Femoral stem neck geometry determines hip range of motion shape

Aims In computer simulations, the shape of the range of motion (ROM) of a stem with a cylindrical neck design will be a perfect cone. However, many modern stems have rectangular/oval-shaped necks. We hypothesized that the rectangular/oval stem neck will affect the shape of the ROM and the prosthetic impingement. Methods Total hip arthroplasty (THA) motion while standing and sitting was simulated using a MATLAB model (one stem with a cylindrical neck and one stem with a rectangular neck). The primary predictor was the geometry of the neck (cylindrical vs rectangular) and the main outcome was the shape of ROM based on the prosthetic impingement between the neck and the liner. The secondary outcome was the difference in the ROM provided by each neck geometry and the effect of the pelvic tilt on this ROM. Multiple regression was used to analyze the data. Results The stem with a rectangular neck has increased internal and external rotation with a quatrefoil cross-section compared to a cone in a cylindrical neck. Modification of the cup orientation and pelvic tilt affected the direction of projection of the cone or quatrefoil shape. The mean increase in internal rotation with a rectangular neck was 3.4° (0° to 7.9°; p < 0.001); for external rotation, it was 2.8° (0.5° to 7.8°; p < 0.001). Conclusion Our study shows the importance of attention to femoral implant design for the assessment of prosthetic impingement. Any universal mathematical model or computer simulation that ignores each stem’s unique neck geometry will provide inaccurate predictions of prosthetic impingement. Cite this article: Bone Joint Res 2021;10(12):780–789.

Optimal Combined Anteversion Range for Obtaining a Wider Range of Motion without Prosthetic Impingement after Total Hip Arthroplasty: A Three-dimensional Analysis Study

Background: Obtaining a larger theoretical range of motion (ROM) is crucial to avoid prosthetic impingement after total hip arthroplasty (THA); however, no reports have examined the permissible range values of combined anteversion (CA) satisfying targeted ROM without prosthetic impingement. This retrospective study aimed to evaluate the possible postoperative CA extent that would allow meeting target ROM criteria according to Yoshimine’s theory using computed tomography (CT)-based three-dimensional motion analysis after THA.Methods: This study included 114 patients (133 hips) who underwent cementless primary THA using a CT-based navigation system and implants (oscillation angle ≥135°). Implant positions were determined using Yoshimine's CA formula. Postoperative evaluation was conducted using a three-dimensional templating software for CT data. The postoperative Yoshimine’s and Widmer’s CA was calculated, and the difference between the target and postoperative values was defined as the error of Yoshimine’s CA and Widmer’s CA. Prosthetic ROM was assessed by Yoshimine’s stringent criteria for activities of daily living. Based on fulfilling these criteria, all patients were divided into the ROM (+) and ROM (-) groups. Evaluation items were compared between the two groups.Results: There were 111 and 22 hips in the ROM (+) and ROM (-) groups, respectively. A significant difference was noted in the absolute error of Yoshimine’s and Widmer’s CA between the two groups. Using receiver operating characteristic analysis, threshold values of 6.0 (higher values indicate greater disability; sensitivity 90.9%, specificity 72.1%) for the absolute Yoshimine’s CA difference (area under the curve [AUC] 0.87, P<0.01) and 6.9 (higher values indicate greater disability; sensitivity 68.2%, specificity 88.3%) for the absolute Widmer’s CA difference (AUC 0.83, P<0.01) were predictors in the ROM (-) group.Conclusions: The target range of Yoshimine’s CA (90.8°±6.0°) and Widmer’s CA values (37.3°±6.9°) was crucial in implant orientation for obtaining theoretical ROM without prosthetic impingement after THA.

Optimal combined anteversion pattern for obtaining a wider range of motion without prosthetic impingement after total hip arthroplasty: a three-dimensional analysis study

Background Obtaining a larger theoretical range of motion (ROM) is crucial to avoid prosthetic impingement after total hip arthroplasty (THA); however, no reports have examined the permissible range values of combined anteversion (CA) satisfying targeted ROM without prosthetic impingement. This retrospective study aimed to evaluate the possible postoperative CA extent that would allow meeting target ROM criteria according to Yoshimine’s theory using computed tomography (CT)-based three-dimensional motion analysis after THA. Methods This study included 114 patients (133 hips) who underwent cementless primary THA using a CT-based navigation system and implants (oscillation angle ≥ 135°). Implant positions were determined using Yoshimine's CA formula. Postoperative evaluation was conducted using a three-dimensional templating software for CT data. The postoperative Yoshimine’s and Widmer’s CA was calculated, and the difference between the target and postoperative values was defined as the error of Yoshimine’s CA and Widmer’s CA. Prosthetic ROM was assessed by Yoshimine’s stringent criteria for activities of daily living. Based on fulfilling these criteria, all patients were divided into the ROM (+) and ROM (-) groups. Evaluation items were compared between the two groups. Results There were 111 and 22 hips in the ROM (+) and ROM (-) groups, respectively. A significant difference was noted in the absolute error of Yoshimine’s and Widmer’s CA between the two groups. Using receiver operating characteristic analysis, threshold values of 6.0 (higher values indicate greater disability; sensitivity 90.9%, specificity 72.1%) for the absolute Yoshimine’s CA difference (area under the curve [AUC] 0.87, P < 0.01) and 6.9 (higher values indicate greater disability; sensitivity 68.2%, specificity 88.3%) for the absolute Widmer’s CA difference (AUC 0.83, P < 0.01) were predictors in the ROM (-) group. Conclusions The target range of Yoshimine’s CA (90.8°±6.0°) and Widmer’s CA values (37.3°±6.9°) was crucial in implant orientation for obtaining theoretical ROM without prosthetic impingement after THA.

High Offset Stems are Protective of Dislocation in High Risk Total Hip Arthroplasty

Background: Spinal stiffness has been shown to increase risk of dislocation due to impingement and instability. Increasing anteversion of the acetabular component has been suggested to prevent dislocation, but little has been discussed in terms of femoral or global offset restoration. The purpose of this study is to quantify dislocation rates after primary THA using standard versus high offset femoral components and to determine how differences in offset affect impingement-free range of motion in a stiff spine cohort using a novel impingement model. Methods: 12,365 patients undergoing THA from 2016-2018 were retrospectively reviewed to determine dislocation rates and utilization of standard versus high offset stems. For 50 consecutive patients with spinal stiffness, a CT-based computer software impingement modeling system assessed bony or prosthetic impingement during simulated range of motion. The model was run 5 times for each patient with varying offsets. Range of motion was simulated in each scenario to determine the degree at which impingement occurred. Results: There were 51 dislocations for a 0.41% dislocation rate. Total utilization of high offset stems in the entire cohort was 49%. Of those patients who sustained a dislocation, 49 (96%) utilized a standard offset stem. The impingement modeling demonstrated 5 degrees of added range of motion until impingement for every 1mm offset increase. Conclusion: In the impingement model, high offset stems facilitated greater ROM before bony impingement and resulted in lower dislocation rates. In the setting of high-risk THA due to spinal stiffness, surgeons should consider the use of high-offset stems and pay attention to offset restoration.

Patient-Specific Safe Zone Based on Daily Positions and Range of Motion

Cup orientation is a challenging step in total hip arthroplasty (THA), to ensure comfort of the patient and durability of the prosthesis. The safe zone defined by Lewinnek is commonly used for cup orientation, but it is nowadays being questioned because it is not patient-specific.We propose to define a new safe zone for cup orientation, that considers patient-specific parameters such as the pelvic tilt and the range of motion (ROM) of the leg.We developed a software that easily computes a unique safe zone from these param- eters in different daily positions, ensuring a cup orientation without risks of prosthetic impingement.

A Preoperative Analytical Model for Patient-Specific Impingement Analysis in Total Hip Arthroplasty

Prosthetic impingement is important to consider during total hip arthroplasty planning to minimise the risk of joint instability. Modelling impingement preoperatively can assist in defining the required component alignment for each individual. We developed an analytical impingement model utilising a combination of mathematical calculations and an automated computational simulation to determine the risk of prosthetic impingement. The model assesses cup inclination and anteversion angles that are associated with prosthetic impingement using patient-specific inputs, such as stem anteversion, planned implant types, and target Range of Motion (ROM). The analysed results are presented as a range of cup inclination and anteversion angles over which a colour map indicates an impingement-free safe zone in green and impingement risk zones in red. A validation of the model demonstrates accuracy within +/- 1.4° of cup inclination and anteversion. The study further investigated the impact of changes in stem anteversion, femoral head size, and head offset on prosthetic impingement, as an example of the application of the model.