J A Travieso-Rodríguez

G Dessein

H A González-Rojas

Improving the Surface Finish of Concave and Convex Surfaces Using a Ball Burnishing Process

Keywords: Burnishing, Manufacturing, Roughness, Surface

The ball burnishing process is done to improve the surface finish of workpieces that have been previously machined. In this article we present the results of tests performed with this process that was applied to workpieces with a convex or concave surface of two different materials: aluminum A92017 and steel G10380. An experiment to do the tests was designed. The results of measurements of surface roughness are presented in this paper as well. These results are compared to those measured in the workpieces before being burnished. After that conclusions are drawn about the improvement of surface roughness applied to the workpieces through the ball burnishing process. The main innovation of this paper is that we work with concave and convex geometries. We also obtain a table of recomended parameter values for the process.

Introduction

A good surface finish on a complex surface geometry, for instance, any part of a three-dimensional (3D) mold or die is a very difficult problem to be solved. A flat or revolution surface can be produced relatively easily in a grinding machine; thereby, improving its finish is relatively simple. When the development surface is complex, improving its quality is not that simple and that is when this process is a problem.

In this case we have developed a study to improve the surface finish of concave and convex configurations through a ball burnishing process developed in the same milling machine in which the workpiece in question has been developed.

Through a ball burnishing operation complex configuration surfaces could be machined to obtain a good surface finish on them, according to Yen et al. [START_REF] Yen | Finite element modelling of roller burnishing process[END_REF]. As shown in Fig. 1, this process is developed using a tool that is mounted on a hydraulic head, which will apply some pressure to a ball. When this ball glides on the workpiece area, it deforms the peaks of the surface irregularities, flattening the surface profile, and producing a much more regular surface than the one that the workpiece had before.

To perform this study, a series of experiments will de developed. The system parameters will vary between three values. The impact of this variation on the results of measured surface roughness is obtained, as well as their best values.

A ball burnishing process is recommended because the tool can be easily installed on the same computer numerical control (CNC) machine. The ball can have diameters between 3 and 12 mm, and it operates under the action of a normal force high enough to deform the peaks of the surface profile to be treated. The ball is in contact with the surface just for burnishing it, but it can freely rotate on itself, because the values of friction forces are very small. As it happens in the cutting process, plastic deformation is produced on the entire surface because the tool is constantly impacting on the workpiece according to Roettger [START_REF] Roettger | Rolling on hard turned surfaces[END_REF], Klocke and Liermann [START_REF] Klocke | Roller burnishing of hard turned surfaces[END_REF], Prevey et al. [START_REF] Prevey | Case studies of fatigue life improvement using low plasticity burnishing in gas turbine engine applications[END_REF], Nemat and Lyons [START_REF] Nemat | An investigation of the surface topography of ball burnished mild steel and aluminium[END_REF], and Yen and Altan [START_REF] Yen | Finite Element Modelling of Ball Burnishing-Prediction of Surface Deformation and Residual Stress[END_REF].

The ball burnishing is considered by Adel and Sulieman [START_REF] Hassan | Improvement in the wear resistance of brass components by the ball burnishing process[END_REF] as a cold working process, which can be used to improve the surface characteristics. According to these authors, most papers which have been published before refer to the effects of burnishing process on surface roughness and hardness. But they considered that it has not been worked enough to prove it. That is why their work was focused on evaluating the increased wear resistance when a burnishing operation is performed.

The same results of previous authors were obtained by Adel and Ayman [START_REF] Hassan | The effects of initial burnishing parameters on non-ferrous components[END_REF] by applying different forces to bronze workpieces with different initial roughness. The force increase reduced the roughness of the burnished surface.

A study of the burnishing process in hard steel workpieces crafted in a lathe machine was developed by Luca [START_REF] Luca | Investigation into the Use of Ball Burnishing of Hardened Steel Components as a Finishing Process[END_REF]. Finite elements method (FEM) was used to develop models to predict the surface behavior in terms of surface finish after the burnishing process.

Four burnishing parameters, such as ball material, lateral pass width, force, and tool feed-rate, were selected by Fang-Jung and Chien-Hua [START_REF] Shiou | Freeform surface finish of plastic injection mould by using ball-burnishing process[END_REF] as factors of Taguchi experimental design to determine the optimal burnishing parameters which have the main influence on surface roughness. The optimal values of burnishing parameters were obtained after performing the experiments. The parameters values were: the tungsten carbide ball, the 200 mm/min feed-rate, the 300 N burnishing force, and the 40 m depth of burnishing. The surface roughness Ra of the sample was improved by about 1 to 0.07 m by using the optimal parameters for flat burnishing.

One year later, Yen et al. [START_REF] Yen | Finite element modelling of roller burnishing process[END_REF] developed a research that focused on the processes of surface finish in a lathe machine. First, the conventional finish through a lathing process and second through a burnishing process. The most critical aspects that determine the surface final state and its properties such as residual stress, microstructure, and microhardness, were obtained. He also developed two models, two-dimensional (2D) and 3D to optimize the selection of burnishing process parameters (ball diameter, pressure, feed-rate), by using FEM, which were subsequently evaluated with experimental data.

More recently Hamadache et al. [START_REF] Hamadache | Characteristics of Rb40 steel superficial layer under ball and roller burnishing[END_REF] shown that the hardness increase is reduced with an increase of the ball rotation frequency and feed-rate in the burnishing process, while the burnishing force as well as the number of passes increase the hardness.

As a last reference recently found, Celaya et al. [START_REF] Celaya | Finite element model of burnishing[END_REF] published a paper that is of great interest to develop a model of the burnishing process to obtain the required results in terms of surface finish and residual stresses in workpieces. This model can help to evaluate the effect of different operating parameters in the burnishing process. In this case, they evaluate a burnishing process of a revolution surface, but taking into account the parallel plane to the cylinder axis.

Most of the research papers, as it can be seen, focused on experimental tests in flat or revolution workpieces. Of course, the burnishing process is shown to be effective to improve the surface quality of these kinds of workpieces. The main advantage is the fact that it is performed in the same machine-tool in which the workpiece has been machined without removing it but avoiding the centering problems that occur if it is necessary to change the workpiece from one machine to another. It also helps to reduce manufacturing. In addition, in complex surfaces, the burnishing process can be convenient because normally their surface finish is only performed manually and in difficult conditions. For this reason, the main purpose and innovation of this article is to analyze how the process parameters influence on the surface quality of burnished convex and concave surfaces of two different materials (aluminium A92017 and steel G10380) and also to make recommendations about their optimal values to be used in this process.

Experiments

Different experiments were performed on workpieces with convex and concave surfaces of two different materials: the aluminium A92017 and the steel G10380. The workpieces have the shape of Fig. 2. Convex or concave surfaces of the workpieces are composed of three areas in which there are three curves of 50, 100, and 50 mm of diameter, respectively. These curves have been machined by using different spherical mills with different diameters, but as a starting condition for each milling operation, the resulting peak height has been considered as a constant parameter.

On each milled area, the burnishing operation is performed. This operation is done every time with three different feed-rates. It means that in each burnished area we can find three subareas in which the difference lies in the values of feed-rates used.

In this case, the system parameters or variables to be evaluated are: the curvature radius of the burnished surface r, the feed-rate of the burnishing tool a, and the machining strategy that changes when the burnishing direction is changed, D. This direction can be parallel (Par) or perpendicular (Per) to the direction of the feed-rate of the milling process.

A 2 3 experimental design is obtained through these evaluation parameters, it means 8 combinations with two replicates, which makes a total of 16 different experiments in each case. The results of variable are shown in Tables 1 to 4. Regarding the results of the measurements four indicators of surface roughness have been taken into account: Ra (average surface roughness) and Rt (peakvalley maximal surface roughness), and both in the parallel direction of tool feed-rate (Ra // and Rt //) and in the perpendicular direction to the tool feed-rate (Ra ⊥ and Rt⊥). Four equations are obtained, each one corresponding to each measured value.

Result Analysis of Surface Roughness Measurements

In the result tables (Tables 1 to 4), we can see that the value of the surface roughness decreases compared to the surface roughness of the prior milling operation in case of convex aluminium workpieces. Ra decreases by 59% in the parallel measurements to the milling feed-rate and 63% in perpendicular measurements. Rt decreases by 52% and 62% in the parallel and perpendicular measurements, For each evaluated workpiece a regression equation can be obtained in form [START_REF] Yen | Finite element modelling of roller burnishing process[END_REF]. Each equation allows to determine the value of the surface roughness index (Ra //, Rt //, Ra ⊥, and Rt ⊥). With the MINITAB experimental design software, the coefficients of each regression Eq. ( 1) can be obtained.

First, these regression equations are usually obtained from a general perspective to analyze the significance degree of each evaluated parameter to the measured roughness index. After the experiment is analyzed again by just taking into account the significant parameters. A second curve is obtained fitted to each particular case. Hence, the coefficients for each regression equation vary in a second analysis. The coefficients for each regression equation in each case are shown in Table 5, as well as the adjusted R-squared parameter (R-Sq adj), which indicates the validity level of each experiment:

R aot = Constant + C1r + C2a + C3D + C4ra + C5rD + C6aD + C7raD (1)
The result analysis is based upon a Pareto chart as shown in Figs. 3 and4. We can observe the significant parameters for the result measurements. The measurements were performed according to a 95% confidence level.

Comments on Experimental Results

In a convex surface with a radius between 50 and 100 mm of aluminium A92017, the burnishing process helps to improve the surface roughness of the workpieces tested. The surface curvature radius is the parameter that most strongly determines the final surface roughness. However, in the perpendicular direction to the previous milling feed rate, if the roughness is measured, the tool feed rate is the most significant parameter. This means that the feed rate is very important for flat surfaces, but in curved surfaces the curvature radius definitely determines the surface quality. We have to keep in mind that this happens when we use a constant burnishing force. If the force is not constant, the results can be certainly different.

If a surface of 100 mm radius needs to be burnished, it is highly recommended to do so before a perpendicular milling process. It means drawing straight lines with the burnishing tool, and thus the process develops as if on a flat surface. It is better to burnish a flat surface than a curved surface. In Fig. 5, to summarize, we can see the influence of the experimental parameter values on the index of measured surface roughness.

In a convex surface with a radius between 50 and 100 mm of steel G10380, the burnishing process also helps to improve the surface roughness of the workpieces tested. The burnishing direction is the most significant parameter for this material. Unlike aluminium, the feed rate is not significant in the parallel direction to the feed rate of the prior milling, but it affects the measurements in the parallel direction.

The best roughness values are also obtained on the surface of smaller radius and in case of higher feed rates. In the case of the direction of burnishing, the measurements in the parallel direction to the milling feed rates are smaller for the perpendicular burnishing and in the perpendicular direction to the milling process. The lower roughness values were obtained in the burnishing parallel to the milling feed rates. In the summary graph of Fig. 6, previous comments are shown.

In a concave surface with a radius between 50 and 100 mm of aluminium A92017, the ball burnishing process is also suitable for improving surface roughness of the workpieces tested. The direction in which the ball burnishing process has been performed is the parameter that generally affects more the index results of surface roughness in this experiment. The curvature radius also influences this index, obtaining better results with a radius of 100 mm. This is the opposite to what happened in the case of convex workpieces (Fig. 5), where the best results were obtained in surfaces with a radius of 50 mm.

The feed rates also play a role in this case, which has also happened previously in experiments with workpieces of the same material. In the summary graph of Fig. 7, previous comments are shown.

Finally, for a concave surface with a radius between 50 and 100 mm, of steel G10380, successful results were also obtained. The burnishing direction and the curvature radius are the most significant parameters in this experiment. These results are quite similar to those obtained with convex workpieces of the same material. The feed rate is not significant for this material.

The best values of roughness were obtained with a radius of 50 mm in the parallel direction to the milling feed rate and with a radius of 100 mm, in the perpendicular direction to the milling feed rate. In Fig. 8, the previous comments are shown.

Regarding experiments with concave workpieces of aluminium A92017, there are two differences. The first difference is the influence of feed rate on aluminium as discussed above. The second one is that the burnishing direction in the Ra and Rt measurements made in the perpendicular direction to the milling feed rate has an opposite effect compared to an experiment with steel.

On the other hand, if we compare these results with those obtained for convex workpieces of steel G10380, the difference is that the best results for Ra and Rt measured in the parallel direction to the feed rate on the convex surfaces are more suitable for burnishing done in the perpendicular direction to the feed rate. On the concave surfaces, the best results are more suitable for burnishing performed in the parallel direction to the feed rate. In the case of concave surfaces, the best results in Ra and Rt measurements in the perpendicular direction to milling feed rate are for surfaces with a radius of 100 mm and in the convex workpieces for surfaces with a radius of 50 mm. The last difference was also found between the concave and convex workpieces of aluminium A92017.

Recommendations on Parameter Values

Experiments performed allow us to draw interesting conclusions about the development of future research papers on ball burnishing with concave and convex surfaces using tested materials. These recommendations are summarized in Tables 6 and7.

Conclusions

Once this article was finished, we could verify that the ball burnishing process is effective to improve the surface finish of workpieces of different materials and geometric configurations with a certain level of complexity. The following conclusions can be drawn:

1. The ball burnishing process can be used to improve the finish of complex surfaces. 2. For aluminium A92017, workpieces with concave and convex surfaces have been used. The main parameter value that affects the surface roughness is the curvature radius, achieving better results with a smaller radius in convex surfaces and with a bigger radius in concave surfaces. The direction of the burnishing process related to the milling feed rate is also relevant in curved surfaces.

3. In steel 1038 workpieces with convex or concave surfaces were evaluated as well. The prior peak height produced by the milling process is the parameter value that affects more the indexes of surface roughness. The feed rate has no influence whatsoever, because this material has a high coefficient of self-hardening. The burnishing direction and the curvature radius are also significant parameters.

Figure 1 .

 1 Figure 1.-Schematic representation of the ball burnishing process.

Figure 2 .

 2 Figure 2.-Workpieces used for testing surface roughness after burnishing.

Figure 3 .

 3 Figure 3.-Pareto chart for standardized effects on aluminium A-92017 convex workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Figure 4 .

 4 Figure 4.-Pareto chart for standardized effects on steel G-10380 convex workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Figure 5 .

 5 Figure 5.-Summary graph of the influence of experimental parameters on the index of measured surface roughness in convex aluminium A92017 workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Figure 6 .

 6 Figure 6.-Summary graph of the influence of experimental parameters on the index of measured surface roughness in convex steel G10380 workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Figure 7 .

 7 Figure 7.-Summary graph of the influence of experimental parameters on the index of measured surface roughness in concave aluminium A92017 workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Figure 8 .

 8 Figure 8.-Summary graph of the influence of experimental parameters on the index of measured surface roughness in concave steel G10380 workpieces: (A) Ra //, (B) Rt //, (C) Ra ⊥, and (D) Rt ⊥.

Table 1 .

 1 -Results of roughness measurements in convex aluminium workpieces.

	Exp. number	r (mm)	a (mm/min)	D	R a// ( m)	Rt // ( m)	Ra ⊥ ( m)	Rt ⊥ ( m)
	1	50	200	Par	0.0712	1.2291	0.5444	2.4399
	2	100	200	Par	0.3441	3.8418	0.3081	1.7671
	3	50	500	Par	0.0925	1.1649	0.4573	2.0082
	4	100	500	Par	0.1619	2.5495	0.2925	1.5053
	5	50	200	Per	0.1353	1.4762	0.3875	2.2829
	6	100	200	Per	0.1987	1.8568	1.1473	4.6075
	7	50	500	Per	0.1921	1.2336	0.0926	0.7101
	8	100	500	Per	0.1933	1.3900	0.1511	0.9595
	9	50	200	Par	0.0657	0.7250	0.3549	2.0802
	10	100	200	Par	0.3511	4.2815	0.5121	1.9860
	11	50	500	Par	0.0658	0.7088	0.3132	1.4091
	12	100	500	Par	0.4283	4.2474	0.3698	1.9344
	13	50	200	Per	0.1676	1.3173	0.2013	0.9631
	14	100	200	Per	0.6284	3.8996	0.4462	3.2392
	15	50	500	Per	0.1370	1.0670	0.0796	0.4902
	16	100	500	Per	0.3112	4.1647	0.1221	1.1243
	Milling				0.8379	5.2101	1.6741	6.7833

Table 2 .

 2 -Results of roughness measurements in convex steel workpieces.

	Exp. number	r (mm)	a (mm/min)	D	R a// ( m)	Rt // ( m)	Ra ⊥ ( m)	Rt ⊥ ( m)
	1	50	200	Par	0.1589	3.2426	0.5230	2.0223
	2	100	200	Par	0.8213	6.3732	0.5319	2.2973
	3	50	500	Par	0.2195	3.2720	0.7963	3.5370
	4	100	500	Par	0.5536	5.3222	0.8184	3.5319
	5	50	200	Per	0.1034	0.7153	0.8283	5.4373
	6	100	200	Per	0.3355	2.1932	1.3076	7.0976
	7	50	500	Per	0.1116	0.7104	0.4285	2.4230
	8	100	500	Per	0.1803	1.4868	0.5155	2.1726
	9	50	200	Par	0.1631	3.4265	0.6241	2.3306
	10	100	200	Par	0.6161	4.5065	0.8296	3.3200
	11	50	500	Par	0.3274	6.4135	0.5770	2.5576
	12	100	500	Par	0.7527	5.2415	0.6645	2.8229
	13	50	200	Per	0.0914	0.5542	0.5985	2.5478
	14	100	200	Per	0.1620	1.1320	0.9259	5.1955
	15	50	500	Per	0.0887	0.5610	0.4047	2.3786
	16	100	500	Per	0.1883	1.1186	0.5030	2.6436
	Milling				0.7132	4.8651	1.6760	7.6036

Table 3 .

 3 -Results of roughness measurements in concave aluminium workpieces.

	Exp. number	r (mm)	a (mm/min)	D	R a// ( m)	Rt // ( m)	Ra ⊥ ( m)	Rt ⊥ ( m)
	1	50	200	Par	0.1506	1.0582	0.1827	1.0502
	2	100	200	Par	0.0964	1.2843	0.1483	0.7812
	3	50	500	Par	0.1243	0.6205	0.1570	0.9910
	4	100	500	Par	0.0847	0.8121	0.2090	1.1357
	5	50	200	Per	0.2217	1.4248	0.1326	1.4239
	6	100	200	Per	0.2179	1.3444	0.0594	0.8050
	7	50	500	Per	0.1918	1.2466	0.0431	0.3916
	8	100	500	Per	0.1307	1.2764	0.0527	0.9787
	9	50	200	Par	0.1731	1.5564	0.1290	1.0432
	10	100	200	Par	0.0906	0.9665	0.1327	0.9322
	11	50	500	Par	0.1151	0.5554	0.1300	1.0337
	12	100	500	Par	0.0953	0.8047	0.1554	1.1627
	13	50	200	Per	0.2564	1.6647	0.1540	0.8191
	14	100	200	Per	0.1743	1.3073	0.0658	0.5035
	15	50	500	Per	0.1306	0.8300	0.0744	0.4634
	16	100	500	Per	0.1566	1.2015	0.0503	0.9324
	Milling				0.2557	1.8200	0.8215	3.6271

Table 4 .

 4 -Results of roughness measurements in concave steel workpieces.

	Exp. number	r (mm)	a (mm/min)	D	R a// ( m)	Rt // ( m)	Ra ⊥ ( m)	Rt ⊥ ( m)
	1	50	200	Par	0.1388	1.0509	0.3179	1.4426
	2	100	200	Par	0.0979	1.0745	0.1530	0.9431
	3	50	500	Par	0.0548	0.3958	0.3552	1.5525
	4	100	500	Par	0.0851	0.7004	0.1680	1.0024
	5	50	200	Per	0.0855	1.0533	0.3253	2.5111
	6	100	200	Per	0.1856	1.7346	0.2663	2.1591
	7	50	500	Per	0.0509	0.4388	0.2317	1.9140
	8	100	500	Per	0.1327	1.0554	0.2370	2.2532
	9	50	200	Par	0.1375	1.0382	0.2996	1.4572
	10	100	200	Par	0.1063	1.0487	0.1695	0.6175
	11	50	500	Par	0.0669	0.3769	0.3674	1.2996
	12	100	500	Par	0.0869	0.9649	0.1504	0.9272
	13	50	200	Per	0.0912	1.1307	0.3412	2.8130
	14	100	200	Per	0.1504	1.6431	0.2853	2.0080
	15	50	500	Per	0.0595	0.4623	0.2389	1.8170
	16	100	500	Per	0.3451	1.7514	0.3223	2.5052
	Milling				0.4281	3.0103	0.9316	4.5837

Table 5 .

 5 -Obtained coefficients for the regression curves of each index of surface roughness in each experiment.

	Term	Ra //	Rt //	Ra ⊥	Rt ⊥
			Aluminium A92017 convex piece
	Constant	-0 0953	-1 0484	0.076058	1.01988
	C1	0.0042	0.0433	0.00773733	0.0289278
	C2	0	0	0.00032283	-0 00018392
	C3	0	0	-0 446375	-2 25440
	C4	0	0	-1 55467 * 10 -5	-4 87967 * 10 -5
	C5	0	0	0.00833433	0.0418605
	C6	0	0	0.000489	0.00296975
	C7	0	0	-1 45767 * 10 -5	-7 511 * 10 -5
	R-Sq adj (%)	70.49	80.63	39.57	73.74
			Steel G10380 convex piece
	Constant	-0 135225	1.30213	0.646113	3.36647
	C1	0.0058645	0.0211962	0.00329	0.0146180
	C2	0	0	-6 0875 * 10 -4	-0 00340883
	C3	0.11625	-1 51433	0.323646	2.38347
	C 4	0	0	0	0
	C5	-0 0035095	-0 00424775	0	0
	C6	0	0	-8 98417 * 10 -4	-0 00547483
	C 7	0	0	0	0
	R-Sq adj (%)	88.39	82.07	55.04	54.5
			Aluminium A92017 concave piece
	Constant	0.261406	1.59744	0.2724	2.09622
	C1	-7 9275 * 10 -4	0	-0 0018105	-0 0153868
	C2	-1 46625 * 10 -4	-0 00135808	-3 74 * 10 -4	-0 0034175
	C3	0.0343688	0.035642	0.0399583	0.618725
	C4	0	0	4 25 * 10 -6	4 40717 * 10 -5
	C5	0	0	-5 565 * 10 -4	-0 00722883
	C6	0	0.000369167	-1 04167 * 10 -4	-0 0022135
	C 7	0	0	0	2 22817 * 10 -5
	R-Sq adj (%)	77.21	63.93	82.84	58.76
			Steel G10380 concave piece
	Constant	0.022525	0.769198	0.400325	3.38701
	C1	0.00126225	0.0100652	-0 0018135	-0 0211513
	C2	0	-0 00151171	0	-0 00353492
	C3	-0 082425	-0 243675	-0 1095	1.11271
	C 4	0	0	0	4 33517 * 10 -5
	C5	0.00137125	0.00543175	0.0016825	-0 00498133
	C 6	0	0	0	-0 00276092
	C 7	0	0	0	2 94617 * 10 -5
	R-Sq adj (%)	42.13	79.95	74.44	94.65

Table 6 .

 6 -Recommendations on burnishing process parameters to be used with convex workpieces.

				A92017		AISI 1038
		Material
	Parameter	Indicator	IL	Optimal value	IL	Optimal value
	Feed	Ra //	NS		NS
		Rt //
		Ra per	HS	500 mm/s	HS	500 mm/s
		Rt per			IS
	Surface radius	Ra //	HS	50 mm	HS	50 mm
		Rt //			S
		Ra per	IS		IS
		Rt per
	Burnishing	Ra //	NS		HS	Perpendicular
	direction					to milling
		Rt //
		Ra per	IS	Perpendicular	NS
				to milling
		Rt per			S	Parallel
						to milling
	IL = Importance level;
	HS = High significant;
	S = Significant;
	IS = Not very significant or significant in combination with another parameter;
	NS = Nonsignificant.

Table 7 .

 7 -Recommendations on burnishing process parameters to be used with concave workpieces. Not very significant or significant in combination with another parameter; NS = Nonsignificant.

				A92017		AISI 1038
		Material
	Parameter	Indicator	IL	Optimal value	IL	Optimal value
	Feed	Ra //	S	500 mm/s	NS
		Rt //	HS		S	500 mm/s
		Ra per	IS		NS
		Rt per
	Surface radius	Ra //	S	100 mm	S	50 mm
		Rt //	NS		HS
		Ra per	IS	100 mm		100 mm
		Rt per	NS		S
	Burnishing	Ra //	HS	Perpendicular	IS	Parallel
	direction			to milling		to milling
		Rt //		Parallel	S
				to milling
		Ra per		Perpendicular	IS
				to milling
		Rt per	S		HS
	IL = Importance level
	HS = High significant;
	S = Significant;
	IS =

Acknowledgments

This research article is the result of a collaboration between the UPC TECNOFAB group (Catalonia, Spain) and the LGV group from ENIT (Tarbes, France). Thanks to all the colleagues who helped us in this work.


Philippe Le Corvoisier
email: philippe.lecorvoisier@hmn.aphp.fr

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MD, PhD Emmanuel Teiger

Ambulatory transradial percutaneous coronary intervention: a safe, effective and cost-saving strategy

Keywords: percutaneous coronary intervention, radial approach, ambulatory procedure, safety, economic evaluation CPK: creatine phosphokinase DRG: diagnosis-related group ECG: electrocardiogram MACCE: major adverse cardiac or cerebrovascular events PCI: percutaneous coronary intervention

Objectives:

The aim of this prospective, multicenter study was to assess the safety, feasibility, acceptance, and cost of ambulatory transradial percutaneous coronary intervention (PCI) under the conditions of everyday practice. Background: Major advances in PCI techniques have considerably reduced the incidence of post-procedure complications.

However, overnight admission still constitutes the standard of care in most interventional cardiology centers. Methods: Eligibility for ambulatory management was assessed in 370 patients with stable angina referred to three high-volume angioplasty centers. Based on prespecified clinical and PCI-linked criteria, 220 patients were selected for ambulatory PCI.

Results:

The study population included a substantial proportion of patients with complex procedures: 115 (52.3%) patients with multivessel coronary artery disease, 50 (22.7%) patients with multilesion procedures, and 60 (21.5%) bifurcation lesions. After a 4 to 6 hours observation period, 213 of the 220 patients (96.8%) were cleared for discharge. The remaining 7 (3.2%) patients were kept overnight for unstable angina (n=1), atypical chest discomfort (n=2), puncture site hematoma (n=1), or non-cardiovascular reasons (n=3).

Within 24 hours after discharge, no patients experienced readmission, stent occlusion, recurrent ischemia, or local complications. Furthermore, 99% of patients were satisfied with ambulatory management and 85% reported no anxiety. The average non-procedural cost was lower for ambulatory PCI than conventional PCI (1,230±98 Euros vs. 2,304±1814 Euros, p<10 -6 ). Conclusions: Ambulatory PCI in patients with stable coronary artery disease is safe, effective, and well accepted by the patients. It may both significantly reduce costs and optimize hospital resource utilization.

INTRODUCTION

The number of percutaneous coronary interventions (PCIs) has increased steadily in recent years [START_REF] Cook | Percutaneous coronary interventions in Europe: prevalence, numerical estimates, and projections based on data up to 2004[END_REF][START_REF] Togni | Percutaneous coronary interventions in Europe 1992-2001[END_REF]. In many interventional cardiology centers, admissions for PCI represent a significant burden on available resources. Shortening the hospital stay required for PCI would be expected to decrease costs and to optimize healthcare resource utilization. Diagnostic coronary angiography is routinely performed on an outpatient basis with no increase in complication rates compared to inpatient coronary angiography [START_REF] Block | A prospective randomized trial of outpatient versus inpatient cardiac catheterization[END_REF]. By contrast, most interventional centers keep patients overnight for observation after PCI (4). This practice reflects concern about the risk of subacute stent thrombosis and vascular complications. However, overnight observation may not be necessary in all patients. Dual antiplatelet therapy has markedly decreased the incidence of stent thrombosis [START_REF] Moussa | Effectiveness of clopidogrel and aspirin versus ticlopidine and aspirin in preventing stent thrombosis after coronary stent implantation[END_REF][START_REF] Schomig | A randomized comparison of antiplatelet and anticoagulant therapy after the placement of coronaryartery stents[END_REF]. Furthermore, subacute stent thrombosis usually occurs several days after the procedure and, therefore, patient outcomes are not modified by 24 hours of observation instead of same-day discharge [START_REF] Daemen | Early and late coronary stent thrombosis of sirolimus-eluting and paclitaxel-eluting stents in routine clinical practice: data from a large two-institutional cohort study[END_REF].

The safety of same-day discharge after PCI needs to be clearly established before this method can be applied in daily practice. To date, few large-scale prospective studies have evaluated the feasibility and safety of this new patient management strategy [START_REF] Banning | Same day discharge following elective percutaneous coronary intervention in patients with stable angina[END_REF][START_REF] Bertrand | A randomized study comparing same-day home discharge and abciximab bolus only to overnight hospitalization and abciximab bolus and infusion after transradial coronary stent implantation[END_REF][START_REF] Heyde | Randomized trial comparing same-day discharge with overnight hospital stay after percutaneous coronary intervention: results of the Elective PCI in Outpatient Study (EPOS)[END_REF][START_REF] Koch | Triage of patients for short term observation after elective coronary angioplasty[END_REF][START_REF] Slagboom | Outpatient coronary angioplasty: feasible and safe[END_REF]. In most of these previous studies, patients were discharged on the day of PCI, but many of them were hospitalized at least the day before the procedure for pre-procedure assessment or transferred from another hospital. Our study was one of the first in which patients were admitted and discharged on the day of PCI. The aim of this multicenter prospective study was to assess the safety, cost, and acceptability of transradial PCI performed on an ambulatory basis.

MATERIALS AND METHODS

Study design

The study protocol conformed to the ethical guidelines of the Declaration of Helsinki and was approved by the institutional review board of our institution. A written informed consent was obtained from each patient. This prospective, multicenter, interventional study was conducted from June 2007 to February 2009 in three high-volume angioplasty centers in France. Eligibility for ambulatory management was assessed in 370 consecutive patients with stable angina and/or silent myocardial ischemia who underwent coronary angiography and ad hoc PCI after admission in a day hospitalization unit. The patients were evaluated for same-day discharge at the end of the procedure, based on pre-specified clinical, social, and PCI-related criteria (Figure 1). Two hundred-twenty patients were selected for ambulatory management and kept under observation for 4-to 6-hours in a day hospitalization unit (Figure 2).

Pharmacological treatment

All patients were pre-treated with aspirin (≥75 mg/day) for more than 3 days. A 600mg clopidogrel loading dose was administered to patients, including those already on maintenance clopidogrel treatment (75 mg/day). During the procedure, anticoagulation was achieved with a single subcutaneous injection of enoxaparin at admission or with a bolus of unfractionated heparin (70 IU/kg) injected immediately after insertion of the arterial sheath.

After PCI, patients were maintained on aspirin indefinitely and on clopidogrel according to current guidelines for bare metal stents and drug-eluting stents [START_REF] Silber | Guidelines for percutaneous coronary interventions. The Task Force for Percutaneous Coronary Interventions of the European Society of Cardiology[END_REF]. Other cardiac medications were continued as clinically indicated.

PCI procedure

The sheath was inserted in the radial artery under local anesthesia using the standard Seldinger technique. A vasodilating mixture of verapamil (2.5 mg) and isosorbide dinitrate (1 mg) was administered intra-arterially to prevent radial artery spasm. Coronary stenting was performed using standard techniques and according to international guidelines [START_REF] Silber | Guidelines for percutaneous coronary interventions. The Task Force for Percutaneous Coronary Interventions of the European Society of Cardiology[END_REF]. The choice of the PCI strategy was at the operator"s discretion. The arterial sheath was removed immediately after the end of the intervention. Hemostasis was achieved using the TR Band ™ radial compression device (Terumo, Somerset, NJ, USA).

Triage and discharge

At the end of the procedure, patients were transferred to the day hospitalization unit and were allowed to walk freely after 2 hours of bed rest. Then, 4 to 6 hours after PCI, an electrocardiogram and clinical examination were systematically performed by a cardiologist [START_REF] Koch | Triage of patients for short term observation after elective coronary angioplasty[END_REF]. Patients were cleared for discharge if they had no puncture site complications, chest pain, or electrocardiographic changes during the observation period. Patients were instructed to contact their cardiologist or the coronary intensive care unit of the hospital where PCI was performed in the event of complications during the following night.

Follow-up

On the day after PCI, a phone interview was conducted by a cardiologist to record all post-procedural complications including re-admissions, chest pain, and puncture site complications. Patient satisfaction and degree of anxiety were evaluated using standardized questionnaires (see below). Patients were asked if they visited their general practitioner or cardiologist for any of the above-mentioned reasons. Serum troponin I and creatine phosphokinase (CPK) levels were measured 18 to 24 hours after PCI in outpatient laboratories and the results were faxed to the investigators. A second phone interview was conducted by a physician 1 month after PCI to assess the incidence of emergency readmission, angina, dyspnea, and puncture site complications.

Patient comfort and satisfaction

Patient comfort and satisfaction were evaluated during the phone interview on the day after PCI using a simple questionnaire that offered three response options for satisfaction ("very satisfied", "satisfied", and "not satisfied") ( 14), three for anxiety ("not anxious", "anxious", and "very anxious"), and two for willingness to accept outpatient PCI in the future ("yes" and "no") [START_REF] Khatri | Safety and cost benefit of same-day discharge after percutaneous coronary intervention[END_REF][START_REF] Glaser | Patient satisfaction is comparable to early discharge versus overnight observation after elective percutaneous coronary intervention[END_REF].

Economic evaluation

French hospitals are financed by a diagnosis-related group (DRG)-based prospective payment system. However, the primary focus of this economic evaluation was to help a decision process at the hospital, not the payer, level. We adopted therefore the perspective of the hospital, assuming that for the present the DRG tariff would remain identical, regardless of the length of stay reduction.

Patient undergoing ambulatory PCI were compared to a group of patients with stable angina, without comorbidities, and undergoing conventional PCI in non-study centers in 2009.

We estimated the cost of ambulatory PCI using microcosting techniques. The total cost of the procedure was divided into non-procedural and procedural costs (cost of supplies, devices, drugs and cardiac catheterization laboratory utilization). The units of physical inputs required for the economic evaluation were collected prospectively at the patient level.

The cost of conventional inpatient PCI was similarly estimated by microcosting. We used the microcosting performed yearly to update DRG tariffs in 1,820 patients with stable angina treated in comparable university hospitals in the Paris region (16). Unit costs are presented in Table 1 (16,[START_REF] Midy | Available at[END_REF]. All costs were inflated to reflect 2012 euros. We tested the robustness of our results in a univariate sensitivity analysis of the duration of conventional PCI admission.

Study objectives

The primary objective of this study was to evaluate the safety of ambulatory PCI as assessed by: the rate of re-admission for any reason within the first 24 hours after ambulatory PCI, the rate of hemorrhagic or thrombotic complications at the radial puncture site, the rate of acute stent thrombosis, patient anxiety and patient satisfaction after the procedure. The cost of ambulatory PCI was also evaluated. Successful ambulatory PCI was defined as absence of cardiac or vascular complications within 24 hours after discharge.

Statistical analyses

Continuous variables were described as mean±SD and compared with a Student t test.

Qualitative variables were compared using a Chi-squared test. Results were considered to be significantly different for p values < 0.05.

RESULTS

Patients

The main characteristics of the 220 study patients are listed in Table 2. Patients mean age was 64+/-9.6 years and 197 (89.5%) were males. Sixty seven patients (30%) had diabetes mellitus and 123 (56%) hypertension. All patients had stable angina and/or silent myocardial ischemia evidenced by treadmill testing, scintigraphy, or stress echocardiography.

PCI procedure

The main angiographic and procedural characteristics are reported in Table 2. The study population included a substantial proportion of patients with angiographically complex lesions and complex PCI procedures: multivessel coronary artery disease (n=115; 52.3%), multivessel interventions (n=31; 14.1%), multilesion procedures (n=50; 22.7%), restenosis (32 of 279 lesions, 11.5%), bifurcation lesions (60 of 279, 21.5%) and chronic total occlusions (24 of 279, 8.6%).

An unfractionated heparin bolus (70 IU/kg) was administered in 177 (80%) patients for intraprocedural anticoagulation. The remaining 43 (20%) patients received a single subcutaneous enoxaparin injection at admission. Coronary stents were placed in 218 (99.1%) patients (312 stents for 279 lesions; range: 0-3 stents per patient). Among implanted stents, 70% (n=217) were drug-eluting stents. PCI was technically successful in all patients.

Observation and discharge

Of the 220 patients, 213 (96.8%) were discharged on the day of PCI and returned home. During the 4-to 6-hour observation period, 4 (1.8%) patients developed medical events requiring overnight admission: one had a clinically significant puncture site hematoma with no need for blood transfusion, two had chest pain without ECG modifications, and one had angina with transient ECG modifications (Table 3). In this last patient, myocardial ischemia occurred despite an uncomplicated PCI and was associated with a moderate increase in serum troponin I level (to twice the upper limit of normal). None of these 4 patients required follow-up coronary angiography during the hospital stay.

The remaining 3 (1.4%) patients were kept overnight for non-cardiovascular reasons.

Among them, 1 had transport problems and 2 were concerned about complications in the event of same-day discharge.

Follow-up and adverse clinical events

Table 4 shows the adverse clinical events recorded on day 1 and day 30. No readmissions and no major adverse cardiac or cerebrovascular events (MACCE) occurred within the first 24 hours. A small increase in plasma troponin-I was observed in 33.5% of patients on day 1, but troponin-I level was more than three times the upper limit of normal in only 20.9 % of patients.

The 1-month follow-up showed that a secondary end point occurred in one patient.

This event was a non-fatal myocardial infarction 4 days after PCI, caused by stent thrombosis and managed by emergency PCI. Interestingly, bedside aggregometry (VerifyNow ® , Accumetrics, San Diego, CA, USA) showed biological resistance to both aspirin and clopidogrel in this patient. Readmission was required in 3 other patients, on days 3, 7, and 9, respectively, for atypical non-sustained chest discomfort without ECG changes or troponin elevation that did not require emergency coronary angiography. All 3 patients were discharged within 24 to 48 hours after re-admission. We found no relationship between the complication rate and either the angiographic characteristics or the postprocedural increase in plasma troponin I.

Patient comfort and satisfaction

Of the 213 patients discharged on the same day as PCI, 212 completed the comfort and satisfaction questionnaire. Among them, 185 (87%) were very satisfied, 26 (12%) were satisfied, and only 1 patient was not satisfied with ambulatory management. Furthermore, 180 (85%) patients reported no anxiety during the first 24 hours after discharge, 28 (13%) reported being anxious, and 5 (2%) reported being very anxious. Of the 212 patients, 203 (96%) were willing to accept ambulatory PCI should they require another PCI procedure. A trend towards less anxiety was observed in patients with a previous history of PCI (12.5% of patients with a previous history of PCI were anxious or very anxious vs 18.9% of patients without history of PCI, ns).

Cost analysis

The average non-procedural cost for all 220 patients included in our study, including the overnight stay for 7 patients, was €1,230±98. In the control group of 1,820 patients who underwent conventional PCI in non-study centres during the same period, the median length of stay was 2 days and the average non-procedural cost per patient was higher at €2,304±1,814 (p<10 -6 vs ambulatory PCI) (Table 5).

When procedural costs were included (€1,890±886 vs €2,026±1,086 for ambulatory PCI and conventional PCI, respectively, p<0.04), the average total cost remained lower for ambulatory PCI (€3,120±888 vs €4,330±1317, p<10 -6 ). The difference in procedural costs between ambulatory PCI and conventional PCI was due to a lower average number of stents (1.4 in ambulatory PCI patients vs 1.6 for conventional PCI patients).

PCI was the length of stay. In daily practice, patients are commonly admitted the day before the procedure and discharged one or sometimes two days later. If the total length of stay were two days or less for all patients in the control group, and assuming that no complication would occur, the non procedural costs for conventional PCI would be reduced to €1,535 and total costs to €3,561. The cost difference with ambulatory PCI would then be "only" €441 but the budget impact would remain important, given the almost 100,000 patients potentially eligible for ambulatory PCI yearly in France.

DISCUSSION

Our results indicate that ambulatory PCI using the radial approach is safe, even in patients with complex coronary artery lesions given a high clopidogrel loading dose.

Moreover, the one-day strategy significantly decreased the cost of PCI, suggesting a potential for optimizing healthcare resource utilization.

Safety and feasibility of outpatient transradial PCI

In recent years, major advances in interventional techniques have radically transformed the management of patients undergoing PCI. Dual antiplatelet therapy has reduced the incidence of cardiac ischemic events and stent thrombosis after coronary stenting [START_REF] Moussa | Effectiveness of clopidogrel and aspirin versus ticlopidine and aspirin in preventing stent thrombosis after coronary stent implantation[END_REF][START_REF] Schomig | A randomized comparison of antiplatelet and anticoagulant therapy after the placement of coronaryartery stents[END_REF][START_REF] Movahed | Decreasing in-hospital mortality of patients undergoing percutaneous coronary intervention with persistent higher mortality rates in women and minorities in the United States[END_REF].

The transradial approach has emerged as an alternative to the transfemoral approach, as it is associated with fewer access-site complications and allows early ambulation [START_REF] Brueck | A randomized comparison of transradial versus transfemoral approach for coronary angiography and angioplasty[END_REF][START_REF] Rao | Trends in the prevalence and outcomes of radial and femoral approaches to percutaneous coronary intervention: a report from the National Cardiovascular Data Registry[END_REF]. These changes have called into question the need for an overnight hospital stay after PCI in patients with stable angina. Although several large-scale prospective studies have assessed the feasibility and safety of same-day home discharge after PCI [START_REF] Banning | Same day discharge following elective percutaneous coronary intervention in patients with stable angina[END_REF][START_REF] Bertrand | A randomized study comparing same-day home discharge and abciximab bolus only to overnight hospitalization and abciximab bolus and infusion after transradial coronary stent implantation[END_REF][START_REF] Heyde | Randomized trial comparing same-day discharge with overnight hospital stay after percutaneous coronary intervention: results of the Elective PCI in Outpatient Study (EPOS)[END_REF][START_REF] Slagboom | Outpatient coronary angioplasty: feasible and safe[END_REF], additional data collected under the conditions of everyday practice are required before ambulatory PCI can be used on a routine basis.

Our study showed that ambulatory transradial PCI was feasible and safe even in patients given aggressive antiplatelet therapy before the procedure. These results are in agreement with previous studies of PCI using the transradial [START_REF] Bertrand | A randomized study comparing same-day home discharge and abciximab bolus only to overnight hospitalization and abciximab bolus and infusion after transradial coronary stent implantation[END_REF][START_REF] Slagboom | Outpatient coronary angioplasty: feasible and safe[END_REF] or transfemoral approach [START_REF] Banning | Same day discharge following elective percutaneous coronary intervention in patients with stable angina[END_REF][START_REF] Heyde | Randomized trial comparing same-day discharge with overnight hospital stay after percutaneous coronary intervention: results of the Elective PCI in Outpatient Study (EPOS)[END_REF][START_REF] Slagboom | Outpatient coronary angioplasty: feasible and safe[END_REF]. In our population of clinically stable patients referred for PCI, most of the patients were eligible for ambulatory management. Our simple patient-selection strategy based on pre-defined clinical and angiographic criteria was associated with an excellent safety profile after uncomplicated PCI. However, a few complications occurred within 4-6 hours after PCI in a limited number of patients, indicating that a short post-procedural observation period remains mandatory. No specific risk factors for complications were identified in our population. Whereas several previous studies enrolled patients selected on relatively conservative criteria [START_REF] Banning | Same day discharge following elective percutaneous coronary intervention in patients with stable angina[END_REF][START_REF] Chambers | Defining the length of stay following percutaneous coronary intervention: an expert consensus document from the Society for Cardiovascular Angiography and Interventions. Endorsed by the American College of Cardiology Foundation[END_REF][START_REF] Laarman | A pilot study of coronary angioplasty in outpatients[END_REF][START_REF] Rao | Prevalence and outcomes of same-day discharge after elective percutaneous coronary intervention among older patients[END_REF], our population included a high percentage of patients with multivessel interventions, bifurcation lesions, and total coronary occlusions and was therefore representative of patients referred to catheterization laboratories in everyday practice. The risk of ischemic events was not increased in the patients who had complex lesions. Thus, our results show that angiographically severe coronary lesions and a technically complex procedure are not limiting factors for same-day discharge after successful PCI. Furthermore, the moderate increase in plasma troponin level observed in our population was similar to that found in previously published studies [START_REF] Feldman | Relation of troponin I levels following nonemergent percutaneous coronary intervention to short-and long-term outcomes[END_REF] and was not associated with short-term post-procedural complications.

Logistical constraints

In most of the previous studies, many patients were hospitalized before the procedure or transferred from another hospital [START_REF] Bertrand | A randomized study comparing same-day home discharge and abciximab bolus only to overnight hospitalization and abciximab bolus and infusion after transradial coronary stent implantation[END_REF]. Similarly, diagnostic coronary angiography was performed before patient inclusion in the study by Heyde et al [START_REF] Heyde | Randomized trial comparing same-day discharge with overnight hospital stay after percutaneous coronary intervention: results of the Elective PCI in Outpatient Study (EPOS)[END_REF]. Our study represents a new step in the management of patients with coronary artery disease. It is one of the first to assess the safety of ambulatory PCI (with same-day admission and discharge of the patients) under the conditions of everyday practice [START_REF] Khater | Contemporary outpatient percutaneous coronary intervention: feasible and safe[END_REF]. Thus, the patients were referred directly by their cardiologist to the cardiology departments on the morning of the procedure and were discharged on the same day. However, ambulatory PCI is associated with a number of logistical constraints. Close collaboration between the referring cardiologists, general practitioners, and interventional centers is required. Before the procedure, patients must be carefully screened for factors that preclude ambulatory PCI. Moreover, the referring cardiologists must provide antiplatelet therapy management, patient education, and post-procedure follow-up [START_REF] Bertrand | Outpatient percutaneous coronary intervention: Ready for prime time?[END_REF][START_REF] Fox | Guidelines on the management of stable angina pectoris: executive summary: The Task Force on the Management of Stable Angina Pectoris of the European Society of Cardiology[END_REF][START_REF] Henein | Statins moderate coronary stenoses but not coronary calcification: results from meta-analyses[END_REF]. In our experience, the feedback from physicians regarding ambulatory PCI was extremely positive. Greater use of ambulatory PCI has allowed in our centers to cope with the increasing number of PCI procedures.

Acceptance

Ambulatory PCI can be deemed feasible in everyday practice only if it is well-accepted by the patients. Our results show that ambulatory PCI was extremely well accepted [START_REF] Glaser | Patient satisfaction is comparable to early discharge versus overnight observation after elective percutaneous coronary intervention[END_REF]. It induced anxiety in only a small percentage of patients and a slight non-significant trend towards less anxiety was observed in patients with a previous history of PCI.

Impact on costs and hospital resources

Our economic analysis showed that ambulatory PCI was associated with cost savings related to the shortening of the hospital stay. Since early discharge did not result in early readmissions, the reduction of non-procedural cost was not offset by subsequent expenditures.

Only few studies have evaluated the economic consequences of these new patient management strategies after PCI. Even if our study was performed in a different population, our findings are consistent with the cost-analysis of the EASY trial [START_REF] Rinfret | Economic impact of same-day home discharge after[END_REF]. This latter trial showed that same day home discharge after transradial PCI and a bolus of abciximab reduced by 50% the post-procedural medical costs compared to overnight admission associated with standard abciximab therapy. Other studies in the Netherlands, Canada and USA have reported savings ranging from €258 to approximately €1,500 ($1.961) [START_REF] Heyde | Randomized trial comparing same-day discharge with overnight hospital stay after percutaneous coronary intervention: results of the Elective PCI in Outpatient Study (EPOS)[END_REF][START_REF] Khatri | Safety and cost benefit of same-day discharge after percutaneous coronary intervention[END_REF][START_REF] Glaser | Patient satisfaction is comparable to early discharge versus overnight observation after elective percutaneous coronary intervention[END_REF]. This heterogeneity could be due to differences in the standard of care (transradial or transfemoral approach), study design and healthcare prices, in addition to the inclusion of procedural costs. Ambulatory PCI requires a specific organization of the departments of cardiology to ensure patient follow-up and evaluation before discharge. The development of ambulatory PCI in daily practice will depend on the level of hospital reimbursement assigned by national health authorities or private insurers and the incentives thus generated. The move from conventional to ambulatory PCI should remain financially attractive for hospitals to promote this strategy.

Moreover, ambulatory PCI may optimize healthcare resource utilization and bed availability. The impact of ambulatory PCI on hospital resource management depends on the characteristics of the cardiology intervention center. In high-volume centers, the shorter hospital stay increases patient turn-over and the number of procedures. In other centers, the increase in hospital bed availability might allow the development of other interventional cardiology activities.

Study limitations

Our study has a number of limitations. It was designed to prospectively evaluate clinical outcomes in a cohort of stable patients selected for ambulatory PCI based on predefined clinical and angiographic criteria. Therefore, our study was an observational assessment of clinical practice and was not powered to identify factors that predicted outcomes. Moreover, the cost analysis was based on reimbursement modalities in France and its results may not apply directly to countries that have different healthcare systems.

CONCLUSIONS

Ambulatory PCI in patients with stable coronary artery disease is safe, feasible, and well accepted by the patients. The development of this strategy holds promise for decreasing costs and optimizing healthcare resource utilization by shortening hospital stay. uncomplicated transradial percutaneous coronary intervention and bolus-only abciximab regimen. JACC Cardiovasc Interv 2010;3:1011-9. Note: figures were rounded to the nearest €

Figure 1 :

 1 Figure 1: Inclusion and exclusion criteria.

Figure 2 :

 2 Figure 2: Patient flow chart.

  between the average non-procedural cost of ambulatory PCI vs conventional PCI.

Table 1 : Unit costs of hospital resources

 1

	Item

Table 2 : Patient characteristics Demographic characteristics

 2

	Patients, n

Table 3 : Management of the 220 included patients after the 4-to 6-hour observation period

 3

	Same day discharge, n (%)

Table 4 : Follow-up data 24 hours and 1 month after ambulatory percutaneous coronary intervention

 4

	24 hours	1 month
	(n=213)	(n=213)

Acknowledgments

The authors thank Ms. Dalila Bitari for technical support. The study was funded by the Assistance Publique-Hôpitaux de Paris [Grant number SCR06003 -P060227].

Conflict of interest: Nothing to report.


Abstract : The in vitro fate of the saturated hydrocarbon fraction (SF) of Arabian Light crude oil has been studied in PVC cores filled with a coastal marine sediment defaunated by sieving. Experiments were conducted in absence or presence of polychaetes Nereis diversicolor. The luminophore tracer technique was used to quantify the mixing of sediment by worms. Presence of crude oil reduced the building of burrows by polychaetes. This work demonstrates the ability of infaunal organisms to stimulate both downard and outward transfers of hydrocarbons from sediment reservoirs. In non-bioturbated sediment hydrocarbons were confined to the sediment surface. Introduction of polychaetes in sediment (1) induced the burying of SF in sediment (2.5 % and 13.5 % of the initial surface input after 15 and 45 days, respectively); (2) enhanced the exportation of SF in the overlying water (plus 59 % and 23.5 % compared to defaunated control sediment after 15 and 45 days, respectively). Buried hydrocarbons were submitted to biodegradation, from 2 cm to 10 cm depth in polychaete burrows, after 45 days.

Bioturbation plays an important role in organic matter decomposition by increasing the water-sediment interface 1 and irrigating sediment. Most burrow constructors maintain contact with the overlying water by ventilating water through their burrow systems 2,3 and thus increase the transport of ions and gases (e.g. oxygen) over the water-sediment interface 4,5,6 .

In the same way, burrowing activity may also alter pollutant concentration in sediments. Through the activities of burrowing populations, deposited pollutants become buried. This may result in long-lasting negative effects on the marine ecosystem. On the other hand, buried components may be released more quickly than can be explained by normal molecular diffusion coefficients 7,8 . Among the pollutants, numerous studies have investigated the fate of petroleum hydrocarbons in marine sediments 9,[START_REF] Flootgate | Petroleum Microbiology[END_REF][START_REF] Bertrand | [END_REF]12 . However, only a few works have taken into account the influence of burrowing macrofauna activity on the fate of hydrocarbons in the sediments 13,14,15 .

In estuarine ecosystem, the infaunal polychaetes from the Nereidae family are known to generate significant changes in the properties of the sediment-water interface 1,[START_REF] Aller | Animal-Sediment Relations[END_REF][START_REF] Meadows | [END_REF] . Our purpose was to study the influence of burrowing activity on the fate of hydrocarbons in relation with the mixing of sediment quantified by the luminophore tracer technique [START_REF] Gerino | Fluxes Between Trophics Levels and Through the Water-Sediment Interface[END_REF]19 . We therefore decided to carry out an in vitro experiment with the polychaete Nereis diversicolor whose burrowing activity results in both the mixing and the irrigation of different layers of the sediment [START_REF] Lee | Analysis, Chemistry, Biology[END_REF] . EXPERIMENTAL -Collection of polychaetes and sediment : During July 1991, sediment and polychaetes were collected by digging in the Carteau cove (Gulf of Fos, France). The sampling site has already been described [START_REF] Gerino | Fluxes Between Trophics Levels and Through the Water-Sediment Interface[END_REF] . An undisturbed sediment core was sampled using a PVC corer (i. d. 7.3 cm) of 30 cm length to examine sediment characteristics before the start of the experiment (A, figure 1). In the laboratory, the sediment was wet-sieved through a 1 mm sieve to remove macrofauna and thoroughly homogenized by gloved hands (B). Then, eight cores (i. d. 7.3 cm; 30 cm length) were filled to a depth of 20 cm with macrofauna-free sediment and placed in an aquarium thermostated at 17° C under a LD 12:12 cycle (C). By sieving of the sediment, Nereis diversicolor individuals were obtained (D). The polychaetes were acclimatized to experimental conditions, 17°C and 30 ‰ S, for 6 d in a recirculating seawater aquarium partially filled with natural sediment before being introduced into cores. Then, six weighed N. diversicolor (0.64 to 0.81 g wet weight) were added (E) to 6 cores (~ 1433 indiv.m -2 ; ~ 1 wt kg.m -2 ). Several animals that did not burrow within 12 hours were removed and replaced with fresh ones of same weight. -Continuous flow system : A peristaltic pump system supplied sediments with seawater. Four multiple-head peristaltic pumps (Cole-Parmer, Chicago, IL) controled filtered seawater flow (0.6 ml min -1 ) to the cores from two 50-litre thermostated tanks. Overlying the sediment of each core was approximatively 0.3 litre of seawater which was kept aerated by a small aquarium pump attached to an airstone. The evacuation of water was allowed through a 1 mm perforation bored 3 cm from the core top.

-Petroleum hydrocarbons : A mixture of Arabian Light crude oil and sediment (F) was used to contaminate cores [START_REF] Bonin | [END_REF] . Then, 12 "hydrocarbon cakes" were made by filling moulds (i. d.: 10 cm, 1 cm thick), and frozen (G).

GC analysis showed that the concentration of the saturated fraction of hydrocarbons (SF) in each cake was 3215± 25 mg SF. kg-1 dry sediment (mean ± CI, n=6). Frozen hydrocarbon cakes were deposited at the sediment surface of six cores (H). The two other cores received a "clean" cake made with sieved sediment to ensure standard sediment height (I).

After 15 and 45 d, sediment cores were sectioned to 2 cm thick segments from the top to 10 cm, where the analyses were performed.

The extraction and fractionation of petroleum hydrocarbons from the sediment, and the saturated fraction analysis were carried out as described by Bertrand et al. 22 .

-Analysis of sediment mixing : The mixing of sediment was quantified with the luminophore tracer technique [START_REF] Gerino | Fluxes Between Trophics Levels and Through the Water-Sediment Interface[END_REF]19 . Two size fractions of luminophores, 40-60 µm diameter (red) and 150-200 µm diameter (yellow) were used. Then, for six cores, a mixture of the two luminophore fractions (0 . 5 g per fraction) was added at the cake surface. When removed, each segment of sediment was dried at 70°C for a week , carefully mixed to homogeneize sediment and luminophores, and sieved through a 500 µm mesh. For each segment, 3 subsamples of 0 . 25 g were taken. The luminophores counts were then conducted under an ultra-violet light source.

RESULTS AND DISCUSSION

There is a significant correlation between the surface of the burrow walls of N. diversicolor and the quantity of luminophores at each level, except the first layer (0-2 cm) because luminophores are deposited in excess at the surface 23 . Table I presents the distribution of luminophores in control defaunated sediment, and experimental sediments (with 6 Nereis diversicolor) contaminated and not contaminated, after 15 and 45 d. It is first shown that the building of burrows by the worms was inhibited in the presence of crude oil. The quantities of luminophores buried in contaminated sediment were on average 77 % lower than those found in noncontaminated sediment, after 15 days.

Table I

Distribution of luminophores in control defaunated sediment and experimental sediments (with 6 Nereis diversicolor). NC, non-contaminated sediment; AL, Arabian Light crude oil contaminated sediment. Values are expressed in milligrams for an 1 g initial luminophore input.

After 45 days, differences between "clean" and contaminated sediments were about 54 %, and could only be seen below 6-cm depth. Research has already shown that the 5-day sediment reworking rate of Arenicola marina was about 70 % reduced in presence of a surface sediment oil concentration of 2086 mg. kg-1 dry sediment 24 , and when natural populations were sprayed with oil 25 at a rate of 0.2 l. m -2 .

Experiments were carried out on a artificially contaminated sediment. Before contamination, the saturated hydrocarbon composition of natural sediment was essentially due to biogenic hydrocarbons (figure 2). Arabian Light crude oil was chosen as source of hydrocarbons because there was no analogy between its n-alcanes distribution of SF (figure 3) and that of the natural sediment one.

In control cores, hydrocarbons were kept at the sediment surface (Table II). In inhabited sediments, the presence of polycheates induced the burying of the crude oil (Table II andfigure 4). Gravimetric results have also shown that the distribution of added hydrocarbons in experimental (with polychaetes) sediment was not equal with depth and time. After 15 d, only very low quantities of crude oil were detected (only by gaschromatography) under 6 cm depth. After 45d, crude oil was detected throughout the whole sediment depth. If we compare the concentrations of SF and the quantities of luminophores in the experimental sediment which have been contaminated by crude oil, no correlation is apparent. However, it is clearly demonstrated that when the surface of burrows is very low (e.g. layers 6-8 cm and 8-10 cm, after 15 d) (Table I), only traces of crude oil are detectable (Table II). We conclude that the burying of hydrocarbons is only qualitatively related with the building of burrows by the polychaetes. Results can be presented as the percentages, in the water-sediment interface, of luminophores and SF compared to quantities initially deposited. Studying the transfers of luminophores (figure 5) seems to suggest that the presence of polychaetes did not have any influence on their exportation in the water column (from 24 % to 28 % in control sediment; 32 % in experimental sediment). Exportation of luminophores in the water column seems to be only due to the hydrodynamic conditions during experimentation, especially circulating water and oxygenation of water near the sediment surface. The burying of luminophores which never occurred in control sediment was respectively of 6 % (15 d) and 19 % (45 d) in experimental inhabited sediment.

Figure 6 shows the estimated hydrocarbon transfers in control and experimental sediment. After 15 d, 2.5 % of the crude oil deposited in the surface of the experimental sediment was buried, as compared to 0 % for the control sediment. The presence of the polychaetes also induced the disappearence in the water column of 63 % of the initial quantity; this exportation was only about 4 % in the control sediment. After 45 d, in control sediment, only 47 % of the initial hydrocarbon input remained in the sediment. In experimental sediment, 13.5 % of the hydrocarbons were buried, 76.5 % exported in the water column and 10 % kept at the sediment surface. The presence of the worms stimulated the removal of hydrocarbons deposited in surface (plus 23.5 % compared to the control sediment). The burial rate for hydrocarbons and luminophores appears to be very similar, 13.5 % and 19 %, respectively, after 45 d. Sediment reworking occurs during the building of burrows by worms, where superficial material is added to burrow walls and trapped by mucus. Luminophores and hydrocarbons adsorbed to sediment particles are subjected to the same process. In the absence of macrofauna, no burying occurs.

On the other hand, exportation in the water column differs in relation with the tracer. Whereas in the burying process, where it is the hydrocarbons adsorbed to sediment particles that are principally concerned, the exportation into the water column involves desorbed hydrocarbons. In contact with the polychaete burrows, hydrocarbons are diffused out of sediment pore water and are transported into the water column by both simple diffusion and the irrigation activity of the worms. Polychaete burrows increase the surface of sediment exposed to the water column and therefore the diffusion exchanges 1,25 .

Biodegradation by microorganisms also appears to be responsible for some of the hydrocarbon removal observed. Among oil components, isoprenoid alkanes, such as pristane and phytane, are more stable and can serve at the corresponding boiling point as references to assess the degradation of the normal alkanes. Thus, the nC17/pristane and nC18/phytane are used as measures of microbial degradation of the normal alkanes 26 . After 15 days, no biodegradation occurred in control and experimental sediments (data not shown). After 45 days, in the surface contaminated layer, nC17/pristane and nC18/phytane ratios remained of the same order as those of the "hydrocarbon cake" (figure 3), respectively of 5 and 3.5 (control sediment), and 5.1 and 2.7 (inhabited sediment). On the other hand, after 45 days, the nC17/pristane and nC18/phytane ratios calculated from SF of buried hydrocarbons decreased respectively to 2.3 and 1.1 (ratios are presented in figure 4) indicating microbial degradation of normal alkanes in polychaete burrows where microbial activities are enhanced 27 .

Several authors have already reported the stimulation of degradation of added hydrocarbons in sediment in presence of polychaetes 13,14,15 , but they used pure PAH mixtures that are more accessible for microorganisms than complex crude oil.

In conclusion, although the reworking activity of Nereis diversicolor is reduced in presence of Arabian Light crude oil, bioturbation by the polychaetes significantly influences the fate of hydrocarbons in sediment. During building of burrows, the polychaetes select particles from the sediment surface, resulting in a downward transport of surface sediment and adsorbed hydrocarbons. However, the major influence of polychaetes on hydrocarbons is their removal from sediment. Associated with the enhanced surface area from burrowing, irrigation of burrows by Nereis diversicolor also contibutes to the diffusion loss of desorbed hydrocarbons from the sediment. Normal alkanes are also degraded in burrows.

  THE IN VITRO INFLUENCE OF THE BURROWING POLYCHAETE NEREIS DIVERSICOLOR ON THE FATE OF PETROLEUM HYDROCARBONS IN MARINE SEDIMENTS. F. GILBERT a , L. RIVET b and J-C. BERTRAND a a Centre d'Océanologie de Marseille, Université d'Aix-Marseille II -URA CNRS n° 41, Station Marine d'Endoume, Rue de la batterie des Lions, 13007 Marseille, France. b Laboratoire de Chimie Analytique de l'Environnement -URA CNRS n° 1049, Boite 312, 13397 Marseille Cedex 20

Figure 1 :

 1 Figure 1: Experimental procedure. The different steps (letters in brackets) are described in detail in text.

Figure 2 :

 2 Figure 2: GC-chromatogram of the saturated hydrocarbon fraction extracted from the natural sediment; Pr: pristane; Ph: phytane;n-alkanes (numbers from 13 to 31) are labelled with the number of carbons in the molecule.

Figure 3 :

 3 Figure 3: GC-chromatogram of the saturated hydrocarbon fraction extracted from the " hydrocarbon cake" used to contaminate sediment. Pr: pristane; Ph: phytane; n-alkanes (numbers from 13 to 31) are labelled with the number of carbons in the molecule.

Figure 4 :

 4 Figure 4: GC-chromatograms of the saturated hydrocarbon fraction extracted from the experimental sediment with 6 Nereis diversicolor , after 45 d. A: 2-4 cm depth; B: 4-6 cm depth; C: 6-8 cm depth; D: 8-10 cm depth; Pr: pristane; Ph: phytane; n-alkanes (numbers from 13 to 31) are labelled with the number of carbons in the molecule.

Figure 5 :

 5 Figure 5: Luminophore transfers in non-bioturbated control sediment (left) and the experimental sediment with 6 Nereis diversicolor (right), after 15 and 45 d. Values are expressed in percentage of the initial luminophore input at the sediment surface.

Figure 6 :

 6 Figure 6: Hydrocarbon transfers in non-bioturbated control sediment (left) and the experimental sediment with 6 Nereis diversicolor (right), after 15 and 45 d. Values are expressed in percentage of the initial hydrocarbon input at the sediment surface.

ACKNOWLEDGEMENTS

This work was carried out in the framework of the programme "Cycles biogéochimiques: évolution de la matière organique en milieu littoral" and supported by the Centre National de la Recherche Scientifique, the Programme Interdisciplinaire de Recherche sur l'Environnement (PIREN) and the Société Nationale Elf Aquitaine.