The Rule of 57: Orthopaedic Trigonometry Made Easy

Roof arc width: The novel calculation method for calculation of patient specific roof arc width in acetabular fractures

Purpose

Roof arc angle (RAA) is determined by measuring angle between a vertical line drawn from center of the acetabulum towards the acetabular dome and a second line drawn from center of acetabulum to the fracture through the acetabulum. Joel and Matta demonstrated that when roof arc angle was less than 45° on Pelvic AP and Judet's views, the fracture line is considered to be passing through the weight-bearing dome and require surgical fixation. The main purpose of the study is to calculate patient-specific angle and width for the better evaluation and management of acetabular fractures.

Methods

Radiographs of normal hips were retrieved from electronic data and parameters were calculated. Two observers calculated the parameters at two different intervals. Pearson correlation formula was used to find a correlation between groups.

Results

Fifty radiographs of 28 patients were reviewed. The mean age of patients was 75.58 years ±13.28. The radius of the acetabulum, the radiographic measurement of sector width for 45° angulation at the roof, and the mathematical calculation for roof arc for 45° angle had significant positive correlation for both observers at two different occasions.

Conclusions

The measured roof arc width ranges from a minimum of 16.20 mm-31.50 mm and the calculated arc width for a 45-degree angle varies from a minimum of 15 mm-25.56 mm. These values are higher than the described values of 10 mm equals to 45 degrees. Hence, the values measured in this study should be considered for decision making in the management of acetabular fractures.