Preoperative Planning for Total Hip Arthroplasty

Introduction

Total hip arthroplasty (THA) improves the quality of life of patients with degenerative disease of the hip. In order to accomplish a successful outcome, hip joint biomechanics must be effectively restored. Preoperative planning is key to executing a successful THA. It allows surgeons to foresee difficulties and prepare solutions ahead of time. Complications in hip replacement surgery might arise from an inadequate preoperative plan and poorly executed preoperative planning.

Preoperative planning is not limited to templating. It also includes understanding of the patient’s diagnosis, selection of surgical approach, consideration of previous approaches, anticipation of the requirement of hardware removal or grafting, accurate assessment of leg length discrepancy (clinically and radiologically), preservation of femoral offset, restoration of the hip center of rotation, and accurate selection of implants. Remarkably, most experienced surgeons have the ability to identify difficult cases from the preoperative plan.

Planning and templating have been an essential part of hip replacement since its origin. Salvati described a technique of preoperative planning and templating used for decades at the Hospital for Special Surgery that was widespread and is still used worldwide. Details of this method were published and validated by Gonzalez-Della Valle et al. demonstrating its usefulness and accuracy. ^, Planning and templating in printed analog radiographs of the hip is the gold standard. ^, ^, ^, However, the advent of digital imaging has led to the development of new methods of preoperative planning that are also useful. Furthermore, three-dimensional (3D) printing and 3D reconstruction from computed tomography (CT) images has gained acceptance.

In this chapter, we will discuss the fundamentals, methods, and special considerations in preoperative planning for THA.

Preoperative Templating

Obtaining Preoperative Radiographs

A basic set of images required for planning a primary THA comprises an anteroposterior (AP) view of the pelvis and an AP and lateral view of the hip. Depending on the case, a femur radiograph of the compromised side and long-leg films to assess leg length or lower limb alignment, or a radiograph of the lumbar spine to evaluate spinal instrumentation or deformities, can also be included.

Either printed or digital, the preoperative AP view of the pelvis must meet the following standards in order to be appropriated for planning and templating: (1) centered in the pubic symphysis to allow visualization of the acetabulum and the proximal third of the femur; (2) perpendicular to the patient in supine position, avoiding inlet/outlet views, hips with 15 degrees of internal rotation to evaluate the femoral neck at its maximum length and discount its normal anteversion; and (3) both obturator foramens should appear symmetric and the sacrum and symphysis pubis are aligned. Asymmetry on the iliac wings or obturator foramens and abnormal pelvic tilt (resembling inlet or outlet views) or rotation might indicate contractures or fixed deformities of the hip and pathology of the lumbar spine. ^, Similarly, the shape of the lesser trochanter must be similar on both sides: a highly exposed lesser trochanter indicates an increased external rotation of the hip and a hidden lesser trochanter denotes internal rotation, altering the true dimensions of the femoral neck and offset.

It is important to obtain a lateral view that allows the alignment of the piriformis fossa with the diaphysis and allows an adequate template of the femoral stem. This can be achieved either with a frog leg (Löwenstein) or true lateral projection.

Another consideration when obtaining preoperative radiographs is the magnification. This is determined by the distance between the beam and the film: the further the object is from the source, the larger it will appear. With the patient supine on the imaging table, the tube 1 m away from the patient and the film 5 cm from the table, the magnification is around 20%. ^, ^, ^, However, this might not be constant for all images. In patients with higher body mass index, the magnification increases. ^, However, this has not been demonstrated precisely. Most printed templates of the available prosthesis assume a standard magnification of 20%.

Surgeons and trainees must be familiarized with the magnification of the radiographs. If the magnification is unknown, the assessment of femoral offset, leg length, and implant size will be less precise. Different methods have been described to assess magnification in both analog and digital images, with heterogeneous results. ^, One of the simplest methods to assess radiograph magnification, whether digital or analog, is the use of a sphere of known diameter positioned at a constant point close to the pelvis at the level of the greater trochanter.

Anatomic and Biomechanical Landmarks

The next step in preoperative planning is the identification of anatomic landmarks and biomechanical parameters ( Fig. 16.1 ). First, the horizontal plane of the pelvis is identified: a horizontal line that crosses the base of both teardrops. Alternatively, this can be the line connecting the most distal part of the ischial tuberosities.

On the acetabular side, anatomic landmarks are the ilioischial line (Kohler), the base of the teardrop, and the superolateral edge of the acetabulum. These parameters are strategic to restoring the center of rotation of the hip and predict the position of the acetabular component. In some cases, it might be useful to obtain the Sharp angle to reduce the probability of acetabular component mispositioning.

Main anatomic landmarks in the femur are the femoral longitudinal axis, the tip of the greater trochanter, proximal corner of the lesser trochanter, the femoral neck, and the center of the femoral head, which corresponds to the center of rotation of the hip when congruity is preserved. Biomechanical parameters are femoral offset, which is the distance between the femoral head center of rotation and the longitudinal axis, and the relationship between the level of the center of rotation and the tip of the greater trochanter. Both are indicative of the lever arm of the abductor mechanism and depend on the neck angle. One should always bear in mind that any rotation of the hip internally or externally might alter these measurements.

The leg length—more accurately, the hip length—is assessed by measuring the distance between the proximal corner of the lesser trochanter and the horizontal plane ( Fig. 16.2 ). ^, This distance is measured in both hips to detect any difference and correct if necessary. According to each case, the leg length might be required to be assessed with orthoradiography and confirmed with clinical examination.

With the introduction of the concept of the spinopelvic relationship and its implications in prosthetic instability, ^, the assessment of pelvic and spine tilt in seated and standing radiographs of the hip in the lateral view might be included. However, the clinical benefit or impact of this relationship is yet to be determined.

Templating and Restoration of Hip Biomechanics

Templating is the act of determining the correct position and size of the implant that allows precise restoration of the anatomy and biomechanics of the hip. ^, ^, ^, ^, ^, The acetabular cup template must be positioned within the triangle formed by the key anatomic landmarks. This helps the surgeon find the native center of rotation of the hip and mark its position ( Fig. 16.3 ). More precisely, the appropriate position of the template is lateral to the Kohler line, the inferior corner at the base of the teardrop, the periphery of the cup resting on the superolateral border of the acetabulum, with an approximate abduction angle of 45 degrees and the equator in line with the center of rotation of the femoral head (if the joint is congruent). If the center of rotation of the femoral head is not in line with that marked with the acetabular cup, the length of the hip should be adjusted. The size of the cup must be chosen accordingly, considering a superolateral coverage of at least 70%. ^, ^,

The femoral template ( Fig. 16.4 ) must fill the medullary canal at the metaphysis for proximally coated cementless stems, at the isthmus for fully coated stems with distal fixation, or allow for a 2-mm cement mantle circumferentially and must be aligned with the longitudinal axis of the femur. Canal morphology according to the Dorr classification is used as a guide for implant selection: a type A canal might be too narrow for a cemented stem, whereas in type C, a cemented stem might be preferable, given the increased risk of intraoperative fractures. There are different types of cemented stems; main variations among designs are the presence of a collar intended to aid positioning and load transfer in the proximal femur, and surface treatments with the purpose of improving the cement-implant interface. One of the most popular cemented stems is Exeter (Stryker; Mahwah, NJ), a collarless, polished, tapered stem that has demonstrated favorable survivorship rates. There are also multiple designs of cementless stems. Khanuja et al. group them into 6 types: type 1, single-wedge proximal tapered stem; type 2, double-wedge proximal tapered stem (fit and fill); type 3, tapered metaphyseal/diaphyseal engaging stem (round, splined, or rectangular); type 4, extensively coated metaphyseal/diaphyseal cylindrical stem; type 5, modular stem; and type 6, anatomic stem. Accolade 2 (Stryker; Mahwah, NJ) and ML Taper (Zimmer Biomet; Warsaw, IN) are examples of taper stems. Corail (Depuy; Warsaw, IN) and Polarstem (Smith & Nephew; Memphis, TN) are double tapered. Wagner (Zimmer Biomet; Warsaw, IN) is a metaphyseal/diaphyseal engaging stem, and Anato (Stryker; Mahwah, NJ) is an anatomic stem.

The position of the stem and prosthetic head should intersect with the center of rotation of the hip defined with the position of the cup, maintaining the length, the offset, and the level with the tip of the greater trochanter defined. Then, the level of the neck osteotomy is marked according to the template. If leg length requires adjustment, the stem can be displaced proximally or distally until the prosthetic head and the marked center of the hip overlap and the level of the neck osteotomy is modified consequently. ^, ^, ^, Modularity allows selection of the stem size, offset, and femoral head length that facilitate restoration of the biomechanical parameters. Transitioning from a standard or high-offset stem usually does not affect leg length unless the neck angle is changed, but larger prosthetic heads increase the length of the hip by a factor of 0.7 by each centimeter enlarged and will require the stem to be inserted more distally. ^,

Accurate restoration of the length, offset, and relationship between the center of rotation and the tip of the trochanter by templating is translated into an adequate balance of the soft tissues. An increase in femoral offset will increase the abductor moment arm but cause trochanteric bursitis and pain. If femoral offset is decreased, a loose abductor mechanism will increase prosthetic instability and limping. Testing trial implants will help to assess the balance of the soft tissues intraoperatively. ^, ^, ^,

Special Considerations

A lateralized acetabulum is frequent in degenerative osteoarthritis due to osteophyte growth in the acetabular fossa. This is evident in the radiographs as an increased distance lateral to the teardrop and a shallow appearance. It is very important not to overlook this finding; lateral placement of the cup increases hip offset, a deficient peripherical fit, and poor superolateral coverage. Thus, cup positioning lateral to the Kohler line is desirable. Intraoperatively, the transverse ligament and cotyloid notch are useful landmarks to achieve the optimal position. ^,

In protrusio acetabuli, the femoral head lies medial to the Kohler line, produces a defect of the medial wall, decreases hip offset, and causes impingement. The ideal position of the cup is lateral to the ilioischial line. However, to achieve this, during the preoperative planning the surgeon must anticipate the need of bone grafts for the medial wall and a larger cup size to obtain adequate peripheral fit. Lateralization of the center of rotation of the hip might lengthen the hip. ^,

Coxa vara femoral stems with extended offset might not be enough to restore the anatomy and biomechanical parameters for some patients. Therefore, the use of larger heads is necessary. However, this can lengthen the hip, in which case the neck osteotomy should be more distal and the stem inserted distally as well. In valgus hips, the femoral offset can be medialized to prevent additional tension of the soft tissues. However, this shortens the leg, and a more proximal neck osteotomy and protruded stem might be required.

Total hip arthroplasty in dysplastic hips is a complex procedure. ^, ^, If there is minor superolateral migration of the center of rotation, restoration of leg length and offset can be achieved by the identification of acetabular landmarks. In cases of complete loss of joint congruence and hip dislocation Crowe type 3 or 4, the ideal position of the cup is also the true acetabulum or 20 mm above in order to decrease mechanical complications. However, high cup implantation is considered to provide acceptable cup coverage without the need of grafts or augments. Moreover, controlled medialization might increase desirable coverage and biomechanics. ^, ^, If anatomic landmarks are difficult to evaluate in preoperative radiographs, one of the most widely used methods is the Ranawat triangle. However, this method is debatable. On the femoral side, it is important to consider that decreasing the level of the center of rotation of the hip more than 4 cm will lengthen the limb, raise stress on the soft tissues, and cause neurovascular injuries. To avoid this, femoral-shortening osteotomies can be performed. Several osteotomy techniques have been described with good results, and it is at the surgeon’s discretion to select the osteotomy. In addition, the proximal femur in dysplastic hips has a short anteverted neck and narrow femoral canal; most implants might not adjust to this atypical anatomy. In these cases, the use of a metaphyseal/diaphyseal tapered stem, such as the Wagner stem, fully coated cylindrical stems, or revision modular stems, will allow modification of the anteversion of the implant and its shape will fit in the narrow canal. It is imperative to assess judiciously the preoperative radiographs of dysplastic hips to anticipate the need for additional procedures, implants or grafts, or to examine complementary images to assess complex dysplastic acetabuli. ^,

Elderly patients also have special considerations during THA: most elderly patients requiring THA have osteoarthritis or intracapsular fractures and often present with a wide femoral canal, such as Dorr type C. In these cases, a cemented stem might be preferable. Several studies published to date have described the benefits of the use of cemented stems in this population since the use of cementless stems has been associated with an increased risk of periprosthetic fractures both intraoperatively and postoperatively. ^, The use of large broaches to fit the enlarged metaphysis might produce inadvertent fractures of the calcar or diaphysis. Some extracapsular fractures in the elderly can be treated with arthroplasty; this can be successfully achieved with the use of cemented calcar-replacing stems or diaphyseal-engaging stems that bypass the fracture site and optimize fixation. Moreover, a substantial rate of failure of internal fixation of extracapsular hip fractures has been described; most of these patients are treated with conversion to arthroplasty. ^, In these cases, the use of both cemented and cementless stems have been described with good results. When choosing cementless implants in patients with good bone stock and prior proximal femoral hardware, use of a long modular diaphyseal fixation stem to bypass screw holes from previous implants should be considered to avoid the stress raiser, along with augmentation of wires and/or plates and screws. If cemented stems are preferred by the surgeon, a calcar-replacing stem might be selected, taking care to seal screw holes to prevent cement leakage. Conventional metaphyseal fixation stems might be considered in cases in which it has been demonstrated that previous hardware or its extraction has not caused significant bone loss to the metaphysis. It is important to always rule out infection as the cause of fixation failure in the process of preoperative planning.

In general, when treating patients with previous hardware, significant deformities, or bone loss in the proximal femur, surgeons must consider the use of diaphyseal fixation stems long enough that allow spanning defects while obtaining adequate fixation. Likewise, for the acetabular side, cementless fixation is preferred. However, it is important to have available acetabular cups with holes, large acetabular cups, grafts, and augments according to the size of the defect.

Digital Templating

Digital imaging in orthopaedics has gradually replaced analog printed radiographs, given its undoubtedly clinical and environmental benefits. Different planning and templating software has become available that allows the surgical team to electronically overlap templates of different implants onto a digital image. ^, ^, The same principles apply to digital radiography; therefore, a reliable calibration system for magnification is also required. ^, ^, Aims, fundamentals, and procedures for digital templating are the same as analog-printed radiographs. Given that outcomes published to date are heterogeneous, it is not possible to conclude that digital planning is superior. However, accuracy and reproducibility are good to excellent, especially for the femoral component, ^, and improve with the expertise of the planner. We believe that as surgeons and trainees familiarize themselves with digital planning, the results will improve and become the standard of care.

Templating Accuracy

Anticipation of pitfalls and solutions is critical during preoperative planning. It is also important that magnitudes and implant sizes obtained during this process are accurate and consistent with intraoperative findings. This helps to reduce perioperative risks and cost. Accuracy is directly proportional to adequate magnification. ^, ^, ^, In both digital and analog radiographs, the accuracy for femoral stems is 77% to 98% and 60% to 89% for acetabular cups when the size of the implant selected in surgery is between ±1 size of that calculated preoperatively. ^,

New Technology

The preoperative planning methodology described to this point considers the restoration of anatomic parameters in the coronal plane. Defining implant positioning in the sagittal plane is substantial for prevention of instability and prosthesis dislocation. ^, ^,

Preoperative planning using 3D images obtained by CT scan and robotic-assisted surgery has been demonstrated to increase accuracy in implant positioning in the sagittal plane and achieve the desired combined anteversion, ^, ^, predicting implant sizes, and proving its usefulness in difficult reconstruction cases. ^, Preoperative planning with this new technology provides a truly patient-specific plan.

Robotic and computer-guided systems such as MAKO and CAOS use CT images to create a 3D reconstruction of the acetabulum and proximal femur. Principles and objectives are the same as traditional planning: restoration of the center of rotation, leg length, and offset adjustment. Anatomic landmarks vary and include acetabular walls, the transverse ligament, and both acetabular and femoral anteversion; implant templating is performed with 3D images. Intraoperatively, robotic assistance helps to reproduce implant positioning as planned. ^, ^,

Conclusion

Preoperative planning is the procedure performed to gain understanding of the indication, objectives, and pitfalls of THA. Templating is the routine of visualizing the result of the reconstruction and making adjustments before the patient is in the operating room. It is the key step to anticipating the needs, solutions, and tools that might be required intraoperatively. Whether analog, digital, or CT based/robotic assisted, surgeons and trainees must formulate a systematic preoperative plan to restore joint anatomy and biomechanics to achieve a successful long-lasting hip arthroplasty.