From the Guidelines
The Merchant's line is used in X-rays to check if the patella is in the correct place. This line is drawn from the anterior superior iliac spine to the lateral aspect of the patella, and then to the tibial tubercle. In a normal knee, the patella should lie on or medial to this line. If the patella is positioned lateral to the Merchant's line, it suggests patellar malalignment or lateral displacement, which may indicate conditions like patellar subluxation or dislocation. The Merchant view X-ray, taken with the knee flexed at 45 degrees and the X-ray beam directed from superior to inferior, is particularly useful for evaluating patellar tracking and alignment, as recommended by studies such as 1. This view allows visualization of the patellofemoral joint and helps assess the relationship between the patella and the femoral trochlea. Proper evaluation of patellar position is important for diagnosing conditions like patellofemoral pain syndrome, chondromalacia patellae, and recurrent patellar instability.
Some key points to consider when evaluating patellar position include:
- The use of weight-bearing axial radiographs to better assess patellofemoral kinematics, as suggested by Baldini et al, cited in 1
- The importance of assessing patellar tilt or subluxation using axial radiographs, as noted in 1
- The value of standing AP and lateral views, as well as tangential axial views of the patellofemoral joint, in evaluating the position of the patella, as recommended in 1
Overall, the Merchant's line and Merchant view X-ray are essential tools for evaluating patellar position and tracking, and should be used in conjunction with other radiographic views to provide a comprehensive assessment of the knee joint, as supported by studies such as 1 and 1.
From the Research
Patellar Height Measurement
To check if the patella is in the correct place using X-ray, several measurement methods can be employed. The most commonly used methods include:
- Insall-Salvati ratio
- Caton-Deschamps ratio
- Blackburne-Peel index
- Patellotrochlear index
Measurement Methods
These methods involve measuring the distance between the patella and the tibia or femur on a lateral X-ray of the knee. The Insall-Salvati ratio, for example, measures the ratio of the patellar length to the distance between the patellar apex and the anterior tibial surface 2, 3, 4, 5.
Reliability of Measurement Methods
Studies have shown that the Insall-Salvati ratio has high inter- and intra-observer reliability on both X-ray and MRI 3, 4. The patellotrochlear index also shows good inter- and intra-observer reliability on MRI 4. However, other measurement methods may have lower reliability and may not be suitable for use on both X-ray and MRI 5.
Simplified Measurement Method
A simpler method, known as the plateau-patella angle, has been proposed as an alternative to the traditional ratios 6. This method involves measuring the angle between the patellar surface and the tibial plateau, and has been shown to have high inter- and intra-observer reliability.
Key Findings
Key findings from the studies include:
- The Insall-Salvati ratio is a reliable method for measuring patellar height on both X-ray and MRI 3, 4
- The patellotrochlear index is a reliable method for measuring patellar height on MRI 4
- The plateau-patella angle is a simple and reliable method for measuring patellar height on X-ray 6
- Different measurement methods may have different reliability on X-ray and MRI 5