Department of Orthopaedics, West China Hospital, Sichuan University, Chengdu 610041, China

Accuracy of a Simple Digital Templating in Primary Uncemented Total Hip Arthroplasty△

Hai-bo Si, Yi Zeng, Fei Cao, Fu-xing Pei, and Bin Shen*

Department of Orthopaedics, West China Hospital, Sichuan University, Chengdu 610041, China

arthroplasty; hip; digital templating; preoperative planning; accuracy

MethodsA prospective study of digital planned THAs was performed on 90 hips with different types of prostheses from August 2013 to Angust 2014. The plastic templates of different types of prostheses were scanned for desired digital templating and the scanned images were then processed to the transparent, digital templates using Photoshop software. We calibrated the scales of the digital templates with the preoperative radiographs, and then selected the right size of the component. The preoperatively digital planned component sizes were compared to the actual sizes used in the operation, as well as the preoperatively traditional planned sizes.

ResultsIn either cup or stem templating, no difference was found among the groups with different types of prostheses, and the accuracy of digital templating was statistically higher than traditional templating. In cup templating, digital planned and implanted cup sizes were identical in 43.33% of the cases, 78.89% of the cup sizes were predicted within 1 size (± 2mm) and 95.56% were predicted within 2 sizes (±4 mm), all significantly higher than the results of traditional planned sizes (30.00%, P=0.044; 61.11%, P=0.007; 82.22%, P=0.004). The exact femoral stem size was predicted in 40.00% of cases using the digital templating, 84.44% of stem sizes were predicted within 1 size, and 93.33% within 2 sizes, better than using traditional templating (25.56%, P=0.028; 53.33%, P＜0.001; 77.77%, P=0.003).

ConclusionThis simple digital preoperative templating is an accurate and reproducible process to preoperatively predict the implant size of uncemented THA.

Chin Med Sci J 2015; 30(3):150-155

TOTAL hip arthroplasty (THA) is one of the most successful procedures for surgical treatment of a variety of hip diseases.1-3With increasing number of THAs and the use of uncemented prostheses, precise preoperative planning has become vital for successful THA, and more emphasis has been placed on selecting the accurate implant size to avoid facture with a component too large or subsidence with a component too small.4,5Traditionally, plain radiographs and plastic templates with fixed magnification factors of 115% or 120% were used to achieve these goals, but the uncontrolled magnification of the plain radiographs from different equipment often result in errors in preoperative templating.The outcomes of the operation would be affected if the unmatched component according to the inaccurate preoperative measurement is used.6-8

Digital radiography is currently replacing traditional radiography in many hospitals, and digital templating was proposed as a method to electronically overlay templates from a digital library on clinical radiographs for arthroplasties.9Combination of digital radiography and digital templating has the potential to eliminate the errors associated with the manual manipulation of plastic templates and plain radiographs. To date, many studies have evaluated the accuracy of digital templating using expensive orthopaedic software in uncemented THA.10,11This prospective study introduces a simple digital templating and determines how well preoperative size selection of THAs of this digital templating correlates with the size actually implanted.

MATERIALS AND METHODS

Preparation of digital templating

1.3 觀察指標 對比兩組患者手術前后社會功能、軀體功能、心理功能、物質(zhì)生活狀態(tài)等生活質(zhì)量變化，手術時間、術后出血量、月經(jīng)恢復時間、住院時間、血清β-hCG恢復時間等圍手術期各項指標水平，下腹疼痛、發(fā)熱、陰道出血、下肢靜脈血栓等術后并發(fā)癥的發(fā)生情況及術后滿意度。生活質(zhì)量評分采用生活質(zhì)量綜合評定問卷-74(GQOLI-74)評估，滿分為100分，分數(shù)越高表示生活質(zhì)量越高，滿意度采用自制問卷調(diào)查進行評分，滿分為100分，其中80分以上為非常滿意，60～80分為滿意，60分以下為不滿意，滿意度=非常滿意率+滿意率。

Plastic templates of Corail, Summit and Tri-lock femoral stem and Pinnacle acetabular cup (DePuy, USA) were used. These template overlays were scanned for desired digital templating using a standard tabletop scanner and the scanned images were saved as JPEG (.jpg) files. The scanned images were then opened in Photoshop CS6 software (Adobe Systems Inc, San Jose, CA, USA), the blank areas were selected by the “Magic Wand tool” and deleted using the “Delete” key. The measure image, ruler, size, and other markers were remained, the “Erase” tool was used to erase the extra spots when necessary. Finally, the transparent, digital templates were named (type-size) and saved as PNG (.png) files (Fig. 1).

Preoperative templating

The preoperative digital radiographs of the hips were obtained by digital radiography. Care was taken to ensure weight bearing radiographs were well centered with the coccyx pointing just above the symphysis pubis, symmetrical obturator foramina, and with both feet in 10° to 15° of internal rotation. The digitized radiographs were stored in the hospital image station, and downloaded to our computer saved as JPEG files.

The JPEG images of hip radiograph and the digital templates were imported into Photoshop software. The preoperative hip radiograph was opened as the initial image and set as under layer, the digital template image was then moved and placed on preoperative radiograph, scaled by the “free transform” tool, with the SHIFT key pressed to keep the same aspect ratio and avoid distortion, until the scale of the digital template layer coincide with the radiograph’s scale (Fig. 2A). The digital template image was moved and rotated to select the right size of the component. Select the acetabular size that matches the contour of the patient’s acetabulum without excessive removal of subchondral bone. The medial position of the acetabular template is at the teardrop and the inferior margin at the level of the obturator foramen (Fig. 2B). Select the femoral size that matches most precisely the contour of the proximal canal, restore the horizontal and vertical offset and fit (Fig. 2C). Template the femur on the lateral view in the similar manner to ascertain whether the implant determined on the anteroposterior radiograph can be inserted without excessive bone removal (Fig. 2D).

All the digital templating was performed prospectively by the first author, who is familiar with the Photoshop software. In addition, preoperative planning was also performed traditionally, using the plain radiographs and plastic templates, by the second author.

Total hip arthroplasty

A total of 90 planned size selections were recorded prospectively for selective uncemented THAs and were compared with the actual size implanted at surgery and with traditional preoperative size selections from August 2013 to August 2014. Patients underwent a staged or simultaneous THA used only template for the first hip. The 90 uncemented THAs were performed by a senior joint surgeon (the corresponding author) who has implemented more than 800 THAs successfully using a posterior-lateral approach, the preoperative diagnoses were osteoarthritis or avascular necrosis of femoral head. All the patients received a Pinnacle cup, high-cross-linked polyethylene liner and ceramic femoral head (DePuy, USA). Thirty patients received Corail femoral stem (Group A), 30 received Summit femoral stem (Group B), and 30 received Tri-lock femoral stem (Group C) (DePuy, USA). The sizes of acetabular cup and femoral stem used in the operation were recorded.

Statistical analysis

Statistical analysis was performed with SPSS 18.0 (SPSS, Inc, Chicago, IL, USA) software by the third author, who did not participate in planning or surgery. All categorical variables, such as gender, operative side and the accuracy of planned sizes analyzed against the final component sizes at surgery, were compared using the Chi-square test. Continuous variables (age) were expressed as means ±SD, and comparisons were employed using 2-sided T-test for paired groups. P<0.05 was considered statistically significant.

Figure 1. Preparation of the transparent, digital templates of Pinnacle acetabular cup (A) and Corail femoral stem (B).

Figure 2. Preoperative digital templating.A. Calibration of the scale of the digital template and the preoperative radiograph layer;B. Digital templating of the acetabular cup size;C. Digital templating of the femoral stem size on the anteroposterior view;D. Digital templating of the femoral stem size on the lateral view.

RESULTS

The average age of group C was statistically lower than that of group A (P=0.041, Table 1), but no significant differences in sex and operated side between the groups were found (all P>0.05). The digital and traditional templating results of either cup or stem showed no difference among group A, B, and C (all P> 0.05, Table 2).

In acetabular cup templating, digital planned and implanted cup sizes were identical in 43.33% of the cases. The increments in size of the manufactured Pinnacle cup are 2 mm, a total of 78.89% of cup sizes were predicted within 1 size (± 2mm), and 95.56% were predicted within 2 sizes (±4 mm) in digital templating. Whereas, traditional planned and implanted sizes were identical in 30.00%, within 1 size in 61.11% and within 2 sizes in 82.22%. The accuracy of digital templating for acetabular cup size was statistically higher than that of traditional templating (P<0.05, Table 3).

The exact femoral stem size was predicted in 40.00% of the cases in digital templating and 25.56% in traditional group. The increments of the manufactured Corail, Summit and Tri-lock stem are 1 sizes. 84.44% of stem sizes were predicted within 1 size, and 93.33% within 2 sizes in digital templating. In comparison, 53.33% within 1 size and 77.77% with in 2 sizes, respectively, were predicted in traditional templating. The accuracy of digital templating for femoral stem size was statistically higher than traditional templating (P<0.05, Table 3).

Table 1. Characteristics of the included patients

Table 2. The templating results of the digital and traditional groups with different types of prostheses [n=30, n(%)]

Table 3. The total templating results of the digital and traditional groups [n(%)]

DISCUSSION

Preoperative templating is regarded as an essential step for successful implantation of the hip prostheses. Traditional templating is mainly based on plastic templates and plain radiographs, the plastic templates have a fixed magnification of 115% or 120% usually, but the magnification of the plain radiographs from different device is difficult to control, and it will result in errors in preoperative templating.6-8Furthermore, if the unmatched component according to the inaccurate preoperative templating was used, especially by inexperienced joint surgeons, intraoperative facture (with too large a component) or postoperative subsidence (with too small a component) might occur, and the postoperative rehabilitation would be affected.4,5The introduction of digital radiography is advantageous in preoperative planning for THA since it shows good intra- and inter-observer reliability, as well as provides higher imaging quality through the possibility of image editing and individual referencing.12,13However, the commercial orthopaedic software is expensive, relies on the digital library and needs to design for each application, and whether the digital templating using these commercial orthopaedic software is superior to manual templating in predicting prosthesis size is still controversial.10,11In this study, we prospectively investigated the accuracy of a simple digital templating in estimating the size of the hip prostheses, and found that its accuracy was significantly higher than traditional manual templating.

A few studies investigated the manual onlay templating of uncemented THA. It has been reported that the cup was accurate in 20%-51% of cases, which increased to 60%-100% within 1 cup size, and the femoral stem was accurate in 40%-79% of cases, which increased to greater than 85%-100% within 1 femoral stem size.12,14-16More studies have investigated digital templating in uncemented THA.10,12,17-20They found that templating of the acetabular cup had an accuracy of 25%-84%, which was 60%-91% within 1 size, and more than 90% within 2 sizes. The accuracy for the femoral stem was 35%-62% in femoral stem, which was 74%-98% within 1 size, and greater than 95% within 2 sizes. The et al20showed a higher accuracy of cup estimation in digital templating compared with manual onlay technique, but there were no differences in stem evaluation. In these studies, Trauma Cad templating system (Orthocrat Ltd, Petach-Tikva, Israel), Orthoview software (Meridian Technique Ltd., Southampton, United Kingdom) or Hyper ORTHO software (Rogan-Delft BV, Venendaal, Netherlands) were adopted. These commercial orthopaedic software have the disadvantage of depending on the digital library, high-cost, and limitations in software design for each application.

The application of Photoshop has been reported in optimizing radiographic images,21preoperative surgical planning,9and three-dimensional reconstruction of anatomicstructures based on segmented MR images.22The techniques for image rotation, sizing, and the use of layers in these studies are similar to those used in the current study. Using this technique, preoperative planning can be performed anywhere and at any time as long as the images can be scanned or downloaded directly from the radiographic archive into the Photoshop software. Given its lower cost and higher flexibility compared with commercial orthopaedic software, Photoshop may be therefore an alternative for the preoperative digital templating of THA. Additionally, the digital preoperative templates can also serve as a permanent data record useful for future THA.

Since the magnification increases proportionally to the distance between hip and film, greater magnification factor variance must be assumed for pelvic overviews due to the influence of corpulence.23,24Nevertheless, Kniesel et al6did not find any evidence of interdependence between planning accuracy and body mass index in the planning of THA, and they also found that there was a relatively low correlation between planned and implanted sizes with or without reference ball. To further improve planning accuracy, attention must be directed not only toward precisely adjusting radiographic views but also toward calibration of the digital radiographies.

No intraoperative fracture occurred in this study, but the incidence of intraoperative fracture is reported as 1.5%-27.8%.15,25To avoid intraoperative fracture with uncemented THA, surgeons are likely to insert a componentsmaller than the planned size once intraoperative stability and prostheticfit appear satisfactory. However, it should be noted that if the actual stem is smaller than the planned size by 2 sizes when the femoral neck osteotomy level is made at the desired level, there is a possibility of varus malalignment, adequate femoral lateral reaming is required. When combined with intraoperative checks, both under-sizing with subsequent subsidence and over-sizing with the risk of intraoperative fracture can be minimized.

This study has several limitations. The angles and reference lines were determined manually on the digital radiograph, on some radiographs, it was difficult to determine the superior point of the tear drop, so an alternative to this would have been the use of the interobturator, which a previous study revealed with the least amount of variance.26The technique used in this study is limited to two-dimensional imaging and each view needs to be templated independently. Despite these limitations, the techniques presented here obtained a significantly higher accuracy of preoperative prostheses size selection compared with traditional templating and reached similar results with previous findings. It may be an alternative for the existing complex preoperative digital templating of THA.

In conclusion, digital preoperative templating in this study is an accurate and reproducible process to preoperatively predict the implant size of uncemented THA.

Acknowledgements

We acknowledge the help from all respondents of this study and all the other people who generously helped us with this study.

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Received for publication November 24, 2014.

△Supported by the Health Research Program of Ministry of Health (201302007).

*Corresponding author Tel: 86-13881878767, E-mail: shenbin_1971@163.com