Background: Implants with different neck offsets for hip replacement surgery are now available from various implant manufacturers and have become a widely used option for achieving postoperative hip stability. This study aimed to compare the impact of neck offset on initial stem fixation and the mechanical effects of different stem lengths when using cementless stems. Methods: We performed a finite element analysis using Mechanical Finder ver. 12.0. CAD models of the Profemur Preserve and Profemur TL cementless stems. Each stem was appropriately sized, and the von Mises stress was calculated. We defined micromotion as the relative displacement between the stem node and the surface of bone contact. The maximum micromotion values of these finite element models were compared under standing conditions. Results: The stress per zone for both stems (Preserve and TL) was the highest in zone 5, followed by zones 3 and 4, which were almost in line with each other. The high offset (HIGH) stress was higher than the standard offset (STD) stress in each stem and zone.The micromotion of each stem was higher at each load in the following order: Preserve HIGH, Preserve STD, TL HIGH, and TL STD, with HIGH being higher than STD at each stem. Conclusions The choice of higher offset or shorter length stems induced higher micromotion at the interface to the bone in the early postoperative period. Therefore, surgeons should be more careful to get appropriate initial fixation using shorter stems with higher offset necks due to the relatively high incidence of loosening or fractures.

Background: Implants with different neck offsets for hip replacement surgery are now available from various implant manufacturers and have become a widely used option for achieving postoperative hip stability. This study aimed to compare the impact of neck offset on initial stem fixation and the mechanical effects of different stem lengths when using cementless stems. Methods: We performed a finite element analysis using Mechanical Finder ver. 12.0. CAD models of the Profemur Preserve and Profemur TL cementless stems. Each stem was appropriately sized, and the von Mises stress was calculated. We defined micromotion as the relative displacement between the stem node and the surface of bone contact. The maximum micromotion values of these finite element models were compared under standing conditions. Results: The stress per zone for both stems (Preserve and TL) was the highest in zone 5, followed by zones 3 and 4, which were almost in line with each other. The high offset (HIGH) stress was higher than the standard offset (STD) stress in each stem and zone.The micromotion of each stem was higher at each load in the following order: Preserve HIGH, Preserve STD, TL HIGH, and TL STD, with HIGH being higher than STD at each stem. Conclusions The choice of higher offset or shorter length stems induced higher micromotion at the interface to the bone in the early postoperative period. Therefore, surgeons should be more careful to get appropriate initial fixation using shorter stems with higher offset necks due to the relatively high incidence of loosening or fractures.

Background: Implants with different neck offsets for hip replacement surgery are now available from various implant manufacturers and have become a widely used option for achieving postoperative hip stability. This study aimed to compare the impact of neck offset on initial stem fixation and the mechanical effects of different stem lengths when using cementless stems. Methods: We performed a finite element analysis using Mechanical Finder ver. 12.0. CAD models of the Profemur Preserve and Profemur TL cementless stems. Each stem was appropriately sized, and the von Mises stress was calculated. We defined micromotion as the relative displacement between the stem node and the surface of bone contact. The maximum micromotion values of these finite element models were compared under standing conditions. Results: The stress per zone for both stems (Preserve and TL) was the highest in zone 5, followed by zones 3 and 4, which were almost in line with each other. The high offset (HIGH) stress was higher than the standard offset (STD) stress in each stem and zone.The micromotion of each stem was higher at each load in the following order: Preserve HIGH, Preserve STD, TL HIGH, and TL STD, with HIGH being higher than STD at each stem. Conclusions The choice of higher offset or shorter length stems induced higher micromotion at the interface to the bone in the early postoperative period. Therefore, surgeons should be more careful to get appropriate initial fixation using shorter stems with higher offset necks due to the relatively high incidence of loosening or fractures.

Background: Implants with different neck offsets for hip replacement surgery are now available from various implant manufacturers and have become a widely used option for achieving postoperative hip stability. This study aimed to compare the impact of neck offset on initial stem fixation and the mechanical effects of different stem lengths when using cementless stems. Methods: We performed a finite element analysis using Mechanical Finder ver. 12.0. CAD models of the Profemur Preserve and Profemur TL cementless stems. Each stem was appropriately sized, and the von Mises stress was calculated. We defined micromotion as the relative displacement between the stem node and the surface of bone contact. The maximum micromotion values of these finite element models were compared under standing conditions. Results: The stress per zone for both stems (Preserve and TL) was the highest in zone 5, followed by zones 3 and 4, which were almost in line with each other. The high offset (HIGH) stress was higher than the standard offset (STD) stress in each stem and zone.The micromotion of each stem was higher at each load in the following order: Preserve HIGH, Preserve STD, TL HIGH, and TL STD, with HIGH being higher than STD at each stem. Conclusions The choice of higher offset or shorter length stems induced higher micromotion at the interface to the bone in the early postoperative period. Therefore, surgeons should be more careful to get appropriate initial fixation using shorter stems with higher offset necks due to the relatively high incidence of loosening or fractures.

Background: In total hip arthroplasty (THA), the ideal stem length remains uncertain; different stem lengths are used in different cases or institutions. We aimed to compare the stress distributions of cementless tapered wedges and short stems in femurs with different femoral marrow geometries and determine the appropriate fit. Methods: Finite element models were created and analyzed using HyperMesh and LS-DYNA R11.1, respectively. The 3-dimensional shape data of the femurs were extracted from computed tomography images using the RETOMO software. Femurs were divided into 3 groups based on the Dorr classification. The computer-aided design data of cementless tapered wedge-type and short stems were used to select the appropriate size. In the finite element analysis, the loading condition of the femur was assumed to be walking. Volumes of interest (VOIs) were placed within the femur model at the internal and external contact points of the stem based on Gruen zones. The average stresses and strain energy density (SED) of the elements included in each VOI were obtained from the preoperative and postoperative models. Results: The von Mises stress and SED distributions of the cementless tapered wedge and short stems were similar in their respective Dorr classifications. In both stems, the von Mises stress and SED after THA were lower than before THA. The von Mises stress and SED of the cementless tapered wedge stem were higher than those of short stems. Cementless tapered wedge-type stems tended to have lower rates of change than short stems; however, Dorr C exhibited the opposite trend. In the Dorr classification comparison, the von Mises stress and SED were greater for both stems in the order of Dorr C > Dorr B > Dorr A, from Zone 2 to Zone 6. Conclusions In Dorr A and B, the short stem exhibited a natural stress distribution closer to the preoperative femur than the tapered wedge stem; however, in Dorr C, the short stem may have a greater effect on stress distribution, suggesting that it may cause greater effects, such as fracture in the early postoperative period, than other Dorr types.

Background: This study aimed to clarify the characteristics of stress distribution caused by the placement of tapered wedge stems in bowed femurs compared with that in normal femurs and the effect of varus stem placement. Methods: Models with normal and enhanced bowing were created from the right-side computed tomography data of a 17-yearold woman with the least bowing among 40 participants who underwent anterior cruciate ligament reconstruction or operative treatment for trauma in our hospital between January 2017 and May 2018. Finite element analysis was performed, assuming the tapered wedge stem was placed in the neutral and varus positions. Results: Varus stem placement on a femur with normal bowing showed a deviation and increase of von Mises stresses in the medial femur. Stem placement on a bowed femur, even when placed in the neutral position, increased stress across the periprosthetic bone. When the stem was placed in the varus position, von Mises stress across the periprosthetic bone increased. Zone 7, with strong bowing, demonstrated 3.6-fold increased stress compared with normal femurs. The maximum tensile principal stress was greatest in zone 6 and increased in zones 3 and 4. Conclusions Surgeons should assess femoral bowing preoperatively and pay particular attention to intraoperative stem alignment for femurs with high bowing.

Background: Patients with developmental dysplasia of the hip (DDH) are known to have abnormal pelvic morphologies; however, rotation of innominate bone features remains unclear. Thus, we investigated innominate bone rotation in patients with DDH by measuring the associated angles and distances using three-dimensional (3D) computed tomography. Methods: We defined four straight lines in pelvic 3D models: from the anterior superior iliac spine to the posterior superior iliac spine, from the anterior inferior iliac spine to the posterior inferior iliac spine, from the pubic tubercle to the ischial spine, and from the pubic tubercle to the ischial tuberosity. Similarly, we measured the angles formed by these lines using the vertical axis of the anterior pelvic plane on the horizontal plane and the horizontal axis on the sagittal plane. Additionally, we measured the distances between the femoral head centers and the acetabular centers in the coronal plane. Results: The difference in internal rotation angle between the superior and inferior parts of the iliac bone was significantly lower, by approximately 1.7°, in the DDH group than in the control group (p = 0.007); the difference between the inferior and superior parts of the ischiopubic bone was significantly higher, by approximately 1.5°, in the DDH group (p< 0.001). In the sagittal plane, the sum of the superior aspect of the iliac bone and the inferior aspect of the ischium was significantly lower in the DDH group (p = 0.001) than in the control group. The distances between the femoral heads and the acetabula were significantly greater in the DDH group than in the control group (p = 0.03, p < 0.01, respectively). Conclusions Patients with DDH had a more internally rotated ilium and ischiopubic bone than normal individuals; however, it should be emphasized that internal rotation was reduced near the acetabulum, and the acetabulum was shifted laterally. Similarly, it was shown that patients with DDH had different rotations of the ilium and ischiopubic bone in the sagittal plane.

Background: There is a paucity of studies on the iliac curvature in developmental dysplasia of the hip (DDH). Here, we examined the iliac curvature in DDH using three-dimensional computed tomography. Methods: We allocated cases with a center-edge angle of < 20° to the DDH group (55 cases) and cases with a center-edge angle of > 25° to the control group (57 cases) and measured the straight line (line A) between the anterior and posterior superior iliac spines. We examined which part of the iliac bone line A passes through and classified the results into 4 categories (type A, inside the iliac bone; type B, through the iliac bone; type C, outside the iliac bone; and type D, both inside and outside the iliac bone) to evaluate the iliac wing curvature. After measuring the area and internal surface of the iliac wing using line A, we examined the correlation between these values, the interspinous distance, the superior iliac angle, and the center-edge angle. Results: Distributions of the four types were compared between the two groups; there was no significant difference. The length of the portion of line A inside the ilium and the area formed by line A and the iliac wing, which shows the degree of iliac wing curvature, were not significantly different between the groups. There were no correlations between these values and the center-edge angle; however, there were weak positive correlations among the interspinous distance, the superior iliac angle, and the centeredge angle. Conclusions The inward nature of the iliac bone in patients with DDH is mainly due to the internal rotation of the entire iliac bone and less likely due to the curvature of the iliac bone.

Background: In total hip arthroplasty, the cup setting angle may affect the postoperative results. In recent years, both computed tomography-based navigation and computed tomography-free (imageless) navigation have been reported to produce high accuracy in cup installation; however, no direct comparison between these two methods has been performed. The present study aimed to directly compare the cup installation angle accuracy between computed tomography-based navigation and computed tomographyfree navigation in patients with Crowe’s classification stage I or II dysplastic osteoarthritis and to examine the factors affecting the cup installation accuracy. Methods: Using both navigation systems for the same technique, primary total hip arthroplasty was performed by the same surgeon in 36 patients. A cup was installed using computed tomography-based navigation, and the installed cup was measured again using computed tomography-free navigation. We compared the error between the target angle and the intraoperative installation angle for each navigation method by performing statistical analyses. Results: For computed tomography-based navigation, errors in the inclination and the anteversion angles compared to the target angle were 3.14° ± 1.55° and 1.47° ± 0.99°, respectively. For computed tomography-free navigation, the inclination and anteversion angle errors were significantly larger, i.e., 6.84° ± 4.78° and 5.43° ± 5.22°, respectively (p < 0.01). The inclination and anteversion angles of computed tomography-free navigation were correlated, and there were no significant factors influencing the error. Conclusions Computed tomography-based navigation is more accurate for cup installation than computed tomography-free navigation. When using computed tomography-free navigation, it is necessary to add technical schemes before and during surgery to improve the cup installation accuracy.

Background: Plain computed tomography (CT) and magnetic resonance imaging (MRI) are useful for diagnosing adverse local tissue reactions after metal-on-metal total hip arthroplasty (THA), but metal artifacts can hamper radiological assessments near the implants. We sought to clarify the usefulness of 18F-fluorodeoxyglucose positron-emission tomography (18F-FDG-PET) CT and MRI in the periprosthetic region, which is difficult to assess after THA due to metal artifacts. Methods: We performed preoperative 18F-FDG-PET/CT and 18F-FDG-PET/MRI, as well as plain CT and MRI, in 11 metal-on-metal THA patients who underwent revision surgery. Results: Most patients showed high FDG uptake in the metal artifact areas and pseudotumors in the 18-F-FDG-PET/CT and 18-FFDG-PET/MRI scans. Intraoperative intra-articular macroscopic and histopathological intra-articular granulation tissue findings were suggestive of adverse local tissue reaction. Conclusions The enhanced uptake in the metal artifact areas seemed to reflect adverse local tissue reaction. Therefore, 18F-FDGPET/CT and 18-F-FDG-PET/MRI can be useful for the auxiliary diagnosis of adverse local tissue reactions after metal-on-metal THA.