In vitro initial fixation of porous-coated acetabular total hip components. A biomechanical comparative study

The optimal screw-hole positions of the eccentric revision cup based on a morphological study

Background

Bridging bone defects in revision total hip arthroplasty is a challenge to orthopedic surgeons. The eccentric revision cup is a progression of jumbo cup. Our aim is to confirm the optimal screw-hole positions of the eccentric revision cup by morphological measurements of three-dimensional pelvic reconstruction.

Methods

Eighty CT images were converted to virtual three-dimensional bones. After simulating the surgery procedure, all available screw holes were inserted with the screws in virtual. By measuring the length of the screw in the pelvic bone, we determined the rich bone stock area. Then the screw holes were designed according to the characteristics of bone stock distribution. The peripheral screw-hole cluster and inner screw-hole cluster were studied respectively.

Results

For peripheral screw-hole cluster, five screw holes were evenly distributed between point A and point B in the thicker rim. For inner screw-hole cluster, screw hole 1 and screw hole 2 are the recommended inner screw holes.

Conclusion

The eccentric revision cup has inherited the strengths of jumbo cup besides several unique advantages, including using the peripheral screws enhancing primary stability; decreasing the shift of hip rotation center and restoring biomechanical function; reducing the risk of dislocation because of the smaller head-cup differences; increasing the contact area between the outer cup and the host bone while maintaining a normal inclination of the inner cup. In this study, we confirmed the optimal screw-hole positions of the eccentric revision cup by surgical simulation and morphological measurement. However, biomechanical tests are still being further explored.

Management of intraoperative acetabular fracture in primary total hip arthroplasty

Background

Intraoperative acetabular fracture(IAF) is a rare complication of primary total hip arthroplasty(THA). The previous reports have lacked a sufficiently large number of subjects to allow for an analysis of the causes and appropriate treatment of this problem.

Methods

Between 2015 to 2018, 4888 primary THA were enrolled. We retrospectively reviewed the records in our Total Joint Registry Database and found that 24 patients (24 hips) had sustained intraoperative acetabular fractures. Twenty-four patients(16 females and 8males)were all treated with a posterolateral approach using uncemented components. Twenty patients(83.3%)underwent supplemental screw fixation, of which 2 patients were treated with steel plate fixation. Two patients’ femoral heads were used as a graft. In 4 patients(16.7%), the acetabular components were judged to be stable despite the fracture and no additional treatment was performed. All patients were evaluated clinically with Harris Hip Scores (HHS) and radiographically with serial X-rays which follow up for a mean period of 34.0 ± 12.6 months. We evaluated the anatomic locations, causes, treatments, and outcome of the fractures to study the treatment method and effect of intraoperative acetabular fracture during operation.

Results

The fracture rate associated with uncemented components was 0.49%. In 17(70.8%) of these patients, the fracture was noted during the impaction of the real acetabular component. Six patients(25%)with Ankylosing Spondylitis had fractures, 4 in the anterior wall, and 1 in the anterior column, because the patient with hip joint fusion needs a to pre-osteotomy before the dislocation. The HHS score increased from 30.8 ± 9.7 preoperatively to 90.2 ± 4.2 postoperatively. All the latest x-ray showed that the fracture did not move, and there is no translucent line formed in the acetabular cup bone interface.

Conclusion

Intraoperative acetabular fractures are rare complications of THA, and most commonly occur during the implantation of the acetabular components. It is necessary to prevent the occurrence of fractures as much as possible even if the fractures are found during the operation. It should be noted that patients with ankylosing spondylitis involving hip joints during THA surgery must be careful to prevent IAFs during dislocation and pre-osteotomy.

Fixation Stability of Uncemented Acetabular Cups With Respect to Different Bone Defect Sizes

BACKGROUND

In total hip arthroplasty, acetabular press-fit cups require a proper bone stock for sufficient primary implant fixation. The presence of acetabular bone defects compromises the primary fixation stability of acetabular press-fit cups. The aim of the present study is to determine the fixation stability of a cementless acetabular cup regarding standardized bone defects in an experimental setup.

METHODS

An acetabular defect model was developed and transferred to a biomechanical cup-block model. The lack of superior cup coverage was divided into 4 stages of superior rim loss (33%, 50%, 67%, and 83%) in the anterior-posterior direction and into 4 stages of mediolateral wall absence (11%, 22%, 33%, and 50%). This resulted in 11 different defect cavities, which were compared to the intact cavity in push-in and lever-out tests of one press-fit cup design (56 mm outer diameter). Thereby, push-in force, lever-out moment, lever-out angle, and interface stiffness were determined.

RESULTS

The determined lever-out moments range from 15.53 ± 1.38 Nm (intact cavity) to 1.37 ± 0.54 Nm (83%/50% defect). Smaller defects (33%/11%, 33%/22%, and 50%/11%) reduce the lever-out moments by an average of 33.9% ± 2.8%.

CONCLUSION

The lack of mediolateral acetabular coverage of 50% was assessed as critical for cementless cup fixation, whereby the contact zone between implant and bone in the defect is lost. A lack of 20% to 30% mediolateral coverage appears to be acceptable for press-fit cup fixation in the presence of primary stability. A defect of 50%/50% was identified as the threshold for using additional fixation methods.

Influence of Synthetic Bone Substitutes on the Anchorage Behavior of Open-Porous Acetabular Cup

BACKGROUND

The development in implants such as acetabular cups using additive manufacturing techniques is playing an increasingly important role in the healthcare industry.

METHOD

This study compared the primary stability of four selectively laser-melted press-fit cups (Ti6Al4V) with open-porous, load-bearing structural elements on the surface. The aim was to assess whether the material of the artificial bone stock affects the primary stability of the acetabular cup. The surface structures consist of repeated open-porous, load-bearing elements orthogonal to the acetabular surface. Experimental pull-out and lever-out tests were performed on exact-fit and press-fit cups to evaluate the primary stability of the cups in different synthetic bone substitutes. The acetabular components were placed in three different commercially available synthetic materials (ROHACELL-IGF 110, SikaBlock M330, Sawbones Solid Rigid). Results & conclusions: Within the scope of the study, it was possible to show the differences in fixation strength between the tested acetabular cups depending on their design, the structural elements used, and the different bone substitute material. In addition, functional correlations could be found which provide a qualitative reference to the material density of the bone stock and the press-fit volume of the acetabular cups.

Acetabular components with or without screws in total hip arthroplasty

AIM

To compare the operation time, blood loss, and early outcomes of acetabular components with and without the screw.

METHODS

Thirty patients who underwent cementless acetabular component with or without screw and whose follow-up exceeded one year period in total hip arthroplasty were evaluated. A posterior approach was used in all surgical procedures by one experienced surgeon. Demographic data, operation time, intra- and postoperative blood loss volume, follow-up clinical score, cup migration, and osteolysis were recorded. The Kolmogorov-Smirnov test was performed for testing the normality of study data. Mann-Whitney U test was used to analyze the inter-group differences. A P-value of ≤ 0.05 was considered statistically significant.

RESULTS

Acetabular components were used in 16 (53.3%) patients with screw and 14 (46.7%) without screw. After one year of follow-up, an osteolytic lesion of 3 mm was found in only one patient in the screw group. No cup migration was encountered. Intra-group mean Harris hip score significantly increased, but there was no significant inter-group difference. While the mean operation time of the screw group was 121.8 min (range; 95-140), it was 102.7 min (range; 80-120) in the no-screw group, and this difference was statistically significant (P = 0.002). The mean intraoperative/postoperative, and total blood loss were 556.6 mL (range: 350-800)/423.3 mL (range: 250-600), and 983.3 mL (range: 600-1350), respectively in the screw group; and 527 mL (range: 400-700)/456 mL (range: 230-600), and 983 mL (range: 630-1250), respectively in the no-screw group. The blood loss difference between the two groups was not significant. In the screw group, the operation time was 19.1 min longer than the no-screw group, and this difference was statistically significant.

CONCLUSION

Acetabular components with or without screw have similar results, but the use of screw increases the operation time significantly, while not changing the blood loss volume.

Screw-hole clusters in acetabular cups: a morphological study of optimal positioning of screw-holes

BACKGROUND

Rigid and safe transacetabular screw fixation in total hip arthroplasty (THA) is achieved by pursuing deeper bone stock and avoiding injuries to the neurovascular structures, but these efforts can be restricted by the distribution of screw-holes on cups by the manufacturer. We therefore tried to determine: (i) optimal screw-hole positions on cups to allow anatomical placement of screws; (ii) rationality of the basic 3-screw-hole cluster on commercial cups; and (iii) the optimum method for placing commercial cups in accordance with acetabular anatomy.

METHODS

Periacetabular osseous structure of 64 hips and arterial structures of 50 hips were three-dimensionally reconstructed. Simulated transacetabular screw fixation during THA was performed in these hips with 3 different screw lengths (15, 25, and 35 mm) to define deeper and safer screw trajectories. Screw-hole locations of 7 commercially available cups were measured and matched with the periacetabular anatomy.

RESULTS

When the cup was placed into the acetabulum at 45° of abduction and 20° of anteversion, the optimal locations of 2 screw-holes on the cups were at 30° and 64° of latitude, with a 35° separation angle. The inversetriangle distribution pattern was safer than the triangle pattern in basic 3-screw-hole-cluster cups. When placing the commercial cups, 5°-10° of anterior rotation can be added to allow better screw trajectories.

CONCLUSIONS

Our study determined optimal screw-hole positions and their distribution pattern on cups. We describe methods to place the commercial cups that are not designed according to acetabular anatomy.

Initial stability of press-fit acetabular components under rotational forces

The primary goal of this study was to determine the initial press-fit stability in acetabular components without screw fixation. Mechanical testing was performed with the implantation of press-fit acetabular components in cadaveric specimens. No significant difference was found in load to failure testing between 1 and 2 mm of under-reaming. However, there was significant variability in bending forces required to create 150 μm of micromotion ranging from 49.3 N to 214.4 N. This study shows that cups implanted in a press-fit fashion, which are felt to be clinically stable, have high degrees of variability in resisting load and may be at risk for loosening. There is a need for more objective intra-operative techniques to test cup stability.

Polyaxial locking and compression screws improve construct stiffness of acetabular cup fixation: a biomechanical study

Bone ingrowth into uncemented acetabular components requires intimate cup-bone contact and rigid fixation, which can be difficult to achieve in revision hip arthroplasty. This study compares polyaxial compression locking screws with non-locked and cancellous screw constructs for acetabular cup fixation. An acetabular cup modified with screw holes to provide both compression and angular stability was implanted into a bone substitute. Coronal lever out, axial torsion and push-out tests were performed with an Instron testing machine, measuring load versus displacement. Polyaxial locking compression screws significantly improved construct stiffness compared with non-locked or cancellous screws. This increased construct stiffness will likely reduce interfacial micromotion. Further research is required to determine whether this will improve bone ingrowth in vivo and reduce cup failure.

Comparison of the plantar pressure distributions at different degrees of tilting: a preliminary report

[Purpose] The purpose of this study was to investigate the amount of plantar pressures on the lower limb during tilt table standing and to indicate the ideal degree of tilting for partial weight bearing. [Subjects and Methods] Fifteen healthy subjects between the ages of 20 and 30 were recruited as volunteers for this study. All the measurements were taken while standing on a tilt table according to different inclination angles. [Results] The plantar pressures for 60° tilt table standing were lower by 7–9% of total body weight than the pressures during tilt table standing at 90°, and the pressures for 30° tilt table standing were lower by 18–20% of total body weight than the pressures for tilt table standing at 90°. [Conclusion] Standing training on a 60° tilt table might be equivalent to 80% of full weight bearing training, and tilt table standing training at 30° might be equivalent to 60% of full weight bearing training.

Press-fit cementless acetabular fixation with and without screws

PURPOSE

Cementless acetabular fixation for total hip arthroplasty (THA) is widely used. The question of using screws for a better primary and secondary acetabular fixation has been discussed in the literature in recent years. The aim of this meta-analysis was to compare fixation of acetabular cups with and without screws in total hip arthroplasty.

METHODS

Electronic databases Embase, PubMed and Cochrane Library were used to search for randomised controlled trials reported through May 2013 of cementless acetabular fixation for THA with and without screws. Two independent reviewers assessed the trials for eligibility and quality. All related data matching our standards were abstracted for meta-analysis by RevMan 5.0. Evaluation criteria included revisions, migration and osteolysis.

RESULTS

A total of 1,130 THAs enrolled into five trials were included in this meta-analysis. All studies compared fixation of acetabular cups with and without screws, and our pooled data showed no statistical significance between the two surgical methods in revision, migration and osteolysis.

CONCLUSION

There is no significant difference between cementless acetabular fixation for THA with and without screws in revisions, migration or osteolysis.

[Reconstruction of large acetabular defects using trabecular metal augments]

OBJECTIVE

Revision of cup and reconstruction of original center of rotation. High primary and secondary stability. Prevention of additional bone loss.

INDICATIONS

Osseous defects at the anterior-cranial, cranial and posterior-cranial rim of acetabulum. Larger cavitary, medial or oval defects (Paprosky IIb-IIIb). Segmental defects (anterior column up to half of host bone, posterior column up to one third of host bone).

CONTRAINDICATIONS

Infection of total hip arthroplasty. Pelvic discontinuity (Paprosky IV).

SURGICAL TECHNIQUE

Exposure of acetabulum and detection of defects. Complete removal of soft tissue from acetabulum, reaming of sclerotic bone, if necessary. Adaptation of trial augments to close an oval defect to a round defect and to reach an uncontained defect, respectively. Adaptation of trial cup. In case of sufficient stability, fixation of final augment with two or three screws in cranial bone stock. The screws should be directed to iliosacral joint. Augmentation with allogenic bone chips is possible in the region of wedge and acetabulum as well. Sealing of rough augment surface with bone cement. Implantation of cup, fixation with screws. Application of insert.

POSTOPERATIVE MANAGEMENT

Depending on bone defects, full weight bearing is possible. In cases of severe bone defects, reduction of weight bearing to 20 kg for 6 weeks is recommended. Postoperative physiotherapy is possible in most cases.

RESULTS

Between 2005 and 2007, 38 patients with acetabular defects type IIIa und IIIb according to Paprosky underwent reconstruction using the TMT system (Trabecular Metal Technology). After 25 months, a significant functional improvement was seen in all patients. The Merle d'Aubigné Score increased from 6 points preoperatively to 13 points postoperatively, the Harris Hip Score from 29 to 78 points. Two revisions were necessary because of loosening or migration of the cup.

The effects of acetabular shell deformation and liner thickness on frictional torque in ultrahigh-molecular-weight polyethylene acetabular bearings

The purposes of this study were to determine if there were differences in the frictional torque generated between spherical acetabular shells and acetabular shells deformed as a result of implantation and to evaluate how changes in polyethylene insert thickness and head diameter affected these frictional torque data. An established bench top model was used for mechanical testing. A total of 70 samples were tested. Acetabular shells were impacted into polyurethane foam that was designed to create spherical or deformed shell models. We found that deformed acetabular shells produced higher frictional torque than spherical shells. Also, larger femoral head sizes produced greater frictional torque than smaller femoral head sizes. For the deformed models, the thicker polyethylene inserts produced greater frictional torque than the thinner polyethylene inserts.

Effects of screw eccentricity on the initial stability of the acetabular cup in artificial foam bone of different qualities

Acetabular cup loosening is one of the major failure models of total hip replacement (THR), which is mostly due to insufficient initial stability of the cup. Previous studies have demonstrated that cup stability is affected by the quality of the host bone and the surgical skill when inserting screws. The purpose of this study was to determine the effects on the initial stability of the acetabular cup of eccentric screws in bone of different qualities. In this study, hemispherical cups were fixed into bone specimens constructed from artificial foam with three elastic moduli using one to three screws. The effects of two types of screw eccentricity (offset and angular) on the stability of the acetabular cup were also evaluated. The experimental results indicate that in the presence of ideal screwing, the cup was stable in bone specimens constructed from foam with the highest elastic modulus. In addition, increasing the number of ideal screws enhanced the cup stability, especially in bone specimens constructed from soft foam. Moreover, the cup stability was most affected by offset eccentric screw(s) in the hard-foam bone specimens and by angular eccentric screw(s) in the soft-foam bone specimens. The reported results indicate that the presence of screw eccentricity affects the initial stability of the acetabular cup. Surgeons should keep this in mind when performing screw insertions in THR. However, care is necessary when translating these results to the intraoperative situation due to the experiments being conducted under laboratory conditions, and hence, future studies should attempt to replicate the results reported here in vivo.

Lining up the liner: 2 case reports of early ceramic liner fragmentation

We present 2 cases of early ceramic liner fragmentation in patients undergoing ceramic-on-ceramic uncemented total hip arthroplasty. The reports highlight the significance of correct liner insertion at the time of operation and the importance of a meticulous evaluation of postoperative radiographs. Tilting of the liner may not be visualized on the anteroposterior roentgenogram alone. Preradiological ceramic fragmentation can occur.

Der Einfluss von Oberflächenmerkmalen auf die Primärstabilität einer zementfreien Hüftpfanne: eine mechanischeIn-vitro-Untersuchung / The influence of different surface treatments on the primary stability of cementless acetabular cups: anin vitrostudy

INTRODUCTION

Long-term stability of cementless acetabular cups depends on osseointegration, which requires primary stability of the implant. The aim of this study was to determine the influence of different surface treatments on the primary stability of press-fit acetabular cups. Mechanical lever-out tests were performed to quantify the stability in vitro.

MATERIALS AND METHODS

A hemispherical press-fit cup design with a flattened pole was used and different surface modifications were applied: smooth, corundum-blasted, titanium plasma spray, rough titan plasma spray, and titanium plasma spray with a rim. The outer diameter of all cups was kept constant. Polyurethane foam was selected as the test material and cup insertion was performed with a maximal force of 6000 N. The excess length between the cup and the surface of the foam blocks was measured. The maximum lever-out force was measured and the lever-out torque was calculated.

RESULTS

The excess length of cups with a smooth surface was significantly less (p<0.001) than for the other cups, with no significant differences among the other surface modifications. The lever-out torque for cups with a smooth surface was significantly less (p<0.001) than for the other cups, with no significant differences among the other surface modifications.

CONCLUSION

Only the cup with a smooth surface showed significant differences for excess length and lever-out torque. The other surface modifications exhibited the same stability. As long as a rough surface is chosen, cup design seems to have a greater influence on stability than surface modification. Although the study did not mimic real in vivo conditions and the lever-out-torques cannot be transferred to clinical situations, initial stability before bony ingrowth occurred could be clearly analysed.

The relation between micromotion and screw fixation in acetabular cup

One of the major causes inducing loosening in the cementless acetabular cup implanting is its insufficient initial stability. In this study, three-dimensional finite element models of the pelvis and acetabular components were developed to investigate the relationship between relative micromotion, initial stability, and screw fixation under six daily activity loadings. A commercial available hemispheric cup with five screw holes was used as the target acetabular cup. The simulation results showed that if screws were placed closed together, when the screw number increased from 1 to 5, the peak micromotion decreased less than 14%, from 126.5 to 108.8 microm, while the stable region, micromotion less than 28 microm, enlarged only by 40%, from 46.1% to 64.7%. However, if the screw could be placed near the cup rim, a single rim screw, 202.1 microm micromotion, could provide better stability than that of four dome screws, 209.6 microm micromotion, placed closed together. To conclude, multiple cup screws should be placed near cup rim and as separate as possible to enlarge the stable region and reduce the peak micromotion between cup and acetabulum.

Effects of screw eccentricity on the initial stability of the acetabular cup

One of the major failure modes of cementless acetabular components is the loosening of the acetabular cup, which is mostly attributable to insufficient initial stability. A hemispherical cup with a porous coating which is inserted with press-fit fixation and secured with several screws is one of the most widely used approaches. Many studies have found that bone screws are very helpful aids for cup fixation, but the optimal surgical technique for inserting screws has not been clearly reported. In this study, hemispherical cups were fixed into blocks of foam bone with zero to three screws. The effects of three types of screw eccentricity (a 1-mm offset and angular eccentricities of 15 degrees and 25 degrees ) on the initial stability of the acetabular cup were evaluated. The experimental results indicate that increasing the number of screws enhances the cup stability in the case of ideal screwing (i.e., with no eccentricity). An angular eccentricity of 15 degrees did not affect the cup stability for fixation with one or two screws. However, the presence of 25 degrees of angular eccentricity significantly reduced the stability of the cup, while 1 mm of offset eccentricity produced an even greater impact.

Acetabular component deformation with press-fit fixation

Acetabular component deformation secondary to forces encountered during insertion is a potential consequence of the press-fit technique. This study characterized the stiffness of Pinnacle 100 cups (DePuy, Warsaw, Ind) via mechanical testing and used this information with intraoperative measurements of cup deformation to calculate the in vivo forces acting on cups inserted during hip arthroplasty in 21 patients. We found that 90.5% of cups had measurable compression deformity, averaging 0.16 +/- 0.16 mm. The corresponding forces acting on these cups averaged 414 +/- 421 N. For hard-on-hard bearing surfaces, such in vivo deformation of acetabular shells may result in negative clinical consequences such as equatorial loading with increased wear and potential seizing of components, chipping of ceramic inserts, or locking mechanism damage.

Der acetabuläre Pressfit bei äquatorialer Beschichtung der zementfreien Hüftpfanne – eine finite-Elemente-Analyse Pressfit of equatorially roughened cementless acetabular components – a finite elements analysis

AIM

Does the pressfit anchorage of cementless acetabular cups depend on the roughness of the pole? To answer this question the primary pressfit of two cementless acetabular cups which differ only with regard to the roughness of their poles were compared by means of finite elements analysis.

MATERIALS AND METHODS

It was assumed that the material properties of bone are homogeneous, isotropic and linearly elastic. Material-specific values of cancellous bone with three different bone densities were used. Assumption of isotropy represents an approximation.

RESULTS

Comparison of the two prosthesis designs revealed that both designs/shapes cause similar patterns of bone deformation and tension.

CONCLUSIONS

It can therefore be concluded that with regard to pressfit anchorage the prosthesis with milled polar surface is according to FEA mechanically equivalent to the prosthesis with non-milled polar surface.

Investigation on stick phenomena in metal-on-metal hip joints after resting periods

Insufficient understanding of tribological behaviour in total joint arthroplasty is considered as one of the reasons for prosthesis failure. Contrary to the continuous motion input profiles of hip simulators, human locomotion contains motion interruptions. These occurring resting periods can cause stick phenomena in metal-on-metal hip joints. The aim of the present study was to investigate the tribological sensitivity of all-metal bearings to motion interruptions on in vitro test specimens and retrieved implants. Friction and wear with and without resting periods were quantified. Unlike the metal-on-polyethylene joints, the static friction of metal-on-metal joints increased up to μs = 0.3 with rest, while wear appeared to be unaffected. This effect is caused by the interlocking of firmly adhered carbon layers, which were generated from the protein-containing lubricant through tribochemical reactions. Since more than 80 per cent of the retrieved implants exhibited macroscopically visible carbon layers, the increase in friction presumably also occurs under physiological conditions, which is then transferred to the bone-implant interface. These recurrent tangential stress peaks should be considered for the design features of the cup-bone interface, in particular when larger-sized implant heads are used.

Failure of a "screw-in" acetabular component: ten-year results, survivorship analysis, and the prediction of failure

The Rotalok screw-in threaded acetabular component was prospectively reviewed with 10-year clinical and radiological follow-up for 60 patients. Nine patients died and 5 were lost to follow-up. Clinically, 28 patients were pain-free, 13 had mild pain, and 3 had moderate pain. Thirteen patients underwent revision for loosening and 3 required revision but were unfit. Superior migration, angular migration, and zone lucency were measured radiologically. Cumulative survival was 70.75% with revision surgery as the end point and 60% with combined clinical failure and revision as the end point. Angular migration of 3 degrees or more was a significant predictor of clinical failure and revision (P < .0001), with 5 degrees being very highly predictive with a sensitivity of 0.72 and a specificity of 1.00. Revision was associated with younger patients (P = .03) and autograft use without screw stabilization (P = .024). The high failure rate of the Rotalok necessitates careful clinical and radiological follow-up, with asymptomatic radiological angular migration often the first predictor of failure.

Effect of augmented cup fixation on stability, wear, and osteolysis: a 5-year follow-up of total hip arthroplasty with RSA

To evaluate different modes of cementless fixation of hemispherical cups, we operated on 87 hips in 81 patients using 4 different means of cup fixation. The hips were randomly assigned to fixation with press-fit technique only (PF), or with augmentation with screws (S), pegs (P), or hydroxyapatite (HA) coating. The patients were evaluated with radiostereometric analysis (RSA) for cup migration and wear, conventional radiography for osteolysis, and Harris Hip Score for clinical outcome over 5 years. The fixation of the cups did not differ between the groups, but HA showed a tendency to decrease proximal migration. HA-coated cups displayed the best interface with hardly any signs of radiolucent lines, indicating a superior sealing effect of the HA coating. Cups with screws or pegs had more radiolucent lines and osteolytic lesions than the other groups. Radiolucent lines were correlated to higher proximal migration, young age, and female gender (r2=.2). The wear rate of the ethylene oxide-sterilized polyethylene liner was high (0.2 mm/y) but did not differ between the groups. Two cups with a perioperative fracture of the acetabular rim showed large initial migration but stabilized thereafter.

Techniques for measuring weight bearing during standing and walking

OBJECTIVE

To classify and assess techniques for measuring the amount of weight bearing during standing and walking.

BACKGROUND

A large variety of weight bearing measuring techniques exists. This review describes their advantages and limitations to assist clinicians and researchers in selecting a technique for their specific application in measuring weight bearing.

METHODS

A literature search was performed in Pubmed-Medline, CINAHL, and EMBASE. Measurement techniques were classified in 'clinical examination', 'scales', 'biofeedback systems', 'ambulatory devices' and 'platforms', and assessed on aspects of methodological quality, application, and feasibility.

RESULTS

A total of 68 related articles was evaluated. The clinical examination technique is a crude method to estimate the amount of weight bearing. Scales are useful for static measurements to evaluate symmetry in weight bearing. Biofeedback systems give more reliable, accurate and objective data on weight bearing compared to clinical examination and scales, but the high costs could limit their use in physical therapy departments. The ambulatory devices can measure weight bearing with good accuracy and reliability in the hospital and at home. Platforms have the best methodological quality, but are mostly restricted to a gait laboratory, need trained personnel, and are expensive.

CONCLUSIONS

The choice of a technique largely depends upon the criteria discussed in this review; however the clinical utilisation, the research question posed, and the available budget also play a role. The new developments seen in the field of 'ambulatory devices' are aimed at extending measuring time, and improved practicality in data collection and data analysis. For these latter devices, however, mainly preliminary studies have been published about devices that are not (yet) commercially available.

Initial stability of a cementless acetabular cup design: experimental investigation on the effect of adding fins to the rim of the cup

Different design solutions have been suggested for improvement of the initial stability of cementless acetabular cups, such as adding threads, spikes, or pegs to the hemispherical geometry, the pore structure of the surface; and screw fixation. This experimental study investigated the effect of fins on the initial stability of the acetabular cup. Three designs were studied, with none, 2, and 12 fins, respectively. The cups were press fit into cavities reamed in 2 different polyurethane foams, used to simulate 2 qualities of cancellous bone. Two millimeter press-fit and exact-fit conditions were investigated. The results show that the type of substrate and the interference value are important in determining the initial stability of the cup. The addition of fins on the cup rim enhances in vitro the initial stability, especially in cases of a poor press fit with a good substrate. This preclinical investigation suggests that the use of a cup design with fins may be beneficial in all cases in which press fit of the cup cannot be assured. However, further clinical studies are required to validate in vivo the efficacy of the fins as additional fixation devices.

Cementless acetabular fixation in total hip arthroplasty using polyglycolide-lactide screws: an in vivo canine study

Bone ingrowth into cementless acetabular components was evaluated in a canine total hip arthroplasty model, comparing components initially stabilized with polyglycolide-lactide screws with those initially stabilized with titanium screws. The acetabular shell was anchored with 2 polyglycolide-lactide screws in 16 dogs and with 2 titanium screws in 12 dogs. The dogs were followed and sacrificed at 7 weeks, 14 weeks, 10 months, or 15 months. Histomorphometric analysis of bone ingrowth into the weight-bearing dome of the acetabular shell was conducted. No difference was detected in mean bone ingrowth into the acetabular shell comparing the 2 screw groups. The results of this study do not support a significant advantage to the use of biodegradable screws for the initial stabilization of cementless acetabular components in canine total hip arthroplasty.

5-year review of second-generation acetabular cup with dome screws

A total of 197 modular cementless acetabular cups with central screw fixation were followed an average of 60 months. Four metal shells were revised (2%), including 2 for dislocations, 1 for pain and osteolysis, and 1 for loosening. An additional 2 cup liners were exchanged for dislocations. The 5-year survivorship of the metal shell was 98.0%. Two different types of polyethylene were used for the liners. Polyethylene wear and osteolysis were associated with the use of Hylamer liners and higher preoperative patient activity levels. Cementless acetabular cups with dome screw fixation can provide excellent clinical results. Hylamer liners sterilized with gamma radiation in air should not be used.

Methods for quantitative analysis of the primary stability in uncemented hip prostheses

Torsional loads of daily activities contribute to the failure of the primary fixation of hip prostheses. Implant torsional stability must be evaluated prior to in vivo clinical trials. Whereas previous work has investigated this phenomenon, descriptions of physiologically accurate and reproducible in vitro methodologies are rare. The present study aimed to detect and control the sources of error and variability that influence in vitro methods. A typical set-up for the analysis of primary stability of hip stems was studied. Load cycles included proximal-to-distal axial force, torque, and bending moment. The effects of loading frequency and strain distribution across the cortical bone were investigated in order to optimize testing conditions and measurement set-up. The relative shear motion at the bone-stem interface was measured transcortically using linear variable displacement transducers. The procedures developed for mounting specimens on the testing machine and for positioning sensors on the specimen were standardized and tested for reproducibility. The protocol was finally tested for repeatability and accuracy. Measurement errors were 2.3 microm between load cycles and 4.9 microm for repeated set-ups, comparing favorably with the literature.

Acetabular fracture associated with cementless acetabular component insertion: a report of 13 cases

Acetabular fracture during insertion of a cementless acetabular component occurred in 13 patients. The preoperative diagnosis was osteoarthritis in 6 patients, rheumatoid arthritis in 2 patients, avascular necrosis in 3 patients, hip fracture nonunion in 1 patient, and developmental dysplasia of the hip in 1 patient. Several different components were used; however, the acetabulum was underreamed by 1 to 3 mm in all cases. The acetabular fracture was identified in 9 of 13 cases intraoperatively. The fracture was identified on postoperative radiographs for the other 4 cases. Fractures were treated by a variety of means, including the addition of augmentation screws in or around the cup, use of autograft bone at the fracture site, modified postoperative weight-bearing status, and immobilization. In 2 cases, the socket needed to be revised after it progressively migrated and failed. One patient had cup migration, and another had a radiolucent line about the cup but was not symptomatic enough to require revision. In 3 of these 4 cases, the fracture was not identified intraoperatively. Underreaming of the acetabulum and use of an oversized acetabular component has been recommended to improve the initial stability of the acetabular component during total hip arthroplasty. Impaction of an oversized component requires bone to undergo plastic deformation if the cup is to be fully seated. Theoretically, this technique provides improved component stability with enhanced osseous ingrowth into the cup. The 13 cases reported in this study demonstrate that acetabular fracture is a complication that may occur in association with uncemented hip arthroplasty, particularly if oversized components are used. The importance of recognizing acetabular fractures intraoperatively and the need to institute appropriate treatment to ensure a stable acetabular component is emphasized. In patients with osteoporotic bone, line-to-line reaming with use of a cementless acetabular component or insertion of a cemented socket may be considered to avoid this significant complication.

Periprosthetic fractures of the acetabulum

Periprosthetic acetabular fractures during and after total hip replacement occur infrequently. Intraoperative fractures have risen with the use of press fit cementless fixation techniques and postoperative fractures are increasing because of the long-term problems associated with osteolysis. This article outlines the classification and management of these fractures.

Cementless acetabular revision for aseptic failure of cemented hip arthroplasty

This study is a 5- to 11-year retrospective followup of 40 hips in 33 patients with cementless acetabular revision for aseptic failure of a cemented total hip arthroplasty. A porous coated, Harris-Galante acetabular component was used in all revisions. Thirty-eight of the 40 hips received acetabular bone grafting at revision. The mean Harris Hip Score improved from 51 points just before the index cementless revision to 87 points at the most recent followup. Twenty-nine of 40 (73%) hips were classified as having a good or excellent result. Radiolucencies were observed in seven of the 40 (18%) hips at the most recent followup, but none of these radiolucencies were complete or progressive. Five of the 40 (13%) hips were rated as failures and required repeat revision. Two (5%) of these failures were caused by aseptic loosening, with both hips having severe acetabular bone damage at the time of the index revision. This failure rate for aseptic loosening was less than that reported for cemented acetabular revision, thereby confirming the efficacy of cementless acetabular components in revision hip surgery in the intermediate term.

Press-fit acetabular cup fixation: principles and testing

Pre-clinical testing of the fixation of press-fit acetabular components of total hip prostheses relies on cadaver or synthetic bone, but the properties and geometry of bone models differ from those of physiological bone. Cup designs use varied mechanisms for initial stability in bone; therefore, using different analogues and tests is appropriate. Press-fit cup stability was tested in the following: firstly, polyurethane (PU) foam modelling cancellous support; secondly, glass-fibre reinforced epoxide (GFRE) tubes modelling acetabular cortical support; thirdly, cadaveric acetabula. Three commercial cups [Harris-Galante II (H-G-II), Zimmer; Optifix, Smith & Nephew, Richards; porous coated anatomic (PCA), Howmedica] and an experimental cup with enhanced rim fixation were tested in three modes: direct pull-out, lever-out and axial torque. The fixation stabilities measured in the PU and the GFRE models showed trends consistent with those in cadaver bone, differing in the oversizing and cup geometry. The experimental cup was significantly more secure in most modes than other cups; the H-G II and Optifix cups showed similar stabilities, lower than that of the experimental cup but greater than that of the PCA cup (analysis of variance and Tukey's highly significant test; p < 0.001). The stabilities measured in cadaver bone more closely approximated those in GFRE. The use of several bone analogues enables separation of fixation mechanisms, allowing more accurate prediction of in vivo performance.

The hemispherical Harris-Galante acetabular cup, inserted without cement. The results of an eight to eleven-year follow-up of one hundred and sixty-eight hips

We studied the results for 168 available hips from a series of 324 consecutive primary total hip arthroplasties that had been performed with insertion of a Harris-Galante-I acetabular component without cement. The acetabulum had been reamed in a so-called line-to-line manner, and the cup had been fixed with one to four screws. A femoral component with a modular alumina-ceramic head had been inserted with cement in all hips. The median duration of follow-up was 112 months (range, 101 to 131 months). Of the original 324 hips, 109 could not be included in the clinical and radiographic follow-up because the patients had died and thirty could not be included because the patients were not available for examination. Seventeen hips had had a revision of the acetabular cup: five, because of infection; five, because of dislocation; three, because of aseptic loosening; and four, because of technical failure. This left 168 hips for clinical and radiographic follow-up; of these, fifteen had had a revision of the femoral component only. Of the remaining 153 hips, which had not had a revision, 147 (96 percent) were considered by the patient to have a satisfactory, good, or excellent result. One hip was found to have a loose cup on radiographic evaluation and was therefore considered to have failed, but the clinical function was good. We concluded that, with an overall rate of aseptic loosening of 1 percent (four of 324) after an intermediate (ten-year) duration of follow-up, use of this cup has good results.

Intermediate clinical follow-up of a dual-radius acetabular component

In this study, 92 primary total hip arthroplasties were performed in 83 patients using a porous-coated, dual-radius, cementless, acetabular component. All hips underwent line-to-line dome reaming with press-fit implantation that was judged to have complete bone contact. This acetabular shell provides a 1-mm oversized peripheral rim, which adds excellent initial stability while allowing complete bone contact in all hips. No fractures occurred. In 83% of hips, adjunctive screw fixation was not necessary. At a minimum of 4 years, follow-up, there were no revisions, no acetabular migration, one case of acetabular erosion consistent with osteolysis, and the average Harris Hip Score was 95. The design features of this new acetabular component have provided excellent fixation with complete initial bone contact, resulting in satisfactory intermediate clinical and radiographic results. The design provides excellent peripheral stability and complete bone contact.

Arthopor 2 acetabular component with screw fixation in primary hip arthroplasty: a 7- to 9-year follow-up study

The first 100 consecutive primary hip arthroplasties with the Arthopor 2 acetabular component (Joint Medical Products, Stamford, CT) with supplemental screw fixation performed by the senior author were prospectively studied. The mean patient age was 57 years. Seventy-seven cemented and 23 cementless stems were placed, with 32-mm femoral heads used in most of the patients. Eighty-six hips were reviewed at 7-8.8 years (mean, 7.8 years). The mean Harris hip score at final follow-up evaluation was 94 points. No cups have been revised, although 2 cups in patients with radiation osteonecrosis were loose radiographically. One patient underwent revision of a severely worn liner. Despite a relatively high mean linear wear rate of 0.27 mm/y (range, 0.13-0.57 mm/y), no radiographic evidence of pelvic osteolysis was seen. The Arthopor 2 cup performed well with respect to pelvic osteolysis, despite the presence of substantial polyethylene debris. The relatively high polyethylene wear rate is attributed in part to the younger patient population and the use of 32-mm femoral heads paired with thin polyethylene liners.

Acetabular screw rings and surface treatment

The Zweymüller truncated self tapered threaded ring is forged in pure titanium with a 3- to 5-mu grit blasted surface roughness. Among 167 consecutive primary total hip arthroplasties prospectively studied, two threaded components failed to achieve initial stability, with one requiring almost immediate revision. One hundred twenty-six hips have been fully documented with a 5-year minimum followup. The mean modified Harris hip score improved from 44.4 points preoperatively to 91 points at 7 years average followup (range, 60-121 months). Bone to implant gap rate decreased from 54.7% after surgery to 2.4% at last review, with only one detectable early migration that had secondary stable osseointegration. Annual linear wear of more than 0.2 mm per year could be detected in only one ceramic on polyethylene bearing surface with one corresponding femoral osteolysis granuloma. At the 9- to 10-year interval, the survivorship with definite loosening as a failure was 98.7%. Delayed loosening by failure of an initially stable reconstruction, progressive lucencies, liner disassociation, or worrisome osteolysis have not been observed. These midterm results are much better than those of former screw rings with smooth surface finishes. The main reason for the general failure and justified abandonment of threaded polished components may not be the screw in mechanism of their primary fixation, but the lack of an appropriate surface for bone ingrowth and osseointegration.

Porous-coated acetabular components with screw fixation. Five to ten-year results

The results of 136 consecutive primary total hip arthroplasties performed by one surgeon with the Harris-Galante-I porous-coated acetabular component were reviewed at a mean of seven years (range, five to ten years). In all hips, the outer diameter of the acetabular component was the same as the diameter of the final reamer used in the preparation of the acetabulum. However, this reamer was used only briefly at the rim of the acetabulum, and therefore the components had so-called press-fit stability. A mean of four screws (range, three to six screws) were used for additional fixation of the component. The clinical evaluation was performed with use of the Harris hip score. Standardized anteroposterior radiographs of the pelvis were assessed for migration of the component, radiolucent and radiodense lines, linear wear of the polyethylene, and osteolysis. No acetabular component had been revised for loosening and none were radiographically loose at the time of the most recent follow-up evaluation. There were no complications related to the use of the screws, and no screw had bent or broken. A non-progressive radiolucent line was seen in one acetabular zone in thirty-four hips (25 per cent) and in two acetabular zones in six hips (4 per cent). No hip had a radiolucent line in all three acetabular zones. The mean rate of linear wear of the polyethylene was 0.1 millimeter per year. There was no dissociation of the acetabular liner from the metal shell. Two hips (1 per cent) had asymptomatic osteolysis in the ischium and adjacent to the rim of the acetabular component; this was treated with grafting at the site of the lesion and exchange of the femoral head and the worn polyethylene liner. Five femoral components inserted without cement and one inserted with cement were revised because of loosening. The data suggest that, at a mean of seven years, fixation of this porous-coated component was uniformly excellent. The low prevalence of radiolucent lines and the absence of loosening are probably related, in part, to the technique of implantation. The low prevalence of polyethylene wear and pelvic osteolysis is a notable improvement compared with the results of arthroplasty with other porous-coated acetabular components. Although the results of the present study are encouraging, longer follow-up is necessary to determine the prevalence of late loosening and osteolysis.

Reamed surface topography and component seating in press-fit cementless acetabular fixation

Thirty-eight paired, size-matched, fresh-frozen cadaver acetabulae were used to study reamed surface topography and the seating of on-line versus oversized noncemented components. Four commonly used constructs were considered: on-line reamed and 2-mm oversized hemispherical cups, and on-line reamed and 1-mm oversized dual-radius cups. Reamed surface topography was indexed by three-dimensional surfaces mathematically fitted to digitized castings. Component seating was indexed by the distribution of bone-cup contact and the thickness of the gap between the component backing and the bone. The reamed surfaces were better approximated by slightly blunt ellipsoids than by spheres, especially for the smaller acetabulae. Seating of the on-line hemispherical cups was best near the pole and poorest near the equator, whereas the converse was true for the 1-mm oversized dual-radius cups. The on-line dual-radius cups offered better overall compromise between polar and equatorial contact than did the 2-mm oversized hemispherical components.

Fracture of the acetabulum during insertion of an oversized hemispherical component

A hemispherical metal-backed acetabular component that was oversized by two or four millimeters was introduced without cement into thirty fresh or embalmed cadaveric acetabula that were fifty, fifty-two, fifty-four, fifty-six, or fifty-eight millimeters in diameter. Manual tapping was used to insert the component into ten fresh and ten embalmed specimens, and a materials testing machine was used for the remaining five fresh and five embalmed specimens. The magnitude of force and the number of pulses necessary to seat the component were determined for the insertions performed with the materials testing machine; 2000 and 3000 newtons of force were required to insert the cups that were oversized by two and four millimeters, respectively. Visual and radiographic assessments were used to identify any resulting acetabular fractures. Over-all, eighteen (60 per cent) of the thirty acetabular specimens fractured. Of the eighteen fractures, only fifteen were detectable radiographically: ten on en face or lateral radiographs of the cup and five only on oblique radiographs of the cup. The remaining three fractures were apparent only after the removal of the acetabular component.(ABSTRACT TRUNCATED AT 250 WORDS)

Revision of the acetabulum without cement with use of the Harris-Galante porous-coated implant. Two to eight-year results

Sixty revision total hip arthroplasties were performed in fifty-nine patients with use of the Harris-Galante-I porous-coated acetabular component. The patients were followed prospectively for a mean of five years (range, two to eight years). Multiple small screws were used for fixation of the component in all of the hips. Bulk allograft or cancellous bone graft was placed in forty-eight hips. The hips were evaluated clinically with use of the Harris hip-scoring system. A component was considered to be loose if there was a change in the cup angle of more than 5 degrees or vertical or horizontal migration of more than two millimeters, as demonstrated on serial radiographs. Forty-eight (80 per cent) of the sixty hips had a score of good or excellent at the time of the most recent follow-up. No component had radiographic evidence of loosening, and there were no broken screws or radio-lucent lines around the screws. Osteolysis of the pelvis was not seen in any patient. One hip was debrided because of a late metastatic deep infection, but the components were not removed. At the time of writing, no acetabular component had been revised and no revisions were pending. There were no complications related to the use of multiple small screws, even in the acetabula in which there was a severe deficiency of bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Ingrowth reduces implant-to-bone relative displacements in canine acetabular prostheses

We examined bone-to-implant relative displacement of acetabular prostheses acutely and after ingrowth in a canine model. Uncemented hemispherical acetabular cups with titanium mesh pads comprising approximately 26% of the surface of the cup were inserted in eight adult canine hemipelves ex vivo. The acetabular prostheses were fixed with 13 mm titanium screws. Zero, one, and two-screw configurations were tested, with the order of testing randomly assigned. A load simulating 1,000 cycles of canine gait as applied to the acetabular component, and relative displacements were measured at three locations between implant and bone to determine acute fixation. A repeated measures analysis of variance showed that two screws produced only 42% of the average relative displacement of one screw and 14% that of zero screws. Eight adult mixed-breed dogs then underwent unilateral total hip arthroplasty. All acetabula were biologically fixed with two cancellous screws. The results at 4 months showed significantly less relative displacement between the implant and bone than was measured in ex vivo implantations (p = 0.014). Bone ingrowth filled 20 +/- 6% (mean +/- SD) of the available space. The relative displacements of these implants were small in all cases (12 +/- 13 microns) and did not correlate with the amount of bone ingrowth. These data suggest that acetabular fixation with two screws can lead to bone ingrowth and reduced relative motion of the prosthesis under functional loading.

Porous-coated total hip arthroplasty in rheumatoid arthritis

Thirty-five porous-coated total hip arthroplasties were implanted in 25 patients with juvenile-onset or adult-onset rheumatoid arthritis and prospectively followed for a mean of 4.5 years (range, 3-6.5 years). Using the Harris hip rating, the mean hip score at the final follow-up evaluation was 91, and 86% of the hips had a good or excellent result. Thirty-one hips had no hip or thigh pain, while four had occasional slight pain. After surgery, 10 patients were employed and the remainder were ambulatory. There were no early or late deep infections. There was one reoperation for recurrent dislocation of the hip, but the components were not removed. Radiographic evaluation of the acetabular component showed that only one component had a complete radiolucent line in all three zones. Three femoral components had nonprogressive subsidence. At this intermediate-term follow-up period, porous-coated total hip arthroplasty is a promising technique that is likely to be successful in young patients with rheumatoid arthritis.

Initial mechanical stability of acetabular prostheses

Secure initial stability is crucial for prostheses designed for biological ingrowth; without it, subsequent tissue ingrowth is impossible. Mechanical testing was performed in vitro to measure the interface shear stability of acetabular cup prostheses. Cemented, threaded, fined, and press-fit cups, both with and without supplemental screw fixation, were tested. A mechanically consistent polyethylene foam was used as a substrate to obviate the variations inherent in cadaver bones. Ramped, eccentric rim loads were applied using a servohydraulic materials testing system. Reaming, insertion, and loading variables were carefully controlled. Cups with external threads were significantly more stable than those fixed by fins or screws. Press-fit cups were significantly looser than all other tested designs. None of the uncemented prostheses approached the initial stability of cups secured with acrylic bone cement.

Cementless hemispheric acetabular components. 2-4-year results

A retrospective clinical and radiographic analysis was performed on 99 patients (106 hips) undergoing total hip arthroplasty with noncemented Harris-Galante (Zimmer, Warsaw, IN) or Optifix (Smith Nephew Richards, Memphis, TN) acetabular components. There were 94 primary and 12 revision procedures with a minimum follow-up period of 24 months (range, 24-52 months). The cups were evaluated for evidence of vertical and horizontal migration, as well as the presence of radiolucencies at the bone-implant interface. Age, sex, component inclination and medialization, cup coverage, and number of fixation screws used were examined to determine the influence of these factors on the incidence of cup migration or radiolucent line formation. A radiolucent line was present in at least one zone in 60% of the Harris-Galante and 45% of the Optifix cups. Progressive radiolucent lines were noted in two of the Optifix and three of the Harris-Galante components. Two Harris-Galante cups (1.9%) were revised. There were no Optifix cup migrations or evidence of instability in the Optifix or remaining Harris-Galante cups. Although there was a trend toward an increased incidence of radiolucencies in those cups lacking complete coverage, no statistically significant radiographic predictors for failure or impending failure could be determined from this short-term follow-up study. With the exception of the two revisions, all acetabular components performed well clinically over the study period.

Histological and radiographic assessment of well functioning porous-coated acetabular components. A human postmortem retrieval study

Nine porous-coated acetabular components were retrieved post mortem. All components had been inserted at our institution and had been in situ for a mean of fifty months (range, seventeen to eighty-seven months). Clinical records revealed that all had been functioning well at the time of death, and clinical radiographs showed signs that all had been stable. Standard backscattered scanning electron microscopy was used to quantitate the amount of bone ingrowth into the porous coating. For each component, the histological appearance of the bone-metal interface was compared with the appearance on clinical radiographs. Light microscopy was used to study the non-ossified areas. Every component had growth of bone into the porous coating, with the ingrowth occupying a mean of 32 per cent (range, 3 to 84 per cent) of the fields that were examined. In areas where bone ingrowth had occurred, the mean area density was 48 per cent (range, 26 to 65 per cent). Use of radiographs consistently led to an underestimation of the presence of gap areas and an overestimation of the occurrence of bone apposition. When fibrous tissue was present in non-ossified areas, it was extremely dense and well organized. Within the limits of light microscopic examination, there was no evidence of granulomatous formation in the non-ossified regions. This is particularly encouraging since the fibrous tissue-bone interfaces seem to prohibit the deposit of particulate debris.

Acetabular micromotion as a measure of initial implant stability in primary hip arthroplasty. An in vitro comparison of different methods of initial acetabular component fixation

Micromotion has been shown to affect bony ingrowth into cementless components. This study was designed both to quantitate initial micromotion at the prosthesis-periacetabular bony interface and to compare different methods of commonly employed acetabular component fixations, ie, a press-fit hemispherical titanium cup, a press-fit hemispherical titanium cup with one and two dome screws, a press-fit titanium hemispherical cup with three spikes, and a cemented chromium-cobalt cup. The press-fit component without screws demonstrated the greatest motion equaling 162 microns at the ilium, 97 microns at the publis, and 54 microns at the ischium. With one and two screws placed into the dome, the mean ileal displacement decreased by 28 microns (17%) and 36 microns (22%), respectively. Dome screw placement demonstrated a minimal effect at the pubis and ischium. Compared to the press-fit component without augmentation, the tri-spike motion was less at the pubis and ischium. The cemented prosthesis provided the least amount of motion in all three areas tested. This experiment demonstrates that the ilium provides the least amount of support to immediate acetabular fixation, while the pubis (anterior column) and ischium (posterior column) provide more stability. One dome screw does not affect the stability of a hemispherical prosthetic cup significantly. A two dome screw fixation provides an added method of support at the ilium, but fails to decrease motion at the pubis or ischium significantly. The tri-spike fixation does not restrict motion at the ilium to the extent as the dome screws, but its effect at the ischium and pubis is much more pronounced. The obvious difference between initial motion seen with cemented versus uncemented components may suggest that before surgery, patients may need a period of protected weight bearing until ingrowth has occurred.

A prospective study of 100 consecutive Harris-Galante porous total hip arthroplasties. 2-5-year results

This is a prospective study of the authors' first 100 Harris-Galante porous total hip arthroplasties (THA) that were followed for 2-5 years. The mean age of the 83 patients was 43 years. Using the Harris hip rating system, 80% of the hips were rated as excellent, 13% as good, 5% as fair, and 2% as poor at final evaluation. Eighty percent of the hips had no pain, 11% had slight or occasional pain, 8% had activity-related pain, and 1% had moderate pain. Eighty-seven percent of the hips had no limp, 8% had a slight limp, and 5% had a moderate limp. One femoral component, placed as a conversion of a loose bipolar hemiarthroplasty, has been removed for loosening. Two other hips have been reoperated, one for lengthening the femoral neck and one for changing the acetabular liner. There were no problems with acetabular screw fixation and no component migrated. Only two hips had 1 mm nonprogressive radiolucent lines in all three zones. Femoral component subsidence of 3 or more mm was measured in 16 hips, but was progressive in only 1. Radiolucent and radiodense lines were frequently seen in the nonporous coated regions of the femoral stem. Loss of proximal medial femoral bone density was seen in 59% of hips, and calcar resorption was seen in 16% of hips. Although the clinical results of this uncemented implant system were good or excellent in 93% of hips, the high incidence of femoral component subsidence is worrisome.

Migration of porous coated acetabular prostheses fixed with screws: roentgen stereophotogrammetric analysis

The fixation of screw-fixed, porous-coated acetabular prostheses was studied during 2 years in 21 patients (22 hips) using repeated roentgen stereophotogrammetric measurements. Migration of the center of the prostheses and/or tilting were recorded in five cups after 6 months. Two years after operation, migration was recorded in three, migration and rotation in six, and only rotation in three cups. Bone quality, prosthetic position, and immediate postoperative radiographic appearance of the prosthesis-bone interface did not correlate with the occurrence of migration. Narrow radiolucent zones had developed in half of the patients after 2 years and was associated with proximal migration. Micromovements of screw-fixed, press-fit cups seem to start later compared with previously presented results from cemented acetabular prosthesis.

Stability of press-fit acetabular cups

Mechanical tests were performed to characterize the initial stability of press-fit cups as a function of cup design, surface structure, and surgical preparation. Eight cups from six manufacturers were press-fit into acetabular cavities prepared in two densities of Sawbones polyethylene foam and in bovine knee trabecular bone. Cavity sizes and cup loading forces were varied. Acetabular defects were simulated in the Sawbones model. Preparations were tested to determine axial-rotatory and tangential ("levering-out") stability. Results suggested that cup geometry and proper surgical technique--in particular, proper sizing and depth of the acetabular cavity--are important in determining initial cup stability independent of adjuvant screw or spike fixation. Stability is a function of the area of interface contact between the cup rim and the substrate. If the cavity is too small or too shallow, and the substrate too dense, the cup will not seat to the rim and stability will be compromised. If there are defects in the rim, the area of interface contact will be diminished and stability compromised. Cups with a true hemispherical design have a greater area of rim interface contact than "low-profile" cups and are therefore more stable. 1 mm undersizing of the cavity (or 2 mm undersizing in less dense substrate) appears to provide optimal stability.