Autologous chondrocyte grafting promotes bone formation in the posterolateral spine

Endochondral Ossification for Spinal Fusion: A Novel Perspective from Biological Mechanisms to Clinical Applications

Degenerative scoliosis (DS), encompassing conditions like spondylolisthesis and spinal stenosis, is a common type of spinal deformity. Lumbar interbody fusion (LIF) stands as a conventional surgical intervention for this ailment, aiming at decompression, restoration of intervertebral height, and stabilization of motion segments. Despite its widespread use, the precise mechanism underlying spinal fusion remains elusive. In this review, our focus lies on endochondral ossification for spinal fusion, a process involving vertebral development and bone healing. Endochondral ossification is the key step for the successful vertebral fusion. Endochondral ossification can persist in hypoxic conditions and promote the parallel development of angiogenesis and osteogenesis, which corresponds to the fusion process of new bone formation in the hypoxic region between the vertebrae. The ideal material for interbody fusion cages should have the following characteristics: (1) Good biocompatibility; (2) Stable chemical properties; (3) Biomechanical properties similar to bone tissue; (4) Promotion of bone fusion; (5) Favorable for imaging observation; (6) Biodegradability. Utilizing cartilage-derived bone-like constructs holds promise in promoting bony fusion post-operation, thus warranting exploration in the context of spinal fusion procedures.

Murine models of posterolateral spinal fusion: A systematic review

BACKGROUND

Rodent models are commonly used experimentally to assess treatment effectiveness in spinal fusion. Certain factors are associated with better fusion rates. The objectives of the present study were to report the protocols most frequently used, to evaluate factors known to positively influence fusion rate, and to identify new factors.

METHOD

A systematic literature search of PubMed and Web of Science found 139 experimental studies of posterolateral lumbar spinal fusion in rodent models. Data for level and location of fusion, animal strain, sex, weight and age, graft, decortication, fusion assessment and fusion and mortality rates were collected and analyzed.

RESULTS

The standard murine model for spinal fusion was male Sprague Dawley rats of 295g weight and 13 weeks' age, using decortication, with L4-L5 as fusion level. The last two criteria were associated with significantly better fusion rates. On manual palpation, the overall mean fusion rate in rats was 58% and the autograft mean fusion rate was 61%. Most studies evaluated fusion as a binary on manual palpation, and only a few used CT and histology. Average mortality was 3.03% in rats and 1.56% in mice.

CONCLUSIONS

These results suggest using a rat model, younger than 10 weeks and weighing more than 300 grams on the day of surgery, to optimize fusion rates, with decortication before grafting and fusing the L4-L5 level.

A timeseries analysis of the fracture callus extracellular matrix proteome during bone fracture healing

While most bones fully self-heal, certain diseases require bone allograft to assist with fracture healing. Bone allografts offer promise as treatments for such fractures due to their osteogenic properties. However, current bone allografts made of decellularized bone extracellular matrix (ECM) have high failure rates, and thus grafts which improve fracture healing outcomes are needed. Understanding specific changes to the ECM proteome during normal fracture healing would enable the identification of key proteins that could be used enhance osteogenicity of bone allograft. Here, we performed a timeseries analysis of the fracture callus in mice to investigate proteomic and mineralization changes to the ECM at key stages of fracture healing. We found that changes to the ECM proteome largely coincide with the distinct phases of fracture healing. Basement membrane proteins (AGRN, COL4, LAMA), cartilage proteins (COL2A1, ACAN), and collagen crosslinking enzymes (LOXL, PLOD, ITIH) were initially upregulated, followed by bone specific proteoglycans and collagens (IBSP, COL1A1). Various tissue proteases (MMP2, 9, 13, 14; CTSK, CTSG, ELANE) were expressed at different levels throughout fracture healing. These changes coordinated with mineralization of the fracture callus, which increased steeply during the initial stages of healing. Interestingly the later timepoint was characterized by a response to wound healing and high expression of clotting factors (F2, 7, 9, 10). We identified ELANE and ITIH2 as tissue remodeling enzymes having no prior known involvement with fracture healing. This data can be further mined to identify regenerative proteins for enhanced bone graft design.

Cyclic Tensile Stress to Rat Thoracolumbar Ligamentum Flavum Inducing the Ossification of Ligamentum Flavum: An In Vivo Experimental Study

STUDY DESIGN

Western blot, reverse transcription-polymerase chain reaction (RT-PCR), radiological, and histological analyses of the rat ossification of ligamentum flavum (OLF) induced by cyclic tensile stress.

OBJECTIVE

The aim of this study was to induce the OLF using cyclic tensile stress to rat thoracolumbar ligamentum flavum, and to investigate the possible molecular mechanism of tension-induced OLF.

SUMMARY OF BACKGROUND DATA

Tensile stress has been considered as an important factor leading to the OLF. So far, however, no OLF induced by tension has been reported.

METHODS

Forty rats were randomly divided into five equal groups. For control groups, the blank and anesthesia groups were not subjected to tension. For experimental groups, the 4-, 8-, and 12-week groups were subjected to cyclic tensile stress of ligamentum flavum after abdominal anesthesia for 4 weeks, 8 weeks, and 12 weeks, respectively, using an original stress apparatus for rats. The radiological and morphological changes of rat spine, as well as the protein and mRNA expressions of CD44, bone morphogenetic protein-2 (BMP-2), integrin β3, collagen protein type I (COL1), osteopontin (OPN), runt-related transcription factor 2 (RUNX-2), and vascular endothelial growth factor (VEGF), were concerned.

RESULTS

The micro-CT showed OLF in the 4-, 8-, and 12-week group. The axial maximum occupied area of ossifications was 1.42 mm2, 3.35 mm2, and 7.28 mm2, respectively. In histopathology, chondrocytes proliferated in the experimental model; woven bone arose in the 8- and 12-week groups, and was more noticeable in the 12-week group. According to western blot and RT-PCR, the expressions of seven osteogenesis-related molecules were all increased in three experimental groups.

CONCLUSION

Cyclic tensile stress to the ligamentum flavum in rats can induce the OLF, and the longer the duration, the more visible the osteogenesis. The upregulation and synergism of osteogenesis-related molecules may contribute to the OLF induced by tensile stress.Level of Evidence: N/A.

Bone defect reconstruction via endochondral ossification: A developmental engineering strategy

Traditional bone tissue engineering (BTE) strategies induce direct bone-like matrix formation by mimicking the embryological process of intramembranous ossification. However, the clinical translation of these clinical strategies for bone repair is hampered by limited vascularization and poor bone regeneration after implantation in vivo. An alternative strategy for overcoming these drawbacks is engineering cartilaginous constructs by recapitulating the embryonic processes of endochondral ossification (ECO); these constructs have shown a unique ability to survive under hypoxic conditions as well as induce neovascularization and ossification. Such developmentally engineered constructs can act as transient biomimetic templates to facilitate bone regeneration in critical-sized defects. This review introduces the concept and mechanism of developmental BTE, explores the routes of endochondral bone graft engineering, highlights the current state of the art in large bone defect reconstruction via ECO-based strategies, and offers perspectives on the challenges and future directions of translating current knowledge from the bench to the bedside.

Revision strategies for failed adult spinal deformity surgery

PURPOSE

The aim of this study is to analyse the results of revision surgery for failed adult spinal deformity patients and to describe the surgical strategy selection process, based on the identification of the main clinical diagnosis responsible for failure.

METHODS

We retrospectively reviewed the clinical and radiological data of 77 consecutive patients treated in a 3-year time (2016-2019) for surgical revision of long fusion (more than five levels fused) for adult spinal deformity in a high-volume spine centre, divided into four groups based on the diagnosis: rod breakage (RB) group, proximal junctional failure (PJF) group, distal junctional failure (DJF) group and loss of correction (LOC) group with symptomatic sagittal or coronal malalignment (including iatrogenic flatback).

RESULTS

Seventy-seven patients met our inclusion criteria, with a female prevalence (66 F vs. 11 M). The mean age at revision surgery was 63. Fused levels before surgery were averagely 12, and revision added averagely two levels to the preexisting fusion area. Clinical status was apparently improved in ODI scores and VAS scores, while it was slightly worsened in SF36 scores. Different diagnosis groups have been addressed with different surgical strategies, according to the different surgical goals: interbody cages and multi-rod construct to improve stiffness and favour bony fusion, "kickstand" rod and "tie" rod to correct coronal and sagittal malalignment, specific rod contouring and proximal hooks in "claw" configuration to reduce mechanical stress at the proximal junctional area. Intraoperative complications occurred in 18% of patients and perioperative complications in 39%.

CONCLUSION

Revision surgery in long fusions for adult spinal deformity is a challenging field. Surgical strategy should always be planned carefully. A successful treatment is a direct consequence of a correct preoperative diagnosis, and surgery should address the primary cause of failure. All the above-mentioned surgical techniques and clinical skills should be part of surgeon's expertise when managing these patients. These slides can be retrieved under Electronic Supplementary Material.