Is Subtalar Fusion Affected by Screw Number and Angle?

Background:
Subtalar arthrodesis (SA) is routinely used for fusion of a painful subtalar joint or fixing underlying deformity, therefore decreasing chronic pain, improving gait and change the overall foot structure. While fusion is achievable, some cases will lead to nonunion. This can be attributable to patients’ factors (pre-existing comorbidities, obesity, smoking etc.) or intra-operative factors (inadequate joint preparation or fixation). Our goal was to investigate the mechanical factors which influence the operation`s results, primarily whether 1 vs 2-screw fixation and the angle effects joint union.

Patients and Methods:
A retrospective computerized cohort study was preformed and demographic and surgical information was obtained. 92 patients who underwent subtalar fixation between 2005-2019 were included. One patient was lost to follow-up.

67% were males, 35.9% patients had pre-existing comorbidities (7.6% DM, 37% smokers). Post-traumatic OA was the most common etiology (44.6%). 64.1% underwent isolated SA while 35.9% underwent Triple Arthrodesis. 33.7% had one screw fixation whereas 66.3% had 2 screws fixation. Of the 2 screws fixation group, 42.6% were fixated by parallel screws and 57.4% were fixated by divergent screws.
The general union rate was 89%, with no substantial difference between the groups (1 to 2 screw fixation,90.3% vs. 88.3%, P=NS, parallel to divergent screws 92.3% vs. 85.3%, P=NS). Complication rate was 7.6% (1 to 2 screws, 9.7% vs. 6.6%, P=NS, parallel to divergent 3.8% vs. 8.6%, P=NS). Removal of hardware was done in 16.3% (1 to 2 screws, 12.9% vs. 18%, P=NS, parallel to divergent 15.4% vs. 20%, P=NS).
14.7% of the smokers had complications when only 3.4% of the non-smokers had complications (P value = 0.096).

Conclusions:
No correlation was found between the screw`s number and screw`s angle to union rates, complications or screw removal. There might be an association between smoking and postoperative complications.