Monoaxial vs. Polyaxial Percutaneous Hybrid Stabilization

Vertebral fractures in elderly patients predominantly affect the thoracolumbar spine and continue to pose a significant clinical challenge. These fractures have distinct characteristics, are increasing in incidence, and represent a major source of morbidity in this population. Patients typically present with impaired bone quality and significant comorbidities, which further complicate management. The optimal treatment of these fractures remains controversial. Treatment options range from conservative therapy and isolated vertebral body augmentation procedures - such as vertebroplasty or ballon kyphoplasty - to various posterior instrumentation techniques, and in selected cases, combined posteroanterior surgical stabilization. The choice of treatment depends not only on the specific fracture characteristics but is also influenced by the individual patient's overall condition. There is general consensus that the goals of surgical treatment include providing sufficient stability to enable early mobilization and prevent progressive deformity, achieving reduction of fracture-induced malalignment to restore sagittal alignment, decompressing neural elements in cases of neurological deficits, and ultimately maximizing clinical outcomes.

Percutaneous bisegmental hybrid stabilization is considered a standard treatment option for thoracolumbar fractures in elderly patients [6; 7], particularly for type 3 fractures or higher, as defined by the classification of the Spine Section of the German Society for Orthopaedics and Trauma (DGOU). This stabilization technique involves posterior cement-augmented pedicle screw instrumentation at one level above and below the fracture, combined with balloon kyphoplasty of the fractured vertebral body. While several studies have reported outcomes following combined bisegmental uncemented pedicle screw instrumentation and cement augmentation of the fractured vertebral body, specific data on percutaneous bisegmental hybrid stabilization - incorporating cement augmentation of both the pedicle screws and the fractured vertebra - remain very limited. In a retrospective study of 113 patients aged 60 years or older who underwent polyaxial percutaneous bisegmental hybrid stabilization, reported five in-patient complications (4.4%) and an average Oswestry Disability Index (ODI) score of 29.9±22.2 (range: 0-80) after a mean follow-up of 48 months (range: 24-78 months). The mean loss of reduction was 7.4°±5.6° (range: 0°-25°), with 22% of patients exhibiting a loss of ≥ 10°. Furthermore, a significant correlation was observed between the extent of reduction loss and ODI scores. In a subsequent analysis, the authors identified the superior intervertebral disc adjacent to the fractured vertebral body and the central portion of the vertebral body itself as the primary contributors to reduction loss. Notably, the relative loss of central vertebral body height was the only parameter significantly associated with the overall loss of reduction.

Hybrid stabilization has traditionally been performed using polyaxial pedicle screws, as cannulated and perforated pedicle screws designed for PMMA cement augmentation were only available in the polyaxial variant from various manufacturers. Moreover, polyaxial pedicle screws are commonly used for percutaneous instrumentation due to the ease of rod insertion. However, from a biomechanical perspective, polyaxial screws have several limitations in effective fracture stabilization, particularly when it comes to achieving and maintaining fracture reduction. In cases of a vertebral body fractures, the load-bearing capacity of the anterior column is typically compromised. In such scenarios, the ability of pedicle screw constructs to resist flexion and anterior compressive forces is critical to maintain the intraoperatively achieved fracture reduction. Monoaxial pedicle screws, when attached to a rod, form an angular-stable construct due the fixed axis between the screw head and shaft. In contrast, polyaxial screws feature a screw head that can swivel freely in multiple planes and is connected to the shaft via a coupling mechanism, potentially reducing angular stability between the screw head and shaft. Biomechanical studies have demonstrated that polyaxial screw constructs exhibit reduced stiffness compared to traditional monoaxial systems and fail at lower load levels, typically due to slippage at the screw-head interface. This polyaxial screw-head angular change may result in loss of fracture reduction and contribute to sagittal malalignment. In addition to reduced construct stiffness and the risk of postoperative reduction loss, intraoperative fracture reduction can also be challenging when using polyaxial screws for thoracolumbar fracture stabilization. With monoaxial screws, fracture reduction is typically achieved through direct screw manipulation and attachment to a lordotically pre-contoured rod. In contrast, due to the mobility of the screw heads in polyaxial systems, screw manipulation is less effective, and fracture reduction relies primarily on patient positioning. Consistent with these biomechanical considerations, several clinical studies - predominantly in younger patients - have demonstrated that overall kyphosis correction and vertebral body height restoration are significantly better following monoaxial instrumentation compared to polyaxial constructs. As a result, it has been questioned whether polyaxial instrumentation is capable of restoring anatomic sagittal alignment in the long term, or whether it should be regarded primarily as a means of preventing further collapse of the fractured vertebral body.

Taken together, hybrid stabilization using monoaxial screws may offer superior correction of fracture-induced kyphotic deformity, improved vertebral body height restoration, and reduced postoperative loss of reduction compared to hybrid stabilization with polyaxial screws.

The overall aim of this study is to compare radiological and clinical outcomes following percutaneous bisegmental hybrid stabilization using monoaxial versus polyaxial pedicle screws in patients aged 60 years or older with thoracolumbar fractures (T11-L4).