The sacrum is a triangular bone located at the base of the spine, situated between the two hip bones, forming the posterior portion of the pelvic ring. It connects the spine to the pelvis, transmitting the upper body weight to the lower limbs. Sacral fractures are relatively uncommon but represent a significant injury, frequently resulting from high-energy trauma such as motor vehicle accidents or falls from a substantial height. Among sacral injuries, the U-type fracture is recognized as one of the most unstable and biomechanically devastating classifications.
Defining the U Type Sacral Fracture
The U-type sacral fracture, sometimes called an H-type, is a highly disruptive pattern involving both vertical and transverse fracture lines. This injury is characterized by bilateral vertical fractures running through the wings of the sacrum (sacral alae). These are connected by a single, horizontal fracture line across the central body of the sacrum. The resulting shape of the fracture lines on a coronal imaging view resembles the letter ‘U’ or ‘H’, defining its name.
This combined pattern effectively separates the upper sacrum, which remains attached to the lower lumbar spine, from the lower portion of the bone and the remainder of the pelvic ring. This complete mechanical separation is termed spino-pelvic dissociation, indicating a loss of structural integrity between the spine and the pelvis. The injury mechanism is typically a severe, sudden axial load, such as when a person lands forcefully on their feet or buttocks after a high-impact fall.
The U-type fracture falls into the Denis Zone III classification, defined by a fracture line extending into the central sacral canal. This zone is concerning because it houses the cauda equina, the bundle of nerve roots controlling sensation and function in the lower body. The degree of displacement of the upper sacral fragment relative to the lower segment can be further classified, ranging from undisplaced to completely displaced or angulated. All types represent profound structural instability.
Immediate Medical Concerns
The instability of a U-type sacral fracture creates several immediate medical concerns, with neurological compromise being the most frequent complication. Because the fracture violates the central sacral canal, the cauda equina nerve roots are directly exposed to crushing or stretching forces from displaced bone fragments. Injury to this bundle of nerves, which controls vital functions, can lead to cauda equina syndrome.
Patients often present with significant functional deficits, including the inability to control the bladder and bowel. Urinary retention or incontinence and loss of anal sphincter control are hallmark signs of severe sacral nerve root damage. Sensory loss, often described as saddle anesthesia, affects the perineal area, buttocks, and inner thighs, reflecting damage to the S2-S5 nerve roots.
Motor weakness in the lower limbs can also occur, though it is usually less pronounced than the bowel and bladder issues. Beyond the neurological injury, the U-type fracture is a component of a high-energy pelvic ring disruption, which can lead to life-threatening hemorrhage. The severe shearing forces required to create spino-pelvic dissociation often tear the extensive venous plexus and arterial branches within the pelvis.
Uncontrolled bleeding is a major cause of early mortality. Initial trauma management often involves immediate measures to stabilize the pelvic ring to tamponade the bleeding vessels. The magnitude of the force involved means patients frequently have other serious injuries, such as internal organ damage or lower extremity fractures, complicating initial resuscitation and surgical planning. A high percentage of patients with U-type fractures exhibit a pre-operative neurological deficit.
Diagnostic Methods and Imaging
A U-type sacral fracture should be suspected in any patient presenting after a significant vertical deceleration injury, such as a fall from a height. The initial clinical examination focuses on assessing stability and conducting a thorough neurological check, including a rectal examination to test sphincter tone and perianal sensation. However, the true extent of the bony injury is often difficult to appreciate on a standard X-ray.
Plain radiographs of the pelvis can frequently miss or underestimate the severity of the fracture, especially due to overlying bowel gas or the angulation of the fracture line. Therefore, Computed Tomography (CT) is the preferred imaging modality for diagnosing and mapping the fracture pattern. CT provides fine detail of the bone structure, clearly showing the vertical and transverse fracture lines, the degree of displacement, and the extent of bone fragments encroaching upon the central canal.
Magnetic Resonance Imaging (MRI) plays a supplementary but important role in the diagnostic workup, particularly when neurological signs are present. While CT excels at bone visualization, MRI is superior for assessing soft tissues, specifically the integrity of the cauda equina and surrounding ligaments. MRI can detect nerve root compression or laceration, soft-tissue edema, and hematoma, which helps guide the decision for surgical decompression. The combination of CT for bony detail and MRI for neural element assessment is often necessary to fully characterize this complex injury.
Treatment Approaches and Stabilization
The mechanical instability and high risk of neurological damage mean that U-type sacral fractures almost always necessitate surgical intervention. Non-operative management is generally reserved only for minimally displaced fractures without neurological deficits, which is rare for this pattern. The primary goals of surgery are twofold: to decompress the compressed nerve roots and to provide rigid mechanical stabilization of the spino-pelvic junction.
Neurological decompression is achieved by surgically removing bone fragments or soft tissue impinging on the cauda equina, typically through a sacral laminectomy. This aims to relieve pressure on the damaged nerves, offering the best chance for neurological recovery, particularly for bowel and bladder control. The timing of decompression is significant, with earlier intervention often recommended to maximize the potential for nerve healing.
Mechanical stabilization is accomplished using specialized internal fixation techniques designed to bridge the instability. The standard method for an unstable U-type fracture is often lumbopelvic fixation, also known as triangular osteosynthesis. This technique involves placing rods and screws that span the fracture site, connecting the uninjured lumbar spine (L4 and L5 vertebrae) to the ilium and the sacrum. This construct creates a stable, three-dimensional framework that restores spinal alignment and allows for post-operative mobilization and rehabilitation.