Design process of cementless femoral stem using a nonlinear three dimensional finite element analysis

Effect of stem position and length on bone-stem constructs after cementless hip arthroplasty

Aims

There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning.

Methods

In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition.

Results

Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition.

Conclusion

The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem.

Cite this article: Bone Joint Res 2021;10(4):250–258.

Comparative analysis of the biomechanical behavior of two different design metaphyseal-fitting short stems using digital image correlation

BACKGROUND

The progressive evolution in hip replacement research is directed to follow the principles of bone and soft tissue sparing surgery. Regarding hip implants, a renewed interest has been raised towards short uncemented femoral implants. A heterogeneous group of short stems have been designed with the aim to approximate initial, post-implantation bone strain to the preoperative levels in order to minimize the effects of stress shielding. This study aims to investigate the biomechanical properties of two distinctly designed femoral implants, the TRI-LOCK Bone Preservation Stem, a shortened conventional stem and the Minima S Femoral Stem, an even shorter and anatomically shaped stem, based on experiments and numerical simulations. Furthermore, finite element models of implant-bone constructs should be evaluated for their validity against mechanical tests wherever it is possible. In this work, the validation was performed via a direct comparison of the FE calculated strain fields with their experimental equivalents obtained using the digital image correlation technique.

RESULTS

Design differences between Trilock BPS and Minima S femoral stems conditioned different strain pattern distributions. A distally shifting load distribution pattern as a result of implant insertion and also an obvious decrease of strain in the medial proximal aspect of the femur was noted for both stems. Strain changes induced after the implantation of the Trilock BPS stem at the lateral surface were greater compared to the non-implanted femur response, as opposed to those exhibited by the Minima S stem. Linear correlation analyses revealed a reasonable agreement between the numerical and experimental data in the majority of cases.

CONCLUSION

The study findings support the use of DIC technique as a preclinical evaluation tool of the biomechanical behavior induced by different implants and also identify its potential for experimental FE model validation. Furthermore, a proximal stress-shielding effect was noted after the implantation of both short-stem designs. Design-specific variations in short stems were sufficient to produce dissimilar biomechanical behaviors, although their clinical implication must be investigated through comparative clinical studies.

Patient-specific design process and evaluation of a hip prosthesis femoral stem

INTRODUCTION

There are several commercially available hip implant systems. However, for some cases, custom implant designed based on patient-specific anatomy can offer the patient the best available implant solution. Currently, there is a growing trend toward personalization of medical implants involving additive manufacturing into orthopedic medical implants' manufacturing.

METHODS

This article introduces a systematic design methodology of femoral stem prosthesis based on patient's computer tomography data. Finite element analysis is used to evaluate and compare the micromotion and stress distribution of the customized femoral component and a conventional stem.

RESULTS

The proposed customized femoral stem achieved close geometrical fit and fill between femoral canal and stem surfaces. The customized stem demonstrated lower micromotion (peak: 21 μm) than conventional stem (peak: 34 μm). Stress results indicate up to 89% increase in load transfer by conventional stem than custom stem because the higher stiffness of patient-specific femoral stem proximally increases the custom stem shielding in Gruen's zone 7. Moreover, patient-specific femoral stem transfers the load widely in metaphyseal region.

CONCLUSION

The customized femoral stem presented satisfactory results related to primary stability, but compromising proximo-medial load transfer due to increased stem cross-sectional area increased stem stiffness.

Research: Design and Analysis of a Low-Stiffness Porous Hip Stem

Two major problems are associated with total hip replacement: 1) stress shielding and 2) the adverse tissue reaction to certain elements of the implant material. In this regard, a porous implant provides lower stiffness and vacancies for bone ingrowth, making it more suitable for the human bone compared with a solid stem. Moreover, second-generation titanium biomedical alloys, such as TNZT (Ti35Nb7Zr5Ta) and TMZF (Ti12Mo6Zr2Fe), have been introduced to prevent the adverse tissue reactions related to aluminum and vanadium elements of the popular Ti6Al4V alloy. In the current work, an analysis was performed based on uniaxial compression testing of cubic Ti6Al4V structures of different porosities to predict the governing equations that relate the relative density of the structure to the mechanical properties of the structure according to the Gibson-Ashby model. A numerical study was conducted to evaluate the change in stress distribution obtained by incorporating the new titanium alloys in porous hip stem implants. Implants modeled with the mechanical properties of TNZT and TMZF showed a minimum safety factor of 1.69 and 3.02, respectively, with respect to the yield strength. The results demonstrated an increase in the equivalent von Mises stresses and maximum principal elastic strain up to 7% and 15%, respectively, compared with the porous Ti6Al4V implant and up to 108% and 156%, respectively, compared with the solid Ti6Al4V implant.

The impact of canal flare index on leg length discrepancy after total hip arthroplasty

INTRODUCTION

The femoral stem should protrude from femur by an appropriate vertical distance to allow leg length equalization at hip arthroplasty; this distance depends on the size/shape of medullary canal and implant. The relationship between femoral morphology and achievability of leg length restoration is currently unclear. Our aim was to examine the impact of the femoral canal flare index (CFI) on the risk of leg length discrepancy (LLD) after total hip arthroplasty with different femoral stems.

MATERIALS AND METHODS

The study cohort included 126 patients with unilateral primary total hip arthroplasty due to idiopathic osteoarthritis and three different types of implanted femoral stems. The impact of CFI on postoperative LLD was assessed with separate logistic regression model for each implant and covariables of age, gender, body mass index and femoral neck resection level.

RESULTS

Higher CFI was an independent risk factor for postoperative LLD ≥ 5 mm with odds ratio 4.5 (p = 0.03) in 49 stems with cementless metaphyseal fixation Implantcast-EcoFit®, regardless of the femoral neck resection level. CFI had no significant impact on LLD in 30 stems with cementless diaphyseal fixation EndoPlus-Zweymüller® or 47 cemented collared stems Link-SPII®. No significant difference was observed between groups in pre/postoperative WOMAC scores, postoperative radiographic LLD, subjectively reported LLD, insole use or complications after mean 6.8 years of follow-up.

CONCLUSIONS

Higher CFI increases the risk of clinically detectable postoperative LLD in single-wedge femoral stems with cementless metaphyseal fixation. CFI has no significant impact on LLD in femoral stems with cementless diaphyseal fixation or cemented fixation.

Pre-clinical evaluation of the mechanical properties of a low-stiffness cement-injectable hip stem

In total hip arthroplasty (THA), the femoral stem can be fixed with or without bone cement. Cementless stem fixation is recommended for young and active patients as it eliminates the risk of loss of fixation at the bone-cement and cement-implant interfaces. Cementless fixation, however, suffers from a relatively high early revision rate. In the current research, a novel low-stiffness hip stem was designed, fabricated and tested. The stem design provided the option to inject biodegradable bone cement that could enhance initial stem stability. The stem was made of Ti6Al4V alloy. The proximal portion of the stem was porous, with cubic cells. The stem was fabricated using electron beam melting (EBM) technology and tested in compression and bending. Finite-element analysis was used to evaluate stem performance under a dynamic load representing a stair descending cycle and compare it to the performance of a solid stem with similar geometry. The von Mises stresses and maximum principal strains generated within the bone increased after porous stem insertion compared to solid stem insertion. The low-modulus stem tested in this study has acceptable mechanical properties and generates strain patterns in bone that appear compatible with clinical use.

Comparative Analysis of the Biomechanical Behaviour of Two Cementless Short Stems for Hip Replacement: Linea Anatomic and Minihip

A comparative study between two stems (Linea Anatomic and Minihip) has been performed in order to analyse the differences in their biomechanical behaviour, concerning stem micromotions and load transmission between stem and bone. From the corresponding finite element models, a parametric study was carried out to quantify ranges of micromotions taking into account: friction coefficient in the stem-bone interface, press-fit and two types of gait cycle. Micromotions were evaluated for each stem at six different levels along repeated gait cycles. An initial and marked stem subsidence at the beginning of the simulation was observed, followed by an asymptotic decrease due to friction forces. Once migration occurs, a repeated reversible cyclic micromotion is developed and stabilized as gait cycle times are simulated. The general motion pattern exhibited higher amplitude of micromotion for Minihip compared to Linea stem. The load transmission mechanism was analyzed, identifying the main internal forces. The results show higher local forces for Minihip stem up to 80% greater than for Linea stem. The differences of design between Minihip and Linea conditioned different distributions of load, influencing the posterior stress-shielding. Consequently, short stems require high bone stock and quality should, being indicated for young patients with high bone quality.

No effect of femoral offset on bone implant micromotion in an experimental model

BACKGROUND

In total hip replacement (THR), the femoral offset (FO) is assessed preoperatively, and the surgeon must determine whether to restore, increase, or decrease the FO based on experience and the patient's clinical history. The FO is known to influence the abductor muscle strength, range of motion (ROM), gait, and hip pain after THR; however, the true effect of FO on bone implant micromotion is unclear. Therefore, we investigated to assess: (1) the muscle loading response during gait, (2) whether FO affects bone implant micromotion during gait.

HYPOTHESIS

A variation of ±10mm from the anatomical FO affects the muscle loading forces.

MATERIALS AND METHODS

We modified a personalized musculoskeletal model of the lower extremity to determine the 3-dimensional contact forces at the hip joint in the presence of a stem with varying offsets during a gait cycle. A detailed finite element (FE) model was then constructed for increased, restored, and decreased FOs. The maximum load obtained during normal walking gait from the musculoskeletal model was applied to the respective FE models, and the resultant stem-bone micromotion and stress distribution were computed.

RESULTS

Increasing the FO to +10mm decreased the peak force generated by the abductor muscles during the cycle by 15.0% and decreasing the FO to -10mm increased the von Mises stress distribution at the distal bone by 77.5% (P<0.05). A variation of the offset within 10mm of the anatomical offset showed no significant differences in micromotion (P>0.05) and peak stresses (P>0.05).

DISCUSSION

Coupling the musculoskeletal model of the gait cycle with FE analysis provides a realistic model to understand how FO affects bone implant micromotion. We found that there was no effect of FO on bone implant micromotion; thus, a surgeon does not need to evaluate the implications of FO on micromotion and can determine a FO that best decreases the work load of abductor muscles, increases ROM, and reduces hip pain.

LEVEL OF EVIDENCE

IV, biomechanical study.

Three-Dimensional Analysis of the Characteristics of the Femoral Canal Isthmus: An Anatomical Study

Purpose. To establish a new approach for measuring and locating the femoral intramedullary canal isthmus in 3-dimensional (3D) space. Methods. Based on the computed tomography data from 204 Chinese patients, 3D models of the whole femur and the corresponding femoral isthmus tube were reconstructed using Mimics software (Materialise, Haasrode, Belgium). The anatomical parameters of the femur and the isthmus, including the femur length and radius, and the isthmus diameter and height, were measured accordingly. Results. The mean ratio of the isthmus height versus the femoral height was 55 ± 4.8%. The mean diameter of the isthmus was 10.49 ± 1.52 mm. The femoral length, the isthmus diameter, and the isthmus tube length were significantly larger in the male group. Significant correlations were observed between the femoral length and the isthmus diameter (r = 0.24, p < 0.01) and between the femoral length and the isthmus height (r = 0.6, p < 0.01). Stepwise linear regression analyses demonstrated that the femoral length and radius were the most important factors influencing the location and dimension of the femoral canal isthmus. Conclusion. The current study developed a new approach for measuring the femoral canal and for optimization of customer-specific femoral implants.

Primary stability recognition of the newly designed cementless femoral stem using digital signal processing

Stress shielding and micromotion are two major issues which determine the success of newly designed cementless femoral stems. The correlation of experimental validation with finite element analysis (FEA) is commonly used to evaluate the stress distribution and fixation stability of the stem within the femoral canal. This paper focused on the applications of feature extraction and pattern recognition using support vector machine (SVM) to determine the primary stability of the implant. We measured strain with triaxial rosette at the metaphyseal region and micromotion with linear variable direct transducer proximally and distally using composite femora. The root mean squares technique is used to feed the classifier which provides maximum likelihood estimation of amplitude, and radial basis function is used as the kernel parameter which mapped the datasets into separable hyperplanes. The results showed 100% pattern recognition accuracy using SVM for both strain and micromotion. This indicates that DSP could be applied in determining the femoral stem primary stability with high pattern recognition accuracy in biomechanical testing.