Developmental dysplasia of the hip: osteonecrosis

Avascular necrosis in the treatment of hip dysplasia: an inevitable or avoidable complication?

Avascular necrosis of the femoral head (AVN) is the most feared complication in the treatment of hip dysplasia . The resulting clinical consequences affecting the hip can be serious and difficult to recover from.

A concept that must guide any treatment is that among the complications, a failed reduction of a hip (which can be recovered with a further reduction attempt) is preferable to persisting in achieving the reduction, causing avascular necrosis (which is difficult to recover from).

Unfortunately, this complication is not entirely avoidable: even in the best centers in the world, the rate of avascular necrosis never reaches zero. It has been hypothesized that the underlying cause is a sort of “fragility” linked to an abnormal conformation of the vessels in the femoral head.

However, the percentage of necrosis can be greatly reduced by respecting some well-known treatment principles.

What does avascular necrosis involve?

In avascular necrosis , the blood supply to the femoral head is reduced, causing cell death and impaired development and growth of structures, which can be more or less severe depending on the extent of the necrosis and the structures involved (femoral head, growth plate, etc.).

Precisely because structures still in the early stages of development are affected, with many years of residual growth remaining, the consequences of avascular necrosis can sometimes be subtle (slowing of nuclear ossification, for example) or manifest radiographically years later (for example, grade 2 necrosis often manifests itself late). This is one of the reasons why it is advisable to schedule a radiographic check-up even in pre-pubertal or pubertal age in cases at risk.

Clinically, in children and adolescents, necrosis is generally a benign condition that does not cause overall physical disability, but merely a limitation of hip function (stiffness, reduced strength, etc.). Over time, however, it can lead to a decline in functionality and rapid joint deterioration.

What is the mechanism by which avascular necrosis occurs?

The main mechanisms that can determine a reduced blood flow to the femoral head are:

• Excessive pressure on the femoral head: for example due to forced reductions
• Occlusion/strangulation of the vessels that supply blood to the head of the femur: due to immobilization in forced positions
• Abnormalities in the conformation of the vessels that carry blood to the head of the femur could explain those cases of necrosis that occurred despite compliance with all the optimal precautions to avoid it.

What can be done to limit the risk of necrosis in the treatment of hip dysplasia?

Although in many cases it is an “inevitable” consequence of the pathology, regardless of the treatment adopted, there are some precautions that must be taken to limit the risks.

Avoid forced reductions. Every forced reduction implies an increased risk of necrosis: for example, typical errors:

• reduce a dislocated hip with adductor retraction by placing it in a rigid spreader that forces the retraction
• reduce a dislocated and re-dislocated hip with a bloodless maneuver without adequate release of the tense structures (or without femoral shortening)

Avoid extreme immobilization positions . In extreme positions, the vessels supplying blood to the femoral head are stretched or compressed. This applies, for example:

• When using braces in small children . Be careful when using braces that require a greater opening of the limbs than is possible for that child.
• When applying casts : avoid casting in extreme positions (especially excessive abduction or internal rotation). Respect the principles of the “Human position” and the “Ramsey safe zone.”

The role of other factors in increasing/reducing the risk of necrosis is not entirely clear:

• Effect of previous unsuccessful attempts at reduction with a spreader: controversial
• The presence or absence of the ossification nucleus at the time of bloody reduction: for some authors the presence of the ossification nucleus would have a protective role but other works have not confirmed this hypothesis
• Role of traction: this is a controversial aspect

How to understand if avascular necrosis is present?

Even in the initial period following the reduction, the femoral head may show some signs of pain. It’s important to recognize these signs to understand the possible evolution of the condition early.

Early signs

The Salter criteria describe useful elements to consider in the first 2 years after a reduction (open, non-open, or with a retractor).

The radiographic aspects most indicative of a possible negative prognosis are:

• S1: failure to appear ossification nucleus for more than 1 year after reduction
• S2: failure to grow the ossification nucleus (already appeared) for more than 1 year after reduction
• Dome-shaped, flared, balloon-shaped metaphysis
• Domed, flared, balloon-shaped metaphysis (Tsukagoshi et al, BJJ 2017)

Other signs of suffering whose outcome is more variable are:

• S3: Widening of the femoral neck one year after reduction
• S4: Increased head density, followed by fragmentation

It is important to underline that in a small percentage of patients (14%), there is no evidence of avascular necrosis 1-2 years after treatment, but over time more or less serious signs appear.

Late signs

To describe the signs of avascular necrosis in later life (usually over 10 years of age and toward the end of growth), the Kalamchi and Bucholz-Ogden classifications are used. These two classifications are quite similar and their reproducibility is not optimal. Beyond this, it is important for an orthopedic surgeon to be able to recognize early signs that may lead to an unfavorable progression over time.

In general, the most serious and difficult to recover from conditions are those in which the development of the epiphyseal surface is affected; damage to the physis, on the other hand, causes variable consequences depending on the affected area (valgus, varus, or short neck).

For example, for Kalamchi:

• Grade I: Only the epiphyseal nucleus is affected: hypoplasia, ossification irregularity. It often has no sequelae and is considered by some authors to be a non-pathological condition.
• Grade II: the lateral part of the physis is affected, progressively causing cervico-epiphyseal valgus which can have serious consequences leading to subluxation of the articular portion of the femoral head.
• Grade III: the central part of the physis is affected, progressively causing a short, widened neck and hyper-growth of the greater trochanter
• Grade IV: global damage, both of the head and of the femoral neck; severe alteration of the epiphyseal surface and of the femoral neck

Essential bibliography

• Tsukagoshi Y, Kamegaya M, Kamada H, Saisu T, Morita M, Kakizaki J, Tomaru Y, Yamazaki M. The correlation between Salter’s criteria for avascular necrosis of the femoral head and Kalamchi’s prognostic classification following the treatment of developmental dysplasia of the hip. Bone Joint J. 2017 Aug;99-B(8):1115-1120.
• Roposch A, Wedge JH, Riedl G. Reliability of Bucholz and Ogden classification for osteonecrosis secondary to developmental dysplasia of the hip. Clin Orthop Relat Res. 2012 Dec;470(12):3499-505
  Bradley CS, Perry DC, Wedge JH, Murnaghan ML, Kelley SP. Avascular necrosis following closed reduction for treatment of developmental dysplasia of the hip: a systematic review. J Child Orthop. 2016 Dec;10(6):627-632.
• Roposch A, Liu LQ, Offiah AC, Wedge JH. Functional outcomes in children with osteonecrosis secondary to treatment of developmental dysplasia of the hip. J Bone Joint Surg Am. 2011 Dec 21;93(24):e145
  Canepa G, Stella G. Treatise on Pediatric Orthopedics. Ed. Piccin
• Tonnis D. Normal values of the hip joint for the evaluation of x-rays in children and adults. Clin Orthop Relat Res. 1976;119:41
• Tachdjian’s Pediatric Orthopedics: From the Texas Scottish Rite Hospital for Children: ELSEVIER –Saunders

Our center has an outpatient clinic that handles all aspects, from ultrasound diagnosis to the application of retractors or casts, to surgical procedures (bloodless/bloody reduction, femoral osteotomies, tectoplasty, etc.)

Please refer to other sheets for specific aspects