20 Amount of Cement or Vertebral Fill Material for Optimal Treatment and Pain Relief
Summary
Vertebral augmentation with injection stabilizing material results in renewed stability of the vertebral body and has been shown to be one of the most effective of all spine procedures in alleviating pain and restoring function. It is presumably the restoration of the structural integrity that results in the prominent decrease in patient pain and this result can be seen in both benign and malignant causes of vertebral instability. The amount of fill material necessary to re-establish the strength of the vertebral body and to be durable over time has evolved over the last decade as new information has indicated that the single most important and modifiable strategy that can be employed for the best and most durable results in vertebral augmentation is the injection of an optimal amount of bone cement. The amount necessary is more than was traditionally thought and is between 15 and 25% of the volume of the uncompressed vertebral level. This will provide optimal strength and stiffness to the vertebral body as well as relieving the patient’s pain. The greater amount of cement appears to more optimally stabilize the vertebral body and the results seem to last longer than injecting smaller cement amounts. In addition to injecting an optimal amount of bone cement, other features of the vertebral body that can lead to instability should be taken into consideration. Any nonunion clefts seen on preoperative imaging or on intraoperative fluoroscopy should be filled to ensure an optimal response to vertebral augmentation.
In addition to pain provoking instability within the vertebral body, degenerative end plate changes can give rise to back pain and prominent disability when present. The pathway of pain transmission through the basivertebral nerve (BVN) can be disrupted using BVN ablation. This technique ablates the BVN and can decrease the patients’ pain and improve their function with a minimally invasive outpatient needle procedure. The type of degenerative end plate changes is important as Modic type 1 and 2 changes are associated more commonly with back pain than type 3 changes. Modic type 1 changes may have some instability associated with them and it has been shown that injection of bioactive resorbable bone cement can result in significantly decreased pain and significant functional improvements in patients with back pain and type 1 degenerative end plate changes.
20.1 Introduction
Vertebral augmentation with cement results in stabilization of fracture fragments in a structurally destabilized vertebral body. The loss of mechanical integrity can be due to benign or malignant disease. In benign disease, a failure point is reached that causes a loss of weight-bearing ability. Failure occurs at a lower force in more severely osteoporotic patients. The force necessary to cause fracture is proportional to the degree of bone density and the quality of the bone. In malignant disease with lytic destructive resorption of cortex and trabecula, weight-bearing ability is also lost. The nature of the fracture or that pain that results from it is dependent on the irregular and random distribution of the metastatic disease. The instability caused by an osteoporotic fracture or a neoplasm can be restabilized with vertebral augmentation, and the pain transmission through the BVN complex that innervates the vertebral body can be subsequently decreased after this procedure. 1 An adequate amount of bone cement must be injected to get optimal and durable pain relief and more recent data have further categorized the amount most commonly necessary to establish adequate pain relief. 2
In addition to an optimal amount of bone cements, other portions of the vertebral body such as nonunion clefts should be filled to ensure optimal stability and response to vertebral augmentation. 3 Within an ostensibly stable vertebral body, there can be pain transmission from the end plate at the junction between a degenerated intervertebral disk and the cartilaginous end plate that gives rise to degenerative end plate changes. This pain transmission through the BVN can be disrupted from ablating the nerve itself prior to its primary point of arborization. 4 Alternatively, degenerative end plate changes, specifically Modic type 1 degenerative changes can be treated with resorbable bone cement, thereby providing improvement in pain and function in patients with pain originating from this condition. 5 Whether the symptomatic improvement is a function of increased stability of the vertebral body or ablation of some of the pain transmitting nerve fibers within the vertebral body will be a topic of further investigation.
20.2 What are the Mechanisms of Pain Relief in Vertebral Augmentation?
The mechanism of pain relief in vertebral augmentation is mechanical stabilization of the vertebral body and possibly, to a lesser degree, the denervation of the basivertebral nerve and its C fibers.
20.2.1 Mechanical Stability Cement Distribution
Tohmeh et al 6 and Belkoff et al 7 published in 1999 and 2002, respectively, that cement injection into fractured cadaveric vertebral specimens could increase the compressive strength of vertebral bodies. Belkoff et al also showed that unilateral cement injection could increase compressive strength of a vertebral body almost as much as a bilateral injection provided the injection was done with the correct technique placing it in the center of the vertebral body. Injection of 6 mL of cement via a unilateral approach was almost as effective as an injection of 5 mL of cement bilaterally.
Get Cement into the Fracture Clefts
While Belkoff et al’s work in cadaver spines showed that unilateral cement injection in an isolated vertebral body has almost the same compression strength as a bilateral injection (as measured by an Instron strain gauge), this may not translate into living human experience. Clinically we know that unless clefts and fracture plains in a vertebra are filled with cement fracture, pain may persist after a vertebral augmentation (▶Fig. 20.1). 3 In fact, a second procedure is sometimes needed to selectively fill fractures that were not successfully injected initially (▶Fig. 20.2). 8 , 9
Careful analysis of preprocedural imaging is strongly suggested, as this will predict for you where cement extravasation may occur due to areas of cortical disruption and it is important to visualize clefts or areas void of cancellous bone within the vertebral body and to make sure these are filled during the vertebral augmentation procedure. It is especially important to recognize and fill clefts and fractures adjacent to the end plate as they will commonly occur in this location and the cement may not extend to the areas adjacent to the end plate even with a large-volume injection. Magnetic resonance (MR) imaging can allow for visualization of the clefts, but computed tomography (CT) scanning can give excellent bone detail and can provide a high level of cortical definition, thereby aiding in the preprocedure planning.
Volume
An Australian group led by William Clark et al published the vertebroplasty for acute painful osteoporotic fractures (VAPOUR) trial 10 with fascinating results. They deliberately treated patients in severe pain (7/10 or higher on visual analog scale [VAS]), within 6 weeks after fracture, and aimed for a high-volume vertebral cement fill from the superior end plate to the inferior endplate in the center of the vertebral body through a bilateral fill technique. At every time point, the treated arm patients did better than the placebo group.
Registry data from Switzerland showed that cement volume was a significant predictor for pain relief after vertebral augmentation and that cement volumes greater than 4.5 mL were recommended for achieving optimal pain relief and durability of the results. 11 The authors found that cement volume was not only an important predictor for pain relief in patients treated with balloon kyphoplasty (BKP) but also the third most important covariate (behind the sex of the patient and the location of the fracture); however, it was the only modifiable factor. They reported a clear dose–outcome relationship between filling volumes of cement and pain relief.
A manuscript by Boszczyk et al, discussing the two vertebroplasty versus sham trials published in 2009 in the New England Journal of Medicine (NEJM), noted that only one of the studies reported the volume of fill material and that was reported to be 2.8 mL. 12 It was the authors’ observation that in order to fill the minimum fill volume of an average thoracolumbar vertebral body, it would require a minimum volume of 4 mL. They derived this amount by referencing an average volume of a thoracolumbar vertebral body, which is 30 mL, and using previously reported minimum fill volumes of 13 to 16% that would be necessary to achieve an adequate biomechanical effect regarding restoring the vertebral body strength. 2 , 13 , 14 The authors also pointed out that there are many lumbar VCFs and the volumes required to adequately fill the vertebral body increases toward the lower lumbar spine. They concluded that based on the available information regarding the minimum amount of fill material the reported average volume of 2.8 mL would only be enough to treat the mid to upper thoracic VCFs and that the available data strongly illustrate that patients treated with vertebral augmentation were not treated in an effective manner.
The minimum volume necessary to provide adequate stabilization to the vertebral body has been studied by many different authors. Nieuwenhuijse et al 14 published a prospective study of 106 patients with 196 osteoporotic VCFs and followed the pain response over the course of a year and compared the pain response to the cemented volume of the vertebral body as determined from a postoperative CT scan of the treated levels. Out of all of the patients treated, they reported that 27.3% or 29 patients were nonresponders. This group had a significantly lower mean cemented vertebral body volume or fraction than the patients that were responsive to the procedure (0.15 vs. 0.21, p = 0.002). The mean volume of cement in all of the treated levels was 3.94 mL. Using this information, they concluded that a vertebral body fraction of 24% was the most optimal amount to inject to optimize the patient’s pain response. This fraction was calculated to correspond to a 93 to 100% specificity for providing pain relief. This amount was based on the observation that there were few to no cases that did not have pain relief and were balanced with the need to keep the risk of cement leakage and adjacent-level fractures to a minimum.
In 2015, Martinčič et al published a study with the stated intent of finding the optimal amount of cement to inject into the vertebral body to restore the vertebral stiffness and adjacent intradiskal pressure without causing an undue amount of cement extravasation. 2 This investigation was done on cadavers in the thoracolumbar spine after having loaded the vertebral bodies sufficiently to cause a fracture. Subsequently, vertebroplasty was performed, four times in the same vertebral body and the cement volume injected ranged from 5 to 20%. Biomechanical testing performed before and after the fracture and after each of the cement injections showed that after a vertebral fracture the compressive stiffness was reduce to just under half that of the prefractured vertebral body and was restored to 61% after a 10% cement injection. The measured intradiskal pressure gradually increased to 71% at a 15% cement fill or the point where no significant increase in compressive stiffness or intradiskal pressure could be detected. Based on these observations, they concluded that 15% was the target amount for a minimum cement fill volume. Based on typical vertebral bodies, this would amount to at least 4 to 6 mL or more of cement to attain the minimum fill amount that would be necessary to reestablish the compressive stiffness of the vertebral body.
The key issue here is the concept of adequate filling of the vertebral body. The VAPOUR trial revealed a fundamental philosophical difference on filling between his study and previous two randomized studies, the Vertoss II study comparing vertebroplasty and nonsurgical management 15 and the Kallmes trial comparing vertebroplasty versus sham treatment. 16 The treating physicians in the VAPOUR trial injected a higher polymethyl methacrylate (PMMA) volume than reported in previous randomized control trials, and used a higher viscosity PMMA cement. Their mean PMMA volume used to treat the fracture in the VAPOUR trial was 7.5 mL and in Vertoss II, the average cement volume was 4.1 mL. In the trial comparing vertebroplasty versus sham by Buchbinder et al, the average cement volume amount was a mere 2·8 mL. 17 Molloy et al reported that pain relief happened first and prior to the reestablishment of the strength and stiffness of the vertebral body. 13 This observation may serve to explain why some practitioners conclude that a lesser amount of cement is preferable to more cement when treating vertebral fractures. As we have seen from the above clinical and biomechanical studies, it is necessary to inject between 15 and 25% of the uncompressed vertebral volume to adequately restore the biomechanical properties of the vertebral body.
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16 Cost-Effectiveness of Vertebral Augmentation
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