Preoperative Templating and Biomechanics in Total Hip Arthroplasty

Abstract

Preoperative templating is an essential ingredient of a successful hip arthroplasty. The socket template is positioned first to establish the center of rotation of the reconstruction. For femoral templating, a surgeon should not only consider the part inside the bone (the size of the component), but should also consider the part outside the bone, which determines limb length and biomechanical parameters such as the abductor muscle and joint reaction forces. Medializing the hip center of rotation and increasing the horizontal femoral offset can improve clinical outcomes and reduce polyethylene wear. Modern modular systems allow limb length adjustment and biomechanical improvement for a range of patients.

The past two decades have produced dramatic improvements in cemented and cementless femoral fixation. Aseptic loosening is now a rare complication, and total hip replacement (THR) is performed on active patients who are challenging the functional capacity of the technology. Improving the biomechanics of the reconstruction can improve function and reduce wear. Several contemporary hip systems offer standard and high offset femoral components that enable a superior biomechanical reconstruction. Preoperative planning is the foundation for a successful arthroplasty. This article incorporates elements of the physical examination, preoperative radio-graphs, and templating for the prosthesis to optimize the biomechanics of the reconstruction.

Limb Length

An important secondary goal of THR is limb length equalization. Physical examination is helpful in assessing the true limb length discrepancy. Common methods include placing blocks of known size under the foot of the shorter extremity until the pelvis is level and measuring the distance from the anterosuperior iliac spine to the medial malleolus. In patients with hip arthritis, the most common cause of apparent limb shortening is a flexion contracture. A surgeon must be wary of fixed pelvic obliquity. In such cases, it is helpful to ask the patient if he or she perceives any limb length discrepancy or uses a shoe lift.

The essential preoperative radiographs include a low anteroposterior pelvis (beam centered on the pubis) and a modified frog lateral. It is critical that the radiographs demonstrate the extent of the femur that will be occupied by the prosthesis to ensure that there is no defect, deformity, or disease that may compromise the reconstruction. With consideration given to the presence or absence of a flexion contracture, the relative “height” of the lesser trochanter is a common means to assess length differences in the proximal femurs. A Johnson’s lateral view is helpful in assessing the anteroposterior dimensions of the acetabulum and reveals the extent of anterior osteophytes.

The information on limb length discrepancy obtained from the physical examination and the radiographs is incorporated into the preoperative plan with the goal of decreasing the discrepancy. Patients, especially women, are sensitive to “over-lengthening.” Leaving the arthroplasty slightly short is better tolerated. Higher offset femoral components and larger diameter bearings enable a surgeon to obtain adequate tissue tension and stability without over-lengthening.

Biomechanical Principles

Femoral offset is defined as the perpendicular distance from the center line of the femoral canal (or stem) to the center of rotation of the femoral head. Medialization of the hip’s center of rotation decreases the moment arm for body weight; increasing the femoral offset lengthens the lever arm for the abductor muscles. These changes in hip biomechanics have a double benefit: a reduction in the required abductor forces and lower joint reaction forces.¹Accumulating clinical evidence indicates that such favorable alterations in biomechanics can improve clinical outcomes² and reduce wear.^3,4Higher femoral offset has been associated with greater hip abduction motion and abductor muscle strength. In two independent studies, higher femoral offset has been associated with a significant reduction in polyethylene wear.^3,4 On this basis, improving the biomechanics is an important goal of THR.

The traditional goal of THR has been to recreate the offset of the operated hip. In an analysis of 41 patients with one arthritic hip and one clinically and radiographically normal hip, the author (TPS) found that the femoral offset of the arthritic hip was, on average, 6 mm less than that of the normal contralateral hip. It is therefore reasonable to make the offset of the reconstruction greater than it was preoperatively. Concurrent medialization of the hip’s center of rotation by an amount that is equal to the increase in femoral offset keeps the lateral aspect of the greater trochanter in the same position (no femoral lateralization).

Templating

For templating, it is preferred that an anteroposterior pelvic radiograph be taken with the extremities positioned in approximately 15° of internal rotation to assess the proximal femur in a neutral profile and better assess the shape of the metaphysis and femoral offset. This may not be possible if the disease process has resulted in an external rotation deformity. In such cases, the preoperative radiograph underrepresents the true offset (Figure 1A).

Acetabular templating is performed first because the acetabular component establishes the center of rotation of the arthroplasty. The primary goal is obtaining adequate bony support for the component. The optimal abduction angle (lateral opening) of the component is partially dependent on the version of the acetabular component. For most cases, a target of 45° of abduction is satisfactory. The acetabular templates made by many manufacturers print the outlines of the acetabular components on a 45° angle on the transparent template sheet. A 45° abduction angle is then obtained when the template sheet is placed parallel to the pelvis on the radiograph. The acetabular teardrop is a consistent reference for the medial wall. The medical aspect of an adequately sized acetabular component will approach the medial wall and be covered by superolateral bone. This is progressively challenging with more shallow sockets, as in developmental dysplasia (Figure 1B). Details regarding the degree of medialization, the amount of acceptable verticality, and/or lack of superolateral support and the need for bone graft supplementation are beyond the scope of this article.

For femoral templating, it is critical to consider the part inside the bone (the fit of the stem) and the part outside the bone, which determines the biomechanics of the reconstruction. The size of the femoral component determines the fit. For both cemented and cementless fixation, the definition of optimal femoral fit depends on details of the specific femoral component and issues of surgeon preference, which are beyond the scope of this exercise. The fit of the femoral component should be checked on both the anteroposterior and lateral radiographs.

Having selected an appropriately sized femoral component, the surgeon places the template transparency on the anteroposterior radiograph, aligning the stem of the femoral component inside the femoral canal (Figure 1B). Relative limb length can influence the vertical position of the component. The “neck length” of a modular femoral head influences both length (vertical component) and offset (horizontal component), whereas the vertical position of the stem influences only length. The vertical position of the stem also influences the location of the neck osteotomy.

The limb length and offset change for the proposed reconstruction can be assessed on the templates by comparing the location of the center of rotation of the acetabulum to the location of the center of rotation of the modular femoral head. For the templating exercise, the acetabular center of rotation should be considered the fixed reference and the femoral head center should be considered variable. When the head center directly overlies the acetabular center, there is no change in the limb length or femoral lateralization when such a plan is executed surgically. If the hip center of rotation is medialized, then the femoral offset will be increased by a corresponding amount. When the head center on the templating is superior to the acetabular center, limb length will be gained when the hip is reduced at surgery. When the head center is medial to the acetabular center on the template, then relative femoral lateralization will occur when such a plan is executed surgically (Figure 1C).


Figure 1: Preoperative anteroposterior pelvis of a young female patient with osteoarthritis of the right hip secondary to mild dysplasia (A). The right femur is slightly shorter with less offset than the normal contralateral hip. On the diseased right hip, a medialized 50-mm Pinnacle acetabular component (DePuy) is paired with a size 5 Summit high-offset stem. The lateral white dot indicates the center of rotation of the modular acetabular component, and the medial white dot indicates the center of rotation of the modular femoral head. The horizontal distance between the dots represents the increase in femoral offset, whereas the difference in the vertical positions of dots represents the change in limb length. Biomechanical parameters: The black dot is the hip center of rotation. A=horizontal teardrop line; B=vertical teardrop line; C=femoral longitudinal axis; D=horizontal center of rotation; E=femoral offset; F=limb length (B). Postoperative anteroposterior view of the pelvis demonstrating a slight increase in limb length and an increase in offset (C).

Limb shortening occurs when the head center is below the acetabular center and femoral medialization occurs when the head center is lateral to the acetabular center on the template. Shortening without femoral lateralization and femoral medialization without limb lengthening reduce soft tissue tension and may predispose to hip instability (dislocation). The use of large diameter femoral heads and outset acetabular components can improve the stability of such reconstructions.

Practical Application

In a series of 40 consecutive THRs involving the use of a high offset femoral component (Summit, DePuy Orthopaedics, Warsaw, Ind), the postoperative horizontal center of rotation was medialized by a mean of 5.6 mm and was more medial than its preoperative position in 30 of 40 hips (P<.00001). Compared to the normal contralateral side, the horizontal center of rotation was medialized by a mean of 1 mm and was more medial than the normal contralateral hip in 13 of 19 THRs (P=.13).

The postoperative horizontal femoral offset increased by a mean of 9.5 mm and was larger than the preoperative measurement in 36 of 40 cases (P>.00001). Compared to the normal contralateral side, the postoperative horizontal femoral offset increased by a mean of 5.2 mm and was larger than the normal contralateral side in 17 of 19 hips (P=.0007). Preoperative and postoperative horizontal femoral offset were correlated (r=0.53, P>.0001).

The net biomechanical advantage was calculated as the percent change in horizontal femoral offset minus the percent change in horizontal center of rotation (an increase in horizontal femoral offset with a decrease in horizontal center of rotation results in a higher net biomechanical advantage). The preoperative to postoperative net biomechanical advantage for the 40 Summit hips was 48.1% (range: -20% to 295%; SD 55.3%). Compared to the normal contralateral side, the postoperative net biomechanical advantage was 12.4% (range: -25% to 51%; SD 17.7%) (n=19). In this series, average lengthening of 2.9 mm resulted in the reconstructed limb being an average of 1.1 mm shorter than the normal side.⁵

References

Charnley J. Low Friction Arthroplasty of the Hip: Theory and Practice. Berlin, Germany: Springer-Verlag; 1979.
McGrory BJ, Morrey BF, Cahalan TD, An KN, Cabanela ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br. 1995; 77:865-869.
Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop. 2001; 388:125-134.
Schmalzried TP, Shepherd EF, Dorey FJ, et al. Wear is a function of use, not time. Clin Orthop. 2000; 381:36-46.
Silva M, Schmalzried TP, et al. The biomechanical results of total hip resurfacing arthroplasty. J Bone Joint Surg Am. 2004; 86:40-46.

Author

From the Joint Replacement Institute at Orthopaedic Hospital, Los Angeles, Calif.