A compact guide for radiology teams on correct calibration sphere placement in orthopedic X-ray imaging. The marker must be positioned in the anatomical plane of the target joint so that digital scaling, measurements and preoperative templating remain clinically reliable.
AP pelvis and hip imaging require the sphere to reproduce the hip joint plane as closely as possible.
Place the sphere on the level of the greater trochanter and as close as possible to the hip joint. In larger soft-tissue envelopes, a medial position in the beam path near the symphysis can be used if it better represents the hip joint plane.
For AP pelvis acquisition, a lateral marker position at greater-trochanter level is suitable when the marker remains in the hip plane and fully visible. The decisive factor is depth-plane equivalence to the joint.
Knee calibration depends on marker placement at the real joint line and on correct acquisition geometry.
For AP knee or partial-leg imaging, place the sphere directly at knee joint-space level. Weight-bearing acquisition is preferred whenever clinically appropriate.
For lateral knee imaging, place the sphere in the popliteal fossa or at patellar level. Both options are valid when they reproduce the knee plane without depth mismatch.
Long-leg imaging for HTO and DFO must combine loaded axis depiction with calibration at knee level.
Acquire the full lower limb in standing position. Position the sphere centrally at knee joint level so that hip, knee and ankle are calibrated against the loaded mechanical axis.
Forefoot and ankle imaging require the marker to match the anatomical level of the measurement target.
For hallux valgus assessment, place the sphere at the level of the phalanges / forefoot. If implant sizing is relevant, align the marker with the metatarsal level.
For AP ankle imaging, position the sphere at talus level. If a malleolar landmark is the target, the medial or lateral malleolus may define the reference level.
For lateral ankle imaging, place the sphere at the level of the talus and midfoot axis, corresponding to the talus / second cuneiform region.
Shoulder calibration must reproduce the glenohumeral plane without ventral or dorsal depth error.
Place the sphere at the level of the greater tubercle and in the beam path of the glenohumeral joint. This keeps the shoulder joint plane correctly represented.