I've been thinking and reading a lot about luxating patella (LP).
It is not a straightforward condition and two parents with normal knees can produce puppies with LP just as a parent with LP can produce puppies with normal knees. It is a complex condition of polygenic inheritance and environmental forces. I've been reading every study I can find on it as well as talking to veterinary orthopedists and I'll summarize what I've learned so far:
1. There does seem to be a heritable component, but it is lower than one would think. In small breeds studied that range is up to 0.25, meaning up to 25% of the CAUSE of LP can be explained by genetic variability and the other 75% of the CAUSE can be explained by environmental factors. Read this carefully. This doesn't mean that 25% of cases of LPs are caused by genetics, it means that up to 25% of the cause of a case of LP is because of genetic variability. Again, read that carefully to understand, and once you do, the cause of LP is even less straightforward than you would think.
2. In the case of genetically caused LP (as opposed to trauma-induced) several defects of the hind limb are involved in the pathogenesis of LP. An important one is the angle of the trochlear groove which stabilizes the patella - as you can imagine the deeper and narrower this angle, the better it can hold in the patella. But in Bulldogs, for example, a 2021 study determined that the width of their pelvis causes a medial sideway pull on their growing femoris muscles which causes plastic deformation that leads to LP! This is not straightforward at all, and likely doesn't apply to Papillons, but it makes you wonder if the root genetic cause of many LPs in Papillons is not so straightforward either.
3. Manual testing for LP by a vet on the 1-4 grading scale is not terribly useful. Multiple studies have found that the results of this manual test are highly dependent on the skill and subjectivity of the vet and age of the dog. Some vets perform the test with too much force and sometimes even cause the LP, while others do not perform it hard enough. To understand why, most vets receive only about an hour's worth of instruction on how to perform this test while in school. One Finnish study found a sided bias of determining LP, meaning that whether the vet was left or right handed determined which side they more frequently decided was affected by LP!
4. Radiography and orthopedics is not currently a tool we can use as breeders for selecting away from LP. As I said above, the root cause of the Papillon-specific genetic cause of LP is not known. But if it did come down to the trochlear groove, there is currently no accepted method of measuring the depth and angle on x-ray and saying what is a good angle or cutoff angle. The only thing we can currently use x-ray for is to determine whether the dog already has LP, rather than whether it has morphology that can cause it.
5. Multiple studies have determined that breeding away from LP by excluding affected dogs is not working well. A 2013 study on flat coated retrievers affected by lateral LP (keep in mind Papillons are more often dx'd with medial LP) found that screening for and eliminating affected dogs initially brought the population prevalence down from 28% to 18% but it has stagnated since then, meaning the scheme is no longer improving the breeding population. Why? In fact, in several commonly affected breeds studied the problem is becoming more ubiquitous now despite screening. LP is also becoming a problem in breeds that previously have not had this issue. Perhaps breeding populations are inadvertently selecting for morphology that predisposes LP because of changing breed standards, or environmental factors are changing, or both, or other.
6. Ultimately LP happens when there is a misalignment of the quadriceps action. The depth and angle of the trochlear groove is just a component. For each case of LP that needs to be surgically repaired there will be different approaches depending on the rear morphology. There is not a one-size-fits all surgical fix for the root of the problem.
Those are all the thoughts I can gather for now and I will add more as I learn more.