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 < 0.001) for the meniscal-bearing knees than for the rotating-platform knees. After the 1-year follow-up, there were no further substantial changes in the ISR in either group. There was also no significant correlation between the ISR and clinical outcomes.
Conclusions: LPT decreased after both mobile-bearing TKAs for up to 1 year postoperatively, although the decrease was more significant for the meniscal-bearing knees than the rotating-platform knees. After 1 year postoperatively, however, these conditions had stabilized, with no further changes. Thus, LPT plays a minimal role in mid-to-long-term clinical outcomes after TKA.
Level of evidence: Therapeutic prospective study, Level II.
Keywords: Hospital for Special Surgery score; Insall–Salvati ratio; Mobile-bearing TKA; Patellar tendon length; Range of motion.