The impact of body mass index on the accuracy of the physical examination of the knee

Purpose: Obesity is a worldwide pandemic; however, no adaptations were made to the physical examination of obese patient’s knees. The accuracy of the physical examination is critical for correct assessment and selection of treatment. We aimed to assess whether body mass index (BMI) affects the sensitivity and specificity of common provocative knee tests. Methods: We studied 210 patients who underwent knee arthroscopy to treat anterior cruciate ligament (ACL) and meniscal pathologies. BMI and the knee’s physical examination were documented pre-operatively. Sensitivity, specificity, and accuracy of ACL and meniscal provocative tests in relation to BMI were evaluated using arthroscopy as a gold standard. Results: The Anterior Drawer, Lachman, and Pivot-Shift tests for ACL tears were significantly less accurate and sensitive, yet more specific, in obese patients when compared to normal and overweight patients. The McMurray, Apley Grind, and Thessaly tests for medial meniscus tears showed greater sensitivity, but lower specificity, in patients with increased BMI. Above normal BMIs, independently of age and gender, were significantly associated with higher odds for positive ACL tests. Conclusion: Tests for ACL tears are less sensitive in obese patients and alternatives to the classic tests should be considered. Medial meniscus tests tend to be more sensitive and less specific in patients with greater BMIs. Their results should be carefully interpreted due to possible false positives. The physician should take into consideration the impact of patient BMI on the accuracy of their physical examination of the knee to optimize treatment decision-making.