Current Insights On Internal Fixation Of Subtalar Fusions
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.