A new trend in hallux abductovalgus (HAV) surgery is identifying frontal plane rotation of the first metatarsal and the sesamoid complex, which surgeons correct with a derotational Lapidus procedure.1-5 However, there is considerable misunderstanding about how the first metatarsal can become “pronated” in HAV deformity and in what plane the sesamoid bones rotate relative to the first metatarsal and the supportive surface.
The inventors and patent holders of the triplane Lapidus surgical procedure published an article, which outlines the radiographic classification of HAV deformity using AP, lateral and axial sesamoid weightbearing radiographs.6,7 This classification system relies upon the axial sesamoid weightbearing radiograph to measure first metatarsal pronation as well as subluxation of the sesamoids.7 Further scrutiny of this article shows that the authors provide no criteria for foot positioning when taking an axial sesamoid weightbearing radiograph. They also provide no method to objectively measure frontal plane position of the sesamoid bones.
A recently published study underscores the importance of overall foot position when taking radiographs and measuring relationships of bone segments in surgical planning for HAV deformity. Researchers at the Kent State University College of Podiatric Medicine studied the effects of whole foot positioning on measurements of frontal plane sesamoid positioning.8 The researchers studied 12 people with hallux abductovalgus (HAV) deformity using a weight-bearing pedCAT cone beam computed tomography (CT) scanner (CurveBeam®). They measured the frontal plane position of the sesamoid bones relative to the supportive surface and defined this as the sesamoid rotation angle (SRA). This angle is formed by a line tangential to the plantar plane of the sesamoids and a line representing the weightbearing surface (see image above).
Using weightbearing CT imaging, the researchers compared the sesamoid rotation angle when moving the study participants’ feet from maximal supination to maximal pronation and also compared this angle with standard axial sesamoid weightbearing radiographs.8 The study showed significant change in the sesamoid rotation angle depending on foot position. In a supinated position, the mean sesamoid rotation angle measured 10.5 degrees. In the pronated position, this angle increased to 22.1 degrees.8 In comparison, the standard axial sesamoid radiograph showed a sesamoid rotation angle of 12.2 degrees, which approximated the supinated position of the foot. The study authors concluded that “the results of this study reliably indicate that the sesamoids significantly vary their position with extremes in weight-bearing foot attitudes. The results of this study strongly suggest that the current imaging standard (axial radiograph) for evaluating the coronal plane degree of sesamoid rotation significantly underestimates the true sesamoid rotation value.”8
Kawalec and colleagues correctly point out that the orthoposer positioning device required for the axial sesamoid radiograph will extend the first MPJ, plantarflex the first ray and supinate the foot (see photo to right), which does not depict the true position of the foot at midstance.8
With the axial sesamoid positioning orthoposer, plantarflexion of the first ray couples with first metatarsal eversion, which will increase the perception of a “pronated position” of the first metatarsal in the axial sesamoid radiograph. This correlates with the well-documented observation that the axis of the first ray provides eversion with plantarflexion.9-12
It is well recognized that the sesamoid bones will become displaced from the first metatarsal or vice versa in the transverse plane as HAV deformity progresses.13-17 It was only relatively recently that authors began measuring frontal plane displacement of the sesamoids in HAV deformity. Kuwano and coworkers were the first to measure the sesamoid rotation angle (SRA) using a weightbearing “tangential” view (identical to the axial sesamoid view) of the sesamoid apparatus in patients with HAV.18 Later, Shibuya and colleagues published a study of several radiographic criteria for measuring HAV deformity and used the SRA angle as previously described by Kuwano and team.18,19
Interestingly, proponents of the triplane Lapidus procedure use a different criteria to identify “subluxation” of the sesamoids as they deviate away from the first metatarsal in the transverse plane rather than the frontal plane.7,21 Kim and coworkers used the classification of Smith and colleagues to grade the transverse plane shift of the sesamoids relative to the crista (i.e. intersesamoid ridge) on the inferior surface of the head of the first metatarsal (see the figure below).20,21
In the study by Kim and coworkers, only the transverse plane shift of the sesamoids fulfilled the criteria for subluxation, which occurred in 73 percent of the study participants, yet the sesamoids appear to be rotated in the frontal plane in all four of their classification groups of HAV deformity.21
Indeed, transverse plane subluxation of the sesamoids relative to the first metatarsal crista is an important aspect of classifying the triplane deformity of HAV according to Hatch and team.7 Yet in the same article, these authors recognize that the sesamoids rotate in the frontal plane as well but make no suggestion to measure the sesamoid rotation angle.7 Furthermore, Hatch and colleagues measure the first metatarsal pronation angle with the axial sesamoid radiograph by using a simple line across the “plantar condyles” of the first metatarsal head while Kim and team used a more detailed set of landmarks including the medial and lateral walls of the first metatarsal head.7,21 Visualizing the plantar surface of the first metatarsal head is difficult using standard axial sesamoid radiographs. As shown in the study by Kawalec and coworkers, overall foot positioning can greatly influence the coronal plane position of key structures around the head of the first metatarsal.8
What The Studies Reveal About First Metatarsal Pronation And The Sesamoid Rotation Angle
In the past, I cautioned about concluding that the first metatarsal independently “pronates” or everts relative to the foot in HAV deformity.22-25 I also recognize that the first metatarsal can appear everted or pronated to the supportive surface in patients with HAV, but this rotation occurs at the talonavicular joint, not at the first metatarsocuneiform joint where surgeons now target correction of this frontal plane deformity.26
I based my assertions upon the work of Kimura and colleagues, who showed the following joint rotations in patients with HAV deformity via weightbearing CT imaging.27 With loading, the study authors observed the following bone segments to rotate in this magnitude and direction:
- the first metatarsal moves 4.9 degrees into inversion;
- the medial cuneiform moves 1.5 degrees into eversion;
- the navicular moves 9.6 degrees into eversion; and
- the net motion of the entire medial column results in 6.2 degrees of eversion.27
The aforementioned triplane Lapidus procedure calls for rotation of the first metatarsal into inversion to reduce a perceived pronation deformity.2-5 However, a Lapidus procedure cannot address the actual joint that caused pronation of the medial column of the foot. That joint is the talonavicular joint. Patients with HAV deformity do not demonstrate pronation of the first metatarsocuneiform joint. Indeed, a Curve Beam® image of a patient with HAV deformity clearly shows that the first metatarsal has rotated into inversion at the first metatarsocuneiform joint with weightbearing (see the image to the right).
Shibuya and coworkers amplified on the misleading appearance of a pronated first metatarsal in patients with HAV deformity.19 Using AP, lateral and axial sesamoid radiographic views of 71 patients with hallux valgus and 43 patients without hallux valgus, the researchers performed multivariate analysis of radiographic variables relevant to HAV deformity and flatfoot deformity. They found that first metatarsal pronation rotation, measured with the axial sesamoid view, was associated with flatfoot deformity but not with HAV deformity. Conversely, the sesamoid rotation angle was associated with HAV deformity regardless of the presence of flatfoot deformity. The magnitude of sesamoid rotation strongly correlated with the tibial sesamoid position on AP radiographs. Conversely, first metatarsal rotation was not correlated with the degree of tibial sesamoid displacement on the AP radiograph.19
It is clear that the first metatarsal pronation angle is not the same as the sesamoid rotation angle and both deformities result from different mechanisms (see the image to the right).
In fact, several studies show that pronation of the first metatarsal does not directly correlate with the magnitude of HAV deformity. Using weightbearing radiographs, Saltzman and coworkers found no direct relationship between the magnitude of first metatarsal pronation position and the degree of hallux valgus angle or intermetatarsal angle in patients with HAV deformity.28 Two recent weightbearing CT studies of patients with HAV also showed no relationship between the magnitude of first metatarsal pronation and severity of hallux valgus deformity.29,30 In the weightbearing CT study by Conti and colleagues, they found no correlation between the amount of first metatarsal pronation and the degree of sesamoid rotation in the frontal plane.30
Thus, the two angular changes are the result of different mechanisms.30 Sesamoid rotation is the result of HAV deformity while first metatarsal rotation is the result of whole foot pronation. Indeed, Shibuya and team concluded from their study, that when planning surgical correction of HAV, “the need for derotation of the first metatarsal may have to be carefully examined, especially in persons with underlying flatfoot deformity.”19
On the other hand, the sesamoid rotation angle highly correlates with the magnitude of HAV deformity according to the works of Kuwano, Shibuya and their respective teams.18,19 Shibuya and coworkers also showed that sesamoid rotation was directly related to transverse plane position of these bones on the standard AP radiograph.19 More importantly, this study points out that surgeons can reduce sesamoid rotation with traditional osteotomy of the first metatarsal without rotation and without a Lapidus procedure. They cite two studies by Lamo-Espinoza and colleagues as well as Ramdass and Meyr, whom achieved significant reduction of sesamoid rotation by Scarf osteotomy or distal metaphyseal osteotomy of the first metatarsal without the need for any frontal plane rotation.31,32 It appears that restoration of alignment of the first metatarsal over the sesamoid complex with simple transverse plane translation will correct the soft tissue imbalance, which causes frontal plane rotation of the sesamoids in HAV deformity.
In summary, with axial radiographs, the sesamoid rotation angle (SRA) has a strong correlation with the magnitude of hallux abductovalgus deformity while the first metatarsal pronation angle does not. The measurement of the sesamoid rotation angle is also influenced by the orthoposer or positioning device required for taking the axial sesamoid radiograph. Although the sesamoid rotation angle is a frontal plane deformity, surgeons can reduce this with transverse plane surgical correction of the intermetatarsal angle without the need for a derotational Lapidus procedure.
Dr. Richie is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is a Fellow and Past President of the American Academy of Podiatric Sports Medicine. Dr. Richie is a Fellow of the American College of Foot and Ankle Surgeons, and the American Academy of Podiatric Sports Medicine. Dr. Richie is the author of a new book titled "Pathomechanics of Common Foot Disorders" available from Springer at https://www.springer.com/us/book/9783030542009 .
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