The effect of ankle oedema on bone ultrasound assessment at the heel

Relatively inexpensive, portable bone ultrasound systems are of particular relevance to disabled or elderly subjects, who may have problems with access to other forms of densitometry. The effects of local soft tissues on ultrasound measurements are poorly understood and, as ankle oedema is common in such subjects, we examined its consequences for bone ultrasound readings at the heel. Eleven elderly subjects (mean age 81 years) with below-knee pitting oedema were assessed using a direct-contact bone ultrasound system (CUBA Clinical). We made a total of 16 series of readings, 6 unilateral and 5 bilateral. In each series an initial reading was followed by repeated pressure over the measurement site to disperse oedema; subsequent readings were thus subject to a progressively lessening degree of local oedema, until a steady state was eventually reached in which no further oedema could be displaced. Heel width fell by a mean of 6.3 mm between initial and steady-state readings; consistent with the clinical appearance of moderate oedema, pitting to a mean depth of only 3.15 mm. Measurements in the presence of oedema were compared with those after its elimination, and oedema was shown to cause a mean reduction of 23.9 m/s in velocity of sound (VOS) and of 5.5 dB/MHz in broadband ultrasound attenuation (BUA). Both changes were equivalent to a fall by a quarter of one standard deviation of the reference range, and were significant at p < 0.05 on paired t-test. As the severity of oedema will vary through the day, and from day to day, measurement protocols for bone ultrasound should pay attention to the confounding effects of oedema.