[Quadriceps dysplasia and patellar tilt in objective patellar instability].

PURPOSE OF THE STUDY

Dysplasia of the vastus medialis, a well-known landmark of patellar instability, is difficult to point out pre-operatively. We propose the measurement of patellar tilt on the CT scans in order to appreciate abnormality.

MATERIAL AND METHODS

We studied 3 well defined groups: 143 knees operated on for a true dislocation of the patella, either for the first time or a recurrent episode, 67 asymptomatic and nonoperated contralateral knees and 54 control knees. The patellar tilt in extension was measured on the CT-scan with the quadriceps relaxed and contracted.

RESULTS

The results showed the increase of the patellar tilt as an characteristic factor of patellar instability (28.8 + 10.5 degrees against 11.8 degrees + 5.7 degrees in the control group). Patellar tilt was not a consequence of the dislocation because it was also significantly increased in the asymptomatic contralateral group (17 degrees + 9 degrees). Quadricipital contraction increased the patellar tilt only in the two groups of patellar instability (+ 6 degrees) and asymptomatic contralateral group (+ 13 degrees) but not in the control group (+ 1.6 degrees). The mean of the relaxed and the contracted quadriceps patellar tilt includes the dynamic trouble. We propose the threshold of 20 degrees to determine a pathological patellar tilt. In this case, sensibility is 90 per cent and the specificity is 91 per cent. In the other patellar instability factors, only severe trochlear dysplasias involved the patellar tilt.

DISCUSSION

We think that the patellar tilt in extension is a landmark of a functional disorder of the whole quadriceps muscle more than the vastus medialis only. The mean of the relaxed and contracted quadriceps patellar tilt measures permitted to point out the border cases of this functional abnormality. This measurement is reliable and can be considered pathologic above 20 degrees. The results of Insall's muscular plasty were only symptomatic because this procedure could not correct the effect of the quadricipital contraction.