Interfacial conditions between a press-fit acetabular cup and bone during daily activities: implications for achieving bone in-growth

Interfacial gaps and relative micromotions during activities are widely believed to restrict the boney in-growth process of non-cemented acetabular cups. Using finite element modeling of the cup-bone system, relative micromotions and interfacial gaps are calculated for walking slow, normal and fast and for climbing upstairs, downstairs and standing up from a chair. A 2mm press-fit is simulated and interfacial conditions in the immediate postoperative period (i.e. prior to boney in-growth) calculated between paired nodes covering the whole of the interface. In regions of 'safe' micromotions and 'allowable' gaps, boney in-growth is simulated by specifying zero relative displacement between nodal pairs. The modified model is then resubjected to the loads associated with climbing upstairs, which was shown to be the worst activity. Interfacial conditions are recalculated for subsequent iterations. The procedure is repeated until no further in-growth is predicted. The final pattern of in-growth calculated with the model compares reasonably well with histological evidence from explanted canine cups (Cha et al., 1998. Transactions of the Orthopaedic Research Society, 23, p. 373). Bridging between adjacent regions of in-growth is observed. Notably, in-growth occurs at most of the periphery but not in the polar region. The lack of polar in-growth is caused by the interfacial gap assumed to exist after cup implantation. It is suggested that increasing/decreasing hip-joint loads would have little effect on this lack of polar in-growth. However, excessive micromotions as a result of high hip-joint loads cause a lack of in-growth in the anterior region of the periphery in the model. Although such results were not found in the canine study, if relevant to the general human population, the avoidance of harsh weight-bearing activities may encourage complete peripheral in-growth but is speculated to do little for polar in-growth.

Definition and determination of acetabular component orientation in cemented total hip arthroplasty

OBJECTIVE

To describe the spatial orientation of the cemented acetabular component in cemented total hip arthroplasty, based on a ventrodorsal and lateral radiographic projection of the pelvis.

METHODS

Equations were derived by using trigonometric relationships that describe the radiographic rotation about the longitudinal pelvic axis (alpha), transverse pelvic axis (beta), acetabular inclination (phi), acetabular inclination corrected for longitudinal pelvic rotation, version (phiC), acetabular version (theta), acetabular version corrected for longitudinal pelvic rotation (thetaC), acetabular inclination corrected for transverse pelvic rotation (phi(beta)), and acetabular version corrected for transverse pelvic rotation (theta(beta))

RESULTS

Alpha was calculated by using the equation alpha = sin(-1) (x/y) where x is the transverse distance between the dorsal spinous processes and the center of the pubis on a ventrodorsal radiograph and y is the distance from the pubis to the dorsal aspect of the first coccygeal vertebra perpendicular to the long axis of the pelvis on a lateral radiograph. Phi was calculated from the long axis (LA) and short axis (SA) of the ellipse formed by the radiopaque acetabular marker ring by using the equation phi = sin(-1) (SA/LA). phiC was calculated by using the equation phiC = phi +/- (alpha - tan(-1) (tan alpha cos thetaC)). Theta was determined as previously described. ThetaC was calculated by using the equation thetaC = tan(-1) (tan theta cos alpha). Theta(beta) and theta(beta) were calculated with the equations phi(beta) = tan(-1) (tan theta cos beta) and theta(beta) = theta - tan(-1) (sin beta), respectively. Beta could not be accurately determined from ventrodorsal and lateral pelvic radiographs.

CONCLUSIONS AND CLINICAL RELEVANCE

These techniques allow for more accurate postoperative radiographic assessment of acetabular component positioning. This information can then be used in retrospective or prospective analyses examining that effects of implant positioning on clinical outcome.

An in vivo determination of total hip arthroplasty pistoning during activity

The purpose of this study was to determine to what extent hip joint separation occurs during normal gait on a treadmill and an abduction/adduction leg-lift maneuver in patients who have undergone total hip arthroplasty (THA). Eight patients who had a total of 10 successful unconstrained THAs (Harris Hip Scores >90) performed successive gait motions on an electronically powered treadmill and an abduction/adduction leg lift while under fluoroscopy. The fluoroscopic video images were analyzed using a 3-dimensional model-fitting technique that converts 2-dimensional fluoroscopic images into 3-dimensional real-time images. Hip joint separation was determined to be present if the amount of separation was >0.75 mm, the calculated linear error. During both activities, all 10 THAs experienced femoral head/acetabular component separation. For gait, the maximum amount of separation was 2.8 mm, while the minimum amount was 0.8 mm (average, 1.2 mm). For abduction/adduction leg lift, the maximum amount of separation was 3.0 mm, while the minimal amount was 1.7 mm (average, 2.4 mm). It appears that the femoral head separates from the acetabular component but remains in contact with the superior-most tip of the component. Potential detrimental effects resulting from hip joint separation include premature polyethylene wear and component loosening. Wear may be enhanced because of the creation of multidirectional wear vectors or excessive loads resulting from eccentric femoral head pivoting. These data may be valuable in hip simulation studies to better duplicate wear patterns observed in retrieval analysis.

The influence of the acetabular labrum on hip joint cartilage consolidation: a poroelastic finite element model

The goal of this study was to investigate the influence of the acetabular labrum on the consolidation, and hence the solid matrix strains and stresses, of the cartilage layers of the hip joint. A plane-strain finite element model was developed, which represented a coronal slice through the acetabular and femoral cartilage layers and the acetabular labrum. Elements with poroelastic properties were used to account for the biphasic solid/fluid nature of the cartilage and labrum. The response of the joint over an extended period of loading (10,000s) was examined to simulate the nominal compressive load that the joint is subjected to throughout the day. The model demonstrated that the labrum adds an important resistance in the flow path of the fluid being expressed from the cartilage layers of the joint. Cartilage layer consolidation was up to 40% quicker in the absence of the labrum. Following removal of the labrum from the model, the solid-on-solid contact stresses between the femoral and acetabular cartilage layers were greatly increased (up to 92% higher), which would increase the friction between the joint surfaces. In the absence of the labrum, the centre of contact shifted towards the acetabular rim. Subsurface strains and stresses were much higher without the labrum, which could contribute to fatigue damage of the cartilage layers. Finally, the labrum provided some structural resistance to lateral motion of the femoral head within the acetabulum, enhancing joint stability and preserving joint congruity.

The acetabular labrum seal: a poroelastic finite element model

OBJECTIVE

The aim of the study is to investigate the labrum's ability to seal a pressurised layer of synovial fluid within the joint, and to study the influence of this sealing mechanism on cartilage deformation, interstitial fluid pressure and collagen solid matrix stresses.

BACKGROUND

Cartilage degeneration has been observed in conjunction with labrum pathology. However, little is known about the function of the labrum. Experimental observations have been reported which are consistent with a sealing function of the labrum.

METHODS

The model was an axisymmetric geometric approximation of the acetabular and femoral cartilage layers and the surrounding labrum. A poroelastic formulation was used to account for the solid and fluid components of these hydrated tissues. A sensitivity analysis of the labrum material properties was carried out.

RESULTS

With a compressive load of 1200 N applied across the joint model, the labrum could seal a layer of pressurised fluid between the femur and acetabulum, thus preventing contact of the articulating surfaces. With this sealing effect, loads were transferred across the joint predominantly by uniform pressurisation of the interstitial fluid of the cartilage layers. In the absence of this sealing, strains within the solid matrix of the cartilage layers were higher (e.g. 20% vs. 3%).

CONCLUSIONS

The labrum can seal against fluid expression from the joint space. This sealing function protects the cartilage layers of the hip.

RELEVANCE

Current treatments for labrum damage and early arthrosis may compromise the sealing function of the labrum. With continued study of the function and importance of the labrum, new surgical repair strategies can be developed to maintain the overall function of the hip joint.

The effect of joint loading on acetabular wear measurement in total hip arthroplasty

All radiographic calculations of acetabular wear assume concentric reduction of the prosthetic articulation. To date, no studies have shown that the femoral head is fully reduced on standard radiographs, and we have seen cases on early postoperative radiographs in which this assumption is not met. Using our computerized radiographic technique, 78 paired anteroposterior pelvic radiographs in 46 patients at a mean of 14 months after surgery (range, 1-92 months) were evaluated with and without joint loading. Displacement with loading was analyzed against time since surgery, a surrogate for acetabular wear. Regression analysis found a statistically significant increase in femoral head displacement after loading with longer duration of follow-up, but the rate of this increase was small (0.027 mm/y). This difference affects calculated wear values by <15%. We conclude that in a low-wear cohort, joint loading does not affect radiographic calculations of acetabular polyethylene wear in a clinically important way.

Significance of the lateral epiphysis of the acetabulum to hip joint stability

Normal development of the acetabulum is crucial to the growth and stability of the hip. Twenty-five New Zealand White rabbits (postweaning) weighing 1.5-2.2 kg were used. Both hips were exposed through an anterolateral approach. On the right hip, a fixed area of superolateral physeal cartilage was damaged with drills. Sham open reduction was performed for the left hip. Radiographic changes of the right hips were evident at 6 weeks. At 12 weeks, the right hip dislocated in a posterior and superior direction. The left hip remained normal. Histopathologic analysis correlated strongly with the radiographic findings. There were thinning of cartilage cells of the acetabulum, with disorganization. The acetabular roof was poorly formed. The lateral acetabular epiphysis is vital to the development of the acetabular roof. Damage to this epiphysis may result in acetabular maldevelopment and subsequent hip instability. We tested the hypothesis that abnormality in this epiphysis can give rise to abnormal acetabular development.

Size and direction of hip joint forces associated with various positions of the acetabulum

When total hip replacement is performed, the position of the acetabular component may affect wear and component survival time. We considered the questions: In what way does displacement of the hip joint center alter (1) the magnitude and (2) the direction of the resultant force? Biomechanical tests were carried out on a human multibody model. After displacement of the hip joint center, the resultant forces were calculated for the single leg stance. With the flexed single leg stance, maximum hip joint forces were observed with lateral, cranial, posterior displacement. The peak forces were affected by the modeling of a gluteus maximus wrapping point at the ischial tuberosity and were overestimated when this was removed. With the straight single leg stance, posterior displacement decreases the total load on the hip joint because of the increased leverage of the rectus femoris. With regard to the direction of the resultant force, medial displacement increases the angles in both planes, cranial displacement increases it in the sagittal plane (cranial, posterior-caudal, anterior), and anterior displacement decreases the angle in the sagittal plane and increases it in the frontal plane (medial, cranial-lateral, caudal). The direction of the force is relatively insensitive to displacement of the hip joint center. The results presented here indicate a marked increase in the force after lateral, cranial, posterior displacement of the center in the flexed single leg stance. To avoid extreme joint loading and to reduce the wear after total hip arthroplasty, the cranial and posterior regions of the acetabulum should be fully reconstructed. A high hip joint center has an adverse effect on the magnitude of the force, although the directions are hardly affected by it.

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).

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.

Type of motion and lubricant in wear simulation of polyethylene acetabular cup

The wear of ultra-high molecular weight polyethylene, the most commonly used bearing material in prosthetic joints, is often substantial, posing a significant clinical problem. For a long time, there has been a need for simple but still realistic wear test devices for prosthetic joint materials. The wear factors produced by earlier reciprocating and unidirectionally rotating wear test devices for polyethylene are typically two orders of magnitude too low, both in water and in serum lubrication. Wear is negligible even under multidirectional motion in water. A twelve-station, circularly translating pin-on-disc (CTPOD) device and a modification of the established biaxial rocking motion hip joint simulator were built. With these simple and inexpensive devices, and with the established three-axis hip joint simulator, realistic wear simulation was achieved. This was due to serum lubrication and to the fact that the direction of sliding constantly changed relative to the polyethylene specimen. The type and magnitude of load was found to be less important. The CTPOD tests showed that the subsurface brittle region, which results from gamma irradiation sterilization of polyethylene in air, has poor wear resistance. Phospholipid and soy protein lubrication resulted in unrealistic wear. The introduction of devices like CTPOD may boost wear studies, rendering them feasible without heavy investment.

[Densitometry for assessment of periacetabular bony changes after Parhofer-Mönch total hip replacement]

Densitometry for assessment of mineral changes after Parhofer-Mönch total hip replacement 10 mm around the acetabular component has been done in 53 patients (31 females and 22 males, mean age 52.1 years) with unilateral hip arthritis. Lunar DPX apparatus has been used, "manual analysis" of "Orthopedic" software and modification of DeLee and Charnley zones for DEXA technique was employed. After 6 months follow-up BMC and BMD decreased in all zones analyzed, especially in zone 2. These values increased in next 6 and 12 months but did not compensate for the primary loss.

Anteversion of the acetabulum in developmental dysplasia of the hip: analysis with computed tomography

Acetabular anteversion was measured by using two-dimensional (2-D) computed tomography (CT) scans in 39 dysplastic and 27 normal hips (patient age range, 3-33 years), and averaged 19.7 degrees in the dysplastic hips and 18.1 degrees in the normal hips. There was no statistically significant difference between the two groups, with a wide range of acetabular anteversion values noted in both groups (8-32 degrees ). Although acetabular anteversion may be increased in some patients with developmental dysplasia of the hip (DDH), it is not a universal finding. We believe that assessment and understanding of acetabular anteversion is needed before performing corrective osteotomies for hip dysplasia to optimize results and avoid the complications of acetabular retroversion.

[Cadaver study of acetabular cup mobility in the healthy hip and prosthesis by monopodal pressure simulation ]

PURPOSE OF THIS STUDY

The purpose of this study was to quantify relative displacement of anterior and posterior horn of the acetabulum lunate surface using Omega strain gauges while increasing loads were applied to the hip joint. Measurement were performed on fresh cadaver bones in unipodal stance using experimental method before and after socket's implantation.

MATERIAL AND METHODS

[corrected] Nine skeletons, from fresh non-embalmed cadavers, including pelvis, the three last lumbar vertebrae and both femurs were maintained in unipodal equilibrium using metallic cables for muscle passive simulation. Loads were applied up to 700 N. Specific extensometric Omega strain gauges were created and tested, with 5 mu of sensibility. Measurement of displacement between horns were studied on two sides of each pelvic before and after implantation of conventional and prototype cemented or press fit implants.

RESULTS

Among 81 loads on 18 acetabulum healthy and implanted, displacement was significant in 58 cases with a spacing of horn of 7 to 140 mu with average 31.1 mu and non significant in 25 cases. On healthy acetabulum, spacing was variable from 12 to 140 mu (average 43.2 mu) in 18 of 26 loads. On implanted acetabulum by different sockets, spacing was variable from 7 to 100 mu (average 27.4 mu) in 40 of 55 loads. Displacement was function of rotation hip, smaller in internal rotation and larger in external rotation. Spacing of horns was reduced in oversize sockets.

DISCUSSION

The data obtains by omega strain gauge suggest acetabular displacement is not univocal under load. There were a widening of the acetabular notch on load. This can be explained by the modification of primary incongruity of the two components of the hip joint because of acetabular deformation. Such results are in agreement with recently published data obtained about length of the transverse acetabular ligament.

Variations of relative anteversion of the femoral neck during walking

Based on the geometric model developed by Netter [11], we determined the different positions of the femoral neck during monopodal support in walking in relation to a fixed frontal plane of reference (relative anteversion). This "relative anteversion" ranges on average from 24 degrees of retroversion at the beginning of support to 15 degrees of anterversion at the end if loading. We then studied the relations possibly existing between relative anteversion and acetabular orientation on the one hand, and the orientation of the resultant of the articular stresses on the other (both being variables during monopodal support in walking). The results showed that relative anteversion is well correlated with variations of position of the acetabulum since, at most, the deviation between the respective axes did not exceed the anatomic deviation due to absolute anteversion of the femoral neck and acetabulum. Lastly, analysis of the relations obtained with the orientation of the resultant of the articular stresses allowed a better comprehension of the functional distribution of forces.

The role of the acetabular labrum and the transverse acetabular ligament in load transmission in the hip

We performed a biomechanical study of seventeen hip joints in the pelves of nine cadavera in order to assess the role that the acetabular labrum and the transverse acetabular ligament play in load transmission. The distribution of contact area and pressure between the acetabulum and the femoral head was measured with the hip in four different conditions: intact (seventeen hips), after removal of the transverse acetabular ligament (eight hips), after removal of the entire labrum (nine hips), and after removal of both the transverse acetabular ligament and the labrum (seventeen hips). The hip joint was loaded in simulated single-limb stance, and the measurements were made with use of pressure-sensitive film. A peripheral distribution of load was seen in the intact acetabula. This pattern was altered only minimally after removal of the transverse acetabular ligament or the labrum, or both. When both of these structures were removed, the only significant change was a decrease in the maximum pressure in the posterior aspect of the acetabulum (p = 0.02). No significant changes were detected with regard to the contact area, load, mean pressure, or maximum pressure in the anterior or superior aspect of the acetabulum under any testing condition.

A calculation of the forces acting on the human acetabulum during walking. Based On in vivo force measurements, kinematic analysis and morphometry

Information about the loading of the human acetabulum during walking is necessary for a functional understanding of the morphology of the pelvic girdle and the hip joint as well as for the optimization of endoprosthetic therapy in osteoarthritis. For this purpose, experimental data of the forces acting on the femur in walking taken from the literature [Bergmann et al.: J. Biomech. 1993;26: 969-990] were combined with our own kinematic and morphometric data, to transform the force vectors from the femoral into a pelvic and an acetabular frame. During the walking cycle, the resultant force vector takes a rather constant course relative to the pelvis and its orientation seems to be highly regulated to act within a small range of angles. Only small deviations occur from the angles against the vertical which the resultant peak force forms in the frontal plane (F = 11 degrees, medially orientated) and in the sagittal plane (S = 5 degrees, ventrally orientated). The experimental results form the basis for a model of the incongruous hip joint as an elastic joint, the femoral head being centered between compliant elements.

Analysis of optimal range of socket orientations in total hip arthroplasty with use of computer-aided design simulation

A three-dimensional computer-aided design model of a total hip replacement was used to study the effects of anteversion and abduction of the acetabular component and anteversion and varus-valgus angulation of the femoral component on the range of hip flexion and extension that could be obtained without component impingement. Impingement of the component was defined as impingement between the neck of the femoral component and the edge of the acetabular component. To achieve an angle of hip flexion greater than 90 degrees and an extension angle greater than 30 degrees without component impingement, the optimal angulations were found to be between 1 and 30 degrees of anteversion and 30 and 50 degrees of abduction of the acetabular component, as well as 10 degrees of anteversion of the femoral component. When the valgus angulation of the femoral component was reduced from 7 to 0 degrees, the allowable range of flexion without impingement increased under the same conditions of acetabular-component orientation and femoral-component anteversion. Significant inverse correlations were found between the anteversion angle of the acetabular component and both the lumbar lordosis angle and the sacrohorizontal angle.

Primary stability of acetabular reinforcement implants in revision surgery

It was the purpose of this biomechanical in vitro study to characterize the initial mechanical stability of 3 different acetabular reinforcement prostheses as a function of implant design and bone stock conditions. Müller and Ganz rings and Burch-Schneider cage (Protek, Münsingen, CH) were fixed using 3 screws in normal acetabuli and in acetabuli with 5 different simulated conditions of segmental bone-stock defects. A servohydraulic testing machine (Instron, Canton, USA) was used for the inquiry. Preparations were tested to determine axial stability (2354 N). Three electromagnetic displacement transducers (Micro-Epsilon, Ortenburg, D) were placed in the 3 main quadrants of the acetabular rim to detect the micromotion of the implant. The amount of micromotion depended on the size of the defect and contact area of the prosthesis. All of the implants were stable (< 123 microns) in all quadrants of normal acetabuli and in case of ectasis, protrusio, and ventral defects. Displacement of more than 200 microns was observed at the ilium with the Muller implant in acetabuli with cranial defect (p < 0.05) and with both rings in acetabuli with dorsal defect. The Burch-Schneider cage was stable in all conditions, but displacement of more than 350 microns was observed in acetabula with pseudoarthrosis. The 3 reinforcement implants showed low displacement rates in most of the acetabular bony defects. The experimental data suggests that careful preoperative evaluation and intraoperative assessment to match bone defects and reinforcement implants are of paramount importance to achieve good stability.

The role of the anterior column of the acetabulum on pelvic stability: a biomechanical study

A study on the effects of the anterior and the posterior column of the acetabulum on pelvic ring stability was carried out by applying a lateral compression force to specially prepared specimens. In group 1, the continuity of the right anterior column was disrupted by removing the acetabulopubic part of the anterior column; the posterior column was left intact in order to measure the strength of the posterior column. In group 2 the continuity of the right posterior column was disrupted by removing the ischioacetabular part of the posterior column leaving the anterior column intact in order to measure strength of the anterior column. The posterior column provided an average maximum strength of 759.43 +/- 229.51 N and the stiffness was 113.19 +/- 22.40 N/nm. The anterior column provided an average maximum strength of 2015.40 +/- 352.31 N and the stiffness was 301.57 +/- 98.67 N/mm. Thus the anterior column provides 2.75 times greater strength to the pelvic ring than the posterior column (P < 0.05). This finding may be important in open reduction and internal fixation of double column acetabular fractures.

Analysis of the kinematics of different hip simulators used to study wear of candidate materials for the articulation of total hip arthroplasties

We developed an analytical technique to determine the paths traced by specific points on the femoral head against the acetabulum in the human hip joint during gait. The purpose of the study was to apply this technique to the mechanical hip simulators chosen to conduct wear tests on polymeric acetabular liners used in total hip replacements. These simulators differ from one another in the type of motion produced, apart from other variables such as type of lubricant and head position. Due to the variation in the kinematics between the machines, the paths traced by the points on the femoral head against the acetabular liner ranged from simple linear traces to figure-8 loops and quasi-elliptical paths during a single simulator cycle. The distances traveled by these points during the same period also varied appreciably among the different hip simulator designs. These results are important when combined with other studies that have shown that kinematics can play an important role in the outcome of in vitro wear experiments. The kinematic differences quantified in this study can partially explain the substantial differences in wear data reported from different simulator designs and also underscore the usefulness of the technique described in this study in judging the results from different hip simulator experiments.

Temporal and spatial distributions of directional counterface motion at the acetabular bearing surface in total hip arthroplasty

The motions of counterface articulation against the bearing surface of the acetabular liner strongly influence polyethylene wear debris production in contemporary total hip arthroplasty. However, the available body of relevant articular force and motion information is largely confined to resultant load excursions measured relative to instrumented femoral components, and/or to global angular motions (flexion, adduction, endorotation) of the joint. Analytical frameworks are here developed to transform such information into temporal and spatial variations of the resultant load and of the local counterface sliding velocity relative to an ordered set of discrete locations (e.g., finite element nodes) on the acetabular bearing surface. Whole-duty-cycle time histories of acetabular resultant load and counterface velocity distributions are presented for two important practical situations: human level walking gait, and a 23 degrees biaxial rocking hip simulation machine. The local counterface motions occurring in the simulator are characterized by higher velocities, smoother motion patterns, and wider directional variation than those occurring in human gait.

[Factors affecting the primary stability of acetabular components. An in vitro study]

The aim of the present study was to investigate the micromotion under physiological loading of various acetabular components with and without screws, to determine the most suitable anchoring, cup design and optimal surface structure. Six acetabular components with varying cup geometry and surface configuration were implanted with a 2 mm press-fit into polyethylene pelves. In the first set of trials, the uncemented cups with two peripheral screws were tested under an axial load of 240 kg (2,354 N). The screws were then removed and the cups, held in place only by press fit, were tested again. None of the uncemented cups achieved the high initial stability of the cemented reference cups. It was not possible to determine an optimal cup design. In this study, titanium plasma-spray-coated cups achieved the best results. Stability is determined only in part by the configuration of the prosthesis. Of equal importance is the quality of the preparation of the cup bed. The use of screws cannot be unreservedly recommended. With a good press fit, the use of screws enhances stability only minimally.