Friction couples in total hip replacement

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.

Selective Laser Melting of PA 2200 for Hip Implant Applications: Finite Element Analysis, Process Optimization, and Morphological and Mechanical Characterization

Polyamide 12 (PA 22000) is a well-known material and one of the most biocompatible materials tested and used to manufacture customized medical implants by selective laser sintering technology. To optimize the implants, several research activities were considered, starting with the design and manufacture of test samples made of PA 2200 by selective laser sintering (SLS) technology, with different processing parameters and part orientations. The obtained samples were subjected to compression tests and later to SEM analyses of the fractured zones, in which we determined the microstructural properties of the analyzed samples. Finally, an evaluation of the surface roughness of the material and the possibility of improving the surface roughness of the realized parts using finite element analysis to determine the optimum contact pressure between the component made of PA 2200 by SLS and the component made of TiAl6V4 by SLM was performed.

3D printed zirconia ceramic hip joint with precise structure and broad-spectrum antibacterial properties

Background: Nowadays, zirconia ceramic implants are widely used as a kind of hip prosthesis material because of their excellent biocompatibility and long-term wear resistance. However, the hip joint is one of the major joints with complex 3D morphological structure and greatly individual differences, which usually causes great material waste during the process of surgical selection of prosthesis. Methods: In this paper, by combining ceramic 3D printing technology with antibacterial nano-modification, zirconia ceramic implant material was obtained with precise 3D structure and effective antibacterial properties. Among which, two technical problems (fragile and sintering induced irregular shrinkage) of 3D printed ceramics were effectively minimized by optimizing the reaction conditions and selective area inversing compensation. Through in vivo and in vitro experiments, it was confirmed that the as prepared hip prosthesis could precisely matched the corresponding parts, which also exhibited good biocompatibility and impressive antibacterial activities. Results: 1) Two inherent technical problems (fragile and sintering induced irregular shrinkage) of 3D printed ceramics were effectively minimized by optimizing the reaction conditions and selective area inversing compensation. 2) It could be seen that the surface of the ZrO2 material was covered with a layer of ZnO nano-particles. A universal testing machine was used to measure the tensile, bending and compression experiments of ceramic samples. It could be found that the proposed ZnO modification had no significant effect on the mechanical properties of ZrO2 ceramics. 3) According to the plate counting results, ceramics modified with ZnO exhibited significantly higher antibacterial efficiency than pure ZrO2 ceramics, the ZrO2-ZnO ceramics had a significant killing effect 8 hours. 4) The removed implants and the tissue surrounding the implant were subjected to HE staining. For ZrO2-ZnO ceramics, inflammation was slight, while for pure ZrO2 ceramics, the inflammatory response could be seen that the antibacterial rate of the ZrO2-ZnO ceramics was significantly better than that of the pure ZrO2 ceramics group. 5) It could be seen that the cytotoxicity did not increase proportionally with the increase of concentration, all of viability were still above 80%. This suggested that our materials were safe and could be applied as a type of potential biomaterial in the future. 6) Further animal studies demonstrated that the implant was in good position without dislocation. This resulted implied that the proposed method can achieve accurate 3D printing preparation of ceramic joints. In addition, the femurs and surrounding muscles around the implant were then sectioned and HE stained. Results of muscle tissue sections further showed no significant tissue abnormalities, and the growth of new bone tissue was observed in the sections of bone tissue. Conclusion: 1) The ceramic 3D printing technology combined with antibacterial nano-modification can quickly customize the ideal implant material with precise structure, wear-resistant and effective antibacterial properties. 2) Two inherent technical problems (fragile and sintering induced irregular shrinkage) of 3D printed ceramics were effectively minimized by optimizing the reaction conditions and selective area inversing compensation. 3) ZnO nano-materials were modified on the ceramic surface, which could effectively killing pathogenic bacteria.

Simultaneous measurement of friction and wear in hip simulators

We propose and have evaluated a method to measure hip friction during wear testing on a popular multi-station hip simulator. A 6-degree-of-freedom load cell underneath the specimen sensed forces and torques during implant wear testing of simulated walking. This included internal–external and adduction–abduction rotations which are often neglected during friction testing on pendulum-type machines. Robust mathematical analysis and data processing provided friction estimates in three simultaneous orthogonal rotations, over extended multi-million cycle wear tests. We tested various bearing couples including metal-on-plastic, ceramic-on-plastic, and metal-on-metal material couples. In one test series, new and intentionally scratched CoCrMo 40-mm-diameter femoral heads were tested against conventional ultrahigh-molecular-weight polyethylene, highly cross-linked, and highly cross-linked with vitamin E versions. The scratching significantly increased friction and doubled the wear of all groups. Before scratching, friction levels for the aforementioned plastic groups were 0.056 ± 0.0060, 0.062 ± 0.0080, and 0.070 ± 0.0045, respectively, but after scratching increased to 0.088 ± 0.018, 0.076 ± 0.0066, and 0.082 ± 0.0049, respectively, all statistically significant increases (p = 0.00059, 0.00005, 0.0115, respectively). In another test series of 44-mm femoral head diameter hips, metal-on-plastic hips with conventional ultrahigh-molecular-weight polyethylene showed the lowest friction at 0.045 ± 0.0085, followed by highly cross-linked with 0.046 ± 0.0035 (not significantly different). In a ceramic-on-plastic design with conventional ultrahigh-molecular-weight polyethylene, higher friction 0.079 ± 0.0070 was measured likely due to that ceramic surface being rougher than usual. Metal-on-metal hips were compared without and with a TiN coating, resulting in 0.049 ± 0.014 and 0.097 ± 0.020 friction factors, respectively (statistically significant, p < 0.001), and the coating wore away on all coated hips eventually. Higher friction mostly correlated with higher wear or damage to femoral heads or implant coatings, except for the highly cross-linked wear resistant ultrahigh-molecular-weight polyethylene which had slightly higher friction, confirming the same finding in other independent studies. This type of friction measurements can help screen for clamping and elevated wear of metal-on-metal and resurfacing total hip replacements, surgical malpositioning, and abraded and otherwise damaged surfaces.

The use of ceramic-on-ceramic bearings in isolated revision of the acetabular component

The practice of removing a well-fixed cementless femoral component is associated with high morbidity. Ceramic bearing couples are low wearing and their use minimises the risk of subsequent further revision due to the production of wear debris. A total of 165 revision hip replacements were performed, in which a polyethylene-lined acetabular component was revised to a new acetabular component with a ceramic liner, while retaining the well-fixed femoral component. A titanium sleeve was placed over the used femoral trunnion, to which a ceramic head was added. There were 100 alumina and 65 Delta bearing couples inserted. The mean Harris hip score improved significantly from 71.3 (9.0 to 100.0) pre-operatively to 91.0 (41.0 to 100.0) at a mean follow up of 4.8 years (2.1 to 12.5) (p < 0.001). No patients reported squeaking of the hip. There were two fractures of the ceramic head, both in alumina bearings. No liners were seen to fracture. No fractures were observed in components made of Delta ceramic. At 8.3 years post-operatively the survival with any cause of failure as the endpoint was 96.6% (95% confidence interval (CI) 85.7 to 99.3) for the acetabular component and 94.0% (95% CI 82.1 to 98.4) for the femoral component. The technique of revising the acetabular component in the presence of a well-fixed femoral component with a ceramic head placed on a titanium sleeve over the used trunnion is a useful adjunct in revision hip practice. The use of Delta ceramic is recommended.

Taper junction failure in large-diameter metal-on-metal bearings

OBJECTIVES

An ongoing prospective study to investigate failing metal-on-metal hip prostheses was commenced at our centre in 2008. We report on the results of the analysis of the first consecutive 126 failed mated total hip prostheses from a single manufacturer.

METHODS

Analysis was carried out using highly accurate coordinate measuring to calculate volumetric and linear rates of the articular bearing surfaces and also the surfaces of the taper junctions. The relationship between taper wear rates and a number of variables, including bearing diameter and orientation of the acetabular component, was investigated.

RESULTS

The measured rates of wear and distribution of material loss from the taper surfaces appeared to show that the primary factor leading to taper failure is the increased lever arm acting on this junction in contemporary large-diameter metal-on-metal hip replacements.

CONCLUSIONS

Our analysis suggests that varus stems, laterally engaging taper systems and larger head diameters all contribute to taper failure.

Taper junction failure in large-diameter metal-on-metal bearings

Objectives

An ongoing prospective study to investigate failing metal-on-metal hip prostheses was commenced at our centre in 2008. We report on the results of the analysis of the first consecutive 126 failed mated total hip prostheses from a single manufacturer.

Methods

Analysis was carried out using highly accurate coordinate measuring to calculate volumetric and linear rates of the articular bearing surfaces and also the surfaces of the taper junctions. The relationship between taper wear rates and a number of variables, including bearing diameter and orientation of the acetabular component, was investigated.

Results

The measured rates of wear and distribution of material loss from the taper surfaces appeared to show that the primary factor leading to taper failure is the increased lever arm acting on this junction in contemporary large-diameter metal-on-metal hip replacements.

Conclusions

Our analysis suggests that varus stems, laterally engaging taper systems and larger head diameters all contribute to taper failure.

Dual mobility design use in preventing total hip replacement dislocation following tumor resection

INTRODUCTION

Total hip replacement (THR) following hip tumor resection incurs a high risk of dislocation. We assessed the incidence of dislocation associated with use of a dual mobility cup,and the functional results achieved.

HYPOTHESIS

Use of a dual mobility cup would reduce the risk of THR instability following hip tumor resection.

MATERIAL AND METHODS

We analyzed dislocation rates in a retrospective series of 71 dual mobility cup THRs implanted following the resection of a tumor hip condition: 33 primary bone tumors and 38 bone metastases. The presenting pathology was diagnosed anatomically, and surgery classified in terms of adopted abductor system strategy. Functional results were assessed in terms of pain (analgesia on the World Health Organisation [WHO] scale), assisted walking and Musculoskeletal Tumor Society (MSTS) score.

RESULTS

An overall rate of 9.8% dislocation was observed, taking into account all etiologies and contexts together. More precisely, this rate resulted from a compound figure of 5.2% in bone metastasis and 15% in primitive bone tumor. Dislocation risk depended lesson etiology than on the surgical management of the abductor system, being 3.5% in the case of abductor conservation, 9.5% in the case of abductor sectioning/reinsertion, and 18%in case of gluteus medius muscle or nerve resection. Functional improvement was consistently observed, especially in bone metastasis. At the maximal follow-up, 32 patients were not using analgesics, six were taking WHO class III analgesics, 10 class II and 23 class I. Mean MSTS score was 68.1% +/- 23.5% in bone metastasis and 59.6% +/- 17.5% in primary bone tumor.Fourteen patients could walk without assistance, 33 with a single cane, 15 with two canes and eight with a walker; one patient had not been able to resume walking.

DISCUSSION

In these indications, dual mobility cups use lead to lower dislocation rates than those reported in the literature. It proved especially effective in the case of bone metastasis and consolidation surgery. In the case of primary bone tumor, it failed to prevent dislocation following acetabular resection, especially when involving the abductor muscles and/or abductor innervation, although it provided lower dislocation rates, comparable to those experienced with other techniques, when applied to limited resection.

LEVEL OF EVIDENCE

IV. Retrospective therapeutic study.