What is partial sacralization?

Partial Sacralization: Definition, Clinical Implications, and Management

Partial sacralization is a congenital anomaly of the lumbosacral spine where there is incomplete fusion (unilateral) between the lowest lumbar vertebra (typically L5) and the sacrum, creating a transitional vertebra at the lumbosacral junction. 1, 2

Anatomical Features

• Partial sacralization typically presents as unilateral fusion of the transverse process of L5 with the sacrum, while the contralateral side remains unfused 2
• The fusion may involve only the transverse process, creating an asymmetrical appearance at the lumbosacral junction 3
• Partially sacralized vertebrae show a gap (mean 1.43 mm) between L5 and S1 bodies with incomplete separation between neural arch elements 3
• The auricular surfaces in partially sacralized specimens are positioned lower ("low-down") than normal, typically at the S2-S3 level 3
• The L5 arches in these cases often resemble lumbar laminae rather than typical sacral elements 3

Prevalence and Classification

• Lumbosacral transitional vertebrae (LSTV) are relatively common skeletal anomalies, with sacralization representing one end of the transitional spectrum 4
• Partial sacralization is classified as Castellvi type IIIa variant, indicating unilateral fusion of the transverse process with the sacrum 1
• Studies have found that approximately 2.1% of sacra present with partial lumbarization, while complete sacralization occurs in about 4.8% of the population 3, 4

Clinical Significance

• Partial sacralization can be associated with low back pain due to altered biomechanics at the lumbosacral junction 1, 2
• The asymmetrical fusion creates abnormal load distribution across the lumbosacral junction, potentially leading to accelerated degenerative changes 4
• Symptomatology may mimic piriformis syndrome with pain and tenderness in the gluteal region 1
• The altered anatomy can affect the stability of the lumbosacral junction, which is a complex transition zone between the mobile lordotic distal lumbar spine and the fixed sacral region 5

Diagnostic Evaluation

• Radiographic imaging is the primary method for identifying partial sacralization 6
• MRI is the preferred imaging modality for evaluating transitional vertebrae and associated neural abnormalities, providing detailed visualization of soft tissue, bone marrow, and spinal canal 7
• Plain radiographs can identify vertebral fusion, spondylosis, malalignment, or spinal canal stenosis associated with partial sacralization 7

Management Approaches

• Conservative management is the first-line approach for symptomatic partial sacralization 1
• Manual therapy, including spinal manipulation, soft tissue therapies, and exercise/stretching, may help address presenting symptoms 1
• For cases with significant mechanical instability or persistent pain, surgical intervention might be considered 5
• When surgical intervention is necessary, the goal is to immobilize the lumbosacral junction to relieve pain originating from this site 5
• Various lumbo-sacral fusion techniques are available, with minimally invasive approaches showing good clinical outcomes and high fusion rates in recent years 5

Prognosis and Outcomes

• Conservative management can provide partial symptom resolution, though complete resolution may not always be achieved 1
• Patients may continue to experience aggravation of symptoms with prolonged standing or certain activities 1
• The altered biomechanics at the lumbosacral junction may predispose individuals to earlier degenerative changes in the region 4

Associated Conditions

• Partial sacralization may be associated with other congenital anomalies of the spine 6
• It represents one end of a spectrum of lumbosacral transitional vertebrae, with lumbarization (separation of the first sacral segment) representing the other end 3, 4

Understanding partial sacralization is important for clinicians evaluating patients with low back pain, as this anatomical variant can contribute to symptoms and may require specific management approaches tailored to the altered biomechanics at the lumbosacral junction.