Insall–Salvati ratio stabilizes one year after mobile-bearing total knee arthroplasty and does not correlate with mid-to-long-term clinical outcomes

Purpose It has not been established whether changes in the length of the patellar tendon (LPT) after total knee arthroplasty (TKA) affect clinical outcomes. Therefore, this prospective cohort study aimed to evaluate changes in the LPT over time postoperatively and clarify their impact on clinical outcomes after bilateral TKA, performed with differently designed mobile-bearing (meniscal-bearing and rotating-platform) implants on contralateral knees. Methods 51 patients who required staged bilateral mobile-bearing TKA were recruited. LPT was evaluated by measuring the Insall–Salvati ratio (ISR) preoperatively and at 1 week, 6 months, and 1, 2, and ≥ 5 years postoperatively. Hospital for Special Surgery score and range of motion were assessed at the final follow-up. Results Based on the ISR, there were no differences in the patterns of change between the two types of implant (n.s.). At the 6-month follow-up, the ISRs of both implants had significantly decreased ( p = 0.002). Throughout the follow-up, the ISR was significantly lower ( p