Lateral luxation of the tendon of the superficial digital flexor (SDF) muscle - (hock injury)
The SDF muscle is one that is responsible for flexion of the hind toes. Its tendon is part of the Achilles' or calcanean tendon group at the hock. Luxation of the SDF tendon causes intermittent hind leg lameness and swelling around the point of the hock. Injury may occur during vigorous activity such as herding or agility. Abnormal shape of the point of the hock (calcaneus bone) may be a predisposing factor [1]. Treatment for most affected dogs is surgical correction. (Two websites describing the condition and treatment: Repair of Repair of SDFT Injury 1 and Repair of SDFT Injury 2).
Shetland Sheepdogs and Collies are over represented in breeds with this condition. A 2020 report on from Sweden on surgical repair of affected dogs found that 14 of the 19 dogs were Shelties. [2]. A 2002 study of over 200 Shelties found that SDF tendon luxation in Finnish Shetland Sheepdogs was likely inherited as a simple autosomal recessive trait. [1]
In 2022, researchers at the Institute of Genetics, University of Bern, Switzerland were studying the condition [3]. They performed a genome wide association study (GWAS) using DNA samples from 20 cases and 41 controls in order to search for genetic risk loci that might influence the risk for lateral luxation of the tendon of the SDF muscle. This analysis yielded no significant association signal. They would need more DNA samples from affected and control dogs to future to increase the power of the GWAS. Because of a shortage of personnel, the study is paused for at least 1 year.
Additional articles regarding this condition in Shelties can be found at: https://www.americanshetlandsheepdogassociation.org/health-references/#ortho .
[1] Solanti S, Laitinen O, Atroshi F: Hereditary and clinical characteristics of lateral luxation of the superficial digital flexor tendon in Shetland sheepdogs. Vet Ther. 2002 Spring;3(1):97-103. PMID: 12050833. https://www.vetfolio.com/learn/article/hereditary-and-clinical-characteristics-of-lateral-luxation-of-the-superficial-digital-flexor-tendon-in-shetland-sheepdogs
[2] Jury A: Surgical repair for luxation of the superficial digital flexor tendon using a temporary restraining pin. J Small Anim Pract. 2021; 62: 272-278. https://doi.org/10.1111/jsap.13288