Current Insights On Internal Fixation Of Subtalar Fusions
Sharing findings from the literature and pearls from their own experience, these authors examine pertinent considerations and keys to facilitating optimal placement of internal screw fixation in subtalar joint arthrodesis procedures.
The subtalar joint (STJ) arthrodesis is a well-documented procedure in elective reconstructive hindfoot surgery, for pain relief in cases of subtalar joint arthrosis and occasionally in the treatment of highly comminuted traumatic fractures. In addition, the subtalar joint fusion may be beneficial in the treatment of talocalcaneal coalitions, adult acquired flatfoot, posterior tibial tendon dysfunction, Charcot neuroarthropathy, and hindfoot varus or valgus deformity.1-10
There are many technical pearls to performing this arthrodesis successfully. One of these pearls involves the fixation construct. Surgeons most commonly utilize large diameter screws for internal fixation of this arthrodesis. Still, there is variety to the number and size of screws one selects as well as the orientation in which the surgeon places these screws.
One can access the subtalar joint from either the traditional lateral approach over the sinus tarsi or from a medial approach between the posterior tibial and flexor digitorum longus tendons. For the lateral approach, make an incision two fingerbreadths inferior to the distal fibula, extending distally to just proximal to the fourth metatarsal base. During the dissection through the subcutaneous tissue, keep aware of the local sural nerve, which is inferior to the incision.
Retract the peroneal tendons inferiorly. Leave the extensor digitorum muscle belly in its fascial layer and reflect it dorsally. Evacuate the sinus tarsi, allowing access in the middle facet of the subtalar joint. Using a sharp osteotomy, gently release the interosseous ligament and work posterior to the posterior facet. The use of a lamina spreader allows direct visualization of the posterior facet. Remove the cartilage with a series of osteotomes and curettes. Then fenestrate the subchondral bone with a 2.5 mm drill bit and pulverize the subchondral bone with a ¼-inch curved osteotome.
The medial approach utilizes the interval between the posterior tibial tendon and flexor digitorum longus tendon. One can readily view the inferior aspect of the talar head and follow the plantar joint line posterior to the middle facet of the subtalar joint. Careful identification of the inferior extensions of the deltoid ligament is important for later repair if release is necessary for exposure. Joint preparation and takedown are identical to the lateral approach.
Appropriate positioning of the hindfoot is crucial to this procedure. A neutral or mild valgus of the hindfoot is the ideal position. When it comes to deformity correction, one frequently needs to lateralize the talus back up on top of the calcaneus in a scissors fashion to correct medial collapse of the hindfoot.
Key Considerations With Performing The ‘Top-Down’ And ‘Bottom-Up’ Approaches
In regard to the anterior to posterior subtalar joint screw placement (aka the “top-down approach”), this technique is generally easier if the patient is supine and there is one surgeon who does not need lifting help for the leg. Holding the hindfoot in the non-dominant hand will allow gentle and accurate correction while placing the screw dorsal to plantar. The anatomical landmarks are just lateral to the tibialis anterior tendon and directed at an oblique angle toward the lateral wall of the calcaneus.
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.
We measured the calcaneus at the level of the posterior facet, dividing the measurement in half to ascertain the midline of the calcaneus. We again divided the now medial and lateral quadrants of the calcaneus in half, respectively identifying distances one and two deviations from midline. Radiographic measurements determined the location of the distal aspect of the screw to be either centrally, medially or laterally deviated. If we detected a deviation, we recorded the extent of variation. The first surgeon performed all measurements using the same ruler in order to ensure accuracy.
Ten percent of the 10 screw placements were within the midline aspect of the calcaneus. Forty percent of the screw placements were medially deviated. Twenty percent were located within one medial deviation of midline, 10 percent were within two medial deviations from midline and 10 percent were located within the medial cortex. Finally, 60 percent of the screw placements were laterally deviated. Ten percent were within one lateral deviation from midline, 20 percent were two lateral deviations from midline and 20 percent were within the lateral cortex of the calcaneus.
As we see from these results, though there was some variation with screw placement in relation to the midline, all screws stayed in bone without any medial or lateral cutouts. With the dorsal to plantar approach, there is more room for variance of screw trajectory due to the larger area of the calcaneus in comparison to the planar to dorsal approach into the talus.
Weighing The Options For Effective Screw Placement
When performing a subtalar joint arthrodesis, accurate placement of the internal fixation is imperative because improper positioning of screws can lead to poor stabilization, nonunion and malalignment, ultimately affecting the function of the foot and the entire lower extremity.11-13 When the screw is not centrally located, this may cause an increase in medial or lateral compression with a gapping on the ipsilateral side. This gapping may delay complete union of the planned subtalar joint arthrodesis or even nonunion. In regard to the use of internal screw fixation in subtler joint arthrodesis procedures, nonunion rates range from 2 to 30 percent in the literature.14,15 To our knowledge, there has been no documented literature revealing the “ideal” placement of the screw into the calcaneus.
The benefits of performing the anterior to posterior approach include ease of placement and the ability to use the long thread pattern of a partially threaded screw via the lag by design concept. Finley and co-workers demonstrated the significance of optimizing contact between the screw and the bone in order to enhance the pullout strength.16 In a study by McGlamry and Robitaille, the anterior to posterior screw approach had a higher torque of insertion and pullout strength/failure load when using the cannulated system in comparison to the posterior to anterior screw approach.11,12,16,17 Researchers have documented that experimental pullout force is highly correlated to the predicted shear failure load.11,12,16,17 Therefore, pullout strength is controlled by the core diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded and a thread shape factor (which accounts for screw thread depth and pitch).
In contrast to those findings, tapping reduced pullout strength by 8 percent in compasrison to non-tapped guide holes.18 Pullout forces utilizing the anterior to posterior screw orientation had a larger mean failure load in comparison to the pullout forces using the posterior to anterior approach. Specifically, mean failure load for the anterior to posterior approach was 1782 N whereas the mean failure load for the posterior to anterior screw orientation was 1245 N.12 Chapman and colleagues proved that the best screw placement is at the entrance point where the cortex is the thickest.18 Thick cortical bone has been critical in assisting with stabilization of internal fixation. Indeed, the thick cortical bone of the talus is another advantage for the anterior to posterior approach.
One can inadequately assess deviations medially or laterally from the central aspect of the calcaneus when utilizing only the intraoperative lateral radiographic view. Accordingly, the calcaneal axial view is required to confirm appropriate placement of the guide wire into the central aspect of the calcaneus prior to screw placement. This tip is critically important in the posterior to anterior screw placement as the landing zone is small. Though our cadaver study demonstrated the “top down” screw to be reproducible and generally central into the calcaneus, we still recommend the additional imaging view.
Dr. Scott is a Fellow of the Orthopedic Foot and Ankle Center in Westerville, Ohio. He is an Associate of the American College of Foot and Ankle Surgeons.
Dr. Hyer is the Fellowship Director of Orthopedic Foot and Ankle Center in Westerville, Ohio. He is a Fellow of the American College of Foot and Ankle Surgeons.
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