The Plumbline: A Novel Radiographic Tool for Assessment of Metatarsus Adductus with Hallux Valgus

Abstract

Category: Bunion; Midfoot/Forefoot Introduction/Purpose: Metatarsus adductus (MTA) is a complex multiplanar deformity of the foot which has a 30% reported approximate radiographic incidence in patients with symptomatic hallux valgus (HV). Various radiographic angles have been proposed for the assessment of MTA, but these measurements can be complex with a broad range of interrater reliability. We propose a simplistic radiographic assessment method termed the ‘plumbline’ to identify the presence of MTA and to ascertain if enough physical space is available to align the first metatarsal to the longitudinal foot axis via an isolated 1st ray procedure, or if necessary, through a combined approach which also addresses the lesser metatarsals. This method provides a visual reference to determine if a physical blockade to 1st ray correction is present. Methods: The plumbline is assessed on standard weight-bearing AP radiographs. Neutral foot positioning is key as a pronated or supinated clinical position of the foot will impact the alignment of this and other radiographic angular relationships. A transverse midfoot axis line is drawn by identifying and linking two individual points marked at the calcaneocuboid joint and the talonavicular joint. A third point is marked at the distal lateral apex of the medial cuneiform at the first tarsometatarsal joint. A 90° line is subtended from the transverse midfoot axis line, crosses the third point, and extends distally to the 2nd metatarsal head. A positive radiographic plumbline occurs when this line intersects the 2nd metatarsal indicating the need to correct the adducted lesser metatarsal position prior to addressing HV. A negative plumbline remains tangential to the 2nd metatarsal and indicates an isolated HV correction is possible. Results: The metatarsus adductus angle (MAA) measured via Sgarlato's method (SA), and the plumbline were recorded in patients (N=20) scheduled for HV surgery. Initially the cohort was subdivided into subjects deemed to have a normal MAA (SA < 15°; N=10) and an abnormal MAA (SA>15°; N=10). A mean pre-operative MAA measurement of 8.1° (SD 2.3; range 5, 12.1) and 26.5° (SD 4.5; range 19.2, 33.7) were found in these respective subsets. All subjects (100%) with an MAA < 15° (N=10) were found to have a negative plumbline radiographically and subjects (100%) with a MAA >15° (N=10) presented with a positive plumbline. The plumbline findings identified when an isolated HV correction would have been possible or impeded based upon 2nd metatarsal position, thus driving the surgical decision making algorithm. Conclusion: The plumbline is used to determine when concomitant MTA and HV correction is beneficial without the need to measure other more traditional radiographic angular relationships. It is a quick and simple method of evaluating the proximity of the metatarsal segments to determine if the first metatarsal is amenable to reduction in isolation or if correction will be limited due to a 2nd metatarsal bone blockade. The visual simplicity of the measurement can also aid in patient education and surgeon satisfaction when describing the surgical approach to this complex three-dimensional anatomic pathology.