- This cadaveric study assessed the ability of stress ultrasonography to quantify the severity of MCL (medial collateral ligament) injuries based on medial compartment gapping
- Under 100 N of valgus force, the correlation between ultrasonographic and fluoroscopic measurements was high at 0° of knee flexion (r, 0.88; r2, 0.775; P<0.001) and very high at 20° (r, 0.95; r2; 0.902; P<0.001)
- When differentiating between stable and unstable injuries, 13.8 mm of medial tibiofemoral distance was the optimal threshold (AUC, 0.97; sensitivity, 100%; specificity, 94%)
- Intrarater reliability for ultrasound measurements was 98% and interrater reliability was 93%
- Dynamic ultrasonography is a promising alternative to MRI and radiography for quantifying medial knee instability
MRI is commonly performed when diagnosing a ligamentous injury to the medial knee. However, since medial knee instability is a dynamic condition which cannot be assessed by MRI, radiography with valgus stress maneuvers is often performed during evaluation.
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Rohan Bhimani, MD, MBA, a research fellow in the Foot & Ankle Research and Innovation Laboratory at Massachusetts General Hospital, Miho J. Tanaka, MD, PhD, orthopedic surgeon and director of the Women's Sports Medicine Program, and colleagues now propose using dynamic ultrasonographic assessment is another option. In a cadaveric study reported in Arthroscopy, Sports Medicine, and Rehabilitation, they found it highly accurate in distinguishing stable from unstable medial knee injuries.
The researchers examined eight unpaired cadaveric knees (three male; five female; mean cadaver age 58; range, 48–82). An orthopedic surgeon obtained ultrasound images from intact knees under two standardized loading conditions created using the Telos device: unloaded (0 N of force) and loaded (100 N of valgus force).
Specimens were then reassessed after each of the following ligaments was transected, in this order to stimulate increasing severity of injury: superficial medial collateral ligament (sMCL, stable injury), deep MCL (dMCL, the beginning of unstable injuries), posterior oblique ligament (POL) and anterior cruciate ligament (ACL).
Fluoroscopic assessment was also performed during the intact and sequential ultrasonographic assessments for comparison.
Under 100 N of valgus force, the results of ultrasonographic and fluoroscopic evaluations were:
- Normal knee—The mean medial tibiofemoral distance was 6.9 mm at 0° of knee flexion and 7.0 mm at 20°
- After complete medial knee injury with additional ACL sectioning—The distance increased to 20.3 mm at 0° and 22.8 mm at 20° (P<0.001 vs. intact state)
- Normal knee—The mean medial tibiofemoral distance was 6.2 mm at 0° of knee flexion and 6.9 mm at 20°
- After complete medial knee injury with additional ACL sectioning—The distance increased to 15.3 mm at 0° and 17.5 mm at 20° (P<0.001 vs. intact state)
The correlation between ultrasonographic and fluoroscopic measurements was high at 0° of knee flexion (r, 0.88; r2, 0.775; P<0.001) and very high at 20° (r, 0.95; r2; 0.902; P<0.001).
Accuracy and Reliability
The areas under the receiver operating characteristic curve (AUCs) for ultrasonography measurements were not statistically different from those performed using fluoroscopy.
The optimal cutoff values for ultrasonographic measurements to distinguish between each type of medial knee injury were:
- For distinguishing between an intact knee vs. sMCL injury—8.3 mm of medial tibiofemoral distance (AUC, 0.98)
- sMCL vs. dMCL injury—9.9 mm (AUC, 0.89)
- dMCL vs. POL—16.7 mm (AUC, 0.88)
- POL vs. ACL—18.6 mm (AUC, 0.84)
- Stable vs. unstable injuries—13.8 mm (AUC, 0.97; sensitivity, 100%; specificity, 94%)
Intrarater reliability for ultrasound measurements was 98% and interrater reliability was 93%.
Opportunities to Improve Care
Dynamic ultrasonography is a promising option for quantifying the severity of MCL injuries. Like radiography, it can provide a contralateral comparison and dynamic views. Additionally, it doesn't expose patients or providers to radiation, is less expensive, and is nearly universally portable for ready availability at the point of care.
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