Fig. 1 Three-dimensional (3D) measurement of pronation. This figure demonstrates a 3D computer-aided design model of the first metatarsal. Pronation was measured with respect to the second metatarsal.
Courtesy of Mark Miller, PhD
Published 10/27/2020
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Terry Stanton
Editor’s note: The following content was originally scheduled for the AAOS Now Daily Edition, which publishes each year onsite at the AAOS Annual Meeting, but this year’s meeting in March was canceled due to COVID-19. Despite the cancellation, members can access virtual content from the Annual Meeting by visiting the Academy’s Annual Meeting Virtual Experience webpage at aaos.org/VirtualAAOS2020.
A study demonstrated that in patients undergoing hallux valgus surgery, the modified Lapidus procedure can be an effective tool to change pronation of the first ray. The study, presented as part of the Annual Meeting Virtual Experience, also demonstrated that reduction of the sesamoids was not associated with postoperative first metatarsal pronation, and these should be considered as separate deformities in the treatment of hallux valgus.
Hallux valgus is a triplanar deformity of the first ray, including pronation of the first metatarsal with subluxation of the sesamoids. The purpose of this study was to investigate whether the modified Lapidus technique, without preoperative knowledge of pronation, changes pronation of the first metatarsal, as well as to determine whether reduction of the sesamoids is correlated with changes in first metatarsal pronation.
Thirty-one feet in 31 patients with hallux valgus had preoperative and at least five-month postoperative weight-bearing radiographs and weight-bearing CT scans. All patients underwent a first tarsometatarsal fusion (modified Lapidus procedure) for the deformity. Pre- and postoperative pronation of the first metatarsal was measured with a three-dimensional (3D) computer-aided design as previously described by Campbell et al.
Preoperatively, surgeons were unaware of the CT-derived pronation of the first metatarsal. Differences in preoperative and five-month postoperative pronation of the first metatarsal, hallux valgus angle, and intermetatarsal angle were calculated with Wilcoxon signed-rank tests. After dividing patients into groups based on sesamoid station, the researchers used Kruskal-Wallis H tests to compare first metatarsal pronation between the groups.
The mean pronation of the first metatarsal decreased by 8.8 degrees with the modified Lapidus technique. The final postoperative pronation of 20.2 degrees was similar to the amount of pronation previously reported in normal control patients (19.1 degrees). “Reduction of the sesamoids was not associated with postoperative first metatarsal pronation, which means that simply reducing the sesamoids does imply that the pronation deformity has been corrected,” Matthew S. Conti, MD, a post-graduate year-5 orthopaedic surgery resident at Hospital for Special Surgery, told AAOS Now.
“The advent of weight-bearing CT scans has recently allowed investigators to better understand that hallux valgus is a 3D deformity,” Dr. Conti explained. “Previous studies have demonstrated increased pronation of the first metatarsal in patients with hallux valgus compared to normal control patients on simulated weight-bearing CT scans. The purpose of our study was to determine if a first tarsometatarsal fusion (modified Lapidus procedure) could effectively change pronation of the first metatarsal and determine if reduction of the sesamoids was correlated with changes in first metatarsal pronation.”
He and his colleagues were surprised to observe that the reduction of the sesamoids was not correlated with change in pronation of the first ray. Additionally, he said, pronation of the first ray was not associated with the intermetatarsal angle or hallux valgus angle, noting, “This suggests that sesamoid subluxation and pronation of the metatarsal are separate deformities that need to be corrected individually.”
The clinical takeaway, Dr. Conti said, is that although there is no current optimal degree of pronation of the first ray, the modified Lapidus technique can be used to correct pronation of the first metatarsal. “Additionally, simply correcting sesamoid subluxation does not guarantee an appropriate change in pronation of the first ray,” he said.
A major limitation of the study, according to Dr. Conti, is “These results cannot guide the appropriate or adequate amount of correction of the pronation deformity. At this time, there is no optimal amount of postoperative pronation of the first ray or position of the sesamoids. Additionally, variations in surgical technique may affect both pronation of the first ray and the position of the sesamoids, but we were not able to determine the effect of these variations in technique.”
Dr. Conti’s coauthors of “The Modified Lapidus Procedure is an Effective Tool to Change Pronation of the First Ray in Patients with Hallux Valgus” are Joseph F. Willett, BS; Jonathan H. Garfinkel, MD; Mark C. Miller, PhD; Stephen V. Costigliola, PA-C; Andrew J. Elliott, MD; Stephen F. Conti, MD; and Scott J. Ellis, MD. The poster and abstract for this study (PO674), along with an audio presentation, can be accessed at https://aaos.scientificposters.com/index.cfm?k=piq9yh7wp4.
Terry Stanton is the senior medical writer for AAOS Now. He can be reached at tstanton@aaos.org.