Werner Schmoelz

Dynamic Stabilization of the Lumbar Spine and Its Effects on Adjacent Segments An In Vitro Experiment

In recent years, nonfusion stabilization of the lumbar spine has gained more and more popularity. These nonfusion systems intend to maintain or restore the intersegmental motions to magnitudes of the intact spine and have no negative effects on the segments adjacent to the stabilized one. This study investigated the DYNESYS, a dynamic nonfusion system, which is designed to stabilize the bridged segments while maintaining the disc and the facet joints. To determine the magnitude of stabilization and the effect of the stabilization on the adjacent segment, six lumbar cadaver spines were fixed in a spine tester and loaded with pure moments in the three main motion planes. For each spine, four different stages were tested: intact, defect of the middle segment, fixation with the DYNESYS, and fixation with the internal fixator. Intersegmental motions were measured at all levels. For the bridged segment, the DYNESYS stabilized the spine and was more flexible than the internal fixator. This difference between the internal fixator and the DYNESYS was most pronounced in extension (P < 0.05), with the DYNESYS restoring the motion back to the level of the intact spine. The motion in the adjacent segments was not influenced by either stabilization method. Our results suggest that the DYNESYS provides substantial stability in case of degenerative spinal pathologies and can therefore be considered as an alternative method to fusion surgery in these indications while the motion segment is preserved.

The effect of in situ augmentation on implant anchorage in proximal humeral head fractures

INTRODUCTION Fracture fixation in patients suffering from osteoporosis is difficult as sufficient implant anchorage is not always possible. One method to enhance implant anchorage is implant/screw augmentation with PMMA-cement. The present study investigated the feasibility of implant augmentation with PMMA-cement to enhance implant anchorage in the proximal humerus. MATERIALS AND METHODS A simulated three part humeral head fracture was stabilised with an angular stable plating system in 12 pairs of humeri using six head screws. In the augmentation group the proximal four screws were treated with four cannulated screws, each augmented with 0.5ml of PMMA-cement, whereas the contra lateral side served as a non-augmented control. Specimens were loaded in varus-bending or axial-rotation using a cyclic loading protocol with increasing load magnitude until failure of the osteosynthesis occurred. RESULTS Augmented specimens showed a significant higher number of load cycles until failure than non-augment specimens (varus-bending: 8516 (SD 951.6) vs. 5583 (SD 2273.6), P=0.014; axial-rotation: 3316 (SD 348.8) vs. 2050 (SD 656.5), P=0.003). Non-augmented specimens showed a positive correlation of load cycles until failure and measured bone mineral density (varus-bending: r=0.893, P=0.016; axial-rotation: r=0.753, P=0.084), whereas no correlation was present in augmented specimens (varus-bending: r=0,258, P=0.621; axial-rotation r=0.127, P=0.810). CONCLUSION These findings suggest that augmentation of cannulated screws is a feasible method to enhance implant/screw anchorage in the humeral head. The improvement of screw purchase is increasing with decreasing bone mineral density.

Higher risk of adjacent segment degeneration after floating fusions: long-term outcome after low lumbar spine fusions.

Study Design We report the long-term outcome after mono-segmental and bisegmental fusions at the lumbar L4-S1 region of the spine. Objective Long-term clinical and radiologic outcome measures were used to determine a lumbar fusions contribution to degenerative changes in adjacent motion segments (ASD). Summary of Background Data The role of low lumbar spinal fusions and their long-term contribution to accelerated degenerative changes in the adjacent motion segments continues to be a subject of controversy. Patients and Methods We followed-up 102 patients with an average age of 54 (22 to 78) years and a follow-up time of 14 (3 to 22) years. Results Overall results in patients were good, the Oswestry-Disability Index (ODI) showed an average of 26% (0% to 70%) at follow-up, the Visual Analog Scale rose from 2.7 (postoperative) and 2.9 (12 wk follow-up) to 3.6 (latest follow-up) points, respectively. Patient satisfaction with their health-related situation at follow-up was 69% (15% to 100%). Patients who underwent fusions of the segment L5/S1 showed a significant (P<0.05) lower risk for ASD than patients with fusions L4/5 (20% vs. 46%). Compared with L4/5 fusions, bisegmental L4-S1 fusions showed a similar trend (P=0.06) with a lower risk for ASD (24%). Objective and subjective clinical results showed no differences between these groups. Patients suffering from ASD showed significant (P<0.05) reduced sacral inclination and lumbar lordosis angles and also significant (P<0.05) higher ODI values compared with non-ASD patients. Conclusions We conclude that floating fusions of single low lumbar segments are more likely to result in ASD and are negatively influenced by sagittal plane abnormalities.

Revision of cannulated and perforated cement-augmented pedicle screws: a biomechanical study in human cadavers.

Study Design. Biomechanical investigation of primary and revised cement-augmented pedicle screws in comparison with unaugmented screws. Objective. To evaluate revision of cannulated pedicle screws and investigate cement-augmented and nonaugmented screws biomechanically, testing the torque of primary screws and axial pullout force of revised screws in cadaver vertebrae. Summary of Background Data. Cement augmentation increases the pullout force and stability of pedicle screws in vertebrae with low bone mineral density, but surgeons are concerned about complications during revision. Methods. Bone mineral density was measured using quantitative computed tomography (CT) in 23 osteoporotic thoracolumbar junction vertebrae from human cadavers. Cannulated pedicle screws, augmented with bone cement (on right) or unaugmented (left), were inserted into each vertebra. After CT control, extraction torque was measured and the pedicles were reinstrumented with larger-diameter screws. The right screws were augmented again, with another CT control, before pullout testing. Results. Mean vertebral bone density was 52.6 mg/cm3. No major screw malpositioning was observed on primary CTs. Cement leakage was observed anterolaterally and into the spinal canal. Mean maximal torque in augmented screws (1.2 Nm, SD: 0.6) differed significantly from nonaugmented screws (0.8 Nm, SD: 0.6). Screw removal did not lead to vertebral destruction. No relevant changes due to positioning or leakage were observed on CT after revision procedures compared with primary findings. Maximal pullout force in revised augmented screws (713.2 N, SD: 254.6) differed significantly compared with nonaugmented screws (554.0 N, SD: 296.5). Bone damage was observed in several vertebrae during pullout force testing in augmented screws. Conclusion. Revision of cement-augmented pedicle screws was feasible without bone destruction, and larger-diameter screws can be used in revision procedures. The pullout force after revision was significantly better in cement-augmented screws. During pullout testing, the cement-bone interface broke before the screw-cement interface in several vertebrae, fracturing the pedicles.

In vitro stabilizing effect of a transforaminal compared with two posterior lumbar interbody fusion cages

Study Design. An in vitro biomechanical flexibility test on different lumbar interbody fusion cages using monosegmental lumbar spine specimens. Objective. To investigate the stabilizing effect of a transforaminal lumbar interbody fusion (TLIF) cage compared with two established posterior lumbar interbody fusion (PLIF) cages. Summary of Background Data. TLIF using interbody fusion cages is gaining more and more popularity in the treatment of degenerative disc disease. However, only little is known on its biomechanical behavior. Methods. Eighteen intact human lumbar spine segments were tested for flexibility in a specially designed spine tester. Pure moments were applied in the three main planes, and range of motion and neutral zone were determined. Then, TLIF using the sickle-shaped MOON cage (AMT AG), PLIF using the cubic Stryker cages (Stryker Orthopaedics), or PLIF using the threaded BAK cages (Zimmer Spinetech) was carried out and the specimens tested again. Results. The stability after implantation of the MOON TLIF cage did not significantly differ from that after implantation of the cubic Stryker PLIF cages (P > 0.05). In contrast, the threaded BAK PLIF cages had a significantly higher primary stability than both the MOON TLIF and the Stryker PLIF cages in lateral bending, flexion, and extension (P < 0.05) but not in axial rotation (P > 0.05). Conclusions. In terms of its stabilizing effect, TLIF using the MOON cage can be recommended as an alternative to PLIF using the cubic Stryker cages. Compared with the threaded BAK PLIF cages, however, the MOON TLIF cage provides a lower primary stability in lateral bending, flexion, and extension.

Angular Stable Anterior Plating Following Thoracolumbar Corpectomy Reveals Superior Segmental Stability Compared to Conventional Polyaxial Plate Fixation

Study Design. Biomechanical in vitro testing of primary and secondary stability in 12 human thoracolumbar spinal specimens using a spine simulator. Objective. In a corpectomy model anterior plate systems were investigated for their ability to restore spinal stability particularly focusing on the influence of angular stability, bone mineral density (BMD) and failure mode. Summary of Background Data. The concept of isolated anterior column reconstruction following thoracolumbar fractures using newly developed minimally invasive spine surgical techniques has attracted major clinical interest. In analogy to angular stable plate systems in long bone fixation the application of locking plates to the spine is aimed to limit loss of reduction and to improve stability. Methods. Twelve human spinal specimens (Th11-L3) were tested in a 6-degree-of-freedom spine simulator under pure moments of 7.5 Nm to investigate primary and secondary stiffness of 2 different anterior reconstruction options: (1) Synex II cage and MACS TL polyaxial anterior plating system, (2) Synex II cage and ArcoFix angular stable anterior plating system. An increasing 4-step cyclic loading model was included. Results. The angular stable plate system showed superior stability compared to the nonangular system in axial rotation and lateral bending. Flexion/extension loading demonstrated no difference between the systems in range of motion. A positive correlation between BMD and the number of load cycles until failure for the nonangular stable system (R2 = 0.90) was found. Different failure modes were investigated for the plating systems. The MACS system showed loosening at the connection between screw and plate inducing tilting under flexural load and final failure. The ArcoFix system revealed increased stability under cyclic loading and failed by parallel sintering to the endplate. Conclusion. Anterior angular stable fixation showed higher primary and secondary stability following thoracolumbar corpectomy. In specimens with lower BMD the use of angular stable systems substantially increased stability. Angular stable systems, however, differ in the way of construct failure.

Vertebroplasty with high-viscosity polymethylmethacrylate cement facilitates vertebral body restoration in vitro.

Study Design. In vitro biomechanical study on 6 fresh frozen human thoracolumbar spine specimens. Objective. Using a novel high viscosity polymethylmethacrylate (PMMA) cement and vertebroplasty kit to correct the kyphosis angle of wedge compression fractures (AO/ASIF 1.2). Summary of Background Data. Vertebroplasty is typically used to stabilize vertebral compression fractures in situ without correcting kyphosis, with the main target to reduce pain and disability. The vertebroplasty system investigated in this study comprises a high viscosity PMMA cement and uses a hydrostatic pressure hand piece for enhanced cement allocation and flow control. A recent clinical trial demonstrated a significantly reduced incidence of cement leakage with this system. Methods. Six spinal segments (Th11-L1 and Th12-L2) were loaded in a spine tester with pure moments of 7.5 Nm in lateral bending, flexion/extension and axial rotation. The segmental range of motion (ROM) was continuously recorded. The tested states of the specimens were: intact (a), fractured (b), treated with vertebroplasty (c), after loading with 50 to 250 N (d), 50 to 450 N (e) and 50 to 650 N (f) of 1000 cycles each. In each state (a–f), the kyphosis angle was documented fluoroscopically. Results. Kyphosis angle was significantly reduced between intact and fractured states (P < 0.02). Between treated and fractured states, we found highly significant difference (P < 0.001), indicating full correction. During 3000 loading cycles (50–250, 50–450, and 50–650 N), the kyphosis angle remained constant compared to the treated state (P = 1.0). We noted a logistic relationship between injected cement volume and extent of kyphosis correction (R = 0.89, P < 0.001). In the fractured state, the ROM in flexion/extension increased to 252% of the intact state (P < 0.001). The vertebroplasty treatment decreased ROM to 72% of fractured state in flexion/extension (P < 0.001). Macroscopic inspection of the vertebrae after testing showed an intact interface and tight mechanical interlocking of cement filling and trabecular bone. Conclusion. High viscosity vertebroplasty effectively reduced and stabilized thoracolumbar wedge compression fractures and may represent a one-step solution for restoring vertebral body dimensions following thoracolumbar compression fractures, while minimizing the risk of cement leakage and associated complications in vivo.

Biomechanical comparison of an angular stable plate with augmented and non-augmented screws in a newly developed shoulder test bench

BACKGROUND The proximal humeral fracture is one of the most common fractures. Although there are a number of treatment options available, the clinical outcomes in geriatric patients are still unsatisfactory. Therefore, the aim of this study was to investigate the biomechanical behaviour of an angular stable plate with either augmented or non-augmented screws using two different fracture models in a shoulder test bench with active muscle forces. METHODS Six paired fresh-frozen humeri were loaded into a shoulder test bench simulating ab- and adduction between 15 and 45° induced by active muscle forces. The bone mineral density was measured by a quantitative CT. A two-part fracture model (stable and an unstable) was used to investigate the different biomechanical behaviours of the PHILOS plate, either utilising cannulated screws, allowing in situ augmentation, or without utilising augmented screws. Four screws were augmented with 0.5ml PMMA cement. FINDINGS The in vitro-measured resulting forces in the glenoid fossa were comparable to the in vivo forces generated in shoulder arthroplasties. Under stable conditions, the per cycle motion and varus impaction tilting showed no significant difference. In the unstable state, the augmented group showed a maximum of 0.81° per cycle motion and a maximum varus impaction of -1.46° compared to the non-augmented maximum of 2.31° per cycle motion and maximum varus impaction of -4.26° (P<0.05). INTERPRETATION In an unstable fracture model under dynamic testing conditions, augmentation leads to a decreased per cycle motion and varus impaction of the humeral head.

Erratum to: Effect of augmentation techniques on the failure of pedicle screws under cranio-caudal cyclic loading

Purpose Augmentation of pedicle screws is recommended in selected indications (for instance: osteoporosis). Generally, there are two techniques for pedicle screw augmentation: inserting the screw in the non cured cement and in situ-augmentation with cannulated fenestrated screws, which can be applied percutaneously. Most of the published studies used an axial pull out test for evaluation of the pedicle screw anchorage. However, the loading and the failure mode of pullout tests do not simulate the cranio-caudal in vivo loading and failure mechanism of pedicle screws. The purpose of the present study was to assess the fixation effects of different augmentation techniques (including percutaneous cement application) and to investigate pedicle screw loosening under physiological cyclic cranio-caudal loading.

Primary stiffness of a modified transforaminal lumbar interbody fusion cage with integrated screw fixation: cadaveric biomechanical study.

Study Design. In vitro biomechanical study using human fresh-frozen vertebrae. Objective. To investigate the influence of the additional screw fixation on the stability of a noncommercially available prototype transforaminal lumbar interbody fusion (TLIF) cage, when used as a stand-alone fusion device and in combination with pedicle screws (PSs). Summary of Background Data. Generally interbody fusion cages are supplemented by additional fixation devices such as PS. However, such posterior instrumented techniques are associated with additional soft-tissue trauma and potentially increased complication rate. To limit such drawbacks, a conventional posterior TLIF cage was modified to allow supplemental screw fixation to the adjacent vertebral bodies, to increase initial stiffness and possibly allow as a stand-alone posterior interbody cage. Methods. Six monosegmental lumbar spine segments were loaded in a spine simulator with pure bending moments of 7.5 Nm in lateral bending, flexion/extension, and axial rotation. The following paradigms were tested: intact spines; a destabilized spine (i.e., after discectomy and unilateral facetectomy); and the modified TLIF cage with (i.e., fixed TLIF cage) and without (i.e., TLIF cage) integrated screw fixation as a stand-alone model and with and without additional posterior fixation with bilateral PS. The range of motion (RoM) was recorded by a 3-dimensional motion analysis system. Results. The TLIF cage with integrated screw fixation had minimal additional stabilizing effect in all motion planes with or without supplemental PS fixation. Moreover, compared with the intact spines, the stand-alone TLIF cage with and without integrated screw fixation did not reduce the RoM in any of the 3 motion planes. Comparison of the TLIF cage with integrated screw fixation to the TLIF cage supplemented with PS showed a significantly greater RoM in all testing conditions (P < 0.05). Conclusion. In several testing paradigms, the prototype TLIF cage with the integrated screw fixation had limited effect in reducing RoM and providing stability. The PS was the main contributor in reducing RoM in the destabilized spine and remains the current “gold standard” in posterolateral spinal fixation. Level of Evidence: N/A