Introduction

• Procedure 4, on templating for primary total hip arthroplasty (THA), outlined the basics of templating and the specific technical steps for primary procedures. This chapter is focused on the specific technical details for templating in revision THA and provides some practical examples.

• At St. Michael’s Hospital in Toronto, we routinely use the EndoMap software system (Siemens AG, Medical Solutions, Erlangen, Germany) for preoperative templating for THA. The accuracy of this software has already been reported (Davila et al., 2006).

• The basic principles of templating are the same regardless of the version of the software used, and these principles can also be applied to traditional templating.

  Indications

• Templating is indicated for every revision hip arthroplasty, whether it is a straightforward or a complicated case.

• Revision THA is a complex procedure with a higher risk of complications and unforeseen circumstances. Templating is an essential part of preoperative planning that is more important and sophisticated for revision than for primary arthroplasty.

• Preoperative planning is required for the type of implants to be used, the method of fixation (cemented, uncemented, or hybrid), and the need for bone grafting and/or special instruments or devices.

  • Larger femoral heads or constrained cups may be required if a higher risk of dislocation is expected.

  • In revision surgery, bone stock is usually deficient, and metal or allograft augmentation may be required. It is useful to know in advance the cup size and the level of femoral neck cut to facilitate minimal bone removal.

  • The anatomy is usually distorted in revision surgery, and planning is required to restore the center of rotation, offset, and leg length and to obtain optimal alignment of the implants.

• Templating may allow the surgeon to predict intraoperative difficulties and possible complications.

   

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  • Implant inventory is another concern. Revision implants and instrumentation are not usually stored at the hospital site. Surgeons, nurses, and manufacturers need to be aware long in advance about unusual implants or instruments.

  • The value of preoperative planning for revision THA has been reported by several authors (Barrack and Burnett, 2006; Bono, 2004; Knight and Atwater, 1992; Morrey, 1992; Seel et al., 2006).

     

    P ITFALLS

    • External rotation gives a false impression of valgus leading to underestimation of the femoral offset.

       

    • Internal rotation gives a false impression of varus leading to overestimation of the femoral offset.

       

    • Abduction may alter the leg length (apparent lengthening).

       

    • Adduction may alter the leg length (apparent shortening).

       

    • Pelvic tilting or asymmetry may alter the leg length.

     

    Examination/Imaging

  • History, clinical examination and laboratory investigations are essential components of preoperative planning and should be done routinely before templating.

    • Information about previous THA (ipsilateral and/or contralateral) should be obtained from old hospital notes.

    • Measurements for leg length discrepancy should be done clinically and radiologically. Patients should be asked if they are aware of leg length discrepancy. Measure leg lengths and account for pelvic obliquity and flexion deformity.

• Good-quality radiographs are essential and should include anteroposterior and lateral views extending beyond the tip of the femoral component and the cement restrictor. The position of the patient and the leg during radiographic examination is critical (see Pitfalls).

• 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 may have occurred during the waiting time for surgery.

  Positioning/Exposures

• The technical steps for templating are the same whether a posterior or a lateral approach is used and whether the patient is positioned on his or her side or supine.

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Procedure

• If templating is found to be practically difficult on the affected side, the opposite normal hip can be used for templating. The determined center of rotation on the affected side can be transferred to the normal side.

  Step 1: Radiographic Assessment

• Look at bone quality, evidence of loosening, osteolysis, cortical thinning, perforation, fractures, implant migration or failure. Look at the polyethylene liner and find out if there is any evidence for polyethylene wear The radiographic assessment of a failed THA in Figure 1 shows loosening of the acetabular component with polyethylene wear and osteolysis of the proximal femur.

• For two-stage revision procedures, it may be useful to do a preliminary templating before the removal of the failed component and then a final templating before the second stage of the procedure. Figure 2 shows a preoperative radiograph before a second-stage revision, with the cement spacer in place and the deficient medial femoral cortex that requires a femoral stem with calcar replacement.

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

• Decide whether distal or proximal loading stems are to be used.

• If a decision is made to revise one component, the surgeon needs to find a suitable new implant compatible with the retained component.

• Information about the manufacturer and the sizes of the old implants can be found in the hospital notes, particularly in the stick-on labels from the manufacturers.

   

  P ITFALLS

  • Radiographic magnification is variable and depends on the radiographic techniques used and the patient’s size.

     

  • Errors in correcting magnification will result in wrong selection of implant types and sizes.

   

  Step 2: Correct Radiographic Magnification

• Eliminate magnification by scaling the anteroposterior (AP) pelvic radiograph using the software facilities.

• Consult radiographers about the percentage of magnification, and be aware that the degree of magnification is related to patient size. Conn et al. (2002) described a simple technique using a coin to determine radiographic magnification.

   

  Digital Templating for Revision THA

   

   

   

   

   

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  FIGURE 1

   

  FIGURE 2

   

   

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  Digital Templating for Revision THA

   

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

   

  P ITFALLS

  • Be aware of the effects of leg position; abduction, adduction, and rotation may alter the appearance and level of the lesser trochanter.

   

  Step 3: Measure Leg Length Discrepancy

• Measure leg length discrepancy using fixed landmarks such as the lesser trochanters, greater trochanters, or teardrops.

• The software of a digital templating system can automatically calculate the leg length discrepancy.

• 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 the long unilateral AP radiograph to template for THA implants.

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

• Start with acetabular templating.

• First select the desired cup from the implant library and then modify the size and position to fit the acetabulum. A larger cup is usually selected to compensate for bone loss. The sizing of the cup may help in restoring the center of rotation by avoiding the use of a smaller implant that may lead to a high hip center.

• Place the cup in a near-anatomic position to reproduce the center of rotation. If the cup is placed proximal or distal to this, then shortening or lengthening of the leg will be seen. 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 the teardrop as landmarks and position the cup lateral to the teardrop.

• For a cemented cup, allow enough space for an adequate cement mantle. Estimate the volume of the cavity in the superolateral part of the false acetabulum. This volume should be reproduced intraoperatively, and the defect must be filled by the appropriate material (bone graft, cement, or metal).

• Figure 3 shows the templating for both acetabular and femoral components with the correction of shortening resulting from the dislocation of the cement spacer.

   

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  Digital Templating for Revision THA

   

  FIGURE 3

   

  Step 5: Template the Femoral Component

  • Select the desired stem from the implant library. Modify the size and position to fit the femoral canal. Figure 4 shows templating of the femoral component in the presence of a fractured femoral stem.

     

     

     

     

    FIGURE 4

     

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

• A calcar replacement may be required, and the height of the calcar can be determined by measuring the distance between the stem collar and the available calcar. Allograft may be considered if bone loss is excessive (30 mm).

• Determine the proper offset.

• Try to use a femoral stem with a zero head to leave you with the flexibility to increase or decrease length intraoperatively and obtain optimal soft tissue tension.

• The stem length should bypass cortical defects.

• Lateral radiographs may provide useful information with respect to the position of the existing femoral stem, the quality of bone, and the degree of femoral anteversion and whether there is excessive ante- or retroversion. They also help in localizing areas of loosening, osteolysis, cortical thinning, perforation, fractures, or implant failure. 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.

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

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

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

  • In case of preoperative shortening, the center of the head should be elevated above the center of the cup by the amount of required lengthening in millimeters. For example, if the shortening was

  20 mm, the center of the head should be placed vertically 20 mm above the center of the cup.

• It may be difficult to obtain full correction of leg length discrepancy, Barrack and Burnett (2006) recommend correcting only two thirds of shortening, since it is difficult to overcome excessive soft tissue tension associated with chronic shortening.

• Measure the femoral neck cut, the distance between the lesser trochanter and stem collar (or to the medial border of a collarless stem).

• Measure the length of the femoral neck resection in relation to the lesser trochanter using a digital ruler. This measurement should be reproduced intraoperatively.

   

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• Measure the position of the shoulder tip of the prosthesis in relation to 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. In Figure 5, the computer screen shows the templating of a distal-loading femoral component and complete data from the manufacturer on the selected implant.

• 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.

• Inform nursing staff about sizes of templated implants and any change in plan or type of implants.

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

FIGURE 5

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• 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 another variable 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.