Instrumentation is an important part of spine fusion surgery as a method to maintain solid fixation until osseous union can occur. Early types of instrumentation failed frequently and had to be removed or replaced in most cases. Advances in instrumentation have made it possible for a patient to maintain the instrumentation permanently. However, cases still exist, such as pseudoarthrosis, in which spinal implants will fail and need to be removed. This chapter reviews the most common reasons for instrumentation failure and details the appropriate management plan for these cases.
Causes of Instrumentation Failure
Medical conditions such as osteoporosis and cancer
Coronal and sagittal imbalance
Increased thoracolumbar kyphosis
Incorrect procedure for patient
Try to alter as many risk factors for instrumentation failure as possible, such as encouraging smoking cessation.
If a long fusion is extended to the sacrum, iliac fixation is indicated.
Pelvic fixation should be strongly considered in patients with osteoporosis when more than four spinal segments are fused to the sacrum.
Pedicle and vertebral body augmentation have been used to increase the pullout strength of the screw.
Concern exists that stopping a long fusion at the L5 level can lead to instrumentation failure.
Sacral sublaminar devices consisting of wires, cables, and hooks do not provide sufficient strength in long fusion ending at the sacrum.
Augmentation may be necessary at the adjacent segments, and augmenting the proximal instrumented vertebra alone may create a stress increase and predispose to fractures at the adjacent levels.
SCOPE OF THE PROBLEM
Since the introduction of spinal arthrodesis by Hibbs and Albee in 1911 as a method for treatment of Pott disease, the failure of fixation and consequently failure to achieve a solid fusion has been a major problem in spinal surgery.
Before the development of stainless steel in 1930s, materials used for implants were suboptimal, and devices often had to be removed or revised because of failure, migration, and other related complications. Since the 1940s, spinal instrumentation has undergone substantial changes with the development of biocompatible materials that can withstand the repeated stresses of weightbearing, flexion, and extension, until arthrodesis occurs.
Lumbar spine instrumentation is now used in various clinical settings; the choice of the device depends on the clinical problem, the anatomic location, as well as the surgeon’s experience and preference.
The instrumentation used in spinal fusion surgery is not designed to replace the bony elements, but to stabilize them during the fusion process, and it is generally well known that instrumentation without intact osseous fusion will eventually fail.
BASIC PRINCIPLES OF INSTRUMENTATION FAILURE
Spinal instrumentation may fail when incorrectly selected or placed. Failure may include migration, dislodgement, and implant fracture. The instrumentation may fail at the site of attachment to the spine (e.g., “dislodged hooks, loose screws, or broken wire”); it may also fail at the junction of the instrumentation components (e.g., “a rod sliding off at a pedicle screw or fracture along middle portion of a plate or rod”). The most common cause of instrumentation failure nowadays is probably failure to achieve a solid bony fusion. With the development of pseudoarthrosis, the continued stress on the implant will eventually lead to instrumentation fracture or loosening ( Fig. 37–1 ), which is inevitable.
Transpedicular instrumentation has become a well-established and widely accepted posterior fixation method. Conical screws have been introduced in an attempt to improve purchase, and decrease breakage and bending failure. It has also been shown that conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength stiffness or work to failure.
Instrumentation failure occurs most commonly as a result of metal fatigue from the repeated stress of flexion, extension, and lateral bending. In general, rod-based constructs are significantly less stiff in flexion, extension, lateral bending, and axial rotation. Three obvious damage modes are evident on the “rods and screws” of removed implants: wear is the most common, whereas corrosion and fracture of fixation screws or longitudinal rod is the least common. In the stainless-steel implant screw-rod connectors that incorporated cavities capable of trapping stagnant fluid, fretting corrosion can occur. Titanium implants did not exhibit corrosion but did exhibit wear.
CAUSATIVE AND RISK FACTORS FOR INSTRUMENTATION FAILURE
The effect of the patient’s age and co-morbidities such as osteoporosis, corticosteroids use, smoking, cancer, prior irradiation, as well as the development of infection can all contribute to poor fixation and also may increase the incidence of nonunion, which will compromise the fixation used.
Osteoporosis will not only lead to poor screw purchase during the initial procedure, but it provides a less stable environment and additional mechanical disadvantages, adding extra stress to an already precarious situation. Even after surgery is completed, bone resorption can occur around the screws and under the implants that are in direct contact; this may lead to screw loosening, pullout, or even fracture of the weakened bone.
Spinal instrumentation has become a necessary tool in spinal deformity correction because it allows the surgeon to achieve and maintain the desired physiologic alignment of various segments of the spine. Coronal and sagittal balance including the alignment of individual spinal segments, as well the global balance, are crucial when evaluating a patient for surgery and during preoperative planning, especially when a long construct is required. Although it has been shown that preoperative coronal and sagittal imbalance did not necessarily have a direct correlation with fixation failure, postoperative positive sagittal imbalance and residual increased thoracolumbar kyphosis should be considered as a red flag for potential failure, especially in adult deformity surgery ( Fig. 37–2 ). The thoracolumbar and the lumbosacral spine are two areas of stress concentration and transition. Failure of fusion and subsequently instrumentation in these areas may be related to the increased tensile and shear forces on the implants and the fusion mass.
The size of the patient is an important consideration; a morbidly obese patient may stress the spinal instrumentation more than an average-sized person. Fortunately, overweight patients are less like to be osteoporotic; however, the combination of osteoporosis and obesity should be strongly considered as a relative contraindication for surgery, and correcting the underlying problem should be strongly considered whenever possible.
Procedure-Related Factors: Decision Making and Execution
Selecting a sound approach and a fusion technique for a particular patient with a specific problem are integral steps in the success of spinal surgery. One of the challenges in spinal surgery is choosing the appropriate approach and surgical procedure for each individual patient. The surgeon’s knowledge, experience, and insight into the patient’s anatomic variations, biologic factors, as well as their goals and expectations, are all crucial to a well-executed surgical procedure.
A patient with poor bone quality who has significant lumbar kyphosis will not be suitable for a long posterior fusion that ends at L5 or at S1 without adding pelvic fixation to the construct. Interbody fusion at the lumbosacral spine is often necessary if a fusion is extended to the sacrum to reduce the chance of nonunion and to avoid excess stresses on the posterior construct. The use of spinal instrumentation for internal fixation of the anterior column is necessary if an anterior-only approach is used ( Fig. 37–3 ) because this has improved the fusion rate.
Use of pelvic fixation has become a standard technique in long construct to the sacrum. Cunningham et al. studied ex vivo porcine spines biomechanically to ascertain the value of anterior column support compared with that of iliac fixation in lumbosacral fusions. They found that iliac screws significantly reduced lumbosacral motion and were more protective for S1 screws and more resistive to motion than interbody cages alone. However, other investigators have argued in favor of adding anterior column support, particularly at L4-5 and L5-S1, in long fusion to sacrum.
Despite the fusion advantage of anterior surgery, concerns exist about device-associated complications, including injury to adjacent vascular structures in addition to the theoretical concerns of the stress-shielding effect of rigid anterior spinal instrumentation and the risk for device-related osteopenia.
Interbody devices can provide anterior structural support to the fused spinal segment. These devices, however, will not sustain repeated stresses and will always fail unless bony fusion is achieved. Understanding some of the factors that lead to interbody device subsidence is necessary to address the problem of fusion failure. For instance, interbody fusion device subsidence results from failure at the bone-implant interface rather than failure of the implant itself. Recent studies on regional end plate strength of lumbar and thoracic specimens have determined some consistent variability, with greater strength posterolaterally and closer to the periphery of the end plate.
The choice of the implant should be adjusted to the patient anatomy and size. Placing a large-diameter pedicle screw in a small pedicle is unlikely to provide superior purchase, but, in fact, will lead to pedicle fracture and early loss of fixation. On the contrary, using a small-diameter unicortical screw in S1 in a large person is likely to fail ( Fig. 37–4 ).