Stress radiography of medial knee instability provides a reliable correlation with the severity of injury and medial joint space opening-A robotic biomechanical cadaveric study.

Abstract

Purpose: The medial collateral ligament (MCL), and posterior oblique ligament (POL) are the primary valgus stabilisers of the knee, and clinical examinations in grading valgus instability can be inherently subjective. Stress radiography of medial-sided knee injuries provides objective diagnosis and was analysed in this study. We hypothesised that (1) medial joint space opening would increase cutting the superficial MCL (sMCL), POL and anterior cruciate ligament (ACL); (2) isolated deep MCL (dMCL) injury would not increase medial joint space opening; (3) medial joint space opening would increase at higher flexion angles.

Study design: Controlled laboratory study.

Methods: Ten human cadaveric knees were dissected, preserving ligamentous structures, muscles and fascia. The femur was secured, and the tibia was attached to the six-degree-of-freedom robot. A 10 Nm valgus torque was applied at 0°-45° of flexion and anterior-posterior (a.p.) radiographs were taken. Sequential sectioning was performed on the dMCL, sMCL, POL and ACL. Medial joint space opening was measured on a.p. radiographs (midpoint technique). Statistical analysis was conducted using a mixed model with post hoc correction (p < 0.05). Intra- and interobserver reliability was assessed by calculating the intraclass correlation coefficient (ICC).

Results: Medial joint space opening significantly increased with cutting state (p < 0.0001) and flexion angle (p < 0.0001). Although isolated dMCL injury did not significantly increase medial joint space opening, sMCL resection gradually increased joint space opening 3.2 ± 1.9 to 6.9 ± 2.7 mm (p = 0.039) between 0° and 45° knee flexion. Following POL deficiency, medial joint space opening further increased 6.4 ± 2.7 to 11.4 ± 6.2 mm between 0° and 45° knee flexion (p = 0.0035). A combined injury (dMCL/sMCL/POL/ACL) increased medial joint space opening 12.0 ± 4.9 to 21.8 ± 7.9 mm (p < 0.0001) between 0° and 45° knee flexion, compared to the intact state. The intraobserver ICC was 0.995 and the interobserver ICC was 0.955 showing excellent intra- and interobserver reliability.

Conclusion: Deficiency of the medial stabilisers of the knee increased medial joint space opening in stress radiography, whereas isolated dMCL deficiency did not significantly affect valgus gapping. This study demonstrated a good concordance between valgus stress radiography and clinical scores (International Knee Documentation Committee and Hughston). Our findings support performing valgus stress tests at 0° and at least 20° of flexion.

Level of evidence: There is no level of evidence as this study was an experimental laboratory study.