Corpectomy fusion technique. Spinal instability after corpectomy or after vertebrectomy in the lumbar spine often requires complex reconstruc- tive procedures. The type and degree of instrumen- tation depend strongly on the number of involved levels and the retained functioning stabilizing structures. Generally, after corpectomy anterior support is mandatory and long-term stability can- not be achieved with rod/pedicle screw instrumen- tation alone. Furthermore, the combination with an anterior tension band device still exhibits a certain instability in extension and rotation. Therefore, from the biomechanical perspective, substantial anterior instability requires “front and back” instrumenta- tion. In the cervical spine, however, single-level cage stabilization is sufficiently supported by an anterior tension band device. Multiple-level cervical corpec- tomies are particularly unstable and anterior plating may be insufficient; consequently additional pedi- cle/lateral mass screw devices must be considered. Anterior tension band technique. Anterior rods/ plates act as tension bands in extension and func- tion as buttress plates in flexion. For the cervical spine, the latest generation of “semi-constrained/ dynamic” plates allow locked angle-stable mono- cortical screw fixation while axial compression of the graft is permitted. This offers increased stability combined with a minimized risk of stress-shielding. In the lumbar spine, anterior rod/double-rod instrumentation increases anterior stability after cage or graft implantation especially in extension. In flexion and lateral bending they are still inferior to pedicle screw devices. Biomechanics of the “adjacent segment”. Unphysi- ologically long and stiff spinal segments increase motion and intradiscal pressure in the adjacent segments. However, it is still unclear if adjacent seg- ment degeneration after spinal fusion is resulting from the changed biomechanics or exhibits simply the progression of the natural history. Disc arthroplasty. Disc arthroplasty offers several advantages such as preservation of segmental motion, potential absence of adjacent segment degeneration and no need for harvesting autolo- gous bone graft. Current prostheses differ in bear- ing materials (metal or polyethylene) and kinemat- ics principles. Constrained prostheses have a fixed center of rotation whereas unconstrained devices allow translational movement and thus respect the physiological helical axis of motion. Kinematics studies have shown that both types successfully re- establish almost the physiological range of motion. Only a few data exist so far on the long-term radio- logical and clinical outcome. Posterior dynamic stabilization technique. Improv- ing primary or iatrogenic spinal instability while “unloading/protecting” certain spine elements without performing a spinal fusion are the objec- tives of posterior dynamic implants. All systems successfully reduce segmental motion. However, rotation is poorly controlled while the posterior devices are particularly stiff in flexion. As it is unknown how much stability is needed in which particular entity of spine pathology combined with the partially undefined clinical indications, an assessment of this technique is currently impossi- ble. Finally, only long-term prospective clinical trials will give the necessary evidence for the efficacy of this particular treatment method. Key Articles Cripton PA, Jain GM, Wittenberg RH, Nol te LP (2000) Load-sharing characteristics of stabilized lumbar spine segments. Spine 25:170 – 179 Biomechanical cadaver study using pressure sensors, strain gauges and an optoelectronic tracking system. Load-sharing between an internal fixator and anatomical structures was assessed in asequential injury scenario. Applied loads were mostly supported by equal and opposite forces between disc and fixator. Based on the results, the paper highlights the fact that an anterior column insufficiency may lead to fixator overloads and implant failure. Laxer E (1994) A further development in spinal instrumentation. Technical Commission for Spinal Surgery of the ASIF. Eur Spine J 3:347 – 352 Introduction of the Universal Spine System with a single set of implants and instruments for various spinal disorders and surgical approaches. Spinal Instrumentation Chapter 3 85 Magerl FP (1984) Stabilization of the lower thoracic and lumbar spine with external skeletal fixation. Clin Orthop Relat Res 125–141 Classic article introducing the concept of a new angle-stable transpedicular fixation device which formed the basis for the development of second generation internal spinal fixation devices. Panjabi MM (1988) Biomechanical evaluation of spinal fixation devices: I. A conceptual framework. Spine 13:1129 – 1134 Panjabi M, Abumi K, Duranceau J, Crisco J (1988) Biomechanical evaluation of spinal fixation devices: II. Stability provided by eight internal fixation devices. Spine 13:1135 – 1140 Abumi K, Panjabi MM, Duranceau J (1989) Biomechanical evaluation of spinal fixation devices. Part III. Stability provided by six spinal fixation devices and interbody bone graft. Spine 14:1249 – 1255 These three publications are milestone papers as they introduced the basic concepts for testing and evaluation of spinal implants. Guidelines for three categorical biomechanical tests are stated: assessment of strength, fatigue and stability. TsantrizosA,AndreouA,AebiM,SteffenT(2000) Biomechanical stability of five stand- alone anterior lumbar interbody fusion constructs. 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Nerlich, Günther Paesold Core Messages ✔ The spinal column degenerates far earlier than other musculoskeletal tissues ✔ Age-related changes of the spine are not syn- onymous with painful alterations ✔ Time course and probability of early disc degeneration are largely determined by genetic disposition ✔ Theintervertebraldiscisthelargestavascular structure of the human body resulting in large diffusion distances to allow for disc nutrition ✔ Compromised disc nutrition is a key factor for disc degeneration ✔ Changes in the matrix components of the inter- vertebral disc, especially the proteoglycans, determine age-related changes of the disc ✔ Orientation and misalignment of the facet joints correlate with development of early osteoarthritis of the joint ✔ Changes in bone architecture of the vertebral bodies and formation of osteophytes alter mechanical properties of the spinal column ✔ Changes in matrix molecules and fiber orienta- tion in ligaments alter behavior of the liga- ments ✔ Age-related changes of the three joint complex lead to disc herniation, osseous and ligamen- tous stenosis Epidemiology Musculoskeletal impair- ments are a predominant health problem in the aging population Musculoskeletal impairments are prevalent and symptomatic health problems in individuals of middle and old age. Naturally, aging of an individual is accompa- nied by decreasing strength, pain and restricted movement. As a consequence, increasing age is concomitant with limited abilities for work and leisure activi- ties. Regular physical activities are important to maintain optimal mobility and general health. Age-related changes in the musculoskeletal system occur due to alteration in a multitude of tissues, such as bone and soft tissue including mus- cles, articular cartilage, intervertebral discs, tendons, ligaments and joint cap- sules [40]. In addition, a decrease in musculoskeletal function increases proba- bility and severity of soft tissue and skeletal damage due to trauma and also enhances the likelihood of complications during surgery. The number of people over 65 years will double within 25 years Considering estimations that predict a doubling of the number of people over 65 years of age during the next 25 years, patients suffering from musculoskeletal impairments will increase significantly [79]. In the USA, musculoskeletal and associated conditions in the elderly caused direct costs of US $51 billion in 1992 [158]. These facts impressively underline the impact on healthcare systems that age-related alterations of the musculoskeletal system will have in the future. Basic Science Section 91 ab Case Introduction This spinal specimen shows the extreme course of the result of aging on the lumbar spine. A sagittal section through the lumbar spine (L3–S1) of an 8-year-old individual ( a) demonstrates that the nucleus pulposus can be clearly distinguished from the anulus fibrosus. The cartilage endplates are composed of a thick layer of hyaline cartilage. The disc height is somewhat less than the vertebral body height. The vertebral bodies demonstrate rounded edges. On the contrary, the parasagittal section ( b) of a 77-year-old individual demonstrates that the disc space has completely collapsed. Anterior or posterior displacement of the vertebral bodies is visible at all levels. The cartilaginous endplates are partially resorbed and exhibit severe sclerotic alterations. The vertebral bodies exhibit severe bridging osteophyte formation. Despite these dramatic changes there is no close link between these alterations and pain. General Age-Related Changes Various mechanisms on a cellular and systemic level have been identified to con- tribute to age-related changes in the musculoskeletal system [45]. At the cellular level: cellular senescence, leading to a decreasing ability of somatic cells to repli- cate, repair, and maintain tissue apoptosis (programmed cell death), leading to decreased cell numbers in the affected tissue accumulation of post-translational modifications of matrix proteins, lead- ing to altered properties of the extracellular matrix increasing generation of oxidative stress due to generation of reactive oxy- gen species (ROS), leading to cell damage genetic predisposition, leading to premature aging or phenotypic changes in the musculoskeletal system At the systemic level: Systemic and cellular factors contribute to musculo- skeletal age-related changes declining levels of trophic hormones, leading to altered tissue environment and response of tissue to use and injury general age-related changes,suchasadecreaseinreactiontime,proprio- ception, vision, hearing, pulmonary and cardiovascular function, leading to decreased mobility and therefore affecting the musculoskeletal system socioeconomic and psychosocial factors alsocontribute,mainlybyinflu- encing the individual variation regarding the age-related impairment of mobility The diversity of contributing factors on cellular and systemic levels underlines the multifactorial nature of age-related changes that will finally lead to alter- 92 Section Basic Science ations of the local environment within the affected tissue. These local alterations can then directly affect the function of the respective tissue. Although the result, i.e. altered tissue function, can be observed and analyzed, the exact relationships and interactions between cellular and systemic changes are not yet clear. The spine is most frequently affected by age-related alterations Although any part of the musculoskeletal system can be affected by age- related alterations, lower extremities and especially the lumbar spine are the most frequently reported locations of musculoskeletal impairment ( Case Intro- duction ). Between 70% and 85% of the population in Western industrialized countries will experience back pain at least once during their lives, underlining the impact of age-related alterations to the spine [33, 35, 86, 151, 152]. These epi- sodes of back pain often lead to sickness leave and sometimes to chronic disabili- ties (approx. 10%) causing an enormous socioeconomic burden on society [80]. In this context, it is important to notice that normal age-related degenerative changes and pathological degeneration leading to back pain have to be distin- guished. Several studies have shown that between 7% and 72% of individuals that exhibit signs of disc degeneration never experienced relevant low back pain [15, 115, 155]. Among age-related alterations of the spine, the so-called “degenerating spon- dylosis” or spinal osteoarthritis is the most common and is probably inevitable Degenerative spondylosis is inevitable with aging with increasing age. This alteration is radiologically characterized by osteophy- tes (bone spurs) arising from the margin of the vertebral body and is usually accompanied by disc space narrowing. The term “spondylosis” was historically an effort to distinguish between degenerative changes in the spine and those in synovial joints (osteoarthritis) such as facet joints [145]. However, it has been shown that pathological changes in the spine and osteoarthritis of the synovial joints coexist and in most cases are interrelated [145]. Autopsy studies by Schmorl and Junghanns [64] reported evidence of spondylosis in 60% of women and 80% of men by the age of 49 years, and in 95% of both sexes by the age of 70 years. Functional Spine Unit The motion segment is the functional unit of the spine The spine is a multi-segmented column, which provides stability and mobility to the body at each segmental level and gives protection to the nerve roots and the spinal cord. The smallest anatomical unit of the spine which exhibits the basic functional characteristics of the entire spine is called the “motion segment”or “functional spine unit”( Fig. 1). It was first described by Schmorl and Junghanns [64]. Each motion segment consists of two adjacent vertebrae, separated dorsally by the zygapophyseal joints or facet joints and anteriorly by the interposed inter- vertebral disc. The vertebrae are further connected by spinal ligaments, joint capsules and segmental muscles. The spinal ligament complex consists of the interspinous, supraspinous intertransverse, yellow, anterior and posterior longi- tudinal ligaments. In contrast to the extrinsic muscles, the intrinsic muscles span over two vertebrae and consist of splenius, erector spinae, transversospinal and segmental muscles. Spine motion, stability and equilibrium are achieved by the antagonist action of the powerful flexor and extensor muscle groups. The motion segment is a three joint complex The normal spinal function largely depends on the integrity of these compo- nents and their coordinated interplay. Kirkaldy-Willis [71] introduced the term “the three joint complex” to highlight the importance of a normal interaction of the three joints in a segment, i.e. the intervertebral disc and the two facet joints. Any alterations in one of these components will result in a disturbance of their interplay with subsequent dysfunction, finally leading to back pain, deformity and neurological compromise. Age-Related Changes of the Spine Chapter 4 93 a b Figure 1. Functional spinal unit Schematic representation of a functional spinal unit (motion segment) in a the cervical and b lumbar spine. 94 Section Basic Science . single set of implants and instruments for various spinal disorders and surgical approaches. Spinal Instrumentation Chapter 3 85 Magerl FP (1984) Stabilization of the lower thoracic and lumbar. (1989) Biomechanical evaluation of spinal fixation devices. Part III. Stability provided by six spinal fixation devices and interbody bone graft. Spine 14 :124 9 – 125 5 These three publications are. changes of the disc ✔ Orientation and misalignment of the facet joints correlate with development of early osteoarthritis of the joint ✔ Changes in bone architecture of the vertebral bodies and formation