The definition of rein lameness is difficult because it’s a lameness that can be appreciated only when a horse is being exercised under saddle or wearing a bridle and being exercised. A link between TMJ inflammation and lameness has been studied by James L. Carmalt, MA, VetMB, MVetSc, PhD, FRCVS, DABVP(Eq), DAVDC(Eq), DACVSMR(Eq), DACVSe, and his colleagues.
If you have TMJ inflammation, horses can have dramatic responses to rein input without being classically lame . . . which makes sense . . . the horse is trying to avoid pain”
Source: Modern Equine Vet, August 2022, pages 4-5. Link. Despite the current interest in TMJ disease, it is important to rule out the more common causes of lameness, such as foot pain, or bone spavin before looking at TMJ problems.