AC Joints AP with Weights is a radiography positioning projection of the Upper Extremity. Erect, bilateral AC joints. Equal weights strapped to both wrists (Bontrager: 8 to 10 lb minimum for large adults, less for smaller patients; Merrill's: 5 to 10 lb). SID 72" to fit both joints on one image (Bontrager: 40" minimum, or 72"; Merrill's: 72"). The centering point is located midline at the AC joint level, about 1 inch above the jugular notch, with weights suspended from both wrists by straps. The central ray is perpendicular to the midline at the level of the AC joints. Image-quality criteria include both AC joints visualized side-by-side for direct comparison, AC joint space clearly demonstrated, widened if ligaments are torn. The SID is 72 inches (183 cm).
Anatomy demonstrated
- Bilateral acromioclavicular joints under gravitational stress.
- Distal third of both clavicles.
- Entire clavicles and SC joints.
Patient preparation
- Verify patient identity using two identifiers.
- Remove necklaces and other radiopaque objects from the region to be radiographed.
- Rule out a fracture first: shoulder or clavicle images, or the non-weighted view checked before the weighted one.
- Position patient erect with the back against the IR, shoulders level.
- Strap equal weights to each wrist: Bontrager gives 8 to 10 lb minimum for large adults, 5 to 8 lb for smaller patients and more for larger ones; Merrill's gives 5 to 10 lb.
- Have the patient relax shoulders and let the weights hang freely.
Position & centering point
Midline at the AC joint level, about 1 inch above the jugular notch, with weights suspended from both wrists by straps.
Central ray
Perpendicular to the midline at the level of the AC joints.
Exposure / technique
- kVp
- N/A
- mAs
- N/A
- SID
- 72" (183 cm)
- Notes
- Pair with the non-weighted AP for direct comparison. Weights must hang freely from wrist straps, never held in the hands. Check department protocol for the amount of weight. Bontrager: minimum SID 40" (102 cm), or 72" (183 cm) to include both joints on the same study or for a broad-shouldered adult. Merrill's text: 72", to reduce magnification and joint-space distortion. Bontrager digital range: 75 to 85 kV. Merrill's chart prints 81 kVp, DR 5.6 / CR 11 mAs for a 14 cm part, but at a 40" SID, so adjust for 72" and your equipment. Neither Bontrager nor Merrill's prints a separate weighted technique.
Image-quality criteria
- Both AC joints visualized side-by-side for direct comparison.
- AC joint space clearly demonstrated, widened if ligaments are torn.
- Entire clavicles and SC joints included.
- Both AC joints on the same horizontal plane.
- No rotation or leaning: SC joints symmetric on each side of the vertebral column.
- Collimation: long, narrow field with the upper border at the upper shoulder soft tissue margin (Bontrager); 6 x 17 inches (15 x 43 cm) for both joints on one exposure (Merrill's).
Common errors
- 1 Patient holding the weights in the hands instead of letting them hang.
- 2 Unequal weights: the same amount must be used on each wrist.
- 3 Patient leaning or rotated: Merrill's requires no rotation or leaning.
- 4 Patient tensing or favoring the injured shoulder: instruct the patient not to.
- 5 CR not midway between the AC joints, 1 inch above the jugular notch.
Clinical indications
- Suspected AC joint separation.
- Joint space comparison: widening on the weighted view usually indicates AC joint separation.
- Bilateral comparison of both AC joints, after fracture is ruled out.