What Is the Sacral Ala? Anatomy, Function, and Injuries

The sacral ala is one of the two wing-like sides at the top of the sacrum, the triangular bone at the base of your spine. These wings form the link through which the weight of your upper body passes into your pelvis and legs. That load-bearing job makes the ala one of the most common places for the sacrum to crack, sometimes with no clear injury at all, especially when bones are thin.¹² And because the pain often feels like an ordinary lower back problem, an alar injury is easy to miss at first.³

Where the Sacral Ala Sits and What It Is Made Of

The sacrum has a wide top edge, called the base, and a large wing projects from each side of it. “Alae” is simply the plural of “ala,” Latin for wing.⁴ Each wing meets the hip bone at the sacroiliac joint, or SI joint, one on the left and one on the right. In the assembled pelvis, the front of each ala blends smoothly into the inner bowl of the hip bone and forms part of the rim of the pelvic opening.⁵ The front surface is slightly hollowed from side to side, and the back is rounded.⁶

The dimples many people have on each side of the low back sit roughly over the SI joints, and the alae lie deep, toward the front, just inside those dimples. The sacrum starts in childhood as five separate vertebrae, which fuse into one bone during the teens and early adulthood. The alae come mainly from the top segment, which is why reports often mention S1, the first sacral level.

Most of the front of the ala, about three-quarters, develops from a rib-like piece of the first sacral bone. Only the back quarter matches the side projections seen on other vertebrae.⁷ Before birth, between about weeks 24 and 32, extra growth centers appear beside the top three sacral segments and build the wings, echoing the way other vertebrae grow ribs.⁸

Thick muscle, ligaments and the hip bones cover the wings, so they can’t be felt through the skin. Imaging is the only way to see the ala, which is why the word first turns up on a scan report. “Left” or “right” tells you which wing the finding is on. “Bilateral” means both wings are involved, which describes location, not seriousness.

What the Sacral Ala Does

The alae connect to the pelvis through the SI joints, so every step sends upper-body weight across them into the hips and legs.⁹ When you walk, one leg carries the body at a time, and the sacrum on that side holds up the upper body plus the swinging opposite leg.¹⁰ The ala behaves like a beam supported at one end, which puts it under the highest stress of any part of the sacrum during walking.¹¹

Muscles and nerves lie against the wings too. The alae support a large hip-flexing muscle that runs from the spine to the thigh, and give attachment to some fibers of a second hip muscle.¹² Together these two are the main hip flexors, lifting your thigh when you climb stairs or step into a car. After an ala injury, pain at the front of the hip during movement can come from these muscles tugging near the injured bone. That kind of pain does not by itself mean the hip joint is the problem.

A nerve bundle that carries signals from the lower back nerves toward the leg, runs down across the front of the ala and the SI joint.¹³ An ala fracture or surgery in this area can sometimes affect leg sensation or strength.

The nerve bundle running over the ala joins the network of sacral nerves that merge to form the sciatic nerve. That nerve controls much of the movement and feeling below the knee, including the foot. Numbness on the top of the foot, or a foot that feels weak or drags, after a pelvic injury is worth reporting to a doctor promptly.

How Sacral Ala Fractures Happen

Sacral fractures happen in young people after road accidents or falls from height, and in older people with osteoporosis after trivial injuries.¹⁴ In older adults, a simple fall onto the buttocks can be enough.¹⁵ The ala is the usual site of these weak-bone fractures, typically on both sides, matching where walking concentrates stress.¹⁶ A fracture on the second side may be found only when a doctor looks for it specifically, and pain on both sides of the buttocks can fit this pattern. In teenage athletes, repeated training can cause fatigue fractures of the ala, since growing bones are flexible but can give way under repeated load.¹⁷

Doctors divide the sacrum into three zones, and a break in the ala, outside the openings where nerves exit, is called a zone 1 fracture.¹⁸ Zone 2 is the region of the nerve openings, and zone 3 is the central part around the nerve canal. Problems with bladder, bowel or sexual function are mainly a worry with breaks closer to the center, where those nerves run. Still, the nerve bundle running over the front of the ala can be affected even in a zone 1 break.¹⁹ It is reasonable to ask which zone your fracture is in and whether the break extends toward the nerve openings.

Symptoms of an Ala Fracture and Why It Gets Missed

A sacral insufficiency fracture is a crack that forms in weakened bone under normal everyday load, usually in older people with osteoporosis, a condition of thinning bones.²⁰ So there may be no remembered injury at all.²¹ The typical complaint is a vague low back ache, or pain spreading into the buttocks, without the shooting nerve pain down the leg seen with a pinched nerve.²² The pain gets worse with standing, walking and other activities that load the spine.²³

The pain mimics common lower back problems, such as narrowing of the spinal canal, which can also be present at the same time.²⁴²⁵ It can even resemble cancer that has spread to bone.²⁶ Imaging adds to the confusion: on plain X-rays, the lines that reveal sacral breaks are easily hidden by gas and contents in the bowel.²⁷ A normal X-ray does not rule out a sacral fracture, and a CT scan is often needed.²⁸

New low back or buttock pain after a small fall in someone with thinning bones deserves a look at the sacrum, not just the lumbar spine. Pain that eases when lying down and returns on standing or walking is a useful clue, though not proof. If an X-ray is normal but the pain keeps going, it is reasonable to ask whether a CT or MRI makes sense. Tell the doctor about any osteoporosis treatment, long-term steroid use, or past fractures, since these raise the odds of a weak-bone break.

The two scans show different things. CT shows the bone in fine detail and outlines the fracture line, which helps judge how the break is shaped. MRI shows swelling inside the bone, so it can reveal a fracture before any line is visible on CT, and it helps tell a fracture apart from a tumor. Which comes first depends on the doctor’s suspicion and what is available, and sometimes both are used.

Treatment and Recovery

For sacral fractures overall, people treated with and without surgery reported similar disability, nerve problems, and sexual function a year later in one series.²⁹ For insufficiency fractures, non-surgical care lowered pain by about 3.7 points on a 10-point scale.³⁰ Sacroplasty, injecting bone cement into the fracture through a needle, lowered pain by nearly 6 points.³¹ Screw or rod fixation lowered pain by about 4 points, not clearly better than non-surgical care.³² Surgery is considered for breaks from major trauma that leave the pelvis unstable, for fractures with nerve damage where relieving pressure may matter, and for severe pain that does not settle with other care.

Day to day, non-surgical care means pain medicine so you can keep walking, since long bed rest weakens muscle and bone and can make recovery harder. A walker or crutches are often used at first to take some load off the injured side. Treating the underlying thinning bones is part of care, because it lowers the chance of another fracture. Physical therapy is added gradually, building from gentle movement to stronger exercise as pain allows.

Rehabilitation after a pelvic fracture can move in stages, from very limited weight on the leg in the first weeks to partial weight around 6 to 8 weeks and full strengthening and stair work by about 8 to 10 weeks, in one reported case.³³ If a sacral fracture fails to heal, it can cause lasting pain, trouble sitting, a limp, nerve problems and inability to work.³⁴ Pain that is not improving after several weeks should be rechecked.

Why Screws Near the Ala Need Careful Planning

Surgeons often fix the pelvis with long screws through the hip bone into the top of the sacrum, passing through the ala region.³⁵ At the top sacral level, a safe channel for a fully in-bone screw existed in about 69% of people in one CT study, and at the second level in 92%.³⁶ In another group, only about 43% of male and 26% of female pelvises could fit a cross-pelvis screw at the second level.³⁷

Some sacrums are shaped unusually. An unusually shaped upper sacrum, which your report may call sacral dysmorphism, was found in 28% of people in one group of pelvic patients and in 44% of Hispanic patients.³⁸ In an unusually shaped sacrum the screw channel is narrower, about 16 mm wide versus 25 mm in a typical sacrum.³⁹ A screw broke through the bone’s outer wall in about 1 in 3 people with this shape, compared with 1 in 10 with a typical sacrum.⁴⁰ A screw that breaks through is no longer fully contained and may come close to nerves or vessels nearby. Plates on the front of the sacrum have little room either: there is roughly a 1 to 2 cm safe strip, with the L4 and L5 nerves about 7 to 11 mm from the joint.⁴¹

Before pelvic fixation, ask whether your CT shows an unusual sacral shape, since that changes how much room the screws have. Ask which level the screws will go in and why that level suits your bones. Ask how screw position will be checked during the operation, such as with live imaging, and which nerve symptoms to report afterward, such as new numbness or weakness in the foot.