Objective: The purpose of this study was to evaluate whether ultrasound allows precise assessment of a focal defect at the lateral patellar retinaculum (LPR) and to determine its anatomical significance. Materials and methods: This work was initially undertaken in four cadaveric specimens and followed by high-resolution ultrasound study in 48 healthy adult volunteers (96 knees) by two musculoskeletal radiologists. Dimension and location of the LPR defect and its relations to adjacent anatomical structures were analyzed. Results: A focal defect of the LPR through which vessels penetrated was constantly identified by ultrasound in our population. The mean transverse diameter of the defect was 2.4 mm (range, 1–6 mm). The defect was located a mean of 23.7 mm (range, 18–30 mm) proximal to the lateral tibial plateau and 6.6 mm (range, 4–9 mm) lateral to the patellar tendon. Anatomical dissection demonstrated that the LPR defect is related to perforating vessels that originate from the peripatellar anastomotic ring. Conclusions: The focal defect of the LPR can be depicted by ultrasonography. Reproducible anatomical landmarks for its detection could be defined. Our data suggest the LPR defect may be considered a normal variant rather than a pathologic chang