Os intermetatarseum presenting as splay foot; Case report and literature review

Abstract

Introduction and importance

This report described the management of a lady with a rare finding of a large size of Os intermetatarseum in a bizarre location. This unique condition resulted in a splay foot which was rarely discussed in the literature.

Case presentation

A lady in her early fifth decade has complained of foot swelling and difficulty wearing shoes for the last two years. Her main concern was the worry of a malignant condition.

Clinical discussion

She had an unusually huge articulated lump in the third web space. Also, it showed a central foot splay. A full battery of radiological investigation gave a narrow list of possible differential diagnoses. The final diagnosis showed that it was Os intermetatarseum. Surgical treatment involved enucleating the mass and correcting foot splay using a mini-tight rope®. The histopathology report confirmed the diagnosis of Os intermetatarseum. The central forefoot splay was treated by a known surgical tool differently. Post-operatively she was put on physical therapy. Five years follow-up showed improved foot anatomy and functional outcomes with no recurrence.

Conclusion

Awareness of this rare condition as a differential diagnosis. Complete excisional biopsy of the lump will be a valid option for this condition, as well as using a mini-tight rope® in dealing with central foot splay.

1. Introduction

Accessory ossicles are a standard variant commonly seen on foot. They are usually discovered incidentally by diagnostic radiology. Gruber et al. were the first to describe os intermetatarseum in 1877 [1]. Its incidence ranges from 1.2 % to 14 % [2], [3], and prevalence is 1–7 % [4]. It is commonly derived from unfused primary and secondary ossification centres. This is why it looks like normal bone histopathologically. The first web space is a common site [2] and is usually small, ranging from 1.4 to 9.7 mm [3]. Unilateralism is the norm, although it can occur bilaterally [5]. The condition is inherited [5] and has a familial preponderance [6], [7]. Clinically, it may confuse with a loose body and fracture.

We present an unusual case of massive articulating os intermetatarseum who came to our military hospital with a splay foot. This case demonstrated clearly how a rare foot lump could present and how a mini-tight rope® could safely treat a challenging central foot splay.

This case was reported following SCARE 2020 updated guidelines [8].

2. Presentation of case

A lady in her early fifth decade came to the outpatient elective clinic complaining mainly of foot swelling and difficulty wearing her left shoe. She used to change her boots frequently to find wide-toes-box shoes. Moreover, she experienced pain in the dorsum of her foot due to dorsal skin irritation. She denied any history of trauma and did not have any medical problems. No similar family history was reported, and no significant drug history.

On physical examination, a static check showed; forefoot widening or splay with valgus deformity of her third toe. She had an ecchymosis patch and swelling over her dorsum foot. Otherwise, normal findings on inspection (Fig. 1). Palpation revealed a dorsal, globular 1.5 cm × 0.5 cm, firm, non-tendered lump in the third web space, which was attached to the bone and not connected to the skin. The dynamic examination phase revealed regular passive and active movements of the Ankle, subtalar and midtarsal joints. She showed a normal gait cycle. No other abnormality was detected, and the ankle was stable. The general examination was unremarkable. The patient noticed her shoe trouble one year before she was presented to our Military hospital. It took her about three months to finish her radiological investigation.

Fig. 1.

Clinical photo of left foot lump. It shows an ecchymosis patch on the dorsal skin. Fourth toe varus deformity.

Serological and immunological blood texts excluded any possibility of infection or inflammatory process.

Standing X-rays were performed, which showed a well-defined bony mass arising from the base of the third metatarsal bone extending to the adjacent fourth metatarsal shaft. There was also subluxation of the fourth metatarsophalangeal joint. X-ray was reported as a possible bony lesion at the third web space for clinical correlation (Fig. 2). Picture archiving and communication system (PACS) version 11.3.2.0224 was used to measure foot splay and dimensions of the lump (Fig. 3), which prove the same. Magnetic resonance imaging (MRI) excluded any chance of inflammatory reaction or trauma and revealed synovial pseudo-articulation between the lump and the shaft of the fourth metatarsal.

Fig. 2.

X-rays of left foot AP and oblique views. The lump in the third web space. There is subluxation and varus deformity at the 4th metatarsophalangeal joint.

Fig. 3.

X-ray of AP view left foot. It shows splay with an angle measured by picture archiving and communication system (PACS).

A differential diagnosis of Osteochondral exostosis, Myositis Ossificans, Parosteal Osteosarcoma, and Os intermetatarseum was put (Fig. 4). Since Osteochondral exostosis lesion is always having peculiar radiology and occurred in young age groups it was excluded first. Myositis Ossificans have a different history, presentation and radiology pictures. Finally, a possibility of Parosteal Osteosarcoma was considered, although an uncommon site, versus Os intermetatarseum. After all these investigations, a professional diagnosis of os intermetatarseum was reached.

Fig. 4.

MRI scan cuts of the left foot. It was multi-sequential and planar including T1, T2 proton density with fat sat.(PDFS) and gradient eccho(T2*) weighted images. It revealed lump dimensions, origin, pseudo- articulation, and dorsal protrusion.

A lengthy discussion and counselling were done with the patient about surgical treatment as she exhausted all chances of conservative management. We agreed to go for an excisional biopsy for a concrete histopathological diagnosis. Routine pre-operative antibiotics and deep vein thrombosis prophylaxis were applied. The senior surgeon did the operation under spinal anaesthesia. She was put in a supine position; a tourniquet was applied around her thigh. A direct third web approach was used and dissected layer by layer until the mass was found. Narrow osteotomes were used to enucleate the lump from the base of the third metatarsal, shy of the cortex. The denuded row bleeding base was covered by bony wax. The other end, articulated with the fourth metatarsal, was removed easily due to the cartilaginous cap and surrounded synovial fluid. The mass was 3.0 cm × 1.5 cm × 1.0 cm, clover-like with a bulbous head and broad neck. After removing the lump and the resulting divergence of the fourth metatarsal, ample dead space was revealed. A clamp was used to close the gap, and mini-TightRope® (Arthrex Inc., Naples, FL) was used to maintain the new position (Fig. 5). Then, after a layered closure, a compressive dressing was applied. The sample was sent, in formalin, to the histopathology department for tissue diagnosis. Her histopathology report came as normal bony tissue, no atypical cells with a cartilaginous cap.

Fig. 5.

Intra-operative clinical photos. 1: proximal part of 3rd metatarsal. 2: proximal part of 4th metatarsal. A) The cartilaginous cap at the head (arrow). B) The intermetatarsal gap was compressed by a clamp. C) The intermetatarsal compression was maintained by a Mini-tight rope.

On discharge home, she was advised on a non-weight-bearing regime for four weeks and assisted weight-bearing for the next six weeks (Fig. 6).

Fig. 6.

Postoperative X-rays show excision of the lump. Mini-tight rope at the base of 3rd web space.

She was put on an extended outpatient clinic follow-up for five years to check for recurrence or complications. The same team assessed the patient clinically and radiologically during the follow-up to check for unexpected events. The patient had an excellent post-operative course, and she was satisfied. Pre-operative AOFAS Lessor Metatarsophalangeal-Interphalangeal scale (35/100) was improved to 88/100. The physical functioning on SF36 (Short Form) yielded a score of 100/100 six months after her surgery.

3. Discussion

Os intermetatarseum is one of the uncommon accessory bones in the foot. Most of the cases are asymptomatic and discovered as incidental findings. Radiologically these appear as a small, well-corticated ovoid, spindle, or nodular near bone or joint [4]. Scarlet et al. were the first to describe painful Os intermetatarseum [3].The cause of pain in his case was due to sports activities. On the other hand, Knackfus et al. found that peroneal nerve branch compression was the cause of pain [9]. Moreover, Reichmister et al. noticed that the cause of the pain could be idiopathic [10]. Conservative treatment is the first line: nonsteroidal anti-inflammatory medication, steroid injection, and wide-toe-box shoes [9]. Prakash noticed good pain relief and restoration of movement on shock wave therapy in two sessions one week apart [11]. There is a consensus that painful intermetataseum can be treated surgically on the failure of conservative management. Although the surgical treatment of os intermetatarseum is an excisional biopsy, little was written on splay foot pathology and its treatment. This abnormal size of the forefoot about the hindfoot is usually associated with first-ray adduction (metatarsus varus primus) [12] and fifth-ray bunionette deformity (Tailer bunion) [13]. Radiologically, this can be diagnosed if the divergence angle between the first and fifth metatarsal or the angle of the first to fourth web space is more than 24 degrees [14]. No detailed procedure was described for central splay foot and its treatment. Our patient presented with a case of a relatively large size os intermetatarseum at an uncommon location along with central splaying of the foot, which denied her from wearing regular shoes. After the lump excision, we used the Mini-TightRope® to correct the divergence angle of the central splay. Lexer was the first to describe using this suture material to fix the first and second intermetatarsal angle (IM). Recently, TightRope® has been popularised in treating hallux valgus and Lisfranc injuries [15]. This contradicts the high complication rate in the first and second ray operations [16]. These drawbacks can be explained by the peculiar biomechanics of the foot's medial rays, which weaken this suture material to withstand the gait kinetics. To our knowledge, Mini-TightRope® was not described before for treating central splay foot. The successful use of this procedure will open the door to its use as an augmentation procedure in the Bunionette foot. We feel an extensive series of Os intermetatarseum is needed to be studied to draw a consensus about its management.

The primary (take-away) lessons from our case report are:

• •Awareness of the condition makes it a differential diagnosis in bony foot lump.
• •An excisional operation can be the gold standard, especially if it is symptomatic, with less chance of recurrence.
• •Mini-Tight rope® can be used as surgical treatment for central foot splay.

Patient consent

Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Provenance and review

Not commissioned, externally peer-reviewed.

Sources of funding

This research was not funded by any organization or sponsors. None of the authors received any funds during the course of the research and publication.

Ethical approval

No ethical approval was required for the case report by Ethical Committee in our hospital.

Funding

It was a voluntary activity not covered by any front or institution.

Author contribution

Conceptualization and paper writing: Khalid Sharafeldin.

Resources and data collection: M. Eltinay.

Project administration and literature review: M. Al Gahtani.

Study concept, design and Supervision: H. Sharahili.

Guarantor

Khalid N Sharafeldin.

Registration of research studies

Not applicable.

Conflict of interest statement

None of the authors has a conflict of interest concerning this paper.