Abstract
In total hip arthroplasty (THA), inappropriate cup alignment cause edge loading and prosthetic impingement, which lead to various mechanical problems including dislocation, excessive wear and breakage of bearing materials, and stem neck fracture. To find the optimal cup alignment, various computer simulation studies have been conducted. However there have been few studies focusing on pelvic coordinate system as a reference of cup positioning. Our hypothesis is that the functional pelvic coordinate system with pelvic sagittal inclination in the supine position is appropriate for a reference frame of cup alignment. To test the hypothesis, we have been investigating preoperative and postoperative kinematics of pelvis and hip of THA patients.
In 25 % of the consecutive 163 patients, the difference in preoperative pelvic inclination angle between the supine and standing positions (positional change of pelvic inclination [PC]) was 10o or more. Patients’ age and age-related spinal disorders including compression fracture and lumbar spondylolisthesis were independent factors associated with large preoperative PC. This raises a concern that large PC might increase the risk of edge loading and posterior prosthetic impingement when cup was positioned referencing supine pelvic position, especially in elderly patients.
We compared kinematics of the hip after THA in patients with a preoperative large PC (≥10°) with that in patients with a preoperative small PC (<10°), assuming that the supine position as a zero position of the pelvis. First, we compared intraoperative passive range of motion (ROM) after implantation of the 91 hips using navigation system. No significant differences in intraoperative hip ROM were observed between the both groups. Next, we compared postoperative ROM of the 50 hips during motion of daily livings using our 4-dimentional motion analysis system within two year after THA. No significant differences in postoperative hip flexion or extension angles were observed between the both groups. These results suggested that if cup was positioned referencing the supine pelvic position, the degree of preoperative PC does not matter early after primary THA.
Regarding long-term change of pelvic inclination after THA, 49 % of 70 patients followed for 10 years showed the change more than 10o in the standing position, although only 9% showed the change more than 10o in the supine position. This means that aging after THA increase discrepancy of pelvic inclination between the preoperative supine position as the reference for preoperative planning and the postoperative standing positions in some patients. However we could not find any preoperative predictors of this long-term change of pelvic inclination in the standing position. Therefore, although it is unclear whether surgeons should change the reference pelvic plane for cup alignment taking the longitudinal change of pelvic inclination in the standing position, at least, strict cup alignment control at primary THA is considered to be important to minimize the risk of edge loading and prosthetic impingement due to longitudinal changes of pelvic inclination.
In conclusion, our current recommendation of pelvic coordinate system as a reference of cup alignment is a functional pelvic coordinate system with pelvic sagittal inclination in supine position.
