BEARING SURFACES

1. CERAMIC ON CERAMIC
CERAMIC COMPOUND MATERIALS - Ceramic Options for Resurfacing & Total Hip Arthroplasty

Historical Review
Polyethylene has been used in the field of endo-prosthetics for more than 40 years. It is considered as the weakest component within existing bearing options. Mostly wear and destruction of the polyethylene bearing options due to pitting are the reason for aseptic loosening and therefore for failure of joint endoprosthesis.

Based on numerable ring-on-disc and simulator tests a composite material has been developed for which the patent DD 272 603 A1 has been awarded and which was launched into the market as ENDOCERAM.

In 1989 ESKA Implants GmbH & Co. KG reserved all rights for the production and sales of this composite material. It was modified then as a result of more scientific examinations and was named ESKA-CERAM®. After further biomechanical examinations the clinical approval for a new ESKA-CERAM® insert was achieved in 1998 by ESKA Implants GmbH & Co. KG.

In 2007, the ESKA-CERAM® product has been extended to a full range of Ceramic Large Femoral Heads (available for both THR and Resurfacing) and ESKA-CERAM® Inserts (available for both THR and Resurfacing).

Fig. 2 ESKA Cera-Metal Resurfacing Head Fig. 1 ESKA Cera-Metal Large Femoral Head (38, 44 & 48mm)

ESKA-CERAM® A new material in hip endoprosthesis
The composite material ENDOCERAM consists of a polyurethane matrix and of a mixed-in glass ceramic powder. By starting manufacturing at ESKA Implants GmbH & Co. KG. ESKA-CERAM® for THR and Resurfacing has been modified to comprise two-thirds polyurethane and one-third Al2O3—Ceramic substituting the glass ceramic.

The new material ESKA-CERAM® is a little lighter in appearance than the original material and has a porcelain-like look. Its wear rates are of a similar low order of magnitude as those reported for metal-metal and ceramic-ceramic combinations. A great advantage of the material is that it permits the production of asymmetrical inserts for acetabular cups.

Fig. 3 ESKA-CERAM Insert (Available for both THR and Resurfacing)

Laboratory Results of 10 Mio Simulator Tests
The performance of the new material was shown in numerable tests using a load and movement simulator.

Run-In
Inlay ESKA-Ceram: 5 µm
Femoral Head Cera-Metal : 5 µm

ESKA-Bionik® System — BS Acetabular Cup with Spongiosa Metal® surface structure

Positive results for the metal Spongiosa cups with ESKA-CERAM® inserts can be reported. For an optimal outcome, ESKA Implants AG proudly provides three types of Metal Shell.

- Cementless anchorage : using surface structure Spongiosa Metal® II.
- Extensive range of sizes : Standard Shell, Screw Fixation Shell & Dysplasia.
- Various insert options : Metal-on-metal, ceramic-on-ceramic, metal-on-polyethylene as well as ceramic- on- polyethylene bearing options.
- Extensive range of sizes : available in 11 sizes (OD=46 to 66 mm, 2 mm increments).
- Material : CoCrMo TiNb-Coat.

2. METAL ON METAL ARTICULATION
10 MILLION CYCLES on MoM wear now available on request...

Historical Review
Metal-on-Metal (MoM) for hip endo-prosthesis systems are characterized by low linear and volumetric wear and long survival.

ESKA Implants AG has over 25 years experience in the manufacturing of implants including hard bearing articulations. Eska’s manufacturing of metal bearings incorporates a UNIQUE patented production process along with attention to surface finish and sphericity for optimal tolerances and clearance. Further optimization of the bearing surface through the Bionik® surface design (Golf Ball) has been developed to enhance hydrodynamic lubrication in the bearing and hence to a reduction in wear.

MoM Standard Bearing for THR
The ESKA metal-on-metal bearing for THR uses the commercially available powder (wrought) metallurgical alloy. This process is used in the production of large metal heads (fig.1) in sizes 38mm, 44mm and 48mm and the matching metal articulating inserts(fig.2). The bearings are manufactured from a cobalt-chromium forged alloy according to ISO5832-12.

The powder metallurgical alloy with its fine-grained structure and evenly spread carbides was especially developed for the hard bearing options. The high-performance composite material is characterized by its high tensile strength and high elongation supporting its potential for long term survival.

MoM - Bearing for Resurfacing
The Hip Resurfacing Femoral Component of the ESKA Bionik System is equipped with Spongiosa Metal® II porous structure for cementless use. The Hip Resurfacing Femoral Head is cast by using the special Precision Homogenous Casting Procedure known as P-HCP TM.

This process dissolves the carbides on the boarder of the kernel grains and spreads them evenly in the structure decreasing potential “Breaking-out” of carbides. As well smaller carbides are produced while maintaining a hard bearing surface. (fig.4a)

High carbon alloys may achieve very low wear rates initially, however the high potential of “breaking-out” of the carbides can lead to an increase in third body wear. This third body wear can potentially lead to the failure of the bearing. Low carbon alloys when combined with a powder metallurgical alloy show very good wear behavior. (fig.5 a, b) When the P-HCP process is used a further reduction in wear rates is achieved after the run in phase.

Wear of metal-on-metal bearing options (metal head and insert) Source Kenneth, R.St John et al.

Wear of metal-on-metal bearing options (metal head and insert ) Source Kenneth, R. St John et al.

Higher wear rates of High Carbon Bearings was due to increased third body wear.

More than Low carbon vs. High carbon
In a complex tribological system like a hip replacement many factors are active and mutually influencing on the life of the bearing. The assumption that only the bearing option of “high carbon on high carbon” is the optimal metal bearing is NOT correct.The share of carbides is one of more than 15 primary factors that need to be considered. Various carbides exist parallel in one alloy e.g. Cobalt, chromium and other parts of the alloy and its complex carbides (mixed carbides). All of them with different properties and effects. The composition of the carbides is dependent on the production process.The size and spreading of the carbides is governed by the share of carbon and the processing. The tribological interplay of the articulation surfaces is decisively effected by these parameters therefore a context like “large carbides = low wear” is NOT always the case. The metallurgical micro structure of the grain borders and crystalline development is another factor to consider. The integration of the carbides in the matrix can be controlled through a special production process. ESKA Implants uses this process in the manufacture of its Resurfacing Femoral Head. This process is known as Precision Homogenous Casting procedure P-HCPTM

ESKA Process for Resurfacing Femoral Head
P-HCP TM (Precision homogenous casting procedure)

Features:
The articulation is made by a combination of materials using a cobalt-chromium- forged alloy according to ISO 5832-12 and the respective cast alloy according to ISO 5832-4. In order to allow for a 100% quality in casting of the material a centrifugal casting technique is used, as well as the development of a special embedding mass.

In the casting process the lapse of time is strictly controlled within a matter of seconds.(fig.9) to replicate the result in every bearing. Immediately following the casting process a homogenization process occurs allowing for the even and fine spreading of carbides . This is achieved by precisely controlling the heating and cooling process.

The cast alloy used is carefully tempered (known as “Homogenizing Glow”), dissolving the carbides on the border of the kernel grains and spreading them evenly.(fig.10) This results in an optimal bearing surface of a low Carbon Alloy. (fig.11)

The Eska MoM articulation manufactured under P-HCPTM has shown excellent wear properties in two independent laboratory studies 6 and clinical results from four investigation centres in Europe.

Clearance

A precise manufacturing of the components is undertaken by ESKA in order to achieve the low wear values. The tolerance of geometry between the femoral component and the insert must be optimal (fig.13). Poor clearance has been identified as one major parameter affecting Metal-on-Metal wear performance. The concentricity should be below 5 μm for both components with a deviation of concentricity <5µm. Eska’s Large Femoral heads and the Resurfacing Femoral heads lie within the optimal clearance band.

Surface Finish + Sphericity

A super finish or low surface roughness and maximal sphericity of the bearing surface for both the heads and inserts is a parameter in controlling wear of metal-on-metal hip implants. The Eska manufacturing process acknowledges this and controls this parameter through a specially developed machine.

3. “Bionik Surface Design” - The GOLF BALL

A Hydrodynamic Lubrication technique in Bearings

To optimize bearing function and longevity Eska reduces friction between the bearings by the use of “the pocket principle”(fig.14) for improved lubrication was investigated. (fig.15)

To allow for a quick influx and an optimal distribution of fluid the articulation surfaces have been manufactured with dimples or lubrication-pockets. Fluid film lubrication plays a major role in further reducing the wear of metal-on-metal implants.

Independent Laboratory test results confirmed that design parameters including lubrication pockets produced a lubrication layer three times as thick as pairings without the pockets. The dimples keep the lubrication which, under pressure, is pressed between the articulating surfaces.

The optimization of design parameters such as material and surface quality of the bearing option, lubrication properties, diameter, depth and position of the dimples as well as load and running speed of articulation surfaces were tested for 2.5 years. (fig.16)

The result was the “Biosurf®” surface or “Golf Ball” design (fig.17) with it’s enhanced hydrodynamic lubrication. This has shown a reduction in wear and maintained a wear rate of 2.26 mg/mio cycles over the Non dimpled surface at 2.5 mg/mio cycles in the same time frame and independent simulator.