JBJS | Accuracy of Detecting Screw Penetration of the Acetabulum with Intraoperative Fluoroscopy and Computed Tomography

The Journal of Bone & Joint Surgery, Volume 83, Issue 9

Scientific Articles | September 01, 2001

Accuracy of Detecting Screw Penetration of the Acetabulum with Intraoperative Fluoroscopy and Computed Tomography

David B. Carmack, MD; Berton R. Moed, MD; Kathleen McCarroll, MD; David Freccero, BS

Investigation performed at Detroit Receiving Hospital and the Department of Orthopaedic Surgery, Wayne State University Medical School, Detroit, Michigan
David B. Carmack, MD
Shock Trauma Orthopaedics, University of Maryland, 182 South Green Street, Baltimore, MD 21201

Berton R. Moed, MD
Department of Orthopaedic Surgery, University Health Center, 7C, 4201 St. Antoine Boulevard, Detroit, MI 48201. E-mail address: bmoed@aol.com

Kathleen McCarroll, MD
Department of Radiology, Detroit Receiving Hospital, 3K, 4201 St. Antoine Boulevard, Detroit, MI 48201

David Freccero, BS
Wayne State University Medical School, 540 East Canfield, Detroit, MI 48201

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

The Journal of Bone & Joint Surgery. 2001; 83:1370-1375

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Abstract

Background: The purpose of this study was to determine the accuracy of computed tomography and fluoroscopy in assessing joint penetration by periacetabular screws.

Methods: A 3.5-mm acetabular periarticular screw was inserted in each of thirty-nine cadaveric hemipelves. Twenty screws were intentionally directed to violate the articular surface, whereas nineteen screws were positioned to avoid the articular surface. Using two fluoroscopic views (tangential and axial) in a manner simulating the clinical setting, an examiner blinded to the actual screw location determined whether each screw was violating the articular surface. In addition, each hemipelvis was examined with computed tomography with use of two different techniques: (1) a 1-mm slice thickness at 1-mm intervals, and (2) a 4-mm slice thickness at 3-mm intervals. Each scan was evaluated by another examiner who was blinded to the actual screw location. Sensitivity, specificity, and percent correct interpretations were then calculated for each method.

Results: The sensitivity, specificity, and percent correct interpretations were 95%, 84%, and 90%, respectively, for axial fluoroscopy; 85%, 89%, and 87% for tangential fluoroscopy; 100%, 84%, and 92% for the computed tomography scans with a 1-mm slice thickness at 1-mm intervals; and 100%, 58%, and 79% for the computed tomography scans with a 4-mm slice thickness at 3-mm intervals. Tangential fluoroscopy was found to be more specific than the computed tomography scans with a 4-mm slice thickness at 3-mm intervals (p = 0.02). No other significant differences were found.

Conclusions: Fluoroscopy and computed tomography are equally accurate for determining intra-articular screw penetration. Computed tomography scans with thick slices (4 mm at 3-mm intervals) have a low specificity. Their use postoperatively may lead to a false-positive interpretation of the scan and unnecessary exploration of a hip for screw penetration.

Clinical Relevance: Both intraoperative fluoroscopy and postoperative computed tomography are accurate methods for evaluating intra-articular screw penetration. If postoperative computed tomography is used, 1-mm slices at 1-mm intervals should be chosen. Fluoroscopy has the advantage of enabling intraoperative diagnosis.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Open reduction and internal fixation of acetabular fractures often requires insertion of screws in close proximity to the articular surface¹. Errant screw placement into the joint can lead to arthrosis with a poor clinical outcome despite otherwise satisfactory fracture reduction and fixation². In addition to using meticulous surgical technique and having a general familiarity with the osseous anatomy of the pelvis, surgeons use intraoperative and postoperative radiographic imaging to evaluate possible joint penetration by periacetabular screws^2-6. Intraoperative detection of errant screw placement is more desirable, as corrective action can be taken before completion of the surgical procedure.

Postoperative computed tomography is generally accepted as the most accurate technique for the detection of intra-articular screws^3-5. Recently, it has been suggested that intraoperative fluoroscopy is as accurate as postoperative computed tomography⁶. However, the accuracy of either method has not been evaluated experimentally, to our knowledge. The purpose of this study was to determine the accuracy of computed tomography and fluoroscopy in demonstrating joint penetration by periacetabular screws.

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Fig. 1:: Representative anteroposterior (left), axial (center), and tangential (right) fluoroscopic images, which were made for each specimen.

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Fig. 2:: Computed tomography images showing the difference between the 4¥3-mm scan (left), which in this case was interpreted incorrectly as IN (screw penetration of the subchondral bone into or through the overlying articular cartilage surface), and the 1¥1-mm scan (right), which was interpreted correctly as OUT (containment of the screw within bone).

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Fig. 3:: Axial fluoroscopic image showing the overlying large head of the screw obscuring the screw shaft and threads, thereby giving the appearance of joint penetration. A false-positive interpretation resulted.

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TABLE I: Probability Data Analysis

*The 95% confidence intervals are shown in parentheses.

	Method of Assessment*
Axial Fluoroscopy	Tangential Fluoroscopy	1 1-mm Computed Tomography	4 3-mm Computed Tomography
Sensitivity (n = 20) (%)	95 (75-99.9)	85 (62-97)	100 (83-100)	100 (83-100)
Specificity (n = 19) (%)	84 (60-97)	89 (67-99)	?84 (60-97)	?58 (33-80)
Correct interpretations (n = 39) (%)	90 (76-97)	87 (72-96)	?92 (79-98)	?79 (64-91)

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TABLE I: Probability Data Analysis

*The 95% confidence intervals are shown in parentheses.

	Method of Assessment*
Axial Fluoroscopy	Tangential Fluoroscopy	1 1-mm Computed Tomography	4 3-mm Computed Tomography
Sensitivity (n = 20) (%)	95 (75-99.9)	85 (62-97)	100 (83-100)	100 (83-100)
Specificity (n = 19) (%)	84 (60-97)	89 (67-99)	?84 (60-97)	?58 (33-80)
Correct interpretations (n = 39) (%)	90 (76-97)	87 (72-96)	?92 (79-98)	?79 (64-91)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

A statistical power analysis was performed prior to the study. Assuming a 100% specificity and sensitivity for computed tomography, we selected a sample size of thirty-nine hemipelves (from twenty embalmed cadaveric specimens), with 80% power to detect a 20% difference between groups. All specimens were screened radiographically prior to the study. The required thirty-nine hemipelves were found to be free of any implants that might obstruct imaging. All selected specimens were also free of arthritic changes in the hip and had a normal preserved joint space.

Through a Kocher-Langenbeck approach⁷, the hip capsule was incised to allow visualization of the articular surface. With use of a 2.5-mm drill-bit, a drill-hole was made along the posterosuperior aspect of the acetabular rim (that is, at approximately the ten o’clock position for a right hip) at a point that placed the screw adjacent to the osseous subchondral surface. An attempt was made to create a similar starting position for all screws. A single 3.5-mm stainless-steel screw (Synthes, Paoli, Pennsylvania) was inserted through the predrilled hole. Twenty screws were directed to intentionally violate the articular surface, approximately 1 to 2 mm into the joint. The remaining nineteen screws were positioned to avoid the articular surface. Positioning of the screw in close proximity to the joint was based on the feel, during drill-bit insertion, of hard subchondral bone, as opposed to the soft surrounding cancellous bone, in combination with the known location of the articular surface. In this way, all nineteen extra-articular screws were placed in close proximity to the joint (within 1 to 5 mm).

Each hemipelvis was then placed on a radiolucent operating-room table (OSI [Orthopedic Systems, Incorporated], Union City, California) and examined with use of c-arm image-intensifier fluoroscopy (model 9600; OEC Medical, Salt Lake City, Utah). An experienced acetabular fracture surgeon, blinded to the actual screw location, performed the fluoroscopic examination in a manner approximating the clinical setting. Three views were obtained: an anteroposterior view of the hip, an axial view of the screw, and a tangential view of the screw (Fig. 1). Using the axial and tangential views, the examiner determined whether the screw was violating the hip joint. Only these views were used in determining screw location; dynamic fluoroscopy was not employed. The determinations were made directly from the fluoroscopy monitor, as would be done in the clinical setting. These observations, as well as the occurrence of any technical problems, were recorded. In addition, all images were stored digitally, and hard copies were created.

All of the hemipelves also were studied with computed tomography (Siemens Medical Systems, Iselin, New Jersey). Two techniques were employed: a 1-mm slice thickness at 1-mm intervals (1 1 mm) and a 4-mm slice thickness at 3-mm intervals (4 3 mm). For both techniques, window and center calibrations were adjusted to minimize metal artifact. Each scan was assessed in random order by another examiner (an experienced radiologist) blinded to the actual screw location. A determination regarding screw penetration into the joint was made for each scanning method.

Finally, all hips were dislocated and a direct visual determination of whether the screw had violated the acetabular articular cartilage was made. It was possible that a screw could have penetrated the subchondral bone and violated the articular cartilage but not have penetrated the chondral surface and thus was not directly visible. If not identified, the position of such a screw, embedded within the acetabular articular cartilage but not visible, would translate into an erroneous false-positive result. To account for this possibility, a Cobb elevator was used to carefully remove all of the articular cartilage overlying the subchondral bone to check for intra-articular screw location. Next, a sharp osteotome was used to remove the subchondral bone to locate the extra-articular screws. This after-the-fact check verified that all screws had been accurately placed in the expected positions, eliminating erroneous screw-positioning as a possible confounding study variable.

The sensitivity, specificity, and percent correct interpretations were calculated for each method. For the purpose of the statistical analysis, the disease state was considered to be screw penetration of the subchondral bone into or through the overlying articular cartilage surface (IN); the nondisease state was considered to be containment of the screw within bone (OUT). Sensitivity was defined as the ability of the selected examination to correctly demonstrate screw penetration (probability of predicting IN when the screw was truly IN). Specificity was defined as the ability of the selected examination to correctly demonstrate when the screw had not violated the joint (probability of predicting OUT when the screw was truly OUT). The percent correct interpretations was defined as the ability of the selected examination to accurately determine true screw location. The false-positive rate was defined by the formula 1 - specificity, and the false-negative rate was defined as 1 - sensitivity. Statistical analysis was performed with use of the binomial test.

Results

Introduction | Materials and Methods | Results | Discussion | References

he sensitivity, specificity, and percent correct interpretations for each method are shown in Table I. Statistical analysis revealed that the tangential fluoroscopic view was more specific than the 4 3-mm computed tomography scan (p = 0.02; binomial test). Otherwise, there were no significant differences among the methods used. There was a trend, although not a significant one, for axial fluoroscopy and 1 1-mm computed tomography scans to have better specificities than the 4 3-mm computed tomography method (p = 0.07; binomial test) (Fig. 2).

Difficulty was encountered during the interpretation of some of the tangential fluoroscopic views. In six instances, it was unclear whether the failure to demonstrate space between the screw and the subchondral bone was due to actual penetration of the joint by the screw or to the inability to obtain a more angulated fluoroscopic view, secondary to impingement of the c-arm on the torso of the cadaver. This is similar to the clinical situation, in that there are times when a true tangential view is not obtainable because of obstruction by the patient’s torso or by the table. In these cases, the best possible view was used. In two of these instances, compromise of the tangential view of a somewhat more posteriorly placed screw by impingement of the c-arm on the torso of the cadaver resulted in a false-positive result. Difficulty was also encountered with the interpretation of the axial view when the large head of the 3.5-mm screw was directly overlying the joint line (Fig. 3).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

The main limitation of this study is the single interpretation of each study by one (although expert) examiner. Therefore, interobserver and intraobserver variability was not evaluated. However, the methodology of this study was directed toward approximating the clinical setting, in which treatment is usually based on a single interpretation by the most experienced physician.

Insertion of periacetabular screws demands precise positioning in order to rigidly reconstruct the articular fragments while avoiding penetration of the articular surface. Early discovery and removal of intra-articular screws is imperative in order to avoid mechanical erosion of the articular cartilage^2,8,9. Despite its known problems with metal artifact, computed tomography is still generally accepted as the best technique for the recognition of intra-articular screws^1-5,9. Norris et al., in their study of the accuracy of intraoperative fluoroscopy for fracture reduction and screw placement, used postoperative computed tomography as the control for screw placement⁶. However, the assumption that postoperative computed tomography is the best modality for this purpose (with a presumed 100% clinical accuracy) is based on minimal data^2,3,5. Metal artifact interferes with the recognition of intra-articular acetabular screws on computed tomography scans. This problem is diminished but not eliminated by artifact-limiting techniques, such as that used in the present study¹⁰. In practical terms, a screw appears enlarged on computed tomography scans. If the screw is in close proximity to the hip joint, it may appear to compromise the joint when in actuality it does not; that is, there may be a false-positive result. Therefore, it should be expected that computed tomography will be highly sensitive (few false-negative results) but will have questionable specificity (an excessive number of false-positive results) in the evaluation of screw penetration of the acetabulum. The findings of the present study provide objective data that support these expectations (Table I).

Intraoperative fluoroscopy is not a new technique for assisting in the safe placement of periacetabular screws^3,8. However, problems with x-ray penetration related to the limitations of c-arm technology combined with patient-related factors (such as residual intracolonic contrast material, bowel gas, overlying drapes, patient positioning, and obesity) compromise its effectiveness^3,11. Recently, it has been reported that intraoperative fluoroscopy is as accurate as computed tomography⁶. In that retrospective study, no problems were noted in obtaining adequate intraoperative fluoroscopic visualization of the screws except in one case in which the surgeons failed to recheck properly after two screws had been changed. This was the only case with joint penetration. As noted previously, computed tomography provided the control for this study⁶. In view of our results, it is not surprising that Norris et al. found intraoperative fluoroscopy to be the equal of computed tomography. What is surprising is the lack of false-positive results in their study.

In the present study, four methods were selected for study: axial fluoroscopy, tangential fluoroscopy, 1¥1-mm computed tomography, and 4¥3-mm computed tomography. There was a reason for each selection. Because of the spherical shape of the acetabulum, if the entire screw is demonstrated to be clear of the joint on any projection it must be extra-articular^6,12. The axial ("barrel") view and the tangential view have been described as the best methods for visualizing the entire screw for this purpose^8,12. Mathematical modeling indicates that the tangential view is best overall¹². The 1¥1-mm computed tomography scan is currently the highest-definition tomographic study generally available. Clinically, 4¥3-mm computed tomography scans are used widely. False-positive results (low specificity) were a problem for all of the methods. Statistical analysis revealed that the tangential fluoroscopy was more specific than the 4¥3-mm computed tomography method (p = 0.02; binomial test). The differences between the specificities of axial fluoroscopy and the 1¥1-mm computed tomography method and the specificity of the 4¥3-mm computed tomography method approached significance (p = 0.07; binomial test). It is possible that, with a larger number of specimens, these differences would have reached significance. Low specificity of a postoperative test is a problem as it will result in an unnecessary reoperation. Therefore, postoperative computed tomography, especially the 4¥3-mm examination, is neither diagnostic nor best for the evaluation of possible joint penetration by a periacetabular screw. Although only screws placed in a posterior location were evaluated in this study, these findings should apply to all periacetabular screw locations, assuming that similar images can be obtained.

The obvious advantage of fluoroscopy is that data are obtained in the operating room. Clinical decision-making can take place immediately, and a reoperation can be avoided. If either the axial or the tangential view clearly shows the screw location to be extra-articular, one can know with some certainty that the joint surface has not been compromised. Statistically, the axial image was as sensitive as the tangential image (95% compared with 85%) and as specific as the tangential image (84% compared with 89%), and it was technically easier to obtain. Encroachment of the subchondral bone by any portion of the screw is presumptive evidence of joint penetration and requires the screw to be redirected. Magnified views can be generated intraoperatively by the c-arm software (model 9600; OEC Medical) and are helpful in this analysis. False-positive interpretations of the axial fluoroscopic view can be minimized by imaging the course of the drill-hole, by placing a drill-bit or an appropriately sized Kirschner wire in the hole, prior to screw insertion. This maneuver eliminates the problem with the axial view caused by the overlying screw head (Fig. 3). If this tactic is used, allowance must be made for the fact that the thread diameter of the screw is larger than the diameter of the drilled hole. False-positive interpretations of the tangential fluoroscopic view can be caused by insufficient c-arm angulation related to the table position, patient body habitus, and screw location. It is important to recognize these limitations when obtaining intraoperative fluoroscopic images. However, a false-positive fluoroscopic image causing unnecessary screw redirection is a much better situation than a false-negative examination (unrecognized joint penetration). Therefore, we believe that all screws of indeterminate position should be redirected.

In conclusion, intraoperative fluoroscopy and computed tomography appear to be equally accurate in determining periacetabular screw location. Although computed tomography has been touted as the method of choice, its specificity is too low to be considered as such. The potential for false-positive postoperative computed tomography scans, which was unacceptably high for the 4¥3-mm study, results in an important, but previously unrecognized, risk of an unnecessary reoperation. Intraoperative fluoroscopy is our preferred method for evaluation of periacetabular screw location as its results can be used immediately, eliminating the risk of reoperation. If postoperative computed tomography is used, the 1¥1-mm technique should be selected.

References

Introduction | Materials and Methods | Results | Discussion | References

Letournel E, Judet R. Fractures of the acetabulum. 2nd ed. New York: Springer; 1993. p 417-520, 664

Kellam JF, Tile M. Surgical techniques. In: Tile M, editor. Fractures of the pelvis and acetabulum. 2nd ed. Baltimore: Williams and Wilkins; 1995. p 355-96

Ebraheim NA, Savolaine ER, Hoeflinger MJ,Jackson WT. Radiological diagnosis of screw penetration of the hip joint in acetabular fracture reconstruction. J Orthop Trauma,1989;3: 196-201. 3196 1989 [PubMed]

Helfet DL, Bartlett CS, Malkani AL. Acetabular fractures: extended iliofemoral approach. In: Wiss DA, editor. Master techniques in orthopaedic surgery: fractures. Philadelphia: Lippincott-Raven; 1998. p 675-96

Mears DC, MacLeod MD. Acetabular fractures: triradiate and modified triradiate approaches. In: Wiss DA, editor. Master techniques in orthopaedic surgery: fractures. Philadelphia: Lippincott-Raven; 1998. p 697-724

Norris BL, Hahn DH, Bosse MJ, Kellam JF,Sims SH. Intraoperative fluoroscopy to evaluate fracture reduction and hardware placement during acetabular surgery. J Orthop Trauma,1999;13: 414-7.. 13414 1999 [PubMed]

Moed BR. Acetabular fractures: the Kocher-Langenbeck approach. In: Wiss DA, editor. Master techniques in orthopaedic surgery: fractures. Philadelphia: Lippincott-Raven; 1998. p 631-56

Tile M. Fractures of the acetabulum. In: Rockwood CA Jr, Green DP, Bucholz RW, Heckman JD, editors. Rockwood and Green’s fractures in adults. 4th ed, vol 2. Philadelphia: Lippincott-Raven; 1996. p 1617-58

Dirschl DR. Postoperative management and complications of acetabular fractures. In: Kellam JF, Fischer TJ, Tornetta P 3rd, Bosse MJ, Harris MB, editors. Orthopaedic knowledge update: trauma 2. Rosemont, IL: American Academy of Orthopaedic Surgeons; 2000. p 305-10

Anderson DJ, Berland L. CT techniques. In: Lee JKT, Sagel SS, Stanley RJ, editors. Computed body tomography with MRI correlation. 2nd ed. New York: Raven Press; 1989. p 31-60

Anglen JO,DiPasquale T. The reliability of detecting screw penetration of the acetabulum by intraoperative auscultation. J Orthop Trauma,1994;8: 404-8.. 8404 1994 [PubMed]

Owen PJ, Johnson PR, Kreder HJ. Efficient intraoperative screening for intraarticular hardware in acetabular fracture surgery: a mathematical model. Read at the 21st Triennial World Congress of the Société Internationale de Chirurgie Orthopédique et de Traumatologie; 1999 Apr 18-23; Sydney, Australia

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Fig. 1:: Representative anteroposterior (left), axial (center), and tangential (right) fluoroscopic images, which were made for each specimen.

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TABLE I: Probability Data Analysis

*The 95% confidence intervals are shown in parentheses.

	Method of Assessment*
Axial Fluoroscopy	Tangential Fluoroscopy	1 1-mm Computed Tomography	4 3-mm Computed Tomography
Sensitivity (n = 20) (%)	95 (75-99.9)	85 (62-97)	100 (83-100)	100 (83-100)
Specificity (n = 19) (%)	84 (60-97)	89 (67-99)	?84 (60-97)	?58 (33-80)
Correct interpretations (n = 39) (%)	90 (76-97)	87 (72-96)	?92 (79-98)	?79 (64-91)

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TABLE I: Probability Data Analysis

*The 95% confidence intervals are shown in parentheses.

	Method of Assessment*
Axial Fluoroscopy	Tangential Fluoroscopy	1 1-mm Computed Tomography	4 3-mm Computed Tomography
Sensitivity (n = 20) (%)	95 (75-99.9)	85 (62-97)	100 (83-100)	100 (83-100)
Specificity (n = 19) (%)	84 (60-97)	89 (67-99)	?84 (60-97)	?58 (33-80)
Correct interpretations (n = 39) (%)	90 (76-97)	87 (72-96)	?92 (79-98)	?79 (64-91)

Letournel E, Judet R. Fractures of the acetabulum. 2nd ed. New York: Springer; 1993. p 417-520, 664

Kellam JF, Tile M. Surgical techniques. In: Tile M, editor. Fractures of the pelvis and acetabulum. 2nd ed. Baltimore: Williams and Wilkins; 1995. p 355-96

Moed BR. Acetabular fractures: the Kocher-Langenbeck approach. In: Wiss DA, editor. Master techniques in orthopaedic surgery: fractures. Philadelphia: Lippincott-Raven; 1998. p 631-56

Anderson DJ, Berland L. CT techniques. In: Lee JKT, Sagel SS, Stanley RJ, editors. Computed body tomography with MRI correlation. 2nd ed. New York: Raven Press; 1989. p 31-60

Anglen JO,DiPasquale T. The reliability of detecting screw penetration of the acetabulum by intraoperative auscultation. J Orthop Trauma,1994;8: 404-8.. 8404 1994 [PubMed]

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These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.

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David B. Carmack, Berton R. Moed, Kathleen McCarroll, David Freccero; Accuracy of Detecting Screw Penetration of the Acetabulum with Intraoperative Fluoroscopy and Computed Tomography. The Journal of Bone & Joint Surgery. 2001 Sep;83(9):1370-1375.

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