Introduction: While conventional acetate x-rays came at a fixed size and magnification, digital x-rays are freely scalable on the screen and thus must be individually calibrated for surgical measurements (templating). The standard technique is calibration by a reference object (ruler, ball, coin) to be placed into the x-ray. In clinical practice, x-rays are often taken without a calibration object, the object may be malpositioned or the bucky system’s objectfilm distance is unknown to scale with a fixed magnification. Thus calibration based on an anatomic dimension would be a useful alternative in clinical practice.

In this study x-ray calibration using the femoral head diameter as derived by an anatomic formula is compared to the standard technique of using a calibration ball and acetate type fixed magnification.

Methods: In a prospective study three calibration Methods: were applied to post-op AP digital x-rays of 42 patients (m/f=12/30, height: 151–185cm) following primary total hip arthroplasty (ABG-II, 28mm heads) using a common templating software (Endomap): M1) 30mm metal ball lateral to the affected side at the height of the trochanter, M2) a fixed magnification of 121% (average of bucky system as derived from a pilot study) and M3) anatomic calibration by assigning to the natural femoral head of the contralateral side its diameter derived by an anatomic formula. The gender specific linear relationship between height (h [cm]) and the bony femoral head diameter (d [mm]) was obtained from a CT study (n=120): Males: d=0.156h+23.941, Females: d=0.154h+20.040). For each method applied by two independent observers, the implant’s metal head was measured and the relative error [%] calculated.

Results: The standard technique, calibration by a reference object produced a relative error (mean +/−SD) of 2.01+/−1.82% (max=7.9%). Fixed magnification had sign. less error at 1.41+/−1.3% (max=6.5%, p< 0.05). Anatomic calibration produced sign. higher errors at 2.77+/−1.96% (max=8.4%, p< 0.05). Inter-rater reliability was highest for the fixed magnification (r=0.93) and less for ball calibration (r=0.67) and anatomic calibration (r=0.52).

Discussion: It was shown once more that in THR fixed magnification of digital x-rays is on average more accurate and reliable than using a calibration object such as a ball. The theoretical benefit of individual calibration is lost by the variability in landmark palpation, object placement and patient movement though last can be limited by taping the object (e.g. coin) to the skin. Anatomic calibration based on the current formula cannot replace the use of calibration objects. However its error is within clinically tolerable ranges and it can be used when no calibration object is available or the system’s magnification is unknown. The error may be reduced by more accurate height measurements.