Mechanical Complications in Adult Spinal Deformity Surgery: Can Spinal Alignment Explain Everything?

STUDY DESIGN

Cohort study.

OBJECTIVE

Our goal was to verify the validity of the global alignment and proportion (GAP) score, SRS-Schwab, and Roussouly theoretical apex of lordosis in predicting mechanical complications in adult spinal deformity (ASD).

SUMMARY OF BACKGROUND DATA

Achieving adequate sagittal alignment is critical to obtain favorable outcomes in ASD surgery. It has been proposed that mechanical complications are largely secondary to postoperative spinal alignment.

METHODS

Retrospective review of consecutive primary ASD cases that underwent deformity correction in the same institution over a 5-year period. Association between the 6-week postoperative spinal alignment classification and occurrence of mechanical complications on the last follow-up was assessed using logistic regressions. The discriminant capacity was assessed using the receiver operating characteristic (ROC) curve analysis.

RESULTS

58.3% (N = 49/84) of patients presented with mechanical complications and 32.1% (N = 27/84) underwent revision surgery. GAP score did not show discriminant ability to predict complications (AUC = 0.53, 95% confidence interval [CI] = 0.40-0.66, P = 0.58). Conversely, the SRS-Schwab sagittal modifier score demonstrated a statistically significant (although modest) predictive value for mechanical complications (AUC = 0.67, 95% CI = 0.54-0.79, P = 0.008). There was a significant association between pelvic tilt (PT) (P = 0.03) and sagittal vertical axis (SVA) (P = 0.01) at 6 weeks postoperatively and the occurrence of later mechanical complications. There was no significant association between matched Roussouly theoretical apex of lordosis and final outcome (P = 0.47).

CONCLUSION

The results point to the complexity of mechanical failure and the high likelihood that causative factors are multifactorial and not limited to alignment measures. GAP score should be used with caution as it may not explain or predict mechanical failure based on alignment in all populations as originally expected. Future studies should focus on etiology, surgical technique, and patient factors in order to generate a more universal score that can be applied to all populations.Level of Evidence: 4.

The Safety and Efficacy of CT-Guided, Fluoroscopy-Free Vertebroplasty in Adult Spinal Deformity Surgery

OBJECTIVE

The goal of this study is to analyze the safety and efficacy of a novel technique of computed tomography-guided, fluoroscopy-free vertebroplasty as an adjunct to help prevent proximal junction kyphosis (PJK) in long-segment posterior spinal fusions.

METHODS

We performed a retrospective analysis of 118 consecutive patients with adult spinal deformity who underwent long-segment fusion with vertebroplasty augmentation from 2013-2016 at a single institution. For each patient, we collected demographics, surgical information, length of stay, discharge disposition, and complications, including reoperation, PJK, and PJK requiring reoperation. We reviewed all postoperative radiographs to assess for cement leakage from vertebroplasty. These patients were compared to a historical control of 253 patients who underwent adult spinal deformity surgery without vertebroplasty augmentation.

RESULTS

The PJK rate of 14% and the PJK requiring reoperation rate of 3% in the cohort of 118 patients who underwent vertebroplasty-augmented fusion was significantly lower than that of the 253 historical controls at our institution who did not undergo vertebroplasty (40% PJK rate, 17% PJK-rate requiring reoperation; both P < 0.001). After controlling for patient and other surgical factors in multivariate analyses, vertebroplasty was significantly associated with lower rates of PJK and PJK requiring reoperation (P < 0.001 and P = 0.003).

CONCLUSIONS

Our novel vertebroplasty technique is safe, and it eliminates the need for additional fluoroscopy in cases already using the O-arm to verify screw placement. In addition, it is an effective technique for reducing PJK in adult spinal deformity surgery compared with historical institutional controls.

Fractures After Removal of Spinal Instrumentation: Revisiting the Stress-Shielding Effect of Instrumentation in Spine Fusion

BACKGROUND

Many practicing spine surgeons believe that instrumentation can be removed during revision surgery in successful posterolateral or anterior spinal fusions, confirmed by computed tomography and intraoperative exploration. The stress-shielding effect of spinal instrumentation was well described in the late 1980s and 1990s but has not received recent attention. Despite the paucity of recent literature, concepts underlying the biology and biomechanics of the spinal fusion mass remain particularly salient given the increasing incidence of revision spinal fusion surgery. The aim of this study was to highlight a potential complication of instrumentation removal owing to stress shielding of instrumentation on the spinal column and fusion mass.

METHODS

A retrospective review was performed, and a small case series was described.

RESULTS

In 3 cases, despite apparent solid fusion demonstrated on preoperative computed tomography and confirmed by intraoperative exploration, new fractures developed after removal of instrumentation. In these cases, fracture occurred at the transition zone between the newly rigid instrumented area and previous fusion. This highlights the relative weakness of the fusion and may be explained by the stress-shielding effect of instrumentation within the fusion mass.

CONCLUSIONS

Spinal instrumentation revision requires careful consideration, and routine implant removal should not be performed. The presence of a solid fusion on computed tomography and/or at intraoperative exploration may not justify implant removal in these cases. In cases of extension of a fusion, use of a bridging connection to the new implants should be considered. The cases presented demonstrate the consequences of the stress-shielding effect of implants on the spine and fusion mass.

Genetic variation in the patterns of skeletal progenitor cell differentiation and progression during endochondral bone formation affects the rate of fracture healing

These studies examined how genetic differences that regulate architectural and bone material properties would be expressed during fracture healing and determine whether any of these features would affect rates of healing as defined by regain of strength. Controlled fractures were generated in three inbred strains of mice: A/J, C57Bl/6J (B6), and C3H/HeJ (C3H). Both the A/J and B6 strains showed faster healing than the C3H strain based on regains in strength and stiffness. Strain-specific architectural features such as moment of inertia, cross-sectional area, and cortical thickness were all recapitulated during the development of the callus tissues. None of these traits were directly relatable to rates of fracture healing. However, rates of healing were related to variations in the temporal patterns of chondrogenic and osteogenic lineage development. The B6 strain expressed the highest percentage of cartilage gene products and had the longest period of chondrocyte maturation and hypertrophy. The slowest healing strain (C3H) had the shortest period of chondrogenic development and earliest initiation of osteogenic development. Although the A/J strain showed an almost identical pattern of chondrogenic development as the C3H strain, A/J initiated osteogenic development several days later than C3H during fracture healing. Long bone growth plates at 28 days after birth showed similar strain-specific variation in cartilage tissue development as seen in fracture healing. Thus, the B6 strain had the largest growth plate heights, cell numbers per column, and the largest cell size, whereas the C3H columns were the shortest, had the smallest number of cells per column, and showed the smallest cell sizes. These results show that (1) different strains of mice express variations of skeletal stem cell lineage differentiation and (2) that these variations affect the rate of fracture healing.

Three-dimensional reconstruction of fracture callus morphogenesis

Rat and mouse femur and tibia fracture calluses were collected over various time increments of healing. Serial sections were produced at spatial segments across the fracture callus. Standard histological methods and in situ hybridization to col1a1 and col2a1 mRNAs were used to define areas of cartilage and bone formation as well as tissue areas undergoing remodeling. Computer-assisted reconstructions of histological sections were used to generate three-dimensional images of the spatial morphogenesis of the fracture calluses. Endochondral bone formation occurred in an asymmetrical manner in both the femur and tibia, with cartilage tissues seen primarily proximal or distal to the fractures in the respective calluses of these bones. Remodeling of the calcified cartilage proceeded from the edges of the callus inward toward the fracture producing an inner-supporting trabecular structure over which a thin outer cortical shell forms. These data suggest that the specific developmental mechanisms that control the asymmetrical pattern of endochondral bone formation in fracture healing recapitulated the original asymmetry of development of a given bone because femur and tibia grow predominantly from their respective distal and proximal physis. These data further show that remodeling of the calcified cartilage produces a trabecular bone structure unique to fracture healing that provides the rapid regain in weight-bearing capacity to the injured bone.