Correlation of valgus stress radiographs with medial knee ligament injuries: an in vitro biomechanical study.

BACKGROUND The amount of medial compartment opening for medial knee injuries determined by valgus stress radiography has not been well documented. The purpose of this study was to develop clinical guidelines for diagnosing medial knee injuries using valgus stress radiography. HYPOTHESIS Measurements of medial compartment gapping can accurately differentiate between normal and injured medial structure knees on valgus stress radiographs. STUDY DESIGN Controlled laboratory study. METHODS Valgus stress radiographs were obtained on 18 adult lower extremities using 10-N.m and clinician-applied valgus loads at 0 degrees and 20 degrees of flexion to intact knees and after sequential sectioning of the superficial medial collateral ligament proximally and distally, the meniscofemoral and meniscotibial portions of the deep medial collateral ligament, the posterior oblique ligament, and the cruciate ligaments. Three independent observers of different experience levels measured all of the radiographs during 2 separate occasions to determine intraobserver repeatability and interobserver reproducibility. RESULTS Compared with the intact knee, significant medial joint gapping increases of 1.7 mm and 3.2 mm were produced at 0 degrees and 20 degrees of flexion, respectively, by a clinician-applied load on an isolated grade III superficial medial collateral ligament simulated injury. A complete medial knee injury yielded gapping increases of 6.5 mm and 9.8 mm at 0 degrees and 20 degrees , respectively, for a clinician-applied load. Intraobserver repeatability and interobserver reproducibility intraclass correlation coefficients were .99 and .98, respectively. CONCLUSION Valgus stress radiographs accurately and reliably measure medial compartment gapping but cannot definitively differentiate between meniscofemoral- and meniscotibial-based injuries. A grade III medial collateral ligament injury should be suspected with greater than 3.2 mm of medial compartment gapping compared to the contralateral knee at 20 degrees of flexion, and this injury will also result in gapping in full extension. Clinical Significance Valgus stress radiographs provide objective and reproducible measurements of medial compartment gapping, which should prove useful for definitive diagnosis, management, and postoperative follow-up of patients with medial knee injuries.