Changes in patella tendon length over 5 years after different types of knee arthroplasty

Purpose Post-operative shortening of the patellar tendon resulting in an abnormally low-lying patella has been described previously, but the degree of change in patella tendon length over time and extent of its progression after different types of knee arthroplasties remains unknown. This study assesses the incidence of patella tendon length change following lateral unicompartmental knee arthroplasty (UKA), medial UKA, and total knee arthroplasty (TKA), and its impact on patient-reported outcome at 5 years post-surgery. Methods Immediate post-operative, 1- and 5-year radiographs were reviewed for 50 patients undergoing each operation ( n = 150), with the Insall–Salvati ratio used as a measure of patella tendon length. Clinical outcome was assessed using the Oxford Knee Score (OKS). Results At 5-year follow-up, no significant change in patella tendon length was found following medial UKA (1.07–1.05), whilst a significant shortening was found after TKA (1.02–0.96), and a significant lengthening found after lateral UKA (1.02–1.05). For the UKA groups, no further change in tendon length was observed after the first year post-surgery, whereas the TKA group continued to shorten between years one and five. OKS was significantly better in the UKA groups as compared to TKA group. Change in patella tendon length within individual groups did not correlate with OKS at 5 years. Conclusion Patella tendon length shortening is more prevalent in TKAs, whilst lengthening is more prevalent in lateral UKAs. Despite the vertical incision through the patella tendon, lateral UKAs do not cause increased tendon shortening at 5 years post-surgery. However, in the medium term, changes in patella tendon length do not affect patient-reported outcome. Level of evidence Retrospective, comparative study, Level III.