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Procedures · Upper Extremity
Forearm Positioning Series
Compare the AP and lateral projections of the routine forearm series, both showing the wrist and elbow.
forearm · Extended
Right
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Hover a bone, name it, then click to check · drag to rotate
The routine forearm series
The projections this region is examined with. Set them up yourself in Practice positioning.
AP forearm
Anteroposterior projection · hand supinated
- Demonstrates
- The entire radius and ulna with both of their joints — the wrist below and the elbow above — plus the proximal carpals, the distal humerus, and the fat pads and stripes at each end. The wrist and elbow joint spaces are only partly open here, and that is expected rather than a fault: the beam is centred at mid-forearm, so it reaches both joints at an angle.
- Part position
- Seated at the end of the table with the hand and arm fully extended and the palm up, and the shoulder dropped so the entire upper limb lies on the same horizontal plane. The forearm is aligned and centred to the long axis of the receptor with both the wrist and the elbow joint included, using as large a receptor as that takes, and the patient leans laterally as needed to bring the wrist, forearm and elbow as near a true frontal position as possible.
- Central ray
- 90° to the image receptor, centered to the mid-forearm.
- Why it is positioned this way
- Everything here serves one demand: get both joints on one image without rotating the forearm. Dropping the shoulder to the level of the wrist is what stops the limb from lying at an angle to the receptor, and leaning the patient rather than turning the arm is how the elbow is brought frontal without pronating the hand — pronation would cross the radius over the ulna and destroy the very relationship this projection is taken to show. The check for it is at the elbow: palpate the two epicondyles and make sure they are the same distance from the receptor, because when they are, the beam meets them edge-on and both are seen in profile. Done correctly the radius and ulna are separated along their whole length, meeting only where they articulate at the distal radioulnar joint.
Also covered by this projection
Why a large receptor, not a tight collimationCollimation is kept tight at the sides but deliberately loose at the ends. Because the beam diverges, anatomy at the edges of the field is projected outward, so at least an inch to an inch and a half beyond each joint has to be on the receptor or a joint that looked included will be clipped off the image.
Lateromedial forearm
Lateromedial projection · elbow flexed 90°, thumb side up
- Demonstrates
- The radius and ulna superimposed along their shafts, with the elbow flexed to 90° and the humeral epicondyles stacked one on the other. The radial head lies over the coronoid process and the radial tuberosity is thrown into profile. It is the projection that answers which way a fracture fragment has displaced — front or back — which is what separates the Colles, Smith and Barton patterns from one another.
- Part position
- Seated at the end of the table with the elbow flexed 90 degrees and the shoulder dropped so the whole upper limb is on one horizontal plane. The forearm is aligned and centred to the long axis of the receptor with both joints included, and the hand and wrist are then rotated into a true lateral, thumb side up, and supported to prevent motion so that the distal radius and ulna lie directly over one another.
- Central ray
- 90° to the image receptor, centered to the midpoint of the radius and ulna.
- Why it is positioned this way
- This projection is set up in two separate movements, and it is worth seeing why neither one can do the other’s job. Flexing the elbow to 90° is what stacks the humeral epicondyles; rotating the hand thumb-up is what stacks the radius over the ulna. They are independent, because forearm rotation turns the radius around a stationary ulna and leaves the elbow untouched — which is exactly why the instruction names them separately. Both have to be right: a forearm rotated correctly but left extended buries the elbow anatomy, and an elbow flexed correctly with the hand still supinated projects the two bones side by side in a picture labelled "lateral", which is the one thing a lateral must not show.
Also covered by this projection
Put the elbow at the cathode endThe forearm is thicker at the elbow than at the wrist. The tube gives off more radiation at the cathode side of the field than the anode side, so laying the elbow at the cathode end puts the strongest part of the beam over the thickest part of the part and evens out the image.
A heavy or muscular forearmA support placed under the hand and wrist raises the distal end until the radius and ulna lie parallel to the receptor. Without it the forearm slopes, and the two bones separate again at the end that is furthest from the table.