Crista Volume Measured from 3D Reconstructions of Weightbearing CT Scans Shows a Relationship to Sesamoid Station

Abstract

Category: Bunion Introduction/Purpose: The progression of hallux valgus (HV) deformities results in dislocation of the sesamoids from their position (station) under the plantar surface of the distal first metatarsal. With this dislocation, the crista separating the two sesamoids erodes as the contact of the medial sesamoid with the crista applies pressure with weightbearing. Recently, three dimensional (3D) reconstructions of the metatarsals and first phalanx have demonstrated a means to find a consistently quantify first metatarsal pronation. These same reconstructions can also produce a reliable technique to compute the volumes of the bones involved. The purpose of the current investigation is to examine the relationship of the volume of the crista to pronation and to sesamoid station. Methods: Eleven HV patients and five normal subjects with weightbearing or weightbearing equivalent CT (WBCT) imaging were randomly selected from the senior author’s patients. Pronation was quantified on WBCT scans using 3D reconstructions as described by Campbell et al. (FAI 2018). Crista volume was determined using a line drawn to connect the nadir of each sulcus on either side of the intersesamoidal crista in each slice of the WBCT image (Figure 1). Sesamoid station was also quantified on WBCT scans using the established four point categorization. A linear regression was performed to determine if the volume of the crista was associated with the pronation angle. The sesamoid station was simplified into two categories - mild medial sesamoid subluxation (less than 50% of the medial sesamoid was lateral to the nadir of the crista) and severe medial sesamoid subluxation - and crista volume between these two groups was compared using a t-test. Results: The regression of crista volume against first metatarsal pronation angle did not show statistical significance (P=0.94, r2=0.03). The mean crista volume in the mild medial sesamoid subluxation group was 156(+-47, range 72 - 231)mm3. The mean crista volume in severe medial sesamoid subluxation was 95 (SD 39, range 35 - 160) mm3. The t-test using simplified sesamoid station to compare crista volumes found that the mean crista volume in the mild medial sesamoid subluxation group was statistically significantly greater the than the mean crista volume in the severe sesamoid subluxation group (P=0.01). Conclusion: In HV, it has been hypothesized that the medial sesamoid erodes the crista resulting in arthritis. This is often overlooked as a source of pain in these patients. Our study found that pronation of the first metatarsal was not correlated with crista volume suggesting that pronation does not affect erosion of the crista. Instead, our results are the first to demonstrate that medial sesamoid subluxation as determined from sesamoid station results in erosion of the crista. This supports the hypothesis that sesamoid subluxation, arthritis, and crista erosion are important components of the HV deformity.