Rapid Prototyping Journal

Rapid Prototyping Journal Emerald Article: An improved methodology for design of custom-made hip prostheses to be fabricated using additive manufacturing technologies Sadegh Rahmati, Farid Abbaszadeh, Farzam Farahmand

Article information: To cite this document: Sadegh Rahmati, Farid Abbaszadeh, Farzam Farahmand, (2012),"An improved methodology for design of custom-made hip prostheses to be fabricated using additive manufacturing technologies", Rapid Prototyping Journal, Vol. 18 Iss: 5 pp. 389 - 400 Permanent link to this document: http://dx.doi.org/10.1108/13552541211250382 Downloaded on: 23-07-2012 References: This document contains references to 45 other documents To copy this document: [email protected]

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An improved methodology for design of custom-made hip prostheses to be fabricated using additive manufacturing technologies Sadegh Rahmati Department of Mechanical Engineering, Majlesi Branch, Islamic Azad University, Isfahan, Iran

Farid Abbaszadeh Young Researchers Club, Science and Research Branch, Islamic Azad University, Tehran, Iran, and

Farzam Farahmand School of Mechanical Engineering, Sharif University of Technology and RCSTIM, Tehran University of Medical Sciences, Tehran, Iran Abstract Purpose – The purpose of this paper is to present an improved methodology for design of custom-made hip prostheses, through integration of advanced image processing, computer aided design (CAD) and additive manufacturing (AM) technologies. Design/methodology/approach – The proposed methodology for design of custom-made hip prostheses is based on an independent design criterion for each of the intra-medullary and extra-medullary portions of the prosthesis. The intra-medullar part of the prosthesis is designed using a more accurate and detailed description of the 3D geometry of the femoral intra-medullary cavity, including the septum calcar ridge, so that an improved fill and fit performance is achieved. The extra-medullary portion of the prosthesis is designed based on the anatomical features of the femoral neck, in order to restore the original biomechanical characteristics of the hip joint. The whole design procedure is implemented in a systematic framework to provide a fast, repeatable and non-subjective response which can be further evaluated and modified in a preplanning simulation environment. Findings – The efficacy of the proposed methodology for design of custom-made hip prostheses was evaluated in a case study on a hip dysplasia patient. The cortical bone was distinguished from cancellous in CT images using a thresholding procedure. In particular the septum calcar ridge could be recognized and was incorporated in the design to improve the primary stability of the prosthesis. The lateral and frontal views of the prosthesis, with the patient’s images at the background, indicated a close geometrical match with the cortical bone of femoral shaft, and a good compatibility with the anatomy of the proximal femur. Also examination of the cross sections of the prosthesis and the patient’s intra-medullary canal at five critical levels revealed close geometrical match in distal stem but less conformity in proximal areas due to preserving the septum calcar ridge. The detailed analysis of the fitting deviation between the prosthesis and point cloud data of the patient’s femoral intra-medullary canal, indicated a rest fitting deviation of 0.04 to 0.11 mm in stem. However, relatively large areas of interference fit of 20.04 mm were also found which are considered to be safe and not contributing to the formation of bone cracks. The geometrical analysis of the extra-medullary portion of the prosthesis indicated an anteversion angle of 12.5 degrees and a neck-shaft angle of 131, which are both in the acceptable range. Finally, a time and cost effective investment casting technique, based on AM technology, was used for fabrication of the prosthesis. Originality/value – The proposed design methodology helps to improve the fixation stability of the custom made total hip prostheses and restore the original biomechanical characteristics of the joint. The fabrication procedure, based on AM technology, enables the production of the customized hip prosthesis more accurately, quickly and economically. Keywords Rapid prototypes, Manufacturing systems, Prosthetic devices, Computer aided design, Total hip replacement, Fill and fit, Medical rapid prototyping Paper type Research paper

femoral canal, THR prostheses can be categorized into two distinct groups of cemented and cementless. Cementless THR stems, which rely on biological fixation, are usually the favourite choice of orthopaedic surgeons for younger patients, due to the ease of implantation and shortcomings attributed to bone cementing (Rubin et al., 1992; Adam et al., 2002). However, a cementless THR stem needs to be designed in order to provide adequate initial stability and to encourage bone to osseointegrate onto or into the implant. Several different design concepts have been put into clinical application for cementless stems,

1. Introduction Total hip replacement (THR) or total hip arthroplasty (THA), is known as a common and highly successful operation in orthopaedic surgery (Bennett and Goswami, 2008). With respect to the fixation method of the prosthesis stem inside the The current issue and full text archive of this journal is available at www.emeraldinsight.com/1355-2546.htm

Rapid Prototyping Journal 18/5 (2012) 389– 400 q Emerald Group Publishing Limited [ISSN 1355-2546] [DOI 10.1108/13552541211250382]

Received: 30 December 2010 Revised: 3 June 2011, 2 November 2011 Accepted: 12 January 2012

389

Custom-made hip prostheses to be fabricated using AM technologies

Rapid Prototyping Journal

Sadegh Rahmati, Farid Abbaszadeh and Farzam Farahmand

Volume 18 · Number 5 · 2012 · 389 –400

over the past years, to fulfil these requirements. In particular, the initial fixation of the stem might be achieved through a press fit in the diaphysis, metaphysis, diaphysis-metaphysis junction, or a combination of the three (Learmonth et al., 2007). Moreover, the metaphyseal-fit prosthesis might be either a wedge-shaped or fit-and-fill. The fit-and-fill principle is one of the earliest design rationales in the evolution of cementless femoral stems. This premise is based upon the belief that maximizing contact of the stem with host bone would provide the greatest fixation stability and the most optimal long-term bone osteointegration with the implant. Although there has been a trend toward other design concepts in recent years, e.g. tapered wedge stems (Marshall et al., 2004), the fit-and-fill approach is still a common basis for design of cementless stems and has been validated to be valuable in providing long-term, pain-free, stable THA function (Jones et al., 2008). An optimal fit-and-fill between the prosthesis stem and the host bone is difficult to achieve in practice, with commercially available marketed prostheses, even with various anatomic designs and sizes. Several researchers (Noble et al., 1988; Philippe et al., 2005) have demonstrated a great diversity in the anatomy of the proximal femur and the size and shape of the femoral cavity in patients. Furthermore, for patients with abnormal femoral anatomy, the geometry of both the intramedullary canal and the extramedullary part of femur are unusual (Husmann et al., 1997). A solution for the problem has been the customized femoral stems, which are designed and manufactured to match the 3D geometry of the femur on a patient specific basis. At the initial step, the geometrical information of the femoral intramedullary canal is acquired via intra-operative molding (Mulier et al., 1989; Mathur et al., 1996), conventional radiography, (Rubin et al., 1992; Ruya et al., 2005), or computed tomography (CT), with the latter well established as the most informative, accurate and feasible technique for clinical practice (Werner et al., 2000; Adam et al., 2002). The following step involves the design of the prosthesis with the highest possible conformity with patient’s geometry while other biomechanical and clinical requirements are also taken into account. In spite of different CAD/CAM methodologies utilised for the design of customized THR prostheses (Bo et al., 1997; Kim et al., 1998; Werner et al., 2000; Viceconti et al., 2001; Adam et al., 2002; Pawlikowski et al., 2003; Guo et al., 2004; Kawate et al., 2008), there are still insufficiencies concerning multiple factors, such as: 1 Inadequate fit-and-fill of the stem due to simplification of the 3D curvatures of the femoral cavity and neglecting the septum calcar ridge (Bo et al., 1997; Viceconti et al., 2001; Kawate et al., 2008). 2 Altered biomechanics of the joint due to 2D analysis of the design parameters of the prosthetic neck, e.g. leg length, femoral offset, and anteversion angle (Werner et al., 2000; Viceconti et al., 2001; Pawlikowski et al., 2003; Kawate et al., 2008). 3 Prolonged design procedure and low repeatability due to the lack of systematic design procedure (Werner et al., 2000; Guo et al., 2004; Kawate et al., 2008). 4 Difficulties in communication between the designer and surgeon due to the lack of an effective preplanning simulation environment and/or a physical prototype (Bo et al., 1997; Werner et al., 2000; Viceconti et al., 2001). 5 Costly fabrication procedure (Werner et al., 2000).

The purpose of this study was to develop an improved methodology for design of customized hip prosthesis, based on the fit-and-fill concept. The design methodology includes an independent design criterion for each of the intramedullary and extramedullary portions of the prosthesis. The intramedullary part of the prosthesis is designed using a more accurate and detail description of the 3D geometry of the femoral intramedullary cavity, including the septum calcar ridge, so that an improved fit-and-fill performance is achieved. The extramedullary portion of the prosthesis is designed based on the anatomical features of the femoral neck in order to restore the original biomechanical characteristics of the hip joint. The whole design procedure is implemented in a systematic framework to provide a fast, repeatable and non-subjective response which can be further evaluated and modified in a preplanning simulation environment. The efficacy of the proposed design methodology is then evaluated through a case study in which a time and cost effective casting technique, based on additive manufacturing (AM) technology, is used for precise fabrication of the designed prosthesis for a hip dysplasia patient, with all of its geometrical details and complexities, while satisfying biomaterial constraints.

2. Design methodology The proposed systematic methodology for design and fabrication of custom-made hip prosthesis is shown in Figure 1. It is suggested that the design procedure to be implemented within the MIMICS (n.d.) Software (Version 10.01, Materialise NV, Leuven, Belgium) environment, in order to utilise the available interactive tools for processing of medical image data, 3D object rendering, and preplanning and simulation of the surgical procedure. The effective 3D virtual environment of MIMICS enables the surgeon and designer/manufacturer to simulate the complex surgeries and make the right decision based on virtual reality prior to surgery. Furthermore, it provides links with rapid prototyping machines for the subsequent fabrication stages (Handels et al., 2001; Lee et al., 2006; Wong et al., 2007). The design procedure is initiated from non-invasive data acquisition of the anatomical geometry of the region of interest (ROI) via CT imaging of the patient’s hip joint. The distal part of the femur is also included in imaging to determine the femoral neck anteversion angle. Since the accuracy of the geometrical model is influenced by imaging quality (Bibb and Winder, 2009; Mallepree and Bergers, 2009), the CT data is to be acquired with at least 1mm slice thickness, 512 £ 512 pixel resolution, and 08 gantry tilt. The image data is then imported into MIMICS and segmented with an appropriate threshold to isolate the bony structures at ROI. In the present experience, a threshold value of approximately 200-2,000 Hounsfield units (HU) satisfies the requirements for isolating the bone well (Winder and Bibb, 2005). The 3D geometry of the bones is then obtained by region growing technique of MIMICS (Figure 2). In order to start the main steps of the design procedure, at first the proximal femur plane which is cut by surgeon during surgery for removing the femoral head, is to be determined. The approximate position of this plane is at the lower resection level of femur, and 20 mm above lesser trochanter (Figure 2). In this design methodology, the cutting plan separates the intramedullary and extramedullary portions of the prosthesis 390




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Figure 1 The proposed methodology for design and fabrication of custom-made hip prosthesis

Design of Intra-Medullary Part

Design of Extra-Medullary Part

CT Scanning

Thresholding 2

3D Reconstruction of Femur

Thresholding 1

Slices Marking by Points

Cutting of Femoral Neck

Femoral Neck Axis Assessment

Extracting Spline Curves from Point Cloud

Assessment of Interface Plane

Aligning Femoral Neck of Prosthesis

Stem Design using Spline Curves

Integrating the design using Interface Plane

Fill and Fit Simulation and Evaluation

Fabrication of Prosthesis using AM

patient’s anatomies, and its crucial effect on the performance of the designed prosthesis, it is strongly recommended that this interface plane to be determined in close consultation with surgeon.

Figure 2 The 3D geometrical model of the hip joint obtained by isolating the bones from CT images through thresholding

2.1 Design of intramedullary portion The intramedullary part of the prosthesis is designed using an accurate and detailed description of 3D geometry of the femoral intramedullary cavity, so that an improved fit-and-fill performance is achieved. This not only ensures a primary acceptable stability for the prosthesis, but it is also essential to reduce the micromotion and enhance the bone-implant osteointegration, providing a durable fixation and thus secondary long-term stability. Furthermore, by restoring the physiological load transfer from the prosthesis stem to the femoral shaft, it can improve the bone stock anchoring and reduce bone resorption which is a main cause of prosthesis failure in long-term. In order to achieve the highest possible fit-and-fill, all details of geometrical data available are incorporated in the patient’s CT images of femoral bone. Each CT slice, from distal femur to the cutting plane is processed to extract the relevant crosssection of the femoral intramedullary canal. The border between the cancellous and cortical bones might be obtained via a thresholding process in MIMICS, using an appropriate threshold value. However, determination of a constant threshold for all bones is challenging since the bone density and image gray level are affected by patient’s characteristics and imaging parameters, respectively. Adam et al. (2002) realized this fact and suggested the threshold value to be

Note: The cutting plan that separates the intra-medullary and extramedullary portions of the prosthesis is shown which is supposed to be designed based on independent design criterion. These two portions are then integrated through a common interface surface, to finalize the prosthesis design. Due to the variability of the appropriate cutting plane in different 391




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chosen in a range slightly higher than the cancellous HU but lower than two-third of the cortical peak. They reported that a thresholding value of 600-800 HU yields a close cortical bone structure, especially in the proximal part of the femur with areas of thin cortex. In the present experience, a threshold value of 700 HU has satisfied to distinguish the cancellous and cortical bones. With the above thresholding procedure, the septum calcar ridge can be also recognized in the lesser trochanteric area of the femoral intramedullary canal. This thick internal septum of cortical bone, which is also known as femoral thigh spur, is a cortical bone ridge protruding from mediodorsal endosteal cortex into the medullary canal. Considering the approximate dimensions of 3 mm thickness and 35 mm length of this cortical bone (Adam et al., 2001), it is believed not to neglect it because of its assistive support which can contribute to the primary stability of the prosthesis. In the proposed design methodology the septum calcar ridge is preserved and the stem geometry is designed so that it conforms to the ridge and provides an improved fixation. After marking the slices in MIMICS, the boundary points are extracted using the software. The operator might be needed to manipulate (move/add/remove) the points manually to make sure that the details of the cortical bone boundary are preserved. The whole data points are then saved in “point cloud” format and exported to Geomagic Studio (Raindrop Geomagic, Triangle Park, North Carolina). In Geomagic, a triangular mesh is used to create NURBS surfaces and construct a 3D solid model (Figure 3(a)). This model, saved in STEP format, is then exported to Catia software (www.3ds. com/products/catia). By splitting (cutting) the model with 5 mm interval transverse planes, smooth spline curves are obtained, that are used to design the prosthesis stem (GeoMagic, 2004).

by Mahaisavariya et al. (2002) is adapted for determining the anatomical features of the complex 3D structure of proximal femur, using CAD tools. In this technique, the shapes of particular parts of the proximal femur are approximated using geometric entities such as circles and spheres, which best fits to the actual anatomy and the geometrical relationships between these entities are investigated to assess the bone anatomical data. At first, fitting a sphere to the acetabulum is suggested, to obtain the position of the joint’s centre of rotation, which is also the femoral head centre. Then, the circular cross-section of the femoral neck perpendicular to the approximated femoral neck axis is investigated, by applying the fit circle function in Geomagic software. Next, the smallest crosssection at femoral neck, known as the neck isthmus, is found. The femoral neck axis can now be determined accurately as the line between the femoral head centre and the neck isthmus centre (Figure 3(b)). By finding the 3D orientation of the femoral neck axis, the relevant geometrical parameters of the proximal femur, such as neck-shaft angle and femoral neck length can be easily found and evaluated. The anteversion angle can be also found by measuring the angle between the femoral neck axis and the line drawn along the posterior border of the distal femoral condyles. The extramedullary portion of prosthesis is then designed so that it is aligned in the same 3D direction of the anatomical neck axis. In case of abnormal geometry, the surgeon provides the corrections needed for the prosthetic neck’s appropriate offset and anteversion, based on the contralateral side (when normal) or on the lever arm ratio and the abductor angle (Flecher et al., 2010) (when the disease is bilateral). A standard 12/14 taper (Swanson, 2005) might then be implemented in the design for fixing the prosthesis’s head (Figure 3(c)).

2.2 Design of extramedullary portion The extramedullary portion of the prosthesis is designed in this methodology based on the original anatomy of the patient’s femoral neck in order to restore the initial biomechanical characteristics of the hip joint, e.g. centre of rotation. Reconstruction of the femoral head’s original centre of rotation, helps to regain the normal range of motion and mechanical loading of the hip joint, and contributes to the restoration of other anatomical parameters such as leg length, offset, and anteversion angle. These parameters have been reported to critically affect the durability and stability characteristics of the prosthesis after THA (Karnezis, 2001). Moreover, an excessively larger or smaller anteversion angle may cause dislocation, reduced range of motion, soft tissue strain, high joint contact forces, and bending moments (Lee et al., 2006). In order to reconstruct the original anatomy of the hip joint in the extramedullary portion of the prosthesis, a sophisticated measurement technique is proposed here to obtain the anatomical parameters from the 3D model of the patient’s bone. Traditionally, these parameters are measured on conventional radiography images, or using 2D images obtained from CT/MRI data. However, in spite of their popularity, these methods are based on over simplification of real 3D anatomy and may lead to large errors, due to the inaccuracies in selection of the measurement plane (Kim et al., 2000; Atilla et al., 2007; Sariali et al., 2008). In this methodology, the 3D measurement technique proposed

2.3 Design simulation and evaluation Based on performing the intramedullary and extramedullary design steps, the two portions are merged, through the common interface surface at the cutting plane, to finalize the prosthesis design (Figure 3(c)). This design is then investigated in a preplanning simulation environment within MIMICS software. The 3D model of the prosthesis with the femoral bone at the background, provides an effective tool to verify the efficacy of the design concerning the fit-and-fill characteristics in the intramedullary portion and the geometrical compatibility in the extramedullary portion. Also, the geometrical parameters of the prosthesis in extramedullary portion can be accurately measured and evaluated using the 3D model of the prosthesis within Catia software. This includes the anteversion angle, measured as the orientation of the neck axis in the transverse plane, and the neck-shaft angle (also called CCD angle), measured as the angle between the neck axis and the stem axis. Finally, the preplanning simulation environment allows the designer/ surgeon to simulate the steps of the surgical operation and in particular insert the prosthesis model in the femoral intramedullary canal to ensure that all design parameters have been taken into account and the clinical and biomechanical constraints have been satisfied (Figure 4). In case of observation of any problem, the designed prosthesis might be modified in close collaboration of the designer and surgeon, before starting the fabrication process. 392




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Figure 3 The proposed methodology for design of custom-made prosthesis

(a)

(b)

Neck

Stem

(c)

Notes: (a) The intra-medullary portion of prosthesis is designed based on the 3D geometry of the femoral cavity; (b) the extramedullary portion of prosthesis is designed based on the anatomical orientation of the femoral neck; (c) the two parts of merged at the interface plane to finalize the design

3. Case study results

femoral neck. The CT slices were taken using a single slice CT scanner (GE, USA) with 1mm slice thickness and zero degree gantry tilt, from the proximal femur and the pelvis, as well as the distal part of the femur. The scanning protocol included a 38 cm field of view and resolution of 512 £ 512 pixel, resulting in a pixel size of 0.742 mm. The gray values (HU) profiles, extracted using “Draw profile line” tool of MIMICS, are shown in Figure 5

A case study was performed to evaluate the efficacy of the proposed methodology for design and fabrication of customized hip prostheses. The patient was a 50 years old man suffering from severe unilateral hip dysplasia at the left side. The anatomy of the dysplastic joint was highly abnormal, including an abnormally small femoral head and short and highly anteverted 393




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Figure 4 The preplanning simulation environment to evaluate the efficacy of the design concerning intra-medullary fit-and-fill and extramedullary compatibility (left) and simulate the steps of the surgical operation (right)

Figure 5 The gray values (HU) profiles of four critical slices of the patient’s CT data at: (A, a) 20 mm above lesser trochanter, (B, b) lesser trochanter level, (C, c) 20 mm below lesser trochanter, (D, d) 50 mm below lesser trochanter 1,226

Cortical bone

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1,076

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for four critical slices of the CT data. It was observed that the proposed 700 HU threshold value could effectively distinct the cancellous and cortical bones. In particular the septum calcar ridge could be recognized in the lesser trochanteric area of the femoral intramedullary canal (Figure 6) and was incorporated in the design of the stem to provide an improved fixation and enhance the primary stability of the prosthesis.

(d)

The design methodology was followed as described in Sections 2.1 and 2.2 for the intramedullary and extramedullary portions of the prosthesis, respectively. The initial neck-shaft angle, neck length, and anteversion angle were 1558, 35 mm, and 478, respectively. The orthopaedic surgeon provided the corrections needed for the prosthetic neck’s appropriate geometry, based on the contralateral side. Designed prosthesis was then analysed in 394




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Figure 6 The cross-section of the femoral intermedullary canal from distal to proximal

in five critical levels of the stem (Nishihara et al., 2003). Close geometrical match was found between the cross-sections of prosthesis stem (Figure 7) and femoral intramedullary canal in the distal parts of the prosthesis stem (levels 3-5). For levels 1 and 2, representing the lower corner of the femoral neck resection and centre of the lesser trochanter, respectively, the conformity was incomplete due to the preserving septum calcar ridge as a primary support for the stem. Figure 8 shows the fit-and-fill characteristics of the femoral stem in more detail. Analysis of the fitting deviation between the CAD model of the designed prosthesis and the point cloud data of the femoral intramedullary canal indicated a fitting deviation of 0.04-0.11 mm of the stem. This small deviation is due to the sub-pixel segmention algorithms embedded within MIMICS, and the fact that NURBS surfaces smoothed and interpolated the data points of sequential slices. However, it is obviously far different from the fitting deviation between the prosthesis itself and the femoral canal, which is also affected by the inaccuracies raised in the fabrication process. Nevertheless, from a design point of view, merely, it suggests a sufficiently good rest-fit, consistent with what has been recommended in the literature for safe and successful implantation of hip prosthesis (Leali and Fetto, 2007). In a close examination, we found localized interference fit, in the range of 20.2 to 20.4 mm, between the designed prosthesis and the femoral canal. Although press fit of the prosthesis stem into the intramedullary cavity have been reported to be associated with bone intra-operative fractures (Leali and Fetto, 2007), interferences smaller than 20.5 mm are considered to be safe and not contributing to the formation of bone cracks (Abdul-Kadir et al., 2008).

Note: The femoral thigh spur is shown using arrows the preplanning simulation environment, in close collaboration with the surgeon. The whole design procedure took 8 h and was performed by a biomechanical engineer, in collaboration with the orthopaedic surgeon. Analysing the lateral and frontal views of the prosthesis, with the patient’s images at the background (Figure 7), it was found that the prosthesis neck was well compatible with anatomy of the proximal femur of the normal contralateral side, and the prosthesis stem closely matched the cortical bone of the femoral shaft. The geometrical parameters of the prosthesis in extramedullary portion, included values of 12.58 for anteversion angle, and 1318 for the neck-shaft angle which are both in the normal reference range (Lee et al., 2006). In order to accurately assess the fit-and-fill characteristics of design, the cross-sectional images of the patient’s femoral shaft and the corresponding sections of the prosthesis stem were investigated

Figure 7 Evaluation of the prosthesis’s fit-and-fill characteristics with the femoral inter-medullary canal and geometrical compatibility with the contralateral normal femoral neck

Lateral view

Frontal view

Notes: Left, lateral and frontal views; right, cross-sections at fives levels 395




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Figure 8 Evaluation of the fitting deviation (in mm) between the designed prosthesis and point cloud data of the femoral intra-medullary canal

taken into account. After approval of the model, it was used as a sacrificial pattern for investment casting process of the prosthesis from titanium alloy, i.e. Ti-6Al-4V (Figure 9). The total time of 25 h was taken for fabrication process of hip prosthesis. In order to complete the manufacturing process and prepare the prosthesis for clinical application, the cast product was subjected to surface treatment procedures as recommended by Philippe et al. (2005). This included polishing the distal part to prevent any adhesion to the distal bone, polishing the extramedullary portion, and grit-blasting and coating the intra-trochanter area with hydroxyapatite applied in 70 mm layers by air plasma spraying. The final product, was completed using standard parts of femoral head and acetabular cup, as shown in Figure 9. In addition to the prosthesis, some customized surgical tools were also designed and fabricated, to facilitate the surgical procedure. They included a femoral neck cutting guide and two femoral bone reamer. The cutting guide, made from polyethylene using CNC machining, is used during surgery to accurately determine the cutting plane on the patient’s femur. This would allow the surgeon to osteotomize the femoral neck exactly at the designated plane using a reciprocating saw blade. The bone reamers, on the other hand, were made from martensitic stainless steel (grade 410) using a casting procedure similar to that of the prosthesis and then quenching. The starter bone reamer was designed to be smaller than the implant, for primary preparation of the femoral canal, but the final reamer exactly matched the stem’s geometry (Figure 10).

After finalizing the design, a time and cost effective investment casting technique, based on AM technology, was used for fabrication of the prosthesis. At first the CAD data of the final model was converted to STL format and sliced into 0.1 mm thickness layers. The sliced data was then transferred into a stereolithography apparatus (SLA5000) to fabricate the prosthesis model using watersheldTM 11120 resin (DSM Somos Co. (2005), The Netherlands) in QuickCast pattern (Figure 9). This model was further evaluated by the surgeon to ensure that all clinical and biomechanical factors have been

4. Discussion In this study, an improved methodology for design of custommade prostheses was proposed and its efficacy was evaluated in a case study. An important feature of this methodology was applying an independent design criterion for each of the intramedullary and extramedullary portions of the prosthesis. Previous studies of custom-made hip prosthesis have been

Figure 9 QuickCast pattern and fabricated product of the prosthesis

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Figure 10 Comparison of fit-and-fill characteristics of design methodology (a) with that of Viceconti et al. (2001) (b)

2007). On other hand, in spite of the criticisms, the fit-and-fill approach is still a common basis for design of cementless stems and has been validated to be valuable in providing long-term, pain-free, stable THA function (Jones et al., 2008). Finally, the systematic framework of this methodology, implemented in a preplanning simulation environment, shortens the design cost and time, improves the repeatability and facilitates the communication between designer and surgeon. Such features are believed to be critical for safer and more successful THR surgeries. However, there is a major deficiency in design and preplanning simulation environment of this methodology due to the fact that it involved a number of different software packages, e.g. MIMICS, Geomagic, and Catia. The main reason for employing other softwares, besides MIMICS, was the fact that the surface generation capability of MIMICS 10.01 was not satisfying. Thus, we had to export the point cloud into Geomagic to construct the 3D solid model, and then export it to Catia to extract the spline curves. This approach provided a simple solution to deal with the limitations of each of these softwares and to employ their specific advantageous, features and tools. However, it is a fact that the communication between them might be technically confusing for the designer and more importantly cause inaccuracies in the geometrical data. For wide practical use in orthopaedic surgery, image processing, design and simulation tools of this methodology should be integrated within a single software environment. Apparently, this has been attempted in the newer versions of MIMICS, e.g. Version 14.1 (www-01.ibm.com/software/applications/plm/catiav5). The AM technology was employed in this case study to fabricate the patient’s customized prosthesis accurately, quickly and economically. Fabrication of custom-made prostheses at rational costs has always been a major concern for their wide practical use in orthopaedic surgery. Recently, with the development of AM technology, fabrication process has been fundamentally changed and new opportunities have been emerged. This technology allows fabricating prostheses with biocompatible materials, e.g. titanium and cobaltchromium, directly or indirectly using a variety of techniques such as QuickCast method, electron beam melting (EBM), and selective laser melting (SLM) (He et al., 2006; Harrysson and Cormier, 2006; Harrysson et al., 2007). In fact, the EBM and SLM techniques are capable of fabricating 10-15 customized prostheses directly from the designed model in less than 15 h at a reasonable cost. Moreover, the finishing process of AM fabricated parts is nearly identical to the conventional finishing processes and would not add to the overall cost significantly. A further advantage of fabricating prosthesis using EBM/SLM technology is the possibility of producing the porous surface, required for bone ingrowth encouragement, simultaneously. The sintering treatment of titanium and cobalt-chromium beads on the prosthesis surface is usually performed manually in multiple steps and is costly and labour demanding (Harrysson and Cormier, 2006; Harrysson et al., 2007). However, the major emphasis of this study was to develop an improved methodology for design of custom-made prostheses, and issues such as casting process details, scaling factor, fabrication accuracy and implementation are going to be covered in future. In spite of this, scaling factor applied for shrinkage of titanium alloy (Ti-6Al-4V) was 2.0 as designated by the manufacturer. Mehsa Tech Company

often limited to the intramedullary portion of the prosthesis, trying to provide good fit-and-fill characteristics for the stem (Bo et al., 1997; Kim et al., 1998; Werner et al., 2000; Viceconti et al., 2001; Adam et al., 2002; Pawlikowski et al., 2003; Guo et al., 2004; Kawate et al., 2008). However, in this research the extramedullary portion of the prosthesis is also designed based on the original anatomy of each individual patient. This is of great importance considering the wide range of normal anteversion angle reported in literature (Lee et al., 2006). For patients with abnormal joint anatomy, the geometry of the contralateral side (when normal) or the normal reference ranges (when the disease is bilateral) are adapted in the design procedure. Furthermore, the design of the intramedullary portion of the prosthesis was improved in this methodology using a more accurate and detailed description of the 3D geometry of the femoral cavity. In particular, the applied thresholding procedure allowed preserving the septum calcar ridge or femoral thigh spur and incorporating it in the prosthesis design. It has been reported that this cortical bone is a part of normal anatomy, regardless of the gender and age (Adam et al., 2001; Decking et al., 2003). So, this strong cortical ridge might be employed as an assistive support for prosthesis fixating in proximal stem, where the bone tissue is mostly of cancellous structure; it is expected to improve the primary stability of the prosthesis and contribute to its long-term durability. However, modelling and experimental studies are suggested to provide detail information on the short-term and long-term mechanical significance of septum calcar ridge in prosthesis fixation. It is to be noted that in the present study, the classical fit-andfill approach for design of custom-made THR stems was followed, in which the stem fixation relies only upon the cortical contact. Thresholding with a 600-800 HU value is quite usual in this approach to distinguish the cortical bone from cancellous (Adam et al., 2002). However, such approach would cause loss of bone stock that is critical in case that a revision surgery is required. This fact, among some other factors, has provided impetus for development of conservative hip implants, e.g. tapered wedge design, which use proximal cancellous bony ingrowth and three-point stem fixation to obtain immediate stability (Marshall et al., 2004). However, although several different conservative implants are currently available, few clinical results have been published (Learmonth et al., 397




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handled the casting process. Detailed casting processing data was never released, and we were not too much concerned about it at this stage, but it is going to be a part of our future investigation. Investment casting process was chosen as fabrication process due to its accessibility and cost benefit over complex processes of EBM or SLM. CNC machining process was also rejected due to its difficulties of control and programming of five axis machining and limitations in tool selection. However, as a future work, it is planned to make a comprehensive comparison between these advanced AM fabrication processes for such customized prostheses. In the present study, the fit-and-fill characteristics of the fabricated prosthesis inside the patient’s femoral intramedullary canal were not evaluated. Measuring the fitting deviation between the CAD model of the designed prosthesis and the point cloud data of the femoral morphology provided information on the efficacy of the proposed design procedure, but not the geometric inconsistencies arising from the use of multiple softwares and/or the fabrication process. The actual fitting deviation between the prosthesis and the femoral canal might be much larger due to the manufacturing tolerances produced at the rapid prototyping, casting and finishing procedures. In general, the accuracy of rapid prototyping and QuickCast method employed in this study is within ^0.1 mm. Also, the effect of finishing procedure on the prosthesis dimensions is less than 0.05 mm. Thus, the actual fitting deviation between the prosthesis stem and the formal canal is not expected to exceed ^0.3 mm in total. This small fitting deviation provides acceptable fit-and-fill characteristics for the implant (Leali and Fetto, 2007; Abdul-Kadir et al., 2008). However, it is a limitation of this study that we did not measure the final deviation between the fabricated prosthesis and the canal. Further work is necessary on cadaveric bones, scanned before and after implantation, to evaluate the overall efficacy of our proposed methodology for design and fabrication of customized hip implants.

modeling using rapid prototyping techniques”, Radiography, Vol. 16 No. 1, pp. 78-83. Bo, A., Imura, S., Omori, H., Okumura, Y., Ando, M., Baba, H., White, P. and Zarnowski, A. (1997), “Fit and fill analysis of a newly designed femoral stem in cementless total hip arthroplasty for patients with secondary osteoarthritis”, J. Orthop. Sci., Vol. 2, pp. 301-12. Decking, J., Decking, R., Schoellner, C., Dress, P. and Eckardt, A. (2003), “The internal calcar septum and its contact with the virtual stem in THR”, Acta Orthop. Scand., Vol. 74 No. 5, pp. 542-6. DSM Somos Co. (2005), WaterShedTM 11120 Resin Data Sheet, DSM Somos Co., New Castle, DE. Flecher, X., Pearce, O., Parratte, S., Aubaniac, J.M. and Argenson, J.N. (2010), “Custom cementless stem improves hip function in young patients at 15-years follow-up”, Clin. Orthop. Relat. Res., Vol. 468, pp. 747-55. GeoMagic (2004), User Manual, Raindrop Geomagic, Research Triangle, NC. Guo, L.J., Song, L.D., Hua, M.W., Ping, Z.Z. and Xiang, X.X. (2004), “Computer assisted reconstruction of three-dimensional canal model of femur and design for custom-made stem”, Chinese Medical Journal, Vol. 117 No. 8, pp. 1265-70. Handels, H., Ehrhardt, J., Ploetz, W. and Poeppl, J. (2001), “Simulation of hip operations and design of custom-made endoprostheses virtual reality techniques”, Method Inform. Med., Vol. 40, pp. 74-7. Harrysson, O.L.A. and Cormier, D. (2006), “Direct fabrication of custom orthopedic implants using electron beam melting technology”, in Gibson, I. (Ed.), Advanced Manufacturing Technology for Medical Applications, Wiley, Chichester, pp. 193-208. Harrysson, O.L.A., Hosni, Y.A. and Nayfeh, J.F. (2007), “Custom-designed orthopedic implants evaluated using finite element analysis of patient-specific computed tomography data: femoral-component case study”, BMC Musculoskeletal Disorders, Vol. 8 No. 91, pp. 1-10. Husmann, O., Rubin, P.J., Leyvraz, P.F., de Roguin, B. and Argenson, J.N. (1997), “Three-dimensional morphology of the proximal femur”, J. Arthroplasty, Vol. 12, pp. 444-50. Jones, R.E., Huo, M.H. and Hashemi, M.T. (2008), “Fit and fill total hip arthroplasty: is it still efficacious?”, Current Opinion in Orthopedics, Vol. 19 No. 1, pp. 24-7. Karnezis, I.A. (2001), “A technique for accurate reproduction of the femoral anteversion during primary total hip arthroplasty”, Arch. Orthop. Trauma Surg., Vol. 121, pp. 343-5. Kawate, K., Ohneda, Y., Ohmura, T., Yajima, H., Sugimoto, K. and Takakura, Y. (2008), “Computer tomography-based custom-made stem for dysplastic hips in Japanese patients”, J. Arthroplasty, Vol. 24 No. 1, pp. 65-70. Kim, Y., Park, Y., Skalski, K. and Suh, J. (1998), “Cementless bony ingrowth total hip prosthesis (anatomical contact porous coated total hip prosthesis) design using computed axial tomography and computer aided design”, J. Med. J., Vol. 29 No. 2, pp. 139-59. Kim, J.S., Park, T.S., Park, S.B., Kim, J.S., Kim, L.Y. and Kim, S.I. (2000), “Measurment of femoral neck anteversion in 3D: part 1: 3D imaging method”, Med. Biol. Eng. Comput., Vol. 38, pp. 603-9.

References Abdul-Kadir, M.R., Hansen, U., Klabunde, R., Lucas, D. and Amis, A. (2008), “Finite element modelling of primary hip stem stability: the effect of interference fit”, J. Biomech., Vol. 41 No. 3, pp. 587-94. Adam, F., Hammer, D.S., Pape, D. and Kohn, D. (2001), “The internal calcar septum (femoral thigh spur) in computed tomography and conventional radiography”, Skeletal Radiol., Vol. 30, pp. 77-83. Adam, F., Hammer, D.S., Pape, D. and Kohn, D. (2002), “Femoral anatomy, computed tomography and computer aided design of prosthetic implants”, Arch. Orthop. Trauma Surg., Vol. 122, pp. 262-8. Atilla, B., Oznur, A., Caglar, O., Tokgozoglou, M. and Alpaslan, M. (2007), “Osteometry of the femora in Turkish individuals: a morphometric study in 114 cadaveric femora as an anatomic basis of femoral component design”, Acta Orthop. Traumatol. Turc., Vol. 41 No. 1, pp. 64-8. Bennett, D. and Goswami, T. (2008), “Finite element analysis of hip stem designs”, Materials and Design, Vol. 29, pp. 45-60. Bibb, R. and Winder, J. (2009), “A review of the issues surrounding three-dimensional computed tomography for 398




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Leali, A. and Fetto, J. (2007), “Promising mid-term results of total hip arthroplasties using an uncemented lateral-flare hip prosthesis: a clinical and radiographic study”, International Orthopaedics (SICOT), Vol. 13, pp. 845-9. Learmonth, I.D., Young, C. and Rorabeck, C. (2007), “The operation of the century: total hip replacement”, Lancet, Vol. 370, pp. 1508-19. Lee, Y.S., Oh, S.H., Seon, J.K., Song, E.K. and Yoon, T.R. (2006), “3D femoral neck anteversion measurement based on the posterior femoral plane in ORTHODOC system”, Med. Biol. Eng. Comput., Vol. 44, pp. 895-906. Mahaisavariya, B., Sitthiseripratip, K., Tongdee, T., Bohez, E.L.J., Sloten, J.V. and Oris, P. (2002), “Morphological study of the proximal femur: a new method of geometrical assessment using 3D reverse engineering”, Medical Engineering & Physics, Vol. 24, pp. 617-22. Mallepree, T. and Bergers, D. (2009), “Accuracy of medical RP models”, Rapid Prototyping Journal, Vol. 15 No. 5, pp. 325-32. Marshall, A.D., Mokris, J.G., Reitman, R.D., Dandar, A. and Mauerhan, D.R. (2004), “Cementless titanium taperedwedge femoral stem: 10- to 15-year follow-up”, J. Arthroplasty, Vol. 19 No. 5, pp. 546-52. Mathur, S.K., Mont, M.A. and McCutchen, J.W. (1996), “Intraoperative press-fit and standard press-fit femoral componenets in total hip arthroplasty: a comparison of surgery, charges, and early complications”, Am. J. Orthop., Vol. 25 No. 7, pp. 486-91. MIMICS Reference Guide (n.d.), MIMICS Reference Guide, Version 10.01, Materialise NV, Leuven, available at: http:// materialise.com/ Mulier, J.C., Mulier, M., Brady, L.P., Steenhoudt, H., Cauwe, Y., Goossens, M. and Elloy, M.A. (1989), “A new system to produce intraoperatively custom femoral prosthesis from measurement taking during the surgical procedure”, Clin. Orthop., Vol. 249, pp. 97-112. Nishihara, S., Sugano, N., Nishi, T., Tanaka, H., Yoshikawa, H. and Ochi, T. (2003), “Comparison of the fit and fill between the anatomic hip femoral component and the VerSys taper femoral component using virtual implantation on the ORTHODOC workstation”, J. Orthop. Sci., Vol. 8, pp. 352-60. Noble, P.C., Alexander, J.W., Lindahl, L.J., Nalty, T. and Tullos, H.S. (1988), “The anatomic basis of femoral component design”, Clin. Orthop. Relat. Res., Vol. 235, pp. 148-65. Pawlikowski, M., Skalski, K. and Haraburda, M. (2003), “Process of hip joint prosthesis design including bone remodeling phenomenon”, Computers & Structures, Vol. 81, pp. 887-93. Philippe, M.P., Martin, E., Hummer, J., Gacon, G., Dambreville, A. and Ray, A. (2005), “The ESOP-HA modular cementless femoral stem: a study of the results of 165 hip arthroplasties with a minimum of 10-year followup”, J. Orthop. Surg. Traumatol., Vol. 15, pp. 275-85. Rubin, P.J., Leyvarz, P.F., Aubaniac, J.M., Argenson, J.N., Esteve, P. and Roguin, B.D. (1992), “The morphology of the proximal femur: a three-dimensional radiographic analysis”, J. Bone Joint Surg., Vol. 74 No. 1, pp. 28-32. Ruya, M., Wendong, X., Dongmei, W., Dai, K. and Chengtao, W. (2005), “Design and manufacture of custom hip prostheses based on standard X-ray”, J. Arthroplasty, Vol. 27, pp. 70-4.

Sariali, E., Mouttet, A., Caglar, O., Pasquier, G. and Durante, E. (2008), “Three-dimensional hip anatomy in osteoarthritis: analysis of the femoral offset”, J. Arthroplasty, Vol. 24 No. 6, pp. 990-7. Swanson, T.V. (2005), “The tapered press fit total hip arthroplasty: a European alternative”, J. Arthroplasty, Vol. 63, p. 66. Viceconti, M., Testi, D., Gori, R., Zannoni, C., Cappello, A. and Lokkis, A.D. (2001), “HIDE: a new hybrid environment for the design of custom-made hip prosthesis”, Comput. Methods Prog. Biomed., Vol. 64, pp. 137-44. Werner, A., Lechniak, Z., Skalski, K. and Kedzior, K. (2000), “Design and manufacture of anatomical hip joint endoprostheses using CAD/CAM systems”, J. Mater. Process Technol., Vol. 107 No. 1, pp. 181-96. Winder, R.J. and Bibb, R. (2005), “Medical rapid prototyping technologies: state of the art and current limitations for application in oral and maxillofacial surgery”, J. Oral. Maxillofac. Surg., Vol. 63 No. 7, pp. 1006-15. Wong, K.C., Kumta, S.M., Chiu, K.H., Cheung, K.W., Leung, K.S., Unwin, P. and Martin, C.M.W. (2007), “Computer assisted pelvic tumor resection and reconstruction with a custom-made prosthesis using an innovative adaption and its validation”, Computer Aided Surgery, Vol. 12 No. 4, pp. 225-32.

Further reading Goetze, C., Rosenbaum, D., Hoedemaker, J., Bottner, F. and Steens, W. (2009), “Is there a need of custom-made prostheses for total hip arthroplasty? Gait analysis, clinical and radiographic analysis of customized femoral components”, Arch. Orthop. Trauma Surg., Vol. 129, pp. 267-74. Goetze, C., Steens, W., Vieth, V., Poremba, C., Claes, L. and Steinbeck, J. (2002), “Primary stability in cementless femoral stems: custom-made versus conventional femoral prosthesis”, Clin. Biomech., Vol. 17 No. 4, pp. 267-73. Pettersen, S.H., Wik, T.S. and Skallerud, B. (2009), “Subject specific finite element analysis of stress shielding around a cementless femoral stem”, Clin. Biomech., Vol. 24, pp. 196-202.

About the authors Sadegh Rahmati is Associate Professor of Mechanical Engineering at the IAU, Majlesi Branch, Isfahan, Iran. He received his PhD in Advanced Manufacturing Engineering from Nottingham University (UK), and his MSc in CIM from Loughborough University (UK). His current research focuses on rapid prototyping and rapid tooling. Sadegh Rahmati is the corresponding author and can be contacted at: [email protected] Farid Abbaszadeh received his Master of Science in Mechanical Engineering from Science and Research Branch of Islamic Azad University (IAU), Tehran, Iran, in 2010. He received his Bachelor in Mechanical Engineering from Tabriz Branch of IAU, Iran, in 2006. His research interests include rapid prototyping, rapid manufacturing, medical rapid prototyping, and reverse engineering. 399




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Farzam Farahmand is Professor and Head of Biomechanics Division of the School of Mechanical Engineering at Sharif University of Technology, Tehran, Iran. He has a joint appointment at the Research Center of Science and Technology in Medicine (RCSTIM) at the Tehran

University of Medical Sciences, Tehran, Iran, where he is the head of the Surgical Robotics Group. He received his PhD in Biomedical Engineering from Imperial College London, (UK) in 1996 and his current research focuses on orthopedic biomechanics, implants design and robotic surgery.

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