[Patella height after total knee replacement: influence of the radiological setting]

Acquired patella baja is a possible cause of painful total knee arthroplasty (TKA) and a poor functional outcome. Standardized lateral radiographs - often missing in the daily routine - are required for evaluating the patella height. It is unknown which patella index is insensitive to the radiological setting so that it can be used efficiently in the clinical routine.

This prospective study included 25 patients after TKA. Their average age was 69.5+/-8.8 (51-80) years. In two cases (8%), resurfacing of the patella was performed. The modified Insall-Salvati index (mIS) and the Caton-Deschamps index (CD) were determined and compared on lateral radiographs with the patient's knee extended while lying down, extended while standing, and 30 degrees flexed while lying down.

The average mIS in extension was 1.63+/-0.31 with the patient lying and 1.68+/-0.31 with the patient standing. In 30 degrees flexion and lying, it was 1.7+/-0.3. The mIS on the recommended radiographs (30 degrees flexion) correlated well with the mIS in extension while lying (R=0.48, p<0.001) and standing (R=0.99, p<0.001). The CD in extension was 0.78+/-0.21 while lying and 0.49+/-0.12 while standing; it was 0.48+/-0.15 in 30 degrees flexion. The CD on the recommended radiographs (30 degrees flexion) correlated satisfactorily with the CD in extension while standing (R=0.93, p<0.001) but not in extension while lying (not significant).

The mIS is less dependent on the radiological setting than the CD is. The mIS can be used efficiently for evaluating an acquired patella baja in radiographs performed in different, not standardized, conditions.