Decreased “Polar Axis Angle” is Associated with Instability after Total Hip Arthroplasty: A New Method to Assess Functional Component Position on Lateral Seated Radiographs

Background Research on hip instability has focused on establishing "safe" ranges of combined component position in supine posture or functional placement of the acetabular component based on the hip-spine relationship. A new angle, the polar axis angle (PAA), of the total hip arthroplasty (THA) components describes the concentricity of both components and can be evaluated in functional positions that confer a greater risk of instability (i.e., sitting). The goal of this study was to compare the polar axis angle in functional positions between patients who experienced a postoperative dislocation, and a matched control group who did not have a dislocation. Methods An institutional database was searched for patients experiencing a dislocation after primary THA. Patients who had postoperative full-length standing and seated lateral radiographs were included in the dislocator group. A control group of non-dislocator patients was matched 2:1 by age, body mass index (BMI), sex, and hip-spine classification. Radiographic measurements of the neck angle, acetabular ante-inclination, and polar axis angle (PAA) were performed by two separate blinded, trained reviewers. Results The lateral seated neck angle and lateral seated polar axis angle measurements were significantly lower in the dislocator groups (n = 37) when compared with the control group (n = 74) (23 versus 33 degrees, P < 0.001; 74 versus 83 degrees, P = 0.012, respectively). Significant differences were also observed in changes in the polar axes and neck angles between standing and seated positions (P < 0.001 and P < 0.001, respectively). When comparing patients who have mobile spines versus stiff spines within the dislocator group, there were no differences in the acetabular, neck, or polar axis angles. The effect of neck angle on the polar axis angle showed a linear trend across cohorts. Conclusions Patients who experience postoperative instability have a significantly lower polar axis angle on lateral seated radiographs when matched for age, sex, BMI, and hip-spine classification. In addition, the lower seated polar axis angle is driven more strongly by decreased functional femoral anteversion, which emphasizes the role of functional femoral version on stability in THA. Research on hip instability has focused on establishing "safe" ranges of combined component position in supine posture or functional placement of the acetabular component based on the hip-spine relationship. A new angle, the polar axis angle (PAA), of the total hip arthroplasty (THA) components describes the concentricity of both components and can be evaluated in functional positions that confer a greater risk of instability (i.e., sitting). The goal of this study was to compare the polar axis angle in functional positions between patients who experienced a postoperative dislocation, and a matched control group who did not have a dislocation. An institutional database was searched for patients experiencing a dislocation after primary THA. Patients who had postoperative full-length standing and seated lateral radiographs were included in the dislocator group. A control group of non-dislocator patients was matched 2:1 by age, body mass index (BMI), sex, and hip-spine classification. Radiographic measurements of the neck angle, acetabular ante-inclination, and polar axis angle (PAA) were performed by two separate blinded, trained reviewers. The lateral seated neck angle and lateral seated polar axis angle measurements were significantly lower in the dislocator groups (n = 37) when compared with the control group (n = 74) (23 versus 33 degrees, P < 0.001; 74 versus 83 degrees, P = 0.012, respectively). Significant differences were also observed in changes in the polar axes and neck angles between standing and seated positions (P < 0.001 and P < 0.001, respectively). When comparing patients who have mobile spines versus stiff spines within the dislocator group, there were no differences in the acetabular, neck, or polar axis angles. The effect of neck angle on the polar axis angle showed a linear trend across cohorts. Patients who experience postoperative instability have a significantly lower polar axis angle on lateral seated radiographs when matched for age, sex, BMI, and hip-spine classification. In addition, the lower seated polar axis angle is driven more strongly by decreased functional femoral anteversion, which emphasizes the role of functional femoral version on stability in THA.