Summary

BMI Impact on the Knee Physical Exam

Abstract

Background

Obesity has become a worldwide pandemic; however, the physical examination of the knee had yet to adapt to the overweight population. While mostly overlooked in orthopaedic literature, the accuracy of the knee physical examination is critical for correct patient assessment and selection of appropriate treatment.

Purpose

To assess whether body mass index (BMI) affects the sensitivity and specificity of common provocative knee tests, using arthroscopy as a gold standard.
Study Design: Case-Series, Level of Evidence IV.

Methods

We analysed 210 consecutive patients who underwent knee arthroscopy with or without anterior cruciate ligament (ACL) reconstruction for the treatment of ACL and meniscal pathologies. Demographic characteristics including preoperative BMI and the knee physical examination were documented preoperatively. Sensitivity, specificity and accuracy of provocative tests for ACL and meniscal tears, and their relationship with BMI, were evaluated.

Results

The Anterior Drawer, Lachman and Pivot-Shift tests for ACL tears were less accurate and sensitive, yet more specific, in obese patients when compared to normal and overweight patients (p<0.001). Overall, the Anterior Drawer test was most accurate with a sensitivity of 88.7% and a specificity of 93.9%. The McMurray, Apley Grind and Thessaly for medial meniscus tears showed greater sensitivity and lower specificity in patients with greater BMI. The most accurate test for overweight patients was McMurray with a sensitivity and specificity of 86.84% and 64.00%, respectively, for a medial meniscus tear. Meniscal tests for lateral meniscus tears also demonstrated greater sensitivity and lower specificity in patients in the higher BMI groups but results were not statistically significant. Greater BMI, independently of age and gender, was significantly associated with positive findings of ACL (Anterior Drawer: Odds Ratio (OR)=4.33-5.02, p<0.008; Lachman: OR=4.60-5.43, p<0.006; Pivot-Shift: OR=3.71-4.23, p<0.026).

Conclusion

Provocative tests for ACL tears are less sensitive but more specific in obese patients. Provocative tests for meniscal tears may be more sensitive and less specific in patients with greater BMI, particularly in the setting of a medial meniscal tear. The physician should take into consideration the impact of BMI on the accuracy of the physical examination of the knee to optimize treatment decision-making.