The effect of the Insall-Salvati ratio on outcome after total knee arthroplasty.

The effect of total knee arthroplasty (TKA) on the Insall-Salvati ratio (ISR) and the effect of the ISR on the outcome of TKA have not been clearly established. A retrospective review of 1055 primary TKAs performed in 1997 to 1998 was performed. Radiographic measurements were made preoperatively and postoperatively, and the ISR was calculated. Regression analysis was performed to determine the effects of these variables on range of motion (ROM), Knee Society Score, and stair, function, and pain scores. Total knee arthroplasty resulted in a decrease in the patella tendon length, as measured by the ISR in 50% of cases. Patella infera (ISR less than 0.8) developed postoperatively in 9.8% of TKAs and was twice as likely to occur in women as men. A decrease in the ISR was associated with diminished stair and function scores (P = .0004 and 0.0081, respectively). There was no effect of the ISR upon ROM, Knee Society Score, or pain scores. Optimal outcomes occurred in patients where the ISR was not decreased after TKA, with superior stair and function scores.