the Clinical Use
A. Cigada, M.F. Brunella
Department of Applied Phisical Chemistry, Politecnico of Milan, Italy
Introduction
Various studies have been conducted on the preparation of hip and knee spacers for the local treatment of infections in terms of choosing the right antibiotic and dose and designing the components. [2, 3, 7]. In fact, in addition to releasing the drug to treat the infection, the spacer’s goal is to enable joint movement [8] and to support a partial load without further bone loss. The advantage of the two-stage technique is that the patient is not obliged to stay in bed for the period in which the infection is being treated and the shortening of his extremity is preserved [5]. The efficacy of the two-stage treatment using the spacer has given positive results in more than 90 % of cases [3, 4] with an average follow-up of 36 months. In some cases the stems broke [6], Failure has not been directly related to the use of the spacer except in a small number of fatigue failure of the stem [2]. Even the process of wearing, tested in vitro with ace- tabular corpse cups, seem to indicate the hip spacer’s improved performance with a limited release of wear particles [1].
In general, the spacer is a preformed device, covered in bone cement loaded with antibiotic. There are examples, as in the spacer shown in Fig. 1, in which the antibiotic
Fig. 1. Hip spacer with antibiotic insertions
was added by the surgeon directly before the implant by inserting it in the preformed spacer. For a better distribution of the drug the loading should take place before the cement’s polymerization. For the hip joint, the device is composed of the femoral stem and head which can be obtained from a mould (Spacer-G – Tecres Spa, Italy) and a hollow stainless steel cylindrical rod is inserted in the stem.
The design of the knee spacer is similar to a normal prostheses but the material modified as an antibiotic-loaded acrylic cement. The tibial component is composed of a flat base and the femoral component matching automatically. This enables repeated rotation movements between the distal femur and the proximal tibia during flexion and extension. Both components are secured to the bone with bone cement.
Acrylic cement is prone to wearing and degradation and therefore will require a sec- ond stage revision when the infection has been controlled [2].
The aim of this paper is to evaluate the wear characteristics of acrylic cement in spacers. They were evaluated at different stages after cycling and the breakdown of areas in contact were recorded.
Experimental
Two hip Spacer-G and two knee Spacer-K (Tecres Spa, Italy) were analysed. The knee spacers were removed after having functioned in vivo for 5 and 6 months, while the two hip spacers were removed at 3 – 6 months. They were compared to new implants to evaluate wear characteristics. The composition of the acrylic cement used in Spacer-G and the Spacer-K is indicated in Table 1. The removed implants were cleaned of organic residue prior to evaluation. Initial evaluation was carried out with an optical microscope. A macroscopic survey using a stereomicroscope (WILD M8) was then utilized in areas that wear was identified. A stereoscan microscope (Cam- bridge Stereoscan 360) and a microanalysis with X-ray spectrometer (Oxford INCA 200) was then carried out. The surfaces’ wearing conditions were evaluated through a laser section measurement (UBM microfocus) to further determine the amount of acrylic wear.
The Spacer-K was also submitted to Gel Permeation Chromatography in order to determine the degradation of the acrylic polymer.
Table 1. Chemical composition of the antibiotic cement of Spacer-G and Spacer-K
Liquid component:
Methylmethacrylate 98.20 % w/w
N-N Dimethyl-p-Toludine 1.80 % w/w
Hydroquinone 75 ppm
Powder component:
Polymethylmethacrylate 82 % w/w Barium Sulphate Ph.Eur. 10 % w/w Gentamicin Sulphate Max: 5 % w/w*
Benzoyl Peroxide 3 % w/w
* Equivalent always to 2.5 % Gentamicin base.
Results and Discussion
Macroscopical Examination
The macroscopical examination of the new Spacer-K highlighted the fact that the surface of the two parts that compose the spacer is completely opaque. No particular finishing touches were observed. However, a certain porosity distributed over the whole surface, even at the back of the parts, with big pores of different dimensions even one-millimetre wide, were clearly observed. These are typical of acrylic cement (Figs 2a, 2b).
The surfaces of the Spacer-K, after explantation, are characterised by the existence of very shiny areas with respect to the remaining opaque surface. These areas were different both in shape and extension in the two explants. Contact areas between the flat shinbone and the thigh bone component are indicative. Therefore in these areas the wearing conditions of the material was observed. It was noticed that in the smaller spacer (5 months) the shiny part extended to almost the whole area covering the two surfaces that were in contact with the thighbone, including an area on the surface of the central part in relief. In the bigger Spacer-K, it was instead noticed that the shiny areas were located on the component sides and this was more noticeable on one side (Fig. 3).
The hip Spacer-G, used as a reference, is smaller than the two explants. But even in this case, like the previous one, it was used as a reference to compare the surface of the cement after in vivo functioning.
a b
Fig. 2. Blank Spacer-K: a tibial component; b femoral component
Fig. 3. Explanted Spacer-K: glossy areas on the tibial component
Similarly to the knee spacer, the surface of the new Spacer–G was totally opaque.
It had macroscopic imperfections, such as open and closed pores. The latter were barely seen underneath a cement film, identified as whiter spots. The line joining the two parts of the mould could also be seen and, in particular, the head showed signs of mechanical workmanship that increased the surface’s roughness. Explanted devices were distinguished for the different dimensions of the head, which was bigger in the spacer explanted after 6 months. In one of the two stems, following the breakage of the cement at the end, the metallic rod could be seen inside. Both stems showed a breakage of the cement coating in the distal area. Observing the heads enabled the identification of contact areas, which were particularly shiny with respect to the rest of the surface which, instead appeared just like the new sample of reference. The shiny areas no longer had the mechanical workmanship that was identified on the new spacer and that was still visible on the rest of the heads’ surface. The shinier areas developed along a circular crown towards the side of the head (Fig. 4). These had more or less the same extension. In the bigger head, the smooth surface extended partly even over the side while in the other one it was mainly distributed on the same cap. The location with respect to the stem was also different. It was towards the upper end in the first and sideways in the second. A summary of the observations conducted during this first stage of analysis is contained in Table 2.
Microscopical Examination
The SEM observation of spacers was conducted on the less bulky parts, in order that these would be better moved inside the instrument’s chamber. In particular, the heads of the Spacer-G devices were tested, after having been cut from the stem and shinbone plates of the Spacer-K devices.
The common aspect found in all new spacers, which characterises their surface, is the existence of prepolymer beads (generally in relief) and the existence of a powder compound, in the polymerized matrix, based on radiopacifying barium sulphate, as shown in the EDS spectrum obtained from an accurate analysis of the area (Fig. 5). It was not possible to highlight even an eventual presence of the Gentamicin sulphate antibiotic, since there were no discriminating elements to detect this additive with respect to the polymer and the radiopacifying agent. On the Spacer-G heads the pre- polymer beads were quite noticeable on the ridges of the holes left by the mechanical workmanship (Fig. 6) and flatter on the plains. This was already highlighted by the macroscopical observation.
Fig. 4. Explanted Spacer-G: glossy areas on the head
Table 2. Summary of the macroscopical and microscopical examination results
Surface Site Spacer type
Macro- scopical exami- nation
Opaque
Presence of open and close pores
Signs of mechanical finish- ing
Head Stem
Spacer-G
Opaque
Presence of pores, even in the order of mm
Femoral component Tibial component
Spacer-K
Very smooth areas on the remaining opaque surface
Femoral component Tibial plate
) Extended smooth areas in the planes of coupling with the femoral component ) Limited smooth areas and
localized at the borders of the components
Spacer-K 5M Spacer-K 6M
Fracture in the acrylic cement coating
Stem in distal zone Spacer-G 3M
Very smooth areas respect the remaining opaque sur- face and absence of mechanical finishing signs
Head
The areas develop in a circu- lar crown shifted towards the rim
Extension of the smooth area beyond the head border
Spacer-G 6M
Spacer-G 6M
Microscop- ical exami- nation by SEM and EDS micro- analysis
Emerging pre-polymer spheres inside a matrix mainly composed of BaSO4particles Flattened pre-polymer
spheres
Head
Tibial component
Head, bottom of mechanical finishing grooves
Spacer-G Spacer-K
Spacer-G
As above + cracks Opaque areas of head and
tibial component Spacer-G 3M Spacer-G 6M Spacer-K 5M Spacer-K 6M Polishes pre-polymer
spheres. Reduction pow- dery material made of BaSO4
Several cracks
Smooth areas in head and tibial component (more in head)
Smooth and opaque (in lesser size) areas Head and tibial plate
Spacer-G 3M Spacer-G 6M Spacer-K 5M Spacer-K 6M
Spacer-G = Spacer-G new; Spacer K = Spacer-K new; Spacer-G 3M = implanted 3 months; Spacer-G 6M = implanted 6 months; Spacer-K 5M = implanted 5 months; Spacer-K 6M = implanted 6 months
a
b Fig. 5. a BaSO4powder in the polymerized matrix on the groove throat b EDS spec- trum of the com- pound in the matrix among the spheres
a Fig. 6. SEI-BSE mix signal images of the blank Spacer-G:
a grooves due to mechanical finishing
b
Fig. 6b. Prepolymer spheres on the groove top
a
Fig. 7. Explanted Spacer-K, SEM micro- graphs of: a the smooth area
The rough areas observed on the spacers after explantation have a similar shape to
the ones of reference on the new spacers, except for the existence of cracks that are
particularly numerous and evident in the more worn out areas, that appear shiny
under macroscopic observation. The worn out areas are characterised not only by a
series of cracks but also by the smoothing out of the prepolymer beads and the
absence of the powder compound between the beads which actually tends to trans-
form into islands between the small spheres (Fig. 7). The same figure also shows how
the cracks are not only distributed over the polymerized matrix, but they also cross
the same prepolymer beads. The worn out surfaces of Spacer-G heads, have another
peculiarity. They show a general flattening out of workmanship signs. It was also
noticed that some prepolymer beads (especially those in relief on the ridges of the
workmanship signs) have been moved to other areas, implanted in the matrix, with
Fig. 7b. The rough area b
Fig. 8. Explanted Spacer-G: SEM micro- graph of the smooth area with a sphere implanted in the matrix
clear formation of little new cracks on the sides (Fig. 8). As in previous observations, the most significant results are indicated in Table 2.
Roughness Measurements
Roughness measurements carried out on spacers have given results expressed in
average roughness Ra, assessed according to regulation DIN4768, indicated in Ta-
ble 3 and in Figures 9 to 11. The average roughness of non implanted spacers has been
indicated as a reference. Average roughness values measured in opaque areas are very
close to the non implanted spacers, which in any case show a different surface finish
between the Spacer-G (R
a= 3.37 µm) and the Spacer-K. The surface roughness of the
latter is actually considerably lower, both on the femoral component (R
a= 0.97 µm)
0 0.5 1 1.5 2 2.5 3 3.5 4
smooth area rough area
Ra
(µm)
5 m 6 m 0 m
smooth area rough area
Ra (m)
5 m 6 m 0 m
0 0.5 1 1.5 2 2.5 3 3.5 4
smooth area rough area
Ra
(µm)
5 m 6 m 0 m
0 0.5 1 1.5 2 2.5 3 3.5 4
smooth area rough area
Ra
(µm)
3 m 6 m 0 m
Table 3. Average roughness Raof the spacer’s surface
Spacer Average roughness Ra(µm)
Rough areas Smooth areas
New Spacer-K (Fem Comp) 0.97 –
New Spacer-K (Tib Comp) 1.25 –
Spacer-K 5M (Fem Comp) 1.52 0.35
Spacer-K 5M (Tib Comp) 1.52 0.24
Spacer-K 6M (Fem Comp) 1.31 0.69
Spacer-K 6M (Tib Comp) 1.31 0.66
New Spacer-G 3.37 –
Spacer-G 3M 3.55 0.21
Spacer-G 6M 3.45 0.48
Fig. 9. Average rough- ness Raof the smooth and rough areas of the Spacer-K tibial component. For com- parison the average roughness of the blank spacer surface is reported
Fig. 10. Average roughness Raof the smooth and rough areas of the Spacer-K femoral component.
For comparison the average roughness of the blank spacer sur- face is reported
Fig. 11. Average roughness Raof the smooth and rough areas of the Spacer-G head. For comparison the average roughness of the blank spacer surface is reported
Table 4. Molecular weights of the mate- rial of the new Spacer- K and after 6 months of implantation
Spacer-K Mw Mn DPM
New 35,6890 66,318 5.381
Implanted for 6 M 23,5856 41,992 5.616
and on the tibial one (R
a= 1.25 µm). It can be noted that the shiny areas found are actually less rough than the opaque ones. This difference is even more noticeable in the Spacer-G rather than the Spacer-K. The former varies between a maximum roughness of over 3 µm to a minimum of less than 0.5 µm, while the latter has a differ- ence of barely 1 µm. Even if there have been difficulties in evaluating the roughness of the femoral component of Spacer-K devices, there is no substantial difference between the wearing of the shinbone plate and the thighbone component. The differ- ences between roughness values measured on glossy areas and rough ones for the explanted devices are 3.34 µm and 2.97 µm for Spacer-G respectively after 3 and 6 months of in vivo functioning. The analogous differences are 1.28 µm and 0.65 µm for Spacer-K tibial component respectively after 5 and 6 months of in vivo functioning.
Molecular Weight
In order to obtain an average evaluation of the cement’s deterioration conditions, the molecular weights of the new Spacer-K cement was compared to the one of the Spacer-K cement explanted after 6 months.
The values obtained were indicated in Table 4. The M
w(weight-average molecular weight) decreased from 356,890 to 235,856 and the M
n(number-average molecular weight) from 66,318 g/mol to 41,992. The polydispersity index (DPM) increased from 5.381 to 5.616.
Discussion
Analyzing the observation results summarized in Table 2 some consideration can be drawn.
From the macroscopical point of view the opaque appearance of the new spacers surfaces allowed to identify more easily the worn areas on the spacers ex vivo. In fact it was observed that the areas that came in contact during the in vivo functioning appear glossy. This aspect can be usefully utilized to evaluate the positioning and the loading conditions of the device. On the spacer-K explanted after 5 months the shiny part extended to almost the whole area covering the two surfaces that were in contact with the thighbone, including an area on the surface of the central part in relief, which indicates an imperfect matching between the two components but, in any case, a good distribution of the load on the surface. In the Spacer-K explanted after 6 months, the different location of the shiny areas at the edges of the components indicates that the surfaces that were in contact were less extensive and therefore the load was concen- trated on a limited surface of the spacer.
The smoothing of the contact areas observed on the heads’ surfaces of the Spacer-
G devices and the consequent disappearing of the mechanical workmanship that was
identified on the rough surface suggests that a large amount of cement could be worn out during the in vivo functioning. The presence of the glossy areas developed along a circular crown towards the side of the head on both the spacers-G shows that the spacer was partly loaded and surely articulating, even if it was not able to support the natural load of a thighbone. The glossy areas extension and the different location on the head and with respect to the stem observed on the two explanted devices suggests a different matching with the acetabulum.
Microscopical observations, carried out by SEM, have not highlighted any detail that shows different wearing conditions of the surfaces of spacers explanted after dif- ferent functioning times. A slight difference was found on the Spacer-K, which was explanted after 5 months with respect to the same type of spacer explanted after 6 months. It consists in the existence of fewer microcracks which, however, could also be caused by the material’s different load conditions as argued also by macroscopical observations. A greater difference can be seen between knee and hip spacers. The lat- ter actually show a more consistent smoothing out effect of the prepolymer beads and a reduction in the radiopaque areas. This fact suggests that if the powder compound based on radiopacifying agents is associated to the antibiotic, the worn out areas are less efficient in releasing the drug, since it is moved or absorbed more deeply by the cement and therefore less available. This adds to the evident wear, even if it is distrib- uted to limited areas of contact, with extirpation and consequent release of waste that can increase the inflammatory response thus reducing the device’s efficacy. The pres- ence of cracks can increase this phenomenon since these can lead to the breakage of small fragments of cement.
Roughness measurements confirmed the microscopical observation results. In fact the extension in time of the device in vivo functioning does not seem to be high- lighted by roughness measurements. If we consider, for example, the two Spacer-G devices explanted after 3 and 6 months you cannot notice a substantial difference in roughness values of the shiny areas. However, apart from not knowing the exact func- tioning conditions that the spacers were submitted to during the time they were implanted, it should also be noted that roughness measurements on the spacer head after 6 months were not taken in the area that appeared shinier after a macroscopical observation. This is because the position could not be reached by the analysis system.
The major roughness values difference between implanted and new devices has been revealed for spacer-G. One should keep in mind, though, that even if the spacers were completely loaded, the weight on the knee is not as heavy as that on the hip joint.
Moreover the spacer-G head surfaces are rougher than the spacer-K ones and rougher surface wears out more easily.
Another effect of the in vivo functioning of the spacer has been highlighted by the
polymer molecular weight measurements. Results show that the polymer, which
spacer-K is made of, has suffered from deterioration during the 6 months of in vivo
functioning. In actual fact, apart from the reduction of the M
w(weight-average
molecular weight), and the M
n(number-average molecular weight), there was also an
increase in the DPM (polydispersity index). This shows there has been a degradation
of part of the molecules with the formation of smaller molecules.
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