Background
Further specialization in orthopaedic and podiatric surgery has, amongst others, led to an increased attention for anatomical details, including the interest in the pathophysiology and mechanism of pain beneath the metatarsal heads (metatarsalgia) [
1‐
3]. Metatarsalgia symptoms such as gradual onset of forefoot pain, edema and a positive drawer sign can be explained by instability of the metatarsophalangeal (MTP) joint [
4,
5], MTP joint instability is described as a dorsal subluxation or dislocation of the base of the proximal phalanx over the metatarsal head. The traditional etiology of instability of the lesser MTP joints in the sagittal and/or transverse plane is described in the literature by plantar plate degeneration and rupture [
6,
7]. Alternative causes described in literature include attenuation of the collateral ligaments and the deep transverse metatarsal ligament, and capsular degeneration [
8,
9]. The first choice in treating instability of the MTP joints is conservative treatment and is accomplished with shoe wear modifications, metatarsal pads, and custom-made orthoses [
5]. Operative treatment may consist of an indirect reconstruction of the MTP joint in which the toe is realigned without reconstruction of the plantar plate [
6,
10]. Lately, studies were published in which a direct repair of the plantar plate is reported [
6,
11‐
13]. To be able to determine the best treatment of metatarsalgia it is paramount to know the anatomy (normal and pathologic) of the MTP joints. In recent publications the anatomy of the plantar plate is described. However, only a few authors addressed the relationship between plantar plate integrity and stability. Furthermore, no systematic review on the anatomy and mechanics of the plantar plate has been published. The biomechanics of the first MTP joint are different from the lesser toes. This difference is caused by anatomical differences e.g. the sesamoids, the abductor and adductor muscles, the position in the foot and a larger metatarsal head. Therefore, only articles concerning the lesser MTP joints were included.
The present systematic review has two objectives. Firstly, it assesses and elucidates the published literature regarding the anatomy of the plantar plate of the lesser toes. Secondly, it reviews the literature about the relationship between the integrity of the lesser plantar plates and MTP joint stability.
Conclusion
Recently the role of repair of lesions of the plantar plate in relation to MTP instability and metatarsalgia is subject of increased interest. When trying to find solutions for clinical problems, knowing the anatomy and its function is of uttermost importance. The subject of this systematic review is the anatomy of the plantar plate of the lesser toes and the relation between the integrity of the plantar plates of the lesser toes and MTP joint stability.
The plantar plate is a firm and flexible disc with a form, varying from rectangular to trapezoidal. The thickness ranges from 2 to 5 mm, the length from 16 to 23 mm, and the width 8 to 13 mm. Its plantar surface is smooth, and is grooved at its outer borders to provide a gliding plane for the flexor tendons. From the (histo-)anatomic information, and in according to Pauwel’s theory of ‘causal histogenesis’ that collagen fibrils are always oriented in the direction of the greatest tension, one can draw three conclusions [
14].
Firstly, the plantar insertional fibers withstand mostly
tensile forces, by providing support to the windlass mechanism with the insertion of the plantar fascia [
8,
14]. In addition, the dorsal-to-mid insertional fibers experience also tensile forces [
14]. Secondly, it withstands
compressive loads acting as a cushion through its fibrocartilage structure.
Thirdly, it assists in MTP joint
stability through its central location and attachments to many surrounding structures. Moreover, plantar plate injury or combined injuries to the plate, with the extensor hood and the collateral ligaments have shown to cause significant instability, just as the plantar plate has proven to be the most important isolated sagittal stabilizer of the MTP joint [
5,
18]. With plantar plate injury, support is lost, contributing or leading to metatarsalgia.
In the context of current evidence, this is the first systematic review regarding plantar plate anatomy related to MTP joint stability of the lesser toes. To date the direct relation between plantar plate lesions and MTP joint instability is still controversial. It stands to reason that once a soft tissue structure, i.e. the collateral ligament or plantar plate undergoes attritional and adaptive changes due to chronic injury, it will in turn lead to deformation and attenuation, resulting in MTP joint instability and/or metatarsalgia [
8,
14,
19]. The adaptive changes can be due to various causes, e.g. chronic hyperextension or chronic synovitis, eventually leading to loss of soft tissue balance [
20].
In our opinion, at least, it is challenging to prove MTP joint instability to be caused directly by plantar plate injury, as MTP joint instability is not pathognomonic for plantar plate rupture.
This review showed that only Suero et al., Bhatia et al. and Chalayon et al. have demonstrated, in an in vitro anatomical study, that plantar plate injury in itself can cause MTP joint instability, as isolated or combined sectioning of the plantar plate showed significant instability [
5,
16,
18]. Caution is advised when using this knowledge in cases with
chronic instability, as a model was created simulating
acute instability of the lesser toes, which cannot account for biological healing as may be seen in chronic situations [
18]. Although much is published regarding plantar plate repair techniques, scarce primary data regarding details of normal plantar plate anatomy of the lesser toes is available in cases of metatarsalgia or instability. Remarkably, only two studies have described the lesser plantar plate dimensions, of which one reference of Deland et al. (1995) was referred to as ‘being checked’ [
8,
15]. Furthermore, only a few studies have addressed the relation of plantar plate injury and normal MTP joint mechanics and stability of the lesser toes [
5,
6,
16,
18].
We attempted to conduct a quality assessment of the included studies, however, it was impracticable due to lack of pre-tested data forms or quality criteria for reviewing studies performed on anatomical specimen. Therefore, we could not perform a standardized critical appraisal. To improve future transparency and quality assessment, we recommend a guideline or quality criteria for reporting (biomechanical) anatomical studies.
We report no geographical or temporal constraints, and strived to minimize our source and publication bias. Only articles in English and published in databased literature were included, creating a scope bias. In addition to the existing literature, we recommend a study re-examining the anatomical dimensions in normal and pathological MTP-joints of the lesser toes, the incidence of plantar plate injury and the relationship between instability, plantar plate injury and metatarsalgia. Furthermore, we are interested in the relationship between a functional instable first ray and plantar plate injury. This can be a topic of further research.
This study shows the lack of primary data regarding plantar plate anatomy of the lesser toes and MTP joint stability. Nevertheless, we endeavoured to elucidate and clarify the importance of plantar plate anatomy and integrity, to provide the necessary building blocks for clinical practice and future research.