A 17-year-old man presented with painless hemihypertrophy of the right foot and calf and was treated at our institution between 10.2017 and 12.2019. The asymmetry of his feet, which has never been progressive, was first noticed at the time of his birth (Figure 1). Since then, the patient has been forced to wear a shoe on his right foot that is two sizes larger (European shoe size) than on his left foot. Moreover, an initial leg length discrepancy of 1 centimeter at the time of this birth increased to 1.5 centimeter. Therefore, he was provided with heightening orthotic insoles.
During maturity, his only physical complaints had been muscle cramps on the lateral aspect of his foot, which occasionally occurred after extensive activities. Apart from that, he experienced no difficulty in walking, running, or even playing football. However, since the age of 15, the patient suffered from typical hallux valgus deformity symptoms: pain over the medial eminence and local skin irritation.
Physical examination revealed a painless, soft mass on the medioplantar, plantar, and lateroplantar surface of the right foot. This soft tissue mass completely obliterated the plantar arch of the foot. Range of motion in the right upper limb's joints did not differ from the left side. Motor function, vascularization, and sensibility were normal. The patient exhibited clinical presentation of a pes transversoplanus in combination with a hallux valgus deformity, which showed a slight hallux valgus interphalangeus component. We also observed a claw deformity of the second toe and a digitus quintus with laterally abducted phalanges; however, neither were causing any clinical symptoms.
Radiographs in two planes (posteroanterior and lateral) confirmed the clinical presentation (Figure 2). An intermetatarsal angle of 18.6  and a hallux valgus angle of 33.6  were measured. MRI imaging showed a constant hypertrophy of the short foot musculature with muscular-isointense nodular structures in the plantar subcutaneous fat tissue. Moreover, an accessory musculus soleus, with muscular portions ventral to the Achilles tendon, was shown to reach up to the calcaneus (Figure 3).
Routine laboratory testing revealed elevated blood creatine kinase levels: 306 units/liter (reference range: <190 units/liter).
The procedure was performed with the patient supine and under general anesthesia using an Esmarch's bandage for surgical hemostasis. First, a crescentic skin incision, approximately 10-centimeter-long, was made on the medial side of the right foot. After dissecting the abductor hallucis brevis muscle, the plantar hypertrophic portion of this muscle was resected with a cautery knife. The plantar portion of this muscle was insufficient, partly due to strong connective tissue fibers toward the subcutaneous fat tissue. Below this muscle, the neurovascular bundle appeared flattened in a delta-shaped manner in the intermuscular fascia (Figure 4). Next, the hypertrophic flexor digitorum brevis muscle was resected subtotally. Thus, the remaining abductor hallucis muscle lost physiological tension, which we addressed by using gathering stitches on its tendon. The next step included a lateral, straight, and approximately 6-centimeter-long approach to the hypertrophic abductor digiti minimi muscle, which was also freed and decreased in volume (Figure 5). The surgery then proceeded to correcting the hallux valgus deformity. After the soft tissue of the hallux metatarsophalangeal joint was released, plication of the medial part of the capsule and proximal osteotomy of the first metatarsal was performed. For osteosynthesis, we chose an ankle-stable plate system and a temporary K-wire fixation (Figure 6). Following hemostasis, a drain was placed both in the medial and lateral wound cavity, and dermal closure was performed with skin staples (Figure 7).
Histological analysis of the resected tissue revealed skeletal muscle fiber hypertrophy as well as focal skeletal muscle atrophy, microfocal skeletal muscle necrosis, and local, chronic inflammation with interstitial fibrosis. Postoperatively, the foot was immobilized with a below-knee cast for six weeks, after which the K-wire was removed (Figure 8). Following, he was provided with a removable cast and we recommended partial stress for three weeks. After this period, he received orthotic insoles with medial arch support and a dynamic ankle-foot orthosis for night use and could move on to full stress. Finally, this led to resolution of his symptoms as well as clinical and radiological correction of his orthopedic toe deformities. Moreover, the blood creatine kinase levels declined to a normal range.