Study of micromotion in modular acetabular components during gait and subluxation: a finite element investigation

Polyethylene wear after total hip arthroplasty may occur as a result of normal gait and as a result of subluxation and relocation with impact. Relocation of a subluxed hip may impart a moment to the cup creating sliding as well as compression at the cup liner interface. The purpose of the current study is to quantify, by a validated finite element model, the forces generated in a hip arthroplasty as a result of subluxation relocation and compare them to the forces generated during normal gait. The micromotion between the liner and acetabular shell was quantified by computing the sliding track and the deformation at several points of the interface. A finite element analysis of polyethylene liner stress and liner/cup micromotion in total hip arthroplasty was performed under two dynamic profiles. The first profile was a gait loading profile simulating the force vectors developed in the hip arthroplasty during normal gait. The second profile is generated during subluxation and subsequent relocation of the femoral head. The forces generated by subluxation relocation of a total hip arthroplasty can exceed those forces generated during normal gait. The induced micromotion at the cup polyethylene interface as a result of subluxation can exceed micromotion as a result of the normal gait cycle. This may play a significant role in the generation of backsided wear. Minimizing joint subluxation by restoring balance to the hip joint after arthroplasty should be explored as a strategy to minimize backsided wear.     

Fracture analysis of a ceramic liner. Is in hip endoprosthesis replacement of ceramic on ceramic components with only one of the corresponding partners justified?]

The good biomechanical and tribological properties, together with the excellent biocompatibility, of ceramic-on-ceramic components, make them a preferential choice for total hip replacement surgery, at least in Europe. We report on a man admitted as an outpatient with painless grating in the hip one year after replacement of a ceramic femoral head, but not of the ceramic inlay. Clinical and radiological findings were indicative of a broken liner. This was confirmed during revision surgery, during which it was replaced by a polyethylene inlay; although the ceramic head appeared intact, it was replaced by a metal head. Inspection of the surface of the broken liner in the scanning electron microscope (SEM) revealed signs of material failure. We recommend careful inspection of ceramic-on-ceramic articulating components during total hip revision surgery and if there is any uncertainty, replacement of both so as to avoid premature failure.     

Ceramic cups for hip endoprostheses. 5: Consideration of designs]

Using the combination ceramic on ceramic for hip prostheses, the wear rate can be reduced to less than 1 micron per million cycles. More than 20 years of experience are now available for various concepts involving alumina ceramics in THR. In the early days, monolithic ceramic cups were employed. However, since alumina ceramic has an unsatisfactory potential for bone integration, soft tissue forms at the interface, with the result that the sockets migrate and penetrate. To improve bony integration, monoblock cups surfaced with ceramic beads were introduced. For the past 10 years, there has been a trend towards the use of modular acetabular components comprising a metal shell and a liner made of Biolox forte ceramic. Two concepts for the fixation of the ceramic liners are employed: locking of the ceramic in the metal shell (CeraLock) and the sandwich concept with a polyethylene layer interposed between liner and shell. The basic design and important aspects such as diameter, range of motion, and the possibility for revision, clinical experience, and trends are discussed.     

Ceramic cups for hip endoprostheses. 6: Cup design, inclination and antetorsion angle modify range of motion and impingement]

The present investigation focuses on total hip replacement using ceramic acetabular components. The relationship between the position of the cup and the range of motion (ROM) was investigated. A limited range of motion may cause impingement, which is defined as contact between the femoral neck and the rim of the acetabular cup. Impingement may result in wear, chipping, fracture or dislocation of the femoral head. Joint movements were simulated in a three-dimensional CAD program. The results obtained underscore the importance of correct positioning and design of the cup for achieving a ROM as close to the physiological situation as possible. With ceramic cups, the inclination angle should not be more than 45 degrees, and the antetorsion angle between 10 and 15 degrees. If the cup is too vertical, the risk of dislocation and fracture of the ceramic increases. If, on the other hand, the angle of inclination is too small, flexion and abduction will be greatly limited. The study shows that acetabular components with non-recessed ceramic inserts should not be used. Slight recession of the insert helps to avoid impingement. The ROM is reduced and the risk of impingement appreciably increased when mushroom-shaped femoral heads (XL heads) or ceramic inserts protected by a polyethylene ring are used.     

A 42-year-old patient presenting with femoral head migration after hemiarthroplasty performed 22 years earlier: a case report

Introduction

Treatment of femoral neck fractures in young adults may require total hip arthroplasty or hip hemiarthroplasty using a bipolar cup. The latter can, however, result in migration of the femoral head and poor long-term results.

Case presentation

We report a case of femoral head migration after hemiarthroplasty performed for femoral neck fracture that had occurred 22 years earlier, when the patient (a Japanese man) was 20 years old. He experienced peri-prosthetic fracture of the femur, subsequent migration of the prosthesis, and a massive bone defect of the pelvic side acetabular roof. After bone union of the femoral shaft fracture, the patient was referred to our hospital for reconstruction of the acetabular roof. Intra-operatively, we placed two alloimplants of bone from around the transplanted femoral head into the weight-bearing region of the acetabular roof using an impaction bone graft method. We then implanted an acetabular roof reinforcement plate and a cemented polyethylene cup in the position of the original acetabular cup. Eighteen months post-operatively, X-rays showed union of the transplanted bone.

Conclusions

Treatment of femoral neck fractures in young adults is usually accomplished by osteosynthesis, but it may be complicated by femoral head avascular necrosis or by infection or osteomyelitis. In such cases, once an infection has subsided, either hip hemiarthroplasty using a bipolar cup or total hip arthroplasty may be required. However, if the acetabular side articular cartilage is damaged, a bipolar cup should not be used. Total hip arthroplasty should be performed to prevent migration of the implant.     

Initial stability of modular acetabular components. Comparative in-vitro study with polyethylene and ceramic liners.

Modular acetabular components with alumina ceramic liners are currently used in total hip arthroplasty, but concerns have emerged regarding their high stiffness, which could cause impairment of stability, stress-shielding phenomena, and loosening. The purpose of the present biomechanical investigation was to compare the in-vitro initial stability of a modular press-fit acetabular component using a polyethylene liner and using an alumina liner. The initial stability was investigated by measuring the micromotion between the implant and the acetabulum during the application of physiological load (2.39 kN). The micromotion of the acetabular component was investigated in 10 acetabuli using a polyethylene liner and in 10 acetabuli using an alumina liner. Micromotion was assessed at the level of the Os ilium, Os pubis, and Os ischium using 3 electromagnetic transducers. The transducers have a sensitivity of 1 micron and a range of measurement of 500 microns. All implants have been fixed on human pelves made of polyurethane. Measurement of implant micromotion showed stable conditions at the level of the three main sectors of the acetabulum during all tests. No statistically significant differences of results were observed between the group of specimens with polyethylene liner and the group of specimens with alumina liner. The mean micromotion values of the uncemented cups were similar to the mean micromotion values of 10 cemented cups investigated to achieve comparative data of stability. In conclusion, the modular acetabular components inserted using an alumina liner showed a satisfactory initial stability in-vitro. The results do not contrast with those achieved using the same cup inserted with a polyethylene liner.     

Contact mechanics of modular metal-on-polyethylene total hip replacement under adverse edge loading conditions

Edge loading can negatively impact the biomechanics and long-term performance of hip replacements. Although edge loading has been widely investigated for hard-on-hard articulations, limited work has been conducted for hard-on-soft combinations. The aim of the present study was to investigate edge loading and its effect on the contact mechanics of a modular metal-on-polyethylene (MoP) total hip replacement (THR). A three-dimensional finite element model was developed based on a modular MoP bearing. Different cup inclination angles and head lateral microseparation were modelled and their effect on the contact mechanics of the modular MoP hip replacement were examined. The results showed that lateral microseparation caused loading of the head on the rim of the cup, which produced substantial increases in the maximum von Mises stress in the polyethylene liner and the maximum contact pressure on both the articulating surface and backside surface of the liner. Plastic deformation of the liner was observed under both standard conditions and microseparation conditions, however, the maximum equivalent plastic strain in the liner under microseparation conditions of 2000 µm was predicted to be approximately six times that under standard conditions. The study has indicated that correct positioning the components to avoid edge loading is likely to be important clinically even for hard-on-soft bearings for THR.     

Design considerations for ceramic resurfaced femoral head: effect of interface characteristics on failure mechanisms

Ceramic hip resurfacing may offer improved wear resistance compared to metallic components. The study is aimed at investigating the effects of stiffer ceramic components on the stress/strain-related failure mechanisms in the resurfaced femur, using three-dimensional finite element models of intact and resurfaced femurs with varying stem–bone interface conditions. Tensile stresses in the cement varied between 1 and 5 MPa. Postoperatively, 20–85% strain shielding was observed inside the resurfaced head. The variability in stem–bone interface condition strongly influenced the stresses and strains generated within the resurfaced femoral head. For full stem–bone contact, high tensile (151–158 MPa) stresses were generated at the cup–stem junction, indicating risk of fracture. Moreover, there was risk of femoral neck fracture due to elevated bone strains (0.60–0.80% strain) in the proximal femoral neck region. Stresses in the ceramic component are reduced if a frictionless gap condition exists at the stem–bone interface. High stresses, coupled with increased strain shielding in the ceramic resurfaced femur, appear to be major concerns regarding its use as an alternative material.     

A comparative study of the characteristics of femoral hip prosthesis heads--study and results ceramic-coated hip joint heads of titanium]

Today, only hard/soft cup and femoral head combinations are employed for hip joint prostheses. Highly polished ceramic is a material with very good tribological properties for femoral heads, being highly resistant to mechanical wear and tear, and highly resistant to chemical reactions in the biological environment. The advantage of metal heads, in contrast, undoubtedly lies in their resistance to breakage and the ease with which their geometry can be modified with respect, for example, to antirotation angle and neck length. The ideal material for femoral heads is a combination of the two materials. The new multi-layer combination of titanium-niobium oxide/nitride ceramic coating applied to a prehardened titanium head combines the positive material properties in an ideal manner. Femoral heads made of CoCrMo, oxide-hardened titanium, aluminium oxide or multilayer titanium-niobium ceramic were compared by means of friction an wear and tear tests. The TiNb-ceramic-metal heads showed similar abrasion at the surface as the ceramic heads. At the high loads of more than 400 kp, which may also be reached under physiological conditions, the specially coated titanium-ceramic heads proved to be superior in terms of resistance to fracture and tribological properties.     

Investigation of explanted hip and acetabulum hip prostheses]

Retrieved ceramic femoral heads and acetabular cups were investigated. On the basis of the case studies, the reasons for revision are discussed. Wear patterns and wear rates were found to differ from those observed in hip simulating testing. Monolithic ceramic cups showed a high wear rate. Owing to their limited range of motion, ceramic "mushroom heads" are associated with impingement that leads to a high risk of cup loosening, high wear rates and in vivo fractures. The combination of ceramic "mushroom heads" and cups is not recommended. An evaluation of complications shows that some can be explained by patient behaviour--e.g. Japanese sitting position, horse riding. Designers need to develop new concepts offering a larger range of motion, for example, with head diameters of 32 and 36 mm that reduce the risk of impingement, subluxation and dislocation, while increasing the range of motion. The potential of ceramic/ceramic coupling has been known since the 70s, and ceramic concepts for total hip replacement are currently experiencing a renaissance, although further developments are still possible.     

Design parameters dependences on contact stress distribution in gait and jogging phases after total hip arthroplasty

We have developed a mathematical model to calculate the contact stress distribution in total hip arthroplasty (THA) prosthesis between the articulating surfaces. The model uses the clearance between bearing surfaces as well as the inclination and thickness of the Ultra High Molecular Weight Poly-Ethylene (UHMWPE) cup to achieve this. We have used this mathematical model to contrast the maximal force during normal gait and during jogging. This is based on the assumption that the contact stress is proportional to the radial deformation of the cup. The results show that the magnitude of the maximal contact stress remains constant for inclination values in the range of [0-35 degrees ] and increase significantly with the cup clearance and liner thickness for inclination values in the range of [35-65 degrees ]. A major use for this model would be the calculation of spatial contact stress distribution during normal gait or jogging for different couples of bearing surfaces.     

Effect of acetabular component anteversion on dislocation mechanisms in total hip arthroplasty

Quantifying soft-tissue tension around the hip joint during total hip arthroplasty remains difficult. In this study, a three-dimensional computer-aided design model was developed to clarify how component position in total hip arthroplasty contributes to the primary cause of posterior dislocation in cases of flexion, adduction and internal rotation. To better understand the influences of anteversion angle of the acetabular component, its effects on the primary causes of dislocations and the range of motion were investigated. Three different primary dislocation mechanisms were noted: impingement of the prosthetic femoral neck on the cup liner; impingement of the osseous femur on the osseous pelvis; and spontaneous dislocation caused by soft-tissue traction without impingement. Spontaneous dislocation could be detected by calculating hip forces at any thigh position using the computer-aided design model developed. In computer analysis, a transition from prosthetic impingement rate to osseous impingement rate occurred with increasing anteversion angle of the acetabular component. Spontaneous dislocation was detected at angles > 10° of anteversion of the acetabular component when flexion occurred with extreme adduction and internal rotation. This study demonstrated the possibility of spontaneous dislocation that results not from prosthetic or bony impingement but from muscle traction with increased range of motion.     

Ceramic acetabulum for hip endoprostheses. 8: Revisions]

For total hip replacement, ceramic femoral heads and acetabular liners are being used with success. However, reports of revision surgery necessitated by fractures or marked wear of ceramic components are still being published. The revision rate due to fracture is less than 0.01%, and much lower than for other complications. Nevertheless, improvement of safety remains a topic of discussion. This article reviews the results of investigations of explanted ceramic heads and liners, and discusses the problems caused by ceramic wear and chipping. Recommendations for revision surgery in such cases are given.     

Prediction of lubricating film thickness in UHMWPE hip joint replacements

An elastohydrodynamic lubrication model developed for a ball-in-socket configuration in a previous studies by the present authors (Jalali-Vahid et al., Thinning films and tribological interfaces, 26th Leeds-Lyon Symposium on Tribology, 2000, pp. 329-339) was applied to analyse the lubrication problem of a typical artificial hip joint replacement, consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup against a metallic or ceramic femoral head. The cup was assumed to be stationary whilst the ball was assumed to rotate at a steady angular velocity and under a constant load. A wide range of main design parameters were considered. It has been found that the predicted lubricating film thickness increases with a decrease in the radial clearance, an increase in the femoral head radius, an increase in UHMWPE thickness and a decrease in UHMWPE modulus. However, the predicted lubricating film thicknesses are not found to be sufficiently large in relation to the surface roughness of the cup and head to indicate separation of the two articulating surfaces. It should also be noted that if the design features are unable to secure full fluid film lubrication, it may be preferable to select them for minimum wear rather than maximum film thickness. For example, an increase in head radius will enhance the film thickness, but it will also increase the sliding distance and hence wear in mixed or boundary lubrication conditions. Furthermore, it is pointed out that an increase in the predicted lubricant film thickness is usually associated with an increase in the contact area, and this may cause lubricant starvation and stress concentration at the edge of the cup, and adversely affect the tribological performance of the implant. The effect of running-in process on the lubrication in UHMWPE hip joint replacements is also discussed.     

Total Hip Replacement for the Treatment of End Stage Arthritis of the Hip: A Systematic Review and Meta-Analysis

Background

Evolvements in the design, fixation methods, size, and bearing surface of implants for total hip replacement (THR) have led to a variety of options for healthcare professionals to consider. The need to determine the most optimal combinations of THR implant is warranted. This systematic review evaluated the clinical effectiveness of different types of THR used for the treatment of end stage arthritis of the hip.

Methods

A comprehensive literature search was undertaken in major health databases. Randomised controlled trials (RCTs) and systematic reviews published from 2008 onwards comparing different types of primary THR in patients with end stage arthritis of the hip were included.

Results

Fourteen RCTs and five systematic reviews were included. Patients experienced significant post-THR improvements in Harris Hip scores, but this did not differ between impact types. There was a reduced risk of implant dislocation after receiving a larger femoral head size (36 mm vs. 28 mm; RR = 0.17, 95% CI: 0.04, 0.78) or cemented cup (vs. cementless cup; pooled odds ratio: 0.34, 95% CI: 0.13, 0.89). Recipients of cross-linked vs. conventional polyethylene cup liners experienced reduced femoral head penetration and revision. There was no impact of femoral stem fixation and cup shell design on implant survival rates. Evidence on mortality and complications (aseptic loosening, femoral fracture) was inconclusive.

Conclusions

The majority of evidence was inconclusive due to poor reporting, missing data, or uncertainty in treatment estimates. The findings warrant cautious interpretation given the risk of bias (blinding, attrition), methodological limitations (small sample size, low event counts, short follow-up), and poor reporting. Long-term pragmatic RCTs are needed to allow for more definitive conclusions. Authors are encouraged to specify the minimal clinically important difference and power calculation for their primary outcome(s) as well CONSORT, PRISMA and STROBE guidelines to ensure better reporting and more reliable production and assessment of evidence.     

Low psoas major muscle area as a risk factor for contralateral hip fracture following intertrochanteric fracture

Objective:This study aimed to investigate the relationship between the psoas major muscle area as a risk factor and subsequent contralateral hip fractures in patients with initial intertrochanteric fractures.Methods:Of 136 treated for intertrochanteric fractures, 104 female patients had computed tomography done to assess their fractures at initial stage and had been followed up for more than 2 years. These patients were then divided into 2 groups: i.e. those who had a contralateral hip fracture (CF) (n=16) and those who did not (NF) (n=88) groups. We mainly assessed the relationship between the corrected psoas major muscle area (CPMA) at initial fracture and the occurrence of contralateral hip fracture.Results:The CF group had significantly lower CPMA than the NF group (p=0.001). There was positive correlation between the CPMA and the period from the initial to the contralateral hip fracture in the CF group. The CPMA cutoff value of 480.98 mm²/m², was showed sensitivity of 63.6% and specificity of 87.5% in receiver operating characteristic curve analysis for all patients.Conclusions:The lower CPMA was associated with the contralateral hip fracture within 2 years from initial intertrochanteric fracture. The low CPMA would be a risk factor for contralateral hip fracture.     

Coffee Consumption and Risk of Fracture in the Cohort of Swedish Men (COSM)

Background

Recent research in a large cohort of women showed that coffee consumption is not associated with increased risk of fracture. Whether this is the case also among men is less clear.

Methods

In the Cohort of Swedish Men (COSM) study, 42,978 men aged 45–79 years old at baseline in 1997 answered a self-administered food frequency questionnaire covering coffee consumption and a medical and lifestyle questionnaire covering potential confounders. Our main outcomes first fracture at any site and first hip fracture were collected from the National Patient Registry in Sweden. The association between coffee consumption and fracture risk was investigated using Cox’s proportional hazards regression.

Results

During a mean follow-up of 11.2 years, 5,066 men had a first fracture at any site and of these, 1,186 (23%) were hip fractures. There was no association between increasing coffee consumption (per 200 ml) and rate of any fracture (hazard ratio [HR] 1.00; 95% confidence interval [CI] 0.99–1.02) or hip fracture (HR 1.02; 95% CI 0.99–1.06) after adjustment for potential confounders. For men consuming ≥4 cups of coffee/day compared to those consuming <1 cup of coffee/day, HR for any type of fracture was 0.91 (95% CI 0.80–1.02) and for hip fracture: 0.89 (95% CI 0.70–1.14).

Conclusions

High coffee consumption was not associated with an increased risk of fractures in this large cohort of Swedish men.     

Bone density and risk of hip fracture in men and women: cross sectional analysis.

OBJECTIVE: To determine the relative contribution of decline in bone density to the increase in risk of hip fracture with age in men and women. DESIGN: Incidence data of hip fracture from the general population were combined with the bone density distribution in a sample from the same population and with a risk estimate of low bone density known from literature. SETTING: The Netherlands. SUBJECTS: All people with a hospital admission for a hip fracture in 1993, and bone density measured in a sample of 581.4 men and women aged 55 years and over in a district of Rotterdam. MAIN OUTCOME MEASURE: One year cumulative risk of hip fracture by age, sex, and bone density measured at the femoral neck. RESULTS: A quarter of all hip fractures occurred in men. Men reached the same incidence as women at five years older. Controlled for age, the risk of hip fracture by bone density was similar in men and women. The risk of hip fracture increased 13-fold from age 60 to 80; decrease in bone density associated with age contributed 1.9 (95% confidence interval 1.5 to 2.4) in women and 1.6 (1.3 to 1.8) in men. CONCLUSIONS: The risk of hip fracture by age and bone density is similar in men and women. The decrease in bone density associated with age makes a limited contribution to the exponential increase of the risk of hip fracture with age.     

The health and cost effects of substituting home care for inpatient acute care: a review of the evidence

BACKGROUND: There is much interest in reducing hospital stays by providing some health care services in patients'' homes. The authors review the evidence regarding the effects of this acute care at home (acute home care) on the health of patients and caregivers and on the social costs (public and private costs) of managing the patients'' health conditions. METHODS: MEDLINE and HEALTHSTAR databases were searched for articles using the key term "home care." Bibliographies of articles read were checked for additional references. Fourteen studies met the selection criteria (publication between 1975 and early 1998, evaluation of an acute home care program for adults, and use of a control group to evaluate the program). Of the 14, only 4 also satisfied 6 internal validity criteria (patients were eligible for home care, comparable patients in home care group and hospital care group, adequate patient sample size, appropriate analytical techniques, appropriate health measures and appropriate costing methods). RESULTS: The 4 studies with internal validity evaluated home care for 5 specific health conditions (hip fracture, hip replacement, chronic obstructive pulmonary disease [COPD], hysterectomy and knee replacement); 2 of the studies also evaluated home care for various medical and surgical conditions combined. Compared with hospital care, home care had no notable effects on patients'' or caregivers'' health. Social costs were not reported for hip fracture. They were unaffected for hip and knee replacement, and higher for COPD and hysterectomy; in the 2 studies of various conditions combined, social costs were higher in one and lower in the other. Effects on health system costs were mixed, with overall cost savings for hip fracture and higher costs for hip and knee replacement. INTERPRETATION: The limited existing evidence indicates that, compared with hospital care, acute home care produces no notable difference in health outcomes. The effects on social and health system costs appear to vary with condition. More well-designed evaluations are needed to determine the appropriate use of acute home care.     

A meta-analysis of cigarette smoking,bone mineral density and risk of hip fracture: recognition of a major effect.

OBJECTIVE: To determine the magnitude and importance of the relation between smoking, bone mineral density, and risk of hip fracture according to age. DESIGN: Meta-analysis of 29 published cross sectional studies reporting the difference in bone density in 2156 smokers and 9705 non-smokers according to age, and of 19 cohort and case-control studies recording 3889 hip fractures reporting risk in smokers relative to non-smokers. RESULTS: In premenopausal women bone density was similar in smokers and non-smokers. Postmenopausal bone loss was greater in current smokers than non-smokers, bone density diminishing by about an additional 2% for every 10 year increase in age, with a difference of 6% at age 80. In current smokers relative to non-smokers the risk of hip fracture was similar at age 50 but greater thereafter by an estimated 17% at age 60, 41% at 70, 71% at 80, and 108% at 90. These estimates of relative risk by age, derived directly from a regression analysis of the studies of smoking and hip fracture, were close to estimates using the difference in bone density between smokers and non-smokers and the association between bone density and risk of hip fracture. The estimated cumulative risk of hip fracture in women in England was 19% in smokers and 12% in non-smokers to age 85; 37% and 22% to age 90. Among all women, one hip fracture in eight is attributable to smoking. Limited data in men suggest a similar proportionate effect of smoking as in women. The association was not explained by smokers being thinner, younger at menopause, and exercising less nor by actions of smoking on oestrogen, but smoking may have a direct action on bone. CONCLUSIONS: Hip fracture in old age is a major adverse effect of smoking after the menopause. The cumulative excess bone loss over decades is substantial, increasing the lifetime risk of hip fracture by about half.