ARM Cast

ARM Cast

The ARM cast was designed to improve the current plaster/fiberglass cast and it solves many of the issues surrounding the plaster/fiberglass cast.

Advantages of the ARM cast:

- Short cast preparation/creation time

- Does not require a skilled physician, it is easily done by anyone - even the patient himself

- It is both adjustable and modular, so it fits many patients; even if it were to be too small/large in overall diameter for the patients arm, the ARM cast can be scaled and printed in different size ranges such that it would fit anyone (The ARM cast in this entry has a diameter range of 7 to 3 cm; which would fit most youth)

- Can be adjusted during the bone’s healing process, in case the bone is shifted or is initially misaligned

- Patient can wash and clean skin inside the cast

- To remove the cast, it is just loosened and removed

- Once sterilized the cast can be reused for other patients

To do this, each pad is connected on both ends to the adjustment slides in the modular ring. To adjust the size of the cast, the slides are slid along the slot in the modular ring and this causes the adjustment pads to move toward the center of the ring. Once the pads are adjusted correctly, locking pins are inserted through the slides and into the matching holes found in the modular ring. The bones in a human’s arm naturally twist along the length of the arm; therefore, the slides can be asymmetrically adjusted and this will allow the adjustment pad to follow the bone’s natural twist (See video below, for demo). Also, each patient’s arm has a unique curvature, so the pads are designed with a “flex point” in the middle of the “C” shape. This flex point is simply a thin section which allows the pads to wrap around the patient’s arm in order to best fit the patient’s arm. The adjustment pads also utilize the FDM printer’s ability to create “in-place” assemblies, since it consists of a three-part moving assembly.

Multiple connected Modular Rings

The ARM Cast also utilizes the printer’s layer thickness attributes in the design of the adjustment pads. The “flex-point” of the adjustment pad is designed to be 1.4mm wide, which is approximately 3-4 horizontal layers wide. This is thin enough that the ABS plastic is both strong and flexible. The rest of the adjustment pad is 3.4mm wide, which does not flex at all, rather it remains rigid in order to keep the patient’s bones from moving.

The ARM Cast was printed with ABS plastic on a FDM 3D printer. ABS was used as it is far stronger than other common types of FDM filament like PLA. The ARM Cast utilizes a FDM printer’s capabilities to create in place assembly’s as seen in both the modular ring and adjustment pad. Printing "in-place" assemblies increases production and manufacturing efficiency since it decreases both the unique part count and post-manufacturing preparation time.

When using an FDM printer (a Makerbot Replicator Mini) to create “in-place” assemblies, a gap of .7mm was left between moving parts (See picture). This gap was enough for the parts to be printed without support and still maintain accuracy and post-print movement.