Application of Rapid Prototyping Pelvic Model for Patients with DDH to Facilitate Arthroplasty Planning: A Pilot Study.

The Journal of Arthroplasty xxx (2015) xxx–xxx

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The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org

Application of Rapid Prototyping Pelvic Model for Patients with DDH to Facilitate Arthroplasty Planning: A Pilot Study Jie Xu, MD a, Deng Li, PhD a, Ruo-fan Ma, MD a, Bertram Barden, MD b, Yue Ding, MD a a b

Department of Orthopaedics, Sun Yat-sen Memorial Hospital, Sun Yat-sen University, Guangzhou, China Department of Orthopaedic Surgery, Düren Hospital, Academic Hospital of University of RWTH Aachen, Düren, Germany

a r t i c l e

i n f o

Article history: Received 29 January 2015 Accepted 15 May 2015 Available online xxxx Keywords: Imaging hip arthroplasty DDH 3-D print

a b s t r a c t Total hip arthroplasty (THA) is challenging in cases of osteoarthritis secondary to developmental dysplasia of the hip (DDH). Acetabular deficiency makes the positioning of the acetabular component difficult. Computer tomography based, patient-individual three dimensional (3-D) rapid prototype technology (RPT)-models were used to plan the placement of acetabular cup so that a surgeon was able to identify pelvic structures, assess the ideal extent of reaming and determine the size of cup after a reconstructive procedure. Intraclass correlation coefficients (ICCs) were used to analyze the agreement between the sizes of chosen components on the basis of preoperative planning and the actual sizes used in the operation. The use of the 3-D RPT-model facilitates the surgical procedures due to better planning and improved orientation. © 2015 Elsevier Inc. All rights reserved.

THA in dysplastic hips is technically demanding due to the distorted anatomy. In many cases, a dysplastic hip cannot provide sufficient osseous support for the cup implanted in the original acetabulum. The main problem in THA for DDH patients is how to restore the normal anatomy and obtain a stable fixation of the prosthetic components. Despite these anatomical obstacles, anatomical and mechanical reasons necessitate implantation of the acetabular component in the true acetabulum [1–5]. This implantation site decreases the reaction forces exerted on the hip joint and thus reduces the loosening rate [6,7]. Adequate coverage of the acetabular component in this shallow acetabulum can be successfully managed by structural bone graft. Femoral head autograft augmentation techniques, which consisted of large-sized parts of the femoral head and neck that were fixed with screws to the iliac bone, were introduced [8,9]. Many studies using this technique have shown good clinical results with solid bone-grafts in primary and revision reconstructions in DDH patients [10–12]. The incorporation process of large solid bone-grafts is unpredictable, however, and may result in absorption of the graft. This resorption can lead to loosening of the acetabular component in the long term [13,14]. Besides structural bone grafting, the techniques for managing the problem of insufficient acetabular bone coverage include the use of an acetabular reinforcement ring [15] and medialization of the acetabular No author associated with this paper has disclosed any potential or pertinent conflicts which may be perceived to have impending conflict with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.05.033. Reprint requests: Bertram Barden, MD, Department of Orthopaedic Surgery, Düren Hospital, Academic Hospital of University of RWTH Aachen, Roonstr. 30, 52351 Düren, Germany. Reprint requests: Yue Ding, MD, Department of Orthopaedics, Sun Yat-sen Memorial Hospital, Sun Yat-sen University, 107 Yanjiang Road West, 510120, Guangzhou, China.

component [16–18], the use of a small cup placed in or near the native acetabulum [19,20] and cotyloplasty [21]. The use of imaging techniques for preoperative planning is helpful to improve the results of complex surgical interventions. Information about the position of acetabulum and femoral head, and bone stock can be derived from standard imaging techniques, but 3-D-reconstruction images are actually displayed two dimensionally. The aim of this study is to describe the process of generating solid anatomical RPT of pelvis structures that could be used to facilitate preoperative planning. Materials and Methods Between June 2011 and June 2012, 10 patients (14 hips) with degenerative joint disease secondary to DDH were treated by primary THA (Table 1). All consecutive patients who had this procedure were included in the study, and no other techniques for acetabular reconstruction of hips with developmental dysplasia were used during this period. The approval of the institutional review board of the hospital and the consent of all patients were obtained prior to the study. There were one male and 9 females with a mean age of 57.8 years (range, 45–68 years). All cases were classified as Crowe type II [22] and Hartofilakidis type II [21]. 2-D Preoperative Planning for Placement of Acetabular Component A high-quality anteroposterior pelvic overview radiograph was essential for preoperative planning and templating, with the patellae facing due upward if allowed by internal rotation of the hip.

http://dx.doi.org/10.1016/j.arth.2015.05.033 0883-5403/© 2015 Elsevier Inc. All rights reserved.

Please cite this article as: Xu J, et al, Application of Rapid Prototyping Pelvic Model for Patients with DDH to Facilitate Arthroplasty Planning: A Pilot Study, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.05.033

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J. Xu et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx

Table 1 Data of the Patients.

Patient No.

Gender

Age (yr)

Diagnosis

Affected Side

Crowe Type

Hartofilakidis Type

1 2 3 4 5 6 7 8 9 10

F F F F F M F F F F

51 66 68 62 62 45 52 66 48 58

DDH DDH DDH DDH DDH DDH DDH DDH DDH DDH

R and L R and L R L R R R L R and L R and L

II II II II II II II II II II

II II II II II II II II II II

Digital images were manipulated using a radiographic marker of standard size which was taped to the skin of the patient at the same level of the greater trochanter of the operative side [23]. Preoperative planning for all cases was carried out using the MediCad-system version 2.06 (Hectec, Niedervieh-bach, Germany). All preoperative radiographs were templated by a consultant orthopedic surgeon who was a member of the surgical team performing the operations. For acetabular component placement, the ideal position was in the true acetabulum as far medial as possible. In cases whose acetabular component were not covered by the bone adequately because of bone deficiency of the superolateral acetabulum, the cup template was minimally moved superiorly and medially so that at least 75% of the porouscoated surface of the implant was in contact with host bone on the anteroposterior pelvic overview radiograph. The size of the acetabular component was chosen to ensure that the inferior aspect was on the level of base of the teardrop and the medial aspect approximated the ilio-ischial line. Adjustments were made intraoperatively if necessary. The abduction angle was 45° relative to the line connecting the bases of left and right teardrops (Fig. 1A). 3-D Preoperative Planning for Placement of Acetabular Component Data obtained by CT-scan [24] were transferred to the planning workstation using a DICOM interface and 3-D reconstructed (Fig. 1B).

Here the surgeon performed the planning using a preoperative planning program. The software allowed the surgeon to ‘navigate’ the prosthetic components into the three-dimensional space of the CT data to the proper position, the surgeon could also dynamically change the available sizes of components. Since the posterolateral approach and metal-on-polyethylene bearing surface were used, the acetabular opening section was reconstructed 15° anteversionally (Fig. 1C) and 45° abductionally (Fig. 1D). This section provided clear information of different diameters which could be easily measured. In many cases, the anteroposterior diameter of the acetabulum is often smaller than the vertical diameter (Fig. 1E). The 3-D-cup template was placed to accommodate the anterior and posterior walls, which were prior to the superolateral margin of the acetabulum, so that the removal of the supportive subchondral bone was minimal and the center of hip rotation was closely restored (Fig. 2). The goal was to restore 15° of acetabular anteversion. 3-D Rapid Prototyping and Preoperative Planning for Placement of Acetabular Component After segmentation of the regions of interest, this model was triangulated and stored as an STL (stereolithography) file. STL is a file format that is used in stereolithography CAD software and is an industry standard for rapid prototyping. STL files described a raw unstructured triangulated surface by the unit normal and vertices of the triangles using a three-dimensional Cartesian coordinate system. Subsequently the STL file was sent to the 3-D-printer. The STL file carried the information of actual size since a 1:1 scale is achieved by CT scanning according to the designed nature of this CT instrument and its matching software. In another research (data not shown), we measured the scanned images, the 3-D printed models and the actual bones of patients with articular tumors, verifying that the scanned images and printed models have same sizes as the excised target bone tumor segments. The printing technology was similar to an ink jet printer. Instead of conventional ink, a mixture of fluid-binding substances and ink was applied. The printer consists of a plaster powder piston and a building piston. A roller transports a 0.1 mm-thick layer of plaster powder from the material piston to the building piston. Then the print

Fig. 1. (A) Conventional 2-D templating: The planned cup component (52mm in diameter) was outlined on the film. (B) 3-D reconstruction of pelvis. (C) Multi-planar reconstruction (MPR) of the acetabulum. Anteversion angle was 15°. (D) Abduction angle was 45°. (E) 3-D reconstruction of acetabulum, with an anteversion angle of 15°and abduction angle of 45°. The vertical diameter of acetabulum was 52mm, while the anteroposterior one was 44mm.



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Fig. 2. 3-D preoperative planning. 3-D template with 40mm-cup (40mm in diameter) was navigated to the proper position. (A) Anterior view of the relative position between the pelvis and the template. (B) Posterior view of the relative position between the pelvis and the template. (C) Superior view of the relative position between the pelvis and the template. (D) The cup was in contact with bone on both anterior and posterior edges (Horizontal section).

heads sprayed the binder on the segmented areas (regions of interest) in the building piston and the first layer is completed. To create a 3-D model, this procedure had to be repeated layer by layer until the 3-D model was finished. The unbound plaster powder which filled in the regions not segmented was removed with compressed air. After drying of the model, the manufacture process of the model was finished (Fig. 3A). Preoperative simulated acetabular reaming and cup placing were conducted on the reconstruction proportional pelvis model, which was created based on the three-dimensional CT scan. Reaming of acetabulum was begun with the smallest reamer (Fig. 3B). Position the initial reamer in a vertical direction to ensure the reamer was taken down to the medial wall. After removing the overhanging osteophyte, the underlying bone may be less dense. The preparation should be performed stepwisely. Care should also be taken not to weaken the thin anterior wall. The position of the reamer was then switched to the desired cup position. A larger reamer that would lead to a larger proportion of the reamer remains uncovered, which will leave the surgeon a first impression that the same would happen to a larger cup (Fig. 4). If the larger one was chosen, acetabular reconstruction and structural bone-grafting were necessary for a broader coverage of cup. Meanwhile, the definite reamer size was determined by the anteroposterior diameter of the acetabulum. In many cases, the anteroposterior diameter of the acetabulum was often smaller than the vertical diameter. If reaming was

continued to just accommodate the vertical diameter, the thin anterior and posterior walls might probably be weaken, or even wrecked (Figs. 5, 6), and cause primary instability.

Surgical Technique Surgical procedures were performed through a direct posteriorlateral approach. The optimal positional of the acetabular component was the anatomic hip center, and the acetabular component was fixed without cement. The target abduction angle of the acetabular component was 40°–50° and the anteversion angle was 10°–20°. Because of the quality of bone, only strict step-by-step reaming could be done. With the final reamer, the position of the cup may be simulated. The portion of this reamer remaining uncovered should be very limited. In general, this was not a protrusion technique. Solid fixation of the acetabular component was defined as that the acetabular component could not be moved easily by fingers. When sequential rasping was performed in the femoral canal, an excessive anteversion angle needed to be managed and reduced to an extent that combined anteversion of cup and stem was in the range from 30°to 45°. The hip was kept internally rotated at 90°. Meanwhile, the knee was kept flexed. The intercondylar line was used as the reference

Fig. 3. (A) Pelvis model was created basing on the three-dimensional CT scan. (B) Reaming of acetabulum.


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Fig. 4. (A) Ream the acetabulum with a 40mm-reamer (40mm in diameter). (B) Ream the acetabulum with a 44mm-reamer. (C) A 48mm-reamer was beyond the anteroposterior accomodation of the acetabulum thus an anterior wall bone defect occurred and the inner wall was enormously thinned. (D) The 40mm-cup was implanted in the desired position (anteversion angle 15°, abduction angle 45°). Portion of the cup that remained uncovered by bone was small. (E) Portion of the 44mm-cup that remained uncovered by bone was larger, compared with the 40mm-cup. (F) the coverage of the cup by the true acetabulum was less than 70%.

for managing the anteversion during broaching of the canal and insertion of the femoral component. Two types of acetabular component were used: Trident acetabular cup (Stryker-Howmedica-Osteonics, Rutherford, New Jersey) in 4 hips, Trilogy Acetabular System (Zimmer, Warsaw, Ind) in 10 hips. All acetabular cups were porous-coated. 3 types of femoral component were used: Secur-Fit HA hip stem (Stryker-Howmedica-Osteonics, Rutherford, New Jersey) in 3 hips, Exeter hip stem (Stryker-HowmedicaOsteonics, Rutherford, New Jersey) in 1 hip, VerSys hip stem (Zimmer, Warsaw, Ind) in 10 hips. Postoperative Regimen Postoperative treatment for all patients included systemic administration of antibiotics for five days, oral anticoagulation therapy for one month.

Passive exercises were allowed twenty-four hours postoperatively, followed by partial weight-bearing walking for six weeks and full weightbearing six weeks after the surgery. Radiographic Follow-Up Radiographs taken at the time of the most recent office visit were available for all patients. The duration of the radiographic follow-up equaled that of the clinical follow-up. The initial and all follow-up radiographs were reviewed. The position of the acetabular component and whether it had migrated were assessed with the method that the position of the center of rotation of the cup was determined with respect to a horizontal line connecting the inferior portions of the teardrops. Loosening was assumed when there was a shift more than 2 mm of the cup position or a change of the cup angle more than 3 degrees or detection

Fig. 5. 3-D template with 52mm-cup was navigated to the acetabulum (52mm-cup perfectly fit the vertical diameter). (A) Lateral view of the relative position between the pelvis and the template, the thin anterior and posterior walls were subsequently weak (marked by red arrow). (B) Anterior view of the relative position between the pelvis and the template. The inner wall was deficient (marked by red arrow). (C) Posterior view of the relative position between the pelvis and the template. The posterior wall was deficient (marked by red arrow).



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Fig. 6. (A) Reaming of acetabulum with a 52mm-reamer. The inner and anterior walls were deficient. (B) A 52mm-cup was implanted in the acetabulum (15° of anteversion, 45° of abduction). (C) A large portion of the 52mm-cup remained uncovered. (D) The superomedial acetabular wall was destroyed.

of radiolucent width more than 2 mm not present on the immediate postoperative radiograph [25,26]. The percentage of the acetabular component covered by the bone was measured using the horizontal distance of the acetabular component covered by the bone (b) and the horizontal distance between the most medial edge and the lateral point of the acetabular component (a) [11]. Coverage percentage = b/a × 100% (Fig. 7). Statistics Statistical analysis was performed by SPSS 12.0 (SPSS Inc., Chicago, IL, USA). Intraclass correlation coefficients (ICCs) were used to analyze the agreement between the size of the component chosen on the basis of preoperative planning and the actual size used in the operation. A value of P b 0.05 was considered statistically significant. Results Intraoperatively, fractures of the pelvis which would lead to central dislocation were not observed. Damage of posterior or anterior acetabular surface did not happen, and cancellous allograft or reinforcement ring was not required for acetabular reconstruction. Fracture of the proximal femur did not happen. Femoral osteoplasty, which was in order to shortening procedures of the femur, was not performed in all cases. Clinical Results All ten patients were available for clinical and radiological follow-up and average postoperative follow-up was 23.1 ± 5.9 (14–30) months. No revisions were needed. No patient had a symptomatic deep-vein thrombosis, pulmonary embolism, deep infection, nerve palsy, or hip dislocation. In the patients, ten hips had no demonstrable limp and

other four had a slight limp. The mean Harris hip score was 37.7 ± 6.8 points preoperatively, and had risen to 83.3 ± 5.7 points (P b 0.01) at the last examination (Table 2). Preoperatively, all patients had a moderate to severe limp. Postoperatively, four patients complained of lower limb length discrepancy, specifically, a feeling of longer affected limb than the contralateral side. No perceptible discrepancy was reported after six months’ follow up. There were complaints of thigh pain by six patients, postoperatively. At the sixth month follow-up the pain was relieved completely. Eight patients did not use any ambulatory aids and the remaining two patients who were treated with bilateral THA used a cane just for long distance ambulation at final follow-up evaluations. Radiographic Results On the discharge radiographs, the coverage of the cup by the bone was measured. For each case, at least 80% of the cup was contained by bone, and the mean abduction angle of the cup was 45.1° (range, 40.2°–53.5°). The mean horizontal location of the hip center was 21.7 mm (range, 15.0–31.2 mm) laterally from the teardrop, and the difference was less than 13.5 mm compared with the nonoperative side. The mean height of the hip center was 18.8 mm (range, 11.5–25.8 mm) vertically from the inter-teardrop-base line, and the difference was less than 6.8 mm compared with the nonoperative side (Table 2). At the latest follow-up, radiographs did not indicate any vertical or horizontal cup migration N 2 mm. The bone coverage of the socket was not less than 83%, as shown in Fig. 7. All implants appeared radiologically and clinically stable. The sizes of the acetabular cup chosen on the basis of 3-D preoperative planning showed a high rate of coincidence with the sizes chosen intraoperatively. The actual implant size for each patient compared with the pre-operative template size and the ICC values were shown in Table 1. The ICC values showed only moderate agreement between the 2-D template and the actual results.

Fig. 7. Diagram showing the coverage of the acetabular component by the true acetabulum. The coverage percentage = b/a × 100%.


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Table 2 Functional Evaluation and Radiographic Measurement.

Patient No.

Hip Score (Preoperation)

Hip Score (Postoperation)

1

35

86

40 (L) 42 (R)

2

40

90

42 (L) 44 (R)

3

32

78

44 (R)

Cup Size

4

45

86

44 (L)

5

32

75

46 (R)

6

38

90

52 (R)

7

46

76

48 (R)

8

48

80

40 (L)

9

31

88

42 (L) 42 (R)

10

30

84

44 (L) 44 (R)

Abduction and Anteversion of Cup (in Degrees) 45.3 (L) 47.2 (R) 21.1 (L) 10.2 (R) 41.2 (L) 53.5 (R) 22.0 (L) 10.1 (R) 46.2 (R) 15.2 (R) 43.5 (L) 10.3 (L) 40.2 (R) 12.5 (R) 41.5 (R) 13.0 (R) 41.0 (R) 16.2 (R) 46.2 (L) 20.1 (L) 43.2 (L) 46.3 (R) 15.2 (L) 14.1 (R) 48.2 (L) 47.4 (R) 18.3 (L) 13.2 (R)

Horizontal Locationa,b (in Millimeters)

Height of Hip Centera,c (in Millimeters)

Discrepancyd (in Millimeters)

Femoral Head Size

Femoral Stem Size

Bearing Surface

19.4 (L) 18.2 (R) 15.0 (L) 15.5 (R) 19.8 (31.5)

25.8 (L) 26.1 (R) 25.0 (L) 19.7 (R) 16.8 (17.4)

7.3 (2.1)

22 (L) 22 (R) 22 (L) 22 (R) 22 (R)

#10 (L)e #10 (R)e #10 (L)e #10 (R)e #8 (R)f

MoPh

2.1 (3.9) 1.2 (3.2)

g

MoP MoP

31.2 (32.0)

18.3 (11.5)

4.2 (8.1)

22 (L)

#6 (L)

27.1 (25.7)

12.4 (11.0)

3.4 (7.3)

22 (R)

#12 (R)e

MoP

27.6 (28.0)

19.4 (16.9)

6.2 (3.2)

28 (R)

#13 (R)e

MoP

28 (R)

e

MoP

e

MoP MoP

25.8 (26.4)

18.9 (15.1)

3.2 (4.0)

MoP

#12 (R)

19.7 (33.2)

11.5 (10.6)

6.3 (11.5)

22 (L)

#14 (L)

17.9 (L) 18.9 (R) 22.9 (L) 24.7 (R)

22.0 (L) 20.6 (R) 15.1 (L) 11.5 (R)

1.3 (1.6)

22 (L) 22 (R) 22 (L) 22 (R)

#12 (L)e #12 (R)e #6 (L)g #6 (R)g

1.0 (1.5)

MoP

a For the single-side involved patients, the values were for the operation side while the values in brackets were for the normal side. For the both-side involved patients, (L) represented left side, (R) represented right side. b Vertical lines from the center of rotation were drawn to the line between the pelvic teardrops. The distance between vertical line and teardrop on the same side was defined horizontal location of the hip center. c The distance between the center of rotation and inter-teardrop-base line was defined the height of hip center. d A line was drawn on the anterior-posterior radiographs of both hips across the lowest part of both pelvic teardrops to meet both femurs as a reference. The leg length discrepancy was determined by measuring the distance from this line to the top of the lesser trochanters. The values are for the post-operation while the values in brackets are for the pre-operation. e VerSys hip stem (Zimmer, Warsaw, Ind). f Exeter hip stem (Stryker-Howmedica-Osteonics, Rutherford, New Jersey). g Secur-Fit HA hip stem (Stryker-Howmedica-Osteonics, Rutherford, New Jersey). h MoP, metal on polyethylene.

In comparison between the types selected according to 3-D preoperative planning and the types actually adopted, it was found that there was complete correspondence in 10 hips (71.4%), a difference of one size in 3 hip (21.4%), and a difference of ≥ two sizes in 1 hip. For the types selected according to 2-D template and the corresponding types actually adopted, there was complete correspondence in 1 hip (7.1%), a difference of one size in 5 hips (35.7%), and a difference of ≥ two sizes in 8 hips (57.1%), as shown in Table 3. The difference of excellence rate (a difference of ≤ two sizes) in the prediction of prosthesis between 3-D preoperative planning and 2-D template measuring method was of statistical significance(χ2 = 8.023, P b 0.05).

Discussion What is the specific indication of conventional THA comparing with the other methods, such as using structural bone graft or cotyloplasty? There is no definite agreement on this matter. This is considered to be more dependent on surgeons’ personal choice. The definite indications of each method need further study with a larger number of patients. But achieving adequate coverage and stability of the acetabular component was the ultimate goal.

Table 3 Template Versus Actual Implant Size. Preoperative Planning

ICC Valuea

Same Size

1 Size/2 mm Different



2-D 3-D

0.313 0.888

1 10

5 3

6 1

2 0

a

ICC, intraclass correlation coefficient.

Planning for a THA is crucial. Comorbidities are identified, perioperative issues are addressed, and appropriate surgical planning occurs. Preoperative templating is one aspect of surgical planning. The benefit of templating in THA surgery is clear. First, the surgeon is able to reliably predict a range of implant sizes. Second, templating provides the surgeon with a reliable starting point in determining implant size and position. The above benefits may decrease surgical time and, therefore decrease complications. Preoperative templating based on 2-D radiographs is generally accurate except in the cases of anatomical variation, e.g. DDH. In these cases, 2-D radiographs do not present a comprehensive acetabulum shape. AS seen in the 3-D CT reconstruction for our group of cases, the superioinferior diameter was greater than the anteroposterior diameter, and if this disparity was not concerned, over-reaming or even bone defect of the anterior/posterior wall could have taken place because an oversized reamer was probably chosen. The appropriate pre-operative selection of the type of prosthesis was crucial in THA. Template measuring method was influenced by multiple factors that could bring about inaccurate selection. The CT-based isthmus-measuring method and CT-based resected-femur-measuring method were feasible with higher accuracy than conventional template measuring method. Rapid-prototype technique is a new digital modeling technique based on the principle of separation and the accumulation of materials to create a prototype. It used computerized control and was based on the computer-aided design (CAD) model or imaging data from CT or MRI. Rapid modeling is a comprehensive and collective application of subjects and techniques such as CAD, laser processing, digital control, and research and development of new materials. Its basic procedure includes repetitive layered processing of a 3-D model of the object to obtain its 2-D sectional data, which are then used to generate a “slide” of the acquired 2-D data, until the accumulation of these slides “grows”



into an actual model. The manufacturing technique is accurate to 0.1 mm even less. The uniqueness of rapid-prototype technique is its suitability for the production of complex, single-unit, or large-batch objects. Because the data scanned by CT or MRI are similar to the data from the rapid-prototype slide, accurate replications of biological objects with similar morphology can be performed through vector transformation of CT data and reverse calculation of the biological object’s surface. The 3-D RPT method tries to support the transfer of 2-D images into a 1:1 geometric model, which can be used for the preoperative planning. Once the pelvis is printed, accurate reaming and implantation of the cup can be rehearsed preoperatively. This method converts a 2-D image into a 3-D structure. The advantage of a 3-D model is particularly opponent when dealing with complex structures that have not been previously viewed. When viewing a 2-D image of a rare complex structure such as the abnormal acetabulum, however, the viewer has no preset 3-D recognition. A 3-D model would, therefore, be helpful when planning or performing an operation on such a complex structure. The 3-D RPT is able to present a comprehensive shape of pseudoacetabulum and its relative position to the true acetabulum, and the thickness of the medial wall as well. The 3-D RPT model can clearly show the position and shape of the acetabulum during mimic implanting via the adjustment of implanting angles. This model can also reveal osteophytes so that they can be removed to avoid misimplantation or femoroacetabular impingement syndrome. This study aims to evaluate the accuracy of 3-D RPT preoperative planning. In summary, the 3-D preoperative planning based on spiral computed tomography and 3-D RPT gains better consistency and precision than the templating from 2-D radiographs which are most widely used currently. In addition to an excellent operation scheme, the ability in executing as planned is also critical to achieve the best operation result, and it is the aim of intraoperative navigation as well. Current cup-positioning methods which have been clinically applied include: 1. Conventional freehand with visual inspection: location and orientation of acetabular prosthesis are determined empirically based on the edge of acetabulum and the orientation of the acetabular axis within the limited operative field. The drawback of this method is that the acetabular prosthesis is likely to deviate from ideal position due to patient’s postural changes and surgeon’s subjectivity. 2. Acetabular alignment guide: currently, the various acetabular alignment guides approved are mainly implanted empirically in reference to the trunk midline. And some guides designed according to the physiological relationship between acetabulum and pelvis used selective anatomic landmarks on the pelvis for guiding acetabular implanting at the selected anteversion and abduction angles. But similarly, postural changes tend to decrease the anteversion angle and increased abduction angle. 3. Computer assisted orthopedics surgery (CAOS), or so-called image-guided system (IGS): it has been introduced into various orthopedic fields. With such robotic-guided techniques, preoperative or intraoperative imaging data can accurately correspond to anatomical structures and surgical instruments can be on track to achieve the real-time navigation during surgery. Intraoperative navigation systems have been used for over a decade. Greater accuracy can be achieved by navigated implantation when compared with traditional manual implantation in respect of cup-positioning [27–33]. Although a number of application researches on navigated cup implantation have been published, there are inadequate data about patients with DDH. Limitations include the difficulties in reducing the variabilities of acetabular cup abduction and anteversion and determining the customized depth of acetabular reaming, suitable size of acetabular component as well as special treatments such as shelf graft on acetabulum. With the help of multi-spots positioning navigation, the structure of acetabulum can be simulated and reconstructed. Novel technologies like computed tomography (CT)-based navigation are also available to provide more information for guiding. However, neither is as intuitive

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as rapid prototyping (RP) technique. In our research, we can require a more intuitive acetabulum model preoperatively by RP technique and simulate operation processes for an optimal operation scheme by using such a model. We do not deny that surgeons’ experience is crucial for the consistency between preoperative plan and intraoperative positioning. But the application of navigation techniques can undoubtedly obtain better effects. By applying navigation techniques, surgeons are able to figure out the relative position between the center of acetabular component and the original center of acetabulum on the computer, avoid repeated implantation and improve positioning accuracy of the acetabular cup [34]. In spite of incomparable advantages, some shortages compared with traditional teaching method should be considered. (1) Preoperative CT-scan or intraoperative multi-dimensional X-ray is imperative. (2) A more complicated process matching patient’s image data and anatomical structure and a longer operation duration. (3) Increased exposure to radiation of both surgeons and patients [28,35]. (4) Other restrictions like high cost, high human and system error rate and long learning curve. Designing and applying personalized positioning device by using RP technique have been reported so far. Chen et al. [36] designed individual templates basing on a three-dimensional (3-D) model generated from computed tomography (CT) scans and formatted with rapid-prototype equipment. The physical template was designed to conform to the contours of patients’ acetabula and to confirm the rotation of the acetabular centers. The postoperative X-ray and CT showed that the locations of the acetabular components corresponded to those in the preoperative plan. By RP technique the anteversion and abduction angles can be well controlled. There are some limitations of this study. Firstly, this is a pilot trial with a limited number of cases and a very short period of follow-up. In order to extrapolate from these findings, further investigations will be necessary to determine the appropriate values for clinical application. This method may significantly facilitate precise acetabular prosthesis implants in THA. Secondly, there was no control group in which patients with osteoarthritis secondary to DDH underwent bulk bone grafting or no bone grafting for acetabular deficiency. Thirdly, the coverage of the acetabular component was measured in only anteroposterior radiographs. To evaluate the exact coverage of the acetabular component by the bone, CT would be more reliable. As regards the time it would take for the printing process and 3-D planning, two days would suffice in advance of the surgery. The cost is 400 US dollars. In our opinion, the cost and time delay are acceptable. In conclusion, the use of the 3-D RPT-model before THA in DDH patients facilitates the surgical procedure, due to thorough preoperative planning and intraoperative orientation of risk structures, and may improve surgical outcomes.

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