Conversion of the Sagittal Functional Safe Zone to the Coronal Plane Using a Mathematical Algorithm

Abstract

Background: Ante-inclination (AI) of the cup is a key component of the combined sagittal index (CSI) for predicting joint stability after total hip arthroplasty (THA). We aimed to (1) validate a mathematical algorithm relating AI to radiographic anteversion (RA), radiographic inclination (RI), and pelvic tilt (PT); (2) convert the AI criterion of the CSI into the coronal functional safe zone (CFSZ) and explore the influences of standing-to-sitting pelvic motion (PM) and pelvic incidence (PI) on the CFSZ; and (3) attempt to locate a universal cup orientation that always fulfills the AI criterion of the CSI for all patients.Methods: In the first phase, a phantom pelvis was designed to simulate a range of PT values, and an acetabular cup was implanted with different RA, RI, and PT settings using a robot-assisted technique and scanned using the EOS imaging system. The second phase involved patient radiographs. We enrolled 100 patients who underwent robot-assisted THA from April 2019 to December 2019, and EOS images before THA and at the 12-month follow-up were recorded. The AI was measured on a lateral radiograph; this angle was used as the reference and compared with the calculated AI to assess the accuracy of the algorithm. Linear regression was conducted to explore the relationship between the size of the CFSZ and the values of PM and PI. We calculated the intersection of the CFSZs of the patients to find a universal cup orientation (RA and RI) for the CSI. Results:The algorithm was accurate according to both the phantom study and patient radiographs using PT at the time of follow-up, with mean absolute errors (MAEs) of 1.5°(width of 95% confidence interval [CI], 2.2°) and 2.8°(width of 95% CI, 3.0°), respectively. However, the preoperatively predicted AI had an MAE of 9.0°(width of 95% CI, 10.5°). In patient subgroups with lower PM or PI, the sizes of the CFSZ and of its intersection with the Lewinnek safe zone were significantly smaller (p < 0.017). No universal cup orientation existed to fulfill the CSI criteria for all patients or for any of the PM or PI subgroups. Conclusions:The target orientation for the cup in THA should be individualized. Our validated algorithm may serve as a quantitative tool for the patient-specific optimization of cup AI on the basis of the functional safe zone.Clinical Relevance: The Lewinnek safe zone fails because it cannot predict the functional orientation of the cup. The concept of a universal safe zone of cup orientation should be abandoned and replaced by a patient-specific surgical target.Disclosure: The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJS/G921).

Effectiveness and Accuracy of a Patient‐Specific Instrumentation System for Total Hip Arthroplasty

“…It is clinically desirable to accurately place the prosthesis in a safe zone to avoid postoperative complications 15,16 . But patients have unique spinopelvic movements that can affect the safe zone of the acetabular cup 17 . Therefore, to find the most suitable prosthesis position for patients, it is necessary to identify, reconstruct, and simulate the multimodal information of patient anatomy, biomechanics, and kinematics and then calculate the personalized specific safe zone of the acetabular cup 18 .…”

“… 15 , 16 But patients have unique spinopelvic movements that can affect the safe zone of the acetabular cup. 17 Therefore, to find the most suitable prosthesis position for patients, it is necessary to identify, reconstruct, and simulate the multimodal information of patient anatomy, biomechanics, and kinematics and then calculate the personalized specific safe zone of the acetabular cup. 18 Moreover, the femoral prosthesis position is influenced by the femoral neck physiological anteversion, proximal femoral anterior arch, proximal femoral morphology of the medullary cavity, and other related factors.…”

Assessing component orientation of total hip arthroplasty using the low-dose bi-planar radiographs

“…We thus recommend routine use of standing view radiographs for evaluation of functional standing cup orientation, especially for those at high risk of dislocation or analyzing the cause of dislocation [31][32][33]. Furthermore, we recommend that preoperative standing view radiography of the pelvis should be evaluated before computer-assisted THA to make accurate planning, as surgeons need to target the cup's functional orientation of standing position, instead of merely based on supine CT scans [34][35][36].…”

Assessing Component Orientation of Total Hip Arthroplasty using the EOS Imaging System

“…We thus recommend routine use of standing view radiographies for evaluation of functional standing cup orientation, especially for those at high risk of dislocation or with analysis of dislocated THA [28][29][30]. Furthermore, we recommend that preoperative standing view radiography of the pelvis should be evaluated for computer-assisted THA to make accurate preoperative planning, including navigation or robotic-assisted THA, as surgeons need to target the cup's functional orientation during standing, instead of merely based on supine CT scans [31][32][33].…”