CASE REPORT
A 60-year-old, 215-lb man had a total hip replacement using a newly introduced cemented collared cobalt-chromium alloy femoral stem. A baseline periprosthetic DXA scan was performed 6 weeks postoperatively. Compared to baseline, the change in BMD in the Gruen zone 5 was −8.2%, +6.5%, +4.9%, and +9.46% at 3, 6, 12, and 24 months, respectively. In contrast, dramatic BMD loss was seen in Gruen zone 7 (calcar region): −33.2%, −40.8%, −37.1%, and −34.1% at 3, 6, 12, and 24 months, respectively. Similar findings in other patients led to discontinuation of use of this stem in favor of a collarless stem in which less BMD loss was seen in Gruen zone 7. Although additional technologist training is required and scans may not be reimbursable, for research purposes or for evaluating new component prototypes, the periprosthetic DXA scan capability can be useful.
Various other custom scans can be used to detect and quantify vertebral fractures (vertebral fracture assessment application), monitor healing of fractures by scanning through radiolucent cast materials, or for research purposes to assess BMD at unusual locations.21-23 Other new innovations, such as the ability to perform full-length scans of the femoral shaft and to quantify focal thickening of the lateral cortex to identify beaking, an abnormality associated with atypical femur fracture after long-term bisphosphonate use, continue to expand the utility of DXA. Using standard software, cadaver bones can be scanned prior to biomechanical testing for a variety of purposes, such as ensuring proper matching specimens in test groups. It has been reported that the common practice of using contralateral bone specimens can lead to bias, as the BMD can be significantly different in right and left bones from the same individual.9,24
TOTAL BODY BMD AND BODY COMPOSITION SCANS
Perhaps the least understood capability of DXA from our experience working with orthopedists is the ability to perform total body scans and to obtain not only total body and regional BMD but also body composition data, namely grams of lean and fat mass.25 Soft tissue (no bone pixels) is partitioned into fat and lean body mass by a calibration procedure (lean mass = total soft tissue –fat mass). DXA has become the standard for body composition assessment given the ease of data acquisition (a total body scan takes only a few minutes), accuracy, and precision of measurements. Compared with other methods (eg, skinfold thickness, bioelectrical impedance, and underwater weighing), it is the only method that gives regional values for fat mass, lean mass, and BMC (this allows the ability to compare left vs right sides).25-27 The ability to perform regional measurements cannot be overstated, as stable body weight belies potential changes with age and disease that relate to redistribution of fat and lean mass. It is not possible to identify, let alone track, such changes by measuring gross body weight on a scale or with BMI calculations. However, redistribution of fat and lean mass can be monitored in great detail using DXA. Figures 2 and 3 show the typical output from a DXA total body/body composition scan.
Total body scans with body composition analyses have many applications. For example, monitoring growth and development or treatment in patients with congenital deformity, metabolic bone disease, osteoporosis, and frailty; patients undergoing rehabilitation; and patients having surgery that could affect the use of a contralateral limb with potential hypertrophy or atrophy. Accurate assessment of percent body fat and fat distribution may help surgeons to improve risk stratification and surgical outcome.28-30 Fracture risk has been associated with muscle area.28 Simple measurements of quadriceps size underestimates atrophy, and total body composition can quantitate lean mass.30
In sports medicine, body composition assessments could be useful to monitor postoperative recovery and effectiveness of rehabilitation protocols after injury, effectiveness of conditioning and training programs, developmental changes due to sports participation, and for obtaining baseline assessment at the time of preseason physicals.27,31-34 In athletes, baseline status and morphological adaptations to training have traditionally been measured by anthropometry (eg, skinfold thickness, BMI, limb circumference, etc.), but DXA total body scanning allows for much more detailed assessments with the possibility of subregional quantitation. There is evidence for sports-specific body composition profiles and characteristic adaptations.27,31-34 Using DXA, adaptive changes as a result of training as well as changes and recovery after surgery or injury can be monitored. For example, quadriceps atrophy usually occurs to some extent after ACL repair, and bone mineral loss and muscle atrophy occur after a limb has been immobilized with a cast. DXA body composition assessment could be used to monitor leg lean mass after surgery for comparison with presurgery values or those of the contralateral noninjured side, or to track recovery of bone mineral and muscle after a cast is removed. Some technical sports, such as tennis and baseball pitching, are known to result in limb asymmetry; DXA body composition could be used to monitor development of right-left arm asymmetry in tennis players or baseball pitchers, and then measures could be taken to balance the asymmetry. Wrestlers and elite dancers are expected to maintain strict weight requirements, but diets are often poor, and as such, DXA body composition could be used to track the effects of dieting and training by comparing serial measurements to baseline to ensure that weight changes include preservation or gain of muscle mass.31
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