Insall and Salvati set out to describe a simpler radiographic measure of patellar height than the methods available at the time.
Radiographic measurement study of lateral films of 114 normal knees.
Methods
Design
Radiographic normative study
Patients
114 normal adult knees
Selected from arthrography cases with meniscal tears
No radiologic osteoarthritis
Index test
Patellar tendon length (LT) on lateral radiograph
Diagonal patellar length (LP)
Outcomes
LT:LP ratio
Patellar length relative to femoral condylar width
Tendon length relative to tibial tubercle insertion height
Lower patellar pole relative to Blumensaat's line at 30° flexion
Key Findings
Patellar tendon length and diagonal patellar length were nearly equal in the 114 normal knees, with a mean LT:LP ratio of 1.02 (SD 0.13).
The lower patellar pole never lay on Blumensaat's line in the 44 knees flexed exactly 30°, and it sat above the line in every case.
Patellar length was close to constant relative to femoral condylar width, with a mean LP:WCBL ratio of 0.95 (SD 0.07).
Tendon length relative to tibial tubercle insertion height was also close to constant, with a mean LT:HI ratio of 1.85 (SD 0.24).
In an illustrative patient with chondromalacia, tendon length was 56 mm and patellar length 44 mm, a 27% difference that the authors called significant.
Board pearl
In a normal knee, patellar tendon length should not differ by more than 20% of diagonal patellar length, which the authors placed within the 98% confidence limits of normal.
Authors' conclusion
The ratio of the patella to the patellar ligament may therefore be used as an index of patella height.
Insall and Salvati set out to describe a simpler radiographic measure of patellar height than the methods available at the time.
Radiographic measurement study of lateral films of 114 normal knees.
Methods
Design
Radiographic normative study
Patients
114 normal adult knees
Selected from arthrography cases with meniscal tears
No radiologic osteoarthritis
Index test
Patellar tendon length (LT) on lateral radiograph
Diagonal patellar length (LP)
Outcomes
LT:LP ratio
Patellar length relative to femoral condylar width
Tendon length relative to tibial tubercle insertion height
Lower patellar pole relative to Blumensaat's line at 30° flexion
Key Findings
Patellar tendon length and diagonal patellar length were nearly equal in the 114 normal knees, with a mean LT:LP ratio of 1.02 (SD 0.13).
The lower patellar pole never lay on Blumensaat's line in the 44 knees flexed exactly 30°, and it sat above the line in every case.
Patellar length was close to constant relative to femoral condylar width, with a mean LP:WCBL ratio of 0.95 (SD 0.07).
Tendon length relative to tibial tubercle insertion height was also close to constant, with a mean LT:HI ratio of 1.85 (SD 0.24).
In an illustrative patient with chondromalacia, tendon length was 56 mm and patellar length 44 mm, a 27% difference that the authors called significant.
Board pearl
In a normal knee, patellar tendon length should not differ by more than 20% of diagonal patellar length, which the authors placed within the 98% confidence limits of normal.
Authors' conclusion
The ratio of the patella to the patellar ligament may therefore be used as an index of patella height.