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Does the Rule of Thirds Adequately Detect Deficient and Excessive Acetabular Coverage?
Vera M Stetzelberger1,2,3,4, Angela M Moosmann1,2,3,4, Guoyan Zheng1,2,3,4
1V. M. Stetzelberger, A. M. Moosmann, M. Tannast, Department of Orthopaedic Surgery and Traumatology, Fribourg Cantonal Hospital, University of Fribourg, Fribourg, Switzerland.
Clinical Orthopaedics and Related Research
|December 10, 2020
Summary
The "rule of thirds" accurately identifies excessive or deficient acetabular coverage for hip surgery, offering high specificity but lower sensitivity. This simple method aids in surgical decision-making for hip-preserving procedures.
Area of Science:
- Orthopedic surgery
- Radiology
- Hip biomechanics
Background:
- Accurate assessment of anterior-posterior (AP) acetabular coverage is vital for successful hip-preserving surgery.
- Current methods for quantifying AP coverage are complex, requiring advanced imaging or software, and may involve additional radiation exposure.
- The
- rule of thirds
- is proposed as a simpler alternative for detecting abnormal AP coverage.
Purpose of the Study:
- To evaluate the validity of the
- rule of thirds
- in detecting excessive and deficient anterior and posterior acetabular coverage.
- To compare its diagnostic performance (AUC, sensitivity, specificity, PPV, NPV) against established radiographic values using Hip2Norm as the gold standard.
Main Methods:
- Retrospective analysis of 161 hips from patients eligible for hip-preserving surgery (2003-2015).
- Patients were categorized into subgroups based on acetabular pathomorphology.
- Anterior and posterior coverage assessed using Hip2Norm software and the
- rule of thirds
- applied to standardized AP pelvis radiographs.
Main Results:
- The
- rule of thirds
- demonstrated high Area Under the Curve (AUC) values for detecting excessive (0.945 anterior, 0.933 posterior) and deficient (0.962 anterior, 0.876 posterior) acetabular walls.
- The method showed high specificity and positive predictive values (PPV) but low sensitivity and negative predictive values (NPV).
Conclusions:
- The
- rule of thirds
- is highly specific for identifying excessive or deficient acetabular coverage, suggesting its utility when the intersection point falls in the lateral or medial third, respectively.
- While valuable for surgical planning, its low sensitivity means that a negative result does not definitively exclude abnormal coverage.
