Clinical Accuracy of J-Sign Measurement Compared to Magnetic Resonance Imaging

The J-sign is defined as lateral patellar translation over the anterolateral femur proximal to the trochlear groove during active leg extension. Dynamic magnetic resonance imaging (MRI) techniques allow for quantification of the J-sign using a variety of published indices. However, to date, clinical quantification of the J-sign has not been reliably described. The purpose of the present study is to assess the accuracy of clinically quantifying the J-sign compared with objective MRI data.

All patients in this case series were indicated for Fulkerson osteotomy due to recurrent lateral patellar instability and examined preoperatively for the presence of J-sign. The J-sign was estimated by placing a finger on the lateral edge of the trochlea and estimating the lateral translation of the patella while the patient actively extended the knee from 30 degrees of flexion to maximum extension. Independent preoperative measurements were obtained by both the senior author and a resident and compared to dynamic MRI measurements read by independent investigators.

Preoperative physical examination for the presence of the J-sign was conducted on 10 patients (10 knees). The average difference between clinical and MRI J-sign measurement was 4.32 mm (range 0.2 - 10.4 mm). There was no significant difference between the clinical and MRI J-sign measurements (p = 0.2579). Clinical measurements of the J-sign differed by an average of 2.2 mm between the two examiners (range 0 - 5 mm).

Clinical quantification of the Jsign showed relative imprecision when compared with MRI measurements of the modified lateral patellar edge (LPE), though in several patients we did achieve accurate J-sign assessment. If further research can validate this technique as accurate and consistent using larger patient populations, it could aid in the development of surgical treatment plans for patients presenting with patellar instability, and serve as an objective assessment of alignment in the postoperative period.