CHAPTER SYNOPSIS:
Reconstruction of the adult spine in the setting of osteoporosis is complex, and the benefits of surgical treatment must be balanced against the potential risks for comorbid medical conditions and increased rates of surgical complications. When surgery does become necessary for the patient with osteoporosis because of fracture instability, deformity, neural compression, or other causes, surgical planning must carefully consider the effects of diminished bone quality on the surgical plan and likelihood of success with surgical treatment. Instrumentation is useful in deformity correction, maintenance of spinal balance, and stabilization of unstable segments until bony fusion occurs. Specialized techniques such as pedicle-screw augmentation, reduction of the bone-implant interface, and use of longer constructs with multiple points of fixation are useful to address the diminished bone density and greater rate of failure in constructs applied to osteoporotic bone.
IMPORTANT POINTS:
- •
Osteoporosis is quite common in the adult scoliosis population.
- •
Diminished bone density is related to a greater rate of complications and instrumentation failure with surgery.
- •
Meticulous preoperative planning and surgical technique are key to successful reconstruction of the osteoporotic spine.
- •
Numerous techniques exist to minimize the risk for instrumentation failure, involving pedicle screw types and insertional techniques, implant design, and application.
CLINICAL/SURGICAL PEARLS:
- •
Augmentation of pedicle screws with polymethylmethacrylate, calcium sulfate, calcium phosphate, or hydroxyapatite improves pullout strength.
- •
Reduction of the bone-implant interface with techniques to spread contact stresses (e.g., pediculolaminar fixation) improves fixation strength.
- •
Use of longer constructs and multiple points of fixation are important to reduce the risk for implant failure at the caudal or cephalad ends of the construct.
- •
Restoration of spinal balance and the techniques designed to facilitate this restoration (anterior column reconstruction, anterior-posterior surgery) are important to achieve successful outcomes.
SCOPE OF THE PROBLEM
Adult spinal reconstruction for traumatic or degenerative disorders involves a complex algorithm that can be affected by multiple variables. Patient factors (medical comorbidities, age, pattern of injury/disease) serve as important determinants of treatment. Reconstruction of the adult spine can be complicated by deformity and decreased bone mineral density. Although conditions of decreased bone mineral density can, in part, be addressed by medical treatment, typically the resulting increase in bone mineral density is not sufficient to offset the treatment delay. The benefits of surgical treatment must be balanced against the potential risks for comorbid medical conditions and increased rates of surgical complications for patients with osteoporosis. When surgery does become necessary for the patient with osteoporosis because of fracture instability, deformity, neural compression, or other causes, surgical planning must carefully consider the effects of diminished bone quality on the surgical plan and likelihood of success with surgical treatment.
Instrumentation is useful in deformity correction, maintenance of spinal balance, and stabilization of unstable segments until bony fusion occurs. Spinal reconstruction in osteoporotic patients is associated with a greater failure rate than the general population, as well as a characteristic class of complications. Early postoperative failures have included pedicle fractures, as well as compression fractures adjacent to the terminus of the construct. Later complications include pseudarthroses with instrumentation failure, adjacent-level disc degeneration with herniation, and late compression fractures. Progressive kyphosis frequently occurs at the end of fusion constructs. Careful surgical planning and specialized instrumentation techniques can help address the challenges presented by the patient with osteoporosis with a spinal disorder resulting from degeneration, tumor, or trauma.
PREOPERATIVE CONSIDERATIONS AND INDICATIONS
Patient Assessment
An assessment of the individual patient’s clinical status, goals of treatment, and operative risks aids the appropriate selection of surgical techniques when surgery is required. This can effectively improve clinical outcomes and reduce the complications of surgical treatment. The adult patient with poor bone quality typically has a medical profile that may increase the risk for a spinal operation or may make the surgical goals difficult to achieve. A complete preoperative medical assessment of patients considered for surgical treatment provides the opportunity to minimize risks by optimizing health status. Identification of factors in the patient history, such as a previous fragility fracture, is key to preoperative assessment.
Modifiable conditions that have been shown to affect surgical risk include not just osteoporosis but also tobacco smoking, a history of asthma or chronic obstructive pulmonary disease, coronary or cerebrovascular disease, diabetes mellitus, nutritional deficiency, depression, and current significant life stressors. Collaboration with consulting medical and anesthesia specialists who are trained in perioperative management is important in minimizing surgical risk and optimizing outcomes through correction of modifiable conditions.
Assessment of Bone Quality
The assessment of bone quality plays a critical role in the design of the operation and the consideration of the instrumentation to be used to achieve surgical goals. For the typical adult patient with scoliosis, osteoporosis is the rule and not the exception. The National Osteoporosis Foundation has published patient characteristics that may predict poor bone quality, including a history of a previous fragility fracture as an adult or fracture in a first-degree relative, white race, advanced age, smoking, low body mass, female sex, dementia, poor health, or fragility. Plain radiographs and computed tomographic images can be used to assess the degree of bone loss. In addition, dual-energy radiograph absorptiometry (DEXA) is often performed for patients with identified risk factors. These studies can be used to tailor the reconstructive techniques to the bone quality of the patient.
Bone Quality and Challenges with Fixation
Bone quality serves as a predictor of the reliability of certain forms of instrumentation used in spinal constructs. Biomechanical studies have demonstrated a correlation between bone mineral density and the pullout strength of both pedicle screws and laminar hooks. After assessment of bone quality via medical and imaging studies (DEXA), an approximation can be made of the likelihood of success with instrumented techniques. Accordingly, the patient can be appropriately counseled as to the probability of success and risks for failure.
SURGICAL TECHNIQUE: FIXATION STRATEGIES FOR OSTEOPOROTIC BONE
Screw Placement Strategies
It has been well established that spinal instrumentation with pedicle-screw fixation is less effective in osteoporotic bone. Osteoporosis is a disorder that primarily affects trabecular bone. Because the pedicle screw has cortical contact limited to the pedicle isthmus, a “windshield wiper” mode of failure can lead to screw loosening as the distal threads of the screw move relative to the well-fixed shaft within the pedicle ( Fig. 31-1 ). Fixation strategies for osteoporotic bone either take advantage of the relatively stronger cortical bone (by increasing the screw diameter and avoiding tapping the screw path) or attempt to augment the fixation of a pedicle screw within the existing trabecular bone. It should be recognized, however, that when sclerosis is associated with degeneration in patients with adult scoliosis, the local bone mineral density may be significantly increased, limiting the local effects of the systemic osteoporosis.
Augmentation Techniques to Improve Pedicle-Screw Fixation
Numerous techniques designed to improve the fixation of pedicle screws within osteoporotic trabecular bone have been developed, including augmentation with polymethylmethacrylate (PMMA) cement, calcium sulfate, calcium phosphate, or hydroxyapatite. The primary goal of these techniques is to improve the pullout strength of the screws ( Fig. 31-2 ). The majority of investigations of these techniques have been in cadaveric vertebrae. Frankel et al. demonstrated in a cadaveric study that PMMA augmentation increased pullout strength of pedicle screws in primary and revision procedures by 119% and 162%, respectively, Burval et al. have demonstrated experimentally in cadavers that both the standard transpedicular technique and the kyphoplasty approach are effective in increasing the pullout and fatigue strength of pedicle screws in osteoporotic bone. Fluoroscopy can be used to visualize the placement of 2 to 3 mL PMMA per pedicle, to ensure that cement does not migrate toward the neural elements. Calcium sulfate paste avoids the exothermic reaction associated with the use of PMMA, reduces the risk for extravasation into the spinal canal, is potentially osteoconductive, and is eventually resorbed by the body. Rohmiller et al. colleagues have compared cadaveric placement of pedicle screws augmented with either calcium sulfate or PMMA and found no significant differences in axial pullout strength. Calcium phosphate has compared favorably with PMMA in augmenting pedicle-screw fixation in cadaveric studies. Other alternatives have been investigated as well, including conical screws, hydroxyapatite-coated screws, and expandable screws. The primary difference between fixation using PMMA and either calcium phosphate or hydroxyapatite involves the relative biologic incorporation and remodeling versus the inert nature of PMMA, which is unable to undergo remodeling after microfracture within the cement.
Related posts:
Rheumatoid Arthritis of the Cervical Spine
Instrumentation of the Lumbar Spine for Degenerative Disorders
Management of Postoperative Lumbar Wound Infections
Failure of Cervical Spine Instrumentation
Failure of Lumbar Spine Instrumentation
Facet Joint Replacement for the Treatment of Degenerative Disorders of the Spine
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
