Is an employment-related injury the sole cause of an employee's disability?

Although the phrase "as the result of injury" in section 24.0501 provides no real guidance as to whether an employment-related injury must be the sole cause of an employee's disability to entitle the employee to a disability retirement, section 24.0501 expressly limits disability retirement to employees whose disabling injuries do not arise from a preexisting medical condition. That limitation is permissible because it is not expressly prohibited by the charter. As this court noted in Grimm v. City of San Diego (1979) 94 Cal.App.3d 33, 38: