HD statistics

HD Statistics PP and OFA

by Anne Marit Berge

Maine Coon is one of the cat breeds that are at risk for FHD – feline hip dysplasia. Breeders are recommended to x-ray their cats before breeding, and select for better hips. We see that the number of MCO tested for HD has increased the last five years. This could be an indication of increased awareness, perhaps also due to the discussion about HD after the FIFe General Assembly 2019. Let us take a look at some HD statistics and what numbers can tell us.

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For a general overview of HD in Maine Coon, see the main HD page

Dysplastic HD vs Normal hips:

Two evaluation systems: PP and OFA

There are two internationally, research based evaluation systems for assessing radiographs for FHD: PawPeds and OFA.

OFA, The Orthopedic Foundation for Animals is based in Missouri, USA and has been scoring dog hips since 1966. They are essentially a health service for dog breeders, but Maine Coon cats have been included in their hip evaluation since 1988.

PawPeds is an independent organization based in Sweden, and the HD health programme was started in 2000 through the Swedish Maine Coon club. It is also open for other cat breeds.

The two evaluation systems have in total assessed 11418 hips of Maine Coon cats (by 31 Des 2023), and the number is growing. This gives broad and significant statistics of prevalence.


PawPeds health program Hip Dysplasia

The PP database has 6834 HD results for Maine Coon per 31 Dec 2023. Images are uploaded to PP by the veterinary digitally, and evaluation cost 150 SEK (ca 13 €). Online registration to fill in for each cat.

All hip evaluations in PP are done by one veterinary radiologist. Since 2020 this is Elisabeth Ball. PP uses a 4-degree scale to assess feline hips: Normal, Grade 1, Grade 2 and Grade 3. Until around 2014 PP also included Borderline in their evaluation. Each hip is evaluated independently and results given for Left and Right hip.

As a health program for breeders, PawPeds publish all HD results in their public pedigree database so results can also be seen related to all pedigree lines. They also give clear breeding recommendations: Grade 2 and 3 should not be used in breeding, Grade 1 only mated to Normal hips.

Overview of the number of evaluations done by the three orthopedists engaged by PawPeds since 2000. Results with different degrees for left and right are listed with the worst result.


In the overview below, all HD results in the PP database are listed by year and degree. For simplicity, Borderline one side is listed together with Normal, and Borderline both sides is listed with Grade 1. As you see from the overview above, the number of borderline results is very low, and does not effect the overall impression much. Click image to open in larger format.

hd-statistics PawPeds degrees by year
Many breeders have claimed that the evaluations after Audell retired are more strict and give a higher degree of FHD than before. The statistics of all PP evaluations can be interpreted that way. On the other hand, more cats with unknown background of hip status are x-rayed in recent years, thus with a higher risk of HD than cats from tested lines. Also more breeders working with “show lines” and higher inbreeding coefficients now test. As HD is polygenetic recessive, with likely hundreds of gene variants involved, lower inbreeding will lessen the risk of doubling bad mutations, whereas higher inbreeding will increase risk of doubling them.

There also seem to be an increase of mild HD, Grade 1, and a possible explanation also seen in the kennel clubs, is that modern digital radiography with higher quality images will reveal small changes of the hip that old x-rays did not.


OFA hip scores Maine Coon Cat

The OFA database has 4584 HD results for Maine Coon per 31 Dec 2023. Images can be sent in the mail, or uploaded digitally if the veterinary is pre-approved by OFA to do so. Evaluations cost 45 USD (for over 24 months age, ca 42 €) or 35 USD (preliminary readings, ca 33 €). Submission form download.

OFA evaluation is based on a 7-degree scale to assess hips. Three degreees are considered non-dysplastic (Excellent, Good, Fair) and three are considered dysplastic (Mild, Moderate, Severe). For ambiguous results, Borderline is used.

For cats under 24 months of age a preliminary result is given. The evaluation is done by one of their in-house orthopedists.

For cats over 24 months of age the images are evaluated by three orthopedists, chosen from a pool of 20-45 collaborating specialists all over the USA (and possibly other countries also). The final score is a median of the three evaluations. OFA is mainly a registry, a database, and do not provide specific breeding recommendations apart from breed normal hips.

The different numbers for preliminary evaluations of cats younger than 24 months, and cats older than 24 months:

hd-statistics OFA degrees

We see from their numbers that most cats are evaluated before they are 2 years old, and are given preliminary results. The OFA guidelines state that Preliminary Fair, Preliminary Borderline and Preliminary Mild should wait with any matings, and be x-rayed again after 24 months’ age and then evaluated by three orthopedists to get a final result. Accordingly results of Preliminary Excellent and Preliminary Good are considered OK for breeding without needing new testing (Corley 1997).

The figure below shows all OFA MCO HD results sorted by degree of evaluation, preliminary results and final/over 24 months grouped side by side with the same colour. Preliminary Fair is marked yellow, caution, as OFA recommends not breeding until a new final evaluation done by a panel of three orthopedists. Note that results are listed as most recent (left) to earliest. Click image to open in larger format.

hd-statistics of all evaluations by OFAThe overview shows that the number of evaluations is increasing, and that most cats are well within accepted results for breeding.

Debbie Sprenger created an alternative and more visually informative overview, based on the same numbers, with stacked columns showing degrees, here from earliest (left) to most recent results:

hd-statistics preliminary-final OFA
This clearly shows how the majority of evaluations are preliminary, cats younger than 24 months. It seems most Preliminary Fair and Preliminary Mild results do not get new x-rays, which is the OFA recommendation.

hd-statistic OFAThe blue lines show the relation between total number of x-rayed cats (light blue) and those in the public database. We see that only around 25-40 % of all HD evaluations in OFA are made public, and the percentage of public results has been lower in recent years.

The purple lines show the difference between cats younger than 24 months (lilac) and cats with a final evaluation over 24 months (dark purple). Years with more preliminary results also have less public results. Graph by Debbie.


OFA-PP comparison and statistics

The next graph shows an overview of all HD readings in PawPeds (orange) and OFA (dark blue for public database, light blue for total numbers) compared. Click image to open in larger format.

hd-statistics PP-OFA

The good news: The number of evaluations is increasing all over! More breeders are checking their cats, to the benefit of all Maine Coons.

We see that the larger number of assessments done in OFA are not public results, and thus not accessible for the breeder community. I will encourage all those who send x-rays to OFA to always sign for release and publication of all results, both preliminary results and dysplastic hips. Preliminary results can be published if the owner fills in the form, and the cat is minimum 12 months old.

By knowing as much as possible of all hip results for a cat’s relatives and lines, a breeder can make better and more informed desisions – this is especially important when deciding to breed a cat whose hips are not normal/healthy/perfect.


Evaluation of hips – the images

The evaluation of hip x-rays must look at many factors of the cat’s anatomy. There will always be a subjective factor, however research shows that for the main part there is quite good consensus (Ball 2022).

OFA has a figure describing how hips are evaluated:

OFA description of points of evaluation

  1. Craniolateral acetabular rim
  2. Cranial acetabular margin
  3. Femoral head (hip ball)
  4. Fovea capitis (normal flattened area on hip ball)
  5. Acetabular notch
  6. Caudal acetabular rim
  7. Dorsal acetabular margin
  8. Junction of femoral head and neck
  9. Trochanteric fossa


Note that the OFA descriptions are based on CHD, HD in dogs – photos below are of dog hips:


The Low-study from 2019 used data from the PawPeds database. The four degrees of evaluation was descibed as follows:

Also note that correct positioning of the cat according to instructions (PP and OFA have identical guidelines here) also affect results. Different x-rays of the same cat can give different results due to this as well. See YouTube video with a good example (external link).

Statistics – The Numbers

HD-statistics for the two evaluation systems give different results:

In PawPeds, a study of 5038 Maine Coon showed prevalence of 37,6 % HD (Low 2019)

In OFA, a study of 2708 Maine Coon showed prevalence of 24,9 % HD (Loder 2017) – their annual statistics also show ca 25 % dysplastic hips for Maine Coon cats.

These differences can have various explanations. In OFA owners are not obliged to sign release before doing the x-ray. A breeder and a veterinary who see directly from the image that a cat’s hips are really bad, do not have to send the image in for evaluation. This means that OFA statistics can be biased towards better results. Another possible explanation for the differences is the actual descriptions of each degree. OFA Fair is a hip with minor irregularities. In PawPeds this would be assessed as Grade 1 and thus counted as HD. OFA Mild is described as hips that will most often develop arthritic changes. These are evaluated as Grade 2 in PP.

In PawPeds, owners are required to sign a release form before x-rays, and the veterinary should submit all images for registration and evaluation.

Another difference is that PawPeds evaluate each hip independently, but the “worst” grade counts in all statistics and all recommendations. OFA evaluate the hips together, and in cases of final results/over 24 months’ age give the result as a median of the 3 evaluations. In OFA if there are signs of HD on one side only, the result is given as unilateral.

HD evaluation is a radiographic assessment, and will thus always have a subjective factor. In the Ball 2022 study three orthopedists/radiologists evaluated the same images, at two different times months apart, and all readings were compared to look at consensus. “These results clearly show that consensus scores in radiology can have large uncertainties, and that future studies in both human and veterinary medicine need to include consensus-uncertainty estimates if we are to properly interpret radiological diagnoses and the extent to which consensus scores improve diagnostic accuracy.”

The conclusion is that the assessments have quite good consensus, but that some of the readings vary by one degree up or down, and this mainly applies to hips in the middle layer between normal and severe degrees. This is in the nature of radiographic assessments and should not be interpreted as if HD reading produces completely random results.


Parallel readings

As second opinion and for research purposes images are sometimes submitted to both OFA and PP. Evaluations of the same images give us information on how to compare the two systems.

Here is an overview of known results that have been evaluated in both systems, same image. Data are taken from the public databases and also some of the preliminary (hidden) OFA results where the report is publically available or known. I have found 60 instances of actual parallel evaluations.

Showing all parallel evaluations we see that there is good consensus for the healthy normal hips:

hd-statistics OFA vs PPThe comparison indicates that PawPeds Normal most often get Excellent and Good in OFA. Grade 1 in PP is most often OFA Fair. PP Grade 2 is often OFA Mild, but with variations.

hd-statistics PP-OFA comparison

The comparison also shows that OFA Fair most often corresponds to PP Grade 1. OFA Mild is more often corresponding to PP Grade 2 than PP Grade 1, but there are variations. There is only one PP Grade 3 result with parallel evaluation, and no OFA Severe. More data would be useful.

I recommend more breeders to send hip x-rays to both systems for parallel evaluation, with release of public OFA results. X-rays should be done after the cat is 12 months, for results in OFA to be public. This will give us all more data to compare the two systems, and more breeders will have a second opinion to make an informed decision.


Combinations and selection

PP publish all HD readings done in their system, and link it to the more or less complete MCO database found in PP. This makes it possible to evaluate genetic progress over generations of selection.

Debbie Sprenger has put together statistics and figures in her article “HD Analysis in the Maine Coon, part 1” (August 2020) Her graph below show relative risk for offspring based on PawPeds HD results and pedigrees:


The Low 2019 study also looked at how selection will improve results over generations:

Response to selective breeding. Despite the cross-sectional data showing no reduction in the prevalence or severity of FHD in Maine Coons over time (Fig. 2), there was strong evidence from the multi-generational longitudinal data that selective breeding reduced FHD scores. Here, the predicted average hip score approximately halved after five generations of selective breeding (Fig. 4a); after eight generations the predicted maximum hip score per individual had declined to 0.268 (95% CI: 0.21–0.33) for females and 0.286 (0.21–0.39) for males (from a starting value at generation 1 of 0.85 (0.75–0.96) & 0.86 (0.73–1.03) respectively; Fig. 4a). This effect of culling animals with high FHD scores and only breeding from animals with normal hips or low FHD scores, thus resulted in a 68% (95%CI: 62–73%) reduction in the mean value of the hip score for females and a 66% (60–72%) reduction for males (assuming hip scores follow an approximately linear ordinal scale) between generations 0 and 8 (see Supplementary Table S3 for model coefficient estimates).”

There are large benefits of sending evaluations to PawPeds where all results are linked to the Maine Coon pedigree database, so that results can be seen in view of the complete pedigree. This also enables studies like Low 2019 to review genetic progress.


Choices and the Future

Breeding of family animals is under pressure, from both national and international authorities, and animal rights organizations. All breeders, of Maine Coon and other breeds, must prepare for more rules controlling breeding, and demands of stronger focus on health and welfare, with obligatory testing routines. Breeders are responsible for the health of the kittens we sell, and expected to minimize all risk of disorders.

Learn why health testing with results shared to the breeder community is essential. Learn how to test and how to breed to lower risk.

Simple flowchart as overview: (click to open in larger format)

Normal healthy hips (green): This will be 60-70 % of all Maine Coons. The cat is good for breeding, and can also be mated to cats with some risk.

Midrange – Mild HD (amber): This will be 20-30% of Maine Coons. Consider the overall risk in the line and include results for relatives: siblings, parents, grandparents. If little tested elsewhere in the line then risk can be higher, if most relatives normal then breeding is more safe. Breeding can be possible under caution. Plan well: Which combination can be done to minimize risk. Do new x-rays to get a second opinion if in doubt, as the midrange degrees is where evaluation has less consensus. A clinical examination can also reveal if the cat has pain or problems.

Dysplastic hips (red): This will be 8-16 % of all Maine Coons. These cats should not be used for breeding. If a breeder still chooses to deviate from the guidelines then he/she should know they are taking a larger risk for their offspring.

Taking more of the bad hips out of the gene pool and mating according to the PP guidelines will reduce incident of HD in the gene pool (Low 2019).

Why are so many breeders still skeptical to HD checks, and “don’t believe” x-raying is useful? It is all about understanding complexity; matters that are not black-and-white, but with a large number of genes and factors that influence results. Long time work over years and generations is needed, gradually improving overall status for a breed with selection.

It is also quite typical that breeders who buy a breeding cat where neither parents nor grandparents are tested, get results that are disappointing. To deal with the disappointment and to refuse taking the financial loss of neutering an expensive breeding cat, it is easy to find excuses, and discard the relevance of the HD results, and also often describe the total research and statistics of HD in cats as unreliable. This kind of skepticism is also known from the kennel clubs.

Instead of skepticism and prejudice, we should as breeders rely on the science, the studies and the HD statistics – and make the best choices for our cats.


Vocabulary and abbreviations

  • Evaluation = assessment = score – different terms for the reading and evaluation of radiograph images
  • HD = hip dysplasia
  • FHD = feline hip dysplasia
  • MCO = Maine Coon
  • OFA = The orthopedic foundation for animals
  • PP = PawPeds


PawPeds website

OFA website and statistics

Ball. 2022. «Repeatability of radiographic assessments for feline hip dysplasia suggest consensus scores in radiology are more uncertain than commonly assumed.» In: Scientific Reports.

Corley. 1997. “Reliability of Early Radiographic Evaluation for Canine Hip Dysplasia Obtained from the Standard Ventrodorsal Radiographic Projection.” JAVMA. Vol 211, No. 9, November 1997.

Loder. 2017. “Demographics of hip dysplasia in the Maine Coon cat“.  In: Journal of Feline Medicine & Surgery, April 2017 DOI: 10.1177/1098612X17705554

Low. 2019. “Demography, heritability and genetic correlation of feline hip dysplasia and response to selection in a health screening programme.” Nature Scientific Report. Direct link.

List of further scientific papers see the main HD page

For information about HD in other breeds see the main HD page


All statistics and numbers are until 31 December 2023 as cutoff date.

Thanks to OFA for providing overviews of all their results in addition to the public database.

Thanks to Ulrika Olsson and the team at PawPeds for publishing all their findings in the public domain

Thanks to Debbie Sprenger, graphic designer, coordinator of the PawPeds health programs and MCO breeder at Macadamia Maine Coons in the Netherlands for good advice on design

Article published 18 June 2024

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