Although infraspinatus (ISP) is a muscle that may be associated with loss of muscle strength in rotator cuff (RC) tear patients, teres minor (TM) should also play a role in the postoperative disturbance of external rotaion. However, few reports have evaluated the role of TM in patients with RC tear. The purpose of this study was to measure the volume of TM using MRI to obtain baseline data that we can use to evaluate the influence of TM in the postoperative results of RC tear. Seventy-four shoulders (45 males and 29 females) with small RC tears were studied. We have measured cross-sectional area (CSA) of RC muscle using the oblique sagital image of MRI. Fatty infiltration of the muscle was evaluated according to the classification of Goutallier. The CSAs of males were larger than those of females. To revise the CSAs under gender gap, we divided the CSAs by each patient's height or weight. However, we could not revise gender gap in this method. Next, we divided the CSA of TM by that of another RC muscle. We can revise this gender gap clearly by dividing by subscapularis (SSC). There is only one patient with fatty infiltration in TM and 5 in SSC, whereas 45 in supraspinatus (SSP) and 31 in ISP. Then we divided the CSA of TM into two groups, with or without fatty infiltration in another RC muscle. We recognized unevenness in the data of the TM/ISP and TM/SSC with infiltration. We have shown that we can revise the CSA of TM by dividing of another RC without fatty infiltration. The CSA of TM by revision of another RC muscle without atrophy was 61.1 and 52.6% of that of SSP, 41.3 and 38.4% of ISP, and 21.5 and 21.3% of SSC, male and female respectively.
