± 11.95 | 18.95 ± 2.76 |
| β2 | 30.71 ± 4.78 | 27.70 ± 5.07 | 30.88 ± 12.57 | 30.76 ± 3.78 |
| γ | 158.10 ± 7.36 , | 175.19 ± 8.56 | 159.93 ± 15.16 | 174.64 ± 6.35 |
| AB(cm) | 13.91 ± 2.48 | 10.51 ± 3.85 | 12.33 ± 3.00 | 10.36 ± 1.64 |
| CP(cm) | 0.56 ± 0.75 | 0.48 ± 0.64 | 2.22 ± 0.66 | 0.44 ± 0.58 |
| GH(cm) | 37.21 ± 6.99 | 38.58 ± 2.38 | 35.40 ± 5.68 | 37.46 ± 5.06 |
| CI(cm) | 27.75 ± 7.08 a | 21.68 ± 4.20 | 24.45 ± 5.94 | 24.88 ± 5.70 |
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