Chemoprotection with botulinum toxin following proximal hamstring-Ischial tuberosity avulsion fracture repair: Running title: Chemoprotection for hamstring avulsion fractures.

Introduction: Surgical treatment for avulsion injuries of the proximal hamstrings has gained increasing popularity over the past decade. Despite good outcomes, early failures have been noted and have been attributed to slipping and falling, postoperative muscle spasm, or early mobilization. In a recent review of hamstring repair rehabilitation protocols, it was shown that there is marked variability in post-operative management. Post-operative bracing with limiting knee extension and hip flexion is the standard of care in most early rehabilitation protocols. Braces with limitation of hip flexion and knee locked in 900 flexion can be awkward, cumbersome and create fall risk.Chemoprotection has more recently been proposed to be an alternative approach to prevent tendon repair failure and controlled mobilization which has been shown to be superior to complete immobilization. We present the first case series of the use of botulinum toxin for chemo-protection of the proximal hamstring ischial avulsion repair, demonstrating its safety and efficacy.

Methods: Retrospective case series at a tertiary children's hospital which included patients <18 years of age who underwent interventional treatment for proximal hamstring avulsion injuries of the ischium utilizing botulinum toxin as a chemoprotective agent. Data collected included demographic data, injury and treatment details, imaging, post-operative rehabilitation and return to activity. Descriptive statistical analysis was conducted.

Results: Five male patients with mean age 14 years (12-17) were included in the study. All were sports related non-contact injuries. Radiographs showed displaced avulsion fractures in all 5 patients. All patients had failed conservative management initially; mean time to surgery from initial injury was 34.4 weeks. 4 patients underwent open reduction and internal fixation (ORIF), 1 patient with less displacement had bone marrow aspirate (BMA) injection; all had chemoprotection using botulinum toxin injected in the hamstrings. No patient required hip immobilization or knee immobilization locked to 90°. We elected to use a brace locked at 20° knee flexion in 2/5 patients. All patients underwent supervised physical therapy and achieved symmetric knee range of motion (ROM). Post-operative radiographs confirmed healing of the avulsion fracture in all 5 patients and they all returned to previous level of activity at mean 32 weeks (21-43) from surgery. None of the patients had a hamstring re-injury at mean follow up of 27 months (11-42).

Conclusion: Our case series is the first in literature that shows the safety and efficacy of chemoprotection with botulinum toxin for the post-operative management of avulsion injuries of proximal hamstrings, by minimizing the need for cumbersome bracing and allowing controlled motion during physical therapy.