Summary

This study provides the first report on the normal values and distribution of posterior tibial displacement using kneeling stress radiography (KSR) in healthy knees, establishing an average displacement of 8.3 ± 3.95 mm, with all measurements showing positive values, supporting the use of KSR when a contralateral healthy knee is unavailable for comparison.

Abstract

Introduction

Posterior cruciate ligament (PCL) tears usually occur in the setting of multiligament knee injuries. Isolated PCL injuries are uncommon and account for not more than 1-5% of these injuries. Grading posterior knee instability is fundamental during the assessment of PCL injuries, it can be evaluated by physical exam or radiography. Physical examination is a very useful tool, but can be highly subjective. Stress radiographs are a validated tool providing an objective quantification of posterior knee laxity. Several techniques to objectify posterior instability have been described, but Telos® and the kneeling stress radiography (KSR) have shown the highest accuracy. One potential limitation of KSR is that the normal values and distribution in uninjured knees have not been described in the literature. This highlights the necessity of comparing findings with the contralateral healthy knee; that in many cases is not possible due to injuries or limitations in the contralateral knee. The aim of this study was to report the normal distribution of the posterior tibial displacement by performing a unilateral KSR on healthy knees.

Methodology

A descriptive cross-sectional study where KSR of adult healthy knees were measured (2017-2019). Inclusion criteria: patients >18 years, with no previous or acute injury and bilateral KSR meeting technical standards. Three observers participated in the study to determine interobserver reliability. To assess the equality of variances between the measurements of the three observers, Levene's test was performed. To assess the normal distribution of the measurements of each observer the Shapiro-Wilks test was performed. To determine the level of agreement that exists between the observers, the intraclass correlation coefficient (ICC) was performed. An alpha level of 0.01 was considered statistically significant.

Results

From the initial sample of 94 patients only 54 met inclusion criteria (23 had previous or concomitant knee injuries, and 17 did not meet KSR technical standards and were excluded). Median age was 36.5 years, 48 males (%). Intraclass correlation coefficient was 0.78 (CI=95% 0.68–0.86). Levene's test showed no significant differences in the variance of the measurements between the observers (p=0.15). The calculated mean posterior displacement was 8.30±3.95mm (0.3-18), with a lower limit of 1.67mm (5th percentile) and a positive value in all measurements. Evaluating normality with the Shapiro-Wilks test, given a non-significant p-value of 0.87, it can be concluded that the data are normally distributed.

Conclusion

Our study is the first to report the normal values and distribution of KSR, showing an average of 8.3 ± 3.95, with normal values consistently being positive in healthy knees. These findings facilitate the use of KSR in patients where a contralateral healthy knee is not available and lay the groundwork for future studies that utilize KSR after establishing its normalcy in healthy knees