In situ stabilization remains the standard of care in the treatment of stable slipped capital femoral epiphysis (SCFE). Screw placement perpendicular to the physis has shown satisfactory results with minimal complications. A prominent screw head may produce femoral acetabular impingement and pain after in situ fixation in severe SCFE. We performed a biomechanical study to establish whether screw head impingement occurs after in situ fixation of SCFE and to define the anatomy of slip severity and screw head position that may lead to impingement. A femoral neck dome osteotomy was created in a human cadaveric model simulating 2 conditions: a moderate and severe SCFEs. We tested the specimens after in situ fixation perpendicular to the simulated physis. The simulated SCFEs and normal control were tested through a full arc of motion. Coverage of the femoral head by the labrum was evaluated at 90 degrees of flexion using fluoroscopy. Impingement occurred at 70 degrees of hip flexion in the simulated moderate SCFE, and at 50 degrees of flexion in the severe simulated SCFE. Anteroposterior fluoroscopy revealed that screw heads lateral to the intertrochanteric line were unlikely to impinge on the acetabulum. Screw head impingement occurred with in situ fixation perpendicular to the physis in simulated moderate and severe SCFEs. Anteroposterior radiographs appear helpful in identifying a hip at risk for screw head impingement after in situ fixation. Alternative in situ fixation techniques (screw head resting lateral to the intertrochanteric line on the anteroposterior radiograph) may decrease the rate of screw head impingement in moderate and severe SCFEs.