INTRODUCTION: There is no comparative evidence for relative motion extension (RME) orthosis with dynamic wrist-hand-finger-orthosis (WHFO) management of zones V-VI extensor tendon repairs. PURPOSE OF THE STUDY: To determine if RME with wrist-hand-orthosis (RME plus) is noninferior to dynamic WHFO for these zones in clinical outcomes. STUDY DESIGN: Randomized controlled non-inferiority trial. METHODS: Skilled hand therapists managed 37 participants (95% male; mean age 39 years, SD 18) with repaired zones V-VI extensor tendons randomized to RME plus (n = 19) or dynamic WHFO (n = 18). The primary outcome of percentage of total active motion (%TAM) and secondary outcomes of satisfaction, function, and quality of life were measured at week-6 and -12 postoperatively; percentage grip strength (%Grip), complication rates, and cost data at week-12. Following the intention-to-treat principle non-inferiority was assessed using linear regression analysis (5% significance) and adjusted for injury complexity factors with an analysis of costs performed. RESULTS: RME plus was noninferior for %TAM at week-6 (adjusted estimates 2.5; 95% CI -9.0 to 14.0), %TAM at week-12 (0.3; -6.8 to 7.5), therapy satisfaction at week-6 and -12, and orthosis satisfaction, QuickDASH, and %Grip at week-12. Per protocol analysis yielded 2 tendon ruptures in the RME plus orthoses and 1 in the dynamic WHFO. There were no differences in health system and societal cost, or quality-adjusted life years. DISCUSSION: RME plus orthosis wearers had greater injury complexity than those in dynamic WHFOs, with overall rupture rate for both groups comparatively more than reported by others; however, percentage %TAM was comparable. The number of participants needed was underestimated, so risk of chance findings should be considered. CONCLUSIONS: RME plus management of finger zones V-VI extensor tendon repairs is non-inferior to dynamic WHFO in %TAM, therapy and orthotic satisfaction, QuickDASH, and %Grip. Major costs associated with this injury are related to lost work time.