History of the Atlas cup

PROMOTIONAL FEATURE

The seeds for the Atlas cup were first sown in 1985, when a French industrialist set up a small factory under the name of OMCI. And after 30 years, the Atlas cup is still holding …

The Atlas cup was one of OMCI’s first products and the chief engineer started work on the design in 1986. Particular attention was paid to the locking mechanism of the liner with the metal shell, which at the time had been highlighted as a point of failure in a number of contemporary designs.

By 1987, surgeons from three university hospitals – Professors Yves Gerard and Marc Ameil (Reims), Professor Mestdagh of Lille and Professor Fischer of Lyon – had joined forces with OMCI and the cup design was finalised.

Early clinical work in 1988 by Jean Louis Dore, of Tours, discovered that the elastic properties of the cup were responsible for the excellent primary fixation observed, negating the need for screws in most cases.

Pierre Lecestre and Phillippe Poilbout of the SOR group adopted the Atlas cup and were instrumental in the next phase of development with the hydroxyapatite coating, which was added to assist in later bony on-growth.

Fournitures Hospitalieres (later to become FH Orthopedics) acquired OMCI in 1992 on the back of the Atlas cup’s popularity. As a result, the body of French surgeons in the research group GECO adopted the Atlas cup and numbers of implantations increased substantially.

In 1994, the Atlas cup was married with the ESOP stem.

In 1997, the Atlas cup received FDA approval in the USA and in 2003, TGA approval for use in Australia.

In 2000, the cup was introduced into the UK. Prospectively collected data was instigated from the start of implantation in Swansea (and surrounding hospitals) allowing accurate real-time assessment of its survivability in almost 2000 implantations to date.

The Swansea data was used in previously published work by Dambreville [1,2] and Phillipe [3] and submitted to the ODEP for which it was granted one of the first 10A ratings for an uncemented acetabular component in 2013.

Results have also been published on the success of the Atlas IIIP cup from the Swansea group [4].

The Atlas cup remains as popular as ever and more than 125,000 Atlas cups have now been implanted worldwide.

 

Forming the Atlas cup

The Atlas cup design philosophy was one of a thin walled elastic shell with a thick polyethylene liner. The locking mechanism is unusual as it utilises cold flow of the polyethylene into a rough “back-side” of the cup. This was revolutionary thinking 30 years ago when nearly all other manufacturers were attempting to polish the back-sides of their cups. In the words of Alain Dambreville, “you only need to polish the back-side if you accept movement – we do not”.

In fact, the locking mechanism has been proven to be a strong positive of the Atlas design. The liner expands the flexible shell (see Figure 1 and Figure 2) allowing a complimentary force to the primary fixation.

The pegs of the cup were also found early on to not only give primary stability against rotation failure but, within weeks, were the first area to receive bony on-growth as proved by early histological retrieval.

On this basis, a number of other features have been developed which remain available for surgeon preference:

The Atlas IIIP has holes and spikes.

The ATLAS IVP has no screw holes

The ATLAS MS has no spikes and is designed for M.I.S.

Head sizes available include 22, 28, 32 & 36mm.

Liners include non-cross linked, highly cross linked
(Trianon); lipped and flat face.

 

Cup wall thickness

Part of the original design philosophy which has been adhered to throughout the history of the Atlas cup, is the desire to have a polyethylene load-bearing as thick as the shell would permit.

The wall thickness is 2.5mm made of flexible titanium. Knowing that the average contemporary cup has a wall thickness of at least 5mm, the additional polyethylene per cup size for the Atlas cup can be calculated.

For females, in our series the average cup size is 53.57mm. Allowing for a 28mm head and a 2.5mm total wall thickness (and standard under-reaming by 2mm), the load-bearing side of the polyethylene is 10.29mm (see Figure 7). Considering a standard, line-to-line reamed, thick walled cup of 5mm, the load-bearing polyethylene would be 6.79mm. This gives the Atlas cup an increase of 65.9 per cent for the average female cup. This is even more important if the sub-surface concentrations of polyethylene stress are considered, allowing the wider separation of head-to-poly contact force from the shear stresses of the shell-to-polyethylene forces.

Work in the Biomechanics laboratory in Cardiff by Paul Lees team [4,5], has shown that the creep of the polyethylene only occurs in the peripheral cylindrical region and not the load-bearing convex part of the polyethylene to cup interface.

Additionally, comparison between cross-linked and non-cross linked PE, after three months of loading, suggested that the cross-linked liner had superior wear properties with lower roughness at the sliding interface following loading.

 

Results of Atlas IIIP cup

The Swansea group has prospectively collected data on Atlas cup implantations since it was first used in 2000 and there are now more than 1,900 implantations in eight centres by 19 different lead surgeons.

All cups implanted are the IIIP form (hydroxyapatite, spiked, multi-hole version). The majority (49.7 per cent) were 28mm internal diameter in keeping with the philosophy of maximising the polyethylene wall thickness. However, 32mm (41.1 per cent) and 36mm (8.0 per cent) were also used. The majority of the cups (90.2 per cent) were used with non-cross linked, augmented posterior wall polyethylene but all the 36mm cups utilise a cross-linked, non-lipped version of the liner (less than five cups included in the series are of 22mm, soft capture design).

Follow-up is crucial to construction of a Kaplan Meier survival analysis, particularly of patients who are censured due to death. A loss to follow-up (using a schedule married to a probability of failure estimate) is less than five per cent in this series. Earlier versions of this work have been included in presentations at multiple international conferences [6].

Figure 9 shows a KM probability survival curve constructed on 1,841 consecutive cases [7]. The number at risk are indicated below the graph and 95 per cent confidence intervals are also indicated. Two curves are shown, “All Causes of Failure” and “Cup only Causes”. Tick marks indicate censured data (due to end of observation or death of the patient). Ten-year survival to all causes is 95.2 per cent (95 per cent CI: 93.73 – 96.58); 16-year survival to all causes is 91.8 per cent.

Figure 10 shows the comparison with simultaneously collected data from our unit on 101 ABG I (Stryker, Michigan) cups. This was a contemporary cup to the Atlas but differed in most design aspects: namely, thick walled shell, irradiated in air non-cross linked polyethylene, two studded peripheral locking mechanism and a face angled hood. The 10-year survival for Atlas is 97.2per cent (95 per cent CI: 98.2 – 96.2) vs ABG I 71.7 per cent (95 per cent CI: 82.56 – 62.81), which is a significant difference on the log-rank test (p<0.0001).

The shape of the curve suggests a half-life of the ABG I cup of 10 years; no estimate can be made on the Atlas cup to date as it has not failed sufficiently during the 16 years it has been followed.

 

Current research using the Atlas cup data

The thin-walled nature of the Atlas cup makes it particularly suitable for analysis of radiographs using measuring software. A research team at Morriston Hospital in Swansea is currently looking into the survival of the Atlas cup to lysis as a cause of failure after six years with respect to differing hard bearings. It is known from Fisher’s work [9] that stainless steel femoral heads are particularly prone to third body damage. It is thought that the resulting defect in the surface produces a raised edge, which then causes accelerated wear to the polyethylene soft bearing of the cup.

Analysis of failures using revision as an end point has determined that in the series examined (1,700 cases), no failures due to lysis occur after six years in the non-stainless steel group, but that in the stainless steel group, a (seven per cent) loss at 16 years was found. Using a log-rank and hazard ratio statistical test, this difference is significant at 16 years. (See Figure 11).

Additionally, in those cups that had not failed, linear wear measurements using Imitri measuring software [8] found a significant difference in calculated yearly wear rates. This work is on-going and standardisation and matching for age, sex, cup inclination and anteversion angle are being determined. See Table 1.

Work on the soft bearing surface has been been a focus of the manufacturing industry but this work tends to suggest that good results can be obtained with non-cross linked polyethylene when a suitable hard bearing surface is utilised.

This work suggests that failure will start to occur at around 10 years with stainless steel.

It is hoped that this work will be ready for publication in a peer reviewed journal within three months.

 

An Atlas retrieval analysis at five years

A well-functioning Atlas cup (size 56 cup with standard polyethylene, 28mm chrome Cobalt head) had been implanted at revision surgery in 2004, in an active female. Five years later, she sustained a periprosthetic Type IIb femoral fracture as a result of simple mechanical fall. At surgery, the femur was revised and the Atlas cup liner replaced leaving the shell behind. The liner was sent to FH Industrie for analysis for wear.

The calculated linear wear was less than 0.1mm after 63 months.

 

FH Orthopedics would like to warmly thank everyone at the Swansea, Morriston Hospital Orthopedics Unit, for its contribution to this article, and for all their datas and results.

 

Testimonial

Why love the Atlas cup?

The Atlas cup is my default choice in most clinical situations because, I believe, its flexibility due to its horseshoe design matches the modulus of elasticity of bone. This feature encourages osteointegration and diminishes shear forces with consequent improved long-term survival.

In addition, the expansion of the cup upon implantation, together with surface roughness and spikes, leads to an excellent primary fixation, negating the use of screws in almost all cases.

Due to its thin wall, a thick polyethylene liner can be used in small cup sizes. This allows one to utilise larger head sizes in small patients to improve stability. Together with excellent liner fixation and almost no backside wear, this leads to an extremely low failure rate.

 

Case example

 

References

1. A Dambreville, G Rolland-Jacob and P Lautridou. (1996). Cotyles metal-back et usure du polyethylene. Rev Chir Orthop 87 (suppl II):134.
2. A Dambeville. (2001). Assessing the stability of metal back acetabular insert. A microscopic study of implants. Eur J Orthop Surg Traumatol 11:213-218.
3. Michel Phillipe and Marc Ameil. Survival Analysis at 10 years of a cohort of 297 Atlas Total Hip prostheses. Eur J Orthop Surg Traumatol (2007) 17:573-578. DOI 10.1007/s00590-007-0236-y.
4. EFORT 2017 presentation poster #2451, Jeetender Peehal, Paul Lee and David Woodnutt.
5. EFORT 2017 presentation #1519, Jeetender Peehal, Paul Lee and David Woodnutt.
6. Paul Lee, M Rachala, Kar Teoh, D Woodnutt. Long-Term Results with the Atlas IIIP Elastic Cementless acetabular component in total Hips Replacement. Int Orthop. 2016 Sep;40(9):1835-42. DOI: 10.1007/s00264-015-3088-9.
7. Abbas, G, Mullins, M, Dodd, M and Woodnutt, D. Use of cementless elastic acetabular cups with non-highly cross-linked polyethylene in total hip replacement (2016). Bone Joint J,98-B(SUPP 11), 26. Retrieved from http://www.bjjprocs.boneandjoint.org.uk/content/98-B/SUPP_11/26.
8. KM curves constructed using MedCal Software v12.3.0.0.
9. H Minakawa and others. Quantification of third-body damage and its effect on UHMWPE wear with different types of femoral head. JBJS. Vol 80-B No 5 Spet 1998; 894-9.
10. Imiti Software, SA. Beta version.