Abstract
Introduction
Sagittal pelvic tilt (SPT) can change with spinal pathologies and fusion. Change in the SPT can result in impingement and hip instability. Our aim was to determine the magnitude of the SPT change for hip instability to test the hypothesis that the magnitude of SPT change for hip instability is less than 10° and it is not similar for different hip motions.
Methods
Hip implant motions were simulated in standing, sitting, sit-to-stand, bending forward, squatting and pivoting in Matlab software. When prosthetic head and liner are parallel, femoral head dome (FHD) faces the center of the liner. FHD moves toward the edge of the liner with hip motions. The maximum distance between the FHD and the center in each motion was calculated and analyzed. To make the results more reliable and to consider the possibility of bony impingement, when the FHD approached 90% of the distance between the liner-center and liner-edge, we considered the hip “in danger for dislocation”. The implant orientations and SPT were modified by 1-degree increments and we used linear regression with receiver operating characteristic (ROC) curve and area under the curve (AUC) to determine the magnitude of SPT change that could cause instability.
Results
SPT modification as low as 7° could result in dislocation during pivoting (AUC: 87.5; sensitivity: 87.9; specificity 79.8; p=0.0001). This was as low as 10° for squatting (AUC: 91.5; sensitivity: 100; specificity 75.9; p=0.0001) and as low as 13° for sit-to-stand (AUC: 94.6; sensitivity: 98; specificity 83; p=0.0001). SPT modification affects hip stability more in pivoting than sit-to-stand and squatting.
Discussion
Our results show the importance of close collaboration between the hip and spine surgeons in treating patients who undergo THA and spinal fusion. The postoperative SPT modification should be considered for preoperative computer simulation for determining the implant safe zone.
