Practice Perfect 723
Frontal Plane Bunion Rotation: A Radiographic Primer

One of the newer developments in the treatment of hallux valgus and bunion deformity is the full appreciation of the triplanar nature of the deformity. Despite some controversy in the past few years, it appears frontal plane rotation as part of the dynamic bunion deformity is broadly accepted. As a result, it is important that each provider have an appreciation for and ability to identify the frontal plane component on radiographs. Here’s a quick radiographic primer to help one identify important frontal plane first ray rotation on standard radiographs.

There are four primary radiographic findings to help us understand the frontal plane position of the first metatarsal. All of these findings are simply a result of the rotational position of the metatarsal and what parts are visible on the radiographic views. Look at Figure 1 which shows a model 1st metatarsal from the right foot in different positions. First, note that the metatarsal itself takes on an increasingly dumbbell shape with wider distal and proximal aspects as it is rotated into increased valgus position. This change in appearance with position will be responsible for the radiographic findings reported in the literature.1,2,3,4,5

Figure 1. Anatomical model rotated from rectus (left) into increasing valgus position.

Let’s now take a look at the appearance on radiographs and discuss the specific findings. Examine Figure 2 for three radiographs of a patient’s foot with hallux valgus and mild to moderate frontal plane rotation (in addition to sagittal elevation and transverse increase in the first intermetatarsal angle).

Figure 2. DP, sesamoid axial and lateral radiograph of a patient with hallux valgus frontal plane deformity.
  1. Lateral round sign (red curve, Figure 3) – An increased rounded appearance to the lateral aspect of the head of the first metatarsal is noted when the metatarsal rotates into valgus. The viewer is seeing the plantar-lateral aspect of the metatarsal head. Compare this to the leftmost image in Figure 1, where the dorsolateral side is slightly concave (which is normal) and one can see the fibular sesamoid.
  2. Concave curvature of the metatarsal shaft (green curve, Figure 3) – As the first metatarsal increasingly rotates into valgus, one can appreciate the curvature on the more plantar aspect of the metatarsal head.
  3. Lateral bulge of the metatarsal base (yellow curve, Figure 3) – Again, as the metatarsal rotates into valgus it is possible to see the lateral base of the first metatarsal. Notice the absence of this finding in the rectus first metatarsal in Figure 1.
  4. Deviated sesamoid position (blue circles, Figure 3) – Most of us have been taught that laterally deviated sesamoids (typically described as a tibial sesamoid position > 3) are caused by the sesamoids being pulled laterally. This may be true in some cases, but it has been shown that this is also a sign of valgus rotation of the sesamoids. Look at the extreme valgus rotated first metatarsal in Figure 1 and note that the fibular sesamoid location appears to be more lateral in respect to the bisection of the first metatarsal head. This rotation is evident in our patient’s sesamoid axial view in Figure 2, where it is easy to see the valgus rotation of the sesamoids as a result of the valgus rotation of the head of the metatarsal.
Figure 3. DP radiographs with identification of frontal plane rotational signs (left) with unmarked image (right) for reference. Lateral round sign (red), lateral metatarsal curvature (green), increased base prominence laterally (yellow), increased sesamoid position (blue).

Use these radiographic relationships to better appreciate the 3-dimensional relationships in your hallux valgus patients to make clinical decisions for their best interests.

Best wishes.

Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor
jarrod@podiatry.com
References
  1. Eustace S, O’Bryne J, Stack J, Stephens MM. Radiographic features that enable assessment of first metatarsal rotation: the role of pronation in hallux valgus. Skeletal Radiol. 1993;22(3):153-156.
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  2. DiDomenico LA, Fahim R, Rollandini J, Thomas ZM. Correction of Frontal Plane Rotation of Sesamoid Apparatus during the Lapidus Procedure: A Novel Approach. J Foot Ankle Surg. Mar-Apr 2014;53(2):248-251.
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  3. Dayton P, Kauwe M, Feilmeier M. Is Our Current Paradigm for Evaluation and Management of the Bunion Deformity Flawed? A Discussion of Procedure Philosophy Relative to Anatomy. J Foot Ankle Surg. Jan-Feb 2015;54(1):102-111.
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  4. Kim Y, Kim JS, Young KW, Naraghi R, Cho HK, Lee SY. A New Measure of Tibial Sesamoid Position in Hallux Valgus in Relation to the Coronal Rotation of the First Metatarsal in CT Scans. Foot Ankle Int. 2015 Aug;36(8):944-952.
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  5. Okuda R, Kinoshita M, Yasuda T, Jotoku T, Kitano N, Shima H. The shape of the lateral edge of the first metatarsal head as a risk factor for recurrence of hallux valgus. J Bone Joint Surg Am. 2007 Oct;89(10):2163-2172.
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