A clamp assembly for bone fixation elements including first and second clamp structures configured to engage first and second bone fixation elements, respectively. The first clamp structure may have a first spherically contoured bearing surface. The second clamp structure may have a second spherically contoured bearing surface seated in sliding contact with the first bearing surface. A spring-loaded mechanism may apply a spring force urging the bearing surfaces together to resist pivotal movement of the clamp structures relative to each other. The clamp assembly may be used with an assembly of fixation elements to create an external fixation frame.