The concept of first metacarpal M1-M2 arch. New interest in trapeziometacarpal prostheses.

Analysis of the M1-M2 arch, otherwise known as the M1-M2 metacarpal hanger, objectively helps all surgeons treating basal thumb osteoarthritis to fit a trapeziometacarpal prosthesis that respects the physiological length of the thumb column and thus the physiological tensions of the APL, EPL, and EPB tendons as well as the interossei and thenar muscles. Kapandji X-ray views are a gold-standard in the radiological study of basal thumb osteoarthritis, to classify progression, measure trapezium height, and assess the deformity. Ledoux's M1/M2 ratio is the only method for assessing the normal length of the thumb column but cannot be easily used during surgery. We describe the first metacarpal arch, obtained by a radiological anteroposterior X-ray of the hand and wrist with the thumb in 45° abduction. It may be broken or conserved, depending on the form of osteoarthritis. It can objectively predict whether a prosthesis must be placed iso-long or so as to lengthen the thumb column. This overcomes the subjectivity of the notion of "intraoperative piston" and avoids excessive tensioning of the prosthesis, which would increase stress on the prosthetic components and thus the risk of wear and complications. We applied this technical principle to 148 dual mobility prostheses fitted between January 2019 and May 2021. By respecting the arch, the right trade-off is found between intraoperative stability and mobility while protecting the long-term performance of the prosthesis.