Abstract
INTRODUCTION
Several papers have reported the efficacy of an imageless navigation system in acetabular cup orientation during total hip arthroplasty (THA). Also, an imageless navigation system is useful for recovering leg length discrepancy. However, no study has evaluated the accuracy of the stem antetorsion angle (SAA) with an imageless navigation system in THA. The purpose of this study was to evaluate the accuracy of the stem antetorsion angles, which were measured by CT with the CT-free navigation system. Also, we evaluate the factors that affect the inaccuracy.
MATERIALS AND METHODS
CT evaluation was performed in 60 patients (60 joints) who underwent primary THA from December 2011 to March 2014. Fifty-nine patients were female. The mean age at surgery was 67 years. The mean BMI at surgery was 24.0 kg/m2. Fifty-four patients had osteoarthrosis, 5 patients had osteonecrosis, and 1 patient had femoral neck fracture. All surgeries were performed in the supine position with the direct anterior approach. The OrthoPilot imageless navigation system was used during surgery. An Excia stem was used in 47 patients and a Bicontact stem was used in the other 13.
Evaluation of SAA was carried out. Instead of SAA, the navigation indicates the rasp antetorsion angle based on the hip-knee-ankle plane during surgery. SAA based on the posterior condylar plane was measured with CT by using 3D THA plannning software. The accuracy of the imageless navigation system was evaluated by comparison of the navigation values obtained during surgery with the CT measured values. Correlations were analyzed with Pearson correlation analysis.
RESULTS
The rasp antetorsion angle was 28.1±9.6 degrees [range 7.7–49.2 degrees]. The mean SAA was 29.9±8.9 degrees [10.7–49.7 degrees] in CT. Strong correlation was found between the rasp antetorsion angle and SAA (p<0.001, r=0.858, Figure). The difference between SAA and the rasp antetorsion angle (SAA – rasp antetorsion angle) was 1.8±4.7 degrees [-6.7–11.9 degrees]. The mean absolute difference between the rasp antetorsion angle and SAA was 4.0±3.1 degrees [0.2–11.9 degrees]. Forty-five cases (75%) showed less than 5 degrees of difference between SAA and the rasp antetorsion angle. Five cases (8.3%) showed more than 10 degrees of difference. In these five cases, SAAs were greater than the rasp antetorsion angles. Patients’ age, height, weight, and BMI did not affect the difference. Deformity of the femoral head, osteoarthrosis of the hip with subluxation, and osteoarthrosis of the knee were found in the patients who showed more than 5 degrees of difference between SAA and the rasp antetorsion angle.
DISCUSSION
This study demonstrated that imageless navigation showed good accuracy for determining SAA during operation. This helps surgeons to recognize combined anteversion during surgery, and to avoid impingement after THA. This navigation system determines rasp antetorsion angle based on the hip-knee-ankle plane. The hip center is defined by setting the trial cup on the acetabulum. The knee center and the ankle center are defined by pointing over the skin. Therefore, deformity of the acetabulum or femoral head may affect the accuracy of the rasp antetorsion angle.
