Bone Density of the First Metatarsal Head: Which is the Best Place to Put the Screws in MIS Hallux Valgus Surgery?

CATEGORY: Basic Sciences/Biologics; Bunion INTRODUCTION/PURPOSE: Hallux valgus minimally invasive surgery (MIS) is becoming more common due to its good clinical and functional results. Third-generation MIS surgery uses screw fixation that is dependent on the bone density of the first metatarsal head (FMH) for optimal screw purchase. There is no information regarding areas of higher bone density (BMD) in the FMH that suggest a stronger area of fixation, nor has it been described whether it varies between different age groups or with gender. The objective of this study is to determine the BMD within four quadrants of the FMH. This finding could lead to identifying the ideal trajectory for screw placement. In addition, we evaluate BMD variation by age and gender. METHODS: Transversal study. From our CT scan database, we obtained 40 patients with no previous history of first metatarsal fractures, pre existing hardware, foot surgery or osteoporosis. We divided the patients into 4 groups, each group containing 5 male and 5 female patients: group 1: <30 years, group 2: 31-45 years, group 3: 46-60 years, group 4: > 60 years. The main outcome is to evaluate the association between BMD in each FMH quadrant. Secondary outcomes are to identify any differences in BMD between gender and age groups. According to what has been previously discussed in the literature, two evaluators measured the Hounsfield units (HU) of each quadrant on a coronal cut located 10 mm proximal to the metatarsophalangeal joint using the Merge PACS system. Measures were averaged. Statistical analysis was performed by measuring normality with the Shapiro-Wilks test, and the ANOVA, Student's t-test, and Mann-Whitney U test. RESULTS: The dorsolateral (DL) quadrant had the highest BMD (353.9hu) of all quadrants, followed by the lateral plantar (LP: 302.4hu), medial plantar (MP: 264.1hu), and dorsomedial (DM: 247.6hu) (p=0.001).When comparing the groups by age, DL (p=0.01) and LP (p=0.01) quadrants had significantly greater BMD in groups 1 and 2 (< 45 years) than groups 3 and 4 (> 45 years).When comparing BMD by gender, differences were only obtained in the group 4, in which mens had greater BMD in the DM (p=0.03), DL (p=0.02), and LP (p=0.01) quadrants.An significant inverse association between age and BMD was obtained for the DL (P=0.004) and MP (P=0.03) quadrants. CONCLUSION: There are differences in BMD between the quadrants of the first metatarsal head, with the greatest BMD being in the dorsolateral region at any age. BMD decreased proportionally with increasing age in the DL and MP quadrants. At the same age, there are no significant differences between gender, except in patients >60 years, where women have lower BMD in almost all quadrants.