Patellar tendon length--the factor in patellar instability?

Patellar tendon length has been compared in 42 knees with a history of patellar dislocation, and 51 control knees. A lateral X-ray and a magnetic resonance image (MRI) were taken of each knee. The mean radiological patellar tendon length was 46 mm in the controls and 53 mm in the dislocation group. From MRI images, the mean was 44 mm in controls and 52 mm in the dislocation group. This means that the patellar tendon is significantly (P<0.0001) longer in patients with a history of patellar dislocation on both MRI and X-ray. There is no significant difference (P=0.52) between X-ray and MRI measurements of tendon length. The distance between the tibial plateau and the point of tendon insertion was also measured and found to be 28 and 29 mm in the control and dislocation groups, respectively. There is no significant difference between these two measures (P=0.19). In conclusion, patella alta is caused by a long patellar tendon rather than a low insertion into the tibia. Measuring the length of the patellar tendon using MRI is more specific and more sensitive than the Caton-Deschamps index for patellar instability.