Lawrence D. Dorr
According to recently published study results, patients with a late dislocation after total hip arthroplasty had abnormal spinopelvic motion that caused the dislocation, especially in combination with cup malposition or soft-tissue abnormalities.
“This study illustrates the importance of aging in causing stiffness of the spine which then has an adverse effect on stability of the [total hip replacement] THR,” Lawrence D. Dorr, MD, told Healio.com/Orthopedics. “It has important findings that the functional safe zone is more important than the Lewinnek safe zone, and that increased femoral motion causes impingement, not the coronal acetabular position.”
Dorr and colleagues studied 20 consecutive patients to determine the reason for their late dislocation after total hip arthroplasty. Investigators used standard pelvic radiographs to measure cup inclination and anteversion. They measured pelvic motion, femoral mobility and sagittal cup position with lateral standing and sitting spine-pelvic-hip radiographs. The spinopelvic motion was the difference between standing and sitting sacral slopes. The functional motion of the hip joint and impingement risk were assessed with the combined sagittal index.
Results showed there were nine anterior dislocations and 11 posterior dislocations. Dislocations occurred at a mean of 8.3 years after primary THA. Of the nine patients with anterior dislocations, eight had spinopelvic abnormalities that included fixed posterior pelvic tilt when standing, increased standing femoral extension and increased standing combined sagittal index. Out of the 11 patients with a posterior dislocation, 10 patients had abnormal spinopelvic measurements which includes decreased spinopelvic motion, increased femoral flexion and decreased sitting combined sagittal index.
“For every 1° decrease in spinopelvic motion, there was an associated 0.9° increase in femoral motion and, in some patients, this resulted in osseous impingement and dislocation,” the authors wrote. – by Monica Jaramillo
Disclosures: Dorr reports he receives royalties from Don Joy Orthopedics; has stock in Joint Development Inc and receives royalties and is a consultant for Zimmer Biomet.