15.1.1 Epidemiology

SK is the most common cause of fixed painful structural kyphosis deformity in the thoracic and thoracolumbar spine among adolescents. ⁵ The prevalence is 1% to 8% in the United States, the male-to-female ratio is at least 2:1 and is most commonly diagnosed in adolescents at 12–17 years of age. ⁶

15.1.2 Etiology

The definitive cause of SK remains uncertain. The heritability in Scheuermann’s disease is 0.74, but the mode of transmission has not yet been defined. ⁷ Other factors that have also been implicated in Scheuermann’s disease development are idiopathic juvenile osteoporosis, elevated growth hormone levels, dural cysts, spondylolysis, vitamin D deficiency, spinal deformities, and infections. ^{8 ,​ 9 ,​ 10} Mechanical hyperpressure on growth cartilage has been proposed as a possible etiology and is based on the weak mechanical interface between stiff bone and the resilient disk. Patients usually present with a higher body mass index and are more active individuals. ¹¹

15.1.3 Histopathology

Histological analysis shows endplate irregularities, narrowed intervertebral disks, a thickened anterior longitudinal ligament (ALL), and Schmorl’s nodes. It is considered to be a form of juvenile osteochondrosis of the spine whereby defective growth of the cartilage endplate leads to disorganized endochondral ossification. The microscopic findings include markedly irregular endplates and endplate disruption with protrusion of disk material into the vertebral body. The ring apophysis does not show avascular necrosis. The intervertebral disk is interpreted as normal both by routine histology and electron microscopy. ¹²

15.1.4 History and Physical Exam

The family usually attributes the deformity to poor posture, which delays the diagnosis and treatment. SK is a structural deformity of the thoracic or thoracolumbar spine, which appears before puberty and progresses to become symptomatic during growth. ^{8 ,​ 13 ,​ 14} The patient usually seeks medical attention between the age of 8 and 12 years old, and those who attend later usually present with more severe and rigid deformity. The initial reason for consultation in adolescents is the cosmetic deformity and in adults because of increased pain. The pain is usually located at the paravertebral region, just caudal to the apex of the kyphosis. ^{15 ,​ 16} The compensatory cervical and lumbar hyperlordosis could also be a cause of pain. Some patients presented with lumbosacral spondylosis or spondylolisthesis because of increasing stress on the pars intra-articular. ^{5 ,​ 8 ,​ 14 ,​ 16}

In rare severe cases, neurological symptoms may appear in the form of radicular pain with progressive weakness of the lower limbs, even spastic paraparesis. ^{10 ,​ 13 ,​ 14 ,​ 15} The neurological symptoms could be secondary to the onset of thoracic disk herniation, dural cysts, or by spinal traction and compression mechanisms in the apex of the kyphosis in cases of severe deformity. Cardiopulmonary symptoms are uncommon among patients with Scheuermann’s disease. ^{5 ,​ 8 ,​ 14 ,​ 15 ,​ 16 ,​ 17 ,​ 18}

15.1.5 Differential Diagnosis

It is crucial to differentiate the SK from postural kyphosis, which is nonrigid, nonprogressive, and correctable in the hyperextension or supine position. The apical vertebrae and adjacent disks have a normal appearance without wedging, irregularities of the endplate, or premature degeneration of the disk.

15.1.6 Radiological Features

The average normal thoracic kyphosis among asymptomatic adolescents is 44° ± 10.9° (measuring from T1 to T12). ¹⁹ However, the relevant literature reflects the existence of a wide variability for what could be considered normal sagittal balance in asymptomatic individuals.

The diagnosis of Scheuermann’s disease is obtained by lateral spine radiography with the patient standing. To measure the angle of kyphosis, the final cranial and caudal vertebrae included in the deformity must be selected. The measurement of the wedging degree is obtained from the angle of intersection of the tangents on the upper and lower plates of each vertebral body. The diagnostic criterion establishes a level of wedging over 5° in at least three consecutive vertebrae in the apex of kyphosis (Fig. 15‑1). Other common findings in radiology include the presence of Schmorl’s nodes, irregularity, and thinning of the vertebral endplates and disk space impingement (Fig. 15‑2). In the classical type I disease, the apex of kyphosis is located between T6 and T9. In type II, the apex of kyphosis is located in the thoracolumbar junction. ^{5 ,​ 8 ,​ 9 ,​ 10 ,​ 14 ,​ 15}

Pelvic incidence (PI) is a key regulator of sagittal balance in normal individuals. Lumbar lordosis usually closely correlates with PI with increasing lordosis seen in patients with higher PI. ²⁰ Intuitively, higher thoracic kyphosis and higher lordosis could be related to a higher PI. SK is, however, not considered a normal state, and therefore, sagittal balance is often disrupted.

Recently, the sagittal spinopelvic alignment in adolescents associated with SK has been studied. Jiang et al reported that SK patients had significantly lower PI and pelvic tilt. They found different compensation mechanisms in these patients to keep their sagittal spinopelvic alignment. A significant correlation was noticed between thoracic kyphosis and cervical lordosis in Scheuermann’s thoracic kyphosis (STK) patients and also between thoracic kyphosis and lumbar lordosis in both STK and Scheuermann’s thoracolumbar kyphosis patients. ^{21 ,​ 22 ,​ 23}