1 Introduction

Demographic change and demand for high quality of life lead to increasing implantation numbers with over 1,000,000 primary total hip replacements worldwide.¹ At the same time, however, the number of replacement surgeries mostly due to aseptic loosening is increasing,² which in turn raises the demands for a more bone-saving primary surgery to achieve a better clinical outcome even after revision.³ Due to the high revision rate of 75.3%,² it is absolutely necessary for manufacturers to constantly improve their implants. These improvements may result from the development of novel anchoring mechanisms, novel bioactive coatings, new surface structures as well as from novel prosthetic designs. Then, in order to quantify the advantages and disadvantages of these newly developed – in this case – acetabular cup systems, in vitro primary stability analyses may be conducted prior to market launch to obtain statements about primary cup stability^4–6 and to determine its prospective longevity. Though, such preclinical in vitro studies are often more expensive and labor-intensive than clinical trials.

So, very often new implants are directly used in everyday clinical practice immediately after their approval without having been tested preclinically in in vitro studies, since in accordance with EU Directive 93/42/EEC the technical characteristics of the implant are known and existing clinical data are sufficiently available. Thus, a renewed clinical trial often is considered superfluous. Ultimately, early statements concerning a positive clinical outcome can usually only be confirmed by time-consuming retrospective or prospective clinical studies in which early migration behavior of the acetabular component is measured. One validated relatively cost-efficient and quick clinical method for early prediction of future performance of acetabular cup systems is the “Ein Bild Roentgen Analyse” (EBRA).^7–9 Therefore, the objective of this retrospective clinical study was to compare the migration behavior of the more recent Plasmafit® cup, introduced in 2012, to the cup systems Allofit® and Plasmacup® with a longer market presence via EBRA-CUP within a short-term radiological follow-up period of 12 months as it is to be expected that the biologic integration of the cup implant into the bone occurred prior to the one-year control.¹⁰ Besides, we exploratively investigated predefined possible migration influencing epidemiologic factors like age, sex, bearing couple, indication for surgery, cup/stem combination or body mass index.

2 Patients and methods

2.2

2.2 Implants

The cementless press-fit acetabular cups examined in this study have on the one hand already been established for decades (Plasmacup® (BBraun, Aesculap, Tuttlingen, Germany) and the Allofit® cup (Zimmer Germany GmbH, Freiburg, Germany)) and on the other hand only been used since 2012 (Plasmafit® cup (BBraun, Aesculap, Tuttlingen, Germany)). The implants used were exclusively press-fit cups causing an equatorial press-fit situation which results in primary stability. The Allofit® cup has a macrosurface structure of wrought titanium alloy (Protasul®-Ti). The Plasmacup® cup has a microsurface structure made of a Plasmapore® coating with a porosity of 40%. By contrast, the Plasmafit® cup consists of a macro and micro surface structure. Here, too, we find a Plasmapore® coating with a porosity of 50% and a fine-tooth geometry (Fig. 1).

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Fig. 1 Illustration of the examined acetabular cups Allofit® (left), Plasmafit® (middle) and Plasmacup® (right).

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2.3

2.3 Radiological examination

After surgery, the collective was followed-up at times t1 = postoperatively, t2 = 6 months control and t3 = 12 months control and X-rays of the pelvic area were taken. EBRA.CUP (University Innsbruck, Innsbruck, Austria)¹¹ was used to measure any progression of the anteversion and inclination angles of the acetabular cups. Reference points here were anatomical landmarks such as the foramina obturata, the symphysis joint, the transition from the last lumbar vertebral body to the first sacral vertebral body and the lateral boundary of the opening of the lesser pelvis. The prosthetic head served as calibration point. Progressive migration behavior of the cup components depending on the anteversion and the inclination angles were then calculated by the system using the marked ellipse (red arrow, Fig. 2).

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Fig. 2 Entire view of the pelvis in the EBRA-CUP software with marked landmarks and marking of the hip prosthesis.

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This measuring method is based on a comparability algorithm of the geometric evaluation of the anatomical landmarks and their projections in a series of images which correspond to the observation period. Images of the series which are not considered comparable by the program are excluded by the program itself.¹² The EBRA-CUP software indicates an accuracy of ≈1 mm.^9,13,14

3 Results

From initially 400 patients, 226 patients could not be included due to lack of follow-up, ineligible views of the entire pelvis, treatment on both sides or due to programmatic exclusion. Therefore, the study included 174 patients, 86 women whose mean age was 61.9 ± 8.7 years (40–77 years) and 88 men whose age was 56.9 ± 10.6 years (18–78 years). 33 patients received a Plasmacup® cup, 68 patients received an Allofit® cup and 73 patients a Plasmafit® cup. The mean age of the patients was 59.4 ± 10 years (18–78 years).

3.2. Migration behavior

As already described, the anteversion and inclination angles served for migration analysis of the individual cups. The results indicate relative differences concerning the mean values for the anteversion angles, but not concerning the inclination angles of the individual cups (Fig. 2). For the Plasmafit® cup, the anteversion angle remained averaged at 17.3° ± 6.2° (range: 4.5°–38.7°) at all follow-ups. For the Allofit® at 26.6° ± 6.2° (range: 5.8°–39.7°) and for the Plasmacup® cup at 11.4° ± 4.9° (6.4°–31.8°). Thus, the general mean anteversion angle was 19.7° ± 8.3° (range: 4.5°–39.7°) (Fig. 3).

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Fig. 3 Anteversion angles at follow-ups t1, t2 and t3.

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The inclination angles for all three cups remained almost unchanged at an average of 41.1° ± 6.1° within a range of 26.1°–60.7° (Fig. 4).

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Fig. 4 Inclination angles at follow-ups t1, t2 and t3.

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In none of the three hip cups examined, an increased change or progression of the inclination angle or the anteversion angle could be demonstrated for any follow-up examination. The Plasmacup® cup inclination angle remained nearly unchanged with ΔI t1/t3 of only 0.6° ± 2.8° and for ΔA t1/t3 of only −0.1° ± 1.7°. The Allofit® cup showed a ΔI t1/t3 of 0.2° ± 2.6° and a ΔA t1/t3 of −0.2° ± 1.7°. The Plasmafit® cup had a ΔI t1/t3 of −0.02° ± 1.6° and ΔA t1/t3 of −0.3° ± 1.2° (Table 1).

Table 1 List of the ΔI and ΔA as well as the associated p-values regarding the three examined press-fit cup systems.

cup	ΔA t1/t3	p-value	ΔI t1/t3	p-value
Plasmacup®	−0.1 ± 1.7	1.00	0.6 ± 2.8	0.73
Allofit®	−0.2 ± 1.7	1.00	0.2 ± 2.6	0.74
Plasmafit®	−0.3 ± 1.2	0.38	0.0 ± 1.6	1.00

4 Discussion

Cementless acetabular cup systems have been clinically established for years. Nevertheless, early loosening often occurs which is promoted by an increased migration of the acetabular systems. EBRA helps identify early loosening and make quick statements about possible future performance e.g. in order to evaluate new acetabular systems. Within the framework of this study, possible progressive inclination as well as anteversion migration of two well established cementless acetabular cup systems and a more recent one were radiologically examined in the first 12 months and compared with each other using EBRA. In addition, possible influencing epidemiological factors of the collective on the migration behavior of the acetabular cups were taken into account correlatively. EBRA was chosen because this method has proven to be sufficiently precise^12,15 and uncomplicated due to the X-ray images of the pelvic overview images taken as part of the follow-up examination. At the same time, EBRA provides good short-term predictive power for early loosening of the cup systems.^12,13

Generally, pre-clinical in vitro and in vivo studies should be raised for testing newly approved acetabular prostheses especially when it comes to discovering possible advantages or disadvantages of different surfaces on the primary stability and the supposed related resulting secondary stability - i.e. osseointegration - of the acetabular cups. The Plasmacup® has a microsurface structure, the Allofit® a macrosurface structure with a tooth geometry and the novel Plasmafit® cup is a combination of both surface textures and could therefore be regarded as hybrid between the Allofit® and the Plasmacup® cup. In a former in vitro study of the working group Jahnke et al. 4 three different acetabular cups with different surface qualities were compared for their primary stability. For this purpose, the cups were implanted in a simplified spherical bone model as well as in an anatomical artificial bone model. Taking into account the resulting frictional moments of CoC and CoP bearings, the relative micromotions between cup and bone were determined and primary stability of the three cups was compared. It could be shown that the respective cup wall thickness and the surface quality of the cups had a significant influence on the primary stability of the cup systems examined.

Another in vitro study by Le Cann et al.¹⁶ likewise considered the question of the influence of micro- and macroporosity on primary stability. It could be shown that equatorial macroporosity generally has a negative influence on primary press-fit, but is indispensable for osseointegration, i.e., secondary stability. The key message of this study was that microporosity, thanks to less bone injury, provides better conditions for primary stability. However, some macroporosity is essential for achieving secondary stability, i.e., osseointegration.

In contrast to above mentioned in vitro studies,^4,16 we were unable to demonstrate any short-term difference between the influences of the different surface structures on the migration of the examined acetabular cups.

In a clinical in vivo study by Ilchman et al.,¹⁷ the influence of a roughened surface of a threaded cup and the possible connection between roughness and early aseptic loosening of the cup was examined using EBRA. The Authors found a high rate of aseptic loosening in the examined collective, but no connection between surface roughness and loosening of the cup could be demonstrated. This finding also coincides with the results of our study. The examined cup systems all have a different surface with different roughness values and structures. Nevertheless, we could not prove any migration of the cups within the first 12 months and the cups remained stable in their initial positions - regardless of their initial, partly non-physiological and irregular anteversion angle malposition. Literature specifies 45° ± 5° for inclination and 15° ± 10° for anteversion.¹⁸ If we look at the different anteversion angles of the respective acetabular cups examined, we find that the Allofit® cup was primarily implanted with a relatively unphysiologically high anteversion angle, followed by the Plasmafit® cup and the Plasmacup® which were implanted in an optimized manner according to values found in literature. These different anteversion angles could result from different insertion guides and the associated different curvatures of the used implantation sets. Nevertheless, during the investigation period, the actual anteversion angles and their possible “malorientation” did not show any negative influence on the course of the cup migration. These results coincide with the results of a certain study in which no influence of malorientation of acetabular cups influenced the migration or led to any early aseptic loosening 17.

At no follow-up control, postoperatively, at the 6-month or 12-month follow-up, any significant value of progressive migration nor any correlation between migration and any of the examined epidemiological factors were found (p-value> 0.05). It appears that the primary stability of the cups, bone quality, anatomical reconstruction 17 and the associated implantation expertise of the surgeon might have a greater impact on the anchoring behavior of the cup systems.

The large number of implantations of the two already established cups, Allofit® and Plasmacup®, is justified by long-term studies.^19,20 To date, neither clinical in vivo nor experimental in vitro studies have been carried out to examine the characteristics and the associated future performance of the relatively novel Plasmafit® cup but our first short-term results permit to expect good long-term results compared to the already established cup systems.

A major limitation of this study is the short follow-up time, although osseointegration and any migration of the cup should already have occurred at the 12-month postoperative follow-up 10. Another limiting factor is the evaluation of CoC bearing couples by means of the EBRA-Cup software because of possible obscuration. However, it was possible to counteract this limitation by contrasting in the best possible manner. Furthermore, there were many exclusions within the collective due to the algorithm-controlled exclusion process within the EBRA software. Nevertheless, due to the continuous recruitment and support of our collective, we consider the results to be meaningful and robust.

5 Conclusion

This study analyzes the migration behavior of the more recent Plasmafit® cup system and compares it with that of the already established Allofit® and Plasmacup® press-fit cup systems. There was no evidence for short-term aseptic loosening in any cup system. Neither do any epidemiologic factors such as age, sex, indication for surgery, cup/stem combination, bearing couple, and body mass index seem to influence the migration behavior of the cup systems. The EBRA method appears to be a fast and reliable tool for providing early predictions about the possible performance of acetabular cups and could therefore be used to provide rapid in vivo results certifying recently introduced acetabular cup systems. Though, subsequent long-term studies have to be carried out to confirm our short-term prognosis.

Level of evidence

2b

Ethics approval and consent to participate

IRB approval was obtained and informed, written consent from all patients (or their parents/guardians).

Consent for publication

Not applicable

Funding

We thank Aesculap AG (BBraun, Aesculap, Tuttlingen, Germany) for the financial support for the EBRA software costs.

Authors' contributions

BAI, JBS, MR, GAA, and AJ conceptualized, analysed and interpreted the patient data and were major contributors in writing the manuscript. JW, CAFU performed the data curation, the statistics as well as the discussion. All authors read and approved the final manuscript.

Availability of data and material

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.