Fixing Posterior Pelvic Ring Instability: Sacrum & SI Joint ORIF

Comprehensive Introduction and Patho-Epidemiology

The Burden of Pelvic Ring Trauma

Pelvic ring fractures represent some of the most devastating and complex injuries encountered in orthopedic traumatology. Associated with a diverse assortment of morbidities and mortality rates ranging from 0% to an alarming 50% in severe, open presentations, these injuries demand a profound understanding of pelvic biomechanics, anatomy, and resuscitative protocols. Fractures and dislocations of the pelvis involve, in broad terms, structural failures of the anterior and posterior elements of the pelvic ring. Injuries to the anterior pelvic ring typically include symphyseal disruptions and pubic body or rami fractures, which often serve as the "hinge" or secondary failure point in the ring construct. Conversely, injuries to the posterior pelvic ring—which are the primary weight-bearing conduits of the axial skeleton—involve iliac wing fractures, sacroiliac (SI) joint dislocations, highly unstable fracture-dislocations, and sacral fractures. The clinical implications and definitive treatment options vary widely depending on the spectrum of fracture patterns, the degree of displacement, the vector of potential instability, and the presence of associated visceral or neurovascular injury patterns. This chapter focuses specifically on the definitive surgical management, specifically open reduction and internal fixation (ORIF) and percutaneous techniques, for displaced sacral fractures and SI joint dislocations.

Mechanisms of Injury and Pathogenesis

The sacroiliac ligaments are among the most resilient and robust connective tissues in the human body; therefore, they must be subjected to substantial forces and massive energy transmission to result in catastrophic disruption. Anteroposterior compression (APC) of the pelvic ring, which causes external rotation of the innominate bones (often resulting in an "open book" morphology), is a frequent cause of anterior SI joint disruption and symphyseal diastasis. Lateral compression (LC) injuries, driven by a lateral-to-medial force vector, typically result in internal rotation of the hemipelvis, crushing the anterior sacrum and frequently causing transverse or oblique fractures of the pubic rami. Vertical shear (VS) injuries represent the most unstable pattern, characterized by complete osteoligamentous disruption of both the anterior and posterior ring, allowing cephalad and posterior migration of the hemipelvis. Sacral fractures, however, can occur in three distinctly different clinical scenarios. First, insufficiency fractures of the sacrum arise secondary to physiological loading through excessively osteoporotic or osteopenic bone, often seen in the elderly or those with metabolic bone disease. Second, stress fractures of the sacrum result from fatigue and cyclic failure of normal bone, frequently diagnosed in high-level endurance athletes or military recruits. Finally, traumatic disruptions result from high-energy lateral or anteroposterior compression and/or vertical shear injuries. The most common mechanisms for these high-energy trauma patterns include, in order of decreasing frequency, motorcycle crashes, auto-pedestrian collisions, falls from significant heights, high-speed motor vehicle accidents, and industrial crush injuries.

Natural History and Associated Systemic Injuries

Pelvic fractures occur in at least 20% of blunt trauma admissions, most frequently affecting young, active males. The clinical spectrum ranges from small, insignificant fractures of the pubic rami with no compromise of pelvic ring stability, to major structural disruptions associated with life-threatening exsanguination and catastrophic visceral injury. The pelvic ring anatomically encloses the true pelvis (containing extraperitoneal organs below the pelvic brim) and the false pelvis (containing both peritoneal and retroperitoneal organs above the pelvic brim). The most commonly associated injuries to structures contained within the true pelvis involve the internal iliac arterial and venous systems, the bladder (occurring in approximately 20% of high-energy cases), the urethra (14%), the lumbosacral plexus, and the rectum and vaginal vault (particularly in open pelvic fractures). Injuries to structures within the false pelvis as a direct result of the pelvic fracture are less common, but severe iliac wing fractures with abdominal wall disruption can result in intestinal injury, herniation, and even bowel entrapment.

Morbidity, Mortality, and Clinical Presentation

Morbidity and mortality from pelvic fractures remain dauntingly high and are most commonly secondary to acute retroperitoneal pelvic hemorrhage. The mortality rate associated with a pelvic fracture coupled with a bladder rupture approaches 35% in some historical series. The mortality rate of open pelvic fractures involving the perineum historically reached as high as 50%, though this has decreased to approximately 2% to 10% with the advent of modern damage-control resuscitation, the liberal use of diverting colostomies, and advanced, rapid stabilization techniques (e.g., pelvic binders, external fixation, and pre-peritoneal packing). Neurologic injury to the lumbosacral plexus can lead to profound and permanent sensorimotor dysfunction involving the lower extremities, bowel, bladder, and sexual organs. Because of these associated neurovascular and visceral injuries, pelvic fractures often result in prolonged recovery periods, significant chronic pain, permanent occupational disability, and a devastating loss of psychological and socioeconomic stability.

FIG 4 • Scrotal ecchymosis indicative of internal pelvic hemorrhage traversing fascial planes.

Detailed Surgical Anatomy and Biomechanics

Osseous Architecture and the Pelvic Ring

The pelvis is a complex, multi-planar ring structure composed of the two hemipelves (innominate bones) and the sacrum. Each hemipelvis is the culmination and fusion of three embryonic bony elements: the ilium, the pubis, and the ischium. The two hemipelves are joined anteriorly at the pubic symphysis (an amphiarthrodial symphyseal joint), whereas posteriorly, they articulate with the wings, or alae, of the sacrum via the SI joints to complete the pelvic ring. The sacrum represents the terminal structural segment of the spinal column, serving as the critical mechanical linkage connecting the pelvis and lower extremities to the trunk and axial spine. Being wedge-shaped in both the coronal and axial planes, the sacrum forms a keystone articulation with the innominate bones. However, unlike a true architectural keystone, the sacrum is wider anteriorly and superiorly; thus, axial loading actually drives the sacrum inferiorly and posteriorly, meaning the bony architecture alone is inherently unstable.

FIG 1 • The three embryonic bones (pubis, ischium, ilium) fusing to form the innominate bone or hemipelvis.

Sacral Dysmorphism and Segmentation Anomalies

The sacrum is the caudal segment of the axial skeleton and is, therefore, a spinal element subject to frequent segmentation abnormalities and dysmorphism. Most commonly, segmentation anomalies such as a lumbarized S1 or a sacralized L5 will be present. The only definitive way to ascertain which defect, if any, is present is to count down from the first thoracic vertebra, identified as the first vertebra to have transverse processes inclined cephalad. As a general clinical rule of thumb, the top of the iliac crest is usually at the same level as the L4 or L5 disc space. This rule can be utilized on standard AP radiographs to judge the presence of dysmorphism. Dysmorphic sacra are characterized by upper sacral segments that remain above the pelvic brim, non-recessed alae, acutely angled SI joints, and anomalous sacral nerve root tunnels. These anatomical variations are critically pertinent to the interpretation of the radiographic landmarks required to safely plan and place iliosacral screws. Failure to recognize sacral dysmorphism frequently leads to catastrophic iatrogenic neurologic injury or vascular compromise during percutaneous fixation.

FIG 2C • Intraoperative fluoroscopy and CT imaging demonstrating a unilateral segmentation anomaly with partial sacralization of L5.

Ligamentous Stabilizers of the Posterior Ring

By virtue of the sacrum's shape and orientation, the SI joints are inherently unstable with respect to bony constraints alone. The maintenance of posterior pelvic ring integrity is wholly dependent on the robust support provided by the surrounding ligamentous structures. The pelvic ligaments are strategically structured and positioned to resist multidirectional deformations as the primary static stabilizers of the pelvis; notably, there are no specific dynamic (muscular) stabilizers of the pelvic ring. The SI ligaments (comprising the anterior, interosseous, and posterior SI ligaments) are the strongest ligaments in the human body. The posterior SI ligaments are the most critical in resisting posterior and cephalad displacement of the hemipelvis. The symphyseal ligaments (which themselves contribute no more than 15% to overall pelvic ring stability), alongside the sacrotuberous ligaments and the sacrospinous ligaments, primarily resist external rotation and shear forces.

FIG 2B • SI joint ligaments. 1, posterior; 2, intra-articular; 3, anterior SI ligaments.

Neurovascular Relationships and Surgical Perils

The intricate neurovascular anatomy surrounding the posterior pelvic ring dictates the surgical approaches and safe zones for hardware placement. The bladder is located immediately posterior to the pubic bodies and symphysis, separated only by a thin layer of fat and the potential space of Retzius. Posteriorly, the relationship of the L5 nerve root to the superior aspect of the sacral ala as it courses to join the lumbosacral plexus is a key anatomic feature that must be vigilantly protected during the reduction and stabilization of posterior pelvic ring injuries. The sacral nerve roots course from posterior to anterior, medial to lateral, and proximal to distal as they traverse the upper sacral nerve root tunnels. As they exit the sacrum ventrally, the nerve roots lay on the inferior and lateral aspect of the foramina. Furthermore, the superior gluteal artery lies immediately lateral to the inferior aspect of the SI joint as it arises from the internal iliac artery to exit the greater sciatic notch alongside the superior gluteal nerve. Iatrogenic injury to this vessel during posterior approaches or misplaced iliosacral screws can result in massive, difficult-to-control retroperitoneal hemorrhage.

FIG 3A • Neurovascular structures around the posterior pelvic ring. Note the intimate relationship of the L5 nerve root to the sacral ala.

Exhaustive Indications and Contraindications

Philosophy of Posterior Ring Fixation

The primary goal of surgical intervention in posterior pelvic ring instability is the restoration of anatomical alignment and the provision of rigid mechanical stability. This allows for early patient mobilization, which is critical in mitigating the systemic complications of prolonged recumbency in the polytraumatized patient, such as deep vein thrombosis, pulmonary embolism, decubitus ulcers, and ventilator-associated pneumonia. The decision to proceed with operative fixation hinges on a careful assessment of the patient's hemodynamic status, the morphological classification of the fracture (e.g., Young-Burgess or Tile classifications), and the degree of displacement. In the acute setting, "damage control orthopedics" may dictate the use of provisional external fixation or pelvic binders to restore pelvic volume and tamponade bleeding. Definitive internal fixation is typically delayed until the patient has been adequately resuscitated, coagulopathy has been corrected, and the systemic inflammatory response has optimized.

Specific Indications for Operative Intervention

Operative fixation is strongly indicated for any pelvic ring injury demonstrating posterior instability. This includes Vertical Shear (VS) injuries, which are globally unstable and require robust posterior fixation, often supplemented by anterior stabilization. Anteroposterior Compression Type II and III (APC-II/III) injuries, characterized by disruption of the anterior SI ligaments and complete SI joint dislocation, respectively, necessitate posterior fixation to prevent chronic instability and pain. Lateral Compression (LC) injuries present a more nuanced decision-making process; while LC-I injuries are generally stable and managed non-operatively, LC-II (crescent fractures) and LC-III (windswept pelvis) patterns often require ORIF of the ilium or percutaneous SI fixation to prevent internal rotation deformity and leg length discrepancy. Furthermore, any sacral fracture with greater than 1 cm of displacement, or those associated with progressive neurologic deficits (indicating ongoing nerve root compression), are absolute indications for surgical reduction and stabilization.

Contraindications and Timing

Contraindications to definitive posterior pelvic ring fixation are primarily dictated by the patient's physiological state and the condition of the local soft tissue envelope. Absolute contraindications include profound hemodynamic instability, uncorrected coagulopathy, and severe systemic sepsis. In these scenarios, rapid provisional stabilization with an external fixator or C-clamp is mandated. Local soft tissue compromise, such as a severe Morel-Lavallée lesion (a closed degloving injury with separation of the subcutaneous tissue from the underlying fascia) over the lumbosacral junction, is a strong relative contraindication to open posterior approaches due to the unacceptably high risk of deep postoperative infection. In such cases, percutaneous fixation techniques are heavily favored once the soft tissue swelling has begun to subside. Severe contamination from open pelvic fractures involving the rectum or perineum requires serial debridement and often a diverting colostomy prior to any consideration of internal hardware placement.

Indications and Contraindications Summary

Pre-Operative Planning, Templating, and Patient Positioning

Advanced Clinical Examination and Assessment

Any patient presenting with a history of high-energy trauma or satisfying criteria for a Trauma Alert should be suspected of having a pelvic fracture until definitively ruled out by radiologic and physical examination. The physical examination must strictly adhere to the advanced trauma life support (ATLS) protocol. Examination of the pelvic ring should focus on palpable internal or external rotation instability. This is assessed by manually applying anteroposterior and lateral compressive forces on the iliac wings and crests. However, in an awake patient or one with hemodynamic instability, multiple forceful attempts at manipulating the pelvis are strongly discouraged, as this will exacerbate patient distress and may dislodge nascent hematomas, precipitating catastrophic hemorrhage. A meticulous neurologic examination is paramount, assessing L1 through S3 distributions. Key findings include weakness in the extensor hallucis longus or foot drop (L5 nerve root injury, highly vulnerable at the sacral ala) or loss of plantar flexion and posterior calf sensation (S1 nerve root injury).

Table 1 / FIG • Manual assessment of external rotation pelvic instability.

Radiographic Imaging and 3D Reconstructions

The standard AP pelvis radiograph remains the cornerstone of the initial trauma screening series. A high-quality AP radiograph should have the pubic symphysis colinear with the sacral spinous processes, allowing for precise side-to-side comparison of bony landmarks. The cortical density of the pelvic brim and iliopectineal line must be traced back to its intersection with the lateral margin of the sacral ala. However, plain radiography is insufficient for definitive pre-operative planning of posterior ring injuries. High-resolution computed tomography (CT) with 2D multi-planar reformats (axial, coronal, and sagittal) and 3D surface-rendered reconstructions is the gold standard. CT imaging allows for the precise characterization of sacral fracture morphology (e.g., Denis zones I, II, or III), the assessment of sacral comminution, and the identification of sacral dysmorphism. The axial and coronal CT slices are critical for evaluating the neural foramina and the available osseous corridors for iliosacral screw placement.

FIG 2D • Three-dimensional CT reconstruction highlighting posterior ring disruption and segmentation anomalies.

Pre-Operative Templating and Safe Zone Calculation

Pre-operative templating is an absolute necessity, particularly when planning percutaneous iliosacral screw fixation. The surgeon must meticulously evaluate the CT scan to identify the "safe zones" within the S1 and S2 vertebral bodies. The S1 safe zone is bounded anteriorly by the presacral cortex, superiorly by the L5-S1 disc space and the L5 nerve root, inferiorly by the S1 neural foramen, and posteriorly by the sacral spinal canal. In patients with normal sacral anatomy, the S1 corridor is typically capacious enough to accept one or two 7.0 mm or 7.3 mm cannulated screws. However, in the presence of sacral dysmorphism, the S1 corridor is often narrow, obliquely oriented, and unsafe for standard transverse screw placement. In these instances, the surgeon must either utilize an "in-out-in" trajectory (where the screw safely exits the anterior sacral cortex and re-enters the vertebral body) or rely on the S2 corridor, which is generally unaffected by upper sacral dysmorphism but is inherently smaller in cross-sectional area.

Patient Positioning and Operating Room Setup

Optimal patient positioning and operating room setup are critical for the successful execution of posterior pelvic ring fixation. For percutaneous iliosacral screw placement or anterior ORIF of the SI joint, the patient is typically positioned supine on a completely radiolucent Jackson table or a flat carbon-fiber table. A bump may be placed under the lumbosacral junction to accentuate the lumbar lordosis and facilitate fluoroscopic imaging. The C-arm must be positioned to allow unimpeded, rapid transitions between the AP, Inlet (typically 25° to 30° caudad), and Outlet (typically 40° to 45° cephalad) views. For open posterior approaches to the sacrum or SI joint, the patient is positioned prone. Meticulous padding of all bony prominences and the avoidance of abdominal compression are essential to prevent ocular ischemic neuropathy and to decrease epidural venous pressure, thereby minimizing intraoperative bleeding.

Step-by-Step Surgical Approach and Fixation Technique

Percutaneous Iliosacral Screw Fixation

Percutaneous iliosacral screw fixation has become the workhorse technique for the stabilization of minimally displaced SI joint dislocations and sacral fractures. The procedure relies entirely on perfect intraoperative fluoroscopy. After closed reduction is achieved (often utilizing skeletal traction, internal rotation of the lower extremities, or a pelvic reduction frame), a perfectly superimposed lateral sacral view is obtained to identify the alar cortical density and the S1 neural foramen. A guide pin is inserted through the lateral ilium and advanced into the sacrum under alternating Inlet and Outlet fluoroscopic guidance. The Inlet view ensures the pin remains within the anterior-posterior confines of the sacral body, avoiding the spinal canal and the presacral space. The Outlet view confirms the superior-inferior trajectory, ensuring the pin is safely above the S1 neural foramen and below the L5-S1 disc space. Once the guide pin is correctly positioned across the SI joint and into the sacral body, a cannulated drill is used, followed by the insertion of a fully or partially threaded 7.0 mm or 7.3 mm screw. Washers are utilized to prevent the screw head from sinking into the thin iliac cortex.

FIG 3B • Intraoperative fluoroscopy demonstrating the precise trajectory required to avoid neurovascular compromise during percutaneous screw insertion.

Open Reduction and Internal Fixation (ORIF) of the SI Joint

When closed reduction is unachievable, or in the setting of a highly displaced SI joint dislocation with interposed soft tissue, open reduction is mandated. The anterior approach to the SI joint is typically performed via the lateral window of the ilioinguinal approach or a modified Smith-Petersen approach. The iliacus muscle is elevated from the inner table of the ilium, exposing the anterior SI joint. Care must be taken to identify and protect the L5 nerve root, which lies mere millimeters medial to the joint line. Reduction is facilitated using specialized pelvic reduction clamps (e.g., Jungbluth clamps or Weber clamps) placed on pre-drilled 4.5 mm cortical screws in the ilium and the sacral ala. Once anatomical reduction of the anterior SI joint line is achieved, fixation is typically accomplished using two orthogonal, low-profile symphyseal or pelvic reconstruction plates positioned across the joint, secured with multiple cortical screws.

Posterior Approaches and Trans-Sacral Plating

For highly comminuted transforaminal sacral fractures (Denis Zone II) or central sacral canal fractures (Denis Zone III) with spinopelvic dissociation, a posterior approach is often required. A midline or bilateral parasagittal incision is made, and the multifidus and erector spinae muscles are elevated to expose

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