Background: Ankle sprain is the most frequently occurring lower limb injury among athletes and physically active individuals. There are no specific treatment guidelines for acute lateral ankle sprain. The purpose of the study is to find the functional outcome of a stirrup ankle brace versus a below-knee posterior slab in the management of acute mild to moderate lateral ankle sprain.

Methods: This is a hospital-based prospective comparative study done at Universal College of Medical Science between January 29, 2025, and July 29, 2025. A total of 60 patients with grade I and grade II ankle sprain diagnosed clinically were included; 30 patients were treated with a stirrup ankle brace and 30 with a below-knee posterior slab. Final follow-up was done at the end of the third week; the primary outcome was measured with the modified Karlsson score and secondary outcomes with the VAS score and measurement of ankle swelling.

Results: The mean age of patients was 31.05±13.01 years. The most common mechanism was sports-related twisting injury 30(50%). The change in girth of the ankle between the ankle stirrup and slab groups at the third week was 0.2±0.11 cm and 0.29±0.06 cm, p = 0.001. The Modified Karlsson score of the stirrup ankle brace group was 81.86±1.77, and the below-knee posterior slab was 80.0±2.28, P=0.001 at third-week follow-up, respectively.

Conclusion: Although both treatment options are effective, functional outcome is better seen with a stirrup ankle brace than a below-knee slab.

Keywords: Ankle sprain; below-knee posterior slab; Stirrup ankle brace