Imaging the femoral sulcus with ultrasound, CT, and MRI: reliability and generalizability in patients with patellar instability

Recent advances in surgical intervention for patellar instability have led to a need for long-term radiological monitoring. The aim of this study is to determine whether or not magnetic resonance imaging (MRI) or ultrasound (US) can replace computed tomography (CT) as the standard of care for the evaluation of the femoral sulcus. This was a prospective study comparing the reliability of CT, magnetic resonance (MR), and US for measuring the femoral sulcus in patients with patellar instability. Twenty-four patients were recruited to undergo a CT, MR, and US examination of each knee. Two observers independently measured femoral sulcus angles from subchondral bone and hyaline cartilage on two occasions. Intraclass correlations and generalizability coefficients were calculated to measure the reliability of each of the techniques. Thereafter, two observers measured the femoral sulcus angle from ultrasound images recorded by two independent operators to estimate interobserver and interoperator reliability. Forty-seven knees were examined with CT and US and 44 with MRI. The sulcus angle was consistently smaller when measured from subchondral bone compared to cartilage (5-7A degrees). Interobserver reliability for CT, MR, and US measurements from subchondral bone were 0.87, 0.80, and 0.82 and from cartilage 0.80, 0.81, and 0.50. Generalizability coefficients of measurements from subchondral bone for CT, MR, and US were 0.87, 0.76, and 0.81 and for cartilage 0.76, 0.73, and 0.05. Most of the variability in the US occurred at image acquisition rather than measurement. In patients with patellar instability, CT and MR are reliable techniques for measuring the femoral sulcus angle but US, particularly of the articular cartilage, is not. MR is therefore the most suitable tool for longitudinal studies of the femoral sulcus.