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Preoperative Templating and Biomechanics in TotalHip Arthroplasty

August 1, 2005

Abstract

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

The past two decades have produced dramatic improvements in cemented and cementlessfemoral fixation. Aseptic loosening is now a rare complication, and total hip replacement (THR) isperformed on active patients who are challenging the functional capacity of the technology.Improving the biomechanics of the reconstruction can improve function and reduce wear. Severalcontemporary hip systems offer standard and high offset femoral components that enable asuperior biomechanical reconstruction. Preoperative planning is the foundation for a successfularthroplasty. 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 helpfulin assessing the true limb length discrepancy. Common methods include placing blocks of knownsize under the foot of the shorter extremity until the pelvis is level and measuring the distancefrom the anterosuperior iliac spine to the medial malleolus. In patients with hip arthritis, the mostcommon 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 limblength discrepancy or uses a shoe lift.

The essential preoperative radiographs include a low anteroposterior pelvis (beam centered onthe 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 commonmeans to assess length differences in the proximal femurs. A Johnsons lateral view is helpful inassessing 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 thediscrepancy. Pati ents, especially women, are sensitive to over -lengthening. Leaving the

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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 hips center of rotationdecreases 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: areduction in the required abductor forces and lower joint reaction forces. 1 Accumulating clinicalevidence indicates that such favorable alterations in biomechanics can improve clinicaloutcomes 2 and reduce wear. 3,4 Higher femoral offset has been associated with greater hipabduction motion and abductor muscle strength. In two independent studies, higher femoraloffset 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 thenormal contralateral hip. It is therefore reasonable to make the offset of the reconstructiongreater than it was preoperatively. Concurrent medialization of the hips center of rotation by anamount 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 theextremities positioned in approximately 15 of internal rotation to assess the proximal femur in aneutral profile and better assess the shape of the metaphysis and femoral offset. This may not bepossible if the disease process has resulted in an external rotation deformity. In such cases, thepreoperative radiograph underrepresents the true offset (Figure 1A).

Acetabular templating is performed first because the acetabular component establishes thecenter of rotation of the arthroplasty. The primary goal is obtaining adequate bony support for thecomponent. The optimal abduction angle (lateral opening) of the component is partiallydependent 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 theoutlines of the acetabular components on a 45 angle on the transparent template sheet. A 45abduction angle is then obtained when the template sheet is placed parallel to the pelvis on theradiograph. The acetabular teardrop is a consistent reference for the medial wall. The medicalaspect of an adequately sized acetabular component will approach the medial wall and becovered by superolateral bone. This is progressively challenging with more shallow sockets, as indevelopmental dysplasia (Figure 1B). Details regarding the degree of medialization, the amountof acceptable verticality, and/or lack of superolateral support and the need for bone graftsupplementation 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) andthe part outside the bone, which determines the biomechanics of the reconstruction. The size of

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the femoral component determines the fit. For both cemented and cementless fixation, thedefinition of optimal femoral fit depends on details of the specific femoral component and issuesof surgeon preference, which are beyond the scope of this exercise. The fit of the femoralcomponent should be checked on both the anteroposterior and lateral radiographs.

Having selected an appropriately sized femoral component, the surgeon places the templatetransparency on the anteroposterior radiograph, aligning the stem of the femoral componentinside the femoral canal (Figure 1B). Relative limb length can influence the vertical position of thecomponent. The neck length of a modular femoral head influences both length (verticalcomponent) and offset (horizontal component), whereas the vertical position of the steminfluences only length. The vertical position of the stem also influences the location of the neckosteotomy.

The limb length and offset change for the proposed reconstruction can be assessed on thetemplates 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 shouldbe considered variable. When the head center directly overlies the acetabular center, there is nochange in the limb length or femoral lateralization when such a plan is executed surgically. If thehip center of rotation is medialized, then the femoral offset will be increased by a correspondingamount. When the head center on the templating is superior to the acetabular center, limb lengthwill be gained when the hip is reduced at surgery. When the head center is medial to theacetabular center on the template, then relative femoral lateralization will occur when such a planis executed surgically (Figure 1C).

Figure 1: Preoperative anteroposterior pelvis of a young female patient with osteoarthritis of theright hip secondary to mild dysplasia (A). The right femur is slightly shorter with less offset thanthe 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 indicatesthe center of rotation of the modular acetabular component, and the medial white dot indicates thecenter of rotation of the modular femoral head. The horizontal distance between the dots representsthe increase in femoral offset, whereas the difference in the vertical positions of dots represents thechange 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=horizontalcenter 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 femoralmedialization occurs when the head center is lateral to the acetabular center on the template.Shortening without femoral lateralization and femoral medialization without limb lengtheningreduce soft tissue tension and may predispose to hip instability (dislocation). The use of large

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diameter femoral heads and outset acetabular components can improve the stability of suchreconstructions.

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 wasmedialized by a mean of 5.6 mm and was more medial than its preoperative position in 30 of 40hips ( P .00001). Compared to the normalcontralateral side, the postoperative horizontal femoral offset increased by a mean of 5.2 mmand 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 femoraloffset minus the percent change in horizontal center of rotation (an increase in horizontal femoraloffset with a decrease in horizontal center of rotation results in a higher net biomechanicaladvantage). The preoperative to postoperative net biomechanical advantage for the 40 Summithips 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 anaverage of 1.1 mm shorter than the normal side. 5

References1. Charnley J. Low Friction Arthroplasty of the Hip: Theory and Practice . Berlin, Germany:

Springer-Verlag; 1979.2. 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.

3. Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoralcomponent offset on polyethylene wear in total hip arthroplasty. Clin Orthop . 2001;388:125-134.

4. Schmalzried TP, Shepherd EF, Dorey FJ, et al. Wear is a function of use, not time. ClinOrthop . 2000; 381:36-46.

5. Silva M, Schmalzried TP, et al. The biomechanical results of total hip resurfacingarthroplasty. J Bone Joint Surg Am . 2004; 86:40-46.

Author

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