Evaluation of new hip prosthesis design with finite element analysis

Usefulness of non-surgical treatment without weight bearing restriction versus surgical treatment for maintaining activities of daily living in patients with peri-prosthetic femoral fractures

PURPOSE

Non-surgical treatment without weight-bearing restriction, even in the early post-injury phase, may have a favourable effect on the activities of daily living (ADLs) in elderly patients with peri-prosthetic femoral fractures (PFFs). This study aimed to assess the effectiveness of surgical and non-surgical treatments for PFFs in terms of ADL maintenance and clinical safety.

METHODS

This retrospective cohort study included 44 patients with PFFs proximal to the stem fixation site without stem loosening. Rehabilitation with weight bearing was initiated after internal fixation of the fracture site in the surgical group (n = 12) and immediately after the injury in the non-surgical group (n = 32). Clinical and radiological outcomes, including time until the first weight-bearing exercise, time until independent walking, ADL deterioration, and bone union rate, were compared between groups. Independent risk factors for ADL deterioration were also evaluated.

RESULTS

The time until first weight-bearing exercise was shorter and the ADL deterioration rate was smaller in the non-surgical group than in the surgical group (8.8 ± 9.2 vs. 21 ± 13 days, P = 0.004; 6.2% vs. 12.5%, P = 0.04, respectively). Bone union rates were similar between groups (91% vs. 83%, P = 0.42), and aseptic loosening of the stem was not observed. Time until first weight-bearing exercise was identified as an independent risk factor for ADL deterioration (odds ratio, 1.13; 95% confidence interval, 1.01-1.26; P = 0.03).

CONCLUSION

Non-surgical treatment of PFFs proximal to the stem fixation site without stem loosening, which does not restrict early weight-bearing exercise after injury, is an effective and safe treatment procedure that maintains ADL performance in elderly patients.

Finite Element Analysis of Normal and Dysplastic Hip Joints in Children

From a surgical point of view, quantification cannot always be achieved in the developmental deformity in hip joints, but finite element analysis can be a helpful tool to compare normal joint architecture with a dysplastic counterpart. CT scans from the normal right hip of an 8-year-old boy and the dysplastic left hip of a 12-year-old girl were used to construct our geometric models. In a three-dimensional model construction, distinctions were made between the cortical bone, trabecular bone, cartilage, and contact nonlinearities of the hip joint. The mathematical model incorporated the consideration of the linear elastic and isotropic properties of bony tissue in children, separately for the cortical bone, trabecular bone, and articular cartilage. Hexahedral elements were used in Autodesk Inventor software version 2022 ("Ren") for finite element analysis of the two hips in the boundary conditions of the single-leg stance. In the normal hip joint on the cartilaginous surfaces of the acetabulum, we found a kidney-shaped stress distribution in a 471,672 mm2 area. The measured contact pressure values were between 3.0 and 4.3 MPa. In the dysplastic pediatric hip joint on a patch of 205,272 mm2 contact area, the contact pressure values reached 8.5 MPa. Furthermore, the acetabulum/femur head volume ratio was 20% higher in the dysplastic hip joint. We believe that the knowledge gained from the normal and dysplastic pediatric hip joints can be used to develop surgical treatment methods and quantify and compare the efficiency of different surgical treatments used in children with hip dysplasia.

Finite element analysis of the mechanical strength of a new hip Spacer

BACKGROUND AND OBJECTIVE

At present, the influence of the internal metallic endoskeleton of Spacer on the biomechanical strength of Spacer remains unclear. The aim of this study was to analyze the mechanical stability of a novel Spacer applying a annular skeleton that mimics the structure of trabecular bone using finite element methods.

METHEDS

The femur models of three healthy individuals and skeletonless Spacer, K-Spacer, and AD-Spacer were assembled to create 15 3D models. Finite element analysis was performed in an Ansys Bench2022R1. Biomechanical parameters such as stress and strain of the Spacer, internal skeleton and femur were evaluated under three loads, which were applied with the maximum force borne by the hip joint (2100 N), standing on one leg (700 N), and standing on two legs (350 N). The mechanical properties of the new hip Spacer were evaluated.

RESULT

The stresses on the medial and lateral surfaces of the AD-Spacer and K-Spacer were smaller than the stresses in the state without skeletal support. The maximum stresses on the medial and lateral surfaces of the AD-Spacer were smaller than those of the inserted K-Spacer, and the difference gradually increased with the increase of force intensity. When the skeleton diameter was increased from 3 to 4 mm, the stresses in the medial and lateral sides of the AD-Spacer and K-Spacer necks decreased. The stress of both skeletons was concentrated at the neck, but the stress of the annular skeleton was evenly distributed on the medial and lateral sides of the skeleton. The mean stress in the proximal femur was higher in femurs with K-Spacer than in femurs with AD-Spacer.

CONCLUSIONS

AD-Spacer has lower stress and higher load-bearing capacity than K-Spacer, and the advantages of AD-Spacer are more obvious under the maximum load state of hip joint.

Finite element analysis of hip implant with varying in taper neck lengths under static loading conditions

BACKGROUND AND OBJECTIVE

Total hip arthroplasty is known as one of the best advancements in orthopedics in the 20th century. Due to age or trauma hip joint has to replace by an artificial implant. After the hip arthroplasty, the patients can return to normal day-to-day activities with a normal range of motion. There are several types and designs are currently available. These designs usually depend upon the anatomy of the patients. There is a need for revision surgery due to dislocation and aseptic loosening in these joints over time in actively younger patients. Minor changes in the design stage can certainly improve the life expectancy of the implant and will also further reduce the revision rate.

METHODS

In this current work, finite element analysis is carried out by varying the neck length with a change in femoral head size for a circular-shaped stem. The effects of using a shorter neck are analyzed. A total of nine combinations are considered for analysis. Modeling is carried out in CATIA V-6 and analysis is performed in ANSYS R-19. A femoral head of 36, 40, and 44 mm and taper neck length of 18, 16, and 14 mm is considered. CoPE is considered as the material combination for all the models.

RESULTS

It was observed that the von Mises stresses in the complete implant tend to decrease with an increase in the femoral head size. Maximum 5% variation in stress values when 36 mm femoral head is compared with 44 mm. The stresses in the taper neck region tend to decrease with a decrease in the neck length. Minimum von Mises stress of 161.83 MPa was found for the complete implant and in the head-neck region, a minimum von Mises stress found 91.9 MPa.

CONCLUSIONS

Performance evaluation of hip implant under static loading conditions gives a clear idea about the behavior of implant. It was found that a decrease in the von Mises stresses with a decrease in the taper length. However, these variations won't affect much in the performance of the hip implant. Also, a reduction in taper length can significantly increase the dislocation in the implant. So it is advised to consider the optimal taper length with an increase in the femoral head size.

Evaluation of the Relation Between Preload Values and Pull-Out Force of the Cortical Screw Used in Bone Fracture

This study aims to examine the relation between pull-out strength and preload values of the cortical screw used in bone fracture fixation. The research question is that "Does the pull-out strength of the cortical screw used in the bone fracture fixation change with the preload values of the screw change?". To perform this purpose, the finite element method was selected due to its ease to evaluate and calculate the stresses on the whole model. Models of a cortical screw, partial plate, and bone were created using the SolidWorks program. The material properties of the bone were selected orthotropic material type. The bone fixed on the distal and proximal ends. The pull-out forces were applied at the bottom of the plate. The screw that has been loaded ranges from 100 N-700 N as preload. The pull-out forces were determined 200-400-600 N as in the literature. The results show that the pull-out strength of the screw was changed when the preloaded values higher than 400 N. However, it was seen that the pull-out strength does not substantially change when the preload values were lower than 400 N. When the preload values were applied ≥500 N, the maximum von Mises stresses on the screw exceeded the critical strength of the screw material. In conclusion, the critical preload value was determined as 500 N for the optimum pull-out strength.

Progress of antibiotic-loaded bone cement in joint arthroplasty

Bone cement, consisting of polymethyl methacrylate, is a bioinert material used for prothesis fixation in joint arthroplasty. To treat orthopedic infections, such as periprosthetic joint infection, antibiotic-loaded bone cement (ALBC) was introduced into clinical practice. Recent studies have revealed the limitations of the antibacterial effect of ALBC. Moreover, with the increase in high infection risk patients and highly resistant microbes, more researches and modification of ALBC are required. This paper reviewed latest findings about ALBC for most popular and destructive pathogens, summarized the influence of antibiotic kind, drug dosage, application method, and environment towards characteristic of ALBC. Subsequently, new cement additives and clinical applications of ALBC in joint arthroplasty were also discussed.