The major finding of the current study is that neither clinical nor statistical differences were revealed in the response to IA versus PA needle positions, while performing facet joint injections. Specifically, none of the primary or secondary outcome measures differed between the IA and PA needle positions. In line with the current results, Lilus et al. [
4] found no difference in the outcomes between IA and PA injections of steroids plus local anesthetics. They explained their negative results by the fact that the drugs were injected at a total volume of 8 ml, which is greater than the facet joint volume (1-2 ml). Thus, 5-6 ml of the solution must have leaked into the surrounding tissues. Despite the use of a lower injection volume in our study, no difference between the two needle positions was found. Clearly, some of the IA injected drugs in our study might have also leaked out from the joint. Yet, we wish to emphasize the fact that our study was not aimed to compare pure IA versus PA injections. Rather, it was designed to test if positioning the needle tip within the joint space and injecting at least some of the steroids/local anesthetics into the joint is advantageous. This bears clinical importance because locating the needle tip within a joint space, especially if it is distorted by arthritic changes, may require repeated attempts, which increase the procedure time length and the irradiation dose.
From the mechanism-related viewpoint, the fact that no differences were found between IA and PA needle positions in the current study may imply that underlying mechanisms additional to joint inflammation are involved in facet pain generation. In line with this assumption, McCall et al. [
13] found no difference in the pain referral areas induced in healthy subjects by IA and PA injections of hypertonic saline.
Regardless of the similarities between the effects of the IA and the PA needle positions, an additional finding of the current study relates to the duration of the analgesic effect of these facet injections. Strong pain relief was evident in all outcome measures for up to four weeks after the injections. This effect subsequently subsided, and at the eight-week follow-up, only the primary outcome measure (overall pain intensity) remained significantly different from the baseline measurement. These results are similar to those reported by Chaturvedi et al. [
14]. In their trial, most patients reported significant pain relief immediately after the procedure. The number of patients increased slightly at one week and reached a peak at four weeks, by which time as many as 93.3% of patients had responded. Pain relief declined at 12 and 24 weeks [
14]. Marks et al. [
15] found a similar short-lived response [
15]. In contrast, a few other studies have reported longer analgesic effects, lasting for three months or more [
10,
16-
18]. Differences in the duration of effect can be explained by differences in the drug type, volume, and dosage used in the different studies. In addition, variations in study methodology may also contribute to these inconsistencies. For example, in our study, questionnaires were completed four times during the eight weeks of follow-up, whereas in Fotiadou et al.'s [
16] study, in which longer analgesic effects were demonstrated, questionnaires were completed only once at three months following treatment.
Both Carette et al. [
19] and Lilius et al. [
4] found no difference in pain reduction between steroid and placebo injections. Yet, in both studies, the placebo arm included an IA injection of normal saline. Notably, saline injection into the facet space may have therapeutic properties, given that inflammatory mediators can presumably be expelled following the injection. Indeed, normal saline has been shown to provide better pain relief than expected with a true placebo in a multitude of invasive procedures [
1,
3].
Determination of diagnosis and selection of the joint for injection are important factors in the treatment outcome. Some authors claim that a positive response to medial branch block is the only accurate test for the diagnosis of faced pain [
20]. However, others find clinical and radiological findings as sufficient for establishing the diagnosis correctly [
7-
9,
18,
21,
22]. In our study we have opted to use the second set of criteria and the diagnosis of facet pathology was based on strict clinical and radiological parameters, including medical history and physical examination, as well as CT and bone scans.
Two study limitations should be considered. First, since no placebo control group was included in the present study, one cannot rule out the possibility that the improvement in the outcome measures found may be attributable to a placebo effect. A future study with random IA versus PA needle tip locations should include injections of an active drug or a placebo for excluding this possibility. Second, some joints were injected with half of the quantity of drugs as compared to others, due to the fact that in some patients only one joint was injected and in others two joints received the treatment. This may have reduced the consistency of the results. However, since this was done in similar numbers of patients from both groups, it is unlikely to have skewed the comparisons between the outcomes of the two injection techniques.
Lastly, two additional points deserve consideration. First, trends for differences between the groups in baseline pain intensities in five of the six tested positions (all but sitting) was found, with higher values for the IA group. Similarly, the magnitudes of drop in pain intensities are seemingly larger for the IA as compared to the PA group, in four of these positions. Although these trends have not reached statistical significance, they seem to justify larger-sized future trials to assure that no real differences in the efficacy between these interventions indeed exist. Second, a large drop in pain intensity in relation to those five positions occurred during the first post-injection week, whereas not similar drop was seen for the sitting position. This can be partially explained by the low baseline pain intensity during sitting relative to most other positions, which is typical for patients with facet pain.
In summary, the current study revealed no differences between IA and PA needle positions during facet joint injections. PA needle position is an easier technique that usually necessitates less radiation for verification of the needle position. Moreover, our clinical experience has demonstrated that in some cases, there are difficulties in inserting the needle into the joint space due to anatomical changes. Since both methods result in similar outcomes, the use of PA needle positions should be considered.
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