An articulated, adjustable, and lockable alignment arm, and associated extensions and accessories, which are user manipulable to model the size and shape of the fractured limb of a patient in the field. The alignment arm includes a first elongated arm segment, a second elongated arm segment, and connector means for interconnecting the first segment to the second segment through respective pivotal receivers having normal axes of rotation. The alignment arm is adjusted for proper orientation by setting it over the injury, and then locking the arm into a selected orientation which models the patient's limb around the area of the injury. Next, first and second arm extenders and protective end pads are used to expand the effective size of the alignment arm to form a custom splint structure. Lastly, the splint structure is secured to the patient's injured limb through the use of flexible cravats.