The Chihuahua is one of the smallest recognized canine breeds and one of the longest-lived, with a life expectancy that can exceed 15 years.

It is also a breed often acquired impulsively, rarely systematically orthopedically examined.

Veterinary literature, however, documents two distinct and very real problems: a marked predisposition to patellar luxation, which is hereditary and common, and nearly universal persistent fontanelles, with concrete clinical implications.

This article presents published data on these two pathologies, without alarmism, but without minimization either.The objective is to enable each owner to make informed decisions about follow-up and prevention.

Breed Profile

Key figures

Nilsson et al., Vet J, 2018 - PubMed 29680386 ; Kiviranta et al., JVIM, 2021 - PMC8295709 ; Nilsson et al., Vet J, 2018

Documented risks

Patellar luxation: the most well-documented predisposition

Medial patellar luxation is the most common orthopedic pathology in Chihuahuas. Several independent studies confirm this.

Prevalence

Nilsson et al. (Veterinary Journal, 2018 - PubMed 29680386) analyzed 7,024 Chihuahuas screened by the Swedish Kennel Club between 1999 and 2014.

The prevalence was 23%, or one in four dogs among those screened. Females had a higher risk than males.

The estimated heritability was 0.46, comparable to that of hip dysplasia in large breeds: the genetic component is strong and selectable.

O'Neill et al. (Canine Genetics and Epidemiology, 2016 - PMC4898461) reported a prevalence of 4.9% in primary care in the United Kingdom, in a cohort of 210,824 dogs.

The Chihuahua had an odds ratio of 5.9 compared to crossbred dogs, placing it second after the Pomeranian (OR 6.5).

The discrepancy between 23% and 4.9% is explained by the method: systematic screening captures all forms, including asymptomatic Grade I, while the clinical cohort only records diagnosed cases.

These figures are not contradictory...

Mechanism

Medial luxation results from an anomaly in the quadriceps axis: the patella does not remain in the trochlear groove of the femur but moves medially.

Breed-specific conformational anomalies have been identified on imaging, with implications for surgical planning if corrective osteotomy is considered.

Luxation is hereditary in 82% of cases reported in the literature for small breeds, with polygenic transmission.

Screening of breeding animals has shown modest but real effectiveness in reducing prevalence in Sweden.

Clinical grades

Grade I : intermittent luxation, often asymptomatic.

Grade II : characteristic skipping, the dog lifts the limb for a few seconds before spontaneous repositioning.

Grades III and IV : permanent or near-permanent luxation, with progressive deformity. Untreated luxation predisposes to early osteoarthritis and, in severe forms, to concomitant cranial cruciate ligament rupture.

Surgical treatment is indicated for symptomatic Grade II, III, and IV cases.

The prognosis is generally favorable, with a return to normal function in the vast majority of operated cases.

Persistent fontanelles: a nearly universal peculiarity

The bregmatic fontanelle (unossified area at the top of the skull) has been known in Chihuahuas for decades.

It even appears in some breed standards under the name "molera."

What recent work has clarified is how widespread it is, often multiple, and potentially associated with other abnormalities.

Prevalence and Location

Kiviranta et al. (JVIM, 2021 - PMC8295709) conducted the first systematic study on 50 adult Chihuahuas using CT imaging.

Result: 92% had at least one persistent fontanelle.

Among them, 38% had fontanelles in locations other than the classic bregmatic area.

Thus, the fontanelle is not an isolated anatomical point in some dogs.

The second part of the study (PMC8295681) showed that fontanelles were more numerous and larger in dogs with low body weight, and in those with syringomyelia, ventriculomegaly, or crowding of the craniocervical junction.

These associations do not prove direct causality but indicate that the fontanelle is not necessarily an isolated finding.

A 2025 study (Rautala et al., JVIM - PMC11836668) confirmed that persistent fontanelles are more associated with more pronounced brachycephaly and lower body weight, two characteristics that reflect breeding choices in the breed.

Fontanelle and Traumatic Risk

Gutierrez-Quintana et al. (JVIM, 2020 - PMC7517851) analyzed 91 dogs with traumatic skull fractures.

Chihuahuas and Yorkshire Terriers alone accounted for nearly 20% of cases.

The authors emphasize that the predisposition of Chihuahuas to persistent fontanelles may contribute to greater cranial fragility than would be expected for their size, including during low-intensity trauma.

Is a Fontanelle Always Pathological?

No.

The vast majority of Chihuahuas with persistent fontanelle(s) live without neurological symptoms related to this peculiarity.

What can become problematic is the co-occurrence of multiple large fontanelles with other cranio-morphological abnormalities (notably Chiari-like malformation and syringomyelia).

Warning Signs to Know

What You Can Do Practically

Before Adoption or Purchase

• Require orthopedic screening status for both parents for patellar luxation. The Swedish Kennel Club has shown that systematic selection reduces prevalence.

• Ask for information on the neurological history of the lineage (seizures, gait disorders).

• Avoid puppies from very small parents, with very round skulls and very low weight: these aesthetic criteria promote the concentration of large fontanelles and associated malformations.

After Adoption

• Have the hind limbs evaluated by the veterinarian in the first few months. Asymptomatic luxation can be identified by palpation during a routine consultation.
• Protect the skull: avoid falls from height, rough contact games, unsupervised handling. The traumatic risk is higher than average due to fontanelles.
• Do not minimize skipping on a hind limb: this sign is indicative of patellar luxation and warrants veterinary evaluation.
• In case of diagnosed luxation: discuss with the veterinarian the grade, appropriate treatment, and weight management, as overweight exacerbates mechanical stress on an unstable knee.
• Implement an annual veterinary check-up from 5 years of age, with neurological evaluation if evocative behavioral signs appear.

________________________________________________________________________________________________

Sources

1. Nilsson K., Zanders S., Malm S. (2018). Heritability of patellar luxation in the Chihuahua and Bichon Frise breeds of dogs and effectiveness of a Swedish screening programme. Veterinary Journal, 234 : 136–141. PubMed 29680386
2. O'Neill D.G. et al. (2016). The epidemiology of patellar luxation in dogs attending primary-care veterinary practices in England. Canine Genetics and Epidemiology, 3 : 4. PMC4898461 / PubMed 27280025
3. Kiviranta A.-M. et al. (2021). Persistent fontanelles in Chihuahuas. Part I. Distribution and clinical relevance. Journal of Veterinary Internal Medicine, 35 : 1760–1770. PMC8295709 / PubMed 34028887
4. Kiviranta A.-M. et al. (2021). Persistent fontanelles in Chihuahuas. Part II : Association with craniocervical junction abnormalities, syringomyelia, and ventricular volume. Journal of Veterinary Internal Medicine, 35 : 1771–1780. PMC8295681 / PubMed 33939205
5. Rautala E. et al. (2025). Association of extreme brachycephaly with persistent fontanelles in adult Chihuahuas. Journal of Veterinary Internal Medicine, 39(2). PMC11836668
6. Gutierrez-Quintana R. et al. (2020). Traumatic skull fractures in dogs and cats : a comparative analysis of neurological and computed tomographic features. Journal of Veterinary Internal Medicine, 34 : 2370–2380. PMC7517851