Miscellaneous orthopedic conditions

Question 1

What are the key features of polydactyly

Answer 1

Polydactyly is defined by the finding of supernumerary digits
- it is an occasional condition with no detrimental consequences
- mostly, the extra digits are found on the front paws only

The trait is inherited as an autosomal dominant trait with variable penetrance
- this trait seems to be more common in cats along the eastern coast of Canada and the USA

Question 2

What is radial agenesis (hemimelia)

Answer 2

Radial agenesis is the unilateral or bilateral congenital underdevelopment or absence of the radial structures (radius, radial carpal bone and/or the first digit
- total absence of the radius is often accompanied by complete or partial absence of the radial carpal bone and first digit

This is the most frequent hemimelia reported in cats

Question 3

What is the mechanical consequence of radial agenesis

Answer 3

As the radius is the main weight bearing bone of the forearm, affected forelimb suffers from severe deformity and impaired function
- the ulnar becomes thickened and curved due to increased local bearing
- the paw develops a varus deformity because of loss of medial support structures
- contracture of extensor tendons
- the elbow joint is also affected due to the presence of humerulnar subluxation secondary to lack of support from the radial head

Question 4

What is the cause for radial agenesis

Answer 4

Many factors may affect the embryonic development of the forelimb and the pathogenesis of radial agenesis is not well understood

The trait may be heritable in some cases
- anecdotal reports of the trait being produced by the deliberate breeding of some polydactyl cats

Another hypothesis is that hemimelia results from an injury to the neural crest by a teratogen in utero

Question 5

What are the key features of hypervitaminosis A

Answer 5

This disorder is now rare
- it was associated with diets largely composed of liver

Excessive levels of Vitamin A are thought to increase the sensitivity of the periosteum to microtrauma
- this results in excessive deposition of periosteal new bone, eventually resulting in ankylosing bridges of bone between adjacent vertebrae (especially in the cervical region) and across appendicular joints (particularly the elbow)

Affected cats present with:
- stiffness due to the ankylosing effects of the skeletal exostoses
- pain likely due to nerve root entrapment which may also lead to neurological deficits (e.g., paresis and urinary incontinence or retention)
- as they cannot groom properly, they have a typically unkempt appearance

Prognosis is poor because skeletal changes are irreversible

Question 6

What are the key features of nutritional secondary hyperparathyroidism

Answer 6

This is now a rare condition
- it is occasionally seen in kittens fed an all meat diet

Clinical signs may be due to hypocalcemia (e.g., muscle tremors and/or seizures) and/or due to osteopenia (e.g., lameness and reluctance to move due to bone pain and folding fractures)

Radiography reveals generalized cortical thinning and skeletal radiolucency
- many cats have pathological fractures, especially of the metaphyseal regions of the long bones, the spine and pelvis

Clinicopathological findings include:
- hypocalcemia
- hypophosphatemia
- elevated alkaline phosphatase
- increased PTH

Correction of hypocalcemia will result in the resolution of signs but animals with severe skeletal changes may require corrective surgery

Question 7

What are the key features of rickets

Answer 7

Cats cannot synthesize Vitamin D from sunlight, thus they rely on their diet for an adequate intake of this vitamin
- this is a very rare cause of rickets in cats
- typically the disease is due to hereditary defects in Vitamin D metabolism

There are four main types of hereditary rickets recognized in cats:
- hypophosphatemic rickets: due to excessive loss of phosphate via the kidneys
- Vitamin D-dependent rickets type 1: due to an autosomal recessively heritable defect in the gene encoding alpha-hydroxylase
- Vitamin D-dependent rickets type 2: due to a genetic mutation leading to defective Vitamin D receptors (autosomal recessive)
- Vitamin D-dependent rickets non-type 1, non-type 2: cause by an unknown abnormality

Treatment of rickets usually consists of calcium and Vitamin D supplementation
- response is quite variable

Question 8

Compare and contrast spondylosis deformans and diffuse idiopathic skeletal hyperostosis

Answer 8

Spondylosis deformans is:
- a degenerative condition involving the vertebral endplates (it is associated with DJD of the intervertebral spaces)
- mean age 8 years
- behavioral signs consistent with lumbosacral pain (the hyperesthesia in this region may be due to cauda equina nerve and nerve root irritation

Diffuse idiopathic skeletal hyperostosis is:
- a systemic, noninflammatory disease of the axial and appendicular skeleton (it involves ossification of soft tissues, specifically the ventral longitudinal ligament of the spine
- three criteria have been described to characterize DISH:
- calcification along the ventrolateral aspect of at least four consecutive vertebral bodies
- preservation of intervertebral disc width and absence of radiographic changes indicative of degenerative intervertebral disc disease
- absence of articular process ankyloses, sacroiliac joint erosion, sclerosis or intra-articular osseous fusion
- may cause stiffness, pain and neurologic signs