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Surgical Pearls

Essential Insights On Imaging For Reconstructive Flatfoot Surgery

Surgeons rely heavily on preoperative weightbearing (WB) X-rays when selecting the ideal procedure combination for reconstructive flatfoot surgery. A full preoperative weightbearing radiographic series typically involves three views of the foot plus a Harris axial or long leg axial view. The addition of weightbearing anteroposterior (AP) and mortise ankle views are commonly indicated to rule out ankle degenerative joint disease (DJD), deltoid insufficiency or ankle valgus deformity. 

Aside from looking for DJD, tarsal coalition and accessory bones, surgeons critically evaluate X-rays from a deformity standpoint to determine planal dominance, apex of deformity, heel valgus and joint faults. Other than evaluating the condition of the posterior tibial tendon or ligamentous structures with magnetic resonance imaging (MRI), weightbearing X-rays play a much more important role than advanced imaging in relation to flatfoot procedure selection. 

Intraoperative imaging is effective for evaluation of fixation devices but leaves much to be desired in relation to deformity correction due to the non-weightbearing nature of imaging in the OR. There is a tendency for surgeons to rely more heavily on the intraoperative clinical exam to determine how each procedure is impacting foot alignment. Surgeons typically cannot trust non-weightbearing imaging to fully assess alignment of the rearfoot, midfoot and forefoot. 

Indeed, relying on non-weightbearing imaging creates the added risk of undesirable alignment related to under-correction or overcorrection that one may not notice until seeing the first post-op weightbearing X-rays six to 10 weeks later. This is particularly true for osteotomy procedures like the Cotton or Evans procedures as opposed to fusion procedures that are fixed. 

Intraoperative imaging also burns up valuable OR time and results in radiation exposure. These factors create a strong desire to obtain the optimal views correctly on the first shot, in the ideal order and by an X-ray technician who is efficient with foot imaging and understands the surgeon’s needs. 

With these points in mind, we would like to share a few imaging and image interpretation pearls to keep in mind with reconstructive flatfoot surgery. 

A Protocol For Preoperative Weightbearing X-rays 

Our preoperative weightbearing X-ray protocol for flatfoot reconstruction includes four foot views, AP and mortise ankle views, and a long leg axial view. This series evaluates the entire deformity and we reserve weightbearing computed tomography (CT) for patients who show signs of tarsal coalition or advanced ankle DJD. The fourth foot view is a lateral oblique view, which is particularly helpful when an accessory navicular is present. We obtain MRI and/or ultrasound preoperatively for specific pathologies including posterior tibial tendon dysfunction (PTTD).

We routinely meet with our clinic X-ray technicians and provide training about the importance of imaging in angle and base of gait. Lack of attention to proper patient positioning can severely distort preoperative foot alignment by making the foot look nearly normal if imaging with the foot in a supinated position. This is especially problematic with flexible flatfoot deformities in comparison to other conditions that are rigid. In cases in which the AP and lateral foot X-rays do not match the clinical presentation, we send the patient back to X-ray for supervised views to capture resting calcaneal stance position and proper angle and base of gait. 

Special preoperative views are sometimes indicated in flatfoot surgery. This typically involves deltoid stress eversion and what we call “deformity correction AP and lateral foot views.” The deltoid is more often insufficient in cases with advanced PTTD or advanced ankle DJD. The first two images above demonstrate the deformity correction imaging technique. We have found this technique to be helpful in more extreme cases in which severe rearfoot deformity makes it hard to select ideal medial column procedures. 

Key Considerations For Intraoperative Imaging 

Over the course of a several hour flatfoot operation, a significant amount of time is devoted to C-arm imaging. The surgeon should therefore have a strategy to use the time wisely and get the most information with the least amount of wasted time and radiation exposure. One perfect AP image is better than three attempts to get the correct angle. It is generally easier for the surgeon to hold the foot and ankle in the desired position while imaging rather than having the tech try multiple complex angles. 

One common approach is to use a metal mallet or equipment tray lid to dorsiflex the foot during imaging, which is effective, but the dense metal object impacts image quality. We have used cutting boards for simulated weightbearing intraoperative imaging for many years and find the stiff, translucent plastic material to be ideal for loading the foot. The images on page 18 demonstrate part of our intraoperative simulated weightbearing lateral imaging protocol. 

Being able to simulate weightbearing foot alignment for intraoperative imaging allows the surgeon to incorporate a more comprehensive assessment of structural alignment. A large C-arm is generally necessary for this approach in order to image enough of the foot to assess rearfoot to forefoot structural alignment. There are intraoperative methods to expand the imaging field of view in order to see more or less of the foot on one screen. Moving the image intensifier/tube closer to the foot shows more of the foot on one screen while moving farther away results in magnification but a smaller field of view.

There is generally a principle view that is most critical for each step in the operation. An example would be the lateral heel view for the posterior screw placement when performing subtalar joint (STJ) fusion. Of note, we do not simulate weightbearing when obtaining imaging for hardware placement. 

Once there is positioning of the guidewire on the principle view, we obtain an AP ankle view to ensure that the guidewire is in the middle of the talar body and not too far medial or lateral. We then proceed to obtain a full simulated weightbearing series including AP, lateral and long leg axial views in order to assess rearfoot alignment prior to placing the screw or second guidewire. 

When using the divergent two-screw fixation construct for STJ fusion, the medial oblique view becomes the principle view during guidewire placement. This allows confirmation of incision placement with the starting point proximal to the calcaneocuboid joint and orientation parallel to the midtarsal joint (see radiographs above). The medial oblique radiograph is also the principle view when performing an Evans osteotomy of the calcaneus.

Deltoid insufficiency can be masked on preoperative imaging in patients with STJ instability since the eversion force can go through the STJ, leading to a false negative ankle stress eversion exam. Our routine is to stress the deltoid under AP ankle imaging intraoperatively after placement of STJ hardware. Clinicians should anticipate repair when they observe excessive talar tilt. 

The AP image of the foot is the most challenging view to obtain intraoperatively while loading to simulate weightbearing. Any attempt to dorsiflex the foot makes it harder for the tech to drop the C-arm far enough toward the knee to get the proper image. Obtaining a proper AP foot image allows assessment of alignment after rearfoot or midfoot fusion, Cotton osteotomy placement, and hardware placement. Our approach to obtain a proper AP image of the foot is to tilt the foot out of varus by holding the fifth toe and applying a mild dorsiflexory lift (similar to casting for orthotics). One would position the C-arm or fluoroscan at 90 degrees to the top of the foot, which accomplishes the traditional 75 degree angle off of the weightbearing surface. 

In Conclusion

A systematic approach to preoperative and intraoperative imaging in reconstructive flatfoot surgery is beneficial to allow more critical evaluation of the preoperative condition and results one can achieve at the time of surgery. 

Flatfoot cases nearly always involve multiple procedures and proper imaging can assist greatly with ideal procedure selection. Knowing the best radiographic view for each step in the procedure helps save time in the operating room, especially with a large C-arm. Being able to simulate weightbearing when taking intraoperative images allows surgeons to rely on imaging in addition to the clinical exam to best assess deformity correction.

Dr. Boffeli is the Foot and Ankle Surgical Residency Program Director and Department Chair at Regions Hospital/HealthPartners Medical Group in St. Paul, Minn. Dr. Boffeli discloses that he is the owner of Surgical Design Innovations and an investor in ExoToe, LLC.

Dr. Luer is Chief Resident with the Foot and Ankle Surgical Residency Program at Regions Hospital/HealthPartners Medical Group in St. Paul, Minn. 

Surgical Pearls
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By Troy J. Boffeli, DPM, FACFAS and Samantha A. Luer, DPM
References

1. Boffeli TJ, Reinking R. A 2-screw fixation technique for subtalar joint fusion: a retrospective case series using a 2-screw fixation construct with operative pearls. J Foot Ankle Surg. 2012;51(6):734-738.

2. Boffeli TJ, Waverly BJ. Angle and base of gait long leg axial and intraoperative simulated weight bearing long leg axial imaging to capture true frontal plane tibia to calcaneus alignment in valgus and varus deformities of the rearfoot and ankle. J Foot Ankle Surg. 2016;55(5):1043-1051.

3. Boffeli TJ, Mahoney K. Intraoperative simulated weight bearing lateral foot imaging: the clinical utility and ability to predict sagittal plane position of the first ray in lapidus fusion. J Foot Ankle Surg. 2016;5(6):1158-1163.

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