A Rare Variation of Plantar Interossei Muscles of the Foot – a Case Report

Abstract

The variations in the attachment of plantar interossei might affect the gait of the individual and also raise questions about the function of this muscle at the tarsometatarsal joints. During routine dissection of the right lower limb, we encountered a unique variation in the attachment of plantar interossei. The first plantar interossei took origin from the lateral side of the base of the first metatarsal and inserted into the lateral side of the base of the proximal phalanx of the great toe. The second and third plantar interossei took origin from the medial surface of the base of the second and third metatarsals, respectively, and inserted into the medial side of the base of the proximal phalanges of the corresponding digits. The third and fourth intermetatarsal spaces were void, indicating the absence of these muscles from those spaces. The clinical significance of this variation has been discussed.

Keywords:plantar interossei, anomalous attachment, hallux valgus, bunion, variation.

INTRODUCTION

Two-legged walking has forced us to examine the anatomical features of the foot, particularly the muscles that support and propagate the foot. Although the foot has a static skeletal structure, its ligament and muscular system transmit power to make it dynamic. The intrinsic sole muscle variations are uncommon and they are often seen during surgery or cadaveric dissections (1). These variations are usually pertained to anomalous origin, insertion, absence of a tendon or muscle or presence of supernumerary tendons and muscle. The muscles in the sole of the foot are arranged in four layers. The abductor hallucis, abductor digiti minimi and flexor digitorum brevis are located in the first layer, the four lumbrical muscles and flexor digitorum accessories (quadratus plantae) in the second layer, the adductor hallucis, flexor hallucis brevis and flexor digiti minimi brevis in the third layer, and the dorsal and plantar interossei make up the fourth layer (2). They act both collectively, to stabilise the arches of the foot, and individually, to govern the movement of the digits (2).

Plantar interossei are a group of three small intrinsic, unipennate muscles in the deepest layer of the sole. They normally arise from the medial surface of the third, fourth and fifth metatarsals and also from the soft tissue like the peroneal sheath and long plantar ligament (2). The plantar interossei are innervated by the lateral plantar nerve. They function as adductors of the third, fourth and fifth toes and are responsible for the flexion at the metatarsophalangeal joints and extension at the interphalangeal joints of the lateral three digits. These actions are crucial when the push-off phase of running or walking begins, because they keep the third to fifth toes in place and stop them from spreading apart once the body weight is absorbed into the forefoot (3). The literature mainly describes variations with regard to the origin of these muscles from the metatarsals and only few published articles report various soft tissue origins. These variations in the attachment of the plantar interossei might affect the gait of the individual and also raise questions about the function of this muscle at the tarsometatarsal joints (4). In the present study we document a case of novel anomalous attachment of plantar interossei and the possible clinical consequence of this abnormal attachment.

MATERIALS AND METHODS

After approval from the ethical committee [AIIMS/BBN/IEC/Nov/2022/131], the present study was conducted on cadavers that are utilized for undergraduate medical teaching. The origins of three plantar interossei muscles are studied by a detailed dissection of 20 limbs belonging to 10 cadavers, including six males and four females, with an approximate age of 60-70 years. The sole of the foot was approached; skin and fasciae were carefully reflected till the muscular layers were found. The muscular layers were dissected layer by layer until the last layer of the central compartment was found and plantar interossei along with the tendon of peroneus longus was exposed. Then, the plantar interossei were carefully dissected to identify the origin and insertion of the muscle along with the nerve supply (4).

CASE REPORT

During routine dissection of the lower limb for undergraduate teaching, we encountered a unique variation of plantar interossei muscle. In the right lower limb of a female cadaver aged approximately 64-65 years, the first plantar interossei took origin from the lateral aspect of the base of the first metatarsal and inserted into the lateral aspect of the base of the proximal phalanx of the great toe. The second and third plantar interossei took origin from the medial surface of the base of the second and third metatarsals, respectively, and inserted into the medial side of the base of the proximal phalanges of the corresponding digits (Figure 1). The third and fourth intermetatarsal spaces were void, indicating the absence of these muscles from those spaces. The attachments of dorsal interossei were normal. In addition to the above variation, the great toe of the right lower limb presented hallux valgus deformity (Figure 2). The attachments of plantar and dorsal interossei on the left lower limb were normal.

DISCUSSION

It is crucial to assess the intrinsic muscles of the foot in cases of diseases of the lower limbs because they play a significant role in foot function. The commonly reported intrinsic foot muscle variations in the literature are commonly associated with lumbricals, dorsal interossei, flexor digitorum brevis and extensor digitorum brevis and presence of accessory muscles (5-7). Here we encountered a unique case of abnormal attachment of the plantar interossei. The first plantar interossei took origin from the lateral side of base of first metatarsal and inserted to the lateral side of the base of the proximal phalanx of the great toe. The second and third plantar interossei took origin from the medial surface of the base of second and third metatarsals, respectively, and inserted into the medial side of the base of the proximal phalanges of the corresponding digits. There was no plantar interossei for the fourth and fifth metatarsals and the digits. Kalin et al documented extensive soft tissue origin of the plantar interossei in addition to their attachment to the base and shaft of the metatarsals, but in all observed specimens the attachments were to the third, fourth and fifth metatarsals (4). Similarly, other studies have also documented the soft tissue origin of plantar interossei but their attachments were confined to the third, fourth and fifth metatarsals (8, 9).

We were unable to find any previous reference pertaining to the present abnormal positioning of the plantar interossei on the first metatarsal and its consequent action on the metatarsophalangeal joint of the great toe. In order to explore the possible clinical significance of this abnormal attachment of the plantar interossei, when we carefully evaluated the other visible deformities of the cadaveric foot, the arches of the foot were normal; however, a prominent hallux valgus deformity was noted. The left foot was completely normal and there were no deformities. A bunion, often referred to as hallux valgus (HV), is among the most prevalent forefoot abnormalities. It shows up as a medial deviation of the first metatarsal head and a lateral deviation of the proximal phalanx due to the adduction of first metatarsophalageal joint (10). It has been related to functional impairment, such as foot discomfort, altered walking patterns, poor balance, and falls in elderly adults, in addition to being a significant contributor to the expenditures associated with forefoot surgery (11). The etiology of HV is multifactorial. Anatomical variations that increase the risk of this condition include flat foot, an asymmetric first MTP joint, a lengthy first metatarsal bone, dorsiflexed first metatarsals, inflexible or flexible forefoot varus, pes planovalgus (rigid or flexible), gastrocnemius equinus, poor foot mechanics, and hypermobility of joints (12). It is worth noting that several types of arthritis, including psoriatic arthritis, gouty arthritis, and rheumatoid arthritis, predispose people to HV deformity (10). Based on our present observation, we postulate that the anomalous plantar interossei attached to the first metatarsal and proximal phalanx of the great toe, causing adduction at the first metatarsophalangeal joint, could have contributed to the HV in this patient. Since the treatment of HV depends upon addressing the underlying etiology, it is necessary to have an understanding of such rarely encountered anatomical variation. In standing and walking, the foot represents the only portion of the body in touch with the ground, and it serves a number of purposes. The primary movable component of the foot, the toes, are crucial for stability when standing and FIGURE 2. Right great toe showing hallux valgus deformity moving. Otake et al have demonstrated that stability in the static standing position is regulated by the ability to flex the toes with extended interphalangeal joints, which necessitates the integrity of plantar interossei and lumbricals (13).

Cihak et al provided a thorough description of the interossei development. He postulated that the flexores breves profundi, a presumed muscle layer, was the source of the whole development of the plantar interossei. The flexores breves profundi layer slightly protrudes into the intermetatarsal gaps and is situated between the deep division of the lateral plantar nerve and the metatarsal bones. This layer, which produces the different interossei, is a continuous sheet that divides into discrete bundles on its dorsal side earlier and wholly. Any deviation from this pattern could result in misplacement of the muscle (14).

CONCLUSIONS

Anatomical variations of foot muscles are crucial, owing to the possibility of their consequent pathological presentation. A thorough understanding of such variations is vital to the examination of foot mechanics, biomechanical modelling of the foot and prosthetic design by orthopaedic surgeons, radiologists and podiatrists.

Conflict of interests: none declared.

Financial support: none declared.

Acknowledgements: The donors whose bodies were given to science so that anatomical study could be done are gratefully acknowledged by the writers. The outcomes of such study may contribute to a greater understanding of humankind and, ultimately, better patient care. Therefore, we are incredibly grateful to these contributors and their families.

Authors’ contributions: MC – conceptualization; MC, HS, VL, NG and RM – methodology; MC, AK, HS, VL, NG and RM – data collection; MC and AK – analysis and interpretation of results; AK and MC – original draft writing; AK and MC – manuscript review and editing; MC and AK – critical revision of the manuscript. All authors approved the final version of the manuscript.

FIGURE 1.

Attachments of plantar interossei in the sole of the right foot

FIGURE 2.

Right great toe showing hallux valgus deformity