Intraoperative Fluoroscopy for Anterior Cruciate Ligament Tunnel Placement

Introduction

Intraoperative fluoroscopy offers the knee surgeon a simple, non-invasive, cost-effective, accurate, and reproducible method to achieve anatomic placement of the femoral and tibial bone tunnels when performing an anterior cruciate ligament (ACL) reconstruction. Image intensifiers are routinely available in most operating rooms, so this technique requires no additional equipment investment. The use of intraoperative fluoroscopy can provide accurate and reproducible results, and can help the surgeon decrease variability in the placement of the ACL bone tunnels ( Fig. 43.1 ). Furthermore the method allows the surgeon to objectively measure and document where the ACL bone tunnels are being placed during surgery. Intraoperative fluoroscopy is particularly useful during revision ACL surgery where it is often the case that the native ACL footprints have been removed and the bony anatomy of the ACL attachment sites has been altered by the previous ACL bone tunnels, notchplasty or bone grafting of the original bone tunnels. In the revision situation, fluoroscopy can be used to guide placement of new anatomic femoral and tibial bone tunnels based on preoperative radiographs or lateral computed tomography (CT) scans using published radiographic anatomy data of the native ACL attachment sites. Fluoroscopy is also extremely useful when performing a remnant sparing or an augmentation technique for a partial ACL tear, as the bone tunnels can be placed without the need to resect intact ACL fibers. The goal of using intraoperative fluoroscopy is not necessarily to place the ACL bone tunnels in the same location in every patient, but rather to avoid unintended bone tunnel placements or unrecognized deviation in bone tunnel placement that might lead to failure of the ACL reconstruction.

Operating Room Setup

Successful use of intraoperative fluoroscopy is dependent on the ability to obtain a true lateral radiograph of the knee. To achieve this, it is important to control abduction-adduction and internal-external rotation of the leg. This is best accomplished by keeping the operating room table flat and using a padded thigh post and hip positioner placed at the level of the patient’s pelvis to prevent the hip from abducting and the leg from externally rotating, and a foot support to maintain the knee between 70 and 90 degrees of flexion. The x-ray beam is aligned parallel to the epicondylar axis of the knee. A trial lateral image is taken, and the position of the C-arm and the knee are adjusted until the proximal, posterior, and distal borders of the medial and lateral femoral condyles overlap. After a satisfactory lateral image is achieved, the C-arm adjustments are locked and the position of the wheels of the C-arm is marked on the floor with wide adhesive tape. The C-arm is moved out of the operative field and sterilely draped. When radiographic images are required during the procedure, the C-arm is moved into the operative field and the wheels of the machine are positioned at the previously marked locations on the floor. When properly performed, intraoperatiave fluoroscopy adds only a few additional minutes to the total operating time.

Operating Room Setup

Successful use of intraoperative fluoroscopy is dependent on the ability to obtain a true lateral radiograph of the knee. To achieve this, it is important to control abduction-adduction and internal-external rotation of the leg. This is best accomplished by keeping the operating room table flat and using a padded thigh post and hip positioner placed at the level of the patient’s pelvis to prevent the hip from abducting and the leg from externally rotating, and a foot support to maintain the knee between 70 and 90 degrees of flexion. The x-ray beam is aligned parallel to the epicondylar axis of the knee. A trial lateral image is taken, and the position of the C-arm and the knee are adjusted until the proximal, posterior, and distal borders of the medial and lateral femoral condyles overlap. After a satisfactory lateral image is achieved, the C-arm adjustments are locked and the position of the wheels of the C-arm is marked on the floor with wide adhesive tape. The C-arm is moved out of the operative field and sterilely draped. When radiographic images are required during the procedure, the C-arm is moved into the operative field and the wheels of the machine are positioned at the previously marked locations on the floor. When properly performed, intraoperatiave fluoroscopy adds only a few additional minutes to the total operating time.

Anterior Cruciate Ligament Femoral Tunnel Placement

Placement of the ACL bone tunnels using fluoroscopy does not depend on identifying the native ACL footprint or the bony ACL ridges, so only minimum preparation of the ACL femoral attachment site is required. The knee is placed at 90 degrees of flexion, and under arthroscopic visualization, the tip of a 45-degree angled microfracture awl is positioned along the lateral wall of the intercondylar notch at the proposed location for the ACL femoral tunnel. When identified, anatomic landmarks such as the native ACL footprint and the lateral intercondylar and bifurcate ridges can be used as references to help position the ACL femoral tunnel. A lateral C-arm image of the knee is taken, and the initial position of the tip of the microfracture awl is evaluated. The microfracture awl should be rotated so that the tip is clearly visible on the C-arm image ( Fig. 43.2 ). The location of the tip of the microfracture awl can be compared with a preoperatively planned ideal ACL femoral tunnel position, or alternatively compared with data from published radiographic anatomic studies of the native ACL femoral attachment site. Any discrepancy in the desired ACL femoral tunnel position can be adjusted under arthroscopic and fluoroscopic guidance until the desired ACL femoral tunnel position is achieved.

Analyzing the Anterior Cruciate Ligament Femoral Tunnel Position

The Bernard and Hertel (BH) grid is the most commonly used method to evaluate the placement of the femoral tunnel in cadaveric radiographic anatomic studies of the native ACL femoral attachment site and in clinical studies. This rectangular grid is applied to the lateral femoral condyle and allows any position along the inner wall of the lateral femoral condyle to be measured in the shallow (distal) to deep (proximal) direction ( x -axis) and the high (anterior) to low (posterior) direction ( y -axis). This method is easy to use and has been shown to be independent of the knee size, shape, and distance between the x-ray tube and the patient. Intra- and interobserver ACL femoral tunnel measurements using the BH grid have been found to be reliable. The BH grid is placed on a lateral knee radiographic image in the following way:

1.

Draw a tangent to the roof of the intercondylar notch (Blumensaat line).
2.

Draw two lines perpendicular to the first line, one at the intersection of the tangent line with the shallow (distal) border of the lateral femoral condyle and the other with the intersection of the tangent line and the deep (proximal) border of the lateral femoral condyle. The lateral femoral condyle can be identified by an indentation at the distal margin (Grant’s notch) and the fact that the medial femoral condyle extends more distally.
3.

Draw another line parallel to Blumensaat line and tangential to the inferior (posterior) border of the lateral femoral condyle ( Fig. 43.3 ).

Fig. 43.3

A, Bernard-Hertel (BH) grid. The depth of the anterior cruciate ligament (ACL) femoral tunnel position is measured parallel to Blumensaat line (d), and the height (h) along a line perpendicular to Blumensaat line. B, BH grid plotted on the intraoperative C-arm image. The tip of the microfracture awl is positioned at a point 28% of the depth of the lateral femoral condyle and 34% of the height of the lateral femoral condyle. This point represents the center of the ACL femoral attachment site based on published radiographic data.

The BH grid method has been used to measure the location of the center of the posterolateral (PL) and anteromedial (AM) ACL bundles in human cadaveric specimens. Using data from these published studies, a weighted average position for the center of the PL and AM bundles can be calculated ( Table 43.1 ). Using these data, the extrapolated weighted average position for the center of the ACL femoral attachment is located at a point that is 28% along Blumensaat line and 34% of the height of the intercondylar notch. The weighted average location for the center of the PL and AM bundles and the extrapolated center of the ACL femoral attachment site can be plotted on a preoperative lateral radiograph or a C-arm image, and these images can be used as a reference to guide placement of the ACL femoral tunnel during surgery.

TABLE 43.1

Radiographic Measurements of the Native Anterior Cruciate Ligament Femoral Attachment Site

Author	N	AMB Depth (%)	PLB Depth (%)	Average Center Depth (%)	AMB Height (%)	PLB Height (%)	Average Center Height (%)
Bernard et al.	10	—	—	24.8	—	—	28.5
Luites and Verdonschot	29	21.0	26.6	23.5	19.2	42.2	32.3
Musahl et al.	8	—	—	27	—	—	28
Yamamoto et al.	10	25	29	27	16	42	29
Columbet et al.	7	26.4	32.3	29.4	25.3	47.6	36.5
Zantop et al.	20	18.5	29.3	23.9	22.3	53.6	38.0
Tsukada et al.	36	25.9	34.8	30.4	17.8	42.1	30.0
Lorenz et al.	12	21	27	24	22	45	34
Forsythe et al.	8	21.7	35.1	28.4	33.2	55.3	44.3
Pietrini et al.	12	21.6	28.9	25.3	14.6	42.3	28.5
Iriuchishima et al.	15	15	32	23.5	26	52	39
Moloney et al.	20	—	—	33.7	—	—	32.0
Lee et al.	15	33.5	38.3	35.9	27.6	55.1	41.4
de Abreu-e-Silva et al.	8	—	—	30.0	—	—	35.3
Luites and Verdonschot	12	23.2	25.2	24.9	15.1	38.1	31.9
Davis et al.	12	27.1	39.3	32.9	24.1	49.3	36.6
Weighted Average	234	23.2	31.4	27.8	21.1	46.3	33.7