Nonunion is a very common complication of intracapsular fractures of the neck of femur. Which of the following is not a very important cause for the same?

(d) Stress at fracture site due to muscle spasm- Causes of non-union in femoral neck fractures are posterior comminution, inadequate immobilization, inadequate vascularity, inhibition by synovial fluid, vertical fracture line and absence of cambium layer of periosteum.