Curr Rev Musculoskelet Med (2009) 2:15–24 Again, the patient is observed for pain relief that is appropriate to the biological properties of the anesthetic. This second injection is referred to as the conﬁrmatory injection. The value of adding a conﬁrmatory block was demonstrated by Schwarzer et al. [7], who by using the protocol outlined above, noted a false positive rate of 38% when a single block was used for the diagnosis of facet joint-mediated pain. Although the value of a conﬁrmatory block is recog- nized, the deﬁnition of a successful block is controversial. In the above study by Schwarzer et al. [7], a 50% or greater reduction of pain was accepted as a positive block. A 50% or greater improvement in pain as criteria for a successful diagnostic medial branch block has also been used by other investigators [93–98]. Studies that employed diagnostic intra-articular blocks as apposed to medial branch blocks have also used similar criteria of 50% or greater improvement of pain [99, 100, 107]. However, others have suggested that a more strict deﬁnition of a conﬁrmatory block is necessary to avoid performing radiofrequency denervation on patients who may have low back pain of other etiologies. When more strict criteria are used, the speciﬁcity of a diagnostic block may improve. Other investigators have employed more stringent cri- teria as the marker for a positive diagnostic medial branch block. For example, in a 10-year clinical audit examining the efﬁcacy of radiofrequency denervation, Gofeld and colleagues [101] accepted a positive response to diagnostic medial branch blocks only if the patient experienced at least a 70% reduction in pain scores. In a study assessing the efﬁcacy of facet joint anesthesia, Revel et al. [102] considered a positive result only if there was a 75% or more relief of pain. More recently, Dreyfuss and colleagues [106] excluded patients from subsequent radiofrequency denervation if they did not experience at least 80% improvement in pain scores. In a randomized double blind trial examining cervical facet joint-mediated pain by Lord et al. [108], patients were required to experience complete 100% pain relief to be considered as having had a suc- cessful diagnostic block. In a review of the evidence based procedural guidelines that included three randomized controlled trials and two systematic reviews, Hooten et al. [103] proposed the use of 80% or more relief of pain as the standard for accepting a diagnostic medial branch block as successful. This criteria, however, was challenged by a multi-center retrospective clinical data analysis of 262 patients with chronic low back pain who had undergone lumbar radiofrequency denerva- tion. The study group was divided into two groups of patients. The ﬁrst group included patients who had experi- enced greater than or equal to 50% but less than 80% pain relief after diagnostic medial branch blocks. The other group represented patients that obtained equal to or 19 greater than 80% pain relief. There was no signiﬁcant dif- ference between success rates of subsequent radiofrequency lesioning in these two groups [104]. By utilizing more strict criteria for diagnosing facet joint pain, we limit the number of false positive results. How- ever, as the authors of the above study suggest, the use of more strict criteria may be counterproductive and may lead to the withholding of a potentially therapeutic treatment from patients with treatable facet joint-mediated pain. As we have illustrated, there is a lack of consensus regarding the deﬁnition of a successful diagnostic block. Similarly, there is no universally accepted method of how best to perform these procedures. Although intra- articular facet joint injections are generally considered to be more technically challenging, both medial branch blocks and facet joint injections have their share of technical complications. Furthermore, even the use of local anesthe- sia with both of these procedures has been called into question. Some physicians will administer a local anesthetic to the muscle and fascia in addition to subcutaneous local anesthesia. This technique is simply an attempt to improve patient comfort by anesthetizing the tissue through which the needle will track. A larger number of false-positive results have been reported with this technique [10, 105]. It is for this reason that some support the use of local anesthetic only superﬁcially or of omitting the use of local anesthetic altogether. It is also important to recognize that in addition to the potential confounders outlined in this review, these methods are only as reliable as the ability of the patient to accurately report their symptoms. With respect to medial branch blocks, most centers utilize a multiple needle approach whereby separate nee- dles are used for each medial branch that is anesthetized. More recently, however, a new technique involving a sin- gle needle has been proposed [109]. Although further studies are needed, initial analysis of this single needle technique suggests similar accuracy to that of the multiple needle approach in ability to anesthetize the medial branch with the advantages of increased efﬁciency and less patient discomfort [110]. It has also been recognized that when performing medial branch blocks, subtle differences in needle position can have important diagnostic conse- quences. In a cadaveric study, Dreyfuss et al. [111] reported evidence of local anesthetic spread to other structures such as the intervertebral foramina as well as consistent spread to the posterior muscles of the back. Local anesthetic spread to other potential pain generators may decrease the diagnostic utility of these injections. Of note, in this study, when the needle tip was positioned at a point along the inferior aspect of the transverse process, aberrant ﬂow was minimized. This suggests that when appropriate technique is utilized, spread of anesthetic can be avoided and diagnostic utility may be enhanced.