**1. Introduction**

In the trend toward minimally invasive surgery, operations for the cervical spine have followed a similar tendency. While microsurgery has been in the lexicon of neurosurgery for ages, one of the earliest uses of the term "minimally invasive" in spine surgery was used by Probst in 1989 to discuss lumbar microdiscectomy [1]. While the term "minimally invasive" has become somewhat of a generic moniker for many different approaches, its intent is to be less traumatic to the patient with lower complication rates.

In the cervical spine, midline sparing posterior procedures such as lateral [2] and far later posterior approaches have afforded the opportunity to use smaller incisions and even endoscopic [3] approaches. Anterior foraminotomy, a disc preserving approach, has also been proposed [4] with favorable results [5]. However, these approaches have given limited access to midline pathologies and offer little benefit for cases with central herniation or instability.

Anterior discectomy alone allows central pathology to be addressed with reasonable success but high reoperation rates [6]. The addition of fusion stabilizes the spine in addition to maintaining distraction and neural foramen patency. Interbody grafts were instrumental in providing this indirect decompression and additional stability. Fusion halts further disc degeneration, preserves sagittal

balance, and eliminates segmental instability. Cervical fusion surgery, particularly anterior approaches has followed this minimally invasive trend and become more streamlined.

The anterior cervical discectomy and fusion (ACDF) was first reported in 1955 by Robinson and Smith [7], and this approach quickly became the dominant approach. While the competing Cloward technique [8] offered a high cancellous surface area for fusion, it had a high rate of graft collapse [9] and subsidence rates of up to 9.6% [10]. The shape of the graft provides some intrinsic stability but endplate preparation is more invasive requiring significantly more native bone removal which also predisposed patients to kyphosis [9, 11] at rates up to 9.6% [10]. Furthermore, higher complication rates include up to 4.8% of neurologic injuries [10].

The Smith-Robinson technique [12] is less invasive given that it is endplate sparing and causes minimal vertebral body destruction. It also provides better visualization of the decompression, particularly the uncovertebral joints. Arthrodesis by either technique has proven an effective treatment for cervical spondylosis and disc herniation [9, 13–15]. Anterior discectomy with fusion has been the prominent surgical treatment for symptomatic cervical spondylosis for over 60 years. Traditionally, bone dowels or spacers were harvested from autologous iliac crest contributing to hip pain rates of up to 39% [16]. This additional procedure has contributed to the relatively longer hospital stays of ACDF patients [17].

Allograft iliac crest provided a respite from further surgical trauma at a second site and was thus less invasive. However, fashioning an appropriate size graft added additional operating time on the back table. Pre-cut fibular strut grafts offered a more convenient and efficient option but are limited in terms of footprint size and have a high cortical to cancellous bone concentration. Machined structural allograft was the next iteration providing greater surface area of the graft and a higher percentage of cancellous bone contact, albeit at a greater cost.

Nonunion and graft subsidence still occurred and titanium plating developed as a more stable option [18]. The plate and four screw construct provided a solid fixation for arthrodesis to occur. With fusion rates of up to 100% [19], this technique became the gold standard for 20+ years. While the uniplate had some early adopters [20], high pseudofusion rates were reported [21].

With load-bearing limitations, limited allograft supply, and concerns over disease transmission from cadaveric bone, titanium cages had a simultaneous rise in popularity particularly in the lumbar spine. The imaging artifact and subsidence of titanium as well as its limited machining options increased the demand for synthetic polymers such as poly ether ether ketone (PEEK). In the cervical spine particularly, PEEK offered the ability to have a number of footprint options as well as height options for corpectomy and multilevel constructs.

The latest stage of cervical fusion has allowed titanium mini plates to incorporate with PEEK spacers as a stand-alone option with internal fixation (**Figure 1**). This integral plate allows for less bony exposure and potentially less issues with longus colli bleeding, recurrent laryngeal paresis, and sympathetic chain injury at lower levels. Furthermore, the zero- or low-profile interbody plate, as opposed to an on-lay plate, has been shown to have shorter surgical times and lower rates of dysphagia [22]. In addition, these implants have been shown to have lower rates of adjacent segment degeneration (ASD) [23–25]. However, these implants have been associated with increased rates of kyphosis [26, 27].

While the technological advances in arthrodesis have given rise to faster procedures and shorter stays, there has been a concordant rise in cost from zero-dollar autograft to modern-day single-level constructs costing \$5–6 K a level. At a time when physician reimbursement is diminishing this rise in per case cost is concerning, although the cost–benefit may be worth it for reduced surgeon's time.

### **Figure 1.**

*Cervical fusion devices showing zero-profile devicie at C5–6 and standard allograft and four screw plate at C6–7.*

### **2. Problems with fusion**

Cervical fusion is known to alter spinal biomechanics by creating abnormal loads and affecting segmental motion at adjacent vertebrae [28, 29]. These changes may accelerate adjacent disc degeneration through the increased stress on the adjacent disc [29–31].

Multiple studies have documented evidence of adjacent segment level disease including radiographic findings of new anterior osteophyte formation or enlargement, increased narrowing of an interspace, new DDD, and calcification of the anterior longitudinal ligament [31]. Fusion has shown an increased rate of these compared to arthroplasty. Similarly, the rate of symptomatic disease along with the need for medical treatments related to such was also greater in the fusion cohort.

Multilevel fusion constructs demonstrate even greater stress [32]. These multilevel procedures had higher rates of reoperation, pseudoarthrosis, and complications [33, 34] compared to single-level constructs.

### **3. The case for arthroplasty**

While the ACDF has been the gold standard for years, the well-known effects of motion loss and adjacent segment breakdown have been driving factors for cervical arthroplasty. One such mechanism is the neighboring intradiscal pressure. Fusion

constructs produce greater neighboring intradiscal pressure [30] compared to arthroplasty which preserves physiologic intradiscal pressures at neighboring levels.

In essence, arthroplasty is itself less invasive than fusion because of maintained motion and reducing the need for adjacent level surgery. Like the ACDF, cervical arthroplasty has followed a similar trend toward less invasiveness with a more streamlined process and less procedural time. The nomenclature for this procedure has varied markedly to include: anterior cervical discectomy and arthroplasty (ACDA, and abbreviated ACA), artificial disc replacement (ADR), total disc replacement (TDR), cervical total disc replacement (cTDR), cervical disc replacement (CDR), and cervical disc arthroplasty (CDA).
