Development and Physical Validation of a Finite Element Model of Total Hip Dislocation

Component-on-component impingement, followed by levering of the femoral head, is a common mode of dislocation in total hip arthroplasty. While there have been many registry-based studies of dislocation incidence, confounding factors and sources of variability in the clinical domain make it difficult to identify specific parameter influences. A three dimensional nonlinear finite element model has been developed for the purpose of studying the dislocation event, to allow determination of how individual factors such as component design and clinical implantation position affect the propensity for dislocation. Also, a laboratory testing apparatus was constructed to provide physical validation of the computational model. The finite element model correctly predicted the range of motion observed in the physical apparatus to within 1%, and predicted the peak resisting moment to within 2.5%. Under even a light joint load of 200 N, the von Mises stresses developed in the polyethylene insert reached 13 MPa, and the contact stresses rose to as high as 30 MPa. These deleterious elevations occurred not only at the site of neck impingement, but also at the site of head egress from the liner.     

Finite element simulation of early creep and wear in total hip arthroplasty

Polyethylene wear particulate has been implicated in osteolytic lesion development and may lead to implant loosening and revision surgery. Wear in total hip arthroplasty is frequently estimated from patient radiographs by measurement of penetration of the femoral head into the polyethylene liner. Penetration, however, is multi-factorial, and includes components of wear and deformation due to creep. From a clinical perspective, it is of great interest to separate these elements to better evaluate true wear rates in vivo. Thus, the aim of this study was to determine polyethylene creep and wear penetration and volumetric wear during simulated gait loading conditions for variables of head size, liner thickness, and head–liner clearance. A finite element model of hip replacement articulation was developed, and creep and wear simulation was performed to 1 million gait cycles. Creep of the liner occurred quickly and increased the predicted contact areas by up to 56%, subsequently reducing contact pressures by up to 41%. Greater creep penetration was found with smaller heads, thicker liners, and larger clearance. The least volumetric wear but the most linear penetration was found with the smallest head size. Although polyethylene thickness increases from 4 to 16 mm produced only slight increases in volumetric wear and modest effects on total penetration, the fraction of creep in total penetration varied with thickness from 10% to over 50%. With thicker liners and smaller heads, creep will comprise a significant fraction of early penetration. These results will aid an understanding of the complex interaction of creep and wear.     

The effects of topology and relative density of lattice liners on traumatic brain injury mitigation

This paper evaluates the effects of topology and relative density of helmet lattice liners on mitigating Traumatic Brain Injury (TBI). Finite Element (FE) models of new lattice liners with prismatic and tetrahedral topologies were developed. A typical frontal head impact in motorcycle accidents was simulated, and linear and rotational accelerations of the head were recorded. A high-fidelity FE model of TBI was loaded with the accelerations to predict the brain response during the accident. The results show that prismatic lattices have better performance in preventing TBI than tetrahedral lattices and EPS that is typically used in helmets. Moreover, varying the cell size through the thickness of the liner improves its performance, but this effect was marginal. The relative density also has a significant effect, with lattices with lower relative densities providing better protection. Across different lattices studied here, the prismatic lattice with a relative density of 6% had the best performance and reduced the peak linear and rotational accelerations, Head Injury Criterion (HIC), brain strain and strain rate by 48%, 37%, 49%, 32% and 65% respectively, compared to the EPS liner. These results can be used to guide the design of lattice helmet liners for better mitigation of TBI.     

Effects of episodic subluxation events on third body ingress and embedment in the THA bearing surface

In total joint arthroplasty, third body particle access to the articulating surfaces results in accelerated wear. Hip joint subluxation is an under-recognized means by which third body particles could potentially enter the otherwise closely conforming articular bearing space. The present study was designed to test the hypothesis that, other factors being equal, even occasional events of femoral head subluxation greatly increase the number of third body particles that enter the bearing space and become embedded in the acetabular liner, as compared to level-walking cycles alone. Ten metal-on-polyethylene hip joint head-liner pairs were tested in a multi-axis joint motion simulator, with CoCrMo third body particles added to the synovial fluid analog. All component pairs were tested for 2h of level walking; half were also subjected to 20 intermittent subluxation events. The number and location of embedded particles on the acetabular liners were then determined. Subluxation dramatically increased the number of third body particles embedded in the acetabular liners, and it considerably increased the amount of scratch damage on the femoral heads. Since both third body particles and subluxation frequently occur in contemporary total hip arthroplasty, their potent synergy needs to be factored prominently into strategies to minimize wear.     

Sharpening of the front of the current through a cylindrical foil liner

Results are presented from experiments on the electromagnetic implosion of aluminum foil liners at the MIG generator with a current rise time of ≈80 ns. Plasma with a density of 10¹⁷ cm^–3 was preliminarily injected into the liner region by using a set of radial plasma guns. The Lorentz force J × B causes plasma acceleration in the radial direction. Since the magnetic field pressure is inversely proportional to the radius squared, the plasma displacement is maximum near the liner surface. As a result, plasma motion becomes two-dimensional, a gap appears between the plasma and the liner, and the generator current is switched over to the liner. The plasma velocity at the liner surface is close to the local Alfvén velocity, while the time during which the current is switched over to the liner is nearly equal to the ratio of the liner length to the Alfvén velocity. The proposed scheme allows one to decrease the rise time of the current through the liner to several nano-seconds and, as a result, to reduce the initial liner radius and improve the stability of liner implosion.     

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.     

Evaluation of the assembling and retention forces of constraint liners for total hip replacement]

Dislocation is a severe complication after total hip replacement which may cause revision surgery in some cases. The use of constraint inserts that are coupled to the femoral head by a snapping mechanism provides an opportunity for treatment of recurrent dislocations. This study was aimed to investigate the assembling and retention forces of a specific constraint liner. Using a universal testing machine the assembling forces were determined for head sizes of 28 and 32 mm and the clinically mostly used as well as the maximum cup size. Subsequently, under variation of load direction and pull-out velocity the retention forces were investigated. For primary assembly of the head the required compressive forces were in a range from 197 N and 283 N depending on head and cup size (each size n = 3). Repeated assembly led to a decrease of these forces up to 29%. The retention forces always were slightly below the assembling forces, i. e. forces to remove the heads from the inserts were between 183 N and 230 N (each size n = 3). Repeated disconnection caused a decrease of the retention forces up to 16%. An increase of load velocity as well as an oblique load direction resulted in an enhancement of the retention forces. For all investigated implant sizes the retention force for the femoral head was approximately ten-times less than the interface strength between the insert and the metal-back. In case of correct implant handling the risk of disconnection between the tested constraint insert and the corresponding metal-back has not to be considered in clinical practice.     

The HEPFIEx simulator, a device for determination of friction between femur head prosthesis and cartilage]

We describe a device designed to investigate friction between various femoral head prostheses and human acetabula. It enables not only the determination of friction and the relevance of the play between the femoral head and acetabulum, but also the evaluation of the kinematic behaviour of bipolar prostheses. In the simulator, the various femoral head prostheses are placed on a special cone and tested against a human cadaveric acetabulum. The swiveling range of the device is uniaxial, and the swiveling angle is +/- 35 degrees. The maximum force produced pneumatically is 5kN. Testing of the simulator with a TEP was successful and friction-coefficients of < 0.1 were measured, as are reported in the literature.     

Kinematics,kinetics, and finite element analysis of commonplace maneuvers at risk for total hip dislocation

Dislocation remains a disturbingly frequent complication of total hip arthroplasty (THA). Over the past several years, increasingly rigorous biomechanical approaches have been developed for studying dislocation, both experimentally and computationally. Realism of the input motion challenge data has lagged behind most other aspects of this body of work, and anterior dislocation maneuvers remain unstudied. To enhance realism of biomechanical studies of dislocation, motion data are here reported for ten THA-aged subjects, each repeatedly performing seven maneuvers known to be dislocation-prone. An optoelectronic motion tracking system and a recessed force plate captured the kinematics and ground reaction forces of these maneuvers. Using an established inverse dynamics model to estimate hip joint loading, 354 motion trials were evaluated using an existing finite element model of THA dislocation. Worst-case-scenario THA constructs were simulated (22 mm femoral head, acetabular cup orientations at the limit of the accepted safe zone), in order to deliberately induce impingement and dislocation. The results showed a high incidence of computationally predicted dislocation for all movements studied, but also that risk was very maneuver-dependent, with patients being six times more likely to dislocate from a low-sit-to-stand maneuver than from stooping. These new motion data hopefully will help facilitate systematic efforts to reduce the incidence of dislocation.     

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.     

Dynamics of Heterogeneous Liners with Prolonged Plasma Creation

Prolonged plasma creation in heterogeneous liners, in which the liner substance is separated into two phase states (a hot plasma and a cold skeleton), is investigated both experimentally and theoretically. This situation is typical of multiwire, foam, and even gas liners in high-current high-voltage facilities. The main mechanisms governing the rate at which the plasma is created are investigated, and the simplest estimates of the creation rate are presented. It is found that, during prolonged plasma creation, the electric current flows through the entire cross section of the produced plasma shell, whose thickness is comparable with the liner radius; in other words, a current skin layer does not form. During compression, such a shell is fairly stable because of its relatively high resilience. It is shown that, under certain conditions, even a thick plasma shell can be highly compressed toward the discharge axis. A simplified numerical simulation of the compression of a plasma shell in a liner with prolonged plasma creation is employed in order to determine the conditions for achieving regimes of fairly compact and relatively stable radial compression of the shell.     

"In vivo" determination of hip joint separation and the forces generated due to impact loading conditions

Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3mm (range 1.9-5.2mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening.     

Translational stiffness of the replaced shoulder joint

Results after a total shoulder arthroplasty in rheumatoid patients are poor, indicated by loosening of especially the glenoid component, bad joint functionality and the possibility of a joint dislocation. The failure mechanisms behind this are multiple, including patient, surgical and design factors. These results must be improved. At present, the optimal geometrical prosthesis component design, focused on joint conformity and constraint, still has to be investigated.

Proper understanding of the effect of geometrical design parameters on the theoretical relationship between joint translations and joint forces may contribute to improved designs. The main objective of this study is to theoretically describe this relationship and to investigate the joint translational stiffness, which can be used to investigate the effect of design parameters on joint motion. Joint translational stiffness is the gradient of the subluxation force with respect to the humeral head displacement.

For this static analysis a potential field is introduced, as the result of a joint compressive force (muscle forces) and a subluxation force (external forces). The positive and negative stiffness during articulation inside and subluxation outside the glenoid cavity, lead to stable and unstable equilibrium joint positions, respectively. A most lateral position of the humeral head centre coincides with a zero subluxation force; at this position the humerus is dislocated and a restoring force is needed to relocate the humeral head.

Joint conformity and compression force influence the joint translational stiffness during articulation inside the glenoid cavity, whereas during articulating outside the glenoid cavity this is influenced by the joint compression force and humeral radius of curvature. The glenoid radius of curvature influences the contact point and, in combination with the glenoid superior–inferior chord length, it also influences the constraintness angle, which influences the maximum allowable subluxation load to prevent a joint dislocation. This constraintness angle together with the joint conformity also influences maximum joint translations before articulation outside the glenoid cavity. Furthermore, the sign of the joint translational stiffness determines the stability of shoulder motion, which is stable and unstable if this stiffness is positive and negative, respectively.     

中医推拿法手部受力的生物力学建模及分析

目的建立中医法的简化生物力学模型,对法过程中手部主要部位的受力进行定量分析。方法应用摄像机和推拿手法测定仪实测推拿专家法推拿的运动学和力信号,建立含手部及桡骨、尺骨的简化的生物力学模型和方程,求解手部桡骨和尺骨远端点处的受力。结果 法外推时,手部桡骨和尺骨远端点处X方向（动方向）受力方向不变,出现两个峰值,近外推结束时受力最大;回收时两处受力大小和方向出现波动。手部桡骨和尺骨远端点处,Y方向和Z方向受力趋势相同,在逐步上升后出现了一个平缓变化的阶段,之后急骤下降。结论建立的简化生物力学模型可对中医法推拿手部受力进行较好的定量分析。     

Method for the evaluation of factors influencing the dislocation stability of total hip endoprotheses]

Dislocation of the artificial joint is a serious complication of total hip replacement. Various factors with an influence on dislocation stability were determined clinically. Our goal was to develop a method for evaluating experimentally the parameters implant design, position and the load situation for their influence on joint stability. With the newly developed testing device the range of motion to impingement and to dislocation can be determined at different implant positions. In addition, the rotational moments on subluxation, i.e. the "levering out" of the femoral head, can be determined. By way of example several hip implants were examined during movements associated with dislocation, e.g. (internal-)rotation in 90 degrees flexion and 0 degrees adduction as well as with (external-)rotation in combination with 10 degrees extension and 15 degrees adduction. Irrespective of implant design and position, the following movement phases can be differentiated: undisturbed motion, impingement, subluxation and, finally, complete dislocation of the head. On the basis of the range of motion of the specific phases, the moments occurring and the direction of dislocation, different implant systems can be compared. In this study the influence of the head diameter on the dislocation stability of the hip endoprosthesis is shown. With the aid of the model presented herein, a data set showing the most favourable and/or most dislocation stable implant position can be acquired for different combinations of the implant components. Additionally, useful information for implant design can be deduced and applied to new developments and/or modifications of existing implant components.     

Semen collection efficiency using an artificial vagina plastic liner

A one-piece plastic single-service liner and collection cone has been developed and tested for use in the bovine artificial vagina semen collection method. The plastic liner was found to be equal to the standard rubber liner in terms of ejaculate volume and more efficient in terms of spermatozoa reaching the collection tube. Additionally, ejaculate volume had a smaller effect on collection efficiency with plastic liners than with standard rubber liners.     

In situ friction measurement on murine cartilage by atomic force microscopy

Articular cartilage provides a low-friction, wear-resistant surface for the motion of diarthrodial joints. The objective of this study was to develop a method for in situ friction measurement of murine cartilage using a colloidal probe attached to the cantilever of an atomic force microscope. Sliding friction was measured between a chemically functionalized microsphere and the cartilage of the murine femoral head. Friction was measured at normal loads ranging incrementally from 20 to 100 nN with a sliding speed of 40 microm/s and sliding distance of 64 microm. Under these test conditions, hydrostatic pressurization and biphasic load support in the cartilage were minimized, providing frictional measurements that predominantly reflect boundary lubrication properties. Friction coefficients measured on murine tissue (0.25+/-0.11) were similar to those measured on porcine tissue (0.23+/-0.09) and were in general agreement with measurements of boundary friction on cartilage by other researchers. Using the colloidal probe as an indenter, the elastic mechanical properties and surface roughness were measured in the same configuration. Interfacial shear was found to be the principal mechanism of friction generation, with little to no friction resulting from plowing forces, collision forces, or energy losses due to normal deformation. This measurement technique can be applied to future studies of cartilage friction and mechanical properties on genetically altered mice or other small animals.     

Passive force characteristics of an architecturally complex muscle

In architecturally complex muscles with large attachment areas, it can be expected that during movement different muscle regions undergo different amounts of length excursions. As a consequence, the amount of passive force produced by the regions will differ. Therefore, we tested the hypothesis that during movement the vector of the passive force of such a muscle, which defines the magnitude, position and orientation of the resultant force of the various regions, has no fixed position, between the muscle's center of origin and insertion. As a model for an architecturally complex muscle we used the masseter muscle. It was expected that during jaw opening anterior muscle regions are more stretched than posterior regions, leading to an anterior shift of the passive force vector. A three-component force transducer was used to measure both the position and magnitude of passive force in the masseter muscle of 9 rabbits. Forces were recorded during repeated cycles of stepwise opening and closure of the jaw. The muscle exhibited a clear hysteresis: passive force measured during jaw opening was larger than that during jaw closing. With an increase of the jaw gape there was an approximately exponential increase of the magnitude of the passive muscle force, while simultaneously the passive force vector shifted anteriorly. Moment arm length of passive force increased by about 100%. This anterior shift contributed substantially to the increase of the passive muscle moment generated during jaw opening. It can be concluded that in architecturally complex muscles the increase of the passive resistance moment which is associated with muscle lengthening might not only be due to an increase of the magnitude of passive muscle force but also to an increase of the moment arm of this force.     

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.     

The influence of friction and interference on the seating of a hemispherical press-fit cup: a finite element investigation.

The formation of gaps in the polar region of acetabular cups is seen as a drawback of press-fit fixation of non-cemented acetabular cups. Recent findings indicate a link between long-term polar gaps and the gaps present directly after implantation. In this study the process of press-fitting is simulated with a linear-elastic two-dimensional axisymmetric finite-element model. The aim of this paper is to investigate the possible importance of friction and interference on the formation of these gaps. A range of cup-bone friction coefficients (mu = 0.1-0.5) is assigned to the cup-bone interface in order to represent the unknown amount of friction occurring during press-fitting. The cup is modeled with a radius of 27 mm, whereas the radius of the cavity is varied between 26.50 and 26.75 mm, thus, creating 0.50 and 0.25 mm radial interference fits. The difference in cavity radius represents the discrepancy between the radius of the last-reamer-used and radius of the cavity it creates. The subchondral plate is considered as being completely removed during reaming. The effects of impact blows via the surgeon's mallet during surgery are modeled as a series of four load pulses, in which peak force is gradually increased from 0.5 to 4.0 kN. The effects of load removal as well as those of load application are investigated. On load application, the cup penetrates into the cavity, and on load removal, the cup rebounds. Depending on the friction, interference and load applied, the position of the cup after the load pulse is somewhere between its position at peak force and its position at the beginning of the pulse. Although the simplifications and conditions involved in the creation of the model necessitate caution when interpreting the results for all clinical cases, it is found that the seating of hemispherical cups in trabecular bone could be more satisfactory for intermediate values of friction (mu = 0.2-0.3) and smaller interference fits (0.25 mm).