Some reports evaluated muscle volume around the hip after THA using CT and reported that muscle volume was increased after THA [9, 10, 12, 13]. Our results also found significantly increased muscle volume after THA in the gluteus maximus and quadriceps. This may be because alleviation of hip pain leads to improvement of daily activity, which thus accelerates the increase of muscle volume. Conversely, in this study, HU of the gluteus maximus and quadriceps did not change from pre- to post-operation. Our surgical approach was intermuscular invasion between the gluteus medius and tensor fasciae latae, which prevented injury to the posterior muscles and quadriceps as well as adipose degeneration of the muscles. This study only assessed the gluteus maximus and quadriceps because these muscles are essential for posture retention and gait and excluded other hip muscles, such as the gluteus medius, tensor fasciae latae, inter obturator, and external obturator, which could have been injured by the surgical approach.
Although CT can evaluate muscle volume and adipose degeneration, examination costs and radiation exposure are concerns, and CT can only assess muscle volume planarly. BIA has been used for evaluating metabolic disease and obesity and can assess body composition safely and simultaneously by taking advantage of microcurrent electricity [16, 32, 33]. BMI also has been used for assessing metabolic disease. However, metabolically health and abnormal obesity can not be distinguished, because the FMI and FFMI can not be distinguished by BMI . Merchant et al. reported that FMI was associated with higher rate of sarcopenia, and there was a possibility that the FFMI and FMI were more useful for predicting functional outcome in prefrail patients than BMI . Preoperative FMI was found to correlate with both muscle volume and fatty degeneration after THA in this study. Conversely, preoperative FFMI was only associated with the muscle volume of the quadriceps. From these results, a detailed assessment of body parameters may be important to assess postoperative muscle volume and degeneration. Additionally, preoperative FMI may be a better predictor of muscle volume and fatty degeneration around the hip after THA than FFMI.
PhA is affected by nutritional status  and several health indicators  and deeply related to muscle mass  and muscle quality . Lower PhA values indicate an increase in the extra water and a decrease in muscle mass [38, 39]. PhA has also been associated with osteoarthritis severity , functional ability [23, 34, 41, 42], and Barthel’s index . It is reported that PhA was associated with muscle strength  and quadriceps strength . Therefore, there is a possibility that PhA might be a useful predictor for screening physical function . In this study, preoperative PhA correlated with postoperative HU of both the gluteus maximus and quadriceps, and muscle volume of the quadriceps. These results also indicate that preoperative PhA may be a useful prognostic tool for evaluating postoperative muscle volume and fatty degeneration around the hip. From our study, Preoperative interventions to decrease FMI (maintaining proper weight) and increase FFMI (maintaining and increasing skeletal muscle mass) and PhA (improving the nutritional balance) may have a positive impact on increasing muscle volume and preventing fatty degeneration of muscles.
There were some limitations to this study. First, this study was a retrospective study and the sample size was small. However, to the best of our knowledge, this study is the first report to assess body composition before and after THA. Thus, we believe that this study provides new insight into the prediction of skeletal muscle volume and adipose degeneration after THA. Second, our patients had multiple diagnoses, which may have influenced the muscle volume and adipose degeneration after THA. Third, although it has been reported that PhA varies dependent on BMI, the limited sample size restricted BMI variance in this study. Westphal et al. reported that PhA tended to increase when BMI was < 35 kg/m2 and decrease when BMI was > 35 kg/m2 . PhA has also been inversely associated with percentage body fat  and the degree of obesity . However, there was no patients whose BMI was 35 > kg/m2 and significant correlation between preoperative FMI and PhA was not seen in this study. Fourth, postoperative BIA analysis can be affected by THA, because BIA is based on the measurement of impedance of body tissues to an applied electric current of low intensity. Although the electric resistance of fat tissue is very high, the resistances of muscle tissue and metal are low. Thus, postoperative muscle mass may be overestimated. Although this factor could not be eliminated completely, we unified implantation to minimize the effect of the implant.