**4.1 Implant placement and rationale**

The cervical hybrid arthroplasty provides the unique opportunity where with one procedure the surgeon can address an area of junctional kyphosis while simultaneously preserving motion at a neighboring disc. When considering all scenarios for a cervical hybrid surgery, there should be a consistent rationale in regards to which level to fuse and in which level to place the arthroplasty.

For the most part there are some straightforward scenarios which dictate which level warrants the cervical fusion implant. If one disc is entirely collapsed, demonstrates significant bony spurs, and/or heterotopic ossification, this level would assuredly justify the fusion implant. If the operative level lies within the inferior aspect of the spine (i.e. Cervical 6–7, C7-T1) sufficient reasoning exists towards the insertion of a fusion spacer at this level as opposed to an arthroplasty. This is because along the inferior limb of the cervical spine, the sub adjacent interspace levels are morphologically larger and well documented as demonstrating less motion [29–31]. Their size and innate stiffness coupled with the stability conferred by their adjoining anatomy makes these levels are ideally suited towards forming the foundation of the hybrid construct and bearing any subsequent transferred loads [31–33]. By contrast, the interspaces along the more cephalad aspect of the spine (Cervical 2–3, Cervical 3–4) routinely comprise a smaller footprint and consequently can only accommodate a smaller implant. As a result these smaller interspaces are often ideally suited towards fusion spacers which tend to come in more sizes and options. Furthermore if there is any indication of ongoing myelopathy or an underlying contiguous myelomalacia, this level would best be served with a fusion implant which would provide a stable postoperative environment. Otherwise in patients who have myelopathy only those without instability and symptoms due to soft disc herniations with or without minor spurs would be good candidates for an arthroplasty.

The core principle behind all arthroplasties is their perceived objective, once implanted, towards minimizing the biomechanical stresses placed on adjacent levels. With this in mind deciding which level should obtain the arthroplasty device is of paramount importance. As a rule of thumb, all efforts are geared towards placing the arthroplasty at the top of the overall construct in order to minimize stress at the superior neighboring and often more mobile disc [29, 34]. When this is not possible, in a circumstance where there are three disc herniations and the decision has been made only to operate on two of the discs because they are the only symptomatic levels, then the arthroplasty should be placed at the level nearest the third disc in hopes of preventing it from further deterioration. Studies have shown that the arthroplasty implant would limit transmission of angular, horizontal, and translation forces experienced by the adjacent third level disc [35–40].

### **4.2 Sequence of implantation**

The sequence of implantation should be considered well in advance during the preoperative planning phase in order to limit complications. During insertion, tapping of the implants with the mallet can lead to an aggravation of an underlying stenotic area, or the migration and loosening of a previously inserted prosthesis [41, 42]. In order to avoid this for all cervical hybrid arthroplasty procedures a

thorough decompression of all the intended disc spaces should be performed prior to any implant insertion, with priority given to the most stenotic level. In all circumstances the C-ADR should be implanted prior to the ACDF portion of the procedure. If implanting more than one arthroplasty, all trialing, rasping, drilling for both prostheses should be performed prior to C-ADR implantation [41, 43].
