PERIACETABULAR OSEOLYSIS MEASUREMENT IN UNCEMENTED THR – A CT BASED STUDY

Abstract

Periacetabular osteolysis is now considered one of the major long term complications following uncemented total hip replacement. Radiographs are inaccurate and lack sensitivity in detecting lesions even with multiple views. Very few clinical studies have shown the use of CTscan for measuring these lesions. We report our clinical experience with CT based algorithm for measuring it.

Twenty two patients (32 hips) who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean follow up was 5.4 yrs. Of the 34 hips,17 were polyethylene bearings and 15 were ceramic bearings. Nine patients had bilateral replacement in this group. Using custom reconstruction software, 3D models were created and volume measurements made after identifying the lesions in the slices and painting them using appropriate tools available in the software.

Accuracy of the method was assessed by measuring the volume of artificial cavities created on polyurethane pelvis with and without the components. In our control experiments, a high correlation between the test and standard measurements was noted in the cavities above the component, while medial to the acetabular component in bilateral cases it was difficult to be accurate, with cavities less than 10mm in diameter being hard to detect reliably.

In our clinical group of 32 hips, degenerative cysts were noted in 13, secondary rheumatoid cysts in 2 and wear cysts were noted in 2, the largest having a maximum dimension of 10mm. All the degenerative cysts were in the peripheral zone and both the wear cysts were seen in the central zone communicating with the screw holes. These cysts were identified by the characteristic absence of sclerosis surrounding the cyst and obvious communication with screw holes. Both the wear cysts were found with polyethylene bearings at a minimum of 5yrs follow up.

The mean volume of the degenerative cysts was 799 mm3 (71–3500) and the mean volume of the wear cysts was 567 mm3 (550–585)

The low dose CT method we describe and the results we report show that cavities can be measured reliably, above or below the acetabular component. On the medial side, in bilateral cases in particular, although location is possible, volumetric analysis of anything less than 10mm in diameter is not.

Regarding surveillance strategy for wear cysts, we have established that in this series the incidence is 14%, with one at 5 yrs and another noted at 12 yrs, with 10mm in maximum dimension. The absence of any wear cysts at all in the ceramic group, albeit after a shorter follow up of only 5 years is encouraging.

Based on these figures, with these implants, we would recommend that there is no need to undertake surveillance more frequently than every 10 years.