Current Insights On Internal Fixation Of Subtalar Fusions
When it comes to the posterior to anterior subtalar joint screw placement (aka the “bottom-up approach”), one can generally perform this technique with the patient either in a lateral or supine position. After appropriate joint reduction, drive the guide wire from the posterior-inferior calcaneus across the subtler joint into the talar body and/or talar neck region. With the guide wire in the central aspect of the calcaneus, place an index finger on the tibialis anterior and aim for that landmark. It is very important for either technique to confirm placement of the screw on AP ankle, calcaneal axial and lateral foot radiographs.
There are several vital structures on the medial and lateral aspects of the calcaneus that can be potentially damaged when one is performing the dorsal to plantar subtalar arthrodesis screw insertion procedure. The high risk medial structures include the tibial nerve and its branches (medial and lateral plantar nerve), the posterior tibial artery, the flexor digitorum and the flexor hallucis longus. The lateral structures at risk include the peroneal tendons and the sural nerve.
Easley and colleagues reported sural nerve damage in 17 of 184 feet secondary to improper subtalar arthrodesis screw insertion.3 The authors stress the importance of using multiple intraoperative radiographic images, including the calcaneal axial view, prior to definitive fixation to ensure appropriate guide pin placement and avoid damage to vital anatomical structures. Intraoperative single lateral view radiographs can be misleading as they are only demonstrating screw trajectory in one plane. Calcaneal axial and AP ankle views are necessary to demonstrate frontal plane angulation of the intended screw path.
Avoiding Common Fixation Mistakes With The Bottom Up Approach
Two common mistakes occur intraoperatively and in cadaver labs with surgeons in training with the bottom up approach. First, when one directs the screw from posterior to anterior across the subtalar joint, the screw goes in a lateral to medial direction on the posterior calcaneus from the start. This allows the screw to violate the medial cortex of the calcaneus near the sustentaculum tali, dangerously close to neurovascular structures. If this screw continues up into the talus, it usually is too medial, enters the medial ankle gutter and does not remain in the talar body.
The second potential mistake is due to overcompensation after making the first mistake. Here the surgeon overcorrects the lateral to medial orientation, captures the more laterally based calcaneal body, and avoids the medial neurovascular structures.
However, the talus distally has a medially directed bias as we approach the neck-body junction. Though this screw purchases well in the calcaneus, it frequently is shallow laterally in the talus or blows out the lateral wall altogether, violating the lateral ankle gutter.
What A Recent Cadaver Study Reveals About Screw Placement Variations
In an effort to examine our theory that an anterior to posterior screw orientation approach might be easier and more reproducible, we performed a cadaver study. We examined 10 fresh frozen cadaver legs (nine female and one male) of similar ages (average 67.3 years) at the San Diego Cadaver Anatomy Research Symposium. Specimens were thawed to room temperature prior to the investigation.
The first surgeon placed the initial guide wire for screw placement, using only a lateral fluoroscopic image on a mini C-arm, in a dorsal to plantar approach. A second surgeon examined the cannulated screw position through a series of radiographic images including a calcaneal axial image, an AP image and a lateral image. The surgeons used that same method for all 10 specimens.