Preliminary biomechanical proof of concept for a hybrid locking plate/variable pitch screw construct for anterior fixation of type II odontoid fractures

Bicortical Compression and Construct Stability With Variable Pitch Locking Screws in Cadaveric Specimens

OBJECTIVES

A variable pitch locking screw is intended to provide interfragmentary compression combined with fixed angle stability of locking plate constructs. The objective of this study was to compare variable pitch locking screws (3.5-mm KreuLock Ti locking compression screws, Arthrex Inc., Naples, FL) with standard locking screws (from the same manufacturer) in bicortical fixation scenarios in cadaver bone by assessing (1) interfragmentary compression and plate-bone compression and (2) construct biomechanical stability.

METHODS

Nine matched pairs of fresh-frozen cadaveric specimens with an average age of 67.2 years (range, 37-83) were used. Interfragmentary compression and plate-bone compression associated with insertion of single bicortical screws were compared between the variable pitch and standard locking screws at increasing levels of torque. The specimens tested were distal tibiae having a simulated longitudinal fracture. Additionally, fibulae were osteotomized to create a stable longitudinal fracture pattern and were fixed with a 5-screw plate construct with either all variable pitch or all standard locking screws. One of the 5 screws was placed across the osteotomy without lagging. Fibulae were tested cyclically with axial with torsional loading to compare displacements, rotation, and loads at failure or tested in 4-point bending to compare construct stiffness and maximum force to failure.

RESULTS

Interfragmentary and plate-bone compression forces in the distal tibia model varied across specimens but were significantly higher with variable pitch locking screws compared with standard locking screws [512 N (SD = 324 N) vs. 79 N (SD = 64 N), P = 0.002, and 242 N (SD = 119 N) vs. 104 N (SD = 123 N), P = 0.028, respectively]. In cyclic loading of fibula constructs, no significant differences were detected in construct axial displacement or angular displacement (P > 0.05). In 4-point bending, no differences were detected in maximum force or bending stiffness (P > 0.05).

CONCLUSIONS

Variable pitch locking screws produced interfragmentary compression between cortices and plate-bone compression that was greater than that produced by standard locking screws. In a stable bicortical fibula fixation scenario under external loading, the stability of variable pitch locking screw constructs was similar to constructs with standard locking screws.

The limitations of fully threaded screws in isolated percutaneous transarticular screw fixation of C1/C2

Demographic aging accompanied by increased falls inevitably leads to an increased incidence of atlantoaxial instabilities (AAI). Minimally invasive surgical procedures decrease the perioperative risk and regarding the treatment of AAI, percutaneous transarticular screw fixation of C1/C2 was more frequently considered in the past. This study aims to investigate the outcome of patients treated for AAI by isolated percutaneous transarticular screw fixation of C1/C2 (IPTSFC1/C2) using 3.5 mm fully threaded screws to identify its chances and limitations. In this retrospective study, data from patients who underwent IPTSFC1/C2 were analyzed. 23 patients (17 females and 6 males) with an average age of 73.1 years (y) were included. Mean VAS decreased significantly from preoperative 3.9 ± 1.8 to the last follow-up 2.6 ± 2.5 (p = 0.020) and neurological functions were preserved. In the radiological follow-up, we saw a single malposition of an inserted screw (2.27%) and one single bony fusion (4.54%). However, in 6 of 7 patients (85.71%), there was a loosening of the inserted screws due course. We demonstrated that the use of 3.5 mm fully threaded screws for IPTSFC1/C2 results in low rates of osseous fusions between C1 and C2. Therefore, their use in IPTSFC1/C2 is not suitable, especially for geriatric patients with impaired bone status.

Cement augmentation of odontoid peg fractures: the effect of cement volume and distribution on construct stiffness

PURPOSE

The cement augmentation of a conventional anterior screw fixation in type II odontoid process fractures for elderly patients significantly increased stiffness and load to failure under anterior-posterior load in comparison with non-augmented fixation. The amount and quality of bone cement are usually taken ad hoc in clinical practise. In this study, we wanted to clarify the role of bone cement amount and its quality to the stiffness of odontoid and vertebrae body junction.

METHODS

Finite-element method was used to achieve different scenarios of cement augmentation. For all models, an initial stiffness was calculated. Model (1) the intact vertebrae were virtually potted into a polymethylmethacrylate base via the posterior vertebral arches. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. (2) The odontoid fracture type IIa (Anderson-D'Alonzo classification) was achieved by virtual transverse osteotomy. Anterior screw fixation was virtually performed by putting self-drilling titanium alloy 3.5 mm diameter anterior cannulated lag screw with a 12 mm thread into the inspected vertebrae. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. The vertebrae body was assumed to be non-cemented and cemented with different volume.

RESULTS

The mean cement volume was lowest for body base filling with 0.47 ± 0.03 ml. The standard body filling corresponds to 0.95 ± 0.15 ml. The largest volume corresponds to 1.62 ± 0.12 ml in the presence of cement leakage. The initial stiffness of the intact C2 vertebrae was taken as the reference value. The mean initial stiffness for non-porous cement (E = 3000 MPa) increased linearly (R2 = 0.98). The lowest stiffness (123.3 ± 5.8 N/mm) was measured in the intact C2 vertebrae. However, the highest stiffness (165.2 ± 5.2 N/mm) was measured when cement leakage out of the odontoid peg occurred. The mean initial stiffness of the base-only cemented group was 147.2 ± 8.4 N/mm compared with 157.9 ± 6.6 N/mm for the base and body cemented group. This difference was statistically significant (p < 0.0061). The mean initial stiffness for porous cement (E = 500 MPa) remains constant. Therefore, there is no difference between cemented and non-cemented junction. This difference was not statistically significant (p < 0.18).

CONCLUSION

The present study showed that the low porous cement was able to significantly influence the stiffness of the augmented odontoid screw fixation in vitro, although further in vivo clinical studies should be undertaken. Our results suggest that only a small amount of non-porous cement is needed to restore stiffness at least to its pre-fracture level and this can be achieved with the injection of 0.7-1.2 ml of cement. These slides can be retrieved under Electronic Supplementary Material.

Computational anatomy of the dens axis evaluated by quantitative computed tomography: Implications for anterior screw fixation

The surgical fracture fixation of the odontoid process (dens) of the second cervical vertebra (C2/axis) is a challenging procedure, particularly in elderly patients affected by bone loss, and includes screw positioning close to vital structures. The aim of this study was to provide an extended anatomical knowledge of C2, the bone mass distribution and bone loss, and to understand the implications for anterior screw fixation. One hundred and twenty standard clinical quantitative computed tomography (QCT) scans of the intact cervical spine from 60 female and 60 male European patients, aged 18-90 years, were used to compute a three-dimensional statistical model and an averaged bone mass model of C2. Shape and size variability was assessed via principal component analysis (PCA), bone mass distribution by thresholding and via virtual core drilling, and the screw placement via virtual positioning of screw templates. Principal component analysis (PCA) revealed a highly variable anatomy of the dens with size as the predominant variation according to the first principal component (PC) whereas shape changes were primarily described by the remaining PCs. The bone mass distribution demonstrated a characteristic 3D pattern, and remained unchanged in the presence of bone loss. Virtual screw positioning of two 3.5 mm dens screws with a 1 mm safety zone was possible in 81.7% in a standard, parallel position and in additional 15.8% in a twisted position. The approach permitted a more detailed anatomical assessment of the dens axis. Combined with a preoperative QCT it may further improve the diagnostic procedure of odontoid fractures. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2154-2163, 2017.

An in vitro biomechanical evaluation of an expansive double-threaded bi-directional compression screw for fixation of type II odontoid process fractures: A SQUIRE-compliant article

Odontoid process fracture accounts for 5% to 15% of all cervical spine injuries, and the rate is higher among elderly people. The anterior cannulated screw fixation has been widely used in odontoid process fracture, but the fixation strength may still be limited under some circumstances. This study aims to investigate the biomechanical fixation strength of expansive double-threaded bi-directional compression screw (EDBCS) compared with cannulated lag screw (CLS) and improved Herbert screw (IHS) for fixation of type II odontoid process fracture.Thirty fresh cadaveric C2 vertebrae specimens were harvested and randomly divided into groups A, B, and C. A type II fracture model was simulated by osteotomy. Then the specimens of the 3 groups were stabilized with a single CLS, IHS, or EDBCS, respectively. Each specimen was tested in torsion from 0° to 1.25° for 75 s in each of 5 cycles clockwise and 5 cycles anticlockwise. Shear and tensile forces were applied at the anterior-to-posterior and proximal-to-distal directions, respectively, both to a maximum load of 45 N and at a speed of 1 mm/min.The mean torsional stiffness was 0.309 N m/deg for IHS and 0.389 N m/deg for EDBCS, which were significantly greater compared with CLS, respectively (0.169 N m/deg) (P < .05 and P < .05). The mean shear stiffness for the EDBCS was 238 N/mm, which was significantly greater than CLS (150 N/mm) and IHS (132 N/mm) (P < .05 and P < .05). All 3 screws only partly restored tensile stiffness, but not significantly.Fixation with the EDBCS can improve the biomechanical strength for odontoid process fracture compared with CLS and IHS, especially in terms of torsional and shear stiffness.

A Novel Anterior Odontoid Screw Plate for C1-C3 Internal Fixation: An In Vitro Biomechanical Study

STUDY DESIGN

A biomechanical in vitro study was performed using a standardized experimental protocol in a biomechanical spine testing apparatus.

OBJECTIVE

The aims of this study were to evaluate the biomechanical stability afforded by 4 cervical fixation techniques: anterior cervical plate+odontoid screw+cage (ACP+OS+cage), anterior odontoid screw plate+bone graft (AOSP+bone graft), posterior C2-3 fixation+odontoid screw (C2PS+C3LMS+OS), and posterior C1-3 fixation (C1PS+C2PS+C3LMS).

SUMMARY OF BACKGROUND DATA

Unstable axis injuries with multiple fracture lines are uncommon injuries, and their management is still challenging for surgeons who aim to achieve primary stability, early mobilization, preserved cervical range of motion (ROM), and favorable outcome. We designed a novel AOSP to assist in this challenging clinical scenario.

METHODS

Eight fresh-frozen cadaveric spine specimens (C1-C3) were subjected to stepwise destabilization of the C1-3 complex, with serial replication of a type II Hangman fracture, a type II odontoid fracture, and a C2 to C3 disc injury. Intact specimens, destabilized specimens, and destabilized specimens with various stabilization techniques including anterior and posterior techniques, some using our AOSP, were each tested for stability. Each spine was subjected to flexion, and extension testing, left and right lateral bending, and left and right rotation.

RESULTS

After AOSP+bone graft fixation, the ROMC2-C3 during all loading modes were reduced to values that were significantly less than normal. During all loading modes, AOSP+bone graft fixation significantly outperformed the ACP+OS+cage fixation in limiting ROMC2-C3. During flexion and extension, AOSP+bone graft fixation significantly outperformed the C1PS+C2PS+C3LMS fixation and C2PS+C3LMS+OS fixation in limiting ROMC2-C3.

CONCLUSION

The AOSP has excellent biomechanical performance when dealing with type I Hangman fractures, type II odontoid fractures, and C2-3 disc injuries. The AOSP+one graft fixation can preserve the function of atlanto-axial joint, which may be a valuable stabilization strategy for these unique injuries.

Assessment of the Breakaway Torque at the Posterior Pelvic Ring in Human Cadavers

Purpose: To enhance the diminished screw purchase in cancellous, osteoporotic bone following the fixation of posterior pelvic ring injuries by iliosacral screws an increased bone-implant contact area using modificated screws, techniques or bone cement may become necessary. The aim of the study was to identify sites within the pathway of iliosacral screws requiring modifications of the local bone or the design of instrumentations placed at this site. Materials and Methods: The breakaway torque was measured mechanically at the iliosacral joint (“ISJ”), the sacral lateral mass (“SLM”) and the center of the S1 (“CS1”), at a superior and an inferior site under fluoroscopic control on five human cadaveric specimens (3 female; mean age 87 years, range: 76–99) using the DensiProbe™Spine device. Results: The measured median (range) breakaway torque was 0.63 Nm (0.31–2.52) at the “iliosacral joint”, 0.14 Nm (0.05–1.22) at the “sacral lateral mass”, 0.57 Nm (0.05–1.42) at the “S1 center.” The “sacral lateral mass” breakaway torque was lower than compared to that at the “iliosacral joint” (p < .001) or “S1 center” (p < .001). The median (range) breakaway torque measured at all superior measurement points was 0.52 Nm (0.10–2.52), and 0.48 Nm (0.05–1.18) at all inferior sites. The observed difference was statistically significant (p < .05). Conclusions: The lateral mass of the sacrum provides the lowest bone quality for implant anchorage. Iliosacral screws should be placed as superior as safely possible, should bridge the iliosacral joint and may allow for cement application at the lateral mass of the sacrum through perforations.

A novel technique of two-hole guide tube for percutaneous anterior odontoid screw fixation

BACKGROUND CONTEXT

Surgical stabilization is recommended for odontoid fractures with mechanical instability. Compared with C1-C2 fusion, percutaneous anterior odontoid screw fixation has the advantages of preserving C1/C2 motion and being a minimally invasive procedure. However, determining the optimal screw trajectory is often difficult. When an initial suboptimal K-wire hole is drilled, it is especially difficult to drill a second optimal K-wire trajectory because the initial hole will be entered inadvertently.

PURPOSE

To design a novel device, two-hole guide tube, to make drilling a second optimal K-wire trajectory easier, and thus, avoid unnecessary additional surgical time and reduce the likelihood of needing to change the procedure to traditional open surgery.

STUDY DESIGN

A technical report.

METHODS

Fifty-three patients with odontoid fractures were treated by percutaneous anterior odontoid screw fixation in our hospital, and the initial K-wire trajectories of 16 cases (12 men and 4 women) among the 53 patients were imperfect. The two-hole guide tube was applied for drilling the second trajectory in each of these 16 cases.

RESULTS

No complications associated with this technique occurred. Satisfactory results and good screw placement was achieved in all patients. Radiographic fusion was confirmed for 15 of 16 patients. None of the patients experienced clinical symptoms or screw loosening or breakage in this study.

CONCLUSIONS

Our novel device, two-hole guide tube, can be used to reduce the difficulty associated with redrilling an optimal K-wire trajectory if the initial trajectory is imperfect during percutaneous anterior odontoid screw fixation. Moreover, by referring to the initial misplaced K-wire, a more accurate trajectory for the second K-wire can be achieved.

Cement-augmented anterior screw fixation of Type II odontoid fractures in elderly patients with osteoporosis

BACKGROUND CONTEXT

Closed reduction and internal fixation by an anterior approach is an established option for operative treatment of displaced Type II odontoid fractures. In elderly patients, however, inadequate screw purchase in osteoporotic bone can result in severe procedure-related complications.

PURPOSE

To improve the stability of odontoid fracture screw fixation in the elderly using a new technique that includes injection of polymethylmethacrylat (PMMA) cement into the C2 body.

STUDY DESIGN

Retrospective review of hospital and outpatient records as well as radiographs of elderly patients treated in a university hospital department of orthopedic surgery.

PATIENT SAMPLE

Twenty-four elderly patients (8 males and 16 females; mean age, 81 years; range, 62-98 years) with Type II fractures of the dens.

OUTCOME MEASURES

Complications, cement leakage (symptomatic/asymptomatic), operation time, loss of reduction, pseudarthrosis and revision surgery, patient complaints, return to normal activities, and signs of neurologic complications were all documented.

METHODS

After closed reduction and anterior approach to the inferior border of C2, a guide wire is advanced to the tip of the odontoid under biplanar fluoroscopic control. Before the insertion of one cannulated, self-drilling, short thread screws, a 12 gauge Yamshidi cannula is inserted from anterior and 1 to 3 mL of high-viscosity PMMA cement is injected into the anteroinferior portion of the C2 body. During polymerization of the cement, the screws are further inserted using a lag-screw compression technique. The cervical spine then is immobilized with a soft collar for 8 weeks postoperatively.

RESULTS

Anatomical reduction of the dens was achieved in all 24 patients. Mean operative time was 64 minutes (40-90 minutes). Early loss of reduction occurred in three patients, but revision surgery was indicated in only one patient 2 days after primary surgery. One patient died within the first eight postoperative weeks, one within 3 months after surgery. In five patients, asymptomatic cement leakage was observed (into the C1-C2 joint in three patients, into the fracture in two). Conventional radiologic follow-up at 2 and 6 months confirmed anatomical healing in 16 of the 19 patients with complete follow-up. In two patients, the fractures healed in slight dorsal angulation; one patient developed a asymptomatic pseudarthrosis. All patients were able to resume their pretrauma level of activity.

CONCLUSIONS

Cement augmentation of the screw in Type II odontoid fractures in elderly patients is technically feasible in a clinical setting with a low complication rate. This technique may improve screw purchase, especially in the osteoporotic C2 body.