Paper of the month April 2023

Sacroiliac Innervation
Eur Spine J. 2022 Nov;31(11):2831-2843.

^{Steinke H, Saito T, Kuehner J, Reibetanz U, Heyde CE, Itoh M, Voelker A.}

Dr. Szadek is going to talk about sacroiliac joint pain from a pain physician's point of view in an upcoming SIMEG Webinar. In this connection, I will present a study that investigated the innervation of the sacroiliac joint in detail.

Honestly, this paper is difficult to read and understand. The various issues are confusing, but I read on, trying to get anatomical facts about the branches of the nerves going to the sacroiliac joint.

I would like to focus on Figure 1B, which is an observation of the posterior branches from L5-S5 and the communicating branch of them, removing the sacrum via ventral approach; the thin posterior branches of L5, S1, and S2 are directed toward the short posterior sacroiliac ligament and the interosseous ligament just dorsal to the SIJ. On the other hand, the thicker posterior branches of S2-S4 are distributed toward the sacrotuberous ligament (STL) after plexus formation, and the latter is more distinct and impressive. The attempt to shave the sacrum from the ventral side to observe the posterior nerves is novel.

In Figure 3B, vessels and nerves are located in the layer between the sacral multifidus muscle and the sacral periosteal layers, and the neurovascular bundle is observed penetrating the posterior sacroiliac ligament and entering the interosseous ligament, but not reaching into the joint, which is important information.

However, the identification of nerve endings by staining with anti-CGRP or substance P antibodies, as done by Szadek et al., is technically difficult and time-consuming (large number of sections must be viewed). Immunohistological staining was not performed in this study (the work of Szadek et al. is more ambitious in this respect). Therefore, it is still not conclusive whether nerve endings really do not exist in the sacroiliac joints.

In the present article, Steinke et al. report that gross anatomy and histology indicate that the pain is not from within the joint. This is consistent with the fact that, as reported by Murakami et al, local anesthetic infiltration into the posterior sacroiliac ligamentous region rather than into the sacroiliac joint cavity is more useful in diagnosing most sacroiliac joint pain in clinical practice.

The authors also mention that sacroiliac joint pain is due to peripheral neurovascular bundle impingement in the periarticular ligaments of the sacroiliac joint. It is certainly conceivable that nerve impingement and adhesions within the ligaments may be part of the pathogenesis of sacroiliac joint pain. As the authors point out, there is collagen around the nerves, and loss of fluid within the collagen could affect the nerves and be associated with pain. In fact, the middle cutaneous nerve runs between the long posterior sacroiliac ligaments, and in cases of buttock numbness, it is theoretically possible that persistent sacral counter-nutation, increased tension in the long posterior sacroiliac ligament, and impingement of the middle cutaneous nerve that runs ventral to it could be causing the pain.

On the other hand, there are cases in which the pain is immediately relieved by manual therapy to correct the dysfunction of the sacroiliac joints This fact means that  pain in the sacroiliac joints cannot be explained by only impingement or entrapment of the dominant nerve due to histologic changes. In addition, the presence of pain from within the joint itself cannot be ruled out, as some patients actually benefit from intra-articular injections. As Dreyfuss et al. have shown, even with posterior lateral branch block of S1-3, much of the discomfort in the SIJ remains due to capsular distension with contrast medium injection into the joint space. In this regard, if one were to try to explain this solely in terms of nerves, another innervation pathway might be involved, such as the nerves that innervate the sacroiliac join from the ventral side (L5 and/or S2 ventral branch). The presumption is that what we have clinically determined to be pain from within the joint cavity could be a blockage of these ventral nerve endings reaching the sacroiliac joint capsule.

In summary, this paper carefully dissects the branches of the nerves leading to the sacroiliac joint. However, the ends of the nerves do not extend into the sacroiliac joint cavity. The content of this article frees us from the assumption that when we talk about sacroiliac joint pain, the pain is coming simply from within the joint cavity. However, the nerve innervation, nerve impingement, or adhesion theory of sacroiliac joint pain alone does not yet explain our clinical experiences.

I would like to hear Dr. Szadek's straightforward opinion.