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Ferguson Core Messages ✔ Spinal instrumentation is usually combined with spinal fusion ✔ The type of instrumentation and the surgical approach should follow the degree of instabil- ity ✔ Consolidated fusion may relieve the implant from stress ✔ Implant failure is a result of instant overload or of cyclic loading (fatigue) ✔ If fusion is delayed and/or the wrong implants are chosen, instrumentation will ultimately fail ✔ Spinal instrumentation should provide early and safe mobilization of the patient ✔ For achieving bony fusion sufficient segmental stability and appropriate load sharing are essential ✔ Absolute stability may interfere with fracture healing due to stress-shielding of the bone graft ✔ Rigid (multi-)segmental instrumentation may cause adjacent segment overload Goals of Spinal Instrumentation Knowledge of biomechanical principles reduces the rate of implant failure and non-union Spinal instrumentation basically means the implantation of more or less rigid metallic or non-metallic devices which are attached to the spine. These devices function to provide spinal stability and thus facilitate bone healing leading to spi- nal fusion (spondylodesis). Fundamental biomechanical knowledge and its application serves to improve the performance of the individual spine surgeon with respect to the rate of bony fusion, implant failure or degree of deformity cor- rection. However, biomechanics is inherently linked with (mechano-)biology. And there is still an incomplete understanding of spinal biomechanics and even more so of the underlying biology. Moreover, apparently advantageous biome- chanical concepts do not necessarily lead to a better patient outcome. While a myriad of spinal stabilization devices and fusion techniques are avail- able to the surgeon today, there are a concise number of underlying fundamental principles. Indeed, whole volumes have been written about the definition and assessment of spinal instability and the biomechanics of spinal stabilization [11, 103]. The reader is encouraged to explore these resources for a more in-depth study of this subject and for an interesting historical perspective of chronological implant development, from the Harrington rod [40] to the first external segmen- tal instrumentation systems by Magerl in 1977 [55], followed by the “fixateur interne” which was developed by Kluger and Dick [27], and the CD (Cotrel/ Dubousset) system [20]. A milestone in the history of spine research was the introduction of universal concepts for the biomechanical testing of spinal implants by Manohar M. Panjabi, taking into consideration three major aspects [65]: Basic Science Section 67 Key properties are material strength, stability and fatigue resistance implant strength (failure load) fatigue (longevity under cyclic loading) ability to restore spinal stability However, in vitro testing for primary implant stability usually comprises non- destructive testing protocols with only a few cycles, and therefore takes into account neither the effect of repetitive loading (fatigue) nor the biological host reaction. Adapt implant and instrumentation technique to the individual case Each spinal pathology which is intended to be treated with a stabilizing surgi- cal procedure has its own unique biomechanical characteristics. For a successful patient outcome it is important that one chooses the appropriate implant and technique, considering the specific nature of each case. Before selecting an instrumentation system to restore or maintain stability of the compromised spine, it is a prerequisite to understand the functions of the respective structures and how the biomechanics are changed by their loss. Thus, the choice of implant is strongly dependent on the indication. For example, the stress on a lumbar translaminar facet joint screw (TLS) in a patient treated with instrumented fusion for arthritis-related facet pain and with only minimal resid- ual segmental mobility is relatively low. However, it is not reasonable to stabilize a complete vertebral body burst fracture with a substantially compromised ante- rior column solely with TLS. In this case, the screws would most likely fail, result- ing in a post-traumatic kyphosis, because anterior support was mandatory. The goals of spinal instrumentation are to stabilize, correct and fuse With the exception of the recent developments in non-fusion devices such as spinal arthroplasty and posterior dynamic systems, spinal stabilization is a means to achieve the end goal of a solid bony fusion. Beyond this, the aims of spi- nal instrumentation are ( Table 1): Table 1. Goals of spinal instrumentation to support the spine when its structural integrity is severely compromised (iatrogenic, traumatic, infectious or tumorous) to prevent progression or to maintain the achieved profile after correction of spinal deformities (scoliosis, kyphosis, spondylolisthesis) to alleviate or eliminate pain originating from various anatomical structures by fusing or stiffening spine segments and thereby diminishing movement Current implants have a wide “safety zone” Each region of the spine has its own anatomical, biomechanical and biological properties. Aspects such as kyphotic or lordotic curve, inherent mobility, loading conditions as well as bone healing potential have an influence on the choice of implant and surgical approach. For this reason spinal implants not only differ in size but also follow different preferred region-specific stabilization principles. The authors’ intention is to outline instrumentation principles based on biome- chanical studies rather than to discuss specific implants. For detailed informa- tion about individual implants and anatomical regions, the reader is referred to the clinical chapters of this book and the literature cited in the references. Since nowadays it is still only approximately possible to assess the individual case in advance concerning spinal stability, individual constitutional and genetic factors as well as biological responses, e.g., bone healing properties, bone quality, toler- ance to foreign materials, the recommendations for instrumentation techniques can only be generalized to a certain extent. The inability to assess complete dis- ease entities has also led to therapy principles which are within “the safety zone” and implants which are generally sufficient for the majority of cases. But this also implies that instrumented fusion is sometimes overpowered (too rigid) or is sometimes not indicated at all. 68 Section Basic Science The extent of stability necessary to achieve fusion is unclear Unlike in biomechanical studies, where spine specimens are tested under “extreme” conditions, in reality very often substantial stabilizing structures are preserved and therefore may make the instrumentation partially redundant. This is one reason why suboptimal (in the biomechanical sense) spinal instrumenta- tion methods may still result in excellent patient outcomes. Furthermore, the “better and the faster the biology” the less rigidity is likely necessary to ensure healing of the spondylodesis. This is impressively demonstrated by the safe and reliable posterior in-situ fusion (without instrumentation) in lumbar lytic spon- dylolisthesis in children [87]. Instrumentation generally aims to exceed physiological segmental stability Another example of the role of the biological and mechanical environment is the cervical spine: unlike in the lumbar spine, where rigid stabilization is manda- tory, the subaxial cervical spine is more tolerant to less rigid instrumentation in terms of bony fusion. Here, for example after discectomy, stand-alone interbody cages or structural autologous bone grafts successfully reestablish physiological stability, which nevertheless results in an approximately 100% fusion rate [37, 83]. Basic Biomechanics of Spinal Instrumentation The following sections are intended to provide insights into the biomechanical principles of spinal instrumentation and should also provide background knowl- edge for the different stabilization techniques treated in the subsequent clinical chapters of this book. Loading and Load Sharing Characteristics Mainly muscle forces have an influence on internal fixator loads while posture is less important Spinal instrumentation and the stabilized spine segment form a mechanical sys- tem, a couple, which shares loads and moments. In-vivo telemetry has provided valuable insights into the complex three-dimensional loading of internal fixa- tors during daily physiological activity [77]. Several interesting conclusions can be drawn from these studies: mainly muscle forces were influencing fixator loads. Flexion/extension movements as well as wearing braces or harnesses did not significantly affect fixator loads. Sitting and standing exhibited similar loads and erect standing and walking resulted in the highest loads. The forces acting were mainly compression forces rather than distraction; moments were mainly flexion-bending types. Support of the anterior column reduced fixator loads postoperatively while later healing of the fusion very often did not.Thusimplant failure such as screw breakage does not necessarily prove pseudarthrosis [76, 78, 79, 81]. However, telemetric fixator load analysis does not provide any information about the overall force flow and load sharing, i.e. how much of the total load is transferred by the implant and how much by the spine. This topic was investi- gated by Cripton et al. [21] using posteriorly instrumented spine segments. By simultaneously measuring intradiscal pressure and the forces in a modified AO internal fixator during physiological loading, analysis of the load distribution The loading pattern of the implant is critically dependent on the motion within the instrumented spinal construct was possible. On this basis, it was dem- onstrated that spinal loads during flexion and extension were carried predomi- nantly by equal and opposite forces in the disc and the fixator constituting a force couple. Only a small portion of the total loading was transferred directly by bending of the implant or through the posterior elements. However, for side bending the majority of loading was transferred through equal and opposite forces in the fixator rods. For torsional loading, the distribution was approxi- mately evenly spread between implant forces, torsional resistance of the disc and Spinal Instrumentation Chapter 3 69 13 Figure 1. Load sharing Load-sharing between rod/pedicle screw instrumentation and the anatomical structures of the spine during spinal motion. In flexion-extension load is mainly transferred by the disc-fixator force couple through equal and opposite forces. In torsion a great fraction of load is transferred by the disc. Therefore, the integrity of the anterior column is crucial for relieving the implants from load and thus to ensure longevity. In lateral bending load transfer is mainly through the implant. forces acting on the posterior elements (Fig. 1). But how does the load distribu- tion change with an insufficient anterior column support, which may be found in various spinal disorders, e.g. vertebral body burst fractures, spondylitis, meta- static vertebral destruction or after disc ruptures? In case of a compromised ante- rior column, the implant must carry the majority of the load in lateral bending, flexion, and extension ( Fig. 1). Furthermore, after discectomy and the complete removal of the posterior structures the segmental range of motion (ROM) is still sufficiently limited (by 64%) in flexion and extension, but torsion is only weakly controlled and increases by more than 230% under these conditions ( Fig. 1). Tak- ing this information into consideration, in the clinical setting postoperative lat- eral bending (and torsion) should be avoided by the patient in any event to mini- mize fixator loads whereas flexion and extension are mostly unproblematic pro- vided there is a functioning anterior column. Anterior column defects require anterior buttressing Combining the in-vivo measurements of implant loading taken by Rohlmann et al., and the force flow analysis in the study of Cripton et al., global moments of up to 30 Nm may act through the spine [21]. If instrumentation devices are exposed to such high moments, the safe limit for many implants may be exceeded. Therefore, in the case of a substantially unstable anterior column, additional anterior support is critical to prevent hardware failure. Further work is required to characterize the force and load transfer through intervertebral devices, corpectomy cages and other stabilization constructs. 70 Section Basic Science Posterior Stabilization Principles The term “posterior instrumentation” is used for any surgical measure with the implantation of a stabilization device acting on the posterior column (according to F.W. Holdsworth’s two-column concept [43]). This is commonly carried out via a posterior approach, which can vary depending on the surgeon’s preferences. However, it does not necessarily mean that the device itself is exclusively acting on the posterior spinal column. Rod/pedicle screw devices or lateral mass screws, for example, also affect the anterior column. On the other hand, implantation of PLIF effectively stabilizes the anterior column by a posterior approach interbody cages through the spinal canal (PLIF = posterior lumbar interbody fusion) is a measure of anterior instrumentation, although it generally makes additional posterior stabilization, e.g. pedicle screws or translaminar screws, necessary due to the iatrogenic destabilization of dorsal structures. Pedicle Screw Technique Pedicle screw/rod systems are now well established forsurgicaltreatment TheintroductionofpediclescrewsbyRoy-Camillein1970[82],thesubsequent development of the external fixator by Magerl [55], the following “fixateur interne”byKlugerandDick[27],theangle-stableinternalAO fixator [4] and the posterior segmental instrumentation systems [20, 51] have all dramatically improved the outcomes of spinal fusion. In contrast to the usage of long rods, now short segment stabilization using pedicle screws and rigid connecting plates or rods has become possible. This technique has been proven to be safe and effective for the surgical treatment of almost all spinal disorders such as congenital, devel- opmental,traumatic,neoplasticanddegenerativeconditions[2,3,13,34,51]. The stabilizing potential of screw/rod systems depends heavily on extent and location of instability The stabilizing properties of pedicle screw/rod spinal fixation systems, such as the Universal Spine System (Synthes, USA and Switzerland) [51], are not exceeded by any other posterior systems but are critically dependent on the degree of spinal instability and thus the pathological condition. Various biomechanical studies have been conducted on further implant characterization and to define accurate clinical indications. For example, after corpectomy and bisegmental instrumenta- tion using a spacer and a cross-linked pedicle screw/rod system, motion is reduced by up to 85% in flexion, 52% in extension, 81% in lateral bending and 51% in axial rotation [7]. Similar results have been reported by Cripton et al. [21]. This applies also for monosegmental instability with destruction of the posterior elements combined with a partial dissection of the intervertebral disc. Here most other pos- terior instrumentation devices also exceed the physiological stability, but with the short segment fixator being the stiffest [1]. However, after complete removal of the posterior structures combined with a complete disruption of the intervertebral disc but with the pedicle screw instrumentation in place, the range of motion for flexion/extension was increased by 21% compared to the intact spine. Further- more,torsionwasonlyweaklystabilizedbyrod/pediclescrewsinposterior(facet joint) and two-column insufficiency [21]. The stability of pedicle screw systems is derived from the solid anchorage of the screw in the pedicle and the inherent rigidity of the connecting hardware. While the pullout strength of pedicle screws is directly related to the bone density [39], Convergent screw positioning increases pull-out strength it can be increased by choosing convergent screw trajectories ( Fig. 2 ). Further- more, in the presence of anterior column instability, the avoidance of parallel ped- icle screw insertion in short segment fixation not only increases the pull-out strength but also prevents an unstable “four-bar” mechanism.Thesamerationale applies for cross-linking the rods. Here, diagonal cross-linking is favorable to the horizontal configuration in terms of rotational stability [29, 100] ( Fig. 3 ). The material, length and diameter of the connecting rods determine their stiffness. Compared to 7-mm rods, using 10-mm rods would increase the stiff- ness 4.1 times and 3-mm rods would have a 30 times lower bending stiffness [80]. Spinal Instrumentation Chapter 3 71 ab Figure 2. Pedicle screw positioning The use of convergent screw trajectories (right) increases the pull-out strength and overall stability of pedicle screw con- structs, in comparison with parallel screw insertion (left). abc Figure 3. Screw assembly a The use of conventional parallel pedicle screws and rods for spine segments with diminished anterior integrity may be insufficient. b Displacement of the stabilized segment by rotation of the pedicle screws – a so-called “four-bar” mecha- nism – may result in instability. Further stability can be achieved by the use of convergent screw trajectories and the addi- tion of cross-linking. c Two cross-links or at least one oblique cross-link provides better stability than one horizontal cross-link. However, greater deformation in smaller rods leads to greater internal stress and may finally result in failure. More rigid rods on the other hand produce higher internal loads in the implant, on the clamping device, and on the pedicle screws, and thus have a higher risk of screw breakage [80]. Therefore, current implant designs are a compromise between an absolutely rigid fixation and a minimal risk of implant failure to provide stable fixation with a proven service life [7]. 72 Section Basic Science Figure4.Thoracicpediclescrew positioning In contrast to the standard intrapedicular screw insertion (left pedicle), an extrapedicular screw trajectory (right pedicle) allows a greater margin of safety with respect to the spinal canal and offers greater pull-out strength and stability. Extrapedicular screw placement in the thoracic spine is safe and reliable While pedicle screws have been accepted as a reliable and safe method for stabi- lizing the thoracolumbar spine, their use in the mid and upper thoracic spine is more complicated and risky, due to the smaller overall dimensions and greater morphological variation of the thoracic pedicle, and the existing spinal cord at this height. A safer alternative to the standard intrapedicular screw placement in Lateral extrapedicular screw positioning is safe and bio- mechanically advantageous in the thoracic spine the thoracic spine is the ext rapedicular screw trajec tory (Fig. 4), first described by Dvorak et al. [28]. The pull out strength is increased by a greater screw-angu- lation, longer screw length, and the penetration of additional cortices. Segmental stability has been shown to be equivalent to that of the conventional intrapedicu- lar technique, without a higher risk of material fatigue [59]. The use of simple laminar hooks in the thoracic spine is safe with respect to the damage of neural structures. However, hook disengagement has been reported in scoliosis correction surgery [38]. To achieve a higher resistance to the complex three-dimensional forces, pedicle hooks with additional supporting screws have been developed [4, 51]. Biomechanical pull-out tests have shown that a significant increase in failureload canbeachieved with the useof screw-augmented hooks [12]. Translaminar and Transarticular Screw Technique Translaminar screws effectively stabilize the spinal segments in conjunction with anterior instrumentation Transarticular screws were first used by D. King in 1948 and later modified by H. Boucherin1959[14].Thenowwidelyacceptedtranslaminar facet joint screw placement ( Fig. 5)wasintroducedbyF. Magerl in the 1980s [58]. Translaminar screws (TLS) are setscrews, have a long trajectory in bone and have a favorable direction with reference to the nerve root. TLS are mostly used supplementary to anterior fusion techniques or in concert with posterior/posterolateral fusion measures in degenerative disorders. Here incompetent facet joints frequently allow pathological shear translation (olisthesis) and segmental multiplanar rota- tion. Biomechanical testing has shown that isolated screw fixation of the facet joints causes a moderate stabilization in all loading directions [72]. Therefore for posterior and posterolateral spondylodesis, the combination with facet fusion is generally recommended as it enhances stability [96]. Stand-alone interbody cages do not sufficiently stabilize the spine in extension and axial rotation Similarly, as anterior fusion (PLIF/ALIF) with stand-alone cagesisparticu- larly weak in controlling extension and axial rotation [54], an additional fixation is strongly recommended to ensure fusion [72]. In one study TLS were applied complementary to paired threaded interbody cages, thereby achieving a reduced angular motion of 30% in flexion and 60% in extension [67]. Spinal Instrumentation Chapter 3 73 ab Figure 5. Translaminar screws Translaminar screw positioning in the coronal (a) and the axial view (b). However, compared to pedicle screws, the stabilizing properties of TLS are fewer, especially in flexion and rotation [49]. Nevertheless, one should emphasize that Thedegreeofstability needed for optimal fusion is still unknown the degree of stability needed to achieve bony fusion is still not known. Further- more, several studies have shown that solid fusion and clinical outcome are not well correlated [33]. Nevertheless, the goal must be to achieve solid fusion and it is much more likely that a poor clinical outcome and “failed surgery” with pseud- arthrosis and implant failure are due to insufficient postoperative spinal stability and improper instrumentation than to excessive stability and thus stress shield- ing. In this context, the related question of “adjacent segment degeneration” is discussed below in detail. Occipitocervical Fixation The evolution of occipitocervical fixation started with pure in-situ bone graf- ting, after which came wire techniques, first without and later with attached steel rods, then followed by plate/screw instrumentation in the 1990s and most recently modular combined plate-rod/screw instrumentation [46, 99, 102]. The major advantage of the latter is its greater stability, allowing the abandonment of supplemental external fixation such as halo fixators or Minerva jackets. Basically the same principles of posterior fixation as described above apply to Lateral mass and pedicular screw fixation is superior to sublaminar wiring or hooks for cervical fusions the occipitocervical junction. Comparative biomechanical in-vitro studies have demonstrated that lateral mass screws, pedicle screws or transarticular screws (C1–C2) are superior to sublaminar wiring or sublaminar hooks [63]. Stability of occipital fixation depends on whether mono- or bicortical screws are used and the local occipital topography to the side of the screw placement. Cortical thick- ness is greatest at the midline and the superior and inferior nuchal lines [75]. Anterior Stabilization Principles The term “anterior instrumentation”isusedforanysurgicalmeasureforthe implantation of a stabilization device acting on the anterior column (according to 74 Section Basic Science . and assessment of spinal instability and the biomechanics of spinal stabilization [11, 103 ]. The reader is encouraged to explore these resources for a more in-depth study of this subject and for an. fibrosus. Spine 15:402– 410 51. Mayer TG, Tencer AF, Kristoferson S, Mooney V (1984) Use of noninvasive techniques for quantification of spinal range -of- motion in normal subjects and chronic low-back. support was mandatory. The goals of spinal instrumentation are to stabilize, correct and fuse With the exception of the recent developments in non-fusion devices such as spinal arthroplasty and posterior