Comparative statement for diametric delamination in drilling of cortical bone with conventional and ultrasonic assisted drilling techniques

Abstract

Drilled hole quality is a significant parameter for successful orthopedic surgery. The present investigation is an effort to reduce the delamination produced drilling with state-of-the-art hybrid drilling i.e. ultrasonically-assisted drilling. A comparative analysis has carried out as per experimental design to assess the ultrasonic drilling with conventional drilling. The novelty of the work is the use of coordinate measuring machine (CMM) for characterization of the delamination during bone drilling. The results revealed that ultrasonically-assisted drilling caused lesser delamination than conventional drilling. The maximum percentage delamination during conventional drilling was found to be 9.153% and 8.541% during ultrasonically-assisted drilling.

Highlights

• •UAD technique shows its significance to control the delamination damage in drilling of bones.
• •Coordinate measuring machine is shows the better practice to measure the delamination.
• •Effect of parameters has been studied for ultrasonic assisted bone drilling technique.
• •Microscopic study of drilled holes is carried out to visualize the delamination around the edges.

1. Introduction

Bone drilling is one of the oldest techniques to fix fractured bones due to injuries, accidents, and aging. Usually, implants such as screws, locking plates, and nails are used to immobilize and align the broken bone parts for faster healing post-operatively in orthopedic surgical operations. A pre-drilled hole is required to fix these inserts into the bone. Therefore, bone drilling plays a paramount role in orthopedic, trauma, and dental surgeries. Bone is an anisotropic material comprises of cortical (hard layer) and cancellous portion (spongy layer). During bone drilling, delamination occurs at the hole's entrance and exit which results in loosening and misalignment of implants due to which it prematurely pulls out from its location, even before the complete repair and reconstruction of bone.1, 2, 3, 4, 5 The delamination significantly amplifies the healing time after surgery which is a major concern for orthopedic surgeons. Pandey and panda⁶ highlighted the importance of drilling parameters spindle speed, feed rate, and types of drill bit for minimum delamination. The results revealed that feed rate is highly governing factor which influences the delamination with 84.99% of contribution. The optimal parameters recommended for minimum delamination were 500 rpm, 30 mm/min, and standard drill bit with point angle 118°. Gupta et al.⁷ reported delamination at the hole's entrance and exit during conventional drilling of porcine femur bone along with the cracks on the inside walls of the hole.

It was found that bone-tool interaction plays a significant role in the delamination during drilling of bone. At present, conventional drilling is used worldwide for orthopedic application but ultrasonic-assisted drilling has shown its potential in the last few decades. Alam et al.⁸ reported that force and torque during the bone drilling mitigates by 55% and 28% respectively in ultrasonic-assisted drilling. Gupta et al.⁹ studied force, torque, tool wear, and temperature during drilling porcine bone with conventional and ultrasonically actuated drilling. On the same context, Shakouri et al.¹⁰ investigated different process parameters in terms of thermal necrosis and cutting forces during drilling with ultrasonically actuated tool and highlighted the importance of vibrational drilling.

Although delamination study is an important aspect of hole quality which ensures the healthy reconstruction and repair of the fractured bone post-operatively. Nonetheless, maximum research has been carried out to evaluate the effect of cutting force, torque, temperature, and tool wear during bone drilling but a very limited literature is available for investigations on delamination. The method to calculate the delamination and evaluation of damage to the surface quality of hole has been reported by various researchers.11, 12, 13, 14 The goal of the present experimental work is to investigate the delamination during drilling of bovine femur bone with conventional and ultrasonic-assisted drilling with different process parameters for instance rotational speed, feed rate, and drill bits with different point angles. First time in the orthopedic history, the percentage increase in the delamination was estimated with the aid of coordinate measuring machine. A comparative analysis has been made for conventional drilling and ultrasonic-assisted drilling in terms of delamination with microscopic images and SEM micrographs.

2. Materials and methods

2.1. Experimental setup

An ultrasonic-assisted drilling (UAD) setup was designed and mounted on CNC vertical machining center to drill holes in bovine femur bone. Fig. 1 illustrates the complete UAD setup along with its elements horn assembly, workpiece, tools, coolant supply, and control panel. To mimic the clinical conditions, experiments were performed with saline irrigation to remove the heat produced during drilling of bone. The spindle rotates as well as vibrates at frequency of 20 kHz so as to bring longitudinal vibration in the tool. The amplitude required during the drilling could be also adjusted in the range of 0–20 μm. In UAD, material from the workpiece is removed due to the two dominating material removal mechanism of conventional drilling and rotary ultrasonic drilling. The process parameters rotational speed, feed rate, and type of tool were varied through control panel interface as per experimental design.

Fig. 1.

Ultrasonically-assisted drilling setup.

2.2. Tooling and workpiece

The experimentation was performed with five drill bits having different point angles (60°, 80°, 100°, 120°, 140°). These drill bit specification were chosen after consulting with orthopedic surgeons. The drilling experiments have been carried out on bovine femur bone which was obtained from the local butcher house. No animal was slaughtered especially for present experimental investigations. Animal bone was used as human bone is not easily available and ethical concerns are associated with it. The periosteum layer from the cortical bone was removed and proximal as well as distal epiphysis portion of the bone was cut with hack-saw and experiments were performed on mid-diaphysis of the bone.

2.3. Process parameters

Three input parameters rotational speed, feed rate, and point angle were used to complete the experimental study. The parameters and their corresponding levels were selected on the basis of widely reported literature.15, 16, 17, 18, 19, 20, 21, 22, 23 The experimentation consists of 25 experiments for conventional drilling and 25 experiments for ultrasonically-assisted drilling with same set of parameters as per L₂₅ design of orthogonal array. The parameters chosen for bone drilling along with their levels are accentuated in Table 1.

Table 1.

Control log of experimentation during bone drilling.

Control parameters	Level 1	Level 2	Level 3	Level 4	Level 5
Rotation speed (rpm) (R)	600	1200	1800	2400	3000
Feed rate (mm/min) (F)	10	20	30	40	50
Drill Point angle (°) (P)	60	80	100	120	140

2.4. Characterization

The values of diameter of drill bit (D) and maximum diameter of hole (D_max) of the maximum area affected by delamination are determined using coordinate measuring machine (CMM). The data collected from the CMM was used to estimate the percentage delamination during drilling of hole in the bone. From the previous decades, CMM has been unceasingly used in industries for checking errors in flatness, circularity, and diameters.24, 25, 26, 27, 28, 29 A well-developed relationship was established between estimated results and measuring points using coordinate measuring machine. This is the first type of study in orthopedic application in which CMM (Accurate 564, spectra) has been used for quantification of delamination (increase in the effected diameter) during bone drilling. The percentage delamination is calculated by using Eq. (1). The complete setup of CMM is shown in Fig. 2.

$$\text{Percentage delamination }=(\frac{D_{max}-D}{D})\times 100$$ (1)

Fig. 2.

(a) Complete CMM setup and (b) Measurement of diametric delamination of hole drilled on cortical bone.

The images of hole after every set of experimentation were taken with digital camera (Nikon D5300) and surface electron micrographs were used for microstructural analysis of delamination at the entrance and exit of drilled hole.

3. Results and discussion

3.1. Delamination measurement

The experimentation was performed on bovine femur bone as per experimental design. The microscopic images were taken after drilling every hole to analyze the delamination at the hole periphery. Fig. 3(a) clearly shows the delamination in terms of diameter change during conventional drilling. It may be due to the fact that the contact at the drill-bone interface is continuous during conventional drilling which generates continuous chipping with greater thickness which is not easily removed during drilling owing to lack of space at tool-bone contact zone. Further these chips distort the geometry of the hole and degrade the hole quality and results in delamination. Moreover, the drilling force and torque produced during CD is more than the UAD which aids in damaging the hole quality and increase in post-operative healing time.^4,30

Fig. 3.

Microscopic images illustrating delamination in terms of change in diameter during (a) Conventional drilling (CD) and (b) Ultrasonically-assisted drilling.

On the flip side, negligible delamination was observed during drilling holes with ultrasonic-assisted drilling process. It happens because of the intermittent contact during vibration-assisted drilling which cause formation of segmented small-size chips which easily evacuates from the drilling zone during the retract of tool due to longitudinal vibrations supplied to the tool during UAD. Hence, chips produced does not erodes the surface of hole which leads to lesser delamination as can be seen in Fig. 3(b).

The experimentation consists of drilling 50 hole in bovine bone, 25 for conventional drilling and 25 for rotary ultrasonic drilling. The delamination obtained during CD and UAD is characterized using CMM in terms of change in the diameter which is further used to calculate percentage delamination during CD and UAD. The results obtained for percentage delamination are summarized in Table 2.

Table 2.

diametric delamination during CD and UAD.

S. No.	Rotational speed (R)	Feed	Drill Point	Diameter of hole	% age delamination using CD	Diameter	% age delamination using UAD
		Rate (F)	Angle (P)			of hole
1	600	10	60	3.328	4.00	3.310	3.438
2	600	20	80	3.349	4.66	3.320	3.750
3	600	30	100	3.399	6.22	3.346	4.569
4	600	40	120	3.418	6.81	3.377	5.525
5	600	50	140	3.399	6.22	3.383	5.706
6	1200	10	80	3.321	3.78	3.312	3.506
7	1200	20	100	3.383	5.72	3.358	4.947
8	1200	30	120	3.386	5.81	3.361	5.016
9	1200	40	140	3.434	7.31	3.421	6.912
10	1200	50	60	3.454	7.94	3.419	6.856
11	1800	10	100	3.346	4.56	3.327	3.953
12	1800	20	120	3.417	6.78	3.408	6.500
13	1800	30	140	3.451	7.84	3.432	7.237
14	1800	40	60	3.407	6.47	3.387	5.828
15	1800	50	80	3.43	7.19	3.412	6.634
16	2400	10	120	3.37	5.31	3.345	4.516
17	2400	20	140	3.445	7.66	3.422	6.925
18	2400	30	60	3.425	7.03	3.407	6.453
19	2400	40	80	3.434	7.31	3.412	6.609
20	2400	50	100	3.437	7.41	3.426	7.063
21	3000	10	140	3.359	4.97	3.340	4.375
22	3000	20	60	3.355	4.84	3.348	4.625
23	3000	30	80	3.442	7.56	3.435	7.331
24	3000	40	100	3.475	8.59	3.465	8.266
25	3000	50	120	3.493	9.16	3.473	8.544

Fig. 4 illustrates the comparative analysis of conventional and ultrasonically-assisted bone drilling. It can be seen that for each experiment UAD cause less damage and less percentage delamination during drilling. The maximum delamination caused during CD was found to be 9.153% for rotational speed of 3000 rpm, feed rate of 50 mm/min, drill point angle of 120° while minimum delamination was 3.791% for rotational speed of 1200 rpm, feed rate of 10 mm/min, drill point angle of 80°.

Fig. 4.

Comparative analysis of % age delamination during ultrasonically-assisted drilling and conventional drilling.

Similarly, the maximum percentage change in delamination during UAD in the terms of percentage increase in the diameter was 8.544% for rotational speed of 3000 rpm, feed rate of 50 mm/min, drill point angle of 120° and 3.438% for rotational speed of 600 rpm, feed rate of 10 mm/min, drill point angle of 60°. Hence the results produced by the UAD are far better in comparison to CD.

3.2. Effect of process parameters on diametric delamination

In order to understand the effect of individual parameter on delamination of drilled holes in bones Analysis of Variance (ANOVA) has been applied. The ANOVA has been applied on the values of percentage delamination individually on conventionally drilled holes and ultrasonic assisted drilled holes shown in Table 2. ANOVA analysis on values of Cd and UAD percentage delamination is shown in Table 3. Response values with respect to each level of the contributing parameter is also evaluated and shown in Table 4. A response curve has been developed using the response values and it shows the variation in delamination with respect to the change in respective parameter.

Table 3.

ANOVA table of diametric delamination.

Source	ANOVA for CD					ANOVA for UAD
	DOF	SS	V	F ratio	P value	SS	V	F ratio	P value
Rotational Speed (R)	4	7.28	1.82	2.54	0.095	12.86	3.21	4.63	0.017
Feed Rate (F)	4	30.8	7.71	10.74	0.001	28.66	7.16	10.3	0.001
Drill Point Angle (P)	4	2.54	0.65	0.88	0.502	2.10	0.52	0.76	0.573
Residual Error	12	8.61				8.32

Table 4.

Response table for individual parameters.

Parameters	Response values of CD					Response values of UAD
	L 1	L 2	L 3	L 4	L 5	L1	L2	L3	L4	L5
R	5.581	6.112	6.569	6.944	7.025	4.597	5.447	6.031	6.313	6.628
F	4.525	5.931	6.894	7.300	7.581	3.957	5.349	6.121	6.628	6.961
P	6.056	6.100	6.500	6.775	6.800	5.440	5.566	5.759	6.020	6.231

Fig. 5(A) shows the comparison of delamination during CD and UAD when the rotational speed was increased from 600 rpm (R1) to 3000 rpm (R5). The conceivable reason could be that at higher rpm, the centrifugal force upsurges due to which chips produced during drilling got struck at interface of drill bit and hole's wall instead of evacuation of chip from drilling zone.³¹ These chips further erode the machined surface and affect the surface hole quality and cause delamination. The average value of delamination escalated on increasing the feed rate from 10 mm/min to 50 mm/min i.e. from level 1 (R1) to level 5 (R5). It may be due to the fact that at higher feed rate, cutting chip thickness per revolution tool increases which cause more shear and frictional forces and leads to damage to the surface of hole produced during drilling. Fig. 5(C) revealed that the change in drill point angle had least effect on change in the percentage delamination during drilling of bone. The average value of the delamination slightly increased on increasing the drill point angle from 60° to 140°.

Fig. 5.

Effect of rotational speed, feed rate, and drill point angle on average diametric delamination during bone drilling using CD & UAD.

4. Microscopic investigation

Two drilled bone specimens have been observed under scanning electrode microscopy (SEM). It is required to understand to the damage of edges under the two different conditions of drilling. For the purpose, specimen no 25 is selected, because of its highest delamination observed under coordinate measuring machine (CMM). Fig. 6(A&B) shows the edges of hole drilled with ultrasonic-assisted drilling under 25X and 200× magnifications. While figure (C&D) shows the delamination of edges of hole drilled with conventional drilling. From comparison, the cutting edge improvement in terms of better hole quality is observed using ultrasonic assisted bone drilling technique.

Fig. 6.

(A,B,C&D): SEM micrographs indicating diametric delamination at the edges of hole.

5. Conclusion

The effect of various process parameters rotational speed, feed rate, and drill point angle was analyzed for percentage delamination at five different levels as per L₂₅ orthogonal array using conventional and ultrasonically-assisted drilling during osteotomy of bovine femur bone. The following key conclusions are drawn out from present investigative work.

• 1.The comparative analysis of the ultrasonically-assisted drilling and conventional drilling showed that UAD produced less delamination during drilling of hole in bovine bone.
• 2.A novel method for the characterization of delamination has been introduced. Moreover, CMM has proved to be a potential method to determine the percentage increase in the delamination precisely during orthopedic drilling.
• 3.It was found that as the rotational speed, feed rate, and drill point angle was increased, the percentage delamination also increases due to the increase in frictional, shear stresses and different chip formation morphology.
• 4.The maximum percentage delamination was found to be 9.153% and 8.544% during CD and UAD respectively. Whereas the minimum percentage delamination was 3.971% and 3.438% during CD and UAD respectively.

Declaration of competing interest

There is no conflict of interest.