Normal hip joint contact pressure distribution in single-leg standing--effect of gender and anatomic parameters.

A practical and easy-to-use analysis technique that can study the patient's hip joint contact force/pressure distribution would be useful to assess the effect of abnormal biomechanical conditions and anatomical deformities on joint contact stress for treatment planning purpose. This technique can also help to establish the normative database on hip joint contact pressure distribution in men and women in different age groups. Twelve anatomic parameters and seven biomechanical parameters of the hip joint in a normal population (41 females, 15 males) were calculated. The inter-parameter correlations were investigated. The pressure distribution in the hip joint was calculated using a three-dimensional discrete element analysis (DEA) technique. The 3D contact geometry of the hip joint was estimated from a 2D radiograph by assuming that the femoral head and the acetabular surface were spherical in shape. The head-trochanter ratio (HT), femoral head radius, pelvic height, the joint contact area, the normalized peak contact pressure, abductor force, and the joint contact force were significantly different between men and women. The normalized peak contact pressure was correlated both with acetabular coverage and head-trochanter ratio. Change of abductor force direction within normal variation did not affect the joint peak contact pressure. However, in simulated dysplastic conditions when the CE angle is small or negative, abductor muscle direction becomes very sensitive in joint contact pressure estimation. The models and the results presented can be used as the reference base in computer simulation for preoperative planning in pelvic or femoral osteotomy.     

Three-dimensional dynamic hip contact area and pressure distribution during activities of daily living

Estimation of the hip joint contact area and pressure distribution during activities of daily living is important in predicting joint degeneration mechanism, prosthetic implant wear, providing biomechanical rationales for preoperative planning and postoperative rehabilitation. These biomechanical data were estimated utilizing a generic hip model, the Discrete Element Analysis technique, and the in vivo hip joint contact force data. The three-dimensional joint potential contact area was obtained from the anteroposterior radiograph of a subject and the actual joint contact area and pressure distribution in eight activities of daily living were calculated. During fast, normal, and slow walking, the peak pressure of moderate magnitude was located at the lateral roof of the acetabulum during mid-stance. In standing up and sitting down, and during knee bending, the peak pressures were located at the edge of the posterior horn and the magnitude of the peak pressure during sitting down was 2.8 times that of normal walking. The peak pressure was found at the lateral roof in climbing up stairs which was higher than that in going down stairs. These results can be used to rationalize rehabilitation protocols, functional restrictions after complex acetabular reconstructions, and prosthetic component wear and fatigue test set up. The same model and analysis can provide further insight to soft tissue loading and pathology such as labral injury. When the pressure distribution on the acetabulum is inverted onto the femoral head, prediction of subchondral bone collapse associated with avascular necrosis can be achieved with improved accuracy.     

Secondary hip dysplasia increases risk for early coxarthritis after Legg-Calve-Perthes disease. A study of 255 hips

Abstract

The biomechanical parameters of the hip joint articular surface were analysed in 141 adult hips after Legg-Calve Perthes Disease, and 114 contralateral unaffected hips (controls), by using HIPSTRESS mathematical models. Geometrical parameters, assessed from anteroposterior and axial radiograms, were used as input to models for resultant hip force and contact hip stress. Results confirm previous indications that head enlargement after the Legg-Calve-Perthes Disease compensates the values of hip stress. Furthermore, it was found that an increased risk for coxarthritis development after the disease is secondary to concomitant hip dysplasia, with considerable and statistically significantly lower centre-edge angle and unfavourable distribution of stress.     

Development of computational wear simulation of metal-on-metal hip resurfacing replacements

As one of the alternatives to traditional metal-on-polyethylene total hip replacements, metal-on-metal hip resurfacing prostheses demonstrating lower wear have been introduced for younger and more active patients during the past decade. However, in vitro hip simulator testing for the predicted increased lifetime of these surface replacements is time-consuming and costly. Computational wear modelling based on the Archard wear equation and finite element contact analysis was developed in this study for artificial hip joints and particularly applied to metal-on-metal resurfacing bearings under simulator testing conditions to address this issue. Wear factors associated with the Archard wear equation were experimentally determined and based on the short-term hip simulator wear results. The computational wear simulation was further extended to a long-term evaluation up to 50 million cycles assuming that the wear rate stays constant. The prediction from the computational model shows good agreement with the corresponding simulator study in terms of volumetric wear and the wear geometry. The simulation shows the progression of linear wear penetrations, and the complexity of contact stress distribution on the worn bearing surfaces. After 50 million cycles, the maximum linear wear was predicted to be approximately 6 and 8 microm for the cup and head, respectively, and no edge contact was found.     

Long-term hip loading in unilateral total hip replacement patients is no different between limbs or compared to healthy controls at similar walking speeds

Variation in hip joint contact forces directly influences the performance of total hip replacements (THRs). Measurement and calculation of contact forces in THR patients has been limited by small sample sizes, wide variation in patient and surgical factors, and short-term follow-up. This study hypothesised that, at long-term follow-up, unilateral THR patients have similar calculated hip contact forces compared to controls walking at similar (self-selected) speeds and, in contrast, THR patients walking at slower (self-selected) speeds have reduced hip contact forces. It was further hypothesised that there is no difference in calculated hip contact forces between operated and non-operated limbs at long-term follow-up for both faster and slower patients. Gait analysis data for THR patients walking at faster (walking speed: 1.29 ± 0.12 m/s; n = 11) and slower (walking speed: 0.72 ± 0.09 m/s; n = 11) speeds were used. Healthy subjects constituted the control group (walking speed: 1.36 ± 0.12 m/s; n = 10). Hip contact forces were calculated using static optimisation. There was no significant difference (p > 0.31) in hip contact forces between faster and control groups. Conversely, force was reduced at heel strike by 19% (p = 0.002), toe-off by 31% (p < 0.001) and increased at mid-stance by 15% (p = 0.02) for the slower group compared to controls. There were no differences between operated and non-operated limbs for the slower group or the faster group, suggesting good biomechanical recovery at long-term follow-up. Loading, at different walking speeds, presented here can improve the relevance of preclinical testing methods.     

Biomechanical and clinical alterations of the hip joint following femoral neck fracture and implantation of bipolar hip endoprosthesis

The implantation of a bipolar partial hip endoprosthesis is a treatment of choice for displaced medial femoral neck fracture. We present an experimental study which asses and compare biomechanical and clinical status through period before and after hip fracture and implantation of bipolar partial hip endoprosthesis. This study encompassed 75 patients who suffered from an acute medial femoral neck fracture and were treated with the implantation of a bipolar partial hip endoprosthesis. Their biomechanical status (stress distribution on the hip joint weight bearing area) and clinical status (Harris Hip Score) were estimated for the time prior to the injury and assessed at the follow-up examination that was, on average, carried out 40 months after the operation. Despite ageing, the observed Harris Hip Score at the follow-up examination was higher than that estimated prior to the injury (77.9 > 69.6; p = 0.006). Similarly, the hip stress distribution was reduced (2.7 MPa < 2.3 MPa; p = 0.001). While this reduction can be attributed to a loss of weight due to late ageing, the principal improvement came from the operative treatment and corresponding restoration of the biomechanical properties of the hip joint. The implantation of a bipolar partial hip endoprosthesis for patients with displaced medial femoral neck fractures improves the biomechanical and clinical features of the hip, what should have on mind during making decision about treatment.     

Drive principles of a triaxial physiological hip joint simulator for wear testing of hip endoprostheses]

The long-term performance of hip joint replacements is a major criterion for quality assessment. Aseptic loosening as a result of wear has a considerable impact on the long-term stability of the prosthesis. Constant improvements in hip joint endoprostheses necessitate the use of hip joint simulator to evaluate and assess the suitability of newly developed materials. In the present paper, a triaxial physiological hip joint simulator is presented, and detailed information provided, on the applied control concepts, adaptation of various hard- and software systems and the accuracy of the kinematic and force parameters achievable.     

Subject-specific hip geometry and hip joint centre location affects calculated contact forces at the hip during gait

Hip loading affects the development of hip osteoarthritis, bone remodelling and osseointegration of implants. In this study, we analyzed the effect of subject-specific modelling of hip geometry and hip joint centre (HJC) location on the quantification of hip joint moments, muscle moments and hip contact forces during gait, using musculoskeletal modelling, inverse dynamic analysis and static optimization. For 10 subjects, hip joint moments, muscle moments and hip loading in terms of magnitude and orientation were quantified using three different model types, each including a different amount of subject-specific detail: (1) a generic scaled musculoskeletal model, (2) a generic scaled musculoskeletal model with subject-specific hip geometry (femoral anteversion, neck-length and neck-shaft angle) and (3) a generic scaled musculoskeletal model with subject-specific hip geometry including HJC location. Subject-specific geometry and HJC location were derived from CT. Significant differences were found between the three model types in HJC location, hip flexion–extension moment and inclination angle of the total contact force in the frontal plane. No model agreement was found between the three model types for the calculation of contact forces in terms of magnitude and orientations, and muscle moments. Therefore, we suggest that personalized models with individualized hip joint geometry and HJC location should be used for the quantification of hip loading. For biomechanical analyses aiming to understand modified hip joint loading, and planning hip surgery in patients with osteoarthritis, the amount of subject-specific detail, related to bone geometry and joint centre location in the musculoskeletal models used, needs to be considered.     

Effect of hip joint angle at seat-off on hip joint contact force during sit-to-stand movement: a computer simulation study

Background

Sit-to-stand movements are a necessary part of daily life, and excessive mechanical stress on the articular cartilage has been reported to encourage the progression of osteoarthritis. Although a change in hip joint angle at seat-off may affect hip joint contact force during a sit-to-stand movement, the effect is unclear. This study aimed to examine the effect of the hip joint angle at seat-off on the hip joint contact force during a sit-to-stand movement by using a computer simulation.

Methods

A musculoskeletal model was created for the computer simulation, and eight muscles were attached to each lower limb. Various sit-to-stand movements were generated using parameters (e.g., seat height and time from seat-off to standing posture) reported by previous studies. The hip joint contact force for each sit-to-stand movement was calculated. Furthermore, the effect of the hip joint angle at seat-off on the hip joint contact force during the sit-to-stand movement was examined. In this study, as the changes to the musculoskeletal model parameters affect the hip joint contact force, a sensitivity analysis was conducted.

Results and conclusions

The hip joint contact force during the sit-to-stand movement increased approximately linearly as the hip flexion angle at the seat-off increased. Moreover, the normal sit-to-stand movement and the sit-to-stand movement yielding a minimum hip joint contact force were approximately equivalent. The effect of the changes to the musculoskeletal model parameters on the main findings of this study was minimal. Thus, the main findings are robust and may help prevent the progression of hip osteoarthritis by decreasing mechanical stress, which will be explored in future studies.

     

In vitro fatigue failure of cemented acetabular replacements: a hip simulator study

Although hip simulators for in vitro wear testing of prosthetic materials used in total hip arthroplasty (THA) have been available for a number of years, similar equipment has yet to appear for endurance testing of fixation in cemented THA, despite considerable evidence of late aseptic loosening as one of the most significant failure mechanisms in this type of replacements. An in vitro study of fatigue behavior in cemented acetabular replacements has been carried out, utilizing a newly developed hip simulator. The machine was designed to simulate the direction and the magnitude of the hip contact force under typical physiological loading conditions, including normal walking and stair climbing, as reported by Bergmann et al. (2001, Hip 98, Freie Universitaet, Berlin). A 3D finite element analysis has been carried out to validate the function of the hip simulator and to evaluate the effects of boundary conditions and geometry of the specimen on the stress distribution in the cement mantle. Bovine pelvic bones were implanted with a Charnley cup, using standard manual cementing techniques. Experiments were carried out under normal walking and descending stairs loading conditions with selected load levels from a body weight of 75-125 kg. Periodically, the samples were removed from the test rigs to allow CT scanning for the purpose of monitoring damage development in the cement fixation. The hip simulator was found to be satisfactory in reproducing the hip contact force during normal walking and stair climbing, as reported by Bergmann et al. Finite element analysis shows that the stress distributions in the cement mantle and at the bone-cement interface are largely unaffected by the geometry and the boundary conditions of the model. Three samples were tested up to 17 x 10(6) cycles and sectioned post-testing for microscopic studies. Debonding at the bone-cement interface of various degrees in the posterior-superior quadrant was revealed in these samples, and the location of the failures corresponds to the highest stressed region from the finite-element analysis. Preliminary experimental results from a newly developed hip simulator seem to suggest that debonding at the bone-cement interface is the main failure mechanism in cemented acetabular replacements, and descending stairs seem to be more detrimental than normal walking or ascending stairs with regard to fatigue integrity of cement fixation.     

Computational hip joint simulator for wear and heat generation

This paper presents a computational simulator for the hip to compute the wear and heat generation on artificial joints. The friction produced on artificial hip joints originates wear rates that can lead to failure of the implant. Furthermore, the frictional heating can increase the wear. The developed computational model calculates the wear in the joint and the temperature in the surrounding zone, allowing the use of different combinations of joint materials, daily activities and different individuals. The pressure distribution on the joint bearing surfaces is obtained with the solution of a contact model. The heat generation by friction and the volumetric wear is computed from the pressure distribution and the sliding distance. The temperature is obtained from the solution of a transient heat conduction problem that includes the time-dependent heat generated by friction. The contact and heat conduction problems are solved numerically with the Finite Element Method. The developed computational model performs a full simulation of the acetabular bearing surface behaviour, which is useful for acetabular cup design and material selection. The results obtained by the present model agree with experimental and clinical data, as well as other numerical studies.     

Computer determination of contact stress distribution and size of weight bearing area in the human hip joint

The mathematical models and the corresponding computer program for determination of the hip joint contact force, the contact stress distribution, and the size of the weight bearing area from a standard anteroposterior radiograph are described. The described method can be applied in clinical practice to predict an optimal stress distribution after different operative interventions in the hip joint and to analyze the short and long term outcome of the treatment of various pathological conditions in the hip. A group of dysplastic hips and a group of normal hips were examined, with respect to the peak contact stress normalized by the body weight, and with respect to the functional angle of the weight bearing area. It is shown that both these parameters can be used in the assessment of hip dysplasia.     

One-legged stance as a representative static body position for calculation of hip contact stress distribution in clinical studies

It was shown in several clinical studies that static one-legged stance may be a relevant body position to describe the loads acting at the hip. However, the stress distribution averaged during movement may better describe hip load than hip contact stress distribution in the static body position. Using data on the resultant hip force during walking taken from the measurements of Bergmann (2001), spatial distribution of contact stress over the articular surface was calculated by the HIPSTRESS method and compared with the stress distribution in one-legged stance. It is shown, that the shape of the contact stress distribution during one-legged stance closely resembled the averaged contact stress distribution during the walking cycle (Pearson's correlation coefficient R2 equals; .986; p < .001). This finding presents a link between the hypothesis that the averaged contact stress distribution during a walking cycle is crucial for cartilage development and the results of clinical studies in which the calculated distribution of contact stress in one-legged stance was successfully used to predict the clinical status of the hip.     

Mathematical modelling of stress in the hip during gait.

A mathematical model is developed for calculating the contact stress distribution in the hip for a known resultant hip force and characteristic geometrical parameters. Using a relatively simple single nonlinear algebraic equation, the model can be readily applied in clinical practice to estimate the stress distribution in the most frequent body positions of everyday activities. This is demonstrated by analyzing the data on the resultant hip force obtained from laboratory observations where a stance period of gait is considered.     

Hip biomechanics in orthopaedic clinical practice

Radiographic and clinical studies, coupled with biomechanical assessment of the hip, are important tools for predicting the development of osteoarthitis of the hip. In order to better understand the treatment of hip dysplasia, it is necessary to determine the contact stress in the hip joint. In this study, a three-dimensional mathematical model was used to determine hip joint contact stress. Because of the discrepancy in the results of analyses of different radiographic indicators of hip dysplasia, the calculation of hip joint contact stress is proposed for a more accurate assessment of the severity of hip dysplasia.     

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.     

The influence of the biomechanical parameters of the hip on the outcome of treatment of hips subject to avascular necrosis of the femoral head

We were interested in whether or not the biomechanical status of the hip influences the course of avascular necrosis of the femoral head. To investigate this, we used a computer aided system based on a three dimensional mathematical model for determining the stress distribution in the hip joint from standard anteroposterior rentgenographs (X-ray images) of both hips and pelvis. Based on the results of our study, we suggest that the biomechanical parameters of the hip play an important role in the outcome of treatment of hips affected by avascular necrosis of the femoral head.     

An improved method of computing the wear factor for total hip prostheses involving the variation of relative motion and contact pressure with location on the bearing surface

A new method of computing the wear factor for total hip prostheses is presented. In the conventional method, only the resultant contact force and the track drawn by the point of its application are considered so that the product of the instantaneous force and sliding increment is integrated over one motion cycle. In the present, improved, method the contact pressure distribution is discretized by a large number of smaller normal forces, and the contribution of each is summed. This is important because the relative motion and contact pressure vary strongly with location, and because the transverse pressure component is substantial. Hence, the present surface integral represents the large contact surface better than the conventional line integral. A prerequisite for the surface integral was the method of computing the relative motion correctly anywhere on the contact surface, developed and published earlier by the present authors. For the pressure discretization, the contact surface was divided into nearly equal-sized surface elements. The contact pressure was modelled with ellipsoidal, paraboloidal and sinusoidal distributions. Two load cases were studied, double-peak and static. When an ellipsoidal contact pressure distribution extending over a hemisphere was discretized by 1000 element forces, the computed wear factor for double-peak load in a biaxial hip wear simulator was 30% lower than in the conventional resultant force case. The present method can be later developed further to involve the temporal variation of size and location of the contact surface.     

Direct comparison of calculated hip joint contact forces with those measured using instrumented implants. An evaluation of a three-dimensional mathematical model of the lower limb

Characterisation of hip joint contact forces is essential for the definition of hip joint prosthesis design requirements. In vivo hip joint contact force measurements have been made using instrumented hip joint prostheses. However, to allow determination of the range of values of joint contact force and their directions relative to anatomical structures in a range of subject groups sufficient to form an agreed data base it is necessary to adopt a different approach without the use of an implanted transducer. The use of mathematical models of the lower limb to examine the forces in soft tissues and at the joints has provided valuable insight into internal loading conditions. Several authors have proposed mathematical musculo-skeletal models. However, there have been only limited attempts at validation of these models. It is possible to use the results of in vivo force measurements from instrumented prostheses to validate the results calculated using the mathematical models. In this study two subjects with instrumented hip joint prostheses were studied. Forces at the hip joints were calculated using a three-dimensional model of the leg. Walking at slow, normal and fast speeds (0.97-2.01m/s), weight transfer from two to one leg and back again, and sit to stand were studied. Direct comparisons were made between the 'gold standard' measured hip joint contact forces and the calculated forces. There was general agreement between the calculated and measured forces in both pattern and magnitude. There were, however, discrepancies. Reasons for these differences in results are discussed and possible model developments suggested.     

Effects of idealized joint geometry on finite element predictions of cartilage contact stresses in the hip

Computational models may have the ability to quantify the relationship between hip morphology, cartilage mechanics and osteoarthritis. Most models have assumed the hip joint to be a perfect ball and socket joint and have neglected deformation at the bone-cartilage interface. The objective of this study was to analyze finite element (FE) models of hip cartilage mechanics with varying degrees of simplified geometry and a model with a rigid bone material assumption to elucidate the effects on predictions of cartilage stress. A previously validated subject-specific FE model of a cadaveric hip joint was used as the basis for the models. Geometry for the bone-cartilage interface was either: (1) subject-specific (i.e. irregular), (2) spherical, or (3) a rotational conchoid. Cartilage was assigned either a varying (irregular) or constant thickness (smoothed). Loading conditions simulated walking, stair-climbing and descending stairs. FE predictions of contact stress for the simplified models were compared with predictions from the subject-specific model. Both spheres and conchoids provided a good approximation of native hip joint geometry (average fitting error ～0.5 mm). However, models with spherical/conchoid bone geometry and smoothed articulating cartilage surfaces grossly underestimated peak and average contact pressures (50% and 25% lower, respectively) and overestimated contact area when compared to the subject-specific FE model. Models incorporating subject-specific bone geometry with smoothed articulating cartilage also underestimated pressures and predicted evenly distributed patterns of contact. The model with rigid bones predicted much higher pressures than the subject-specific model with deformable bones. The results demonstrate that simplifications to the geometry of the bone-cartilage interface, cartilage surface and bone material properties can have a dramatic effect on the predicted magnitude and distribution of cartilage contact pressures in the hip joint.