Surface treatment method for dental implants and dental implants surface-treated by the method

Abstract

A method for treating the surface of a dental implant to impart surface roughness is disclosed. The method for treating the surface of a dental implant according to the present invention includes a first surface treatment step of forming grooves in a regular pattern on the entire or part of the implant surface using a laser, and a second surface treatment step of imparting roughness to the grooves and their surroundings through acid etching. The first surface treatment step may include a first laser treatment step of processing a plurality of first grooves that are continuous in a first direction and spaced apart in a second direction, and a second laser treatment step of processing a plurality of second grooves that are continuous in the second direction and spaced apart in the first direction, forming craters at each intersection between the first and second grooves.

Description

[Technical Field]

[0001] The present invention relates to a method for treating the surface of a dental implant and a dental implant surface-treated by the method. [Background technology]

[0002] Dental implants are originally meant to be a substitute for lost human tissue, but in dentistry they refer to artificial teeth. This type of implant procedure is widely used as a replacement procedure that can restore or reconstruct the function of your original teeth when teeth are lost or their function has deteriorated.

[0003] The key to dental implant surgery is how well the implant integrates with the bone to form a stable bond. This is called osseointegration, and excellent osseointegration allows the implant to be fixed to the bone, achieving initial stability and then creating a permanent bond between the implant and the bone.

[0004] Bone fusion characteristics are closely related to the surface area of ​​dental implants. This is because a larger surface area increases the bone fusion area and the fixation strength. In the field of dental implants, a method of increasing the surface area by properly treating the surface of the dental implant to give it roughness is commonly used. Known surface treatment methods for giving roughness to the surface of dental implants include blasting and etching using acid.

[0005] Blasting is the process of spraying atypical ceramic powder such as Al2O3 or TiO2 at high pressure onto the machined surface of a dental implant to create a rough texture. However, blasting alone can cause problems with residues and burrs remaining on the surface of the dental implant, so etching using acid is performed as a post-treatment to remove surface residues and burrs and create a micro-rough texture.

[0006] However, in the case of the method of roughening the surface of a dental implant by blasting, although it has the effect of increasing the surface area of ​​the dental implant, the morphology of the implant surface treated by the amorphous ceramic powder sprayed at high pressure as in the example of Figure 1 is irregular, and the design of the threads and cutting edges on the machined dental implant surface as in Figure 2 may be changed.

[0007] In particular, if the surface of a dental implant has an irregular surface morphology, it may cause deviations in fusion depending on the area during the bone fusion process. Also, design changes to the threads or cutting edges on the surface of a dental implant due to blasting may apply excessive pressure to the surrounding alveolar bone when the dental implant is placed, or may affect the self-tapping function.

[0008] [Prior art documents] [Patent documents]

[0009] (Patent Document 1) Korean Registered Utility Model Publication No. 20-0386621 (Publication Date: June 2, 2005) Summary of the Invention [Problem to be solved by the invention]

[0010] The technical problem to be solved by the present invention is to provide a dental implant surface treatment method that can eliminate the influence of surface treatment on the design of the dental implant and can form a regularly patterned surface morphology on the surface of the dental implant so that stable bone fusion performance can be achieved without variation from site to site, and a dental implant surface-treated by this method. [Means for solving the problem]

[0011] According to one embodiment, there is provided a method for treating the surface of a dental implant to impart roughness to the surface, the method including: a first surface treatment step of forming grooves in a regular pattern on the entire or part of the implant surface using a laser; and a second surface treatment step of imparting roughness to the grooves and surrounding areas thereof through acid etching, wherein the first surface treatment step includes a first laser treatment step of processing a plurality of first grooves that are continuous in a first direction and spaced apart in a second direction; and a second laser treatment step of processing a plurality of second grooves that are continuous in the second direction and spaced apart in the first direction, thereby forming craters at each intersection of the first grooves and the second grooves.

[0012] In one embodiment of the present invention, the first direction may be a circumferential direction of the implant, and the second direction may be a height direction of the implant that intersects with the first direction.

[0013] In one embodiment of the present invention, the width w1 and depth d1 of the first groove may be the same as the width w2 and depth d2 of the second groove. Preferably, the widths w1 and w2 of the first and second grooves may be 5 to 30 μm, and the depths d1 and d2 may be 5 to 10 μm.

[0014] In one embodiment of the present invention, the second direction separation distance D1 between one first groove and the other adjacent first groove may be 5 to 20 μm, and the first direction separation distance D2 between one second groove and the other adjacent second groove may be 5 to 20 μm.

[0015] In one embodiment of the present invention, the second surface treatment step may involve acid etching the implant for 5 to 10 minutes.

[0016] According to another embodiment, as a means for solving the problem, a dental implant can be provided that is implanted into alveolar bone to form an artificial tooth root, and that includes an internal groove formed at a specific depth from the upper end surface to which an abutment for supporting a prosthesis is coupled, and an external surface formed with an external thread for implantation into the alveolar bone, and the entire or a part of the external surface is surface-treated by the surface treatment method according to the one embodiment.

[0017] In another embodiment of the present invention, the external thread may be composed of an external bottom thread section formed at a first depth over a first height upward from the bottom of the implant, and an external top thread section formed at a second depth lower than the first depth over a second height above the external bottom thread section, wherein the external top thread section may have a lower density of craters formed through surface treatment than the external bottom thread section. [Effects of the Invention]

[0018] According to a surface treatment method for a dental implant according to an embodiment of the present invention, roughness is imparted to the surface of the implant through precision processing using a laser, thereby preventing unintentional damage or alteration of designs such as threads or cutting edges on the surface of the implant during the surface treatment process for imparting roughness.

[0019] In addition, when treating the surface of an implant with a laser, a grid pattern surface morphology is formed first and second times, and then additional surface treatment is performed through acid etching. This allows the overall surface morphology to be regular and the surface area to be greatly increased, thereby achieving stable bone fusion performance without variation from site to site after implant placement and shortening the healing period. [Brief explanation of the drawings]

[0020] [Figure 1]1 is a diagram showing a surface treatment process by blasting, which is one method of surface treatment of an implant according to the prior art, and a schematic diagram showing the surface of an implant surface-treated by blasting.

[0021] [Figure 2] FIG. 1 is a diagram for explaining problems associated with conventional blast surface treatment.

[0022] [Figure 3] 1 is a process schematic diagram of a surface treatment method for a dental implant according to one embodiment of the present invention.

[0023] [Figure 4] FIG. 2 is a schematic diagram of the laser surface treatment performed in the first surface treatment step.

[0024] [Figure 5] FIG. 1 shows an SEM image and a cross-sectional schematic diagram showing the surface condition of an implant after the first surface treatment step.

[0025] [Figure 6] 1 shows test data showing the rate of increase in implant surface area when the implant surface is treated using a laser compared to SA (Sandblasted with alimina and acid etched), a conventionally known implant surface treatment method.

[0026] [Figure 7] 1 is a graph showing a comparison of surface roughness after surface treatment of an implant using blasting (a conventional implant surface treatment method), laser, and laser + acid etching (the present invention).

[0027] [Figure 8] FIG. 1 is a diagram showing a comparison of SEM images of an implant surface treated by blasting and an implant surface treated by a surface treatment method (laser+acid etching) according to one embodiment of the present invention.

[0028] [Figure 9] 1 is a front view of an implant surface-treated by a surface treatment method according to an embodiment of the present invention.

[0029] [Figure 10] 10 is a cutaway perspective view of an implant according to another embodiment of the present invention shown in FIG. 9. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0031] The terms used in this specification are merely used to describe specific embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless otherwise clearly indicated in the context.

[0032] As used herein, terms such as "comprise" or "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof set forth in the specification, and should be understood as not precluding the possible presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0033] Furthermore, terms such as first and second may be used to describe various components, but the components should not be limited by the terms, and the terms are used only to distinguish one component from another.

[0034] In the description with reference to the accompanying drawings, the same components will be given the same reference numerals and duplicate descriptions thereof will be omitted. In addition, in the description of the present invention, if it is determined that a detailed description of related prior art may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted.

[0035] FIG. 3 is a process schematic diagram of a surface treatment method for a dental implant according to one embodiment of the present invention.

[0036] Referring to FIG. 3, a surface treatment method according to one embodiment of the present invention is a method for increasing the surface area by providing a regular pattern of roughness to the surface of an implant so as to achieve improved bone integration performance without affecting the design of the implant that is implanted into the alveolar bone to form an artificial tooth root. The method may include a first surface treatment step S100 using a laser and a second surface treatment step S200 using acid etching.

[0037] In the first surface treatment step S100, a laser can be used to form grooves in a regular pattern on the entire or part of the implant surface. The regular pattern can be a grid pattern, and the grid pattern can include both patterns in which two groups of grooves arranged in different directions intersect each other at various angles, including a right angle.

[0038] Specifically, the first surface treatment step S100 can be composed of a primary laser treatment step S102 for processing a plurality of first grooves (hereinafter referred to as "first groove group G1") that are continuous in a first direction and spaced apart in a second direction, and a secondary laser treatment step S104 for processing a plurality of second groove groups G2 (hereinafter referred to as "second groove group G2") that are continuous in the second direction and spaced apart in the first direction, to form a crater c at each intersection of the first groove g1 and the second groove g2.

[0039] In an embodiment of the present invention, the first direction may be the circumferential direction of the implant, and the second direction may be a height direction of the implant that intersects with the first direction, but is not limited thereto. For example, the first direction may be the same as or opposite to the direction in which a thread is formed on the surface of the implant, and the second direction may be a direction obliquely inclined relative to the height direction of the implant.

[0040] When performing the primary surface treatment using a laser, the implant 100 to be surface treated is clamped in a rotating device (not shown), and the rotating device is rotated or moved in a second direction relative to a laser generator 200 (a device that irradiates a laser at a set intensity toward the implant), which is a relatively fixed body, as shown in the schematic diagram of surface treatment in Figure 4(a), thereby forming grid pattern grooves.

[0041] Conversely, as shown in the schematic diagram of surface treatment in Figure 4(b), the implant 100 to be surface treated is fixed to a fixation device (not shown), and a relatively movable laser generator 200 (a device that irradiates a laser at a set intensity toward the implant) is rotated relative to the implant 100 fixed to the fixation device or moved in a second direction, thereby forming grooves in a grid pattern.

[0042] 5 shows an SEM image and a cross-sectional view of the surface of an implant after the first surface treatment step, in which the first grooves g1 and the second grooves g2 are preferably formed in a grid pattern that intersects with each other at right angles. The width w1 of the first groove g1 and the width w2 of the second groove g2 are the same (w1=w2), and the depth d1 of the first groove g1 and the depth d2 of the second groove g2 are also the same (d1=d2).

[0043] Here, it is preferable to understand that the width w1 of the first groove g1 and the width w2 of the second groove g2 mean the average width of the grooves g1 of the first groove group G1 and the grooves g2 of the second groove group G2, which are formed in plurality, and it is also preferable to understand that the depth d1 of the first groove g1 and the depth d2 of the second groove g2 mean the average depth of the grooves g1 and g2 included in the corresponding groups.

[0044] The widths w1 and w2 of the first groove g1 and the second groove g2 may preferably be 5 to 30 μm. If the widths w1 and w2 of the first groove g1 and the second groove g2 formed in directions intersecting each other are less than 5 μm, the effect of increasing the surface area may be insufficient, resulting in reduced bone fusion properties, whereas if the widths of the first groove g1 and the second groove g2 exceed 30 μm, the durability of the implant itself may be reduced.

[0045] The depths d1 and d2 of the first groove g1 and the second groove g2 are preferably 5 to 10 μm. If the depths d1 and d2 of the first groove g1 and the second groove g2 are less than 5 μm, the effect of increasing the surface area is insufficient. If the depths of the first groove g1 and the second groove g2 are more than 10 μm, the uniformity of the microgrooves g3 formed on the surfaces of the first groove g1 and the second groove g2 through a subsequent secondary surface treatment step may be reduced.

[0046] In one embodiment of the present invention, the second direction separation distance D1 between one first groove g1 and the other adjacent first groove g1 is preferably 5 to 20 μm, taking into consideration the density of the craters c per unit area and the associated bone fusion characteristics, and the first direction separation distance D2 between one second groove g2 formed in a direction intersecting the first groove g1 and the other adjacent second groove g2 is also preferably 5 to 20 μm.

[0047] Figure 6 shows test data showing the rate of increase in implant surface area when laser-treated implants are compared to the conventional implant surface treatment method, SA (Sandblasted with alumina and acid etched). The SA method, exemplified as a conventional implant surface treatment method, involves blasting the implant surface with small alumina powder and then etching it with sulfuric acid or hydrochloric acid to impart surface roughness.

[0048] Referring to Figure 6, when a specific pattern of grooves (grid pattern grooves) is formed on the surface of an implant using a laser, the surface area increases by at least 1.16 times to at most 1.3 times compared to the conventional SA (Sandblasted with Alumina and Acid Etched) method, based on the same area.

[0049] In particular, when a 7μm deep groove is formed while maintaining a laser processing speed of 300mm / s, the surface area is increased more than when a laser processing speed of 450mm / s and a groove depth of 5μm are used. Based on this test data, the most preferable conditions for the first surface treatment stage are a laser processing speed of 300mm / s and a groove depth of 7μm.

[0050] Meanwhile, the implant, which has been given an initial surface roughness by forming grid pattern grooves on the surface through laser processing, can be subjected to a subsequent process of further roughening by exposing it to acid in the second surface treatment step S200. In the second surface treatment step S200, an acid etching process can be performed using an etching solution such as sulfuric acid or hydrochloric acid to further give micro-roughness.

[0051] In this way, if the surface of the primary laser-processed implant is additionally treated through an additional surface treatment step (second surface treatment step S200) in which the implant is exposed to an etching solution such as sulfuric acid or hydrochloric acid and etched, additional roughness is imparted to the surfaces of the grooves g1 and g2 and their surroundings by micro grooves g3, as shown in the schematic diagram of Figure 3, which can further improve the bonding strength with bone (bone fusion characteristics).

[0052] In the second surface treatment step S200, the etching solution for acid etching may be, but is not limited to, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, or hydrogen peroxide, or a mixture of two or more of these.

[0053] In one embodiment of the present invention, the second surface treatment step S200 preferably involves acid etching the implant 100 for about 5 to 10 minutes. If the etching time is less than 5 minutes, the time for the acid to chemically react with the implant surface is too short to adequately form microgrooves on the implant surface, whereas if the etching time is more than 10 minutes, the microgrooves are adequately formed, but the strength of the implant may be significantly reduced.

[0054] Of course, the acid etching time may vary depending on the type of etching solution, and therefore it should be made clear that it is not limited to the time mentioned above (5 to 10 minutes).

[0055] FIG. 7 is a graph showing a comparison of the surface roughness after surface treatment of implants using blasting (a conventional implant surface treatment method), laser, and laser + acid etching (the present invention).

[0056] As shown in Figure 7, when the implant surface was treated with a laser or laser + acid etching method, the overall surface roughness increased compared to surface treatment using blasting. In particular, it was found that surface treatment using the laser + acid etching method according to the present invention had a clear effect in terms of increasing surface roughness compared to surface treatment using only a laser.

[0057] FIG. 8 shows a comparison of SEM images of a blast-treated implant surface and an implant surface treated by a surface treatment method (laser + acid etching) according to one embodiment of the present invention. It can be seen that the surface morphology of the implant treated by the surface treatment method according to the present invention is more regular and has a much larger number of microgrooves than the surface morphology of the blast-treated implant.

[0058] According to the surface treatment method for a dental implant according to one embodiment of the present invention, roughness is imparted to the surface of the implant through precision machining using a laser, thereby preventing unintentional damage or alteration of designs such as threads or cutting edges on the surface of the implant during the surface treatment process for imparting roughness.

[0059] In addition, when treating the surface of an implant with a laser, a grid pattern surface morphology is formed first and second times, and then additional surface treatment is performed through acid etching. This allows the overall surface morphology to be regular and the surface area to be greatly increased, thereby achieving stable bone fusion performance without variation from site to site after implant placement and shortening the healing period.

[0060] Next, as another embodiment of the present invention, we will briefly look at the structure of a dental implant that has been surface-treated by the surface treatment method according to the embodiment of the present invention described above.

[0061] FIG. 9 is a front view of an implant that has been surface-treated by a surface treatment method according to one embodiment of the present invention, and FIG. 10 is a cutaway perspective view of the implant according to another embodiment of the present invention shown in FIG.

[0062] 9 and 10, a dental implant 100 according to another embodiment of the present invention is a structure to be implanted into alveolar bone to form an artificial tooth root, and may include an outer surface 101 having an outer thread 103 formed thereon for implanting the alveolar bone. Here, the outer thread 103 formed on the outer surface 101 may be divided into two thread sections having the same pitch P in the height direction in the drawing but different bone depths.

[0063] The two thread sections constituting the external thread 103 can preferably be divided into an external lower end thread section 104 formed at a first depth d3 over a certain height upward from the lower end of the outer peripheral surface of the implant 100, and an external upper end thread section 106 formed above the external lower end thread section 104 at a second depth d4 lower than the first depth d3.

[0064] The two thread sections 104, 106, which have the same pitch P but different bone depths, can be formed by a machining operation in which two cutting bits having different machining surface shapes are sequentially inserted into the machining surface of the base material of the implant 100 before the threads are machined, and then the implant 100 body is rotated while the bits are moved from the lower end to the upper end.

[0065] In this way, by configuring the bone depth d4 of the thread formed in the external upper end thread section 106 of the implant 100 to be lower than the bone depth d3 formed in the external lower end thread section 104, the implant 100 body can be more firmly fixed to the compact bone region on the surface of the alveolar bone, and by increasing the wall thickness of the upper region of the implant 100, the rigidity of that region can be increased, thereby preventing vertical cracks from occurring in the upper portion of the implant 100.

[0066] The external upper thread section 106, in which the bone depth of the threads is formed relatively low, may preferably be formed to descend from the edge of the upper surface of the implant 100 to the lower section by 2 to 5 threads. In this case, when a punching process is performed to form a polygonal or torx-shaped binding portion in the internal groove 110 formed inside the implant 100, it is possible to indirectly reinforce the corners of the binding portion, which may become the starting point of fatigue fracture.

[0067] The external upper thread section 106 of the external thread may have a non-threaded section H in which the threads are not machined over a certain section downward from the top end of the implant 100. Such non-threaded section H may be a section provided to ensure the strength of the upper end of the implant 100, where vertical cracking may be induced.

[0068] Since no threads are formed in the non-threaded section H, when implant 100 is implanted into the alveolar bone, the non-threaded section H is not inserted into the inside of the alveolar bone and may be partially exposed outside the upper surface of the alveolar bone. However, since non-threaded section H is a section formed with a length (0.2 to 0.3 mm) that is very small compared to the overall length of the body of implant 100, when non-threaded section H is not implanted inside the alveolar bone, the effect that it has on the supporting rigidity of the body of implant 100 within the alveolar bone is extremely small.

[0069] By forming a non-threaded section H at the upper end of the implant 100 in this manner, when the implant 100 is implanted into the alveolar bone, a certain level of wall thickness is ensured at the upper end of the implant 100 body without significantly reducing the support force of the implant 100 body within the alveolar bone, thereby increasing the structural rigidity of the upper end of the implant 100.

[0070] The outer surface 101 of the implant 100 according to another embodiment of the present invention may be surface-treated by the above-described surface treatment method. This allows the implant to have a regular surface morphology while having a much larger surface area than an implant surface-treated by conventional blasting of the same area standard. In this case, craters c may be formed in the outer upper thread section 106 at a lower coarseness density than in the outer lower thread section 104.

[0071] Incidentally, the crater refers to a recessed groove formed at the intersection of the first groove and the second groove.

[0072] In this way, by configuring the external lower thread section to have a higher density of craters than the external upper thread section, i.e., by configuring the external lower thread section to have a rougher surface than the external upper thread section, it is possible to achieve a more balanced improvement in the bacterial growth inhibitory effect and bone fusion characteristics.

[0073] An implant 100 according to another embodiment of the present invention may also include an internal groove 110 formed at a certain depth from the upper surface to allow an abutment for supporting a prosthesis to be coupled thereto. The internal groove 110 includes an upper inclined portion 115, a polygonal portion 120, a boring section 130, and an internal thread portion 160, and the upper inclined portion 115, the polygonal portion 120, the boring section 130, and the internal thread portion 160 may be formed sequentially from the upper end of the implant.

[0074] The upper inclined portion 115 is formed at a predetermined height from the entrance of the upper end surface of the implant 100, and may have a circular cross-sectional shape with an inner diameter that narrows downward. The polygonal portion 120 is formed at a predetermined height below the upper inclined portion 115, and may have a polygonal (e.g., square, hexagonal, octagonal) or Torx cross-sectional shape.

[0075] The internal thread portion 160 may be formed to a predetermined depth below the polygonal portion 120. A thread for abutment connection may be formed in the internal thread portion 160 with a diameter smaller than that of a circle inscribed in the polygonal shape of the polygonal portion 120, and a boring section 130 having a circular cross section and a diameter equal to or larger than the diameter of a circle circumscribed in the polygonal shape of the polygonal portion 120 may be formed between the polygonal portion 120 and the internal thread portion 160.

[0076] The polygonal portion 120 may prevent rotation of an abutment (not shown) inserted into the internal groove 110, and a screw (not shown) for fixing the abutment to the implant 100 may be threadedly coupled to the internal threaded portion 160. Furthermore, the boring section 130 may prevent compressive residual stress and the concentration of crack nuclei from occurring by pushing the base material of the upper inclined portion 115, which is plastically deformed by the impact of a punching tool during processing of the polygonal portion 120, downward from the polygonal portion 120.

[0077] In the foregoing detailed description of the present invention, specific embodiments thereof have been described. However, it is to be understood that the invention is not limited to the specific forms set forth in the detailed description, but rather includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0078] [Explanation of symbols]

[0079] 100: Implants

[0080] 101: External surface

[0081] 103: External thread

[0082] 104: External bottom thread section

[0083] 106: External upper thread section

[0084] 110: Internal groove

[0085] g1: 1st groove

[0086] g2: Second groove

[0087] c: Crater

Claims

1. 1. A method of treating a surface of a dental implant to impart roughness to the surface, comprising: a first surface treatment step in which a regular pattern of grooves is formed on the entire or part of the implant surface using a laser; a second surface treatment step of providing roughness to the grooves and surrounding areas through acid etching; The first surface treatment step includes: a first laser processing step for processing a plurality of first grooves that are continuous in a first direction and spaced apart in a second direction; a secondary laser treatment process for processing a plurality of second grooves that are continuous in the second direction and spaced apart in the first direction, and forming a crater at each intersection of the first groove and the second groove.

2. the first direction is a circumferential direction of the implant; The surface treatment method for a dental implant according to claim 1 , wherein the second direction is a height direction of the implant that intersects with the first direction.

3. A surface treatment method for a dental implant, wherein the width w1 and depth d1 of the first groove are the same as the width w2 and depth d2 of the second groove.

4. 4. The surface treatment method for a dental implant according to claim 3, wherein the widths w1 and w2 of the first and second grooves are 5 to 30 μm, and the depths d1 and d2 are 5 to 10 μm.

5. a second direction separation distance D1 between one first groove and the other adjacent first groove is 5 to 20 μm; 4. The surface treatment method for a dental implant according to claim 3, wherein a distance D2 in the first direction between one second groove and the other adjacent second groove is 5 to 20 μm.

6. 2. The surface treatment method for a dental implant according to claim 1, wherein the second surface treatment step comprises subjecting the implant to acid etching for 5 to 10 minutes.

7. A dental implant that is implanted into alveolar bone to form an artificial tooth root, an internal groove formed at a specific depth from the upper end surface, to which an abutment for supporting a prosthesis is coupled; an outer surface having an external thread formed thereon for implantation into the alveolar bone; A dental implant, the outer surface of which has been entirely or partially surface-treated by the surface treatment method according to any one of claims 1 to 6.

8. The external thread is 8. The dental implant of claim 7, comprising an external lower end thread section formed at a first depth over a first height upward from the lower end of the implant, and an external upper end thread section formed at a second depth above the external lower end thread section over a second height that is lower than the first depth.

9. The dental implant according to claim 8 , wherein the outer upper thread section has a lower density of craters formed through surface treatment than the outer lower thread section.

Images