Assessment of First Metatarsal Head Bone Density for Optimal Screw Placement in Minimally-Invasive Hallux Valgus Surgery

Abstract

Category: Bunion; Basic Sciences/Biologics Introduction/Purpose: Minimally-invasive surgery (MIS) for hallux valgus correction has become increasingly common. This technique involves an osteotomy of the first metatarsal, followed by fixation with two cannulated screws. Since screws are typically not bicortical, they rely upon bone quality within the metatarsal head for fixation strength. However, bone mineral density (BMD) within different regions of the metatarsal head is unknown. Measuring the BMD in the target region may predict the strength of the bone-screw fixation. Similar to previous work which determined the optimal position for lag screw placement in the femoral head during hip fracture fixation, this study aimed to determine average BMD within four quadrants of the metatarsal head using CT and thus predict the optimal trajectories for cannulated screws during the MIS bunion procedure. Methods: All patients between 18-75 years of age scheduled to undergo MIS hallux valgus correction by one of two surgeons experienced in the MIS technique were eligible to participate. Patients were excluded if they had a prior first metatarsal surgery, pre-existing hardware, previous first metatarsal fracture, or a history of osteoporosis treatment. Patients were enrolled prospectively, and a weight-bearing CT scan of the affected foot was obtained pre-operatively. Demographic factors including age, sex, laterality, body mass index (BMI), comorbidities, and smoking status as well as standard three-view weight-bearing radiographs were collected for all patients.Using the coronal CT slice at maximal metatarsal head diameter, each head was divided into equal quadrants. Hounsfield units (HU) within each quadrant were measured independently by three study investigators using our hospital’s radiology viewing software (Merge PACS; IBM Corporation, Armonk, NY), and these density measurements were averaged. Statistical analysis was conducted using ANOVA and Student’s t-test. Results: Fifteen patients were included for preliminary analysis. All patients were female. The average age was 45.7 years. 9 of the 15 included feet were right feet. Average BMI was 28.0. One patient reported active smoking prior to surgery. Comorbidities included obesity in three patients; none were diabetic. One had a history of diplegic cerebral palsy. The average HVA on a weight- bearing AP foot x-ray was 28.2°, and the average IMA was 12.6°. The BMD within the metatarsal head varied by quadrant, with the two combined dorsal quadrants having higher average BMD than the two combined plantar quadrants (122 vs 85 HU; p<0.001). The dorsal lateral quadrant had the highest average BMD of any quadrant (132 HU, p<0.001; Table 1). Conclusion: The density of the metatarsal head did vary by region within the head. The highest BMD was found in the dorsal lateral quadrant, and the lowest in the plantar lateral and plantar medial quadrants, which did not differ significantly from each other. Because strength of screw fixation is predicated upon screw design as well as bone density, these results suggest that surgeons may wish to direct screws toward the dorsolateral region of the metatarsal head in order to achieve optimal fixation. Further work is needed to determine whether this varies with patient age, gender, or hallux valgus angle.