Inadequacy of computed tomography for pre-operative planning of patellofemoral arthroplasty

Purpose: To evaluate the accuracy of preoperative planning for patellofemoral arthroplasty (PFA) by comparing: (1) virtual implant positioning simulated on pre-operative images versus (2) real implant positioning from post-operative images. Methods: The authors prospectively studied 15 patients that received a PFJ implant (Tornier, Montbonnot France). A pre-operative planning software was established to determine the size and position of the trochlear component. Pre-operative scans were used to perform virtual implantations by two different operators, which were then compared to the post-operative scans to calculate errors (ε) in implant positioning and intra-class correlation coefficients (ICC) for intra- and inter-observer repeatability. Results: Analysis was performed for 13 patients, for whom agreement between virtual and real surgery was excellent for anteroposterior (AP) position (ICC = 0.84; εmax = 3.5 mm), fair for proximodistal (PD) position (ICC = 0.50; εmax = 9.5 mm), and poor for mediolateral (ML) position (ICC = 0.07; εmax = 9.0 mm). It was fair for flexum–recurvatum (FR) alignment (ICC = 0.53; εmax = 8.2°), poor for varus–valgus (VV) alignment (ICC = 0.34; εmax = 10.0°), and internal–external (IE) rotation (ICC = 0.34; εmax = 10.6°). Conclusions: Pre-operative planning was insufficiently accurate to follow intra-operatively, the greatest errors being angular alignment (VV and FR). The clinical relevance of these findings is that PFA is difficult to plan pre/operatively due to non-visibility of cartilage on CT scans and to trochlear dysplasia in most cases. Level of evidence: Prospective evaluation of operative tools on consecutive patients, Level III.