The Mechanical Axis of the First Ray: A Radiographic Assessment in Hallux Abducto Valgus Evaluation

An Imaged-Based Three-Dimensional Study of First Metatarsal Protrusion Distance in Women with and Without Hallux Valgus

“…In addition to using a 3-D methodology, this study quantified MPD using a mechanical axis approach advanced by LaPorta et al 16 In introducing a paradigm to better measure the intermetatarsal angle, they described how the anatomic axis of a long bone differs from its mechanical axis. The anatomic axis is simply defined by the middiaphysis shaft of a bone, whereas the mechanical axis is represented on an image with a line drawn by an examiner that connects the distal and proximal ends of any combination of bones that function as a single segment in supporting weight.…”

“…The anatomic axis is simply defined by the middiaphysis shaft of a bone, whereas the mechanical axis is represented on an image with a line drawn by an examiner that connects the distal and proximal ends of any combination of bones that function as a single segment in supporting weight. 16 After making this distinction, LaPorta and coworkers 16 represented the mechanical axis of the first and second rays with lines drawn on a radiograph. The lines bisected the heads of the first and second metatarsals distally, and converged on the talus proximally.…”

“…The study, however, did not report MPD in the populations sampled. 16 To apply this mechanical axis reasoning to the measurement of MPD, an examiner must first decide on what constitutes a single-functioning segment. Though the second ray has been less studied, the first ray behaves as a single segment because the metatarsocuneiform joint surfaces interlock, 26 while allowing the first ray to move independent of the second when the foot carries weight.…”

“…2) falls within a normal range for adults. [15][16][17][18] Despite the experimental nature of this study, one immediate application taken from this research is that future study of MPD and hallux valgus should employ methods that capture the 3-D spatial relationship of the anatomy of the foot.…”

“…Though the selection of the proximal point of reference is dependent on method, 14 MPD averages between plus or minus 2 millimeters (mm) in adults. [1][2][3][4][5][6][15][16][17][18] A positive value indicates protrusion of the first metatarsal. 15,19 Metatarsal protrusion distance was measured in this study using a technique called for by LaPorta et al, 16 but with modification to record the relative difference in the actual length of the first and second rays.…”