Spinal Surgery Biomechanics: Principles for Residents and Students

This chapter provides an overview of biomechanical in vitro testing of pedicle screws. Several aspects, such as specimen selection, test setup, and loading modalities for the investigation of screw anchorage are discussed. In general, cement augmentation is an effective technique to improve pedicle screw anchorage. However, in clinical practice, it should be considered that augmentation is most effective in the osteoporotic bone while in healthy bone, the improvement of screw anchorage is only marginal.

Low back pain is a prevalent medical condition. Although most patients improve conservative treatments, some need surgery. The traditional procedure, the spinal arthrodesis, fixes a spinal segment, forcing the adjacent ones to undergo an extra load and a mobility excess that is the cause of middle and long-term discal degeneration and zygapophyseal joint arthritis changes. All this can be the source of further low back pain and require a new surgical procedure with a new spinal fusion in an average of ten years.

Joint mobility preservation is a must in all areas of surgery, and the spine is no exception. Disc arthroplasty has provided better results than spinal arthrodesis, particularly in patients under 50 with discal degeneration and no concurrent zygapophyseal joint arthritic changes. The patient selection must be accurate to get adequate results. No zygapophyseal joint damage must be present as otherwise, low back pain is common after disc arthroplasty.

The surgical technique must concentrate on every detail. The retroperitoneal approach is challenging even in the best hands. In this respect, the assistance of an access vascular surgeon is of particular help. The prosthetic disc's final position inside the discal must be no more than 2mm from the midline and 4 mm from the posterior aspect of the vertebral body. The anterior longitudinal ligament and annulus fibrosus removal induce an excess of mobility not controlled by the commercially available discal prosthesis. It is an area that still needs improvement.

The choice of which discal prosthesis to use depends on the surgeon's preferences, and new designs steadily improve the features, results, and complication rate of the previously existing ones. But there is still plenty of room for further improvement.

Fusion is frequently considered when planning for a spinal surgery procedure; nowadays, such a fusion is preferred between the intervertebral bodies (socalled interbody fusion) because there is a bigger surface for the bone to grow and make contact (increasing fusion rates), improving also overall spine alignment (trying to get a balanced spine), which in turn protects the adjacent segment, as there is also less wobbling at the fused space and mechanic aspects are less deleterious.

From a mechanical point of view, a fusion is an ankylosing procedure that eliminates any movement between vertebral bodies, so the Functional Spine Unit (FSU) is abolished; both kinetics (forces at stake), kinematics (displacements caused by those forces) and stiffness (deformations by those same forces) within FSU should be considered. 

Osteoporosis is the most frequent metabolic bone disease, affecting particularly women. Due to the progressive ageing of the population, the number of patients with this condition requiring spine surgery is increasing, while new techniques and implants are in development to help this particular population: apart from percutaneous augmentation techniques (such as vertebroplasty and kyphoplasty), fenestrated pedicle screws which can be cemented have changed the spinal management of these patients. 

Degenerative spondylolisthesis (DS) is a common entity in the fifth-sixth decade of life, and it is assumed that there is a biomechanical rationale behind the pathogeny as it will not develop in all individuals. There are several causes that could initiate its natural history: strong lumbopelvic anatomical fixations, ligamentous laxity, sarcopenia, spinopelvic parameters, etc. In the end, it will stabilize by itself due to the Kirkaldy-Willis cycle. The issue arises when it becomes symptomatic because of the facet deformity and hypertrophy together with the endplate spondylotic osteophytes - even with small displacements-, producing a central and lateral stenosis with a concomitant pluriradicular involvement. The biomechanical background is analyzed to provide clues to understand the natural history of DS and set the rationale for treatment

Several interspinous devices have been incorporated into the spinal implant market. There have been several reasons for their wide use, including that they can be implanted using a minimally invasive approach even under local anesthesia. This chapter reviews the biomechanical studies about interspinous devices to allow the reader a better comprehension of the effects of these devices, not only on the treated segment but also on the adjacent segments of the spine. Unfortunately, the use of these implants is often not associated with a thorough understanding of their biomechanical behaviour, which is useful to address both indications and contraindications for this procedure

It is critical for any doctor dealing with spinal trauma cases to be able to reliably and quickly determine the stability of traumatic injuries throughout the subaxial spine. Ultimately, the stability of the spine is dependent on numerous, complex biomechanical relationships between bone, ligament, disc, and muscle. There have been many attempts at classifying different traumatic injuries in the spine based on mechanism, morphology, and a combination of the two, which are presented for review along with two commonly used systems in modern practice.