34 Construct Failure and Failure Prevention: The Decision Making Process
In this chapter, focus is placed on construct failure and failure prevention. Seven scenarios have been defined as worthy of discussion:
Construct failure
Preoperative decision making–related failure
Intraoperative decision making–related failure
Subsidence-related failure
Implant–bone interface integrity–related failure
Implant fracture–related failure
Postoperative management–related failure
Each scenario is introduced with a case or cases that portray the principles to be clarified in the section that follows. Obviously, these cases simply represent examples. Many others exist. Each scenario relates, to one degree or another, to principles portrayed elsewhere in this book.
34.1 Construct Failure
We begin by focusing on construct failure. Such a complication is commonplace and, more often than not, related to surgeon issues. Our first case is that of a patient who underwent a multilevel cervical corpectomy for cervical spondylotic myelopathy. A bridging implant and a fibular allograft strut were employed to fix the spine (Fig. 34.1a). Repetitive stresses were applied to the implant–bone interface during flexion, extension, lateral bending, and rotation. This caused fatigue of the screw–bone interface and, eventually, failure. An intermediate fixation point into the native spine (e.g., a retained intermediate vertebral body) was not employed. This would have permitted both the attachment of more anchors to the native spine and the addition of a three-point bending mode of fixation (see Chapters 19 and 26). As a result, the spine, under a variety of loading conditions, moved in dyssynchrony with the implant, causing fatigue of the screw–bone interface and, ultimately, failure (Fig. 34.1b). Of note, such is often seen with long bridging implants.
Construct failure is an all too common consequence of modern-day spine surgery. In reality, constructs do not fail; surgeons fail, as exemplified in Fig. 34.1. The surgeon did not recognize the limitations of long bridging implants. In fact, the surgeon likely felt comforted by the apparent security of fixation observed at the time of surgery. What the surgeon did not realize were the nature, magnitude, and number of load cycles to be applied to the construct during the bone fusion and healing process. The surgeon also did not realize the importance of using as many modes of fixation as possible. In this case, the omission of a three-point bending component proved to be a critical omission.
Regardless of the semantics involved, the term construct failure is used here because this terminology is so deeply embedded in the spine surgeons’ repertoire. In reality, construct failure should be considered surgeon failure. Regardless, this chapter “drills down” into the phenomenon of construct failure, with a focus on prevention. Although the management of construct failure is critically important, it is addressed in prior and subsequent chapters of this book, particularly in Chapter 19.
Spine surgery is unique among the surgical disciplines in that it involves both the application of physical and biomechanical principles and the protection of eloquent neurologic tissues. As such, it is the only surgical discipline that requires of its practitioners (1) a deep understanding of biomechanical and physical principles, (2) the skills of a master carpenter, and (3) the requisite knowledge and finesse to protect and restore neurologic function.
Construct failure is often the end result, via either direct or indirect means, of a failure to appreciate and/or possess the aforementioned requisites of an accomplished spine surgeon. Hence, construct failure is deserving of particular attention.
34.1.1 Prevention of Construct Failure
Construct failure prevention strategies are myriad, and some are better than others at achieving the anatomical and clinical goals of surgery without complications. The surgeon, in achieving the goal of construct failure prevention, should in general focus on the aforementioned three requisites of a spine surgeon that cause him or her to stand apart from other surgical specialists. The surgeon must harbor a solid foundation of knowledge regarding anatomy and biomechanical and physical principles. A surgeon who is a true student of the discipline of surgery will have mastered surgical technique. Again, unique to the spine surgeon and the orthopedic surgeon is the need to harbor a high level of biomechanical knowledge and the surgical skills unique to the management of structural pathologies. Such a foundation of knowledge and skill sets most certainly should alert the surgeon to the potential adverse consequences of using a long bridging implant such as the one portrayed in Fig. 34.1. This includes the ability to appreciate three-dimensional anatomy and the ability to negotiate the three-dimensional anatomical environment of the regional pathology at hand. Finally, the surgeon must be adept at the act of surgery—a skilled surgeon who has mastered not only the regional neuroanatomy but also the means to protect and restore neurologic function.
34.2 Preoperative Decision Making–Related Failure
Decision making–related failures abound. In reality, all seven categories discussed in this chapter are, in one way or another, related to decision-making failures. Let us, for a moment, consider an L4–5 fusion operation performed for back pain ( Fig. 34.2a). We might substitute any number of imaging examples here. The problem here may not be the choice of operation, the technique employed, or the number of levels fused. It may instead be related to the fact that surgery was performed in the first place. This patient had a chronic pain syndrome that manifested as back pain. She also had an imaging finding that in some ways correlated with her back pain. The error made by the surgeon in this case was the inadequate consideration of “other causes” of back pain. This patient had multiple ongoing and long-term life stresses, with chronic fatigue and a sleep disorder. In addition, multiple other somatic complaints were voiced by the patient, but ignored by the surgeon (see Chapter 37). The surgeon focused on the back pain and ignored the rest. Such misguided decision making can lead to multiple operations and a very unhappy patient. The end result of such a scenario is depicted in Fig. 34.2b.
34.2.1 The Decision to Operate
The decision to operate, or rather more appropriately the decision not to operate, is perhaps the most important decision a surgeon makes. The decision not to operate is unequivocally the best strategy to ensure the prevention of construct failure. It is argued that spine surgery is performed in excess. This is particularly so in the case of surgery for pain, particularly axial back pain. The decision-making process is very complex in this arena. It is complicated by multiple factors. These include, but are not limited to the following: (1) Truly objective criteria by which surgical candidates can be defined are lacking; (2) the surgeon makes the ultimate decision, and this decision is based on multiple clinical, patient-derived, economic, academic, and intellectually related influences; (3) outcome assessment instruments have been and still are suboptimal and relatively infrequently employed; and (4) the decision is related to other external influences, including implant vendors and hospital economics.
The Absence of Criteria for Surgical Candidacy
It most certainly is difficult to objectively quantify the candidacy for surgery. Multiple clinical, anatomical (imaging-based), and psychosocial factors affect candidacy for surgery. A patient may harbor anatomical criteria for surgery (e.g., a degenerative L4–5 spondylolisthesis) but not manifest the commensurate clinical findings and symptoms. On the other hand, the patient may have both, but be so adversely affected by the ravages of a chronic pain syndrome that surgery, even though anatomically and clinically indicated, provides limited hope for ultimate clinical success (see Fig. 34.2b). It is in this vein that the surgeon must have a solid grasp of the entirety of the decision-making process, including a commitment to ensure that clinical and anatomical/imaging correlation exists. This involves an assessment of the “character” of the pain.
Pain that has no anatomical/imaging correlate falls into several categories: (1) nonradicular extremity pain (e.g., pain in a stocking–glove distribution that may be associated with peripheral neuropathy); (2) pain whose “character” does not describe a syndrome that is expected to respond to surgical intervention (e.g., axial myofascial pain); (3) pain that is related to another, but not surgically treatable, syndrome (e.g., early-onset spondyloarthropathic symptoms that may be characteristic of ankylosing spondylitis); (4) pain that is not somatic in origin (e.g., burning or neuropathic pain); and (5) pain that has not been adequately addressed by nonoperative means (e.g., a trial of membrane stabilizers in a patient with neurogenic claudication). All of these are expected either not to respond to surgical intervention or possibly not to require surgery because alternative treatment strategies obviate the need for surgery (e.g., the successful treatment of neurogenic claudication symptoms with gabapentin).
Pain without Anatomical/Imaging Correlation
Pain that is not associated with an anatomical or imaging correlate cannot be expected to respond to surgery. Traditional spine surgery for pain addresses, and hence must be aligned with, the anatomical and imaging pathology and findings. Without such correlation, there exists no hope of a positive surgical yield. An L5–S1 discectomy for nondermatomal extremity pain is doomed to failure—that is, failure to achieve a satisfactory response to surgery. Although this goes without saying, the surgeon can be coerced by extraneous influences (e.g., economic and persuasive patient surgically oriented maneuvers) to perform surgery despite knowing in his or her “heart of hearts” that surgery is not indicated. The rationale that “nothing else has worked, so why not try surgery?” is lame and without foundation. The surgery can most certainly be justified to third-party payers on the basis of the anatomical/imaging findings. This unfortunately further complicates the already complex decision-making process. Simply stated, “the absence of response to other treatment modalities should in no way be suggestive of the notion that another modality (i.e., surgery) will meet with success.” Many a patient has lived asymptomatic for decades, until death from natural causes, with significant anatomical/imaging findings.
Pain whose “character” does not describe a syndrome that is expected to respond to surgical intervention (e.g., axial myofascial pain) will rarely respond to surgical intervention. Again, a patient with myofascial back pain and a large L5–S1 herniated disc is not likely to respond to discectomy. Although the chance of surgical success is admittedly greater in this scenario than in the scenario from the prior paragraph, that does not justify surgery in the majority of cases. Myofascial back pain is best treated by an aggressive physical restoration program, with a focus on core strengthening and flexibility.
A discussion with the patient regarding “hurt versus harm” is relevant in such cases. The patient must understand that the provider indeed understands that the patient “hurts.” No one is denying that. Furthermore, the patient must clearly and unequivocally appreciate the fact that the pain is not an indicator of “harm,” if indeed such is the case (and it nearly always is). Further activity, such as physical therapy, will not be “harmful,” although it may very well hurt. Once this barrier is overcome, the patient can embark on a physical restoration program that usually meets with success. If this barrier is not overcome, further unsuccessful treatments (including misguided surgery) can pave the way toward a chronic pain syndrome, which would be very unfortunate.
A surgical approach to pain that is, unbeknownst to the surgeon, related to another, not surgically treatable syndrome (e.g., early-onset ankylosing spondylitis) is also doomed to failure. Ankylosing spondylitis is a not uncommon cause of back pain in young adults. It is characterized by early-morning back pain that dissipates with activity by midmorning. This pattern of inflammatory pain is strikingly different from that of mechanical back pain (pain that is deep and agonizing in nature, worsened with activity or loading of the spine, and improved by inactivity or unloading of the spine). Mechanical pain does not dissipate in the morning and usually escalates as the day and activity progress. The patient with ankylosing spondylitis may also have a degenerated lumbar spondylolisthesis, surgery for which would not improve symptoms that are characteristic of inflammatory and not mechanical back pain.
Pain that is not somatic in origin (e.g., burning or neuropathic pain) does not respond to decompressive or stabilization surgical intervention, regardless of the imaging findings. Management by a physician specializing in the treatment of chronic pain, with the use of membrane-stabilizing medications (e.g., gabapentin) and selected antidepressant medications (that function in the capacity of central serotonin-mediated pain inhibitory pathway modulators), is likely the most appropriate strategy, not decompressive or stabilization surgery.
Pain for which a reasonable trial of treatment via nonoperative means (e.g., a trial of membrane stabilizers in a patient with neurogenic claudication) has not been attempted is not yet optimally amenable to surgical intervention. More than 50% of patients with symptoms of neurogenic claudication, who would otherwise be candidates for lumbar decompression surgery, respond to membrane stabilizers (e.g., gabapentin). 1 , 2 This response is usually sustained for years, and the medication can often be tapered and discontinued after 4 to 6 months (author’s observations). The treating physician must be diligent in the administration of these medications and must ensure that adequate doses (gradually increased) are employed. There exists a substantial individual dose–response effect, particularly with gabapentin.
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