Which Pauwels classification type for femoral neck fractures represents the highest degree of vertical shear and thus the highest risk of nonunion?

Explanation:
The Pauwels classification is based on the angle of the fracture line relative to the horizontal. Type I is < 30 degrees, Type II is 30-50 degrees, and Type III is > 50 degrees. A higher angle (more vertical) results in greater shear forces and lower compressive forces across the fracture site during weight-bearing, leading to a significantly higher risk of nonunion and avascular necrosis. Reference: Rockwood and Green's Fractures in Adults, 9th Edition.