Kifosis

01Pathophysiology

Kyphosis refers to an excessive posterior convexity of the thoracic spine in the sagittal plane. The normal thoracic kyphosis ranges from approximately $20\mathrm{°}20°$ to $45\mathrm{°}45°$ as measured by the Cobb angle on a standing lateral radiograph. When this angle exceeds the upper limit, the condition becomes pathological.

In postural kyphosis, there is no structural vertebral abnormality. The curve results from habitual slouching and weakened paraspinal extensor musculature. The hallmark is that the curve is flexible and fully corrects when the patient actively extends the spine or lies prone. Because the vertebral bodies themselves remain normally shaped, radiographs show a smooth, rounded curve without wedging or endplate irregularities.

Scheuermann disease is the most common cause of structural kyphosis in adolescents, and the type most heavily tested. The underlying mechanism involves abnormal endochondral ossification of the vertebral ring apophyses. During the growth period, the anterior portion of the vertebral endplates becomes biomechanically vulnerable. Repetitive axial loading on these weakened endplates causes the anterior vertebral body height to grow less than the posterior height, producing anterior wedging. Over time, compressive forces also drive disc material through the weakened endplates, forming Schmorl nodes (intravertebral disc herniations). The endplates become irregular and the intervertebral disc spaces narrow. This creates a rigid, angular kyphotic deformity that does not correct with extension. Tight hamstrings are a frequent associated finding, likely a compensatory response to the altered pelvic tilt and sagittal balance.

In congenital kyphosis, the deformity arises from embryological vertebral anomalies during weeks 4 to 8 of gestation. These anomalies fall into two categories. Failure of formation (Type I) involves absence or hypoplasia of the anterior part of the vertebral body, producing a progressive angular kyphosis with a high risk of spinal cord compression. Failure of segmentation (Type II) involves an anterior bony bar that tethers growth anteriorly while the posterior elements continue to grow. Type I carries a worse prognosis because the deformity is inherently unstable and tends to progress rapidly, particularly during growth spurts.

In degenerative or senile kyphosis, the pathophysiology centers on age-related disc degeneration and, critically, osteoporotic vertebral compression fractures. The anterior cortex of the vertebral body, being thinner and more cancellous, fails under axial load, leading to anterior height loss and progressive wedging. Multiple sequential compression fractures can accumulate into a severe thoracic kyphosis colloquially known as a "dowager's hump."

02Classification and Clinical Manifestation

Clinical notes on exam presentation:

The classic Scheuermann disease vignette presents an adolescent (typically male) with a visible thoracic roundback, back pain that worsens with prolonged sitting or physical activity, and tight hamstrings on examination. The Adam's forward bend test reveals a sharp angular deformity that does not flatten with hyperextension. Compensatory lumbar hyperlordosis is often present below the kyphotic segment. Neurological deficits are uncommon unless the kyphosis is severe ($>80\mathrm{°}>80°$) or complicated by thoracic disc herniation.

03Diagnostic Workup

The workup begins with a thorough physical examination. Inspection from the side reveals the degree of thoracic roundback. The Adam's forward bend test is essential: the patient bends forward at the waist with arms hanging. In postural kyphosis, the curve smooths out. In Scheuermann disease, the angular prominence persists. Hamstring tightness should be tested, as it is characteristically present in Scheuermann disease.

The best initial test is a standing full-length lateral spine radiograph. The Cobb angle is measured from the superior endplate of the most tilted proximal vertebra to the inferior endplate of the most tilted distal vertebra. Normal thoracic kyphosis is $20\mathrm{°}20°$ to $45\mathrm{°}45°$. Values above $45\mathrm{°}45°$ are abnormal.

For Scheuermann disease, the diagnosis requires the Sorensen criteria on lateral radiograph:

• Anterior wedging of $\ge 5\mathrm{°}\backslash geq5°$ in at least 3 consecutive vertebrae

• Irregular vertebral endplates

• Schmorl nodes (present in the majority of cases)

• Narrowing of the intervertebral disc spaces

This is a frequently tested point: Scheuermann disease is a radiographic diagnosis. The combination of clinical rigidity on forward bend testing plus the Sorensen criteria on lateral radiograph confirms the diagnosis. MRI is not needed for diagnosis but is reserved for patients with neurological findings, suspected thoracic disc herniation, or atypical pain patterns that raise concern for infection or tumor.

For congenital kyphosis, CT with 3D reconstruction provides the best delineation of bony anatomy and helps classify the anomaly (failure of formation vs. segmentation).

For elderly patients with progressive kyphosis, a DEXA scan should be obtained to evaluate for osteoporosis as the driving etiology, and radiographs should be reviewed carefully for compression fractures.

04Management and Treatment

Postural kyphosis is managed nonoperatively. Physical therapy focusing on core extensor strengthening and pectoral stretching corrects the posture over time. Bracing is not indicated because there is no structural deformity.

For Scheuermann disease, management is dictated by the Cobb angle and skeletal maturity. Mild curves below $50\mathrm{°}50°$ require observation and physical therapy alone. A structured hamstring stretching program is important because hamstring contracture contributes to pelvic retroversion and worsens the kyphotic posture.

For moderate curves between $50\mathrm{°}50°$ and $75\mathrm{°}75°$ in a skeletally immature patient (Risser sign 0 to 2), bracing is the standard treatment. The Milwaukee brace (cervicothoracolumbosacral orthosis) has the longest track record but is poorly tolerated due to the cervical ring. A TLSO (thoracolumbosacral orthosis) with an anterior sternal pad is an alternative that provides comparable correction with better compliance. Bracing is prescribed for 16 to 23 hours per day and continued until the patient reaches skeletal maturity. The goal is not curve reversal but prevention of progression. Bracing is ineffective in skeletally mature patients.

Surgical intervention is reserved for curves exceeding $75\mathrm{°}75°$, progressive deformity despite bracing, intractable pain unresponsive to at least 6 months of conservative treatment, or neurological compromise. The standard procedure is posterior spinal fusion with segmental pedicle screw instrumentation. For rigid curves (those that do not correct to below $50\mathrm{°}50°$ on hyperextension films), a combined anterior release (discectomy at multiple levels) followed by posterior instrumentation and fusion may be needed to achieve adequate correction.

For congenital kyphosis Type I (failure of formation), early surgical intervention is recommended because these curves have the highest risk of progression and spinal cord compression. Posterior in situ fusion performed early (even in infancy) can prevent catastrophic progression. Delaying surgery increases the complexity and morbidity.

For osteoporotic kyphosis, the treatment targets the underlying bone disease. Bisphosphonates (e.g., alendronate 70 mg weekly or zoledronic acid 5 mg IV annually), calcium supplementation (1000 to 1200 mg/day), and vitamin D (800 to 1000 IU/day) form the pharmacological backbone. For acute, painful compression fractures that fail to respond to conservative pain management after 4 to 6 weeks, vertebral augmentation (vertebroplasty or balloon kyphoplasty) provides pain relief and partial height restoration.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap on this topic is confusing postural kyphosis with Scheuermann disease. Both present as an adolescent with a visible thoracic roundback. The critical discriminator is the Adam's forward bend test: postural kyphosis corrects with active hyperextension; Scheuermann disease does not. On radiograph, the giveaway for Scheuermann is the triad of anterior vertebral wedging in 3 or more consecutive vertebrae, irregular endplates, and Schmorl nodes. If a vignette describes a teenager with a "fixed" or "rigid" thoracic kyphosis that persists on forward bending, the answer is Scheuermann disease, not postural roundback.

A second common mistake involves management thresholds. Students frequently select bracing for every Scheuermann case, but bracing is only effective in skeletally immature patients with curves between $50\mathrm{°}50°$ and $75\mathrm{°}75°$. A skeletally mature patient (Risser 4 to 5) will not benefit from bracing. Likewise, surgery is reserved for curves $>75\mathrm{°}>75°$ or cases with neurological compromise or refractory pain, not merely because the curve looks severe.

Third, vignettes featuring a young adult with progressive kyphosis and morning stiffness are designed to lure students toward Scheuermann disease. The discriminator is the pattern: ankylosing spondylitis begins in the sacroiliac joints with bilateral sacroiliitis, progresses cranially, and is associated with HLA-B27 positivity. Scheuermann disease does not cause sacroiliitis and is not associated with inflammatory markers.

Fourth, for congenital kyphosis, the high-yield pearl is that Type I (failure of formation) has the worst prognosis and requires early surgical intervention. Exam writers test whether you recognize that delay in treatment leads to progressive neurological deterioration.

Finally, when an elderly patient presents with acute back pain and a new thoracic kyphosis, the next best step is a radiograph to look for a compression fracture followed by DEXA scanning. The treatment of the fracture alone is insufficient; you must also initiate osteoporosis treatment (bisphosphonates, calcium, vitamin D) to prevent recurrence. Failing to address the underlying bone disease is a commonly tested management error.

The core competency being tested across all kyphosis questions is the ability to classify the deformity by etiology (postural vs. structural vs. congenital vs. degenerative), apply the correct diagnostic criteria (particularly the Sorensen criteria for Scheuermann disease), and match the management to both the severity and the skeletal maturity of the patient.