Four-year results of Summit stems for total hip arthroplasty in Japanese patients

Relationship between Stress Shielding and Optimal Femoral Canal Contact Regions for Short, Tapered-Wedge Stem Analyzed by 2D and 3D Systems in Total Hip Arthroplasty

Although tapered-wedge short stem has been widely employed with its availability for minimally invasive surgeries in total hip arthroplasty (THA), post-operative stress shielding matter remains unresolved in cementless procedures. This study aimed to clarify the most optimal femoral canal contact regions of the stem design taking stress shielding incidence into consideration. This investigation included 60 joints from 60 patients (mean age at operation: 65.9 years), of which follow-up duration after primary THA had been more than 2 years. Frequencies of spot welds, subsidence, and stress shielding were examined 2 years after surgery. The most suitable femoral canal contact regions were evaluated by plain radiograph (2D) and 3D-computed tomography analyses according to Nakata's division for fitting manners. Spot welds were observed in 38 cases (63.3%), and no subsidence case was seen. Respective number of stress shielding cases, based on Engh's classification, categorized as degree 0, 1, and 2, were 2 (3.3%), 31 (51.7%), and 27 (45.0%), while no cases for degree 3 or 4 were found. When assessed by 3D fitting analysis, 27 cases of stress shielding degree 2 were constituted by 13/42 cases of mediolateral (ML) fit, 2/4 cases of flare fit, and 12/14 cases of multi point fit. In 42 cases of ML fitting, stem contact rate of the most proximedial region in stress shielding degree 0 and 1 was significantly higher compared to stress shielding degree 2 cases. Meanwhile, the rates of distal regions were significantly lower or absent in stress shielding degree 0 and 1 cases. The initial fixation of this stem design was very good in our cohort regardless of fitting manners. This study successfully revealed that ML fitting with femoral component, especially the most proximedial calcar site restricted fitting, would be optimal for reducing stress shielding occurrence in cementless short, tapered-wedge stem THA. Thus, the ideal stem contact region should be considered during THA procedures in light of the reduction of stress shielding development.

Patients with a Dorr type C femoral bone require attention for using a Summit cementless stem: Results of total hip arthroplasty after a minimum follow-up period of 5 years after insertion of a Summit cementless stem

BACKGROUND

The Summit cementless stem has been used as a device to occupy the proximal medullary canal space in total hip arthroplasty (THA). The purpose of this study was to evaluate the mid-term clinical and radiographic results of the Summit stem and the differences in the results as a function of medullary cavity shape.

METHODS

This study analyzed the 90 consecutive patients who underwent THA by using the Summit cementless stem. The minimum clinical and radiographic follow-up period for the patients was 5 years (mean, 5.5 years; range, 5.0-7.1 years). The radiographic and Harris hip scores (HHS) were evaluated across the three Dorr type classifications of femoral bone.

RESULTS

The postoperative HHSs were significantly higher than the preoperative HHS scores (p < 0.05). No significant differences in preoperative and postoperative HHSs were found among the three Dorr types. Stress shielding was observed in 58 hips. Spot welds and cortical hypertrophy were observed in various zones in 53 and 11 hips, respectively. No significant difference in the number of occurrences of cortical hypertrophy was found among the three Dorr types. However, the number of occurrences of severe stress shielding in Dorr type C was higher than that in Dorr type B. The number of occurrences of spot welds in Dorr type C was lower than that in both Dorr types A and B.

CONCLUSIONS

Mid-term clinical results were good regardless of the medullary cavity shape. However, severe stress shielding in Dorr type C was more frequently than that in Dorr type B. Therefore, attention should be paid to the types of medullary cavity shapes for Summit stem use.

Influence of trabecular bone quality and implantation direction on press-fit mechanics

Achieving primary stability of uncemented press-fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces ("Beaded" and "Flaked") were implanted onto trabecular bone specimens of varying quality in a press-fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push-in and pull-out forces were measured to assess stability. Microstructural changes in the bone were determined from μCT analysis. For force-defined implantation analysis, push-in and pull-out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position-defined implantation analysis, pull-out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull-out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224-233, 2017.