Genetic Welfare Problems of Companion Animals

Pomeranian

Distal Fractures of the Radius and Ulna

Related terms: antibrachial fracture, fracture non-union, fracture malunion, fracture delayed union.

Outline: Pomeranians, like dogs of other miniature and toy breeds, have a high prevalence of fractures of the distal radius and ulna, that is, of the lower part of the forearm. Not only do they occur more commonly in Pomeranians than in larger dogs, but they also often take a long time to heal or even fail to do so. These fractures can occur as a result of relatively minor trauma and it has been found that the bones are relatively weaker, in relation to body size, than in larger dogs. These fractures cause acute and severe pain and their pain and discomfort can be of days to months in duration depending on how well they heal. Whether predisposition to these fractures is an unavoidable by-product of selecting for small size or whether selecting for stronger bones (by not breeding from dogs that have had fractures) might prove successful in tackling the problem, is not known. As far as we are aware there are no coordinated efforts to address the issue.

Summary of Information

(for more information click on the links below)

1. Brief description

All toy breed dogs, including the Pomeranian are vulnerable to fractures of the distal (lower end of the) radius and ulna (Denny 1985, Muir 1997). Generally, these types of fracture are the result of major trauma, such as road traffic accidents, but in toy breeds they can commonly result from only rather minor trauma (Waters et al 1993, Muir 1997, Rochat 2010). The commonest cause appears to be jumping or falling from a height eg from the owner’s arms (Sumner-Smith 1974a, Harasen 2003a, Rochat 2010, McCartney et al 2010). These fractures occur because, in relation to their body weight, of the lower mechanical strength of the distal radius and ulna in toy breeds compared to other breeds. This is due to size-related differences in the cross-sectional structure of the bones (Brianza et al 2006).

Delayed (non-union) healing of this type of fracture is also more common in toy than in larger breeds of dogs (Vaughan 1964, Sumner-Smith & Cawley 1970, Sumner-Smith 1974a, b, Muir 1997, Harasen 2003a, Welch et al 1997, McCartney et al 2010). Factors associated with delayed or incomplete healing of these fractures include: reduced blood supply to the distal radius in toy breeds; the absence of soft tissues enveloping the fracture site in these breeds; and the ‘exuberant’ nature of these dogs resulting in their wanting to use the limb soon after surgery or before external stabilisation of the fracture.

Casts and splints are generally no longer used to treat distal radial fractures in toy breeds, as the number of cases in which the break fails to heal can be as high as 80% (Muir 1997, Welsh et al 1997; Harasen 2003b). Surgical interventions that rigidly fix the bones are necessary to allow these fractures to heal.

2. Intensity of welfare impact   

Fractures of long bones are extremely painful, from the time of fracture until appropriate veterinary intervention. Treatment is likely to involve travel to a veterinary practice, surgery, and subsequent medication and all these may cause significant stress to the affected animal. During healing, pain and discomfort may occur and restriction of normal activities is likely.

Some dogs with fracture that fail to heal may have to undergo repeated surgical interventions and, occasionally, amputation of the limb is necessary (Hunt et al 1980, Muir 1997).

3. Duration of welfare impact

It can take months for these fractures to heal. If the broken ends fail to unite (non-union), the healing process can be protracted and may last many months.

4. Number of animals affected

Denny (1985) suggested that fractures of the distal antebrachium (the lower part of the front leg between the elbow and wrist), in dogs weighing less than 3 kg, are relatively common. However, we are unaware of any published data on the prevalence of these fractures in Pomeranians.

5. Diagnosis

A fracture will be suspected in any Pomeranian showing signs of acute forelimb lameness, especially if there is swelling or deformity, and particularly after a fall or a jump. The diagnosis can be confirmed, and the site and nature of the fracture determined, by radiography (x-rays).

6. Genetics

Many miniature or toy dog breeds, including Pomeranians, are apparently predisposed to distal fractures of the radius and ulna and to subsequent non-union (failure to heal) of these fractures. A single gene for an insulin-like growth factor (IGF1) has been identified that influences variation in body size between many breeds of dog. One form of this gene is associated with small body size, another with larger size (Sutter et al 2007). However, other genes may also be involved in determining body size among breeds of dogs.

7. How do you know if an animal is a carrier or likely to become affected?

All Pomeranians are vulnerable to radial and ulna fractures that are the result of relatively mild trauma and all are at risk of the delayed healing of these fractures.

8. Methods and prospects for elimination of the problem

The relatively high risk of these antebrachial fractures, that tend to heal poorly in toy breeds, appears to be associated with small body size. It may be that reducing their prevalence will therefore be difficult without also selecting for an increase in body size.

For further details about this condition, please click on the following:
(these link to items down this page)

• Clinical and pathological effects
• Intensity of welfare impact
• Duration of welfare impact
• Number of animals affected
• Diagnosis
• Genetics
• How do you know if an animal is a carrier or likely to become affected?
• Methods and prospects for elimination of the problem
• Acknowledgements
• References

1. Clinical and pathological effects

Dog breed organisations, such as The Kennel Club, group together breeds of very small dogs into a group called the toy group.  Included in this group are breeds such as the Pomeranian, Chihuahua, Papillon, Miniature pinscher, Italian greyhound, toy Fox terrier, Yorkshire terrier, Griffon and toy and miniature poodle. All are vulnerable to fractures of the distal radius and ulna (Denny 1985, Muir 1997).

The Pomeranian was developed by miniaturising a dog breed of the Spitz-type– a larger dog originally used for sheep herding in Pomerania (now part of Germany and Poland) (http://www.akc.org/breeds/pomeranian/). It is thought that the smallest individuals of this breed were kept as companions and then bred for their small size. Pomeranians are classified as a toy breed by both the American and UK Kennel Clubs. Their ideal weight (from the UK standard) is 1.8 to 2kg for males and 2 to 2.5kg for females (http://www.thekennelclub.org.uk/item/197).

The radius and ulna are the paired long bones of the antebrachium – the lower part of the foreleg, from the elbow to the carpus (the wrist joint in humans). Like other long limb bones they are essentially supporting columns that have to carry the weight of the animal. Each long bone consists of a central, roughly cylinder-shaped, shaft of bone called the diaphysis and two rounded ‘head’ sections at either end, called the epiphyses, which articulate with other bones as joints which allow movement. These epiphyses have an outer layer of compact bone and spongy bone in their centre. Between the epiphyses and the diaphysis in young animals are the growth or epiphyseal plates which are the zones in which bone lengthening occurs.

The diaphysis (the shaft of the bone) has, in cross section, an outer thin fibrous layer of tissue called the periosteum and, inside this, a wide layer of compact bone which provides the strength. In the centre of the shaft is the bone marrow cavity that contains the soft, blood cell producing, bone marrow. Bone is a dynamic, living tissue which can slowly respond to the forces exerted on it, through changing its shape and strengthening. It can also repair itself.

Fractures of the antebrachium are relatively common, accounting for 17% of all fractures in dogs (Phillips 1979). These are usually caused by major trauma incidents such as road traffic accidents, but in toy breeds they can commonly result from only rather minor trauma (Waters et al 1993, Muir 1997, Rochat 2010).

In a review of cases by Muir (1997), 16 were caused by jumping or falling (“often from a minimal height”); three by being trodden on; one by an attack from another dog, one by being shut in a door; one by being in a car accident; and in four cases the cause was unknown. Many agree that jumping or falling from a height (including from the owner’s arms) is the commonest cause of fractures in toy breeds (Sumner-Smith 1974a, Harasen 2003a, Rochat 2010, McCartney et al 2010).

Fractures of the antebrachium in toy dogs tend to occur in the distal third of the bones and within the main shaft, the diaphysis (Waters et al 1993, Muir 1997, Brianza et al 2006). Muir (1997) found that fractures due to jumping or falling from minimal heights tended to be transverse (straight across the bone, at right angles to the shaft) or oblique (at an angle across the bone). He speculated that this might be because the radius (which bears 90% of the load) and which is slightly bowed in shape, may experience a bending force on landing from a high jump, with the concave side of the bone being compressed with the front, convex, side simultaneously being stretched (put under tension).  It is this convex side that fails – fractures - first (as bone is weaker under tension than compression). After it has cracked, the concave side then fails. Interestingly, larger dogs tend to suffer hyperextension injuries of the carpus (wrist) joint when falling from a height– where the ligaments that hold the joint in place are overstretched and damaged, rather than fractures (Brianza et al 2006). This suggests that the radius and ulna bones are inherently weak in toy breeds. Until recently this apparent weakness was only a hypothesis but in 2006, Briana and others found that, in relation to the body weights, the antebrachii of toy breeds were more susceptible to fracture than those of larger dogs, and that this was due to a weaker cross-sectional structure of the bones.

Surgical treatments of these fractures by coaptation, using external splints and casts, have high rates of delayed or failed healing in toy breeds– of up to 80% (Muir 1997, Welsh et al 1997; Harasen 2003b); certainly a much higher prevalence than in larger breeds (Vaughan 1964, Sumner-Smith & Cawley 1970, Sumner-Smith 1974a, b; Muir 1997, Harasen 2003a, Welch et al 1997;, McCartney et al 2010). Various surgical treatments have been recommended. A delayed union refers to the healing of the fracture taking longer than expected as assessed by radiographs (x-rays) or on palpation of the stability of the bones at the fracture site. Non-union means that the bone fragments have not healed together at the first attempted treatment and that further surgical intervention is needed (Langley-Hobbs 2003).

Normal bone healing occurs in two ways. If the broken ends of the bones are closely opposed, stable in that position, and compressed together, primary (direct) healing occurs as osteoclasts (cells which absorb bone) cross from one side of the fracture to the other creating channels for new blood vessels to grow across the fracture followed by osteoblasts (the bone cells which lay down new bone).

If the bone ends are less intimately held together then spontaneous (also called secondary intention or secondary) healing occurs instead. In this, a fibrous or fibro-cartilaginous callus (junction) is formed and then this tissue is ossified (turned into bone). Ultimately this bony callus is then reshaped (over years) (Langley-Hobbs 2003). Spontaneous healing requires that there is adequate healthy tissue, with a healthy blood supply, surrounding the bone fragments. This is because the new fibrous/ fibro-cartilaginous tissue is formed from cells which come from this surrounding tissue and from the periosteum (the fibrous external lining membrane) of the bone (Langley-Hobbs 2003). The commonest type of non-union of the distal radius and ulna in toy breeds is called non-active, atrophic non-union. In this, no callus forms and the ends of the bones actually slowly disappear (Langley-Hobbs 2003).

Various factors are thought to be involved in the high incidence of delayed or non-union fractures in toy breeds. These include:

• Dogs of toy breeds have very little soft tissue enveloping the bones at the lower end of the antebrachium to provide support or to be a supply of callus forming cells (Vaughan 1964, Langley-Hobbs 2003, Welch Fossum 2007).
• External splinting and casting often fail to provide adequate stability of these fractures in toy breed dogs (Vaughan 1964, Muir 1997, Palmer 2005). Excessive movement of the ends of the fracture affects the formation of an ossified callus.
• Biomechanical instability occurs with oblique fractures (Fossum 2007) and the movement that results prevents adequate callus formation.
• Welch et al (1997) found a decreased density of blood vessels within the distal diaphysis of small dogs as compared to large dogs, which is implicated in the poor healing and atrophy of the bone ends. This is often considered the most important factor.

Dogs with non-union fractures require further surgical interventions. Occasionally amputation of the leg may be necessary (Hunt et al 1980, Muir 1997).

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2. Intensity of welfare impact

Fractures of long bones are extremely painful. Where they are suspected, animals need prompt veterinary treatment. Dogs may be in acute pain for hours until there is appropriate veterinary intervention and pain relief. Over 20% of fractures may be bilateral i.e. both front legs are fractured at the same time (Muir 1979). This severely affects mobility during the recovery period and obviously intensifies suffering.

Veterinary interventions including travel to and from the practice, surgery, and medication can also cause significant stress.

Individuals in which a fracture fails to heal may have to undergo repeated surgical interventions. Occasionally amputation of the limb is necessary (Hunt et al 1980, Muir 1997).

Due to owners' concerns about the risks of fractures occurring, some dogs may have activities such as running and jumping restricted or closely controlled throughout their life.

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3. Duration of welfare impact

All toy dogs, including Pomeranians, tend to be young when they sustain these fractures (Campbell 1980). In Muir’s (1997) study of 26 dogs, nine were less than a year old and 16 were between one and four years (the age of one dog was unknown).

Normal healing after surgery can take from a few weeks to a few months. Despite these dogs often being young, distal radial and ulna fractures in toy breeds usually take a relatively long time to heal (Langley-Hobbs 2003). In cases of delayed or non-union fractures, healing may be protracted over many months.

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4. Number of animals affected

We are unaware of any published data on the prevalence of these fractures in Pomeranians but Denny (1985) suggested that they were relatively common in dogs weighing less than 3 kg. Harasen (2003a) found distal radius and ulna fractures to be the third most common canine long bone fracture at his referral practice in Canada –and most of these cases were in toy breeds.

It appears to be common for these fractures to be bilateral ie for both legs to be fractured at the same time. In Muir’s (1997) review of cases, six out of 26 dogs had bilateral fractures.

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5. Diagnosis

A fracture may be suspected in any Pomeranian showing acute signs of front limb lameness and pain, particularly after a jump or fall. On examination, a fracture may be indicated by swelling, pain on palpation, detection of abnormal movement along the limb or of crepitus (a grating feeling of bone rubbing on bone). The diagnosis can be confirmed by radiography (x-rays), often under sedation or anaesthesia, after any life-threatening concurrent conditions or injuries have been stabilized.

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6. Genetics

Dogs of all miniature and toy breeds, including Pomeranians, appear to be predisposed to fractures of the distal radius and ulna and to delayed or failed healing of them.

The fact that the prevalence of this type of fracture appears to be linked to (small) body size suggests that during selection for small size, the slenderness of the bones and consequential reduced amount of bone tissue means the biomechanical strength of the bones has been impaired, such that the bones are disproportionately weak in relation to body weight.

When animals are selected for larger or smaller body size, size changes can be brought about in several ways and the balance of relative tissue and organ sizes, strengths and capacities can be disturbed. It is possible that selecting for stronger bones by not breeding from any dog that has had a fracture might help in tackling the high prevalence of this disease.

A single gene for an insulin-like growth factor (IGF1) has been identified that influences variation in body size between many breeds of dog. One form of this gene is associated with small body size, another with larger size (Sutter et al 2007). However, other genes may also be involved in determining body size among breeds of dogs.

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7. How do you know if an animal is a carrier or likely to become affected?

All Pomeranians are vulnerable to radial and ulna fractures even with relatively mild trauma and all are at risk of their delayed or failed healing.

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8. Methods and prospects for elimination of the problem

The high prevalence of antebrachial fractures with poor healing is associated with toy body size, so it may be difficult to select for reduced prevalence (ie for relatively stronger bones) without selecting for increased body size at the same time.  It may be that relatively weak limb bones are an unavoidable correlation of very small body size. Detailed comparisons of fracture prevalence rates between various breeds in relation to body size might help answer that.

Out-crossing to larger dogs, with the normal form of the IGF1 gene, to increase the breed's body size might improve this issue. As far as we are aware there are no coordinated efforts to address this problem. It is possible that it may be made worse through fashions for further miniaturisation of dog breeds e.g. the teacup and tiny toy poodles (http://www.dogbreedinfo.com/t/teacuppoodle.htm).

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9. Acknowledgements

UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section.

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10. References

Aspinall V and O’Reilly M (2004) Introduction to Veterinary Anatomy and Physiology. Butterworth Heinemann, London

Brianza SZ, Delise M, Maddalena Ferraris M, D'Amelio P and Botti P (2006) Cross-sectional geometrical properties of distal radius and ulna in large, medium and toy breed dogs. Journal of Biomechanics 39(2): 302-11

Campbell JR (1980) Healing of radial fractures in miniature dogs. Veterinary Annual 20: 106–112

Denny HR (1985) A Guide to Canine orthopaedic Surgery. 2nd ed. pp 179-185. Blackwell Scientific Publications, Oxford, UK

Harasen G (2003a) Common long bone fracture in small animal practice — Part 2 Canadian Veterinary Journal 44(6): 503–504

Harasen G (2003b) External coaptation of distal radius and ulna fractures. Canadian Veterinary Journal 44(12): 1010–1011

Hunt JM, Aitkin ML, Denny HR, and Gibbs C (1980) The complications of diaphyseal fractures in dogs: A review of 100 cases. Journal of Small Animal Practice 21: 103–119

Langley-Hobbs S (2003) Biology and radiological assessment of fracture healing. In Practice: 26-25

McCartney W, Kiss K and Robertson I (2010) Treatment of distal radial/ulnar fractures in 17 toy breed dogs. Veterinary Record 166: 430-432

Muir P (1997) Distal antebrachial fractures in toy-breed dogs. Compendium of Continuing Education for the Practicing Veterinarian 19: 137–145

Palmer RH (2005) Fractures of the Radius/ Ulna. On-line. http://wvc.omnibooksonline.com/data/papers/2005_V312.pdf. accessed 18.7.11

Phillips IR (1979) A survey of bone fractures in the dog and cat. Journal of Small Animal Practice 20: 661–674

Rochat M (2010) Fracture Fixation: Distal Radius and Ulna. On-line. http://www.vetstream.com/canis/Content/Technique/teq00831. Accessed 18.7.11

Sumner-Smith GA and Cawley AJ (1970) Non union of fractures in the dog. Journal of Small Animal Practice 11: 311–325

Sumner-Smith GA (1974a) A comparative investigation into the healing of fractures in miniature poodles and mongrel dogs. Journal of Small Animal Practice 15: 323-328

Sumner-Smith GA (1974b) A histological study of fracture nonunion in small dogs. Journal of Small Animal Practice 15: 571-578

Sutter NB, Bustamante CD, ChaseK, Gray MM, Zhao K, Zhu L, Padhukasahasram B, Karlins E, Davis S, Jones PG, Quignon P, Johnson GS, Parker HG, Fretwell N, Mosher DS, Lawler DF, Satyaraj E, Nordborg M, Lark KG, Wayne RK and Ostrander EA (2007) A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs. Science 316: 112–115

Vaughan LC (1964) A Clinical Study of Non-union Fractures in the Dog. Journal of small Animal Practice 5: 173 -177

Waters DJ, Breuer GJ and Toombs JP (1993) Treatment of common forelimb fractures in miniature and toy-breed dogs. Journal of American Animal Hospital Association 29: 442–448

Welch JA, Boudrieau RJ, Dejardin LM and Spodnick GJ (1997) The Intraosseous Blood Supply of the Canine Radius: Implications for Healing of Distal Fractures in Small Dogs. Veterinary Surgery 26: 57–61

Welch Fossum T (2007) Management of Specific Fractures. In Welch Fossum T. Ed. Small Animal Surgery 3rd Ed. Mosby Elsevier, Philadelphia, USA

http://www.akc.org/breeds/pomeranian/
http://www.dogbreedinfo.com/t/teacuppoodle.htm
http://www.thekennelclub.org.uk/item/197

© UFAW 2011

Credit for main photo above:

By Jpkollman (Own work) [CC0], via Wikimedia Commons