Biomechanical Evaluation of Cervicothoracic Junction Fusion Constructs

Mechanical Analysis of 3 Posterior Fusion Assemblies Intended to Cross the Cervicothoracic Junction

STUDY DESIGN

This was a biomechanical comparison study.

OBJECTIVE

The objective of this study is to evaluate the mechanical properties of 3 posterior spinal fusion assemblies commonly used to cross the cervicothoracic junction.

SUMMARY OF BACKGROUND

When posterior cervical fusions are extended into the thoracic spine, an instrumentation transition is often utilized. The cervical rod (3.5 mm) can continue using thoracic screws designed to accept the cervical rods. Alternatively, traditional thoracic screws may be used to accept thoracic rods (5.5 mm). This requires the use of a 3.5-5.5 mm transition rod or a separate 5.5 mm rod and a connector to fix the 3.5 and 5.5 mm rod together. Fusion success depends on the immobilization of vertebrae, yet the mechanics provided by these different assemblies are unknown.

MATERIALS AND METHODS

Three titanium alloy posterior fusion assemblies intended to cross the cervicothoracic junction underwent static compressive bending, tensile bending, and torsion as described in ASTM F1717 to a torque of 2.5 Nm. Five samples of each assembly were attached to ultrahigh molecular weight polyethylene blocks via multiaxial screws for testing. Force and displacement were recorded, and the stiffness of each construct was calculated.

RESULTS

The 2 assemblies that included a 5.5 mm rod were found to be stiffer and have less range of motion than the assembly that used only 3.5 mm rods.

CONCLUSIONS

The results of this study indicate that incorporating a 5.5 mm rod in a fusion assembly adds significant stiffness to the construct. When the stability of a fusion is of heightened concern, as demonstrated by the ASTM F1717 vertebrectomy (worst-case scenario) model, including 5.5 mm rods may increase fusion success rates.

LEVEL OF EVIDENCE

Level V.

Is the fusion order of the cranial and caudal levels different in two-level anterior cervical discectomy and fusion for cervical spondylopathy? A retrospective study

STUDY DESIGN

Retrospective study.

OBJECTIVE

This study aimed to compare the fusion order between the cranial and caudal levels in two-level anterior cervical discectomy and fusion (ACDF) with a zero-profile device in the treatment of cervical spondylopathy. Fusion is the standard used to judge the success of ACDF. However, the fusion order in two-level ACDF remains uncertain. The mechanical environment of different levels is different, which may affect the fusion rate or fusion order.

METHODS

From 2014 to January 2019, data of consecutive patients with two-level cervical disk degenerative disease who underwent ACDF were retrospectively reviewed. Radiological assessments were based on the range of motion of the fusion level, segment slope, and disk height, and complications were assessed. Data were analyzed using the paired t, Mann-Whitney U, χ2, Fisher exact, and rank-sum tests and logistic regression analysis.

RESULTS

In total, 118 patients were ultimately enrolled for analysis in the study. The respective fusion rates of the cranial and caudal levels were 26.27% and 10.17% (p < 0.05) at 3 months, 58.47% and 42.37% (p < 0.05) at 6 months, 86.44% and 82.20% (1 0.05) at 1 year, and 92.37% and 89.83% (p > 0.05) at the last follow-up. Multivariate logistic regression analysis indicated that the preoperative segmental slope and cranial level were independent risk factors for non-fusion. The adjacent segment degeneration (ASD) and subsidence rates were comparable between the two levels.

CONCLUSION

The caudal level had a slower fusion process than the cranial level. A higher preoperative segment slope was a risk factor for fusion. However, the subsidence and ASD rate were comparable between the caudal and cranial levels in the two-level ACDF.

Crossing the cervicothoracic junction in complex pediatric deformity using anterior cervical discectomy and fusion: a case series

PURPOSE

Proximal instrumentation failure is a challenge in posterior spinal fusions (PSFs) crossing the cervicothoracic junction. High rates of proximal junctional kyphosis (PJK) and loss of fixation have been reported. In this single-center retrospective cohort study, we evaluate the utility of anterior cervical discectomy and fusion (ACDF) in addition to traditional PSF crossing the cervicothoracic junction in order to mitigate implant-related complications.

METHODS

All patients who underwent PSF across the cervicothoracic junction with ACDF with 2 years of follow-up data were reviewed. We analyzed clinical, surgical, and radiographic measures such as operative details, presence of PJK, complications, instrumentation migration, curve angles, and vertebral translation. Measurements were compared statistically using paired samples t-tests.

RESULTS

Ten patients (6 girls, 4 boys) met inclusion criteria with a mean age at surgery of 12.8 ± 3.3 years and follow-up of 3.38 ± 0.9 years. All patients underwent ACDF (range 1-3 levels), and 8 (80%) underwent traction. The average number of levels fused posteriorly was 16.7 ± 4.7 and anteriorly was 2.4 ± 0.7. The major coronal curve averaged 48.8 ± 34.7° preoperatively and 23.3±13.3° postoperatively (p = 0.028). The average major sagittal curve was 83.5 ± 24.2° preoperatively, resolving to 53.9 ± 25.5° (p=0.001). One patient suffered rod breakage at T7, and another developed symptomatic PJK 19 months postoperatively.

CONCLUSION

Our data suggest that ACDF procedures added to PSFs crossing the cervicothoracic junction offer promise for reducing risk for instrumentation-related complications. ACDF also significantly helps improve and maintain both coronal and sagittal correction over 2 years.

LEVEL OF EVIDENCE

4.

Surgical challenges in posterior cervicothoracic junction instrumentation

The cervicothoracic junction (CTJ) is a region of the spine submitted to significant mechanical stress. The peculiar anatomical and biomechanical characteristics make posterior surgical stabilization of this area particularly challenging. We present and discuss our surgical series highlighting the specific surgical challenges provided by this region of the spine. We have analyzed and reported retrospective data from patients who underwent a posterior cervicothoracic instrumentation between 2011 and 2019 at the Neurosurgical Department of the Geneva University Hospitals. We have discussed C7 and Th1 instrumentation techniques, rods design, extension of constructs, and spinal navigation. Thirty-six patients were enrolled. We have preferentially used lateral mass (LM) screws in the subaxial spine and pedicle screws (PS) in C7, Th1, and upper thoracic spine. We have found no superiority of 3D navigation techniques over 2D fluoroscopy guidance in PS placement accuracy, probably due to the relatively small case series. Surgical site infection was the most frequent complication, significantly associated with tumor as diagnosis. When technically feasible, PS represent the technique of choice for C7 and Th1 instrumentation although other safe techniques are available. Different rod constructs are described although significant differences in biomechanical stability still need to be clarified. Spinal navigation should be used whenever available even though 2D fluoroscopy is still a safe option. Posterior instrumentation of the CTJ is a challenging procedure, but with correct surgical planning and technique, it is safe and effective.

How I do it: tapered rod placement across the cervicothoracic junction for augmented posterior constructs

BACKGROUND

Posterior instrumentation techniques are commonly employed for cervicothoracic fixation. The pedicles of the upper thoracic vertebrae can typically accommodate larger diameter screws than the subaxial cervical vertebrae. In many construct systems, this requires the use of a tapered rod, which can be technically challenging to place.

METHOD

Using a three-dimensionally printed biomimetic spine simulator, we illustrate the stepwise process of instrumentation and tapered rod placement across the cervicothoracic junction (CTJ).

CONCLUSION

Tapered rod systems can augment the biomechanical stability of cervicothoracic constructs. Ease of rod placement across the CTJ hinges upon a systematic method of instrumentation.