Femoral capital physeal dysplasia

What it is

Femoral capital physeal dysplasia, also called slipped capital femoral epiphysis or spontaneous capital physeal fracture, is a separation of the femoral head from the neck through the growth cartilage (capital physis) without significant trauma. In these cats the physis is thickened and disorganised, with irregular clusters of chondrocytes instead of the normal orderly columns, and it stays open longer than usual. Early neutering is thought to delay physeal closure, because sex hormones take part in it, and excess body weight to increase shear forces on the weak cartilage; over time the epiphysis slips and the femoral neck is resorbed.

Who it affects

Young cats, usually neutered males that are overweight, from the last months of growth to about two years of age. Some breeds, such as the Maine Coon and Siamese, are represented in the studies. It frequently affects both hips, simultaneously or weeks to months apart.

Anatomy involved

• Proximal femur: the head epiphysis separates from the neck through the capital physis; the femoral neck undergoes resorption and sclerosis, with the radiographic look of a bitten "apple core".
• Hip joint: the head usually stays within the acetabulum while the neck moves; secondary osteoarthritis develops.

Clinical signs

Acute or progressive lameness of one or both pelvic limbs, often without a history of trauma or after a trivial jump; pain on hip manipulation, especially extension and abduction, crepitus, difficulty jumping and thigh muscle atrophy in chronic cases.

Diagnosis

• Orthopaedic examination: hip pain and crepitus.
• Ventrodorsal and frog-leg radiographs of both hips: displacement of the capital epiphysis, irregular widening of the physis and, in chronic cases, resorption and sclerosis of the femoral neck. Other physes should be checked too, as they may remain open at ages when they should be closed.
• Histopathology of the excised head confirms the physeal dysplasia.

Treatment

• Femoral head and neck excision arthroplasty: the most common treatment; most cats regain good function thanks to their low body weight.
• Total hip replacement: an alternative in centres experienced with cats.
• Pin fixation of the epiphysis: possible in very acute cases with little neck resorption, but with a higher risk of failure because of the poor quality of the physis.

Body weight is also controlled and the contralateral hip monitored, as it may become affected later. The functional prognosis is good.

Key studies

• McNicholas et al. (2002), a retrospective study of 26 cats older than one year with spontaneous capital physeal fractures. Mean age was 22.5 months, 25 of the 26 were neutered males, and they weighed significantly more than age- and sex-matched controls. Of the 16 cats with a known age at neutering, 14 had been neutered before 6 months. Nine had bilateral fractures, and other physes that should already have closed were still open (the distal femoral physis in 19 of 24 and the proximal tibial physis in 24 of 24). On histology, the growth cartilage had lost its normal columnar arrangement.
• Borak et al. (2017), 17 Maine Coon cats from an Austrian clinic. Slipped capital femoral epiphysis was diagnosed in 8.2% of the Maine Coons seen (17 of 208) versus 0.67% of all cats (29 of 4348), about 12 times more. 16 of the 17 were male, with a mean weight of 7.5 kg; 7 had both hips affected and four were littermates. It is a single-centre series.
• Butts et al. (2023), a radiographic measurement study of 28 affected cats and 5 controls, each read twice by five observers. Excluding fully displaced epiphyses, the S-sign on the frog-leg view was the most accurate (92.4%) and sensitive (93.9%) method, while Klein's line detected only 30.6% of cases. The control group is small.
• Vink et al. (2022), 39 cats (46 fractures) stabilised with cortical positional screws. 45 of the 46 fractures showed signs of healing and 35 of the 39 cats walked normally at 6 weeks, although femoral neck osteolysis and osteoarthritis increased on radiographs and 7 of 25 owners reported a gait abnormality in the longer term. The technique worked best when preoperative neck osteolysis was mild.
• Rodiño Tilve et al. (2022), a multicentre registry of 44 cats with 56 total hip replacements. Slipped capital femoral epiphysis was the most common indication (34 of 56 hips). There were 11 complications in 56 hips, 9 of them major, and owners rated the outcome as very good in 30 of 33 cases after a median follow-up of 752 days.