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Impact of surface structure and screw fixation on primary stability in three acetabular cup systems: A biomechanical study
, Samar Hamada, Jessica Lokea, Hannes Laua, Markus Rickerta,b, Bernd Alexander Ishaquea,b, Gafar Adam Ahmeda,b
⁎Corresponding author: Alexander Jahnke. Alexander.Jahnke@ortho.med.uni-giessen.de
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
Primary stability is essential for successful uncemented total hip arthroplasty (THA), as it facilitates early osseointegration and reduces the risk of implant failure. Acetabular cup design, surface morphology, and fixation strategy significantly influence this initial stability. This biomechanical study assessed the primary stability of three uncemented acetabular cup systems - Allofit-S®, EcoFit®-Epore®, and Revisio®-S - each with distinct surface characteristics and materials.
Using a standardized polymethacrylimide bone model and a custom-built hip acetabulum simulator, micromotions were measured under physiological torque. Rotational, translational, and total micromotion were evaluated across four screw fixation configurations (0–3 screws) using eddy current sensors with 0.1 μm precision. Statistical analysis included ANOVA and post-hoc LSD tests with Bonferroni correction (α = 0.05).
Rotational micromotion did not show any significant variation with the number of screws (p > 0.05). Translational and total micromotions varied significantly across implant types and fixation levels. EcoFit®-Epore® displayed the highest initial micromotion, which was noticeably reduced with screw augmentation (p = 0.003, p = 0.018). In contrast, Allofit-S® and Revision®-S demonstrated superior intrinsic stability with minimal benefit from screw fixation.
Implant design and surface morphology critically affect primary acetabular cup stability. Macrostructured and roughened surfaces (Allofit-S®, Revisio®-S) provide greater inherent stability, while highly porous implants (EcoFit®-Epore®) benefit evidently from screw fixation. These findings support tailored implant selection and fixation strategies based on patient-specific anatomy and surgical context to optimize outcomes in uncemented THA.
Keywords
Hip arthroplasty
Cementless
Friction torques
Acetabular component
Primary stability
1 Introduction
Achieving primary stability is a critical determinant of long-term success in uncemented total hip arthroplasty (THA), as it facilitates early osseointegration and reduces the risk of implant failure.1 The acetabular component, in particular, must withstand complex multidirectional loads and micromotions that arise during physiological activity.2,3 Inadequate initial fixation can impair bone ingrowth and contribute to aseptic loosening, which remains a leading cause of implant revision of cementless acetabular cup systems.4
The stability of acetabular cups is influenced by multiple design factors, including geometry, surface characteristics, and the use of supplementary fixation such as screws.5 Surface topology plays a pivotal role in promoting mechanical interlock with host bone, with roughened or porous structures often employed to enhance osseointegration.6 In revision scenarios or cases with compromised bone quality, additional fixation strategies, such as the application of cancellous screws, are frequently employed to augment primary stability and minimize micromotion.7–9
Therefore, this study aims to investigate the biomechanical performance of three clinically established uncemented acetabular cup systems - Allofit-S®, Ecofit®-Epore®, and Revisio®-S - each with distinct material compositions and surface morphologies. Using a standardized bone substitute model and a custom-designed hip acetabulum simulator,3,9 micromotions of these implants under physiological loading conditions were examined. The effects of varying degrees of screw fixation (0–3 screws) on rotational, translational, and total micromotion were systematically analyzed. By assessing how implant design and fixation strategy affect primary stability, this study seeks to inform clinical decision-making in both primary and revision THA settings.
2 Methods
2.1 Acetabular cup systems
In this study, the acetabular cup systems Allofit-S® (Zimmer Biomet Holdings Inc., Warsaw, Indiana, USA), EcoFit®-Epore® (Implantcast GmbH, Buxtehude, Germany), and Revisio®-S (AQ Solutions GmbH, Hürth, Germany) were examined, each with a standardized diameter of 54 mm. The Allofit-S® is an uncemented hemispherical acetabular cup with a flattened pole region. It features a coarse-grit-blasted macrostructure composed of the titanium forging alloy Protasul®-Ti, which is intended to facilitate long-term osseointegration and secure fixation. The EcoFit®-Epore® cup incorporates a highly porous surface structure based on a TiAl6V4 titanium alloy, featuring characteristic rod-like structures that mimic the trabecular bone architecture. This design aims to enhance osseointegration by improving bone ingrowth. The Revisio®-S acetabular cup system is a specialized uncemented revision cup constructed from cobalt-chromium-molybdenum (CoCrMo) with a TiNb coating. It possesses an open-porous tripod structure designed to optimize primary stability and promote long-term osseous integration.3 All acetabular cup systems in this study were capable of supplementary fixation using cancellous screws in cases where primary stability via press-fit implantation was deemed insufficient. This was particularly relevant for compromised bone conditions or revision procedures (Fig. 1).
2.2 Bone model
To simulate trabecular bone in the pelvic region, rigid polymer foam blocks made of polymethacrylimide (ROHACELL® 200WF, Evonik Goldschmidt GmbH Rewo, Steinau an der Straβe, Germany) were used. These blocks were modified to replicate the anatomical constraints of the acetabulum by introducing cranial and caudal notches.10,11 A standardized milling protocol was applied to prepare the implantation site, using a self-centering drill operating at a rotational speed of 150 rpm and a feed rate of 10 mm/min. The surgical preparation process closely resembled clinical implantation procedures. The acetabular cups were implanted using a line-to-line milling approach, ensuring a press-fit of 2 mm. The cups were then impacted into the foam block using a guided drop weight (2.5 kg) released from a height of 0.5 m, with three successive impacts applied per cup. Subsequently, screw holes were pre-drilled using a 3.2 mm drill bit, following the standardized protocol.
2.3 Measurement protocol
A hip acetabulum simulator was employed to apply physiological friction torque (Mx, My, Mz) of a ceramic-on-polyethylene pairing via a three-dimensional lever system.3 This system incorporated synchronized servomotors (ISK 3104.30/230 IGAS, Echterdingen, Germany) to reproduce the loading conditions arising from a ceramic-polyethylene bearing couple. To minimize frictional interference, couplings (Giunti Oldham GOS-32, Orbit GmbH, Wolfenbüttel, Germany) were integrated into the setup. Motion analysis was performed using an array of nine eddy current sensors (NCDT 3010-S2, Micro-Epsilon Messtechnik GmbH & Co. KG, Ortenburg, Germany), each with a precision of 0.1 μm. These sensors continuously recorded spatial displacement, including translational and rotational motion of both the acetabular cup and bone substitute model. The difference between the maximal recorded displacements represented the total micromotion of each implant system3,9,12 (Table 1).
| Measurement | Number of screws |
| #1 | Zero |
| #2 | cranial |
| #3 | cranial + anterior |
| #4 | cranial + anterior + posterior |
2.4 Hip acetabulum simulator
A hip acetabulum simulator was employed to apply physiological torque (Mx, My, Mz) via a three-dimensional lever system. This system incorporated synchronized servomotors (ISK 3104.30/230 IGAS, Echterdingen, Germany) to reproduce the loading conditions arising from a ceramic-polyethylene bearing couple. To minimize frictional interference, couplings (Giunti Oldham GOS-32, Orbit GmbH, Wolfenbüttel, Germany) were integrated into the setup.
Motion analysis was performed using an array of nine eddy current sensors (NCDT 3010-S2, Micro-Epsilon Messtechnik GmbH & Co. KG, Ortenburg, Germany), each with a precision of 0.1 μm. These sensors continuously recorded spatial displacement, including translational and rotational motion of both the acetabular cup and bone substitute model. The difference between the maximal recorded displacements represented the total micromotion of each implant system (Fig. 2).3,9,12
2.5 Statistical analysis
Descriptive statistics, including mean values (MEAN) and standard deviations (SD), were used to evaluate the primary stability of the acetabular cup systems. Primary outcome measures included maximum total micromotion, as well as translational and rotational displacements. The impact of screw fixation on primary stability was evaluated using a multifactorial analysis of variance (ANOVA). Pairwise comparisons were performed using the least significant difference (LSD) post-hoc test, with Bonferroni correction applied to account for multiple comparisons. A p-value of <0.05 was considered statistically significant.
3 Results
Rotational movement exhibited minimal variation across screw configurations. For Allofit-S®, rotational movement ranged from 23.1 ± 4.4 μm without screws to 18.2 ± 2.3 μm with two screws. A similar trend was observed for Ecofit®-Epore®, decreasing from 23.2 ± 7.8 μm to 19.0 ± 3.2 μm, while Revisio®-S showed the highest initial rotational movement (25.7 ± 5.9 μm) and a gradual reduction to 20.8 ± 5.7 μm with three screws. However, no statistically significant differences were observed between screw configurations (p > 0.05), suggesting that rotational stability was largely independent of additional fixation.
Translational movement showed significant variation between cup types, particularly in the case of Ecofit®-Epore®. Without screw fixation, Ecofit®-Epore® exhibited the highest translational movement (92.6 ± 4.6 μm), which significantly decreased with increasing screw fixation (p = 0.003). In contrast, Allofit-S® (65.0 ± 10.7 μm) and Revisio®-S (64.2 ± 8.6 μm) demonstrated lower initial translational movement, indicating greater intrinsic stability. The introduction of screw fixation further improved stability across all systems, with the most pronounced reduction observed for Ecofit®-Epore®, suggesting that its smoother surface and structural properties required additional fixation for sufficient primary stability.
Total microrelative movement followed a similar pattern to translational movement. Ecofit®-Epore® exhibited the highest baseline total movement (94.8 ± 5.2 μm), whereas Allofit-S® and Revisio®-S displayed lower values (68.2 ± 6.6 μm and 74.1 ± 10.0 μm, respectively). The addition of screws significantly reduced total movement for Ecofit®-Epore® (p = 0.018), while changes in Allofit-S® and Revisio®-S were less pronounced. These findings indicate that primary stability is significantly influenced by the surface roughness and cup thickness, particularly when no additional fixation is used.
Overall, the results suggest that while rotational stability remains largely unaffected by screw fixation, translational and total movement are highly dependent on cup design and surface characteristics. The Ecofit®-Epore® system demonstrated the highest initial instability, which was significantly mitigated by screw fixation. In contrast, the Allofit-S® and Revisio®-S systems showed lower baseline movement, suggesting superior intrinsic stability. These findings emphasize the importance of surface roughness and cup design in ensuring primary acetabular stability, particularly in uncemented configurations where screw fixation may be necessary to compensate for smoother implant surfaces (Table 2).
| Rotation [μm] | Translation [μm] | Total movement [μm] | ||||||||||
| Number of screws | zero | 1 | 2 | 3 | zero | 1 | 2 | 3 | zero | 1 | 2 | 3 |
| Allofit-S® | 23.1 ± 4.4 | 23.1 ± 9.1 | 18.2 ± 2.3 | 19.6 ± 7.1 | 65.0 ± 10.7a | 70.1 ± 15.3 | 63.6 ± 3.6 | 67.7 ± 16.8 | 68.2 ± 6.6a | 75.0 ± 19.6 | 68.4 ± 2.0 | 71.9 ± 21.5 |
| Ecofit® | 23.2 ± 7.8 | 20.9 ± 3.3 | 19.0 ± 3.2 | 19.9 ± 9.7 | 92.6 ± 4.6a,b | 77.2 ± 8.5 | 71.9 ± 7.7 | 76.3 ± 14.3 | 94.8 ± 5.2a | 82.5 ± 5.8 | 76.8 ± 8.0 | 82.1 ± 19.4 |
| Revisio®-S | 25.7 ± 5.9 | 22.2 ± 9.7 | 21.5 ± 6.4 | 20.8 ± 5.7 | 64.2 ± 8.6b | 67.3 ± 16.1 | 66.3 ± 16.7 | 72.8 ± 13.9 | 74.1 ± 10.0 | 75.4 ± 20.3 | 72.2 ± 20.3 | 81.4 ± 16.2 |
| p-values | – | – | – | – | a = 0.003, b = 0.002 | – | – | – | a = 0.018 | – | – | – |
4 Discussion
This study investigated the primary stability of three uncemented acetabular cup systems -Allofit-S®, Ecofit®-Epore®, and Revisio®-S - under simulated physiological conditions using a standardized polymethacrylimide bone model. The implants, each with distinct surface topographies and materials, were evaluated with varying levels of supplementary screw fixation to determine their performance in terms of micromotion, a critical predictor of osseointegration and long-term clinical success.13,14
The results revealed that translational and total micromotion were significantly influenced by cup design and the number of screws used, while rotational micromotion remained largely unaffected. The EcoFit®-Epore® cup exhibited the highest micromotion in the absence of screws, which was significantly reduced upon screw augmentation. In contrast, the Allofit-S® and Revisio®-S systems showed relatively low micromotion even without additional fixation, with only marginal improvement when screws were added.
These findings reflect the underlying differences in macrostructure, surface roughness, and stiffness of the implant materials. The EcoFit®-Epore® system features a trabecular-like, highly porous surface made from TiAl6V4 alloy. While its rod-like architecture promotes biological integration over time, the lower stiffness and smooth macrogeometry likely compromise the mechanical interlock necessary for primary fixation.15 Levine et al. support this interpretation, noting that although porous metals enhance long-term osseointegration, their initial fixation is often less robust if not supplemented appropriately.16
Conversely, the Allofit-S®, with its Protasul®-Ti coarse-grit-blasted macrostructure, exhibited excellent press-fit characteristics and intrinsic primary stability. The Revisio®-S system, designed for revision scenarios, also demonstrated high stability, likely due to its tripod macrostructure, CoCrMo core, and TiNb coating. These features might increase resistance to micromotion through enhanced mechanical strength and favorable osseointegrative potential.17
Screw fixation significantly reduced translational and total micromotion, particularly for the EcoFit®-Epore® system. This aligns with previous findings by Iorio et al., who demonstrated that screw augmentation improves acetabular cup fixation when primary mechanical stability is insufficient.18 In our study, screw fixation effectively compensated for the EcoFit®-Epore® cup's lower intrinsic stiffness and press-fit.
Notably, across all systems, the number of screws had a minimal impact on rotational micromotion. This results is consistent with the findings of Jasty et al., who demonstrated that press-fit geometry is the predominant factor influencing rotational stability, with screws providing only limited contribution.19
The magnitude of micromotion observed in this study - particularly the values below 150 μm -falls within the range considered favorable for bone ingrowth and implant stability, as reported by Stiehl et al..2 The consistent reduction in micromotion with screw fixation, especially for more porous designs, is supported by earlier findings from Bobyn et al., who demonstrated that although porous surfaces support long-term osseointegration, their initial mechanical fixation may require augmentation.20
Additionally, other studies emphasized that macrostructure and roughness are critical factors for minimizing micromotion and promoting early implant stability21,22 - findings that corroborate the performance of the Allofit-S® and Revisio®-S systems. These characteristics appear especially relevant when screw fixation is limited or not desired due to surgical considerations.
5 Clinical implications
These results carry important clinical implications. In patients with good bone quality and favorable acetabular geometry, implant systems like Allofit-S® or Revisio®-S may provide sufficient primary stability without supplementary fixation, potentially reducing operative time and the risk of stress shielding or screw-related complications.
In contrast, for patients undergoing revision surgery or those with osteoporotic bone, highly porous implants like EcoFit®-Epore® may require screw augmentation to achieve adequate early fixation. This is critical in preventing micromotion-induced fibrous tissue formation during the early postoperative period when biological fixation has yet to occur.
Importantly, the observed differences between systems underscore the need for patient-specific implant selection based on individual anatomy, bone quality, and surgical context. Surgeons should consider both mechanical and biological factors when choosing acetabular components for uncemented total hip arthroplasty.
6 Limitations
Despite the robust findings, several limitations must be acknowledged. The use of rigid polymethacrylimide foam blocks, although standardized and reproducible, does not replicate the heterogeneity and viscoelasticity of the human cancellous bone. Therefore, absolute micromotion values might differ in vivo, particularly in cases involving osteopenic or sclerotic bone.
Second, the loading conditions used in the study represent average physiological torque but did not incorporate dynamic or cyclic loading, which is typical during activities such as walking or stair climbing. Future research incorporating long-term, multidirectional loading scenarios would provide a more comprehensive evaluation of implant behavior.
Third, the study standardized all testing to a single cup size (54 mm), which enabled consistent comparisons but limited generalizability across sizes. In vivo, varying cup diameters and bone contact areas may alter fixation behavior.
Finally, while micromotion is an established surrogate for implant fixation, the biological response - such as osteoblast activity or fibrous tissue formation - could not and was not assessed. In vivo or cadaveric models would be needed to fully understand how micromotion values translate into long-term clinical outcomes.
7 Conclusion
In summary, this study demonstrates that acetabular cup design and surface morphology significantly influence primary implant stability. Macrostructured and roughened surfaces, as in the Allofit-S® and Revisio®-S systems, provide excellent press-fit performance, reducing reliance on screw fixation. Porous designs like EcoFit®-Epore® may require supplementary fixation to achieve comparable stability. These findings emphasize the necessity of tailoring implant selection and fixation strategy to each patient's clinical and anatomical circumstances to optimize outcomes in uncemented total hip arthroplasty.
Guardian/patient's consent
Informed consent was not required for this study as it did not involve human participants or personal data that required consent. The study was conducted in accordance with applicable ethical guidelines.
Level of evidence
Basic Science Study.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
GAA, MR, BAI, and AJ conceptualized, analyzed and interpreted the data, the statistics as well as the discussion and were major contributors title = "au8">href = "\#au10″ title = "au10">in writing the manuscript.
LJ, LH, HS performed the data curation, All authors read and approved the final manuscript.
Availability of data and material
The datasets used and/or analyzed during the current study are
available from the corresponding author on reasonable request.
Author contributions statement
All authors contributed to the conception, development, planning, execution, analysis or interpretation of the research and/or writing of the manuscript.
Jahnke, Alexander – research design, analysis and interpretation of data; drafting the paper.
Hamad, Samar - data acquisition.
Jessica Loke – data acquisition
title = "au8">Hannes Lau – data acquisition.
Ishaque, Bernd Alexander - research design; interpretation of data
title = "au8">href = "\#au5″ title = "au5">Rickert, Markus - research design; interpretation of data; revising the manuscript.
Ahmed, Gafar Adam - research design; interpretation of data; revising the manuscript.
All authors agree and accept responsibility for the contents of the submitted manuscript and give approval of the submitted and final version.
Author's contribution
Jahnke, Alexander – research design, analysis and interpretation of data; drafting the paper.
Hamad, Samar - data acquisition.
Jessica Loke – data acquisition.
Hannes Lau – data acquisition.
Ishaque, Bernd Alexander - research design; interpretation of data.
Rickert, Markus - research design; interpretation of data; revising the manuscript.
Ahmed, Gafar Adam - research design; interpretation of data; revising the manuscript.
All authors agree and accept responsibility for the contents of the submitted manuscript and give approval of the submitted and final version.
Ethical statement
Ethical approval was not applicable.
Funding
No funding
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