Role of Fluoroscopic Imaging and Image Guidance

Preetesh D. Patel

Preston W. Grieco

Jeremy M. Gililland

Trevor M. Owen

Introduction

Accurate component positioning is of the utmost importance in THA. Malpositioned implants can lead to dislocation, impingement, wear, and early failure. One frequently discussed advantage of the DAA in the supine position is the ease with which fluoroscopic imaging can be used to aid in component positioning, component sizing, assessment of leg length, and offset and to identify intraoperative complications such as canal perforation or fracture. This visual feedback is provided in real time, and adjustments can be made to improve the final implant position. With the patient in a supine position on a radiolucent table, the pelvis and hip are in an ideal position to obtain routine anteroposterior (AP) imaging with minimal interference from the fluoroscopy unit. This contrasts with procedures performed in the lateral position where the pelvis can roll forward or backward and imaging requires the fluoroscopy unit to be rotated 90°, leading to difficulty accessing the operative field.

Equipment and Operating Room Setup

Fluoroscopy units come in many configurations and sizes with common image intensifier sizes of 9 and 12 in. We recommend using the larger 12-in units because these will typically allow the surgeon to visualize the entire pelvis on the AP view, providing the most data for the assessment of component positioning, leg length, and offset. Patients with a large body habitus may limit the ability to obtain a full AP pelvis even on the larger units because the image intensifier cannot be brought close enough to the pelvis without impinging on soft tissue. Similarly, anteriorly placed retractors can also limit the ability to bring the unit low enough to obtain the desired view and obscure the hip as well.

The fluoroscopy unit is positioned on the nonoperative side of the patient and can be brought in over the patient, limiting interference with the surgical team. If an assistant is used on this side of the table (ie, the nonoperative side), it is necessary to move that individual before bringing in the unit to obtain imaging. Standard plastic draping of the unit is used; however, it is helpful to add additional sterile draping over the body of the unit to prevent contamination of the side of the surgical field. The latch allowing the unit to be rolled to the lateral view is not needed and thus can be covered with this draping. The unit is positioned perpendicular to the patient and can be tilted cranially or caudally to adjust for pelvic tilt. Specialized tables such as the Hana table (Mizuho OSI) can easily accommodate a fluoroscopic unit. However, if using a standard operating room table, one must ensure that the base of the table is positioned such that it does not interfere with the path of the unit and an acceptable pelvis image can be obtained. Other equipment such as electrocautery and suction can be placed at the proximal aspect of the patient on the nonoperative side to minimize their interference.

Technical Tips for Fluoroscopy

Image intensifiers can be used throughout the DAA THA surgical procedure to confirm or improve on surgical steps. This can be performed with or without special surgical tables or attachments based on surgeon preference. The key steps on how to properly incorporate fluoroscopy to the DAA THA are as follows.

Preprocedural Physical Assessment of the Patient’s Leg Length Discrepancy

Before the procedure, the patient’s leg length discrepancy (LLD) is assessed. The medial malleoli can be used as a reference and have been established as a valid and reliable landmark for measure.¹ True leg length is a complex interaction between soft tissues, bones, and implants. One must consider any knee and hip contractures as well as spinal deformity with subsequent pelvic obliquity when determining the true LLD. The physical assessment can then be compared with the patient’s perceived LLD, and a planned limb length correction is formulated. It is our routine practice to ask the patient before surgery of any perceived LLD.

Preprocedural Fluoroscopy (Pelvic Tilt, Pelvic Rotation, and Leg Length)

Once the patient is positioned on the preferred operating table, fluoroscopy is performed. An AP pelvis image
is obtained with the pelvic tilt matched to the patient’s preoperative standing radiograph using the relationship between the coccyx and the pubic symphysis as well as the appearance of the obturator foramen. To accomplish this, the patient position or the fluoroscopic beam can be tilted in either the cephalad or caudad direction. This becomes essential in order to place the acetabulum in the true “functional position.” It has been demonstrated that, if the acetabular component is positioned appropriately from a supine pelvis radiograph, up to 31% will become malpositioned upon standing.² This is a change that can occur because of the dynamic interaction between the lumbar spine, pelvis, and hip with postural changes.

The image should also demonstrate neutral pelvic rotation by assessing the symmetry of the obturator foramina and ischial spines as well as the alignment of the coccyx with the pubic symphysis. Again, either the patient or the fluoroscopic machine can be tilted to obtain an optimal image.

Lastly, a radiopaque rigid bar is placed across the pelvis to recreate the bi-ischial line, and its intersection to the lesser trochanters is identified (Figure 14.1). This measurement is not used to determine LLD between the two sides but rather is used as a reference point to assess for any changes that occur on the ipsilateral side once the trials or final implants are placed. A comparison is made between this preprocedural image and the same image with trial or final implants in place to assess for the change in leg length that occurred during component implantation. The change in ipsilateral leg length can then be objectively calculated.

FIGURE 14.1 A preoperative image of a patient undergoing right THA with a radiopaque bar placed across the bi-ischial line.

Confirmation of Femoral Neck Osteotomy

The level of femoral neck resection can be confirmed intraoperatively and modified based on preoperative templating. There is a propensity to underresect the femoral neck, which makes acetabular exposure and preparation more difficult.

Reaming

Fluoroscopy can be used during acetabular preparation but is not something of routine use for some of the authors. It can specifically be used to determine sizing as well as depth and medialization and is helpful during a surgeon’s learning curve. It can also be used in more difficult cases, such as severe dysplasia or revision THA. It is important to note that fluoroscopy during this step does not substitute for good acetabular exposure. Overreliance on fluoroscopy in cases of poor exposure could lead to errant reaming and unintended acetabular bone loss.

Acetabular Component Position

Acetabular component position including inclination and version is determined using fluoroscopy. This can be done on a pelvis or hip image depending on the size of the intensifier (Figure 14.2). A key step before impaction is to confirm the proper preoperative pelvic rotation. There is a tendency with traction or manipulation that malrotation of the pelvis occurs toward an iliac oblique view. This will result in improper assessment of acetabular version. In addition, one can confirm that the component is fully “seated” as well as screw placement if needed.

FIGURE 14.2 Acetabular cup insertion under fluoroscopic guidance allowing for “fine-tuned” adjustments to version and inclination.

Final Leg Length Assessment

Once trials or final components are inserted, another AP pelvis image is obtained in neutral rotation and pelvic tilt matching the preoperative standing pelvis radiograph. The radiopaque rigid bar is again placed across the bi-ischial line. Comparison of this image with preprocedural fluoroscopy can give objective data on the change in leg length on the ipsilateral side (Figure 14.3A and B).

FIGURE 14.3 A, With the trial implants in, the radiopaque bar is placed across the bi-ischial line. A comparison is made with the preoperative image to quantify the change in leg length as well as offset. B, As done with the trial components, the radiopaque bar can be used again after the insertion of the final implants.

Reconstruction of Global Offset

In addition to providing information on limb length, fluoroscopy provides valuable information on offset. Reconstruction of global offset, a sum of both acetabular and femoral offsets, has been shown to lead to improved stability, range of motion, and abductor strength while decreasing wear.³^,⁴ In conjunction with accurate limb length restoration, proper offset restoration has been shown to lead to improved patient outcomes with an additive effect.⁵ The introduction of fluoroscopy to the DAA provides real-time information regarding offset. To accomplish this, a measurement is made between the teardrop and a longitudinal line down the center of the femoral shaft (femoral shaft axis) while keeping in mind a consistent abduction or adduction position of the limb. The global offset is maintained within ±4 mm. Ideally, this is compared with a contralateral normal hip if possible but may not be feasible if the patient has bilateral hip arthrosis at presentation during the index THA.

Femoral Component Position

Dedicated AP and lateral images of the hip with trial components are obtained. Assessment can be made to ensure appropriate sizing and alignment before opening of the final implants.

Final Implant Position

Images including a lateral hip are taken with the final implants in place to confirm femoral implant size and position, to ensure the prosthetic hip is fully reduced without any interposed tissue, and to rule out any obvious fractures (Figure 14.4). It is important to note that these images are not meant as a substitute for direct visual confirmation of hip reduction, inspection for interposed soft tissue, and evaluation of the neck and calcar for fracture. The major advantage with this technique is that it reduces any “surprises” one may encounter with films taken in the recovery room or during the first office visit.

FIGURE 14.4 Imaging of the hip can be obtained to confirm size and positioning as well as rule out an obvious fracture or perforation.

An AP image only is shown, but typically a lateral image is obtained as well by externally rotating the leg 90°.

Fluoroscopic “Illusions”

Distortion is a phenomenon that can alter the image in image-intensified fluoroscopy. Image-intensified fluoroscopic machines, which are commonly used in orthopaedic operating rooms, are susceptible to external electromagnetic fields (EMFs) that cause image aberrations. This effect is well documented in the radiology and applied physics literature but has not been discussed much at all in the orthopaedic literature.

Fluoroscopic machines are susceptible to two main types of image distortion due to the design of the image intensifier. The first is pincushion distortion arising from x-ray photons being projected onto a curved input phosphor and then transitioned to a flat output surface. The second is S-type distortion originating from the effect of EMFs on the electrons being accelerated through the image intensifier between the input phosphor and the output phosphor. This type of distortion is most pronounced at the periphery of images where the electrons are closer to the surface of the intensifier and thus more subject to EMF forces (Figure 14.5). These EMFs may come from the earth’s magnetic field and may be generated more locally by pieces of electrical equipment within the operating room or nearby. Items such as the motor driving the operating table or nearby magnetic resonance imaging scanners, even a floor above or below, generate powerful EMFs. The proximity of the image intensifier to such equipment affects the degree of image distortion. Distortion is also pose dependent, meaning that the amount of image distortion varies with C-arm positioning (tilt, rollover, translation, etc.) because the position of the image intensifier changes the impact of the EMF vector on the electron path. Image-intensified units have shielding to help protect against EMF disturbances, but this does not remove the impact entirely. It has been stated that distortion in fluoroscopy is location dependent, pose dependent, and machine dependent and therefore is entirely unpredictable. This is distinctly separate from the phenomenon of parallax, which is not true image aberration but rather altered image interpretation due to variations in the viewing angle.