Identification and biometry of horizontal extraocular muscle tendons using optical coherence tomography

To purpose if this study was to determine whether the horizontal rectus muscle tendons (HRMTs) can be observed using anterior segment optical coherence tomography (AS-OCT) and to determine the repeatability of its measurements. Also, this study aimed to observe and measure the different external ocular structures at the level of the horizontal rectus muscle (HRM) insertion. This was a retrospective, observational, descriptive and comparative study. Images were obtained utilizing the RTVue 100 CAM system. Eyes were analyzed at the three and nine o’clock position. Scans were performed for three different locations: the limbus, the ciliary body and the equator. All scans were analyzed by two graders, separately and blinded. Measurements were performed for: HRMT length; HRM thickness; conjunctival epithelium thickness; conjunctiva and Tenon’s capsule thickness; scleral thickness; and external ocular thickness. Results were obtained from twenty eyes of ten volunteers. The conjunctival epithelium thickness was 52.33 μm, the total conjunctiva/Tenon’s capsule thickness was 313.54 μm, the medial rectus (MR) thickness was 136.63 μm and the lateral rectus (LR) thickness was 181.65 μm. The MR tendon length was 1,426.88 μm, the LR tendon length was 1,433.65 μm, the scleral thickness was 489.91 μm and the total external ocular structure thickness was 785.17 μm. Intra-observer reproducibility (intraclass correlation coefficient [ICC]) for tendon length was 0.993 for grader #1, 0.989 for grader #2; the muscle thickness ICC was 0.990 for grader #1 and 0.981 for grader #2. The inter-observer reproducibility ICC for tendon length was 0.557; the ICC for muscle thickness was 0.834. It is possible to visualize and measure HRMTs using AS-OCT. Measurements of the HRM, as well as the surrounding external ocular tissues, can be achieved.