This metalwork portfolio has been appropriately supported by a range of Spacer products, taking the business into the area of the management of two stage revision of infected joint replacement. This complication of surgery has devastating consequences for the patient, and a return to home, mobile and weight bearing, has been seen to be of great benefit in their rehabilitation from such a complication.

The custom option offers an extended choice of fixation scenarios as summarised: -

• Cemented (with the option of proximal cement ‘restriction’) in the metaphyseal femur
• Cementless fixation
• Standard 90 micron plasma coating of Hydroxyapatite
• Roughened titanium plasma coatings
• Bi- coating- roughened and hydroxyapatite combined finish

The case below (Figs 3 and 4) is an example of a custom component designed in response to a 47-year-old male who suffered a fracture neck of femur with ipsilateral distal shaft fracture, originally treated with a reconstruction nail. The femoral head failed to unite, requiring hip replacement with continued stabilisation of the distal fracture. The custom stem shown matched the diameter and anterior bow of the femur, extending beyond the distal fracture, with the option to lock at this, or subsequent procedure.

Where the design must involve the provision of adequate fixation often with minimal distal bone stock available, then fully HA roughened coated components are best indicated in this situation, and special challenges lay ahead for the operating surgeon, often choosing to re- inforce the femur with strut allograft (Fig 6). Distal locking with careful consideration to stem length is achieved. HA coating in apposition to host bone at the isthmus ensures the best possible circumstances for oseointegration to proceed.

Case study 2
Indications for our Cannulok solution often involve cases of massive proximal bone loss (Fig 7). Diaphyseal interference fit with HA coated and a distally locked prostheses, can create a bio-mechanically favourable environment for proximal femoral bone stock recovery (Fig 8). Trikha et al² describe the case of a 59-year-old rheumatoid woman who underwent revision hip surgery to a long stem cemented implant following early aseptic loosening. Seven years later, the patient sustained an atraumatic unstable peri- prosthetic fracture. The stem was revised to a fully coated implant.

Two months post-operatively, a second stable peri-prosthetic fracture was sustained, and fixed with cable plate fixation with use of autogenous iliac bone graft. 2 years later, the patient presented with an infected non-union. All metalwork was removed leaving a grossly deficient proximal femur (Fig 7), ectatic in nature with combined cavitary and segmental bone loss and discontinuity mid-shaft. 16 months postoperative, reconstruction with an HA coated Cannulok stem confirms fracture union and bone ingrowth in all areas of the prosthesis (Fig 8).

Early surgical intervention should therefore be a consideration. Cementation, with the aid of proximal cement restriction allows fracture healing to progress unhindered preventing cement migration into the fracture line. On the other hand, increasingly popular is the selection of an appropriately sized, HA coated stem which facilitates a diaphyseal scratch fit of HA in apposition to viable host bone. Fracture healing and oseo- integration is seen to progress rapidly, largely as a result of prosthetic bio- mechanical stability made possible through the use of cross locking screws.

The device is a two part-interlocking nail, facilitating the approach to the knee through the original incision following previous knee surgery. The design allows for compression and the prompt course to union as reported in the above series (Figs 13 and 14).

In September 2003, there followed a comprehensive review⁴ of the available devices for consideration of arthrodesis of the knee from Mr Manoj Sood, Clinical Lecturer, Institute of Orthopaedics, Stanmore, who reported encouraging results even with end staged, infected cases arthrodesed as a single stage.

Matching the needs of the surgeon who requires a wide range of lower limb revision solutions and knee salvage devices, remains the driving force behind the product portfolio of Orthodynamics.

1. Charnley G, Anderson G. Preliminary experience of the Cannulok revision hip prosthesis in late periprosthetic fracture management. Hip International, Vol. 12, n. 1, 2002 – pp. 1-10
2. Trikha P, Singh S, Raynham O, Lewis J, Edge J. Use of an interlocking hydroxyapatite-coated stem in a patient with an infected non-union of a periprosthetic femoral fracture with massive bone loss. J Bone Joint Surg Am, 2004; 86: 1783 - 86.
3. SP White, AJ Porteous, JH Newman, W Mintowt-Czyz, V Barr. Arthrodesis of the knee using a custom-made intermedullary coupled device. J Bone Joint Surg [Br] 2003; 85-B:57-61.
4. M Sood. Knee arthrodesis using short, two-part, modular intramedullary nails. Orthopaedic Product News. Sept/October 2003

A combination of scientific improvements and an ageing population have led to incredible growth in the number of THAs’ completed each year. In 2003, some 650,000 hip implant procedures were completed on a global basis.

The alternative material combinations in THA are metal on metal or ceramic on ceramic, which can offer reduced wear characteristics. However they come with associated risks, which would include:
metal on metal

• Adverse biological effects to increased metal ion levels in the body.

• Chipping of ceramic cup liner and the risk of fracture.

With the aim of improving surface texture characterisation knowledge, this article compares the use of two instruments (contact & non-contact) in the measurement of hip implant components. It looks at the instrument factors affecting nanometric accuracy and the parameters available, which can be used to improve functional performance knowledge.

Advantages of different gauging techniques
Contact probe	Non contact probe
Can deal with contamination - dust/oil films.	Can achieve rapid 3D measurement.
Defined ISO standards for parameters used	Will not damage the component or cause contamination
Measures the mechanical surface	Typically probe cannot be damaged during measurement
Table 1: Benefits of contact and non-contact probes

Fig.1 - Phase Grating Interferometric gauge head

A laser is directed down a beam splitter onto a convex diffractive grating. This splits the beam into two elements, which are reflected back along different paths onto detectors. Movement of the stylus beam – as it moves across the surface – creates a phase difference between the reflected beams which is measured.

The PGI offers both good vertical range and resolution which makes it ideally suited to measuring form, radius and surface texture, all of which are important in THA components. It should be noted that for stable and accurate measurement of form and radius on hip implant components, the stylus trace should cover an angle of at least 60 degrees.

Before considering the parameters available in 2D analysis it is worth briefly reflecting on the factors that affect accuracy in contact profiling systems. The factors include:

• Stylus tip size and geometry
• Gauge linearity
• Gauge frequency response
• Vertical resolution of gauge
• Data logging in the Z direction
• System noise

A standard parameter used in 2D analysis of hip implants is Ra (average roughness), which is the arithmetic average of the absolute departure of the profile from the reference line throughout the sampling length. In mathematical terms:

Due to the nature of its calculation, Ra is a reasonably useful tool for process control however it is less useful as a predictor of functional performance. The reason for this can be explained by reviewing fig.2, which shows a number of different profiles. It is quite feasible for each of these profiles to have the same Ra figure, however it is easy to see that they will have quite different tribological characteristics.

Fig. 2 - Ra calculation on four different profiles

There are alternative 2D parameters available which can offer more insight into the bearing characteristics of hip implant components. These would include Rq and Rsk.

Rq is the root mean square of the distance of the filtered or unfiltered profile from its mean line. In mathematical terms:

Because this parameter squares amplitudes, it is more sensitive to peaks and valleys. Another parameter is Rsk (Skew) which is a measure of the symmetry of a profile about a mean line. It can distinguish between asymmetrical profiles with the same Ra or Rq. Negative Skew indicates a predominance of valleys, while positive Skew will be seen on “peaky” surfaces. Examples of this are shown in fig. 3.

Fig. 3: Skew can distinguish between good and poor bearing surfaces

An upper beam splitter directs light from a white light source towards the objective lens. The lower beam splitter splits the light into two separate beams, one is reflected onto a reference surface, the other onto the surface. The two beams recombine to create an interference pattern which is monitored by a CCD (Charged Coupled Device). The interference pattern is processed and a digital image of the surface comprising (X,Y,Z) data points over the measurement area is obtained.

Fig. 4: Schematic of optical head in Talysurf CCI

As with contact profiling systems, there are a number of factors that affect the accuracy of areal interferometric systems. The factors include:

• Vertical resolution of gauge
• Amount of returned light
• Data points in X,Y (pixel array on CCD)
• System noise

It is possible to look at hip implant components using 3D equivalents of the 2D parameters we have discussed, i.e. Sa, Sq and Ssk. As these parameters are over an area they are double integrals (summations) in the x and y direction. For example in mathematical terms Ssk would be defined as:

As with the 2D analysis Sa is a limited parameter for gaining functional information from surfaces. However parameters like, Ssk and analysis related to volume properties of the surface can be used to gain a greater insight into functional performance.

2D (Contact) & 3D Non Contact Surface Characterisation of 36mm CoCr femoral heads
Heads	2D Parameters			3D Parameters
	Ra	Rq	Rsk	Sa	Sq	Ssk
	m	m		m	m
No.1	0.0094	0.0129	0.1780	0.0111	0.0213	7.0854
No.2	0.0053	0.0168	-15.572	0.0303	0.0624	-6.4239
No.3	0.0111	0.0178	-2.000	0.0399	0.0792	-5.1018
No.4	0.0035	0.0077	-10.017	0.0412	0.0846	-4.7668
Table 2: A comparison of some 2D and 3D parameter calculations on femoral heads

The 3D calculations were completed on a 0.9mm² area taken from the apex of each femoral head. The Sa and Sq parameters for head no.1 are within acceptable limits, however the values for the other three heads are very high. Closer inspection of these three heads using 3D visualisation tools highlights a severe problem with pitting on these heads, which is invariably due to a poor process. It is interesting to note how a 3D tool can identify issues with bearing surfaces that can be masked by a 2D profiling system.

This article has provided a brief insight into 2D and 3D surface characterisation on hip implants. It has highlighted that there are more parameters available than the average mean (Ra/Sa) for understanding and quantifying the functional performance of hip implants.

Further study should be considered into detailing the best parameters to be used and the acceptable limits on these parameters. There should also be some consideration into defining surface texture direction (lay) on hip implant components as this could have a large influence on the ability to generate a thick lubrication film.

1. D.W. Murray, N Rushton, “Macrophages stimulate resorption when they phagocytose particles”, J Bone Surg 205H (1990)73-79
2. Heisel H. Et al.: J Bone Joint Surg-Am 2003; 85-A: 1366-79
3. D.J.R. Cooper and J Fisher , Clin Matls 14 (1993) 295
4. Smith, S.L., Dowson, D. and Goldsmith, A.A.J., (2001), ‘The Lubrication of Metal-on-Metal Hip Joints: A slide Down the Stribeck Curve’, Proceedings of the Institute of Mechanical Engineers, Part ‘J’ Engineering Tribology, Volume 215, No. J5, 483-484
5. Z.Jin, D. Dowson, J. Fisher. ‘Analysis of fluid film lubrication in artificial hip joint replacements with surfaces of high elastic modulus’. Proceedings of the Institute of Mechanical Engineers, Vol 211, Part H, 247-256, 1997
6. L. Blunt and X Jiang “Three dimensional measurement of the surface topography of ceramic and metallic orthopaedic joint prostheses” J. Matl’s Sci; Matl’s in Medicine 11(2000) 235-246.
7. D. Whitehouse, “Surfaces and their Measurement”, Hermes Penton Science, London.
8. L. Blunt and X. Jiang, “Assessment Surface Topography”, Kogan Page Science, London.

ZAPP-Group products are made of stainless steel, alloyed tool steels, titanium and special materials and come in the form of wire, bars, profile and precision strip in semi-finished condition. The Group also trades in special materials consisting of nickel and nickel alloys, cobalt alloys, reactive metals, high performance alloys and powder-metallurgical steels.

Zapp’s origins date back to 1701, since then they have become an international group with four steel processing mills in both Europe and the USA. I was kindly shown round their mill and in Ergste just before the New Year.

Now, unless you are biomechanical engineer you have probably never seen a working steel mill, let alone even have visited one. I remember studying them at school, so I knew the basics, but there is no substitute for a hands-on approach in my opinion. So here is my guided tour, which I will leave deliberately light on the technical details, so if you are an engineer, I apologise in advance.

The factory is approached by a historic frontage that really deceives you to what lies behind it. The plant is massive and modern, and this is just a small mill. Despite the size of the site, the distances between production facilities, laboratories and the sales and administration offices are extremely short. Communications between the various departments are central to the Company's success and they have obviously spent time planning the site over several years.

Zapp is a steel-processing mill. That is to say they do not do the hot melting of steel and rolling it out. They simply process the raw product into something that the orthopaedic manufacturers can work with easily. From a chat to their marketing people, it is obvious that their selling point is producing a high quality product consistently, so that there is less further processing required further down the line. For medical metals, quality seems to be bar or sheet straightness, tight tolerances and surface finish.

The management at Zapp talk about wire pins, bars (for hip implants and screws), profiles (for fixation plates), strips (saw blades), sheets (bone plates) and tubes (cannulated screws) – admittedly, I find it difficult to differentiate between the products. When does a square bar become a profile, for example. Basically, though, the same processes are applied to all the raw material, it is just that the sheet products (sheets and strips) require different machinery.

Lets look at the sheets first. Outside in the yard there are massive rolls of sheet steel for clients. These come in a few fairly standard sizes. Zapp take these and roll them to the required thickness and sizes. This is done on huge rolling machines. Now, although this mill only does cold work, do not be fooled. Compressing metal produces a fair amount of heat.

The interesting thing is that there are limits to how much you can flatten a sheet in one pass. So it is usually required to have several passes to achieve the required specification. To make life more complicated, as you flatten the sheet the tensile strength changes, making it stiffer and less pliable. To make the metal soft and pliable it requires heating. This is done in line in furnaces on the move. After that, more flattening can be performed. It is all very clever, and at the end you get a sheet or strip to the correct sizes within required tolerances with the specified tensile strength. The products are then put through a straightening machine that looks like it would do the exact opposite. Rows of rollers above and below the sheet bend the sheet back and forth and amazingly after the last roller a bent sheet becomes a straight product.

The bars, wires and profiles work along similar lines. In the yard huge coil of raw material are processed. These coils are fed through machines with a die and drawn out. The dies require water-cooling as so much heat is generated, and a lubricant is applied to before drawing to make it easier. Again, as with the sheets, there are limits to how much you can draw in a single pass, so several passes are usually required. Obviously, huge annealing furnaces heat up the product when required, and bars are then straightened in a similar way to sheets with the wire normally coiled at the end. Some of the wire produced was so fine it was barely visible.

Profiles are really machined bars, and the round bars are subjected to pressure to give them a required shape - like a curved shape for fixation plates, for example. This reduces further processing.

During my tour of the laboratories, it became clear that the quality assurance and Research & Development Departments are not simply working on today's surface treatments and quality, but also on the materials of the future.

One of the interesting things about the building was the lack of people. Obviously, hundreds are employed, but considering the size and amount of machinery it was amazing to see it looking so empty. Another interesting point was the speed - this is not a fast process. To give you an idea, you can work along a wire drawing machine at the same sort of speed that it draws the material out.

Back in the smart office building I was told about lead times! Typically, the orthopaedic manufactures are thinking a year ahead. Zapp can actually process the steel very quickly – the long lead time is understandably due to the time required to actually process the raw project. If anyone can supply an actual time frame from ordering the steel to supplying a hospital with a product I would love to know.

Another point of interest is that medical grade steels are very low in contaminants. This is good for implants, but bad for machining. The lack of sulphurs make machining the products slow and difficult. We will look at how medical steels are produced in a later issue.

One obvious thing that stood out from my visit is that titanium is rare stuff. There was, I would guess, hundreds of rolls of steel - granted not all medical - and I would say only a couple of titanium. The lead times, I am told, are even longer for titanium and the price has doubled in the last year. So you basically order blind without knowing how much you will pay by the time it turns up! The steel market itself makes an interesting topic which we will also cover in a separate issue.

Finally, it is important to me to mention that ZAPP makes a great deal more than products for medical technology. In addition to these, ZAPP manufactures products for the automotive, chemical, electrical and consumer goods industries, food manufacturers, aerospace systems, the textile industry and the watch and jewellery industries. In their showrooms, I also spotted very intricate and delicate components for Rolex watches, so they obviously have the skills and technology to go well beyond what is required in the orthopaedic market place.