Sacral Plexus

00:01 Now let's turn our attention to the sacral plexus.

00:05 So the sacral plexus has a similar arrangement to the lumbar plexus, except it's further down, it's more inferiorly within the spinal cord.

00:14 So here, we can see, we have the sacral plexus really coming from L4, L5, S1,2,3,4, before we get into the coccygeal plexus.

00:24 And ultimately, we have a series of nerves coming away from it similar to the lumbar plexus.

00:29 So L4, L5 and then S1, we have the superior gluteal nerve, L5 to S2, we have the inferior gluteal nerve.

00:38 L4 through to S3, we have the sciatic nerve, we have the lumbosacral trunk then formed by all of these nerves running together in a cluster.

00:48 Coming away from this much inferiorly, we have the posterior femoral cutaneous nerve, that's an S1, S3 contributions.

00:56 And then we have the pudendal nerve, S2 and S4 also forming within the pelvis.

01:03 So now let's have a look at these nerves in situ as they pass and interacting with the various muscles and the foramina of these bony landmarks.

01:12 Here, we have L4, S1, giving rise to the superior gluteal nerve.

01:16 And what we're going to do to appreciate this is actually look at the postural lateral aspects of the pelvis and the thigh region to see how this nerve passes out of the pelvis.

01:29 Here we can see a passage out of the suprapiriform foramen.

01:33 This is a space associated just superior to the piriformis muscle.

01:38 So here we can see piriformis muscle running across the screen, and we can see how above it, so we have the suprapiriform foramen.

01:46 Ultimately, this is leaving the greater sciatic foramen to leave the pelvis and enter into the gluteal region where supplies gluteus medius, gluteus minimus, and the tensor fascia lata.

01:59 If we then turn to the inferior gluteal nerve, this nerve is coming away from the spinal cord, we can see it's coming L5 to S2, and it follows a similar course, it leaves the greater sciatic frame and to exit the pelvis to leave the gluteal region.

02:16 But this time it does so bypassing inferior to piriformis and this is the infrapiriform foramen.

02:22 A small hole, inferior, it's piriformis, still passing through the greater sciatic foramen, but this time it goes on to supply gluteus Maximus muscle.

02:32 Injury to this nerve can lead to a weakness of the gluteal muscles like medius and minimus.

02:37 And also tensor fasciae lata muscle.

02:40 A way in which you can examine that is why the Trendelenburg test.

02:44 So here we ask the patient to stand on one leg.

02:47 And typically if there's no injury, gluteus muscles contract and they stabilize the pelvis.

02:54 So when you're standing on one leg, what happens is the gluteal muscles will contract and they move your center of gravity over towards that one leg you're standing on.

03:03 Therefore, they have to stabilize the pelvis.

03:06 This indicates a negative Trendelenburg test.

03:09 However, if there was to be injury of the superior gluteal nerve, and you have weakness of therefore gluteus medius and minimus, the pelvis would drop as you're unable to stabilize the pelvis.

03:22 You're unable to move your body over that standing leg and you would drift to the unaffected side.

03:29 So the opposite side would be the direction you move of where the injury is damaged.

03:35 So injury as in this case on the right side would lead to a pelvis drop on the left side.

03:40 And this is a positive Trendelenburg test.

03:44 When you ask the patient to walk, a gluteal limp or waddling appearance as a compensatory movement of the body is observed within the patient.

03:53 If we then look at the posterior femoral cutaneous nerve, this nerve also passes out of this region, and it runs all the way down to supply the posterior aspect of the thigh.

04:04 And here we can see in green, the distribution of the cutaneous innervation of the posterior aspect of the thigh via this nerve.

04:13 Importantly, it also leaves the greater sciatic foramen, inferior to piriformis muscle, similar to the inferior gluteal nerve.

04:22 Now let's have a look at some muscles that are supplied not by specific named branches, you can often find they're got just name nerve of the specific muscle, but they don't have a specific name like obturator, femoral, etc.

04:37 They're kind of named very specifically after the nerve they supply.

04:41 And these are direct branches that are coming from the sacral plexus.

04:45 So I mentioned previously piriformis, here we can see it highlighted and this is supplied by branches that come directly off the sacral plexus and it's called nerve to piriformis.

04:56 We also have other nerves in this region that sit inferior to piriformis, the superior and inferior gemelli muscles.

05:03 And between those we have obturator internus.

05:06 And here we have nerves to these muscles.

05:09 So we have nerve to obturator internus.

05:12 And that will give branches to nerve to gemellus superior, nerve to gemellus inferior, but they're all very small fine branches that pass to these muscles.

05:22 The final one in this region is quite a flat broad muscle, quadratus femorris, and here, we have nerve to quadratus femorris.