Templating for Primary Total Hip Arthroplasty

Introduction

• Templating is a familiar terminology to orthopedic surgeons whether it is used preoperatively or intraoperatively.

  • Templating has been extensively used for preoperative planning of fracture fixation and total joint arthroplasty.

  • Intraoperative templates have been routinely used to know the size and the shape of metallic plates before the latter are selected and contoured to fit the corresponding bone surfaces.

  • Templating in total hip arthroplasty (THA) is not new, and it has long been used with traditional radiographic films and printed templates (acetates).

• Digital templating has become possible with the introduction of digital radiography and computers into clinical practice.

   

  

   

• This chapter outlines the indications and rationale for templating, explains different methods used, and then describes in more detail the technical steps and possible pitfalls.

  Indications

• Templating is indicated for every primary THA whether it is a straightforward or a complicated case.

  • The technical success of THA requires accurate and reproducible preoperative planning (Knight and Atwater, 1992).

  • Prevention of complications such as dislocation and leg length inequality is dependent on the precision of alignment and position of prosthetic components (Morrey, 1992).

  • The hip joint is deep and, even with maximum exposure, many of the anatomic details and landmarks are not visible.

  • The introduction of limited and minimally invasive techniques for THA has increased the demand for accurate and reliable preoperative planning.

• Templating can help in deciding the type of fixation needed, whether cemented, cementless, or hybrid.

  • Bone stock is important, and it is useful to know in advance the cup size and the level of femoral neck cut to facilitate minimal bone removal.

  • Center of rotation has to be restored, offset needs to be optimized, and alignment of the stem should be anatomic.

     

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    • Leg length, ideally, should be equal particularly after the increasing rate of litigation in North America due to leg length discrepancy.

      • Templating alerts the surgeon to otherwise unexpected intraoperative difficulties and complications.

    • Surgeons should be aware well in advance regarding unusual implants or instruments. Larger femoral heads or constrained cups may be required if a higher risk of dislocation is expected.

    • The difficulty related to keeping a complete inventory of implants and instrumentation is another concern. In smaller institutions or centers where the turnover of THA cases is not large, manufacturers may supply a limited stock of implants. In such cases, templating is useful in predicting implant sizes and in providing adequate inventory.

      • Several authors have found that preoperative planning is useful in predicting implant size, position, and alignment, as well as in restoring the center of rotation, and equalizing limb length (Bono, 2004;

        Carter et al., 1995; Davila et al., 2006; Della Valle et al., 2005).

      • The accuracy of templating increases gradually with the level of training (Carter et al. 1995).

        Materials and Methods

      • Radiographs should include an anteroposterior (AP) view of the pelvis (both hips) and both AP and lateral views of the affected hip that include the acetabulum and the proximal third of the femur.

    • Patient positioning is critical to avoid misleading information.

    • The x-ray magnification has to be taken into account and be corrected before templating is started (Conn et al., 2002).

    • Templating can be done whether using printed acetates or specific software. The printed acetates may be applied to radiographic films or digital images.

      • Radiographic films are still frequently used in many hospitals around the world, but the use of digital imaging is on the rise.

    • In the United States, it has been estimated that 60% of hospitals have digital imaging (filmless). Digital images allow the use of powerful software

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Templating for Primary THA

with better functionality and more accurate measurements (Murzic et al., 2005).

Traditional Templating

• In this method, surgeons lay and match printed acetates (templates) of implants over radiographic films. The acetates are usually magnified to a certain number of degrees to compensate for x-ray magnification. Manufacturers usually provide information about the percentage of magnification of the templates.

  Templating with Acetates over

  Digital Images

• This technique involves the use of printed acetates with digital images rather than radiographic films. The technique has been found to be accurate and reproducible (Oddy et al., 2006), but White and Shardlow (2005) found digital images could reduce the magnification of the film and, therefore, reduce the accuracy of preoperative templates supplied

  by the manufacturers of implants, resulting in incorrect selection of implant.

   

  

   

  Digital Templating

• In this method, the templating is entirely performed using specific software. There is automatic scaling once the degree of magnification is selected, thus correcting the magnification on the displayed radiographic images.

• The software has a library of implants from different

  manufacturers in various sizes, which can be imported and superimposed on the radiographic images in coronal or sagittal views. The implants can be manipulated by translation or angulation until the optimal position is achieved. Measurements of leg length, distances, and angles can be done in decimals.

• Several software systems are available, such as OrthoView, OrthWork, VAMP, Sectra, mdesk, Merge, mediCAD, IMPAX, and EndoMap.

• At St. Michael’s Hospital in Toronto, we use the EndoMap software system (Siemens AG, Medical Solutions, Erlangen, Germany) routinely in preoperative templating for THA. Although this chapter describes the use of digital templating using the EndoMap system (Davila et al., 2006), the principles can be applied to any software and also to traditional templating.

   

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  Examination/Imaging

  • Ideally, templating should be done in outpatient clinics to give enough notice to obtain the required implants and instruments. For straightforward cases, templating can be done in the operating room just before surgery.

  • History and physical examination are indispensable to preoperative planning and templating.

  • Patients should be asked if they are aware of leg length discrepancy. Do they notice it (symptomatic or not)? Has it been measured or corrected by a heel lift?

  • Inquire about a history of previous surgery for the same or contralateral side.

  • Read old hospital notes to obtain implant sizes for previous THA (contralateral).

  • Measure leg lengths and account for pelvic obliquity and flexion deformity. In the case of pain or spasm, the measurement should be deferred until the patient is anesthetized.

  • Information obtained from history and examination, particularly leg length measurement, should be applied during templating.

  • Good-quality radiographs are essential and should include anteroposterior and lateral views

    extending beyond the expected tip of the femoral component and the cement restrictor. The position of the patient and the leg during radiographic examination is critical (see Pitfalls above).

    Templating for revision procedures should be done in the outpatient clinic and should be repeated just before surgery to take into consideration any changes that occurred during the waiting time for surgery.

     

    Procedure

    Step 1: Radiographic Assessment

  • Perform routine radiographic assessment looking at the quality of bone, amount of bone stock, dysplasia, osteophytes, and other abnormalities.

  • Make a preliminary decision on what type of implants to be used, whether cemented, cementless, or hybrid implants.

  • For uncemented femoral components, decide whether distal or proximal loading stems are to be used.

     

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• In case of a contralateral total hip replacement, determine the type of implants used and consider templating for the same implant type and size.

  Step 2: Correct Radiographic Magnification

• Scale radiographs to eliminate magnification.

• Consult radiographers about the percentage of magnification and be aware that the degree of magnification is related to the patient size.

• Follow the specific instructions of the software used to scale radiographs.

• In case of traditional templating, the printed acetates are usually magnified and the percentage of magnification is usually printed on the acetates.

• Conn et al. (2002) described a simple technique to determine radiographic magnification.

  Step 3: Measure Leg Length Discrepancy

• In addition to clinical measurement, measure leg length discrepancy on the AP pelvic radiograph using fixed landmarks such as lesser trochanters, greater trochanters, or teardrops.

• The software of a digital templating system can automatically calculate the leg length discrepancy, even in the presence of pelvic obliquity (Fig. 1). Note the acetabular templating in the presence of old metal hardware in the AP pelvic radiograph in Figure 1.

• Be aware of the effects of leg position, such as

  abduction, adduction, and rotation, that may alter the appearance and level of the lesser trochanter.

• Compare between radiographic and clinical measurements and differentiate between true and apparent discrepancy.

• Repeat clinical and radiographic measurements and record the final discrepancy in millimeters.

  Step 4: Template the

  Acetabular Component

• Use long unilateral AP radiographs that include the upper femur to template for THA implants (Fig. 2).

• Identify landmarks such as the ilioischial line, teardrop, acetabular margins, center of rotation, and greater and lesser trochanters.

• Start acetabular templating first by selecting the desired cup from the implant library and modifying the size and position to fit the acetabulum.

• Place the cup in a near anatomic position to reproduce the center of rotation.

• Align the cup according to the required angle for abduction (e.g., 45°).

• Consider minimal bone removal and sufficient bone coverage laterally. Use the ilioischial line and teardrop as landmarks and position the cup lateral to the teardrop (see Fig. 1).

• For a cemented cup, allow enough space for an adequate cement mantle (2 mm).

• In case of a dysplastic hip (see Fig. 2B), position the cup in the anatomic acetabulum and visualize the volume of the cavity in the superior lateral part of the false acetabulum. This volume should be reproduced intraoperatively, and the defect is then covered by bone graft or cement in case of cemented implants.

  Step 5: Template the Femoral Component

• Select the desired stem from the implant library.

• Modify the size and position to fit the femoral canal. In some cases each hip will require a different stem size. The cases shown in Figure 2 illustrate the variation in templating for a wide femoral canal (Fig. 2A) versus a narrow canal (Fig. 2B) that required a very small stem from a different manufacturer.

• Compare different offsets (standard or high) to find a better match for the patient’s original offset.

• Adjust height to correct leg length discrepancy based on the center of rotation of the acetabulum.

• In case there is no leg length discrepancy, the center of the head should be at the same level as that of the acetabulum.

  Step 6: Correct Leg Length Discrepancy and Measure Length of Neck Resection

• In case there is no preoperative leg length discrepancy, level the center of the femoral head with the center of the acetabulum.

• In case of leg length discrepancy, adjust the height of the femoral stem to correct this discrepancy (Figs. 3 and 4). For example, if the affected leg is 20 mm short, place the center of the head 20 mm above the center of the cup. Therefore, the neck cut will be higher and the length discrepancy will be exactly corrected when the hip is reduced.

• Mark the level of the neck resection, which

  corresponds to the level of the stem collar (or the upper medial border of a collarless stem)

• Measure the level of the prosthesis shoulder in relation to the level of the tip of the greater trochanter using a digital ruler. This measurement should be checked intraoperatively.

• Measure the center of the femoral head in relation to the greater trochanter. This measurement should be checked intraoperatively.

  Outcome Data and Operative Application

• The computer screen displays the relevant information regarding the implants, such as component sizes, stem length, offset, neck height, neck length, and the like (see Figs. 3 and 4).

• The entire plan can be saved as an electronic file or printed and attached to the patient notes, thus providing a permanent record for clinical, research, audit, or inventory (reordering) purposes.

• The relevant information should be recorded by the surgeon and used during surgery.

• Nursing staff needs to know about types and sizes of implants and any changes from the original plan.

• During surgery, the surgeon should adequately expose the lesser trochanter and mark the level of neck resection according to the preoperative templating.

• Prepare the acetabulum and the femur for the types and sizes of the implants predetermined by templating.

• It is not unusual to deviate from the plan and select sizes above or below the predetermined sizes.

• The soft tissue tension and the stability of the joint are other variables that should be borne in mind. Stability should not be compromised at the expense of leg length equality; further adjustment of the level of the femoral stem with the selection of the appropriate femoral neck length (head) may be required to optimize the stability of the hip joint.

  Additional Functions

• Lateral radiographs may provide useful information with respect to femoral anteversion and whether there is excessive ante- or retroversion. The images also show the shape of the femoral canal and the degree of bowing as well as the entry point and expected alignment of the stem.

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• • Most of the available software has other functions that determine the rotational center and pelvic rotation, medialization/lateralization, biometric calculation of joint geometry, and coxometry (analysis of hip values). In addition, the software allows for planning of other procedures about the hip and knee, such as corrective osteotomy (Fig. 5).

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  FIGURE 5

   

   

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