Originally Posted by pstinard  To add to the conversation, I found this 2010 research article. The meatiest parts of the article are copied below. Parts of the article that I did not copy are represented by [...]. I bolded the parts that are related to spaying and/or Yorkshire Terriers, as well as the main conclusions. With respect to the effect of spaying on the development of luxating patella and CCL rupture, the authors note an increase in the rate of these two conditions in spayed dogs. HOWEVER, they do not draw any conclusions as to cause and effect. Clearly, more research is needed to determine if there is cause and effect, and if so, what the underlying mechanism is.
Severity of patellar luxation and frequency of concomitant cranial cruciate ligament rupture in dogs: 162 cases (2004–2007). Courtney A. Campbell, dvm; Christopher L. Horstman, dvm, ms, dacvs; David R. Mason, bvetmed, dacvs; Richard B. Evans, phd. AVMA, Vol 236, No. 8, April 15, 2010.
Summary:
Objective—To evaluate severity of medial patellar luxation (MPL) and frequency of con- comitant cranial cruciate ligament rupture (CCLR) in dogs.
Design—Retrospective case series.
Animals—162 dogs (266 stifle joints).
Procedures—Medical records of 162 small-breed dogs with MPL were reviewed. Signalment, body weight, luxation grade, bilateral or unilateral MPL, CCLR, and difference in luxation grades between stifle joints were evaluated. Association between severity of MPL and CCLR was investigated.
Results—58 dogs had unilateral MPL, and 104 dogs had bilateral MPL. Dogs ranged from 8.4 months to 16.7 years of age (mean, 5.7 years), and mean body weight was 5.45 kg (12 lb). Forty-one percent of all dogs had concomitant CCLR. Mean age for dogs with MPL alone was 3.0 years, which differed significantly from mean age of dogs with MPL and concomitant CCLR (7.8 years). Dogs with grade IV MPL were significantly more likely to have concomitant CCLR than were dogs with any other grade of MPL. In dogs with bilateral MPL and unilateral CCLR, there was a significantly higher grade of luxation in the stifle joint with CCLR.
Conclusions and Clinical Relevance—Small-breed dogs with MPL and concomitant CCLR were older than were dogs with only MPL. Dogs with grade IV MPL were more likely to have CCLR than were dogs with other grades of MPL. Most dogs with concomitant CCLR had a higher MPL grade in the affected stifle joint than in the intact joint. These findings should be beneficial in client education and clinical diagnosis. (J Am Vet Med Assoc 2010;236:887–891)
Introduction
Medial patellar luxation is one of the most common conditions affecting the stifle joint in dogs.1–4 Small-breed dogs are 12 times as likely to be affected by MPL as are large-breed dogs.5,6 Breed predilections have been reported for the Boston Terrier, Chihuahua, Pomeranian, Miniature Poodle, and Yorkshire Terrier.1,4,6 In these breeds as well as in others, luxation of the patella is primarily a developmental condition, with traumatic luxation being less common.3,7 A low-grade MPL may not result in clinical signs and frequently is an incidental finding during physical examination.3,7
Similarly, CCLR is a common disease affecting the stifle joint in dogs.3,7–9 Cruciate ligament disease has been recognized in both large- and small-breed dogs, with dogs of the Miniature Poodle, Lhasa Apso, Maltese, and Pomeranian breeds being the most frequently affected small-breed dogs.3,7,9
Definitive causes for MPL and CCLR have not been elucidated; however, it has been suggested that they have different causes.1–3,5 Classically, skeletal abnormalities associated with MPL are a shallow trochlear sulcus and medial displacement of the tibial tuberosity.2,3,5 Other skeletal abnormalities suggested to contribute to this condition are genu varum, hypoplasia of the medial femoral condyle, medial bowing of the proximal portion of the tibia, coxa vara, and internal rotation of the pes.3,7,8,10 Many causes of CCLR have been investigated, and common etiopathogeneses include trauma, age-associated degeneration of the ligament, immune- mediated disease, conformational abnormalities, and processes associated with breed, sex, and tibial plateau angle.3,7,8
Patients with bilateral MPL may have clinical signs in a chronic or intermittent manner or may be com- pletely devoid of clinical signs of the condition.3,7,11 When a dog with chronic MPL develops an acute hind limb lameness, concomitant CCLR should be considered.3,7 The suggested pathogenesis for dogs with MPL that develop concomitant CCLR is an increase in strain on the ligament as a result of anatomic abnormalities associated with MPL.3,7,12 Conversely, investigators have hypothesized that dogs with CCLR with no previous history of an MPL may acquire an MPL as a result of the increased internal rotation of the tibia once the cranial cruciate ligament has ruptured.3,12
To our knowledge, there are no published studies confirming an association between the grade of MPL and frequency of CCLR in small-breed dogs. The objective of the study reported here was to determine whether there was a relationship between the grade of MPL and frequency of CCLR in a population of dogs. We hypothesized that dogs with an increase in the grade of MPL would be more likely to have a concomitant CCLR and that the grade of MPL would be greater in the stifle joint with the concomitant CCLR.
[...]
Discussion
The most common breeds affected with MPL in the study reported here were the Chihuahua and Yorkshire Terrier, which corroborates results of a report14 in which there was overrepresentation of Chihuahuas. In another study,4 the Miniature Poodle was the breed most commonly affected. This finding may be explained by the hospital population evaluated in our study.
The sex distribution for patellar luxation in the present study was a male-to-female ratio of 1:1.3, which is consistent with the ratio of 1:1.5 in small-breed dogs reported in other studies4,7,14 but is in contrast to the sex distribution (male-to-female ratio, 1.8:1) reported in large-breed dogs.4,15 In the present study, spayed or neutered dogs were more likely to have MPL. These findings are in agreement with those of another report16 in which investigators detected a higher incidence of patellar luxation in spayed females.16 The findings of the present study may be attributable to our study population, an increase in spaying and neutering, or a true increase in the prevalence of MPL in dogs that have been spayed or neutered. We believe that our study population is representative of that at most referral institutions because there is a great number of similarities between our study population and the study populations in other investigations.
In the study reported here, we hypothesized that higher grades of MPL would be associated with an increase in the frequency of concomitant CCLR. We found that dogs with grade IV MPL were significantly (P = 0.02) more likely to have concomitant CCLR than were dogs with all other grades of luxation.
[...]
To our knowledge, there have been no reports of the small-breed dogs most commonly affected or the sex of dogs with MPL and concomitant CCLR. In the study reported here, the Yorkshire Terrier and spayed female dogs were slightly overrepresented. The study revealed a significant association between age and concomitant CCLR. Mean age at which dogs with concomitant CCLR were identified (7.8 years) was significantly higher than the mean age of dogs in which MPL alone was identified (3.0 years). This is consistent with results of studies3,4 in which middle-aged to older dogs with patellar luxation appeared to be at an increased risk of developing CCLR. The overall incidence of MPL with concomitant CCLR of 25% in the study reported here was higher than that in other reports4,15,24,25; however, those studies did not have strict exclusionary criteria with regard to breed.
[...]
In conclusion, middle-aged to older dogs and dogs with grade IV MPL were at an increased risk for developing CCLR. Dogs with bilateral patellar luxation and concomitant unilateral CCLR were more likely to have a higher score for grade of luxation in the cranial cruciate ligament–deficient stifle joint. Studies are warranted to evaluate the reproducibility of the results reported here, in addition to results for follow-up monitoring, radiography, physical examination findings, and other factors that influence MPL and concomitant CCLR. |