Background Multi-slice computed tomography (CT) is frequently used to assess bone morphology and bone arrangement, among other items. 3D volume rendering (3D-VR) enables three-dimensional rendering of soft tissue such as skin, tendons, and ligaments by arbitrarily changing CT values.
Objectives Painful calluses often require surgery in cases of forefoot deformation associated with rheumatoid arthritis (RA). Therefore, the relationship between calluses and the bones and joints was investigated in RA patients using 3D-VR.
Methods A total of 14 feet (2 right and 10 left) was examined in 13 patients (3 men and 10 women). The patients' mean age at the time of CT scans was 58.0 (36–71) years, and preoperative scans were taken without loading. Volume Analyzer SYNAPSE VINCENT (Fujifilm Holdings Corporation, Tokyo, Japan) was used for 3D-VR reconstructions. CT values were adjusted, and calluses were detected from rendered soft tissue. CT values were then gradually changed, and the relationships between flexor tendons and the bone and joints were examined.
Results All subjects had plantar calluses on the second and third metatarsophalangeal (MTP) joints. A total of 26 calluses was rendered: on the third MTP joint for 13 feet, on the second MTP joint for 5 feet, on the fifth MTP joint for 3 feet, on the first MTP joint for 2 feet, on the first interphalangeal (IP) joint for 2 feet, and on the fourth MTP joint for 1 foot. All were plantar calluses. Only seven subjects had a fallen horizontal arch at the metatarsal head level, and six subjects had callus formation despite no fallen arch. The path of the flexor tendon ran more to the hallux side of each bone head in 11 cases where the callus was on the third MTP joint and three cases where the callus was on the second MTP joint. In those subjects whose postoperative CT scans could be compared, the path of the flexor tendon had been normalized.
Conclusions Calluses are thought to form as a result of MTP joint arthritis, which loosens the joint capsules and ligaments, thereby leading to gradual dislocation and pushing down of the metatarsal head toward the plantar side by the proximal phalanx. While the present results were obtained without loading, some subjects had calluses despite having no fallen horizontal arch at the metatarsal head level. These findings suggest that a dynamic element is involved in callus formation. On examining calluses and the path of the flexor tendon, the flexor tendon followed a path that avoided the callus, although this was improved with surgery. 3D-VR is useful in assessing calluses and bone, joints, and tendons.
Acknowledgements The authors thank Miss Sasaki, for her assistance with this project.
Disclosure of Interest None declared