The lens which is situated in the middle of the eye behind the pupil, changes its shape to allow the eye to focus. This is achieved by tightening of fine ligaments called zonules attached around the perimeter of the lens which suspend it within the eye. When these zonules are damaged or broken the lens can luxate (shift out of place).

 The lens may luxate backwards into the posterior chamber or forward into the anterior chamber.

Early clinical signs, include inflammation within the eye (uveitis), and pain with the surrounding white area of they eye (sclera) becoming inflamed and red in appearance.
The pressure within the eye can increase producing swelling of the eye (glaucoma) especially with anterior luxations. If the lens falls forward onto the cornea (the clear surface of the eye) then it can cause swelling (edema) and a blue-cloudy appearance at the site of contact.
The lens is often visible in front of the eye with anterior luxations whereas with posterior luxations the anterior chamber appears deepened and the coloured part (iris) may move abnormally.

Causes of lens luxation include trauma; disease within the eye including tumors, glaucoma and occasionally cataracts; or a genetically inherited weakness in the zonules, Border Collies, any Terrier breeds, Poodles and the Australian Cattle Dog are all known to have an increased risk of developing lens luxation.

Treatment of lens luxation will nearly always require surgery to remove the lens. If the problem has been identified early enough vision may be saved, although somewhat diminished after the lens is removed. If vision is already lost then options such as eye removal or eye prosthesis may be considered. If one eye is affected then there is a higher risk in the future of the other eye developing a luxation.

Dogs affected with primary lens luxation should not be used for breeding. Since this is a late onset disease, the breeder may encounter a situation where a dog who has already produced a number of litters develops a lens luxation. This is disconcerting since it would be advisable not to use any of the second generation for breeding either.

 Routine eye certification examinations will not, in most cases, detect a dog pre-disposed to lens luxation, unless the ligaments have already started to weaken and a slight wobble of the lens is detected by the ophthalmologist during the examination.