Causes and Clinical Outcomes of Patellar Post Impingement

Patellar post impingement (PPI), which occurs when the post impinges on the patella, may reportedly cause poor total knee arthroplasty (TKA) outcomes. The causes of PPI and its effect on clinical outcomes and symptoms were investigated.

The study subjects were 100 patients who underwent TKA (65 posterior stabilized TKA, 35 bi-cruciate stabilized [BCS] TKA). Whether PPI occurred during surgery was investigated; the patients were then classified into a PPI+ group and a PPI- group, and whether the implant or patellar shape was related to the occurrence of PPI was examined. The measurement parameters included patellar shape, joint range of motion (ROM), and clinical outcome using the new Knee Society Score.

There were 38 knees in the PPI+ group and 62 knees in the PPI- group. There was no difference in patellar shape between the 2 groups, but PPI was more frequent in patients with short patellar tendon and those with low patellar height. In terms of implant type, the PPI+ group included 12 patients (18.5%) who underwent posterior stabilized TKA and 26 (74.3%) who underwent BCS TKA. There was no difference between the 2 groups in either joint ROM or new Knee Society Score. These results suggest that the position of the patella and implant shape that causes the post to be positioned anteriorly may contribute to PPI.

PPI occurred more frequently in knees with low patellar height and in patients who had undergone BCS TKA. PPI had no effect on joint ROM or clinical outcome.