Merchant et al. (1974) introduced a standardized axial radiographic technique for patellofemoral imaging and defined the congruence angle to quantify patellar position relative to the intercondylar sulcus. The study established a normal reference range from 100 asymptomatic subjects and validated a threshold for abnormal lateral subluxation against a pathologic population.
Before 1974, no standardized method existed to objectively quantify mild lateral patellar subluxation on plain radiographs. Prior techniques (Settegast, Hughston) required painful acute flexion, introduced beam-angle distortion, or lacked rotational control — and minor subluxation had no objective threshold.
When you order a patellofemoral axial view, use this technique: 45° flexion, 30° beam, quadriceps relaxed. Tell the technologist explicitly. A contracted quad during exposure is the most common source of false-negative films in patients with instability, and it happens silently.
When you measure the congruence angle on a Merchant view, +16° is your cutoff. Values beyond this are abnormal at the 95th percentile and support lateral subluxation in the appropriate clinical context.
The congruence angle is one piece of a larger puzzle. Some dislocators fall within the normal range, and the authors frame it as one component of extensor mechanism evaluation alongside patella alta and sulcus angle. No single number replaces clinical judgment.
Merchant et al. (1974) introduced a standardized axial radiographic technique for patellofemoral imaging and defined the congruence angle to quantify patellar position relative to the intercondylar sulcus. The study established a normal reference range from 100 asymptomatic subjects and validated a threshold for abnormal lateral subluxation against a pathologic population.
Before 1974, no standardized method existed to objectively quantify mild lateral patellar subluxation on plain radiographs. Prior techniques (Settegast, Hughston) required painful acute flexion, introduced beam-angle distortion, or lacked rotational control — and minor subluxation had no objective threshold.
When you order a patellofemoral axial view, use this technique: 45° flexion, 30° beam, quadriceps relaxed. Tell the technologist explicitly. A contracted quad during exposure is the most common source of false-negative films in patients with instability, and it happens silently.
When you measure the congruence angle on a Merchant view, +16° is your cutoff. Values beyond this are abnormal at the 95th percentile and support lateral subluxation in the appropriate clinical context.
The congruence angle is one piece of a larger puzzle. Some dislocators fall within the normal range, and the authors frame it as one component of extensor mechanism evaluation alongside patella alta and sulcus angle. No single number replaces clinical judgment.