To understand where the slips of the plantar aponeurosis insert, we first need to look at the MP joints, and at some structures nearby: the flexor tendon sheaths, the plantar ligaments of the MP joints, and the ligament that connects the metatarsal heads, the deep transverse metatarsal ligament.
Here’s the deep transverse metatarsal ligament. It goes all the way from the first MP joint, to the fifth. The flexor tendon sheaths, which we’ll see in a minute, are attached along these lines.
To take a look at a typical MP joint, and the structures around it, we’ll look at a toe and its metatarsal in isolation. Here’s the MP joint with its capsule intact. Here it is with the loose parts of the capsule removed. There’s a broad collateral ligament on each side. The MP joint can’t flex much beyond a straight position, but it can extend all the way to here.
Here’s an MP joint divided longitudinally. The joint capsule is thin on the dorsal aspect, and massively thickened on the plantar aspect. This thick part of the capsule is the plantar ligament of the MP joint, it’s fixed to the proximal phalanx here, so when the joint is extended, the plantar ligament is pulled forward.
Here’s the plantar ligament in the intact joint. The tendon sheath is attached to the plantar ligament here, and here. Here’s a short piece of the tendon sheath intact. It runs the whole length of the toe, as we’ll see later. Also attached to the plantar ligament of the MP joint is the deep transverse metatarsal ligament: here's its attachment on one side, here it is on the other side.
Here’s the MP joint of the big toe, the first MP joint. It’s much larger than the other MP joints, and it has two additional structures, a pair of sesamoid bones, which are enclosed within the plantar ligament. One of them’s here, the other one’s here.
Now that we’ve looked at the MP joints and the structures around them, let’s go back to the plantar aponeurosis, and see how it’s inserted. As we’ve seen, each division of the aponeurosis gives rise to two slips. These lie on each side of the flexor tendons. The two slips are inserted here, and here, on each side of the plantar ligament of the MP joint.
Since the plantar aponeurosis is inserted into a set of movable structures, the plantar ligaments of the MP joints, its tightness varies depending on the position of these joints. When the MP joints are straight, the plantar aponeurosis is slack, but when the’re extended, it becomes much tighter.
The plantar aponeurosis acts as a continuation of the achilles tendon. When it’s tight, as it is when the MP joints are extended, it enables the pull of the calcaneal tendon be transmitted directly to the metatarsal heads. That’s why the arch of the foot remains an arch, even at the moments when we place the heaviest loads on it.
The plantar aponeurosis is the central part, and much the strongest part, of a layer of fascia, the plantar fascia, which covers the entire sole of the foot. We’ll see the whole of the plantar fascia when we've looked at at the muscles of the foot, which we’re going to do shortly.