3D printing method for osteogenesis technology and personalized box type titanium plate

Abstract

The invention discloses a 3D printing method for an osteogenesis technology and a personalized box-type titanium plate, and relates to the field of manufacturing of medical instruments, and the 3D printing method specifically comprises the following steps: S1, obtaining and preprocessing three-dimensional data of a jaw bone of a patient; s2, three-dimensional reconstruction and virtual repair of an ideal contour before bone defect; s3, digitally designing the personalized box type titanium plate; s4, the personalized box type titanium plate is manufactured through 3D printing; and S5, clinical application in an operation. According to the 3D printing method for the osteogenesis technology and the personalized box-type titanium plate, the personalized box-type titanium plate can be passively placed in place in an operation, that is, the shape of the personalized box-type titanium plate is naturally matched with a bone defect area, and an ideal bone arch outline can be perfectly recovered after the personalized box-type titanium plate is placed. The dependence on the personal skill of doctors is reduced, the operation result has predictability and repeatability, and the fixing stability of the box-type titanium plate is far better than that of a bone block because the box-type titanium plate is directly fixed on the residual healthy basal bone through the screws.

Description

Technical Field

[0001] This invention relates to the field of medical device manufacturing, and in particular to a 3D printing method for osteogenic technology and a personalized box-shaped titanium plate. Background Technology

[0002] In the field of oral and maxillofacial surgery, osteogenic techniques for large bone defects are mainly divided into two directions: guided bone regeneration technology and bio-bone augmentation technology. This invention focuses on the latter, namely, revolutionizing the framework construction of bio-bone augmentation technology.

[0003] Traditional bioplastic bone augmentation techniques rely on transplanting autologous bone grafts to create a sufficiently rigid scaffold. This scaffold must support the soft tissue at the bone defect site, forming a stable space to accommodate more bone graft material and ultimately achieve high-quality bone regeneration. However, this traditional method suffers from a series of inherent and insurmountable limitations: The surgical procedure is complex and highly invasive: the framework is constructed entirely from autologous bone grafts taken from other parts of the patient's body. This means that a single surgery involves at least two surgical areas, increasing surgical time, blood loss, and postoperative pain. The harvested bone grafts also need to be manually sculpted and shaped by the surgeon during the operation to fit the shape of the bone defect as closely as possible. This process is extremely dependent on the surgeon's experience and touch, making it both tedious and difficult to guarantee precision.

[0004] Limited applicability: Traditional methods fall short when dealing with large and complex bone defects. On the one hand, patients may not be able to provide enough autologous bone blocks to construct a large framework. On the other hand, for severe bone defects, especially those spanning anatomical quadrants, there is a lack of sufficiently healthy and robust basal bone to stabilize these frameworks pieced together from bone blocks. The inflexible nature of bone blocks also makes it difficult for them to adapt to complex three-dimensional contours, rendering bio-bone augmentation techniques almost unusable in such cases. Physicians are forced to turn to guided bone regeneration techniques, which have relatively lower osteogenic quality.

[0005] Postoperative outcomes are unstable: Even if the surgery is successful, the mechanical strength of the bone-framed structure is limited. During the postoperative healing period, the powerful maxillofacial muscles, especially the masticatory muscles, continuously exert pressure on the repaired area. Traditional bone frames cannot withstand this pressure for long periods, and are prone to micromovements or even collapse, leading to changes in the painstakingly restored bone contour, loss of graft material, and ultimately affecting the quality and aesthetic results of bone formation.

[0006] Therefore, it is necessary to propose an osteogenic technology using 3D printing and personalized box-shaped titanium plates to solve the above problems. Summary of the Invention

[0007] The main objective of this invention is to provide a 3D printing method for osteogenic technology and a personalized box-shaped titanium plate, which can effectively solve the problems in the background art.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A 3D printing method for osteogenic technology includes the following steps: S1: Acquisition and preprocessing of three-dimensional data of the patient's jawbone, used to obtain high-precision three-dimensional data of the bone defect area and the surrounding normal anatomical structure; S2: Three-dimensional reconstruction and virtual repair of the ideal contour before bone defects, used to restore the normal physiological morphology before bone defects occur; S3: Digital design of personalized box-shaped titanium plates, used to design box-shaped titanium plates; S4: 3D printing manufactures personalized box-shaped titanium plates to transform S3's digital designs into physical objects; S5: Clinical application during surgery, installation of box-shaped titanium plates is performed according to standard surgical procedures.

[0009] Preferably, step S1 specifically includes the following steps: S101: Medical image acquisition, using one of the commonly used clinical cone-beam computed tomography (CBCT) or multi-slice spiral CT equipment. The slice thickness must be set to no more than 0.5 mm. For defects involving fine structures, a scanning protocol with a slice thickness of 0.3 mm is used. The scanning range must fully cover the bone defect area and a sufficient range of normal bone tissue around it. After the scan is completed, the obtained raw image data is exported and stored in the internationally recognized DICOM format. S102: Data preprocessing and preliminary segmentation: Import DICOM data into professional medical image processing software; using the software's threshold segmentation function, separate bone tissue from soft tissue by setting the range of Henle units.

[0010] Preferably, step S2 specifically includes the following steps: S201: 3D model generation: Convert the data segmented in S102 into a 3D mesh model, perform a light smoothing process on the model to eliminate the jagged edges of the mesh caused by image noise, while preserving all key anatomical landmarks. S202: Virtual reconstruction of the ideal contour, specifically including: Application of mirror method: For unilateral bone defects, the mirror method is used to flip the three-dimensional model of the mandible and maxilla on the healthy side to cover the defect side. Through the registration function of the software, the mirror model and the defect model are precisely aligned in the unaffected anatomical area. After alignment, the outline presented by the mirror model is the ideal outline before the bone defect. Database reference method: When the mirror method cannot be used for midline cross- and bilateral defects, an anatomical morphology database based on population statistics is used as a reference. The remaining part of the patient's defect model is matched with the normal anatomical model in the database, and the software automatically generates an ideal contour model that best conforms to the physiological morphology after repair. Generate a virtual repair model: Finally, this reconstructed ideal contour model is defined as the virtual repair model.

[0011] Preferably, step S3 specifically includes: designing the main body shape of the box-shaped titanium plate, using the virtual repair model generated in S202 as a reference, and generating a three-dimensional curved surface that closely matches the ideal bone arch contour through curved surface modeling technology. The three-dimensional curved surface is the outer surface of the box-shaped titanium plate; the inner surface of the box-shaped titanium plate needs to achieve maximum fit with the actual surface of the bone defect area collected by the patient in S101, and using the software's wrapping and offset functions, a fitting surface consistent with the bone surface morphology is generated; the space between the outer surface and the inner surface is solidified to form the initial three-dimensional model of the box-shaped titanium plate.

[0012] Preferably, step S4 specifically includes the following steps: S401: Printing equipment and materials. The printing equipment uses selective laser melting printing equipment in metal additive manufacturing technology; it uses Ti-6Al-4VELI grade titanium alloy powder that meets medical device standards. S402: During the printing process, the final 3D model of the box-shaped titanium plate determined in S3 is imported into the control system of the selective laser melting printing equipment. Inside the printing chamber, a layer of titanium alloy powder is laid. The high-power fiber laser of the selective laser melting printing equipment selectively melts the powder particles according to the cross-sectional information of each layer of the model. The printing platform descends by one layer thickness, the powder is re-laid, and the laser scanning process is repeated. This process is repeated layer by layer until the entire box-shaped titanium plate is printed. The entire printing process is carried out in a protective atmosphere filled with high-purity argon. S403: Post-processing and sterilization. After printing, the box-shaped titanium plate is removed and subjected to stress-relieving heat treatment. Sandblasting and electrochemical polishing are used to remove unmelted powder adhering to the surface and smooth the surface. The screw holes are then finely machined to ensure smooth threads. Finally, the finished box-shaped titanium plate is thoroughly cleaned and sterilized by high-pressure steam sterilization and gamma ray irradiation to meet surgical use standards.

[0013] Preferably, step S5 specifically includes the following steps: S501: Surgical exposure. Following standard surgical procedures, the soft tissue flap is incised and raised to fully expose the bone defect area. S502: Positioning and Fixation: Place the sterilized box-shaped titanium plate in the bone defect area, and then insert titanium alloy screws through the pre-designed screw holes to firmly fix the box-shaped titanium plate to the healthy basal bone below. S503: Forms a rigid frame; once fixed, the box-shaped titanium plate immediately forms a stable and ideal rigid support space above the bone defect area. S504: Implant autologous bone graft material. The prepared bone graft material is filled into this scaffold space. S505: Close the wound. Finally, reposition the soft tissue flap and suture it.

[0014] A personalized box-shaped titanium plate, the shape of which is consistent with the contour height before the bone defect, is used to restore the normal bone arch shape. In the design, the inner surface of the box-shaped titanium plate is attached to the surface of the basal bone in the bone defect area, while the outer surface forms a smooth arc for filling and fixing bone graft materials.

[0015] Preferably, the box-shaped titanium plate has a thickness of 0.5-2.0 mm to provide sufficient rigidity and prevent postoperative collapse due to muscle pressure.

[0016] Preferably, the edge of the box-shaped titanium plate is designed with 4-8 fixing holes with a diameter of 1.5-2.0 mm for fixing titanium alloy screws. The position of the fixing holes is determined based on the bone density distribution of the surrounding healthy bone, with priority given to areas with high bone density.

[0017] Compared with existing technologies, this invention provides a 3D printing method for osteogenic technology and a personalized box-shaped titanium plate, which has the following beneficial effects: This osteogenic technique uses 3D printing and personalized box-shaped titanium plates. Through preoperative medical imaging data, mirror reconstruction, and virtual design, it transfers the most complex and experience-dependent framework shaping process from the operating room to a computer. Surgeons no longer need to harvest, sculpt, and fix bone blocks within the tight surgical timeframe, greatly shortening the operation time and reducing surgical trauma and patient stress.

[0018] This osteogenesis technique utilizes 3D printing and personalized box-shaped titanium plates. These plates are passively positioned during surgery, naturally conforming to the bone defect area and perfectly restoring the ideal bone arch contour. This reduces reliance on the surgeon's individual skill, making surgical outcomes more predictable and repeatable. Because the box-shaped titanium plates are directly fixed to the remaining healthy bone fragments with screws, their fixation stability far surpasses that of bone blocks. This allows the invention to be successfully applied to complex cases that traditional techniques cannot handle, such as large-scale bone defects, cross-quadrant defects, and cases with poor basal bone conditions.

[0019] This osteogenic technology utilizes 3D printing and personalized box-shaped titanium plates. 3D printing can manufacture components of virtually any complex shape, unrestricted by traditional processing methods. Therefore, regardless of the irregularity of the bone defect's shape, this method can design and manufacture precisely matched personalized box-shaped titanium plates, broadening the indications for bio-bone augmentation technology. The rigidity provided by titanium alloy materials is unparalleled by bone blocks. This rigid framework effectively resists the contractile pressure of surrounding muscles, maintaining the stability of the osteogenic space throughout the healing period, preventing contour collapse, and providing crucial mechanical support for the growth of new bone in the bone graft material, thereby directly promoting higher quality and faster bone regeneration. Attached Figure Description

[0020] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0021] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0022] Example 1: like Figure 1 As shown, a 3D printing method for osteogenic technology includes the following steps: S1: Acquisition and preprocessing of the patient's jawbone 3D data, used to obtain high-precision 3D data of the bone defect area and surrounding normal anatomical structures, specifically including the following steps: S101: Medical image acquisition, using either a cone-beam computed tomography (CBCT) scanner or a multi-slice spiral CT scanner commonly used in clinical practice. To ensure sufficient data accuracy to support subsequent modeling and printing, the slice thickness must be set to no more than 0.5 mm. For defects involving fine structures, such as alveolar ridge crests or near the nerve canal, a scanning protocol with a slice thickness of 0.3 mm is used. The scanning range must fully cover the bone defect area and a sufficient area of ​​normal bone tissue around it. For example, the remaining bone in the defect area is used to fix the box-shaped titanium plate to ensure that there is enough healthy bone area for the subsequent box-shaped titanium plate fixation design. After the scan is completed, the obtained raw image data is exported and stored in the internationally recognized DICOM format. S102: Data preprocessing and preliminary segmentation. Import DICOM data into professional medical image processing software, such as Mimics or the open-source software 3DSlicer. Using the software's threshold segmentation function, bone tissue and soft tissue are initially separated by setting an appropriate range of Henle units. Due to the irregular shape of the bone defect area, it is necessary to combine manual or semi-automatic region growth algorithms to accurately delineate the boundary of the defect.

[0023] S2: Three-dimensional reconstruction and virtual repair of the ideal contour before bone defect, used to restore the normal physiological morphology before bone defect occurs, specifically including the following steps: S201: 3D model generation: Convert the data segmented in S102 into a 3D mesh model, perform a light smoothing process on the model to eliminate the jagged edges of the mesh caused by image noise, while preserving all key anatomical landmarks. S202: Virtual reconstruction of the ideal contour, specifically including: Application of mirror method: For unilateral bone defects, the mirror method is used to flip the three-dimensional model of the mandible and maxilla on the healthy side to cover the defect side. Through the registration function of the software, the mirror model and the defect model are precisely aligned in the unaffected anatomical areas, such as the lower edge of the mandible and the coracoid process. After alignment, the outline presented by the mirror model is the ideal outline before the bone defect. Database reference method: For midline-crossing and bilateral defects where the mirror method cannot be used, an anatomical morphology database based on population statistics is used as a reference. The remaining part of the patient's defect model is matched with the normal anatomical model in the database, and the software automatically generates an ideal contour model that best matches the physiological morphology after repair. Machine learning algorithms can be introduced into this process to improve the accuracy of matching and prediction. Generating a virtual repair model: Finally, this reconstructed ideal contour model is defined as the virtual repair model, which serves as the direct blueprint for the subsequent personalized outer surface shape of the box-shaped titanium plate.

[0024] S3: Digital design of personalized box-shaped titanium plates, used to design box-shaped titanium plates, specifically including: design of the main shape of the box-shaped titanium plate, based on the virtual repair model generated by S202, using surface modeling technology to generate a three-dimensional curved surface that closely fits the ideal bone arch contour, the three-dimensional curved surface being the outer surface of the box-shaped titanium plate; the inner surface of the box-shaped titanium plate needs to fit as closely as possible to the actual surface of the bone defect area acquired by S101, using the software's wrapping and offset functions to generate a fitting surface consistent with the bone surface morphology; the space between the outer and inner surfaces is solidified to form the initial three-dimensional model of the box-shaped titanium plate.

[0025] S4: 3D printing manufactures personalized box-shaped titanium plates to transform S3's digital designs into physical objects, specifically including the following steps: S401: Printing equipment and materials. The printing equipment uses selective laser melting printing equipment in metal additive manufacturing technology; it uses Ti-6Al-4VELI grade titanium alloy powder that meets medical device standards; the particle size distribution of the titanium alloy powder is usually between 15-45 micrometers to ensure printing accuracy and surface quality. S402: During the printing process, the final 3D model of the box-shaped titanium plate determined in S3 is imported into the control system of the selective laser melting printing equipment. Inside the printing chamber, a layer of titanium alloy powder is laid. The high-power fiber laser of the selective laser melting printing equipment selectively melts the powder particles according to the cross-sectional information of each layer of the model. The printing platform descends by one layer thickness, the powder is re-laid, and the laser scanning process is repeated. This process is repeated layer by layer until the entire box-shaped titanium plate is printed. The entire printing process is carried out in a protective atmosphere filled with high-purity argon. S403: Post-processing and sterilization. After printing, the box-shaped titanium plate is removed and subjected to stress-relieving heat treatment to eliminate the internal stress generated during printing. Sandblasting, such as with alumina or glass beads, and electrochemical polishing are used to remove unmelted powder adhering to the surface and smooth the surface. The screw holes are then precision machined to ensure smooth threads. Finally, the finished box-shaped titanium plate is thoroughly cleaned and sterilized using high-pressure steam sterilization and gamma ray irradiation to meet surgical use standards.

[0026] S5: Clinical application during surgery. The installation of the box-shaped titanium plate is performed according to the standard surgical procedure, specifically including the following steps: S501: Surgical exposure. Following standard surgical procedures, the soft tissue flap is incised and raised to fully expose the bone defect area. S502: Positioning and Fixation. The sterilized box-shaped titanium plate is placed in the bone defect area. Since it is designed strictly according to the patient's anatomical shape, it can usually achieve passive positioning, that is, perfect fit with the bone surface, without the need for or with minimal force to adjust the deformation. Then, titanium alloy screws are implanted through the pre-designed screw holes to firmly fix the box-shaped titanium plate to the healthy basal bone below. S503: Forming a rigid frame, the fixed box-shaped titanium plate immediately forms a stable and ideal rigid support space above the bone defect area, which completely restores the contour before the bone defect; S504: Implantation of autologous bone graft material. Prepared bone graft material, such as autologous bone particles, allogeneic bone, or synthetic bone substitute material, is filled into this scaffold space. S505: Close the wound. Finally, reposition and suture the soft tissue flap. The rigidity of the box-shaped titanium plate ensures that the contour of the bone graft material inside will not collapse during the healing period, regardless of the movement of the surrounding muscles, providing a crucial prerequisite for high-quality biological bone regeneration.

[0027] Example 2: A personalized box-shaped titanium plate, the shape of which is consistent with the contour of the bone defect before it is removed, is used to restore the normal bone arch shape. In the design, the inner surface of the box-shaped titanium plate is attached to the surface of the basal bone in the bone defect area, while the outer surface forms a smooth arc for filling and fixing bone graft materials. The box-shaped titanium plate is 0.5-2.0 mm thick to provide sufficient rigidity to prevent postoperative collapse due to muscle pressure, and to promote vascular ingrowth and bone integration. The graft material inside the box-shaped titanium plate is in contact with the alveolar ridge tissue. The box-shaped titanium plate is not a sealed box; it has one open side to provide conditions for tissue vascularization. The edge of the box-shaped titanium plate is designed with 4-8 fixing holes, with a hole diameter of 1.5-2.0 mm, for fixing titanium alloy screws. The position of the fixing holes is determined based on the bone density distribution of the surrounding healthy bone, with priority given to areas with high bone density.

[0028] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A 3D printing method for osteogenic technology, characterized in that: The following steps are included: S1: Acquisition and preprocessing of three-dimensional data of the patient's jawbone, used to obtain high-precision three-dimensional data of the bone defect area and the surrounding normal anatomical structure; S2: Three-dimensional reconstruction and virtual repair of the ideal contour before bone defects, used to restore the normal physiological morphology before bone defects occur; S3: Digital design of personalized box-shaped titanium plates, used to design box-shaped titanium plates; S4: 3D printing manufactures personalized box-shaped titanium plates to transform S3's digital designs into physical objects; S5: Clinical application during surgery, installation of box-shaped titanium plates is performed according to standard surgical procedures.

2. The 3D printing method for osteogenic technology according to claim 1, characterized in that: S1 specifically includes the following steps: S101: Medical image acquisition, using one of the commonly used clinical cone-beam computed tomography (CBCT) or multi-slice spiral CT equipment. The slice thickness must be set to no more than 0.5 mm. For defects involving fine structures, a scanning protocol with a slice thickness of 0.3 mm is used. The scanning range must fully cover the bone defect area and a sufficient range of normal bone tissue around it. After the scan is completed, the obtained raw image data is exported and stored in the internationally recognized DICOM format. S102: Data preprocessing and preliminary segmentation: Import DICOM data into professional medical image processing software; using the software's threshold segmentation function, separate bone tissue from soft tissue by setting the range of Henle units.

3. The 3D printing method for osteogenic technology according to claim 2, characterized in that: S2 specifically includes the following steps: S201: 3D model generation: Convert the data segmented in S102 into a 3D mesh model, perform a light smoothing process on the model to eliminate the jagged edges of the mesh caused by image noise, while preserving all key anatomical landmarks. S202: Virtual reconstruction of the ideal contour, specifically including: Application of mirror method: For unilateral bone defects, the mirror method is used to flip the three-dimensional model of the mandible and maxilla on the healthy side to cover the defect side. Through the registration function of the software, the mirror model and the defect model are precisely aligned in the unaffected anatomical area. After alignment, the outline presented by the mirror model is the ideal outline before the bone defect. Database reference method: When the mirror method cannot be used for midline cross- and bilateral defects, an anatomical morphology database based on population statistics is used as a reference. The remaining part of the patient's defect model is matched with the normal anatomical model in the database, and the software automatically generates an ideal contour model that best matches the physiological morphology after repair. Generate a virtual repair model: Finally, this reconstructed ideal contour model is defined as the virtual repair model.

4. The 3D printing method for osteogenic technology according to claim 3, characterized in that: Specifically, S3 includes: designing the main body shape of the box-shaped titanium plate, using the virtual repair model generated in S202 as a reference, and generating a three-dimensional curved surface that closely matches the ideal bone arch contour through curved surface modeling technology. The three-dimensional curved surface is the outer surface of the box-shaped titanium plate; the inner surface of the box-shaped titanium plate needs to achieve maximum fit with the actual surface of the bone defect area collected by S101 on the patient. Using the software's wrapping and offset functions, a fitting surface consistent with the bone surface morphology is generated; the space between the outer surface and the inner surface is solidified to form the initial three-dimensional model of the box-shaped titanium plate.

5. The 3D printing method for osteogenic technology according to claim 1, characterized in that: S4 specifically includes the following steps: S401: Printing equipment and materials. The printing equipment uses selective laser melting printing equipment in metal additive manufacturing technology; it uses Ti-6Al-4VELI grade titanium alloy powder that meets medical device standards. S402: During the printing process, the final 3D model of the box-shaped titanium plate determined in S3 is imported into the control system of the selective laser melting printing equipment. Inside the printing chamber, a layer of titanium alloy powder is laid. The high-power fiber laser of the selective laser melting printing equipment selectively melts the powder particles according to the cross-sectional information of each layer of the model. The printing platform descends by one layer thickness, the powder is re-laid, and the laser scanning process is repeated. This process is repeated layer by layer until the entire box-shaped titanium plate is printed. The entire printing process is carried out in a protective atmosphere filled with high-purity argon. S403: Post-processing and sterilization. After printing, the box-shaped titanium plate is removed and subjected to stress-relieving heat treatment. Sandblasting and electrochemical polishing are used to remove unmelted powder adhering to the surface and smooth the surface. The screw holes are then finely machined to ensure smooth threads. Finally, the finished box-shaped titanium plate is thoroughly cleaned and sterilized by high-pressure steam sterilization and gamma ray irradiation to meet surgical use standards.

6. The 3D printing method for osteogenic technology according to claim 1, characterized in that: S5 specifically includes the following steps: S501: Surgical exposure. Following standard surgical procedures, the soft tissue flap is incised and raised to fully expose the bone defect area. S502: Positioning and Fixation: Place the sterilized box-shaped titanium plate in the bone defect area, and then insert titanium alloy screws through the pre-designed screw holes to firmly fix the box-shaped titanium plate to the healthy basal bone below. S503: Forms a rigid frame; once fixed, the box-shaped titanium plate immediately forms a stable and ideal rigid support space above the bone defect area. S504: Implant autologous bone graft material. The prepared bone graft material is filled into this scaffold space. S505: Close the wound. Finally, reposition the soft tissue flap and suture it.

7. A personalized box-shaped titanium plate, using a 3D printing method for osteogenic technology as described in any one of claims 1-6, characterized in that: The box-shaped titanium plate has a shape that matches the contour height before the bone defect, and is used to restore the normal bone arch shape. In the design, the inner surface of the box-shaped titanium plate fits the surface of the basal bone in the bone defect area, while the outer surface forms a smooth arc for filling and fixing bone graft materials.

8. A personalized box-shaped titanium plate according to claim 7, characterized in that: The box-shaped titanium plate is 0.5-2.0 mm thick to provide sufficient rigidity and prevent postoperative collapse due to muscle pressure.

9. A personalized box-shaped titanium plate according to claim 7, characterized in that: The edge of the box-shaped titanium plate is designed with 4-8 fixing holes with a diameter of 1.5-2.0 mm for fixing titanium alloy screws. The position of the fixing holes is determined based on the bone density distribution of the surrounding healthy bone, with priority given to areas with high bone density.

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