Background
The extensor hallucis longus (EHL) is a thin muscle situated deep between the tibialis anterior muscle (TAM) and the extensor digitorum longus (EDL). The EHL arises from the middle half of the fibula and from the interosseous membrane, medial to the origin of the EDL. The muscle belly becomes a long tendon, passes behind the superior and inferior extensor retinaculum, crosses the anterior tibial artery and vein from the lateral to the medial side near the ankle, and finally inserts on the dorsal aspect of the base of the distal phalanx of the big toe [
1]. The function of the EHL is to extend the big toe, dorsiflex the foot, adjunct foot eversion and inversion and stretch the plantar aponeurosis [
1,
2].
The EHL is characterized by morphological variability with regard to the number of its additional bands and their insertion [
1,
3‐
7]. Most previous research has focused on its possible morphological variations, particularly those regarding the additional bands. One classification has been proposed for these variations [
3]; however, it requires systematization and upgrading to account for the identification of new band types.
The aim of our study is to systematize the classification of EHL insertion and course of its tendons.
Discussion
The most important asset of this study is that it presents a new systematic classification of the EHL tendon based on anatomical dissectionSuch systematisation of existing classifications is needed to allow effective planning of surgical procedures.
A considerable degree of morphological variation can be attributed to phylogenetic development. Certain parts of the body, such as the palmaris longus or plantaris, can become reduced in response to evolutionary pressures [
13‐
17], while others, such as the fibularis tetrius, can become more developed [
4]. Hence, the presence of a high degree of variation in origin or insertion for a particular muscle may suggest that it has not yet completed its evolutionary development [
4]; such a situation may observed for the EHL tendon, which was found to display a wide range of courses and insertion types, as well as additional bands [
3‐
5,
18,
19]. However, the occurrence of such additional bands varies considerably from 80% [
19], 81.25% (26 ft) [
20] and 78.7% (34 ft) [
18] to 35% (21 ft) [
3], 26.5% (26 ft) [
5] and 10% (six feet) [
4]. In the present study, additional bands were found in 42.3% of cases (44 ft), this being the total number of types II and III.
More consistent results have been achieved regarding the course of the additional bands, the most frequently-observed variant being the medial course in relation to the main tendon, ranging from 93.4 to 100% of all cases [
3,
5,
21]. This is confirmed in the present study, where the medial course of additional bands occurred in 98.1% (102 ft).
The first accurate classification was based on a study of 60 lower limbs and comprised three types of patterns (I-III) with subtypes [
3]. We propose the addition of two further types/subtypes: Pattern II should be supplemented with Type II c identified in the present study, characterized by the auxiliary tendon inserting into the dorsal aspect of the base of the first metatarsal (six feet, 5.7%); in addition, Pattern III should be supplemented by our present Type III, characterized by a tendon split into three bands: a main tendon inserting into the dorsal aspect of the distal part of the distal phalanx, a medial auxiliary band inserting to the distal phalanx but more proximally, as well as a lateral auxiliary band (the stronger of the two auxiliary bands) fusing with the extensor halucis brevis and attaching to the proximal phalanx (two feet, 1.9%). A comparison of the Al-Saggaf classification with ours is presented in Table
2.
Table 2
Comparison results of the insertion of the extensor hallucis longus
Pattern I – single tendon, inserts on the dorsal aspect | 39 (65%) | 60 (57.5%) |
Pattern II |
AT inserted into the dorsal aspect of the base of the proximal phalanx, just distal to the insertion of EHB. | 9 (15%) | 5 (4.8%) |
AT joined to the termination part of the EHB and inserting into the dorsal aspect of the base of the proximal phalanx of the big toe. | 3 (5%) | 0 |
AT inserted into the dorsal aspect of the proximal phalanx of the big toe, medial to the insertion of the EHB. | 2 (3.33%) | 31 (29.9%) |
AT joining the middle of EHB and forming a common tendon, and inserting into the dorsal aspect of the base of the proximal phalanx of the big toe. | 2 (3.33%) | 0 |
AT inserting into the dorsal aspect of the base of the first metatarsal. | 0 | 6 (5.7%) |
Pattern III |
Two ATs arising from the medial side of the main tendon and inserting into the capsule of the joint | 3 (5%) | 0 |
Two ATs arising from the medial and lateral sides of the main tendon and inserting into the capsule of the first metatarso-phalangeal joint | 2 (3.33%) | 0 |
Medial AT also inserts to the distal phalanx but more proximally, and lateral AT (the stronger of the two auxiliary bands) fuses with the extensor halucis brevis and attaches to the proximal phalanx | 0 | 2 (1.9%) |
Ankle extensor compartment trauma is commonly overlooked yet requires expeditious diagnosis and treatment in order to preserve function and provide positive outcomes. All components of the ankle extensor compartment are susceptible to injury, but the tendons are most frequently injured [
22]. Of the different types of EHL lesions, one of the most typical is closed tendon ruptures, which are caused by active tendon contraction against resistance [
23,
24]. Bone spurs or tight-fitting, high-laced boots which reduce one’s mechanical mobility can be predisposing factors; injuries resulting from sports-related activities such as ultramarathon running [
25] and iatrogenic injuries after arthroscopic thermal ablation have also been described [
26]. EHL tendon lacerations that have ocurred distal to the extensor expansion compartment may not significantly reduce function or tendon retraction and can thus be treated conservatively. Conversely, lacerations that are more proximal result in significant tendon retraction and reduced or lost function and require surgical intervention. The introduction of a new system of classification seems necessary because it is possible to speculate that additional bands prevent tendon laceration and retain normal muscle function at the site of the main tendon rupture.
Our study has some limitations. Its main weakness is that the sonographic and anatomic assessment methods were not applied on the same samples. Moreover, considering the heterogeneous nature of the anatomical area, the type of insertion or presence of accessory bands are the morphological details that our proposed classification is mostly focused on. Being within the limits of ability and realization of Ultrasound, MRI or biomechanics tests, these are the ones that should be used in further studies to develop this purpose. Offering a uniform system of classification and terminology we are willing to enlighten of “what and where” to look for in case of various injuries. Hopefully surgeons could also find this research useful for unified communication. Understanding these variants and implementation of the proposed classification system may be part of everyday specialists practice and could modify current surgical techniques. In addition, this work might serve as a segue into studies on diagnostic imaging to determine EHL morphological variations. Acceleration of future biomechanical and physiotherapeutic research could be possible due to our findings on EHL insertion types.
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