Aseptic necrosis of the femoral head, also known as Legg-Calvè-Perthes syndrome and avascular necrosis of the femoral head, is an orthopaedic disease involving the hip joints of small or toy breed dogs. It presents during the animal’s growth period and is usually unilateral. Aseptic necrosis of the femoral head is caused by remodelling of the bone of the femoral head and neck, with arthritic degeneration and, consequently, varying degrees of chronic lameness of the hind limbs.
AETIOPATHOGENESIS
Aseptic necrosis of the femoral headwas first described in a child in 1910 by three different scientists, Legg, Calvè and Perthes. Each of them thought the disease was due to different causes. Legg thought it was due to compromised vascularisation of the femoral head, Calvè that it was secondary to rickets and Perthes though it was due to infectious arthritis. So far, in human medicine, various epidemiological studies have described Legg-Calvè-Perthes syndrome as a defect of endochondral ossification (see under Osteochondrosis), even though this theory has still not been universally accepted.
In veterinary medicine, the syndrome was first reported in the 1930s in toy breeds and in the Wire Haired Fox Terrier (Spicer 1936, Schnelle 1937, Moltzen-Nielsen 1938). The aetiopathogenesis is not clear although it has been shown to be hereditary in the West Highland White Terrier (Wallin 1986, Robinson JSAP 1992), Yorkshire Terrier and toy Poodle (Wyne-Davis and Gormley 1978).
The best accredited hypotheses include vascular anomalies and avascular necrosis with subsequent bone remodelling. The veterinary literature also reports other possible causes, such as trauma, infection, hormone imbalances and osteochondrosis. Most of these have been abandoned in favour of hypotheses regarding changes in vascularisation. Whatever the aetiology, the pathogenesis of this orthopaedic disease retraces some stages related to a vascular insult, with necrosis and consequent bone revascularisation, resorption and remodelling until the femoral head collapses. In the initial stages of the disease, ischaemia occurs in the centre of the femoral head and is manifested by trabecular bone and medullary necrosis and bleeding of the epiphysealcancellous bone, without cartilage involvement and without macroscopic changes in the trabecular structure. The next stage is characterized by attempted neovascularisation, formation of new bone tissue and osteoclastic remodelling phenomena. As the disease progresses, the ischaemic damage extends to the subchondral bone which is replaced by fibroconnective tissue resulting in bone loss. In this stage, there is loss of the normal morphological structure of the epiphysis with areas of lysis, fibrosis and bleeding, and involvement of the joint cartilage. The disease process weakens the bone structure and this leads to collapse of the femoral head under the animal’s weight.
EPIDEMIOLOGY
Aseptic necrosis of the femoral headaffects growing dogs of small or toy breeds, usually between 4 and 12 months of age. The peak incidence is at 6-7 months with no gender-related difference in the incidence. The West Highland White Terrier, Yorkshire Terrier, toy Poodle, Pug, Jack Russell, Chihuahua, Pinscher, Maltese, Pomeranian, Cairn Terrier and Lhasa Apso are the breeds most frequently affected.
CLINICAL SIGNS
Aseptic necrosis of the femoral headis the cause of varying degrees of lameness of the hind limbs, with a progressive onset and course. In 85% of cases the condition is unilateral and consequently lameness is seen in only one limb. In bilateral cases, lameness may affect one limb more than the other and may be only slightly evident or not present at all in the contralateral limb; in other cases it can involve both limbs equally and is manifested by the dog’s reluctance to move or jump. In cases in which lameness is slight, it is nearly always possible to see partial shifting of weight from the limb involved when the dog is standing still. Clinically, a difference must be made between this disease and patellar dislocation, which is also frequently seen in the same breeds. It should, however, be remembered that these orthopaedic conditions can also occur in association, thus complicating the clinical picture.
DIAGNOSIS
Physical examination shows pain during extension and abduction of the hip joint and in unilateral cases a difference in muscle bulk can be noted, with the atrophy becoming more evident as the disease becomes chronic. A diagnostic suspicion of aseptic necrosis of the femoral head can be confirmed by X-ray of the hip joint. The diagnosis can usually be made from a ventro-dorsal view of the pelvis with the limbs in extension (Fig. 1a and b). X-rays show irregular widening of the joint space and foci of radiolucency in the femoral head and neck, associated with changes in the shape and contour of the femoral head. As the lesion becomes chronic, degenerative phenomena can be seen; these lead to intense resorption of bone tissue and collapse of the femoral head. Aseptic necrosis of the femoral head can be classified radiographically into five grades according to the severity and progression of disease, as proposed by Ljunggren in 1967.
Grade I (Figs. 2a and b)
- the contour of the femoral head and neck is normal;
- the joint space is widened;
- the acetabulum is normal;
- there are a few foci of radiolucency.
Grade II (Fig. 3)
- flattening of thefemoral head;
- larger and more numerous foci of radiolucency;
- involvement of the femoral neck;
- production of osteophytes.
Grade III (Fig. 4)
- greater changes in the contour of the femoral head;
- flattening and irregularities of the joint surfaces;
- irregular radiodensity.
- production of osteophytes.
Grade IV (Fig. 5)
- loss of the femoral head contour;
- large areas of radiolucency together with small areas of normal density.
Grade V (Fig. 6)
- extensive fragmentation of the femoral head;
- clear discontinuity in the joint surface.
When doubt remains, X-rays should be repeated 2-3 weeks later or tomography performed, which can help to reach an early diagnosis before radiographic evidence of the very first stages of the disease.
TREATMENT
Surgery is the usual treatment for aseptic necrosis of the femoral head and neck. This involves resection of the femoral head and neck or excisionalarthroplasty (Figs. 7a and b, Fig. 8). Another surgical option is the use of prosthetic mini-implants. A conservative approach with medical treatment does not provide satisfactory results given the tendency for the disease to worsen over time (Figs. 9a and b). Although bone resorption phenomena may stop before the femoral head collapses, the osteoarthritis that results from the loss of congruity of the joint surfaces will progress and contribute to the pain and lameness.
PROGNOSIS
Functional recovery of the femoral head and neck after ostectomy is usually good, considering the small size of the dogs affected by aseptic necrosis of the femoral head, in which excisional arthroplasty gives better functional results. Treatment failure can be caused by a poor surgical technique (incomplete ostectomy of the femoral neck with a remaining bone spur) (Fig. 10) or excessive muscle hypotrophy because of the limb’s prolonged lack of weight-bearing. In this latter case, functional recovery may be slow or not completely satisfactory
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