Irradiating allograft bone may compromise the mechanical stability of the prosthesis-bone construct, potentially having adverse effects on the outcome of femoral impaction grafting at revision hip replacement. This in vitro study aimed to determine the effect of irradiation of allograft bone used in femoral impaction grafting on initial prosthesis stability.

Morsellised ovine femoral head bone was irradiated at 0 kGy (control), 15 kGy and 60 kGy. For each group, six ovine femurs were implanted with a cemented polished double taper stem following femoral impaction bone grafting. Dynamic hip joint loading was applied to the femoral head using a servo-hydraulic materials testing machine. The primary outcome was stem micromotion. Tri-axial micromotion of the stem relative to the bone at two sites was measured using linear variable differential transformers and non-contact laser motion transducers. Statistical analysis was performed using SPSS.

Compared to the control and 15 kGy groups, specimens in the 60 kGy group demonstrated statistically significant greater micromotion in the axial, antero-posterior and medio-lateral axes. A multi-factorial post-hoc power analysis based on the overall effect of group size indicated a power of 0.7. There was no difference in micromotion between the control and 15 kGy groups. The average micromotion in the axial axes was 63μm in the control and 59μm in the 15 kGy group.

The results of this study suggest that a maximum irradiation dose of 15 kGy may not affect initial prosthesis stability following femoral impaction grafting in this model and provide the basis for us to now proceed to in-vivo studies examining the effect of irradiated bone on implant stability over time.

Introduction: Disc degeneration causes structural and biochemical tissue changes resulting in altered stresses that may affect vertebral bone remodelling. We hypothesized that disc degeneration alters vertebral cortical strains and disc mechanics of the motion segment, with and without the presence of zygapophyseal joints.

Methods: Twenty human lumbar functional spinal units (FSUs) were strain gauged on the lateral and anterior vertebral cortices, below the inferior endplate. Each FSU was preloaded overnight (0.2 MPa) in a bath and subjected to dynamic compression (1 MPa), flexion/extension/lateral bending (500N + 5 Nm), and axial rotation (5 Nm), before and after removal of the zygapophyseal joints. After testing, discs were macroscopically assessed and graded (1–4) for degeneration. Stiffness, phase angle (energy absorption) and principal strains were calculated. ANOVAs with the dependent variable of principal strain/stiffness/phase angle versus disc grade were performed for each testing direction.

Results: Assessment of disc degenerative condition revealed six grade 2 discs, eight grade 3, and six grade 4. Age and degeneration were highly correlated (r=0.80, P< 0.0001). The effect of disc grade on stiffness was significant overall in most loading directions, before and after removal of zygapophyseal joints (P< 0.008), apart for axial rotation (P> 0.587). Post-hoc multiple comparisons for all loading directions apart for axial rotation revealed that the stiffness of grade 4 discs was significantly larger than grades 2 and 3 discs in most loading directions.

For phase angle (approximate magnitude 5°), no significant overall effects due to degeneration were found across any loading direction (P> 0.2). ANOVA analyses on maximum/minimum principal strains found no significant effect due to disc grade (P> 0.063). However, a small number of significant effects due to disc grade were found at particular strain gauge locations for the isolated disc in flexion, the intact FSU in extension, and the intact FSU/isolated disc in right lateral bending.

Discussion: This study represents the first of its kind to investigate the effects of disc degeneration on vertebral bone cortical strain and disc mechanical properties. Significant increases in stiffness were found with increasing degeneration in all test directions apart for axial rotation. Changes in disc stiffness were consistent with other studies and may be a result of the structural and biochemical changes within the disc that accompany the degenerative process.

The non-significant small phase angles suggest that the disc behaves more like an elastic solid than a poroelastic material, and that dehydration associated with degeneration does not adversely affect damping. Principal strains were not significantly affected by disc degeneration overall, suggesting that the cortical shell adjacent to the disc-endplate boundary maintains a relatively homeostatic condition, with more dramatic architectural changes probably occurring within the trabecular bone. Applications of this research include providing important validation data for analytical/finite element models of the intact FSU and isolated disc segment, and a better understanding of the magnitudes of cortical strains that need to be maintained in order to avoid damaging vertebral bone stress-shielding effects after treatments for disc degeneration.

Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body has a potentially important role in the mechanism of subsequent biological changes in disc and bone. The disc is a complex structure, exhibiting visco-elastic behaviour that is highly dependent on its condition and fluid content. Studies have shown that the stiffness of the disc is altered by its water content in human, ovine and bovine discs. It has also been shown that disc stiffness or modulus can be preserved if the level of water in the disc is kept constant. The importance of maintaining a reproducible state of stress in the disc during sequential testing of the same specimen is crucial to ensuring consistency of results and minimising systematic experimental errors. The aims of this study were to assess the reliability of sequential testing of the same specimen, and to determine whether stiffness, strains and pressure distribution can be restored to pre-testing levels under a uniform hydration loading environment.

Methods Six ovine FSUs with isolated discs were used in this study. Eight, 1-mm strain gauge rosettes were then bonded to the inferior VB of each FSU at lateral and anterior positions and three heights. FSUs were equilibrated for four hours in a saline bath at room temperature in a materials testing machine. A real-time pressure sensor was placed under the VB. FSUs were tested in axial compression at 0.1 Hz to 1 MPa for 5 sinusoidal cycles. Once tested, the FSU was placed under 0.25 MPa preload for one hour in the water bath for re-equilibration and tested again. Pilot studies by this group have shown that one hour is sufficient to return the disc to its original equilibrium state in a bath after testing, with no associated change in stiffness. This sequence was repeated four times to produce a total of five tests on each FSU. Outcome measures were FSU stiffness, axial strain, peak pressure, average pressure and contact area. Data was statistically analysed using intra-class correlation coefficients (ICC), and repeated measures ANOVA or paired t-tests.

Results The ICC for the five repeated stiffness measures was 0.24 (i.e 24% of the variation in the results was due to between-specimen tests with 76% of the variation due to within-specimen tests). Repeated measures ANOVA found no significant effect on stiffness due to repeating the test five times (P = 0.445). The ICC for the eight axial strains ranged from 0.8 to 0.99. There were no significant differences within any of the eight axial strains over the five repeats (P > 0.287). ICCs, and P values (in brackets) from repeated measures ANOVA, were 0.91 (0.179) for peak pressure, 0.85 (0.44) for average pressure and 0.99 (0.077) for contact area.

Discussion The largest systematic variation was seen for stiffness and this may be due to the tissue changes over the 9 hours of testing. Axial strains showed good to excellent agreement over the five repeated tests as did all pressure parameters. We conclude that the method of allowing one hour for re-equilibration in ovine discs produces a reproducible state of stress in the disc and minimises experimental errors.

Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body (VB) has a potentially important role in the mechanism of subsequent biological changes in disc and bone. It is postulated that changes in the disc may result in increased or abnormal spinal segment motion, modified load distribution across the spinal joint and altered cancellous bone architecture. There have been no studies investigating the direct effect of disc injury on functional spinal unit (FSU) stiffness and the distribution of pressure immediately adjacent to the disc inferior endplate. The aim of this study was to determine whether minor and severe radial tear injuries to the disc alters FSU stiffness and VB surface pressure distribution.

Methods Six ovine FSUs were used in this study. The posterior elements were removed leaving the isolated disc in each FSU. The inferior VB was transversely cut immediately inferior to the endplate and the neutral axis of bending (NAB) identified and marked. FSUs were equilibrated in a saline bath at room temperature for four hours under a constant preload of approximately 0.25 MPa prior to testing. After equilibration, FSUs were transferred to a saline bath in a materials testing machine (Instron 8511, Instron, High Wycombe, UK) and a real-time pressure sensor (I-Scan 5076, Tekscan Inc., MA, USA) placed under the inferior VB.

While maintaining the preload, FSUs were loaded in axial compression at 0.1 Hz through the NAB to 1 MPa in a saline bath for 5 sinusoidal cycles. Once tested, a radial tear was introduced via scalpel injury into the left postero-lateral region of the annulus and tested after one hour of re-equilibration. A final, more severe injury, in the form of removal of a 5 mm x 2 mm window of annulus at the same location was performed and tested after re-equilibration.

Outcome measures were FSU stiffness, peak pressure, average pressure, contact area, and centroid of force location. Data was statistically analysed using repeated measures ANOVA or paired t-tests.

Results No significant differences in stiffness was found as a result of disc injury (P = 0.857), nor for peak and average pressure, contact area and centroid location (P > 0.179).

Discussion These results may not be surprising given that the disc has been shown to be remarkably resilient under axial compression, even with a severe annular or nuclear injury. Further insight will be revealed when other modes of loading are performed in both ovine and human discs for the main study planned to be undertaken in the near future.

Introduction Dynamically identifying the distribution of pressure between any two given surfaces such as articulating joints is of fundamental importance in understanding their interaction. The purpose of this laboratory study was to assess the potential of a dynamic pressure measurement system, Tekscan. ( I-Scan 5076, Tekscan Inc., MA, USA) via a study which observed the changes in the load profile through the vertebral body of harvested ovine lumbar functional spinal units (FSU’s) with a created defect in the intervertebral disc.

The system was used to determine pressure distributions in isolated vertebral bodies inferior to the disc, during axial compression of normal and injured discs of an ovine functional spinal unit.

Methods Four ovine lumbar segments L1-L3 were harvested The superior vertebral body (VB) remained complete, whilst the inferior VB was sectioned 2mm from the endplate and the surface smoothed using emery paper in order to achieve maximum contact area. The neutral axis of bending for each specimen was identified and marked. In accordance with the manufacturer guidelines, the sensor was conditioned and calibrated between 20-200N of load. Testing was carried out in a materials testing machine (Instron 8511, Instron, High Wycombe, UK), where 200N of axial load was applied through the FSU and a snapshot of the instantaneous pressure distribution was taken. A 12 x 2 mm gap defect was created in the right ventro-lateral (2 specimens) and the right lateral (2 specimens) aspect of the IVD. The specimens were returned to the Instron and 200N of load was applied axially through the NAB. A recorded image of the pressure footprint was taken.

Results Comparing the recorded colour-coded images together with their centroids of force of the pre- and post-injury pressure distributions of the vertebral bodies, it was clearly evident that there was a major shift of the load through the IVD. As predicted and as seen in the pressure footprint, the pressure shifted in the opposing direction of the injury in order to maintain a balanced system. A pressure reading validation was also carried out with the use of the Instron, where the experimental pressure of the sensor was within 3% of the NATA calibrated load cell.

Discussion The system was used to sample pressure in real time and display it as a 3D colour-coded map, allowing for visualisation of normal pressure distributions. The associated software has numerous aids and functions, allowing real-time visualisation of the dynamic forces and the balance of forces across two interacting surfaces, making the system an invaluable analytical tool.

The Tekscan system will be used to observe the effect of disc injury on the pressure distribution of the adjacent vertebral body. The relationship between the pressure distribution across the vertebral body and bone architecture will also be studied

This study illustrated that this system is a valid tool for qualitatively and quantitatively assessing dynamic pressure distributions.

Introduction Vertebral deformity, disc disorganisation, and change to vertebral bone architecture are morphological features that are associated with degeneration of the spine and with back pain. Observations from our earlier studies found that the BV/TV is always a maximum in the inferior third of the vertebral body (VB), and minimum in the central third. Animal model studies have reported that the strain in loaded vertebra is a minimum in the central third of the vertebra. There have been no studies investigating the direct affect of VB removal on functional spinal unit (FSU) stiffness, strain magnitude and the distribution of pressure immediately adjacent to the sectioned VB. There were a number of aims for this study. The first aim was to determine whether the strain varies between supero-inferior locations on the VB. The second aim was to determine if strain symmetry was present across the normal VB. The third aim was to determine whether transverse sectioning of the VB alters the stiffness, strains and pressure distributions of the functional spinal unit (FSU) and VB.

Methods Six ovine FSUs with isolated discs were used in this study. Eight, 1-mm strain gauge rosettes were then bonded to the inferior VB of each FSU at lateral and anterior positions and three heights. FSUs were equilibrated in a saline bath at room temperature in a materials testing machine. A real-time pressure sensor was placed under the VB. FSUs were tested in axial compression at 0.1 Hz to 1 MPa for 5 sinusoidal cycles. The inferior VB was then sectioned transversely at 1/3 of its height and placed under preload for one hour for re-equilibration and re-tested. This procedure was repeated at 2/3 of VB height and immediately adjacent to the endplate. Outcome measures were FSU stiffness, axial strain, peak pressure and average pressure. Data was statistically analysed using repeated measures ANOVA or paired t-tests.

Results The results of the first aim found no significant difference in strains within the right lateral or left lateral (P > 0.134) columns of strain gauges. However, for the anterior column of strain gauges, the superior strain was 30% higher than the inferior strain (P = 0.047). The results of the second aim found no significant differences between laterally opposing strain gauges (P > 0.139). For the third aim, transverse sectioning of the VB over three levels produced no significant differences for FSU stiffness (P = 0.275), strains for any strain gauge (P > 0.087), or peak and average pressures (P > 0.076).

Discussion This complex pilot study has shown that overall, axial cortical strain in a normal, ovine FSU did not vary with VB supero-inferior location laterally, but did vary anteriorly. Strains were symmetrical between laterally opposing VB locations at each of three levels, and was not affected by transverse sectioning of the VB at three levels. The finding that anterior column strains differ, may relate to changes in load distribution governed by VB surface second moment of area differences (laterally compared to anteroposteriorly), and the absence of a disc inferiorly. Further insight will be revealed when other modes of loading are performed in both ovine and human discs for the main study planned to be undertaken in the near future.

Introduction There are no reports on the epidemiology of revision hip arthroplasty in Australia. The aim of this study was to characterise the epidemiology of revision hip arthroplasty in relation to primary hip replacements in Australia.

Methods This study covered the seven year period 1993/1994 to 1999/2000. Data on all primary hip replacements and revisions done were obtained from the Australian Institute of Health and Welfare using the ICD-9 (81.53) and ICD-10 (Block No. 1492) cartegorisation. The data was stratified by age (five year age groups), sex, year and state or territory. Log linear modelling was used to examine the rate of revision procedures out of the total number of procedures (primary and revision). The effects of gender, age and year were examined in a series of hierarchical log-rate models (Poisson Loglinear Regression).

Results For the period of the study there were 18,027 revision cases and 122,595 cases of primary hip replacement, representing a revision rate of 14%. The rate of increase of primary hip replacements was significantly higher than the rate of increase of revisions (t= −12.1, p< 0.0005). The number of primary hip replacements performed nationally increased by 810 (95% confidence intervals 658,964) a year. The number of revisions increased by only 62 (95% confidence intervals 21,104) a year. The proportion of revisions decreased by 0.3% per year as determined by regression analysis. The hierarchical log-rate models indicate significant interactions between age and gender and beween age and year.

Conclusions The number of primary and revision hip replacements has been increasing with time. The rate of increase of revision hip replacements has been lower. The proportion of hip replacements that are revisions has been gradually dropping, probably due to a greater increase in the number of primary hip replacements.

Purpose: A variety of second generation femoral interlocking intramedullary nails, in which the proximal lag screw is engaged in the femoral head, are now available for the treatment of complex comminuted pertrochanteric femoral fractures. Jamming of the lag screw results in a rigid device which is more likely to cut-out of the femoral head. The aim of this study was to determine the sliding characteristics and jamming potential of the lag screws of five different devices used to treat these fractures.

Method: The devices examined include; the single lag screw devices: the DHS, the Gamma nail and the Intramedullary hip screw (IMHS), and the double lag screw devices: the Russell-Taylor Reconstruction nail (RTN) and the Austofix Hip nail. The devices were mounted in a servo-hydraulic testing apparatus and examined by two different techniques. The first set-up looked at lag screw motion with respect to loads applied which were representative of the single limb stance phase of gait (SLSPOG). The second set-up which, was first described by Kyle in 1980, looked at the forces required to initiate sliding.

Results: For the first set up (SLSPOG), all single lag screw devices demonstrated sliding across the normal physiological range of applied load. The Russell Taylor Reconstruction nails demonstrated conflicting results with the lag screws of two nails sliding and one nail jamming. All the Austofix nails jammed at the higher angles of the normal physiological range (1590, 1640).

Using the Kyle set-up, the forces required to initiate sliding were found to be lowest with the Synthes DHS (42.33±5.77N), Zimmer CHS (52.67±26.56N), and the IMHS (45.33±10.97N). These were closely followed by the Gamma nail (79.33±8.39N) and the Richards Classic hip screw (82.00±16.37N). The highest forces were for the RTN (98.00±18.52N) and the Austofix hip nail (283.00±70.62N). These results were significantly different. (p< 0.001, ANOVA)

Conclusion: The results demonstrate that double lag screw implants require greater loads to initiate sliding and have a greater potential for jamming. Whilst all single lag screw nails slide, barrel length does alter the forces required to initiate sliding. Further testing using a lubricant is currently being undertaken.

Introduction: Accelerated rehabilitation programs following ACL reconstruction require adequate fixation strength. Staple fixation of grafts outside the tibial tunnel has been shown to have fixation strength comparable to interference screws. The use of bioresorbable polymer implants has potentially significant advantages especially if revision is required. The purpose of this study was to evaluate a new bioresorbable fixation staple using an ovine model.

Materials and Methods: Forty-eight mature sheep underwent unilateral cranial cruciate ligament (CrCl) reconstruction. The reconstruction comprised a loop of superficial digital flexor tendon (autograft) joined to a prosthetic ligament (LK-15). Femoral fixation was by endobutton. Tibial fixation of the LK-15 was with either a new Poly-L-lactic acid (PLLA) staple (Zimmer Japan/Gunze Ltd.) or a Cobalt-chrome (CoCr) staple. Biomechanical and histological responses were evaluated at 0, 6, 12 and 24 weeks.

Results: At all times post-reconstruction there were no significant differences between staple types for construct strength or stiffness (p> 0.05). The staple was not the site of reconstruction failure, and there were no adverse tissue reactions, for either staple type. Fibrous tissue was more often found at the interface of the CoCr staple.

Conclusions: The PLLA staple performed biomechanically as well as the metal staple for tibial fixation of cruciate ligament reconstructions. There were no significant observable adverse histological responses over the time intervals examined.

Purpose: The aim of this study was to examine the intervertebral disc (IVD) biomechanics in a sheep model with concentric tears.

Methods: Fifty two adult merino wethers were randomly allocated into two groups with circumferential tears introduced by injection with saline (group 1) or needle stick with no saline (group 2). They were then sacrificed at 0, 1, 3, 6, 12 and 18 months for biomechanical testing. An additional ten sheep were used as an unoperated control at time 0 (Group 0). Biomechanical tests on each functional spinal unit (FSU) and IVD were performed.

Results: The effect of procedure overall was significant for torsion (P< 0.022), axial compression (P< 0.014), extension (P< 0.001) and left lateral bending (P< 0.004) for both the FSU and IVD. In almost every case, both groups 1 and 2 were significantly stiffer than group 0 but no different to each other. The effect of time overall was significant for flexion (P< 0.0028) and right lateral bending (P< 0.022) for both the FSU and IVD. In torsion, twisting to the left was significant for the intact FSU (P=0.008) and twisting to the right for the isolated IVD (P=0.009).

Discussion: The results of this study show that any intervention in the disc alters the biomechanics compared to an unoperated control group. To our knowledge this has not been shown before and these findings may have relevance to any intervention into the disc in the patient.

Introduction: The purpose of this study was to evaluate the micromotion of a femoral prosthesis relative to the femur in a revision hip replacement model.

Methods: A series of Ovine hip hemiarthroplasties were mechanically tested to detect micromotion of the femoral prosthesis relative to the femur 12 weeks following implantation. A mechanical testing device utilising muscle simulation of the major groups around the femur was designed. A 3D targeting system was developed using non-contact LASER transducers on the implant referenced to a second target on the overlying femur. Movement of this second target was measured with three LVDT’s (linear variable differential transformers).

Results: The system error was quantified in each femur to a resolution of the order of 15 microns. The mean micromotion, in 3D at two points assuming rigid body mechanics, was less than 50 microns for clinically stable implants. One stem was determined to be clinically loose and had a corresponding mean micromotion of 150 microns.

Conclusion: The method enabled measurement of 3Dmicromotion of a femoral prosthesis within the femur, during a laboratory approximation of normal physiological load cycles. The micromotion values corresponded to clinical outcomes, in a manner consistent with other reports in the literature. This system can be modified to allow targeting of different implants within a variety of bone types.

Aim: To determine the intra operative biomechanical properties of a semitendinosus graft used in ACL reconstruction.

Introduction: ACL reconstruction has become a commonly performed operation with 1,139 of these procedures being performed in South Australia in 1997 (SA Health Commission)

The majority of the scientific literature is based on data obtained from elderly cadaveric material. Little is known about the biomechanical properties of the soft tissue grafts currently used prior to implantation. The correct preconditioning and intraoperative tensioning of the soft tissue grafts has also not been investigated.

The initial graft biomechanical properties are important. Inadequate tension will lead to continuing instability whilst excessive tension may cause accelerated joint arthrosis. The tension in the graft may decrease by 30% if it has not been cyclically pretensioned.

Methods: A machine has been designed that will allow the intraoperative biomechanical testing of soft tissue grafts immediately prior to their implantation into the patient during ACL reconstruction. Data will be available on creep, stress relaxation, and tensile testing.

This device will also allow the accurate preconditioning of the graft, providing objective data that can then be compared to the subsequent clinical progress of the patient.

All testing will be accomplished during the time it takes to prepare the tunnels for insertion of the graft, and as such will not prolong unnecessarily the operative time.

Procedure: Once the graft has been prepared prior to fixation, it will be placed between two clamps. One is fixed to a load cell whilst the other is coupled to a linear actuator. The linear actuator will be driven by a computer controlled stepper motor under close-loop control. Custom software will cyclically load the autograft between two definable load points. A linear variable differential transformer (LVDT) will be used to monitor displacement of the autograft and load will be monitored with a load cell of capacity 125Kg.

This set-up will be immersed in a saline water bath maintained at body temperature during testing. The load cell will be hermetically sealed, with clamps and water bath being autoclavable. With the facilities for draping, the test area will remain sterile. The auto graft clamps will be designed to allow fixation of various graft materials (eg semitendinosus, gracilis, bone-patella tendon-bone) and adjustable for graft lengths. The water bath will house a thermocouple, heating mat and controller to maintain the saline temperature to within 1°C.

The testing system will be mounted on a stainless steel trolley for mobility in the operating room with an underlying shelf to house the associated electronics and a retractable side draw for storage of the laptop computer.

The autograft will be preconditioned between two known loads for 20 cycles recording load and displacement simultaneously on a laptop computer. Once preconditioned, the autograft will then be used for the ACL reconstruction in the standard way.

Summary: Objective data on preconditioning of ACL grafts, has never before been available intra-operatively. We outline the experimental set-up which has been designed and is undergoing testing prior to its use in a prospective study.

Introduction: THR and TKR have been shown to be successful treatments for moderate to severe osteoarthritis of the hip and knee. The requirement for total joint replacement will increase as the population ages. This study reports on the incidence of THR and TKR in an Australian population.

Method: Age and gender specific numbers of THR and TKR for the Australian population, 1994–1998 were obtained from the Australian Institute of Health and Welfare. The same data for South Australia, 1988–1998 were obtained from the Department of Human Services Epidemiology Branch. The incidences were calculated and tested for changes over time.

Results: For the Australian population in 1994 there were 9,120 THR and by 1998 this had increased by 25.9% to 11,488 THR. There were 10,132 TKR in 1994 and by 1998 this had increased by 42.8% to 14,472 TKR. Stratified by age group changes in incidence rate with respect to time was statistically tested using regression analysis. For the eleven year data from South Australia there was a significant increase in the overall incidence of THR (p=0.012). There were significant increases in TKR incidence, although this increase was not uniform across all age groups (p< 0.001). The increase in TKR incidence was greater than that for THR. For both THR and TKR there were no significant differences on the basis of gender.

Conclusion: The incidence of THR is increasing in Australia and TKR incidence is increasing at a greater rate. Future projections must take into account these changing incidences as well as changes in population demographics.

Adult human cadaver pelves were tested to determine micromotion at the prosthesis-bone interface in cementless hemispherical acetabular components during simulated single-limb stance. The micromotion of non-press-fit components with screw fixation in response to cyclic loads to a maximum of 1500N was compressive (interface closing) at the superior iliac rim and distractive (interface opening) at the inferior ischial rim; that of press-fit components was compressive all around the acetabular rim regardless of screw fixation. Adding screws to the component decreased the micromotion at the site of the screw, but sometimes increased it at the opposite side. Two dome screws with the press-fit component decreased the micromotion at the superior iliac rim but at the inferior ischial rim there was either no change or increased movement. A press-fit cup shows less micromotion than a non-press-fit cup with screw fixation. The addition of screws to a press-fit cup does not necessarily increase the initial stability.