Final answer:
The sternoclavicular joint is the one that directly attaches the shoulder complex to the ribcage, whereas the glenohumeral joint, known for its extensive range of motion, is supported mainly by the rotator cuff muscles.
Step-by-step explanation:
The joint within the shoulder complex that attaches directly to the ribcage is the sternoclavicular joint. This joint is formed by the articulation between the sternal end of the clavicle and the manubrium of the sternum. In contrast, the glenohumeral joint is a ball-and-socket joint that provides the shoulder with a wide range of motions, but it does not directly connect to the rib cage. The sternoclavicular joint attaches directly to the ribcage within the shoulder complex.
The glenohumeral joint's primary support comes from the rotator cuff muscles.