Correlation Between the Insall-Salvati Ratio and Functional Outcomes After Total Knee Arthroplasty

TL;DR

This study found that changes in patellar height after knee replacement surgery are linked to worse stair-climbing ability in patients.

Contribution

The study identifies a correlation between reduced patellar height (measured by the Insall-Salvati ratio) and poorer functional outcomes after total knee arthroplasty.

Findings

Reduced Insall-Salvati ratio correlated with lower stair-climbing and functional scores after surgery.

Patella baja developed in 17.5% of knees postoperatively, with females twice as likely to develop it.

Patients with normalized patellar height showed improved stair-climbing function.

Abstract

Background

Total knee arthroplasty (TKA) is a highly successful and cost-effective orthopedic procedure. This study aimed to assess the correlation between the Insall-Salvati ratio (ISR), a reliable measure of patellar height, and TKA outcomes.

Methodology

This prospective observational study was conducted at a tertiary care hospital in India for one year. Consecutive patients undergoing primary TKA for degenerative knee conditions were included. A total of 200 knees were analyzed. Patellar height was assessed using the ISR on standardized lateral knee radiographs obtained at approximately 30° of flexion, both preoperatively and postoperatively. Clinical outcomes were assessed using range of motion, pain scores, Knee Society Scores (KSS), and stair-climbing ability. Statistical analysis was performed to evaluate the correlation between ISR changes and postoperative functional outcomes.

Results

The study evaluated 200 TKAs in 122 males and 78 females with a mean age of 67.8 years. Postoperatively, 17.5% of knees showed patella baja, 79% had normal ISR, and 3.5% exhibited patella alta. Among knees initially classified as normal or patella alta, 10.67% developed postoperative patella baja. Females were twice as likely as males to develop new-onset patella infera, with a higher incidence (12.8% vs. 8.19%). No significant difference in ISR reduction or clinical outcomes was observed between posterior cruciate substituting and retaining implants. However, reduced ISR correlated with poorer stair-climbing and functional scores, while KSS, pain, and flexion remained unaffected. Patients whose preoperative patella baja normalized postoperatively showed functional improvement in stair-climbing.

Conclusions

TKA led to a reduction in patellar tendon length, reflected by a decreased ISR, in approximately half of the patients. This reduction was associated with significantly lower postoperative stair-climbing and function scores, while KSS, pain levels, and range of motion remained unaffected. Patella infera was twice as prevalent in female patients compared to males.