Total hip arthroplasty wear simulation using the boundary element method

In this paper an application of the boundary element method for simulating wear in total hip prosthesis is presented. Several examples including different update periods of the worn acetabular cup, various femoral head sizes and various materials for both the femoral head and the acetabular cup are simulated under the same variable loading conditions for up to 20 years of service. Moreover, two different femoral models are considered in order to investigate the influence of the femoral modelling. The analysis demonstrates that due to the boundary only modelling requirement, the computational time and storage remains low, allowing large service periods to be simulated. Generally, the results obtained are in good agreement with other researchers findings. Moreover, ignoring the bending of the femoral neck in the model, results in a small overestimation of the maximum wear depth, while the volumetric wear is slightly underestimated. However, these differences are trivial considering the reduction of the computational effort.     

Cementless Vektor-titan stem in total hip arthroplasty.

INTRODUCTION: Cementless THR is a well established, and a widely accepted optimal procedure for younger patients. The cementless Vektor-Titan stem is made of Ti6AI7Nb, has got the shape of a three-dimensional cone, and an optimal proximal anchoring property. MATERIALS AND METHODS: The aim of this prospective study was to scrutinise the outcome of 250 Vektor-Titan stems in cementless THRs with an average follow-up time of 3.0 years (Min: 1, Max: 6). The average age of the patients including 148 women and 102 men was calculated with 54.6 years (Min: 22.5, Max: 77.7). RESULTS: The score according to Merle d'Aubigné improved from preoperative 9.3 (Min: 7, Max: 13) to postoperative 17.0 (Min: 14, Max: 18). Distal cortical hypertrophy and proximal atrophy was detected in 4 cases. Single atrophy of the proximal femur was found in additional 3 cases. Progressive radiolucent lines in zone 1 and 7 according to Gruen were observed in one case. Postoperative local and general complications were seen as two subfascial hematomas, two single dislocations, two recurrent dislocations of the hip prosthesis, 6 lesions of the sciatic nerve (one persisting), two deep venous thrombosis, two pneumonias, and one lethal pulmonary embolism. A stable proximal fixation was achieved in 242 of 250 cases (96.8%). CONCLUSION: The results of this study using the Vektor-Titan stem in cementless total hip arthroplasty showed that the principle of proximal fixation was optimized. Long term follow-up studies are needed to confirm these good results.     

Osteodensitometry in uncemented total hip arthroplasty using computertomography.

The aim of this investigation was to evaluate a new method developed for the measurement of bone mineral density and bone remodelling phenomena after total hip arthroplasty using computer tomography. Computertomography is a radiological technique to examine bone structures in high resolution. Using an extended scale it is possible to investigate bone scans and implants with fewer metal artifacts. For osteodensitometry measurement a special software (IMPact HIP) for the analysis of the data was used. The measured parameters were the overall bone mineral density (mg Calcium-Hydroxyapatite/ml) and the cortical bone structure. A standard scan mode enable to compare the computertomography scans at follow-up. Nineteen total hip arthroplasty patients (20 hips) with a mean age of 58 years (31-70) were operated on using an uncemented titanium alloy stem with a tapered design. The periprosthetic bone was assessed using computertomography-assisted osteodensitometry two weeks and one year after surgery. We observed a decrease of the overall bone mineral density (15%) and of the cortical bone structure (20%) one year after insertion of the stem in the proximal part of the femur. The area corresponds to the Gruen zones 1 and 7. On the other hand, a decrease of mineral density of 5% for the overall bone and of 3% for the cortical bone was found at the level of the tip of the stem, which corresponds to the Gruen zones 3, 4 and 5. Computertomography-assisted osteodensitometry allows to investigate the bone remodelling after total hip arthroplasty by separating the analysis of the overall bone mineral density and of the cortical structure. The present method is a reliable tool for quality-control in total hip arthroplasty.     

Mixing and matching in ceramic-on-metal hip arthroplasty: An in-vitro hip simulator study

The clinical success of second-generation metal-on-metal hip replacement and the good tribological performance of alumina ceramic revived an interest in hip articulation as a solution to reduce wear.This study was aimed at characterizing the wear behaviour of new hybrid ceramic-on-metal bearings. In particular, this study investigated the wear behaviour of ceramic-on-metal hip components (three different diameters configurations: 28, 32 and 36 mm), not specifically proposed to be coupled, in order to compare them with ceramic-on-ceramic, which is considered to be the gold standard for wear resistance. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear.After five million cycles, our results showed significantly greater wear-in ceramic-on-metal compared with ceramic-on-ceramic, and significant greater wear for the 32-mm diameter compared with the 36-mm one. Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the ceramic-on-ceramic ones confirming that even in the case of ceramic-on-metal bearings, mixing and matching could not prove effective wear behaviour, not even comparable with that of the ceramic-on-ceramic gold standard. Wear patterns and roundness tolerances certainly discourage the coupling of components not specifically intended to be coupled. Unsuitable geometrical conformity could, in fact, result in a poor dynamic behaviour and lead to clinical failure.     

Finite-element modelling of femoral shaft fracture fixation techniques post total hip arthroplasty.

The presence of a femoral prosthesis superior to a shaft fracture severely complicates fixation and treatment. This study uses two-dimensional, multithickness, plane stress finite-element models of a femur with prosthesis to investigate the stresses developed with the application of three popular fixation techniques: revision to a long stem prosthesis, lateral plating with a cortical bone allograft strut and cerclage wires, and custom plate application with proximal Parham band fixation with distal cortical screws (Ogden plate). The plate and bone contact as well as the fracture site contact were modelled by using orthotropic elements with custom-fit moduli so that only the normal stress to the interface was significant. A thermal analogy was used to model the cerclage and Parham band preloads so that representative preloads in the proximal fixation of the two types of plate treatments could be modelled. A parametric study was performed with the long-prosthesis model to show variations in stem lengths of one, two and three femoral diameters distal to the fracture site. The Ogden plate model showed a transfer of tensile stress near the proximal-most band, with the highest tensile stress being at the fracture site with evidence of stress shielding of the proximal lateral cortex. The cortical bone strut model showed a transfer of tensile stress to the bone strut but showed less shielding of the proximal cortex. The cerclage wires at the base of the bone strut showed the highest changes in load with the distalmost wire increasing to almost four times its original preload.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pelvic instability and trunk and hip muscle recruitment patterns in patients with total hip arthroplasty

Hip and lumbar spine disorders often coexist in patients with total hip arthroplasty (THA). The current study aimed to reveal pelvic motion pathology and altered trunk and hip muscle recruitment patterns relating to pelvic motion in patients with THA. Twenty-one women who underwent THA and 12 age-matched healthy women were recruited. Pelvic kinematics and muscle recruitment patterns (i.e., amplitude, activity balance, and onset timing) of the gluteus maximus, semitendinosus, multifidus, and erector spinae were collected during prone hip extension. Compared with healthy subjects, the patients showed increased pelvic motion, especially ventral rotation, decreased multifidus muscle activity relative to the hip extensors, and delayed onset of multifidus activity, despite reaction times and speeds of leg motion not being significantly different between the groups. Furthermore, while contributing factors associated with ventral pelvic rotation were not found, delayed onset of multifidus activity was detected as a factor related to the increased anterior tilt of the pelvis (r = 0.47, p < 0.05) in patients with THA. These results suggest that patients with THA have dysfunction of the stabilizer muscles of the lumbopelvic region along with increased pelvic motion.     

Hypertensive patients show delayed wound healing following total hip arthroplasty

Background

Prolonged wound-discharge following total hip arthroplasty (THA) is associated with an increased risk of infection. However, the potential role of hypertension in prolonging the duration of wound healing in this population has not yet been investigated. The aim of the present study was to compare healing in this population that has not yet been investigated. The aim of the present study was to compare hypertensive and normotensive THA patients in terms of the length of time required to achieve a dry wound and the length of stay in the hospital.

Methods

One hundred and twenty primary THA patients were evaluated. Pre-operative clinical history and physical examination revealed that 29 were hypertensive and 91 were normotensive. The two groups were statistically matched using optimal propensity score matching. The outcomes of interest were the number of days until a dry wound was observed and the duration of hospital stay.

Results

The average systolic blood pressures were 150.1 mmHg and 120.3 mmHg for the hypertensive and normotensive groups, respectively. The mean number of days until the wound was dry was 3.79 for the hypertensive group and 2.03 for the normotensive group. Hypertensive patients required more days for their wounds to dry than normotensive patients (odds ratio  = 1.65, p<0.05). No significant difference in the duration of hospital stay was found between the two groups.

Conclusions

Hypertensive patients had a higher risk of prolonged wound discharge after THA than their normotensive counterparts. Patients with prolonged wound drainage are at greater risk for infection. Clinicians should pay particular attention to infection-prevention strategies in hypertensive THA patients.     

Comparison of patient and surgeon expectations of total hip arthroplasty

Objectives

Analysis of discrepancies between patient and surgeon expectations before total hip arthroplasty (THA) should enable a better understanding of motives of dissatisfaction about surgery, but this question has been seldom studied. Our objectives were to compare surgeons'' and patients'' expectations before THA, and to study factors which affected surgeon-patient agreement.

Methods

132 adults (mean age 62.8+/−13.7 years, 52% men) on waiting list for THA in three tertiary care centres and their 16 surgeons were interviewed to assess their expectations using the Hospital for Special Surgery Total Hip Replacement Expectations Survey (range 0–100). Patients'' and surgeons'' answers were compared, for the total score and for the score of each item. Univariate analyses tested the effect of patients'' characteristics on surgeons'' and patients'' expectations separately, and on surgeon-patient differences.

Results

Surgeon and patient expectations'' mean scores were high (respectively 90.9+/−11.1 and 90.0+/−11.6 over 100). Surgeons'' and patients'' expectations showed no systematic difference, but there was little agreement on Bland and Altman graph and correlation coefficient was low. Patients had higher expectations than surgeons for sports. Patients rated their expectations according to trust in physician and mental quality of life, surgeons considered disability. More disabled patients and patients from a low-income professional category were often “more optimistic” than their surgeons.

Conclusion

Surgeons and patients often do not agree on what to expect from THA. More disabled patients expect better outcomes than their surgeons.     

A novel formulation for scratch-based wear modelling in total hip arthroplasty

Damage to the femoral head in total hip arthroplasty often takes the form of discrete scratches, which can lead to dramatic wear acceleration of the polyethylene (PE) liner. Here, a novel formulation is reported for finite element (FE) analysis of wear acceleration due to scratch damage. A diffused-light photography technique was used to globally locate areas of damage, providing guidance for usage of high-magnification optical profilometry to determine individual scratch morphology. This multiscale image combination allowed comprehensive input of scratch-based damage patterns to an FE Archard wear model, to determine the wear acceleration associated with specific retrieval femoral heads. The wear algorithm imposed correspondingly elevated wear factors on areas of PE incrementally overpassed by individual scratches. Physical validation was provided by agreement with experimental data for custom-ruled scratch patterns. Illustrative wear acceleration results are presented for four retrieval femoral heads.     

The release of metals from metal-on-metal surface arthroplasty of the hip

The aim of the study was to evaluate the serum and urine levels of cobalt (Co), chromium (Cr), manganese (Mn), molybdenum (Mo) and nickel (Ni) in patients who had undergone metal-on-metal hybrid surface arthroplasties on the supposition that a release of metals would occur due to the large head size of this type of implant. Metal levels were determined by using an analytical method based on sector field inductively coupled plasma mass spectrometry. Results showed a significant difference between patients and control subjects in mean levels of Co and Cr in serum (p<0.0001 and p=0.02, respectively) and in urine (p<0.0001 for both). No significant differences were observed in mean serum and urinary levels of Mn, Mo and Ni. Although the clinical consequences of these changes, if any, are unknown, further studies could be performed in a larger number of subjects implanted with a total surface arthroplasty at follow-up times over different periods.     

Molecular and immune toxicity of CoCr nanoparticles in MoM hip arthroplasty

Theoretical, desirable features of second-generation metal-on-metal (MoM) hip prostheses have led to their widespread use. However, the bearing surfaces, consisting of complex cobalt-chromium alloys, are subject to wear and the release of cobalt and chromium (CoCr) nanoparticles. These nanoparticles can reduce cellular viability, induce DNA damage, lead to chromosomal aberrations, and possibly stimulate increased metal hypersensitivity. Clinically, the effects can be both local (soft-tissue reactions) and systemic (arthroprosthetic cobaltism). This review assesses the literature concerning the in vitro and in vivo cytotoxic, genotoxic, and immunotoxic effects of CoCr wear particles, which is increasingly important in view of the large number of MoM arthroplasties performed.