Monoaxial vs. Polyaxial Percutaneous Hybrid Stabilization

Monoaxial vs. Polyaxial Percutaneous Hybrid Stabilization for Thoracolumbar Fractures in Elderly Patients: A Randomized, Single-Blind Pilot Study

Vertebral body fractures in older adults mostly affect the thoracolumbar junction and are challenging to treat due to osteoporosis and other comorbidities. Treatment options range from conservative approaches to minimally invasive procedures like vertebroplasty or balloon kyphoplasty, as well as surgical techniques such as posterior or combined stabilization. A common method for more severe fractures (from OF3 onwards) is percutaneous bisegmental hybrid stabilization using cement-augmented pedicle screws and balloon kyphoplasty. Polyaxial screws are easier to implant but offer less biomechanical stability. In contrast, monoaxial screws provide greater stiffness, allowing better correction of kyphosis and restoration of vertebral body height. The study compares radiological and clinical outcomes in patients aged 60 and older with thoracolumbar fractures (T11-L4) between monoaxial and polyaxial hybrid stabilization. The primary goal is to evaluate and compare the degree of kyphosis correction six months after surgery.

Study Overview

• Status: Not yet recruiting
• Conditions: Fractures in the Elderly; Thoracolumbar
• Intervention / Treatment: Device: Monoaxial instrumentation - the M.U.S.T. Monoaxial Pedicle Screw Cannulated (Medacta International SA, Switzerland); Device: Polyaxial instrumentation - the M.U.S.T. LT Pedicle Screw System (Medacta International SA, Switzerland)

Detailed Description

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).

• Study Type: Interventional
• Enrollment (Estimated): 44
• Phase: Not Applicable

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

Patient-related criteria:

1. Age ≥ 60 years (male and female patients)
2. Ability to understand nature, risks, and benefits of study participation and to provide written informed consent.
3. Psychosocially, mentally, and physically capable of complying with all study requirements, including scheduled follow-up visits, completion of forms, and participation in all procedures as defined by the protocol.
4. Ambulatory prior to fracture, with or without the use of assistive devices

Fracture-related criteria:

1. Acute vertebral fractures located between T11 and L4 (inclusive)
2. Fractures deemed suitable for bisegmental hybrid stabilization based on clinical and radiological assessment - primarily OF type 3 and type 4 fractures according to DGOU classification; type 2 and 5 fractures may also be included based on individual evaluation, with the final treatment decision left to the discretion of the treating surgeon.

Exclusion Criteria:

Patient-related criteria:

1. Body mass index (BMI) > 35
2. Permanent or progressive neurologic deficits (including upper motor neuron disease and myelopathy)
3. Rheumatoid arthritis or known disorders of bone metabolism (excluding osteopenia/osteoporosis, Vitamin D deficiency)
4. History of radiation therapy to the spine
5. History of alcohol or drug abuse within the last 2 years prior to randomization
6. Current therapy with high-dose systemic corticosteroids (e.g., >10 mg/day prednisone equivalent for more than 3 months)
7. Ongoing or planned treatment with cytotoxic chemotherapy (excluding biologicals, immunotherapy, or targeted agents) at the time of enrollment
8. Known allergy to any component of the spinal instrumentation
9. Participation in other clinical investigations (drug or device) that may interfere with the outcomes of this study

Fracture-related criteria:

1. Vertebral fractures associated with neurological deficits
2. Fracture age > 4 weeks at the time of enrollment
3. Pathological fractures due to spinal infection or metastatic bone disease
4. Planned major spinal surgery within 12 months of enrollment
5. Severe spinal deformities or fusion at the index or adjacent segments
6. Previous spinal surgery involving the index or adjacent vertebrae, with the exception of minor non-instrumented procedures, such as simple sequestrectomies or decompressions

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Active Comparator: Arm A: Monoaxial hybrid stabilization; Monoaxial pedicle screw hybrid stabilization is a routinal instrumentation technique and will be compared to the polyaxial hybrid stabilization system, that is an routinal instrumentation at the site as well.	Device: Monoaxial instrumentation - the M.U.S.T. Monoaxial Pedicle Screw Cannulated (Medacta International SA, Switzerland); In this study, fractures between the 11th thoracic (T11) and 4th lumbar (L4) vertebra (both included) will be treated surgically using either monoaxial or polyaxial bisegmental hybrid stabilization. Hybrid stabilization consists of posterior cement-augmented pedicle screw instrumentation combined with balloon kyphoplasty of the fractured vertebral body. For bisegmental hybrid stabilization, standard cannulated and perforated pedicle screws, either monoaxial (M.U.S.T. Monoaxial Pedicle Screw Cannulated (Medacta International SA, Switzerland)) or polyaxial (M.U.S.T. LT Pedicle Screw System (Medacta International SA, Switzerland)), will be placed in the vertebrae adjacent to the fractured vertebral body.
Active Comparator: Arm B: Polyaxial hybrid stabilization; Polyaxial pedicle screw hybrid stabilization is a routinal instrumentation technique and will be compared to the monoaxial hybrid stabilization system, that is an routinal instrumentation at the site as well.	Device: Polyaxial instrumentation - the M.U.S.T. LT Pedicle Screw System (Medacta International SA, Switzerland); In this study, fractures between the 11th thoracic (T11) and 4th lumbar (L4) vertebra (both included) will be treated surgically using either monoaxial or polyaxial bisegmental hybrid stabilization. Hybrid stabilization consists of posterior cement-augmented pedicle screw instrumentation combined with balloon kyphoplasty of the fractured vertebral body. For bisegmental hybrid stabilization, standard cannulated and perforated pedicle screws, either monoaxial (M.U.S.T. Monoaxial Pedicle Screw Cannulated (Medacta International SA, Switzerland)) or polyaxial (M.U.S.T. LT Pedicle Screw System (Medacta International SA, Switzerland)), will be placed in the vertebrae adjacent to the fractured vertebral body.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Overall bisegmental kyphosis correction	Overall bisegmental kyphosis correction (OKC) at 6 months postoperatively: The difference between preoperative bisegmental Cobb angle (BCA) upright and BCA at 6 months (upright) postoperatively describes the overall bisegmental kyphosis correction (OKC) at 6 months postoperatively.	6 month

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Radiological outcome - Loss of bisegmental kyphosis correction	Defined as: BCA after postoperative mobilization minus BCA at 6-month FU in degrees	6 month
Radiological outcome - Vertebral wedge angle (VWA)	Defined as the angle between the superior and inferior endplates of the fractured vertebra in degrees.	1 Year
Radiological outcome - Vertebral body height (VBH) in %	The anterior, central, and posterior VBH of the fractured vertebra (V2) will be calculated relative to the adjacent intact vertebrae above (V1) and below (V3), using the following formula: VBH = V2/((V1+V3)/2)*100%.	1 Year
Radiological outcome - Vertebral body compression percentage (VBCP) in %	The percentage of anterior, central and posterior compression of the fractured vertebra (V2) in relation to V1 and V3, calculated as: VBCP = (((V1 + V3)/2 - V2)/(V1 + V3)/2))*100%	1 Year
Radiological outcome - Disc height (DH) [mm]	Anterior, central and posterior DH will be measured for the intervertebral discs immediately above and below the fractured vertebra. Implanted rods will serve as a reference to correct for potential magnification differences between radiographs. Results in mm	1 Year
Radiological outcome - The upper disc angle (UDA) and lower disc angle (LDA)	UDA: Angle between the superior endplate of the fractured vertebra and the inferior endplate of the vertebra above. LDA: Angle between the inferior endplate of the fractured vertebra and the superior endplate of the vertebra below in degree.	1 year
Radiological outcome - Accuracy of pedicle screw placement	Evaluated on postoperative CT using multiplanar reconstructions. Cortical pedicle breaches will be classified by grades based on 2 mm increments: Grade A - Grade D	1 week Prior to post-op mobilization
Radiological outcome - cement leakage	defined as the presence of any extravertebral cement, will be assessed on postoperative CT imaging at the upper and lower instrumented vertebra (following screw augmentation) as well as the fractured vertebra (following kyphoplasty). The route of leakage will be classified according to Yeom et al. as type B (leakage via the basivertebral vein), type S (leakage via the segmental vein), or type C (leakage through a cortical defect). The anatomical location of leakage will be categorized as perivertebral, spinal canal, intraforaminal, intradiscal, paravertebral soft tissues, or mixed type of leakage.	1 week Prior to post-op mobilization
Radiological outcome - Vertebral body height (VBH at Computertomography) in mm	Anterior, central, and posterior VBH of the fractured vertebra will be measured and compared across time points to assess height restoration and maintenance over time.	6 month
Radiological outcome - Fracture consolidation	Defined as the presence of bridging trabecular bone across fracture lines and disappearance of visible fracture lines, and no progressive displacement. Consolidation status will be independently assessed by two observers using the 6-month CT scans.	6 month
Radiological outcome - Pedicle screw loosening	Assessed on 6-month CT or earlier if symptomatic screw loosening is suspected. Loosening will be defined as the presence of radiolucent zones around the screws exceeding typical metal artifact patterns, as well as any evidence of screw migration or pull-out when compared to the immediate postoperative CT Screw loosening will be independently assessed by two observers.	6 month
Complications	All complications that occur within 1 year time frame compared between the groups.	1 year
Functional outcome - AO Spine PROST	Patient reported functional outcome score measured with AOSpine Patient Reported Outcome Spine Trauma compared outcome between the two groups. The scale ranges from 0 to 100 points. 0 indicates no function and 100 best function that can be imagined.	1 year
Functional outcome - Oswestry Disability Index (ODI)	Patient reported functional outcome score measured with the Oswestry Disability Index (ODI) compared between the two groups. The score has its range from 0 - 50 points, while 50 points is stated as 100%! 80-100% will be scored as bedridden/functionally impaired while 0 point = 0% means mild disability (0-20%). The less the score result the better the function.	1 year
Functional outcome - numeric rating scale	Pain level compared between the groups during the postoperative treatment. 0 - 10 points, while 10 points means worst pain imagined and 0 points = no pain.	1 year
Functional outcome -EQ-5D-5L	Patient reported functional and general health outcome score measured with EQ-5D-5L compared between the two groups. Level 1 = indicating no problem, 5= indicating unable to/extreme problems. A unique code of 5 digits is given. The higher the level the worse is the funtion/general health. The EQ VAS score is rated on a scale of 0-100 points. 0 points correspond to the worst possible state of health, while 100 points correspond to the best possible state of health.	1 year
Functional outcome - Barthel index	Patient reported functional outcome score of daily living and selfcare measured with the Barthel Index.The total score ranges from 0 to 100, with 100 points indicating complete independence and lower scores indicating increasing dependence.	1 year
Functional test - Timeg up and Go score	Functional test of gait speed measured with TUG compared between the two groups postoperatively. The results are given in seconds. Scores under 10 seconds are generally considered normal, while scores over 13.5 to 14 seconds indicate an increased risk of falls. A score above 20 seconds often signals good mobility but with potential issues, and a score over 30 seconds typically indicates a high risk of falls and a need for assistance with mobility.	6weeks - 1 Year
Functional outcome - Parker mobility score (PMS)	Patient reported functional outcome score of patients mobility measured with them PMS compared between the two groups. 0-9 point assessment of mobility, with 9 being the highest score for the best mobility	1 year
Radiological outcome - Bisegmental Cobb Angle (BCA)	Measured preoperatively (on both supine (BCApreop supine) and upright (BCA) radiographs), after postoperative mobilization, and at 6 weeks, 3 months, 6 months, and 12 months postoperatively Defined as the angle between the superior endplate of the vertebral body above and the inferior endplate of the vertebral body below the fractured vertebra.	1 Year
Radiological outcome - Surgical bisegmental kyphosis correction (SKC)	Defined as: Preoperative BCApreop_upright minus BCA after postoperative mobilization in degrees. Both measured on upright radiographs.	1-2 weeks after post-op mobilization
Radiological outcome - overall bisegmental kyphosis correction (OKC) supine	Defined as: Preoperative BCApreop supine (measured on preoperative supine radiographs) - BCA at 6 months postoperatively in degrees.	6 month
Radiological outcome - Surgical bisegmental kyphosis correction (SKC) supine	Defined as: Preoperative BCA preop_supine - BCA after postoperative mobilization in degrees.	1-2 weeks after post-op mobilization

Collaborators and Investigators

• Sponsor: Medical University Innsbruck

Study record dates

Study Major Dates

• Study Start (Estimated): May 1, 2026
• Primary Completion (Estimated): September 1, 2028
• Study Completion (Estimated): November 1, 2028

Study Registration Dates

• First Submitted: September 17, 2025
• First Submitted That Met QC Criteria: February 6, 2026
• First Posted (Actual): February 13, 2026

Study Record Updates

• Last Update Posted (Actual): April 9, 2026
• Last Update Submitted That Met QC Criteria: April 3, 2026
• Last Verified: September 1, 2025

More Information

Terms related to this study

• Keywords: Thoracolumbar fractures; Percutaneous hybrid stabilization
• Other Study ID Numbers: Mono vs polyaxial PHS

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No