Orthopedics

Feature Article 

Quantification of Imaging Error in the Measurement of Cup Position: A Cadaveric Comparison of Radiographic and Computed Tomography Imaging

Ran Schwarzkopf, MD, MSc; Jonathan M. Vigdorchik, MD; Theodore T. Miller, MD; Eric A. Bogner, MD; Jeffrey M. Muir, MSc, DC, MSc (Clin Epi); Michael B. Cross, MD

Abstract

Postoperative radiographs remain the standard for assessment of component placement following total hip arthroplasty (THA), despite the known limitations of radiographs. Computed tomography (CT) scanning offers improved accuracy, but its costs and radiation exposure are prohibitive. The authors performed a cadaver study to compare the error associated with radiographs with that of CT scans following THA. The authors also compared imaging with a novel mini-navigation system. Three board-certified orthopedic surgeons each performed 4 THA procedures (6 cadavers, 12 hips) via the posterior approach using a mini-navigation tool to assist with component placement. Cup position from imaging was compared with corrected CT values for anteversion and inclination, created by correcting the initial scan to align the anterior pelvic plane coplanar with the CT table, thus representing cup position not distorted by imaging or positioning. Anteversion from standard CT scans was within 2.5° (standard deviation [SD], 1.5°) of reference values (P=.25); radiographs showed an average error of 7.8° (SD, 4.3°) vs reference values (all values absolute means) (P<.01). The mini-navigation system provided anteversion values within an average of 4.0° (SD, 4.0°) of reference anteversion (P<.01). Standard CT values for inclination were within 2.4° (SD, 2.0°) of reference values (P=.53), whereas radiographic inclination values were within 2.5° (SD, 2.3°) (P=.12). Mini-navigation values for inclination were within 3.9° (SD, 3.2°) of reference inclination (P=.26). This study demonstrated that cup position as measured by radiographs is significantly less accurate than CT scans and that the mini-navigation system provided anteversion measurements that were of comparable accuracy to CT scans. [Orthopedics. 2017; 40(6):e952–e958.]

Abstract

Postoperative radiographs remain the standard for assessment of component placement following total hip arthroplasty (THA), despite the known limitations of radiographs. Computed tomography (CT) scanning offers improved accuracy, but its costs and radiation exposure are prohibitive. The authors performed a cadaver study to compare the error associated with radiographs with that of CT scans following THA. The authors also compared imaging with a novel mini-navigation system. Three board-certified orthopedic surgeons each performed 4 THA procedures (6 cadavers, 12 hips) via the posterior approach using a mini-navigation tool to assist with component placement. Cup position from imaging was compared with corrected CT values for anteversion and inclination, created by correcting the initial scan to align the anterior pelvic plane coplanar with the CT table, thus representing cup position not distorted by imaging or positioning. Anteversion from standard CT scans was within 2.5° (standard deviation [SD], 1.5°) of reference values (P=.25); radiographs showed an average error of 7.8° (SD, 4.3°) vs reference values (all values absolute means) (P<.01). The mini-navigation system provided anteversion values within an average of 4.0° (SD, 4.0°) of reference anteversion (P<.01). Standard CT values for inclination were within 2.4° (SD, 2.0°) of reference values (P=.53), whereas radiographic inclination values were within 2.5° (SD, 2.3°) (P=.12). Mini-navigation values for inclination were within 3.9° (SD, 3.2°) of reference inclination (P=.26). This study demonstrated that cup position as measured by radiographs is significantly less accurate than CT scans and that the mini-navigation system provided anteversion measurements that were of comparable accuracy to CT scans. [Orthopedics. 2017; 40(6):e952–e958.]

Total hip arthroplasty (THA) is a common surgical procedure1 associated with generally good outcomes, although inaccuracies in component placement can be associated with postoperative complications ranging from low back pain to instability and dislocation.2–4 Accuracy in component placement is the key factor in ensuring the long-term stability and survival of the implant and limiting costly revision procedures. Compounding revision THA costs, which can reach millions per year in the United States alone,5–7 is the growing threat of litigation against surgeons, which has increased awareness regarding component implantation accuracy.8,9

Radiographs represent the current standard of care imaging for confirming component placement postoperatively. Despite drawbacks such as distortion and/or artifact,10,11 the ease of use, accessibility, and low cost of radiographs ensure that they remain the standard for postoperative follow-up. Computed tomography (CT) scanning provides more detailed images than radiographs, but does so at a higher cost and with more radiation exposure.12,13 Newer imaging modalities such as EOS (EOS Imaging, SA, Paris, France) offer improved detail with minimal radiation exposure, although are not yet in widespread use.14,15 The challenges associated with radiographs and the lack of a viable alternative result in an inherent inaccuracy in the imaging used to plan and confirm component placement in THA. Although several authors have attempted to develop corrections for radiographic artifact,16,17 no universal method for counteracting distortion caused by image rotation or tilt is currently available.18 As such, surgeons are left with a potentially significant source of error when seeking to confirm accurate component positioning in THA.

Few studies are available quantifying or comparing the relative error associated with radiographs or CT scanning as methods for confirming component positioning during THA. Given the ongoing use of radiographic images in THA and the lack of definitive evidence quantifying the error associated with radiographs in postoperative monitoring, the authors sought to determine the relative error of plain radiographs and standard CT scanning in measuring cup position when compared with position-corrected CT scans representing the gold standard.

The authors also compared the accuracy of each of these measurement techniques with a novel, 3-dimensional mini-navigation device available to assist surgeons with component placement during THA that has shown excellent accuracy in both clinical and cadaveric studies.19,20 The authors hypothesized that the error associated with CT scanning and the mini-navigation system would not be significantly different and that both modalities would be associated with significantly less error than plain radiographs.

Materials and Methods

This was a cadaver study in which 3 board-certified orthopedic surgeons (R.S., J.M.V., M.B.C.) each performed 4 THA procedures via the posterolateral approach on 6 cadavers (12 hips total) using the mini-navigation tool for all procedures.

Procedure

The Intellijoint HIP mini-navigation system (Intellijoint Surgical, Inc, Waterloo, Ontario, Canada) is a novel device to assist surgeons during THA.21 The device is composed of a camera, a tracker, and a workstation. Intraoperatively, the camera is magnetically attached to a pelvic platform that sits atop 2 surgical screws inserted along the ipsilateral iliac crest. The tracker can be magnetically attached to a platform fixed to the greater trochanter or other objects (eg, impactor, probe) to measure their position. The camera captures the movements of the tracker and relays data to the workstation, which remains outside of the sterile field but within sight of the surgical team.

As part of the current study, 3 fiducial screws were inserted bilaterally into the pelvis of each specimen prior to primary incision to create a common reference plane for comparison of device and image measurements (Figure 1). During each case, the fiducial screws were probed with the device to record their positional coordinates. This recording occurred after implantation of the acetabular cup and again after implantation of the femoral components.

Three fiducial screws (arrows) were inserted into the pelvis of each specimen as pictured. Fiducial screws provided a consistent plane of reference used for comparison of component position coordinates.

Figure 1:

Three fiducial screws (arrows) were inserted into the pelvis of each specimen as pictured. Fiducial screws provided a consistent plane of reference used for comparison of component position coordinates.

For the current study, surgeons performed THA via the posterior approach. Four target orientations representing clinically relevant positioning within Lewinnek's safe zone were prepared by a study member (J.M.M.) not involved in the surgical procedure. Prior to each procedure, surgeons were randomly provided with 1 of these 4 targets for anteversion and inclination, change in leg length, and offset. Following initial set-up on each specimen, the workstation was turned away from the surgeons so that they received no intraoperative feedback from the device regarding component position. Final placement of the components was revealed to the surgeons only at completion of the procedure. Cup position was recorded by the device via attachment of the tracker to a probe, which was used to probe the face of the implanted cup at 3 points to provide orientation data.

Imaging

Both radiographs and CT scanning were used in this study. Computed tomography images were analyzed by 2 board-certified radiologists (T.T.M., E.A.B.) with fellowship training in musculoskeletal imaging who did not participate in the surgeries and were blinded to radiographic and device measurements. Postoperative CT scans were obtained using a GE Lightspeed 16 imager (GE Healthcare, Chicago, Illinois; 140 kV, 600 mA at 0.8-sec revolution time and 0.625-mm slice thickness). Specimens were placed on the imager table in a supine position to mimic the standard of care for lumbar spine and pelvic CT imaging. Specimens were oriented so that their longitudinal axis was parallel with the long axis of the table and their trans-ASIS line was perpendicular to the long axis of the table. As such, the position of the specimen during the CT scan adequately represents patient positioning during a standard CT scan.

Radiographic imaging was obtained using a Viztek portable radiographic unit (Konica Minolta, Garner, North Carolina). Radiographs were analyzed by a licensed health care practitioner (J.M.M.) not involved in the surgical procedure and blinded to CT and device measurements. Anteroposterior (AP) radiographs (Figure 2) were obtained as follows: the specimen was placed in the lateral decubitus position and secured in place using standard surgical bolsters. The radiograph unit was positioned so that the central ray was aimed toward and perpendicular to the center of the anterior pelvic plane (APP) demarcated by the bilateral ASIS and the symphysis pubis. As such, the radiograph closely approximates the standing AP radiographs taken following traditional THA. A scaling object was incorporated into each radiographic image.

Anteroposterior radiographs were obtained pre- (A) and postoperatively (B). The pelvic platform and pelvic screws (X), tracker and its femoral platform (Y), and sizing object (Z) are visible. In the preoperative radiograph, 2 of 3 fiducial screws (arrows) used to demarcate the reference plane are also visible, with the third and superior-most screw out of the image frame. In the postoperative radiograph, 2 of 3 fiducial screws (arrows) are again visible, with the screw inserted posterior to the acetabulum obscured by the acetabular cup component. Radiographs were obtained with the specimen secured to the operating table with standard bolsters, which are also visible.

Figure 2:

Anteroposterior radiographs were obtained pre- (A) and postoperatively (B). The pelvic platform and pelvic screws (X), tracker and its femoral platform (Y), and sizing object (Z) are visible. In the preoperative radiograph, 2 of 3 fiducial screws (arrows) used to demarcate the reference plane are also visible, with the third and superior-most screw out of the image frame. In the postoperative radiograph, 2 of 3 fiducial screws (arrows) are again visible, with the screw inserted posterior to the acetabulum obscured by the acetabular cup component. Radiographs were obtained with the specimen secured to the operating table with standard bolsters, which are also visible.

Image Analysis

Acetabular cup position (anteversion and inclination) was measured on radiographs using TraumaCad (Brainlab, Chicago, Illnois), via the interischial line method.22 Each measurement was made in triplicate and averaged to provide a final measurement for anteversion and inclination.

Cup position from CT scans was measured using 3-dimensional renderings created postoperatively using image analysis software (Mimics and 3-Matic; Materialise, Leuven, Belgium) (Figure 3).23 The study radiologists, working independently, identified the APP, the reference plane, and the plane of the acetabular cup face by marking landmarks on the renderings. The APP was defined by marking the bilateral ASIS and the symphysis pubis; the acetabular cup orientation by marking 3 distinct points on the face of each cup component; and the reference plane by marking the 3 fiducial screws. These coordinate data were analyzed using MATLABS software (Math-works, Natick, Massachusetts) to determine the 3-dimensional orientation of the APP, the reference plane, and the acetabular cup component. Each landmark was defined in triplicate and then averaged to provide final values.

Computed tomography scans were used to create 3-dimensional renderings. Anterolateral (A) and supine anterolateral (B) representations are illustrated. The implanted acetabular cup (purple) and fiducial screws (arrows) were used to gather coordinate data.

Figure 3:

Computed tomography scans were used to create 3-dimensional renderings. Anterolateral (A) and supine anterolateral (B) representations are illustrated. The implanted acetabular cup (purple) and fiducial screws (arrows) were used to gather coordinate data.

To create reference values for anteversion and inclination, the orientation of the pelvis on each CT scan was corrected in the rendering to an orientation where the APP was coplanar with the CT imager table. The cup position measurements obtained from these images thus represent the ideal orientation, with the image not subject to rotation or deflection. These values were used as reference values and compared with the uncorrected CT scans, to represent the standard of care CT scan, and with the plain radiographs, to represent the standard of care for radiographs.

Statistical Analysis

Alpha was set a priori at 0.05 for all comparisons. Means were compared using independent or dependent samples t tests and/or single factor analysis of variance. Mean values are presented as mean (standard deviation [SD]; range). Intraobserver reliability was evaluated using the intra-class correlation coefficient.24 All statistical analyses were completed by an independent statistician (J.M.M.) not involved in the radiographic/CT image analysis or the surgical procedures.

Results

Intra- and interrater reliability were both excellent. Intrarater reliability for individual radiologists was 0.996 and 0.998 for ante-version and 0.959 and 0.983 for inclination, whereas interrater reliability was 0.996 and 0.908 for anteversion and inclination, respectively. For radiographic measurements, the intrarater reliability was 0.994 for anteversion and 0.988 for inclination.

Anteversion

Mean anteversion as calculated from the corrected CT scan—the reference value—was 24.2° (SD, 9.8°). Mean anteversion calculated from the alternate modalities was 23.4° (SD, 9.9°) for standard CT (P=.24 vs reference values), 15.4° (SD, 9.1°) for radiographs (P=.10), and 19.7° (SD, 9.6°) for the mini-navigation device (P=.41).

Standard CT measurements were most closely related to reference values, with a mean absolute difference of 2.5° (SD, 1.5°). The mini-navigation device was associated with a mean absolute difference from reference values of 4.0° (SD, 4.0°), whereas radiographs differed from reference values by 7.8° (SD, 4.3°). Radiographs were significantly worse than both the standard CT (7.8° vs 2.5°, P<.01) and the mini-navigation values (7.8° vs 4.0°, P<.01) when comparing the mean difference from reference values. No significant difference was observed between the error associated with CT scans and the mini-navigation device (2.5° vs 4.0°, P=.22) (Table).

Results of Comparisons of Mean Absolute Differences Between Measurement of Cup Position From Radiographs, Standard Computed Tomography, and Mini-navigation With Corrected Computed Tomography Measurements

Table:

Results of Comparisons of Mean Absolute Differences Between Measurement of Cup Position From Radiographs, Standard Computed Tomography, and Mini-navigation With Corrected Computed Tomography Measurements

Inclination

Mean reference value for inclination calculated from the corrected CT scan was 43.4° (SD, 5.6°). Mean inclination measured from other modalities was 42.7° (SD, 8.5°) for standard CT (P=.53), 41.8° (SD, 7.2°) for radiographs (P=.26), and 41.7° (SD, 7.4°) for the mini-navigation device (P=.12).

Mean absolute error associated with radiographs and standard CT scans was similar for inclination (2.5° and 2.4°, respectively; P=.81). No statistically significant difference between the mean absolute error of the mini-navigation device when compared with either radiographs (3.9° vs 2.5°, P=.17) or standard CT scans (3.9° vs 2.4°, P=.18) (Table).

Discussion

Anteroposterior radiographs remain the imaging standard of care following THA. However, there is error associated with their use. Although CT scanning improves accuracy, the increased radiation exposure and cost make it prohibitive. In this cadaver study, the authors compared the accuracy of CT scanning and radiographs in measuring postoperative cup position. The authors also compared the accuracy of a novel mini-navigation tool in measuring these parameters and found that radiographs were associated with a significantly higher error than either standard CT or the mini-navigation device when measuring anteversion.

In comparing measurements from radiographs and CT scans with reference values, the authors found that CT scans were significantly more accurate than radiographs. The authors noted that radiographs underestimated anteversion by a mean absolute value of 7.8°, error that aligns closely with the recommendations of Grammatopoulos et al,25 who have suggested that surgeons should target an anteversion position 8° less than their desired final orientation to account for radiographic error. However, this degree of error approaches the maximum error of 10° associated with the safe zone of Lewinnek et al.26 By targeting a position this close to the edges of the safe zone, surgeons may be increasing the chances for instability and dislocation. As such, the problematic nature of the reliance on radiographs and the lack of a viable alternative is underscored.

The error associated with CTs was predictably less than that of radiographs; however, the high cost and radiation exposure associated with CT continues to limit their use.27,28 Currently, no imaging option offers the accuracy of CT with the low radiation and cost of radiographs. EOS technology, which creates 3-dimensional images from low-dose biplanar radiographs, has been used in some studies,29–31 and shows potential. Studies have demonstrated that EOS is able to measure cup position to within 0.30° to 3.43° of radiographic measurements32 and 1.5° to 1.7° of CT measurements30; however, despite these results, the technology remains in the early stages of adoption.

In the current study, the authors observed that the mini-navigation tool, when compared with both CT and radiographs, provided accuracy values more closely mirroring those of CT than radiograph. The absolute measurements for anteversion from the mini-navigation tool differed from that of CT scans by an average of only 1.5° and were up to 3.8° more accurate than radiographs. Currently, computer navigation is used sparingly in THA, with only 1% to 3% of procedures performed with its assistance.33 Although the ability of computer navigation to accurately measure cup position and leg length (when compared with radiograph) has been confirmed in several studies,34–38 the cumbersome nature of these systems, plus the additional time required for their use, has limited their widespread adoption. However, the accuracy demonstrated by the mini-navigation device in this study combined with an ease-of-use documented in previous clinical studies20,39 suggests that this tool may be a viable option of monitoring cup position and leg length/offset.

This study indicated that the error associated with imaging is more pronounced in anteversion than inclination. Several authors have investigated the relationship between cup position and dislocation rate, and although the evidence is somewhat contradictory,40 it indicates that there is a significant difference in anteversion and inclination in patients who dislocate, with some authors observing that errors in anteversion are more highly associated with dislocation than are errors in inclination.41–43 In a review of more than 2000 THA procedures, anteversion outside of a range of 40° to 60° was associated with a 6.9 times greater rate of dislocation.44 A similar review found that those cups placed outside of an anteversion range of 10° to 30° were 1.9 times more likely to dislocate.43

Komeno et al,42 in a smaller study, found that not only cup anteversion, but also the combined anteversion of the acetabular cup and femoral stem were contributory to dislocation, with the combined angle significantly less in dislocated hips than nondislocated hips. Although some authors have suggested that other factors besides cup orientation contribute to the likelihood of dislocation,40 the weight of evidence suggests that errors in anteversion are more closely related to the potential for dislocation.41–44 As such, the error in anteversion the current study demonstrated that using radiographs should raise concerns for surgeons and may necessitate the need for improved methods of monitoring cup position intraoperatively.

This study is limited somewhat by the use of cadaveric specimens. Renkawitz et al45 have raised concerns regarding the use of frozen cadavers (as used in this study) and whether these specimens allow for movement truly analogous to that of live human tissue. The authors mitigated this concern by letting their specimens thaw sufficiently prior to use to allow for movement that appropriately mimics that of normal human tissue. An additional benefit of using cadaveric specimens was the ability to properly position the specimens during surgery and imaging and the ability to use fiducial screws to create a consistent reference plane for measurements. Another limitation could be that, because the authors' specimens contained only the torso and lower limbs, there could be difficulties in orienting the specimen in a position consistent with that of a patient during THA. However, the authors were able to mitigate this concern by using the mini-navigation tool to monitor the positioning of the specimen and surgical bolsters to ensure that there was minimal movement of the specimen intraoperatively.

This study demonstrated the relative error in measuring cup position in CT scanning and radiographs following THA, indicating a large discrepancy between the 2 modalities. This study also demonstrated the ability of a novel mini-navigation device to accurately measure cup position intraoperatively, providing these measurements more accurately than radiographs. Given the ongoing use of radiographs as the primary imaging modality in THA, the results of this study provide important information for surgeons regarding the potential errors associated with reliance on radiographs. Surgeons should consider this error when reviewing postoperative radiographs to confirm component positioning and may consider the addition of a navigation tool or other imaging (EOS, CT) to improve both intra- and postoperative measurement of cup position. The results of this study have led to an increased use of navigation among the surgeon authors, as well as pre- and postoperative EOS imaging. However, clinical studies are required to further elucidate the broader impact on clinical practice.

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Results of Comparisons of Mean Absolute Differences Between Measurement of Cup Position From Radiographs, Standard Computed Tomography, and Mini-navigation With Corrected Computed Tomography Measurements

MeasurementRadiographsStandard Computed TomographyaMini-navigationPb



Signed ΔAbsolute ΔSigned ΔAbsolute ΔSigned ΔAbsolute Δ
Anteversion, mean (SD)−7.8° (4.3°)7.8° (4.3°)−1.0° (2.8°)2.5° (1.5°)--<.01
−7.8° (4.3°)7.8° (4.3°)--−3.5° (4.5°)4.0° (4.0°)<.01
--−1.0° (2.8°)2.5° (1.5°)−3.5° (4.5°)4.0° (4.0°).22
Inclination, mean (SD)−1.5° (3.1°)2.5° (2.3°)−0.6° (3.1°)2.4° (2.0°)--.81
−1.5° (3.1°)2.5° (2.3°)--−1.7° (4.9°)3.9° (3.2°).17
--−0.6° (3.1°)2.4° (2.0°)−1.7° (4.9°)3.9° (3.2°).18
Authors

The authors are from the Department of Orthopaedic Surgery (RS, JMV), NYU Langone Medical Center–Hospital for Joint Diseases, and the Department of Radiology and Imaging (TTM, EAB) and the Department of Orthopaedic Surgery (MBC), Hospital for Special Surgery, New York, New York; and Intellijoint Surgical, Inc (JMM), Waterloo, Ontario, Canada.

Drs Vigdorchik, Miller, and Bogner have no relevant financial relationships to disclose. Dr Schwarzkopf is a paid consultant for Smith & Nephew, Gauss Surgical, and Intellijoint Surgical, Inc. Dr Muir is a paid employee of Intellijoint Surgical, Inc. Dr Cross is a paid consultant for Acelity, Exactech, Inc, Intellijoint Surgical, Inc, Link Orthopaedics, Smith & Nephew, Theravance Biopharm, and Zimmer.

The authors thank Thomas DiCarlo of Radiological Services A+ for his assistance with radiographic imaging for this study.

Correspondence should be addressed to: Michael B. Cross, MD, 535 East 70th St, New York, NY 10021 ( crossm@hss.edu).

Received: March 09, 2017
Accepted: July 31, 2017
Posted Online: September 22, 2017

10.3928/01477447-20170918-03

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