Non-fusion Surgeries of Lower Cervical Spine

Traumatic, degenerative, or malignant conditions that involve the removal of intervertebral discs or vertebral bodies often necessitate anterior spinal fusion, which is among the most frequently performed procedures on the cervical spine. For single-level surgery, autogenous bone grafting has demonstrated a fusion rate of 95% or higher.

The utilization of artificial spacers or bone substitutes has alleviated donor-site pain, while anterior cervical plating has been associated with increased fusion rates and eliminated the requirement for external immobilization following surgery.

Despite achieving satisfactory results, it is theoretically possible that spinal fusion places additional strain on neighboring segments and potentially speeds up degenerative processes.

Contrarily, there is an argument that degeneration of adjacent segments is a natural course of the disease in individuals who are genetically susceptible. Nonetheless, it remains crucial to prioritize the preservation of functional spinal unit mobility following disc excision or non-fusion procedures.

The concept of substituting the intervertebral disc is not a novel one. Within the last five decades, more than 100 designs, encompassing simple metallic balls to intricate composite polymers with various metal backings, have been documented.

As the comprehension of the biomechanics and kinematics of the functional spinal unit has improved, modern designs have become more advanced and closely mimic the natural mechanics of the spine.

While the immediate outcomes seem to be acceptable, there is still a lack of available medium- and long-term results. Interestingly, the most favorable clinical results were observed in segments that naturally underwent fusion, which raises the irony that the artificial disc, despite its cost, primarily functions as an expensive intervertebral spacer for fusion purposes.

Several prospective randomized controlled trials have asserted the superiority of artificial disc replacements in terms of results. However, it is worth noting that the control group in these trials comprised standalone intervertebral cages, which are widely recognized for yielding unfavorable outcomes. The cervical spine inherently presents a less hostile environment compared to the lumbar region when it comes to various conditions.

Disc replacement should theoretically have a higher likelihood of success in the cervical segments due to notable differences in factors such as disc size, loading forces, facet joint anatomy, and kinematics when compared to the lumbar segments.

Undoubtedly, the literature on the cervical spine has seen a rise in the number of reports in recent years, approaching the level of those concerning the lumbar spine. Generally, a range of approximately 5º to 7º of segmental motion can be retained.

However, it remains uncertain whether this is the contributing factor to the limited occurrence of adjacent segment degeneration, as most of the follow-up periods have been relatively brief. Hence, there is a growing concern regarding the implantation of artificial discs in progressively younger patients who have several decades of life expectancy remaining.

Insights into the future of artificial disc replacement can potentially be gleaned from the historical progression of joint replacement for hips and knees. The higher the level of resemblance achieved in replicating the anatomy and kinematics of the joint, the greater the likelihood of success.

Over time, the fixation methods of the implant to the recipient bed have undergone evolution, including various generations of cement pressurization, surface in-growth, anatomic press-fitting, with or without screw augmentation, and more. Wear debris osteolysis, mechanical loosening, and implant migration impose limitations on the long-term success of artificial disc replacement.

The frequency of revision surgeries, if not surpassing primary implantations, is increasingly common in the field. The endeavor to reconstruct major bone defects is challenging and entails the possibility of adverse outcomes. Patients may need to be prepared to undergo multiple revision surgeries if such a scenario occurs in the cervical spine after an artificial disc replacement.

Surgeons possess the capability to salvage these situations, restore bone stock, or determine if the bridges have been irreversibly compromised. It is crucial to remember the primary concern, which is to avoid endangering the spinal cord at all costs.

At the University of Hong Kong, a team has adopted a unique approach to address this matter, examining the viability of biologically regenerating or replacing the disc while maintaining its mobility.

The team firmly upholds the belief that the most optimal design for a disc is one that has been naturally provided. Taking inspiration from the achievements seen in major organ transplants, a series of experiments have been conducted on primates to investigate the feasibility of intervertebral disc autografting, allografting, and fresh frozen allografting.

The implanted disc possesses the potential to fulfill the objectives of achieving mechanical stability and preserving mobility within the segment, all while minimizing the risk to neural tissues. However, during the initial months, there is a decrease in the height of the transplanted disc, implying a degenerative process. A study has been conducted on humans in Beijing by the team since the year 2000, and the outcomes have demonstrated remarkable promise.

The healing of the bony endplates is nearly assured, and there is no indication of any immunoreactive response. Apart from the anticipated findings from the animal studies, two additional phenomena have been identified.. The first phenomenon observed was that malpositioned allografts were capable of remodeling over time to achieve a completely normal position.

Unlike artificial disc replacements, where the misplacement of an implant can lead to failure, such an outcome would not have been feasible in this case. Currently, studies are being conducted to determine whether the restoration of anatomical alignment also results in the restoration of kinematics.

Furthermore, during the post-5-year follow-up period, a significant proportion of the transplanted discs exhibited a prominent T2 signal within the nucleus pulposus, indicating active metabolic processes and adequate hydration within the transplanted discs.

From a clinical perspective, the transplant yielded motions that were on par with those achievable through artificial disc replacement. Moreover, none of the cases led to unintended fusion, demonstrating the success of the transplant in maintaining segmental mobility. There were no instances of herniation in the transplanted disc, segmental instability, or adjacent segment degeneration observed. Neither osteolysis nor implant migration were observed.

Although the research group has not encountered any instances necessitating revision surgery thus far, they anticipate that if the need arises, it can be adequately addressed through alternative options such as another allograft transplantation, artificial disc replacement, or the well-established spinal fusion procedure. It is crucial to address concerns regarding the sourcing of allografts, screening for transmittable diseases, and implementing reliable preservation technology, as these are all significant factors to consider.

In order to achieve biological repair or regeneration of the disc, it becomes imperative to enhance the understanding of the underlying molecular mechanisms that contribute to disc degeneration.

Consequently, the research team at the University of Hong Kong has established an extensive research program that focuses on investigating the impact of epidemiology, genetics, functional genomics, proteomics, and nutrition on the nano-structure and mechanics of the disc.

The studies reported in literature firmly maintains the viewpoint that addressing degenerative disc disease through non-fusion strategies, such as biologic repair and/or disc replacement, plays a crucial and noteworthy role in managing the condition. It is anticipated that artificial disc replacement will continue to be a prominent approach for a considerable period until alternative methods are further advanced.