This article has been amended to include a factual correction. An error was identified subsequent to its original printing (2013; 36:776-777), which was acknowledged in an erratum printed in 2014; 37(1):16. The online article and its erratum are considered the version of record.
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Mobile image intensifiers are more readily used to assess acetabular cup placement when performing a total hip arthroplasty through a direct anterior approach. However, patient positioning affects pelvic tilt and rotation, which in turn requires proper C-arm fluoroscopy adjustments for image interpretation. The authors provide insight into the interpretation of acetabular cup positioning based on the fluoroscopic images obtained to ensure appropriate cup positioning using the direct anterior approach for total hip arthroplasty.
The authors are from the Department of Orthopedic Surgery, Cleveland Clinic Florida, Weston, Florida.
The authors have no relevant financial relationships to disclose.
Correspondence should be addressed to: Andres M. Alvarez, MD, Cleveland Clinic Florida, 2950 Cleveland Clinic Blvd, Weston, FL 33331 (email@example.com).
Click here to read erratum on this article.
Two main causes of postoperative total hip arthroplasty (THA) dislocation are malposition of the acetabular component within the prosthesis and soft tissue imbalances.1 The direct anterior approach for THA is viewed as advantageous compared with other approaches because, in addition to preserving many of the inherent restraints within the hip joint,2 it allows C-arm fluoroscopy image intensifiers to be readily used for intraoperative assessment of the acetabular cup positioning. However, proper execution and interpretation of fluoroscopic images may not be straightforward and requires experience to be performed effectively. The authors present some key points to consider when using a portable image intensifier for the interpretation of acetabular cup orientation during THA using a direct anterior approach.
Mobile image intensifiers are more apt for use in a direct anterior approach for THA because patients are in a supine position. However, evaluation of acetabular coverage is difficult because the projected morphologic features of the acetabulum and routinely used hip parameters are dependent on pelvic positioning, an area that is rarely standardized in the operating room. Furthermore, C-arm fluoroscopy placement can vary considerably between image acquisitions during the course of a surgical procedure.
Inconsistency among all of the aforementioned factors makes the interpretation of intraoperative radiographs for acetabular orientation misleading and difficult. Variability with radiography beam offset and centralization, as well as differences in pelvic orientation, can increase the likelihood of errors in the interpretation of projected images and should receive proper recognition during the procedure.
Traditional inlet and outlet views are defined as radiographs directed 45° caudal and 45° cranial from the anteroposterior view, respectively.3 During THA, a tendency toward an inlet view (a longer distance between the pubic symphysis and the sacrococcygeal joint) predisposes an overestimation of the femoral head anterior coverage and, in so doing, decreases the perceived anteversion. Conversely, a tendency toward an outlet view overestimates posterior coverage of the femoral head, thereby increasing perceived anteversion.4 Figure 1 illustrates the change in acetabular version inlet and outlet views, and Figure 2 provides a true anteroposterior view of the pelvis.
Fluoroscopic images showing the change in acetabular positioning for an inlet (A) and outlet (B) view of the pelvis.
Fluoroscopic image showing a true anteroposterior view of the pelvis.
When pelvic orientation has been controlled and a true anteroposterior view of the pelvis and hip is obtained, moving the position of the beam in a caudal-cephalad direction will also affect the appearance of the acetabular cup position. Shifting the direction of the central beam from the center of the femoral head creates considerable variability in acetabular version estimates. A distal anteroposterior view of the hip will skew cup orientation to implicate increased anteversion. Moving the beam toward the center of the femoral head will decrease this effect. Figure 3 depicts the difference in acetabular cup version as the mobile image intensifier is moved in a caudal-cephalad direction.
Fluoroscopic images showing the difference in acetabular cup positioning as the mobile image intensifier is moved in a caudal-cephalad direction. When the fluoroscopy beam is moved in the cephalic direction, the cup position appears less anteverted (A). When the fluoroscopy beam is moved in the caudal direction, the cup position appears more anteverted (B).
Furthermore, identifying pelvic rotation is important in evaluating cup placement. Pelvic rotation will lead to an internal or external oblique radiographic view. Radiologic criteria defines an internal oblique view as a radiograph taken with the radiograph beam directed vertically toward the affected hip that is rotated 45° anteriorly.3 When a patient is positioned with the surgical hip rotated anteriorly or when C-arm fluoroscopy has excessive orbital travel toward the affected hip, anteversion angles will be overestimated (Figure 4A). If the converse is true regarding patient or C-arm placement (more of an external oblique view), anteversion angles will be underestimated (Figure 4B).
Fluoroscopic images showing overestimation (A) and underestimation (B) of anteversion angles.
In a true anteroposterior view of the pelvis, the central beam is directed toward the midpoint between the upper border of the symphysis and a horizontal line intersecting the anterior iliac spines. This can be assessed by aligning the tip of the coccyx with the middle of the pubic symphysis and evaluating the symmetry of the teardrop signs, the obturator foramina, and the iliac wings.4 With excessive pelvic rotation, asymmetry exists in the abovementioned signs, and the ischial spines become visible. At this point, C-arm fluoroscopy repositioning through orbital travel should occur until all signs are accounted for.
Many of the perceived benefits in acetabular cup placement during a direct anterior THA rely on the proper use and interpretation of fluoroscopic images. It is important to recognize that patient positioning affects pelvic tilt and rotation. Therefore, image intensifiers must be adjusted accordingly to acquire proper images. It is also important to avoid misinterpretation in cup positioning by considering the key points that have been presented.
- Garcia-Cimbrelo E, Munuera L. Dislocation in low-friction arthroplasty. J Arthroplasty. 1992; 7(2):149–155. doi:10.1016/0883-5403(92)90008-E [CrossRef]
- Siguier T, Siguier M, Brumpt B. Mini-incision anterior approach does not increase dislocation rate. A study of 1037 total hip replacements. Clin Orthop Relat Res. 2004; (426):164–173. doi:10.1097/01.blo.0000136651.21191.9f [CrossRef]
- Bucholz RW, Heckman JD, Court-Brown CM, eds. Rock-wood and Green’s Fractures in Adults. 6th ed. Philadelphia, PA: Lippincott Williams and Wilkins; 2006.
- Siebenrock KA, Kalbermatten DF, Ganz R. Effect of pelvic tilt on acetabular retroversion: a study of pelves from cadavers. Clin Orthop Relat Res. 2003; (407):241–248. doi:10.1097/00003086-200302000-00033 [CrossRef]