Why Frontal Plane Correction Is A Vital Component Of Bunion Surgery

For decades, foot and ankle surgeons have focused primarily on the transverse and sagittal planes to define and plan surgical correction for hallux abducto valgus (HAV). We have seen a growing body of research identifying the significance of the frontal plane component of the deformity, which surgeons recognized decades ago.¹

Despite a growing knowledge base on the triplane nature of HAV, algorithms for procedure selection remain largely based on two-dimensional radiographic angular relationships. In the current paradigm, we make procedure selections based on the degree of deformity as determined by the intermetatarsal angle, hallux valgus angle and distal metatarsal articular angle as opposed to evaluating the anatomical level of deformity or the frontal plane of the deformity. This has resulted in high variability with hundreds of procedure choices and little agreement among surgeons on best practice. As we will discuss, we believe this lack of consideration for all three planar components of the HAV deformity is at least, in part, the reason for high recurrence and complication rates.

It is well established that valgus rotation or eversion of the first metatarsal is a component of HAV deformity in a high percentage of patients. Mizuno and colleagues first described frontal plane rotation as a component of HAV in 1956.¹ This has continued to be the focus of much research with eversion of the first metatarsal being consistently present in bunion deformities.^2-11 Sesamoid axial radiographs have shown pronation of the first metatarsal in HAV to range between 12.7 and 14.5 degrees in comparison to the average pronation of 3.1 degrees in a normal foot.2,12 Researchers have utilized the radiographic position of the inferior process of the first metatarsal to measure pronation and found a linear relationship with metatarsal medial deviation.³

More recently, computed tomography (CT) studies have assessed pronation of the first metatarsal. In 2013, Collan and colleagues found consistent pronation of the first ray in 10 patients with HAV on weightbearing CT.¹⁰ In 2015, Kim and coworkers found first metatarsal pronation to average 21.9 degrees in 166 HAV feet and 13.8 degrees in 19 normal feet.¹¹ They concluded that first metatarsal pronation exceeding 15.8 degrees is abnormal.

Challenging The Conventional Assessment Of HAV On AP Radiographs

What we visualize on AP radiographs and use in our procedural algorithms may not in fact be a true representation of the three planes of the deformity. We commonly use tibial sesamoid position in pre-surgical planning as an indication of deformity severity. While observation of the tibial sesamoid position on AP radiographs has correlated with transverse plane severity of HAV, medial deviation of the metatarsal off the sesamoids accounts for only the transverse plane of deformity.^13,14 Authors have used the consistent positional relationship between the sesamoids and second metatarsal or the proximal phalanx and second metatarsal to defend the notion that the first metatarsal is displaced medially in HAV while the sesamoid apparatus remains stationary.^14-18

This thought process, however, is lacking consideration of frontal plane influence on anatomical changes in HAV visible on AP radiographs. In many but not all instances, researchers have shown the sesamoids to be in their anatomical grooves on the plantar aspect of a pronated first metatarsal despite the appearance of sesamoid subluxation on an AP view.^6,7,19-21 Tibial sesamoid position, as assessed on AP radiographs, is in reality a representation of the combined findings of adduction of the first metatarsal and first metatarsal eversion. Determination of which deformity components are truly responsible for the AP tibial sesamoid position requires axial imaging (sesamoid axial radiograph or CT scan).

When considering the effect of frontal plane rotation on sesamoid position, one must also recognize that soft tissue and capsular balancing cannot resolve the deformity of the sesamoids on a pronated metatarsal. At best, we are pulling the sesamoids into a more medial position under the metatarsal to make the AP radiograph look normal while, in fact, the sesamoids are now medially displaced from their grooves and their normal anatomic relationship with the metatarsal. We believe this is a major factor in both recurrence and poor function of the metatarsophalangeal joint (MPJ) following many bunionectomy procedures.

What The Research Reveals On Frontal Plane Deviation

If the goal of HAV correction is to restore normal anatomy completely, one must address the frontal plane when it is present as a component of the deformity. The lack of attention and understanding of the frontal plane is reflected in the inconsistent results reported following HAV correction and low rates of patient satisfaction.^22-25

This raises an important question: Why and how could frontal plane deviation affect the structure and function of the first ray? Koller and coworkers noted that weightbearing is reduced under the hallux and first metatarsal in feet with HAV due to pathologic positioning, which interferes with the normal windlass mechanism of the ginglymoarthrodial first MPJ.²⁶ As the first metatarsal rotates in the frontal plane to a valgus position, end dorsiflexory MPJ range of motion (ROM) decreases as well, likely due to this same mechanism.²⁷ This highlights the effect frontal plane position may have on the kinematics of the first ray.

Although many have argued that tarsometatarsal joint motion is necessary for normal gait, studies have proven that first ray maximum movement actually occurs at the naviculocuneiform joint, not the tarsometatarsal joint.^28,29 Rush and colleagues found a 26 percent increase in plantarflexion of the first metatarsal after performing a corrective tarsometatarsal joint fusion despite, as some surgeons say, “sacrificing a perfectly good joint.”³⁰ We think this improved function is likely due to realignment of the first ray with subsequent improvement of the windlass mechanism of the first ray in which normal motion continues proximal to the tarsometatarsal joint. Perez and colleagues offered additional support for this concept, demonstrating a 25 percent increase in MPJ ROM after tarsometatarsal joint fixation due to restoring the first ray to normal anatomical alignment.³¹ Motion at the tarsometatarsal joint is simply not a necessary movement for normal gait. It is alignment and stability that are paramount.²⁹

Taking this thought process one step further, we hypothesize that pronation of the first metatarsal could be the initial deviation that sets into motion the chain of events resulting in HAV. When the first metatarsal everts relative to the plane of the lesser metatarsals around the longitudinal axis of the forefoot, the pull of the flexors on the hallux begins to exert a net lateral torque on the hallux, resulting in abduction and valgus rotation of the hallux. Mortier and coworkers explored this concept and showed that on a pronated first metatarsal, the hallux is angulated and rotated laterally, exerting a medial force on the distal first metatarsal, therefore increasing the intermetatarsal angle.² Although our theory of HAV deformity progression has not been experimentally proven, our empirical experience with triplane HAV correction, which includes first metatarsal inversion, consistently shows adequate correction of the deformity. This accordingly provides a compelling observation of the connection between the frontal plane of deformity and the more commonly recognized transverse and sagittal planes of deformity in HAV.

Addressing The Notion Of An ‘Enlarged Medial Eminence’

There are additional indicators and findings that support the need to address the frontal plane. Although this may not be intuitive due to our reliance on AP radiographs to image the first metatarsal, the “enlarged medial eminence” is not due to excess of bone but, in many cases, is rather simply the planar perspective of the everted metatarsal.

Thordarson and Krewer demonstrated that there is no difference in medial eminence width when comparing normal feet and those with HAV.³² The rotation of the first ray that occurs with HAV causes the medial eminence to appear larger on radiographs by bringing the dorsal medial surface into profile.33 Traditional eminence resection, which removes a section of bone medial to the sagittal groove, is unnecessary and undesirable after the metatarsal is supinated into normal anatomical alignment. In the case of HAV with degenerative joint disease, we may see hyperostosis but we rarely find a significant eminence in non-degenerative cases. In many cases, the medial eminence appearance is due to pathologic frontal plane rotation so one need not remove the medial eminence and it will normalize when one derotates the metatarsal.³⁴

In Conclusion

During a recent literature review, we found that metatarsal osteotomy procedures that did not address the frontal plane of deformity had very high recurrence rates. Proximal opening wedge osteotomies had recurrence rates as high as 64.7 percent, scarf osteotomy recurrence rates were 78 percent, distal chevron osteotomy recurrence rates were 73 percent and modified Mitchell procedure recurrence rates were 47 percent.^22-25

Our personal conclusion as to the high recurrence rates is that failure to correct all planar components of the deformity, including frontal plane rotation, is the basis for many of the shortcomings of osteotomies. Additionally, the vast majority of osteotomy procedures occur far distal to the deformity center of rotation of angulation (CORA), thus affecting the ability to achieve complete correction.

It is becoming quite clear that many of our traditional clinical and radiographic findings that drive procedure selection are, in many cases, observation of projectional artifacts resulting from frontal plane rotation. We wholeheartedly recognize that evidence-based medicine has not established a consensus understanding of the pathologic basis of HAV and the optimum recommendation for correction. We are, at the same time, convinced that attention to the CORA of the deformity and correction of all three planes of the deformity will take us to the point in which we will have the true answers, and thereby provide the best results for our patients.

Dr. Dayton is affiliated with UnityPoint Clinic Foot and Ankle in Fort Dodge, Iowa. He is an Assistant Professor in the College of Podiatric Medicine and Surgery at Des Moines University. Dr. Dayton is a Fellow of the American College of Foot and Ankle Surgeons.

Ms. Cifaldi is a podiatric medical student in the College of Podiatric Medicine and Surgery at Des Moines University.

Ms. Egdorf is a podiatric medical student in the College of Podiatric Medicine and Surgery at Des Moines University.

References

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