Here, we have reported a rare type of complex polydactyly that is postaxial polydactyly between the 4th and 5th metatarsals with an abnormally large forefoot and its treatment method, called modified on-top plasty technique. Many scholars have attempted to apply the existing polydactyly classifications to guide the surgical treatment of polydactyly. However, there is no standard classification[1, 2, 4–7]. Based on the metatarsal morphology, Venn-Watson et al. proposed the classification of metatarsal polydactyly into complete duplication, Y metatarsal type, T metatarsal type, enlarged metatarsal type, and soft tissue vegetative type. However, the polydactyly involved in this study could not be classified with this system. Better classifications systems is required.
Uda et al. have discussed in detail the principles and methods of excision procedures for polydactyly. According to their recommendations, decisions about excision should be made based on the appearance of the extra digits. That is, if the appearance of the medial toe is satisfactory, the lateral toe should be removed, and if the outer toe has a satisfactory appearance, the inner toe should be resected. If both toes are similar in terms of appearance, the lateral toe should be removed. With regard to metatarsal polydactyly, the toe column to be removed is mainly assessed based on the imaging findings: if the medial toe column is well developed, the lateral column should be removed, and if the lateral column is well developed, the medial column should be removed. For this type of polydactyly in our study, there was no well-developed bone in the medial row (metatarsal dysplasia), but the appearance of the toe was good and the metatarsal joint had a normal axis. Further, the lateral column had a stable metatarsal tarsus and the proximal part of the metatarsal bone was well developed, but the metatarsophalangeal joint was malformed. According to our previous experience, neither medial nor lateral column resection could have achieved good results. Instead, we opted for the modified on-top plasty technique, with which we were able to achieve good results.
The on-top plasty technique is widely used in the treatment of repeated bunions and has achieved very good results[8, 9]. Based on the promising results, several scholars tried to apply this technique to the treatment of polydactyly. Usami et al. were the first to report the use of the on-top plasty technique to correct short toe deformity in four patients. However, none of their patients had an unusually wide forefoot, and the author performed osteotomy and transposition at the phalangeal level. Their final results showed that the length of the reconstructed toe was increased to different degrees and its appearance was improved. In another study, Han et al. used metatarsal transfer to treat a series of rare types of polydactyly and achieved good results. Since the lateral column malformation in the polydactyly involved in the study occurred at the metatarsal neck level, osteotomy and metastasis at the phalangeal level could not solve the wide anterior foot malformation. Therefore, we used a different approach in which the medial and distal part of the medial row was preserved (which improved the appearance of the good toe and the normal axis of the metatarsophalangeal joint), and the proximal end of the lateral row metatarsophalangeal was “spliced” to form a perfect toe row. With this modified technique, the complete osteoarticular structure and weight-bearing structure of foot were well reconstructed.
Many scholars are concerned that surgery of the fifth metatarsal will affect weight-bearing activities of the foot. The fifth metatarsal bone is an important part of the transverse arch and plays a key role in walking function. In our study, the malformation occurred at the metatarsal neck, so the malformed metatarsal-toe joint in the lateral column needed to be removed in order to correct the forefoot width and relieve pain. During the operation, we not only removed the malformed metatarsal-toe joint in the lateral column, but also reconstructed the distal part of the medial column to ensure that the appearance and bone structure were good. Specifically, the medial column and lateral column were used to reconstruct a complete toe column, and the results were good. For the metatarsal neck deformity, we used metatarsal neck osteotomy and transfer to correct the appearance and reconstruct the transverse arch of the foot so that it was close to the normal physiological state (as shown in Fig. 4).
In the present type of polydactyly, simple excision of the digits did not yield good results. Therefore, we selected the beneficial parts of the medial and lateral columns for combined reconstruction. We performed the first metatarsal osteotomy and transfer at the neck level using the modified on-top plasty technique to improve appearance and function. In addition, the complete osteoarticular structure and weight-bearing structure of foot were well reconstructed. The short-term results in our study were excellent, but long-term follow-up is needed to identify complications that might be missed.