Statistical shape modeling of the large acetabular defect in hip revision surgery

The assessment of three-dimensional bony defects is important to inform the surgical planning of hip reconstruction. Mirroring of the contralateral side has been previously used to measure the hip center of rotation (CoR). However, the contralateral side may not be useful when diseased or replaced. Statistical Shape Models (SSMs) can aid reconstruction of patient anatomy. Previous studies have been limited to computational models only or small patient cohorts. We used SSM as a tool to help derive landmarks that are often absent in hip joints of patients with large acetabular defects. Our aim was to compare the reconstructed pelvis with patients who have previously undergone hip revision. This retrospective cohort study involved 38 patients with Paprosky type IIIB defects. An SSM was built on 50 healthy pelvises and used to virtually reconstruct the native pelvic morphology for all cases. The outcome measures were the difference in CoR for (1) SSM versus diseased hip, (2) SSM versus plan, and (3) SSM versus contralateral healthy hip. The median differences in CoR were 31.17 mm (interquartile range [IQR]: 43.80-19.87 mm), 8.53 mm (IQR: 12.76-5.74 mm), and 7.84 mm (IQR: 10.13-5.13 mm), respectively. No statistical difference (p > 0.05) was found between the SSM versus plan and the SSM versus contralateral CoRs. Our findings show that the SSM model can be used to reconstruct the absent bony landmarks of patients with significant lysis regardless of the defect severity, hence aiding the surgical planning of hip reconstruction and implant design.