Orthopedics

Feature Article 

Total Hip Arthroplasty Templating: A Simple Method to Correct for Radiograph Magnification

Stanley E. Asnis, MD; Yonah Y. Heller, MD

Abstract

Several factors can cause magnification errors when templating. Parallax, distance from the plate, and printing or monitoring the radiographs can lead to errors. Steel balls, 25.4 mm, underwent radiography on a standard radiograph plate in different positions from the center of the x-ray beam for evaluation of parallax and at different heights from the x-ray table for evaluation of the effects of distance on magnification. Difference in size between the center steel ball and the peripheral balls was less than 0.3 mm. As a metal ball was brought from table top to a 6-inch elevation, there was up to a 25% magnification. A simple correction factor can be calculated to correct magnification error. The diameter of the surgically removed femoral head divided by the diameter of the femoral head on the preoperative radiograph yields a correction factor. This factor multiplied by the template-determined lesser trochanter-to-center yields the length to be created during surgery. This adjusts for all causes of magnification. [Orthopedics. 2019; 42(3):e322–e325.]

Abstract

Several factors can cause magnification errors when templating. Parallax, distance from the plate, and printing or monitoring the radiographs can lead to errors. Steel balls, 25.4 mm, underwent radiography on a standard radiograph plate in different positions from the center of the x-ray beam for evaluation of parallax and at different heights from the x-ray table for evaluation of the effects of distance on magnification. Difference in size between the center steel ball and the peripheral balls was less than 0.3 mm. As a metal ball was brought from table top to a 6-inch elevation, there was up to a 25% magnification. A simple correction factor can be calculated to correct magnification error. The diameter of the surgically removed femoral head divided by the diameter of the femoral head on the preoperative radiograph yields a correction factor. This factor multiplied by the template-determined lesser trochanter-to-center yields the length to be created during surgery. This adjusts for all causes of magnification. [Orthopedics. 2019; 42(3):e322–e325.]

Total hip arthroplasty (THA) is one of the most successful and cost-effective interventions in medicine.1 The goals of THA are pain relief, improvement of function, and restoration of normal hip biomechanics. Preoperative templating relies on standardized measurements taken from radiographs of known magnification to provide accurate information pertaining to implant size, implant position, offset, and limb length.2 The lesser trochanterto-center (LTC) distance is an important measurement used to restore leg-length equality. Typically, this distance is measured from the unaffected hip on the pelvic anteroposterior radiograph and is used to guide resection or component sizing on the pathologic side.3 Accurate measurement of this distance enhances the ability to maintain the appropriate leg length. Differences in radiograph magnification can often lead to unreliable measurements. This article provides a simple and expedient method for predicting the femoral head center of rotation and LTC regardless of radiograph magnification.

Technique

Preoperative planning for hip arthroplasty requires standardized radiographs, with an attempt to standardized magnification. These typically include an antero-posterior view of the pelvis and antero-posterior and lateral views of the affected hip. Conventional templating methods are performed using an anteroposterior view of the pelvis centered over the pubic symphysis. On a proper pelvic anteroposterior radiograph, the hips are internally rotated 10° to 15° to match the hip's normal anteversion and to provide proper perspective of the femoral neck. The x-ray tube is set at a distance of 40 inches from the table, and the film cassette is placed 5 cm below. Maintaining these parameters will approximate a magnification of 20%. Any alteration in these parameters will alter magnification.4 In an effort to comply with the magnification of the radiograph, prosthesis templates are made with a standard magnification. Although standardized preoperative radiographs are typically attempted, they often fall short of the ideal goal for a variety of reasons. Variations due to pathologic conditions such as an acute femoral neck fracture, a history of open reduction and internal fixation, or developmental hip dysplasia must be taken into account. A magnification marker, such as a metal sphere of known dimensions, can be included in a radiograph. However, magnification markers are subject to the error of parallax and are influenced by their position relative to the x-ray beam. Ideally, the center of the sphere must be the same distance from the radiograph plate as the center of the femoral head. If it is not, it may misrepresent the true pelvic measurements.

This study describes a templating method that minimizes the error associated with radiograph magnification and illustrates variables such as parallax, distances from the radiograph plate, and printer or monitor magnifications that contribute to magnification errors.

Method for Correcting Measurements

A magnification correction factor can be calculated using both preoperative and intraoperative measurements. The desired LTC is first measured and recorded, usually using the opposite, more normal hip. This is the “uncorrected” desired LTC. With the same ruler, the diameter of the arthritic femoral head is measured. During the surgical procedure, the arthritic femoral head is removed and the diameter measured with a caliper. The ratio of the true diameter of the arthritic femoral head to the preoperative measurement of the arthritic head is the correction factor. This factor is then multiplied by the initially measured LTC, yielding the corrected true LTC. Surgically, the corrected LTC can then be re-created on the operative hip by using different modular head/neck lengths or re-cutting the femoral neck.

Alternate and the Authors' Preferred Method

After templating the opposite hip, determining the uncorrected LTC, and measuring the femoral head on the operative side, a relation was often found. In the authors' vast experience, the uncorrected measured LTC on the opposite side and the radiographic measurement of the diameter of the operative femoral head are commonly equal or almost equal (ie, the diameter of the femoral head and the LTC are often equal or near equal). If they are equal, then the desired LTC on the operative side should be exactly the same as the anatomic measurement of the femoral head taken on the operating room table. If there is a difference of a few millimeters, then that difference can be added to or subtracted from the measurement of the anatomic femoral head. This method corrects for magnification with but a few millimeters being added or subtracted. It will yield an extremely close approximation of the desired corrected LTC.

Effect of Parallax and Distance From the Radiograph Plate

Radiographs of multiple 25.4-mm radio-opaque metal balls were obtained to determine the effect of parallax and distance from the radiograph plate on magnification. A standard radiograph plate was used at a distance of 40 inches from the x-ray tube and 5 cm below the table surface. The effect of parallax was tested by arranging 9 metal balls on the table at a uniform height (Figure 1). One sphere was placed in the center of the plate, 1 was placed in each corner, and 1 was placed at the center of each side but at the periphery of the film cassette. The x-ray beam was centered over the center ball and a radiograph was obtained. The diameter of each metal ball was measured using a digital caliper on the printed radiograph.

One metal sphere was placed in the center and the other 8 were placed around the periphery of the film cassette. This showed only a minor effect.

Figure 1:

One metal sphere was placed in the center and the other 8 were placed around the periphery of the film cassette. This showed only a minor effect.

The metal balls were placed at differing heights relative to the x-ray table (+0, +1.5 inches, +3 inches, and +6 inches) and again their diameters were measured (Figure 2). Each configuration was performed 3 times.

The metal spheres were placed in the center of the table, then elevated 3 inches above the table, and then elevated 6 inches above the table. This causes significant magnification.

Figure 2:

The metal spheres were placed in the center of the table, then elevated 3 inches above the table, and then elevated 6 inches above the table. This causes significant magnification.

Results

Mean diameters for the center and peripheral balls when placed at the same height from the radiograph plate were 24.47±0.19 mm and 24.71±0.13 mm, respectively (P=.15). As the height from the metal ball to the plate increased, the projected diameter (magnification) increased. At the level of the x-ray table, +1.5 inches, +3 inches, and +6 inches, the mean diameters measured 26.94±0.17 mm, 28.21±0.27 mm, 29.53±0.36 mm, and 31.95±0.36 mm, respectively. All values were significantly different (P<.05) at each interval.

Using preoperative imaging to represent a patient's true anatomy enables the surgeon to accurately and precisely restore normal hip biomechanics. In this study, the error of parallax alone did not create significant variability in magnification within the confines of a standard x-ray cassette. However, distance from the x-ray cassette did generate significant variability in magnification. Values were greater than 25% at a height of 6 inches above the x-ray table but only 6% at the level of the table. Thus, it is clear that using a metal ball marker whose center is at a different height from the radiograph plate than the anatomic center of the true femoral head yields imprecise and inaccurate measurements. Another major factor is the magnification variability in printing. This error is also true when measuring from a projected radiograph on a monitor. The method used here corrects for these factors.

Discussion

Preoperative planning is critical to the success of hip arthroplasties and hemiarthroplasties.5 Correcting limb-length inequality without compromising hip stability is one of the intraoperative challenges that surgeons face while performing hip arthroplasty. In addition to pain relief, the restoration of normal hip biomechanics facilitates normal gait and function.6 Discrepancy in leg length is common after hip arthroplasty, with a reported incidence ranging from 1% to 27%. Leg-length discrepancy following primary THA varies between 3 and 70 mm, with lengthening being more noticeable to patients than shortening.7 Absolute equalization is difficult to achieve after hip arthroplasty, and the boundary between acceptable and unacceptable disparity remains undefined.8 Despite this, it is clear that leg-length equality should remain the goal during hip arthroplasty. Numerous methods, both preoperative and intraoperative, for overcoming leg-length discrepancy have been described. Preoperative templating techniques generally rely on the location of certain landmarks from which to measure the level of femoral neck resection. Relying on preoperative templating alone as a method to correct leg-length discrepancy is often limited. Knight and Atwater9 concluded that correct sizing of components in THA only matched in 60% of cases, illustrating the inaccuracy of using preoperative templating alone. Techniques to evaluate leg lengths intraoperatively typically use a distance between 2 reference points. Conventionally, a fixed pelvic reference point is used, while the femoral reference point is normally the variable point. Ranawat et al10 discussed a new technique using a vertical Steinmann pin in the infracotyloid groove of the acetabulum. By choosing a reference point in close proximity to the hip center of rotation, they believed that the accuracy of measurements would be improved because of the decrease in variability of limb position. Various authors have described the use of LTC as a reliable leg-length check. Lesser trochanter-to-center methods avoid the need to place Steinmann pins or other various markers into the pelvis. González Della Valle et al5 reported outcomes after cemented THA using intraoperative LTC measurements, finding leg-length discrepancy within 5 mm in 87% of hips.11 Lesser trochanterto-center measurements are particularly helpful in hemiarthroplasty for displaced femoral neck fractures when there are no reliable intraoperative landmarks.

The current authors' technique uses both preoperative and intraoperative measurements to create an accurate correction factor for magnification and yields the desired LTC. Using LTC techniques with the authors' calculated value, neck lengths can be specifically adjusted to match the contralateral hip if desired. This simple method eliminates the need for implant-specific templates, expensive digital templating software, standardized radiographic magnification, or placement of magnification markers. This universal template/technique offers a simple, concise, and cost-effective means to restore correct leg lengths during arthroplasty.

Conclusion

For THA, preoperative templating corrected for magnification leads to more reliable restoration of normal hip biomechanics. The correction factor is the actual calipered measurement of the diameter of the femoral head divided by the measurement of the femoral head on the preoperative radiograph. The principles and method described in this article minimize errors associated with radiograph magnification.

References

  1. Daigle ME, Weinstein AM, Katz JN, Losina E. The cost-effectiveness of total joint arthroplasty: a systematic review of published literature. Best Pract Res Clin Rheumatol. 2012;26(5):649–658. doi:10.1016/j.berh.2012.07.013 [CrossRef]
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  10. Ranawat CS, Rao RR, Rodriguez JA, Bhende HS. Correction of limb-length inequality during total hip arthroplasty. J Arthroplasty. 2001;16(6):715–720. doi:10.1054/arth.2001.24442 [CrossRef]
  11. Matsuda K, Nakamura S, Matsushita T. A simple method to minimize limb-length discrepancy after hip arthroplasty. Acta Orthop. 2006;77(3):375–379. doi:10.1080/17453670610046280 [CrossRef]
Authors

The authors are from North Shore University Hospital (SEA) and North Shore LIJ Hospitals (YYH), Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, Great Neck, New York.

The authors have no relevant financial relationships to disclose.

Correspondence should be addressed to: Stanley E. Asnis, MD, North Shore University Hospital, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, 611 Northern Blvd, Great Neck, NY 11021 ( sasnis@northwell.edu).

Received: May 22, 2018
Accepted: September 11, 2018
Posted Online: March 12, 2019

10.3928/01477447-20190307-01

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