Slippage of the capital femoral epiphysis tends to occur between the ages of 10 and 17 years, and is about 2.5 times more common in boys than in girls, suggesting the possibility of a traumatic aetiology. Overweight children are at greatest risk, although very tall children may not be.

As children reach adolescence, the physis, or growth plate, undergoes certain physical changes that make it more vulnerable to stress. The plate can be made more vulnerable by an abnormal endocrine balance, although numerous studies on large numbers of patients with slipped capital femoral epiphysis have no endocrine abnormalities. However, excess androgen or oestrogen leads to rapid maturation and closing of the plate, and defects such as Turner's syndrome, lead to prolonged survival of the physis. Hypothyroidism or hypopituitarism also lead to delayed maturation, and increases in levels of somatomedin, or of pituitary growth factors, can lead to increased growth of the physis. During treatment of the abnormalities slippage occurs much more frequently; this is probably due to further weakening of the plate.

Other changes are also associated with adolescence. For example, the physis is horizontal in young children, but becomes more vertical as adolescence approaches. The perichondrial ring also becomes thinner. Blunt trauma and removal of this ring in puppies leads to slippage of the epiphyses. If the ring is left intact, a fracture of the femoral neck ensues.

Patients with renal osteodystrophy, who effectively have rickets, occasionally suffer slippage not only of the hip, but of almost any weight-bearing joint, including the ankle and distal femur. The predisposing factors are clearly mechanical, and probably provide good support for the theories listed above.

There are two broad categories of slipped capital femoral epiphyses: chronic and acute. The latter is less common. The patient with a chronic slip usually presents with a limp or a lurch to the side, sometimes associated with a low-grade ache in the groin or in the anterior thigh, although pain may affect the knee alone. Adolescents presenting with knee pain should certainly undergo examination of the hip.

Characteristically, the patient loses internal rotation, and on flexion of the hip, the hip goes into external rotation. This is almost a pathognomonic sign. There is also limited abduction and flexion. Sometimes the slip cannot be seen on anteroposterior radiographs, but only on a frog-leg lateral projection. Radiographs of the opposite hip help in reaching the diagnosis; about 30 per cent of patients have bilateral slips. The hip slips both posteriorly and medially. Techniques to determine the degree of slippage all revolve around the degree to which the head has slipped off the neck. This can be determined by drawing angles along the base of the capital head and the axis of the neck, and measuring the angle change.

Mild slips are characterized by slippage of less than 30 per cent; severe cases have slipped by at least 50 or 50 per cent. An acute slip is usually superimposed on a chronic slip: the child has often had low-grade symptoms for a while, followed by acute and usually severe pain. Some authors arbitrarily define an acute slip as symptoms lasting less than 3 weeks. From a practical standpoint, an acute slip represents a rapid change in the position of the capital head on the physis, and the rapid change is always accompanied by severe pain.

Definitions of the extent of the slip provide a logical framework for managing the condition. Most chronic slips should not be reduced. The two major complications of slipped epiphysis treatment are avascular necrosis and chondrolysis. Chondrolysis represents a gradual dissolution of the hip joint cartilage, which leads to stiffness and pain. Its exact aetiology is unclear: it is sometimes idiopathic, but may result from surgical manipulation and treatment. Chronic slips should not be manipulated, or openly reduced. Such treatment is associated with a significant incidence of avascular necrosis (Fig. 3) 2593. Although advocates of the open or closed reduction maintain that it prevents the development of late arthritis, few studies have shown early, severe and painful arthritis in patients with severe slips.

Further studies have indicated that even if the growth plate is closed, most patients regain a functional range of motion. Therefore, the tendency to undertake immediate osteotomies to regain motion has diminished. The major loss is in flexion, abduction, and internal rotation. As the spasm decreases with traction, the motion usually improves dramatically. If 1 year after surgery the patient still has significant functional problems, a Southwick-type osteotomy, performed at the base of the neck or in the intertrochanteric area to increase flexion and abduction, may be appropriate. The correction in these two planes also seems to improve the internal rotation.

Questions concerning manipulation and open reduction for chronic slips only concern patients with severe slips. Mild slips are invariably treated with some internal fixation device. A number of years ago, slips were fixed with a central, non-threaded pin. This method was abandoned, as the growth plate was often strong enough to grow away from a central pin, and the slip recurred. Surgeons subsequently started using three or four threaded pins to ensure fixation. However, multiple pins are located away from the centre of the capita head and if located near the surface they often penetrate the bone. While the incidence of chondrolysis in non-treated slips has been well documented, a multicentre study by the Pediatric Orthopaedic Society of North America showed that most cases of chondrolysis were caused by undiagnosed pin penetration.

There is now a strong tendency to react against this procedure, and many surgeons are urging single pin fixation. There have been isolated reports of the heads growing away from single pins, and a complete shift to that tactic may lead to recurrence of the earlier complications. We prefer to use two threaded pins, of the Knowles pin variety (Fig. 4) 2594. The hollow pins, while easier to use, are much harder to remove later on. Mild slips can be approached through a straight lateral incision. More severe slips need an anterior approach, such as the Watson-Jones procedure. An incision is made from the anterior superior iliac spine to the trochanter, and then laterally along the femur. The tensor fascia lata is swept anteriorly, and the gluteus medius and minimus muscles are retracted posteriorly. The pin is driven down the anterior neck.

Another approach is the removal of a core from the head, neck, and lateral cortex, which is then filled with a bone graft. The obvious advantage of this technique is that there are no pins to protrude into the joint. The disadvantage is that there is little structural support until the joint heals, and the patient has to be placed in a spica cast for a few months. Steele has been treating patients with slips non-operatively in spica casts. While this method presumably allows the slip to heal, the factors which caused the hip to slip in the first place have not been altered, and there is a risk of recurrence.

Chronic slips are usually fixed with one or two threaded pins. Traction is employed initially to enable the patient to regain motion. Failure to regain motion suggests chondrolysis. Significant abnormal motion after fixation should not be treated immediately; many patients regain their own motion.

Acute slips can be associated with avascular necrosis irrespective of any treatment. It is important to distinguish acute slips from acute growth plate fractures of the capital femoral epiphysis. The acute slip is usually caused by fairly minor trauma, while a fracture is caused by severe trauma. Children with acute slips often have a history of having some symptoms or limp previously.

Treatment of acute slips should aim to prevent avascular necrosis. Gentle reduction can be achieved by placing the hip in skin traction only. Severe muscle spasm prevents much movement, but the hip shifts slowly over a period of days or a couple of weeks as the spasm eases. The traction direction is in flexion, then increased abduction, and finally, internal rotation (Fig. 5) 2595. Fixation is not normally performed for about 3 weeks after the acute slip, to allow the hip to heal partially.

If avascular necrosis occurs intra- and extra-articular fusion is usually required. The hip is fused in 30° flexion, with neutral rotation and abduction. Female patients sometimes have the hip fused in 5 to 10° abduction. The hip is approached through an anterior Smith-Peterson incision, and the hip is dislocated anteriorly. The head and acetabulum are denuded using reamers. The head is then fixed to the pelvis with several large cancellous screws. A large dynamic compression plate is contoured to the front of the acetabulum between the anterior iliac spines, and then down on to the anterior neck and shaft. The bone is denuded both from the acetabulum and the neck, and around the lateral side down to the trochanter, and iliac graft is added. The patient remains in a spica for 3 months.

There is no good treatment for chondrolysis, other than fusion. We always first remove any hardware present, and keep the patient non-weight-bearing, in traction, and on anti-inflammatory agents. Since there is no evidence that salicylates block protease action, there is no special value in selecting them. This regimen often produces improvement.

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