Achilles insight

The BMD (g/cm²) of the calcaneus in the non-dominant foot was measured using an Achilles InSight ultrasound device (Achilles InSight, GE, Fort Myers, FL, USA). The T-score was defined as follows: (individual’s BMD—young-adult mean BMD)/standard deviation of the young-adult normal population. A T-score > −1.0 was defined as normal BMD, osteopenia as between −1.0 and −2.5, and osteoporosis as <−2.5.

The speed of sound (SOS), broadband ultrasound attenuation (BUA), and stiffness index of the calcaneus (SI) were measured using an ultrasound bone densitometer (Achilles InSight; GE Healthcare, Little Chalfont, UK). I used an ultrasound bone densitometer because it has no side effects and correlates well with the dual-energy X-ray absorptiometry (DXA)-measured bone mineral density (BMD) [19 –24 ] or bone mineral contents (BMC) [24 ] as well as quantitative computed tomography (QCT) [25 (link)]. The densitometer was operated by a researcher experienced with taking measurements, and the final intra-sample mean coefficient of variation for the stiffness index was 0.7%.
The subjects’ height (determined with a DST-210 N; Muratec-KDS Corp., Kyoto, Japan) and weight, body fat percentage, and muscle mass (determined with a DC-320; Tanita Corp., Tokyo, Japan) were measured after shoes had been removed, and then the body mass index (BMI) was calculated.

The estimated BMD (g/cm²) was evaluated by using ultrasound (Achilles InSight, GE, Madison Heights, USA) at the heel calcaneus [23 (link)]. The formula used was T score = (individual’s BMD—mean BMD in young adults)/standard deviation (SD) of a normal young adult population. The presence of osteoporosis was defined as a T score ≤ −2.5 SD below the young adult level.

The calcaneus QUS-SOS, the calcaneus QUS-BUA, and the calcaneus QUS-SI were measured using an ultrasound bone densitometer (Achilles InSight; GE Healthcare, Little Chalfont, UK). The calcaneus QUS-SOS was used to assess bone density in this study since this value is more important for supporting the value of the calcaneus QUS-SI than the calcaneus BUA and is also used as an indicator for bone density and clinically as an evaluation index. We measured the calcaneus which mostly consists of cancellous bone. If such cancellous bone is frangible, then it will often cause osteoporosis. We used an ultrasound bone densitometer because it has no side effects and correlates well with the DXA-measured BMD^{23 (link),24 (link)} or bone mineral contents (BMC)^{23 (link)}. The densitometer was operated by one researcher experienced in taking such measurements.

BMD (g/cm²) of the calcaneus was measured in the non-dominant foot using ultrasound (Achilles InSight, GE, USA). The T-score was calculated as follows: (the participant’s BMD—mean BMD in young adults)/standard deviation of a normal young-adult population.

Ultrasound measurements were performed on both calcanei using a bone ultrasonometer (Achilles InSight, GE Medical Systems Ultrasound; GE Health Care, Chalfont St Giles, United Kingdom). Two devices without systematic differences were used. Alcohol was used as the coupling agent. The broadband ultrasound attenuation (BUA), which is the frequency-dependent attenuation of the sound waves, and the speed of sound (SOS) were measured. The stiffness-index was calculated as (0.67 × BUA + 0.28 × SOS) − 420. It describes the individual stiffness, which is then compared to the reference mean stiffness of young adults. This approach is analogue to the concept of the T-score value estimated from dual-energy X-ray absorptiometry (DXA). The reference stiffness was provided by Achilles InSight and contains sex-specific data for Germany. Based on the stiffness-index, the risk of osteoporotic fractures was divided in three categories: high risk (below reference mean minus 2.5 SDs), moderate risk (reference mean minus 1 to 2.5 SDs) and low risk (higher than reference mean minus 1 SD). For all statistical analyses, data from the foot with the lower stiffness-index was used. All measurements were performed by trained examiners on both feet on seated participants. All examiners were certified and underwent annual recertifications.

In the cross-sectional analysis, we scrutinized the relationship between BMI and the prevalence of osteoporosis. In the longitudinal analysis, the primary end point was the development of osteoporosis. The definition of osteoporosis in this study involved measuring estimated BMD (g/cm²) through calcaneus quantitative ultrasound (QUS) (Achilles InSight, GE, USA). The T-score was calculated using the formula: the individual’s BMD minus the mean BMD in young adults, divided by the standard deviation (SD) of a normal young-adult population^{21 (link)}. Osteoporosis was diagnosed when the T-score was ≤ − 2.5 SD below that of young adults.