Skeletal Dysplasias

Klane K. White, MD, MSc

Vernon T. Tolo, MD¹

¹Guru:

As a group, these disorders are commonly encountered by the pediatric orthopaedist, but individually, each disorder is quite rare. Orthopaedic training programs often provide little education in the area of skeletal dysplasia, except perhaps a few facts that may appear on in-training or board certification examinations. Many orthopaedists relegate this subject to the arena of orthopaedic trivia. With the growth of support groups and Internet sites for unusual disorders, the skeletal dysplasia patient or parents often know the latest facts about their disorder, a fact that the orthopaedic surgeon can use in their favor. This chapter is designed to suggest a way to approach the evaluation and treatment of your patients with skeletal dysplasia and to help you successfully navigate the world of rare disease.

There are now more than 400 recognized genetic disorders of bone. The skeletal dysplasias, more formally referred to as the “osteochondrodystrophies,” represent a subset of this group in which the primary defect is in the function and development of the growth plate. Genetic mutations of the growth plate result in an array of cytokine, enzyme, and structural malfunctions of the tissues in which they are expressed. The genetic aberrations found in the skeletal dysplasias can be broken up into four different groups: (1) proteins that are structural, (2) proteins that regulate developmental signaling pathways, (3) proteins responsible for metabolite processing or transport, and (4) proteins that regulate cell replication and tumorogenesis.

If the skeletal dysplasia diagnosis has not been established or is unclear, the first step is always to do a skeletal survey, which includes radiographic evaluation of the skull, spine, and all extremities. Long-bone radiographs will establish whether there is epiphyseal or metaphyseal involvement, or both—which in turn will help to narrow down the possible diagnoses to consider. If there is platyspondyly, the term spondylo appears somewhere in the name of the condition (Fig. 20-1). If there is primarily epiphyseal involvement, early joint degeneration is likely to occur. If there is primarily metaphyseal involvement, angular deformity with growth is often seen, which may mimic rickets. Often, the morphology of the ilium or acetabulum reveals unique finding specific to a particular disorder. Even though the X-rays and the physical findings may seem characteristic for a given skeletal dysplasia, it is mandatory to obtain the opinion of a geneticist for confirmation of diagnosis, which has significant implications with regard to treatment options, health maintenance, and genetic counseling.

Figure 20-1 Lateral spine radiograph demonstrates the finding of platyspondyly (height of vertebrae less than normal in relation to width) in a child with spondyloepiphyseal dysplasia congenita.

There is a paucity of literature on many of these dysplasias as far as effective treatment is concerned. Rather than memorizing obscure names and irrelevant dysmorphology facts (which nowadays can be found on many private and government-funded websites, as well as sites for patient advocacy groups), these patients should be approached in a systematic, anatomical fashion. Using known orthopaedic principles, only then should the orthopaedist pursue the specific findings and concerns for each disorder.

Keeping in mind what is known about the natural history of orthopaedic problems in a particular skeletal dysplasia condition, the best orthopaedic approach often here is to use tested orthopaedic principles for joint problems, spine deformity or instability, and limb realignment in the average-sized person, and then apply them to the dysplastic skeleton. Examples would be decompression for spinal stenosis, custom-made implants for total joint replacement, and guided growth or osteotomies to regain mechanical axis alignment.

There are generalized and specific pitfalls with each of these procedures that can occur more often in patients with skeletal dysplasia, but the underlying orthopaedic principles stay constant. This group as a whole has an increased risk for
perioperative complications related to the anatomy of their upper airway, abnormalities of tracheal-bronchial morphology and function; deformity of their chest wall; abnormal mobility of their upper cervical spine; and associated issues with general health and body habitus. There is extreme phenotypic variability in this population. When combined with the rarity of each condition, make it becomes important that patients are assessed and managed in facilities that are aware of these potential complications, and have the skill and resources to anticipate and manage them.

Achondroplasia

Achondroplasia is the most common form of skeletal dysplasia and is recognizable at birth. Upregulation of fibroblast growth factor receptor 3 (FGFR3) leads to the growth inhibition here. Rhizomelia is present in all extremities and the facial appearance includes frontal bossing and nasal bridge depression. Head size is large for the body and, while hydrocephalus may be suspected, this is usually not present. Delayed milestones and hypotonia are common and should be compared to published norms available for achondroplasia from the American Academy of Pediatrics. An anteroposterior spinal radiograph will show interpediculate narrowing in the lumbar spine, with short pedicles and thoracolumbar kyphosis, is common on the lateral view in the first few years of life (Fig. 20-2).

The primary orthopaedic concerns in this condition are foramen magnum stenosis and spinal stenosis. As an infant, the main medical concern centers around sleep apnea, which may be due to either upper airway obstruction or hypotonia resulting from foramen magnum narrowing and compression of the upper cervical spinal cord (Fig. 20-3). Sleep studies and regular neurological exams are recommended to detect and follow these issues. Screening MRI should be reserved for infants with upper motor neuron signs, significant hypotonia, or central sleep apnea. Posterior impingement of the cord is common and typically asymptomatic. While published reports have noted improvement in respiratory function after foraminal decompression, there is a substantial morbidity associated with this
procedure in infancy. If the foramen magnum stenosis is severe enough (anterior and posterior impingement or cord signal change), neurosurgical decompression of the foramen magnum is considered. For asymptomatic stenosis, we know that over the first few years of life, there is a predictable enlargement of the foramen magnum relative to the spinal cord so that cord impingement becomes less of a problem by age 3 or so. Unless there is clear-cut and life-threatening respiratory compromise from this foraminal stenosis, it may be wiser to wait to see if mild sleep apnea and hypotonia resolve with time, without surgery.

Figure 20-2 Anteroposterior (A) and lateral (B) spine radiographs of a child with achondroplasia demonstrate the interpediculate narrowing and short pedicles, most obvious in the lumbar spine. The lateral also demonstrates thoracolumbar kyphosis that may persist in children with achondroplasia.

Figure 20-3 A: Sagittal plane MRI of the upper cervical spine demonstrates narrowing of the spinal canal at the foramen magnum and C1 often seen in young children with achondroplasia. B: Foramen magnum decompression provides ample space for the spinal cord.

Other medical issues are often present in early childhood, especially due to the mid-face hypoplasia present in achondroplasia. Respiratory function can be compromised not only by the foramen magnum stenosis, but also by a small thoracic cage and a redundant tissue in the retropharynx. Because of mid-face hypoplasia, drainage of the Eustachian tubes is impaired and otitis media is common. If unrecognized or not treated aggressively enough, these ear infections can result in significant, permanent hearing loss and delay in speech development.

KYPHOSIS

Spinal problems are common in achondroplasia. In infancy, there is nearly always a thoracolumbar kyphosis present, with the kyphosis accentuated clinically when the child is sitting. Independent walking generally does not take place until 18 to 24 months of age in achondroplasia, but the standing position seems to lead to resolution of this thoracolumbar kyphosis in about 90% of the children. As lumbar lordosis increases with standing and walking, sagittal plane balance occurs with improvement in the thoracolumbar kyphosis. Progression of deformity is associated with developmental motor delays or vertebral body wedging beyond that typically seen in achondroplasia. A fraction of these patients will develop symptomatic, fixed deformities requiring surgery. Clinical surveillance (with periodic plain radiographic imaging when concerned) is encouraged to monitor for progression to a symptomatic deformity requiring treatment. Persistence of kyphosis beyond childhood may lead to myelopathy, paraparesis, or back pain. With that said, the majority of thoracolumbar kyphoses occur below the level of the conus medullaris, and consequently the risk of spinal cord compression or other neurologic compromise in young children is extremely low. In older children and adolescents the presence of thoracolumbar kyphosis can exacerbate the symptoms of neurogenic claudication. Intervention for thoracolumbar kyphosis should be reserved for deformities that are progressive or symptomatic (i.e., neurological compromise or secondary muscular back pain). Bracing has not
been proven effective. Published indications for surgery are for a thoracolumbar kyphosis exceeding 60° with more than 10° of progression in 1 year.

Successful treatment of symptomatic kyphosis relies on achievement of sagittal alignment at the site of correction and overall spinal balance. For less severe and more flexible deformities, such as those found in young children, isolated posterior fusion and instrumentation can be successful. However, surgical treatment in this group should be an uncommon occurrence.

Typically speaking, older children, adolescents, and adults with symptomatic deformities require a more aggressive surgical correction, using either an anteroposterior approach, or an all-posterior, posterior column shortening. The approach to achieve appropriate sagittal alignment and balance relies less on any prescribed recommendation, and more on the experience of the surgeon and the perceived risk of surgery in that surgeon’s hands in attaining appropriate correction. Be aware that the risk of neurologic injury or dural tear is much higher in these patients because of the limited canal size.

The most common problem in achondroplasia is spinal stenosis, which can occur anywhere throughout the spine. It is most commonly symptomatic in the lumbar spine (Fig. 20-4). There is characteristic narrowing of the interpediculate distances in the coronal plane of the spinal canal and pedicle shortening on the sagittal plane. Spinal claudication signs and symptoms can occur at any age but are uncommon before the second decade. In children and adolescents, spinal stenosis is almost always associated with thoracolumbar kyphosis. For adults, spinal stenosis becomes increasingly a problem as degenerative changes occur, further narrowing the spinal canal and compressing either the cauda equina or the spinal cord. Multiple levels of compression may be present, so it is very important to evaluate lower extremity symptoms of spinal stenosis with a magnetic resonance image (MRI) of the entire spine.

Figure 20-4 Persistent thoracolumbar kyphosis exacerbates spinal stenosis in adolescents with achondroplasia. A 13-year-old girl with neurogenic claudication and modest kyphosis (A) has multilevel stenosis (B), requiring decompression and instrumented fusion (C).

In general, decompression is needed over multiple levels, commonly T11 to S1, to relieve the lower extremity neurologic symptoms. Using a high-speed burr to transect the lamina adjacent to the facet and then lifting the lamina dorsally appears to be a safer technique of laminectomy than placing rongeurs within the tight spinal canal, but neurologic deficit as a complication of laminectomy is not rare, and dural tears occur in up to 40% of surgeries. Fusion is needed after multilevel laminectomy when there is persistent thoracolumbar kyphosis or skeletal immaturity. If thoracolumbar kyphosis is present in the area of laminectomy, this will worsen postlaminectomy. Since the pedicles are of adequate size to accept screw placement, pedicle screw instrumentation and posterior fusion at the time of the laminectomy is required to adequately treat this kyphosis. Placement of hooks into the canal is discouraged.

The most obvious orthopaedic concern to surface in achondroplasia is progressive bowing of the legs. In achondroplasia the fibula is longer than the tibia, and some degree of bowing is present in most with this condition. It is unclear why some get worse and some do not, but it is expected that about 20% of the tibiae in achondroplasia will at some point have corrective surgery. It is a curious finding that for older adults with achondroplasia and mild genu varum, medial compartment degenerative arthritis rarely occurs, and as such does not need to be treated prophylactically. The primary reasons for surgical correction, which usually is done between the ages of 4 and 12, are lateral leg pain with walking or running activity and the physical finding of a lateral thrust of the knee in early and mid-stance. Surgery may be needed in either the femur, the tibia, or both. Gait analysis studies have shown excellent improvement and normalization of gait parameters from this relatively simple proximal tibial/fibular valgus osteotomy. The need to repeat the tibial osteotomy at a later age is uncommon. Guided growth can be attempted at younger ages (<8 years). Because of slow growth in these patients, complete correction may take longer than generally expected (up to 3 years), or may not occur at all (Fig. 20-5).
Addressing the lateral distal femur, proximal tibia, and proximal fibula (by placing a cannulated screw across the growth plate) can be attempted. Older children are less likely to benefit from guided growth as a result of limited growth. In these patients, osteotomy is likely the better option.

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