By Mr Ahmed-Ramadan Sadek, Neurosurgeon · Last reviewed October 2026 · 7 min read

What is spondylolysis?

At the back of each vertebra, a narrow strip of bone links the joint above to the joint below. This strip is the pars interarticularis. In spondylolysis, a crack (stress fracture) forms across it on one or both sides. Doctors often call this a pars defect.

Two things tend to contribute. Some people are born with slender bone in this area, so the tendency can run in families. Repeated arching and twisting of the lower back also places heavy strain on the pars, which is why the condition is linked with sports such as gymnastics, weightlifting and football. It is a frequent cause of back pain in young athletes.

When the vertebra slips

If the fracture leaves the vertebra without its normal bony anchor, the bone can begin to slide forward over the one below. This is spondylolytic spondylolisthesis, and it most commonly happens at the bottom of the spine, where L5 sits on the sacrum. A slip can range from very small to severe. A larger slip can stretch the nerve roots where they leave the spine. (Slippage caused by joint arthritis in older adults is a different condition, covered in our guide to degenerative spondylolisthesis.)

Symptoms

Plenty of people have a pars defect without ever knowing. When it does cause trouble, common symptoms include:

• Low back pain, often worse when leaning backwards or after sport
• Pain spreading into the buttocks and thighs
• Stiffness, tight hamstrings and tender back muscles
• If a nerve is stretched, sciatica: pain below the knee with tingling, numbness or weakness in the leg

Diagnosis

X-rays may show the defect and any slip. CT gives the clearest view of the fracture itself, while an MRI scan shows the discs and nerves. Because a pars defect can be present without causing pain, your specialist may suggest a pars injection to check whether it really is the source of your symptoms.

Non-surgical treatment

Most people do not need an operation. Treatment usually includes physiotherapy to strengthen the core and stretch tight muscles, pain relief, and sometimes manual therapy. Lifestyle changes help too: avoiding contact sport, heavy weightlifting and repeated over-arching of the back (hyperextension) while symptoms settle, and losing weight if you are carrying extra, which reduces the load through the spine.

Pars injections

If pain is severe and has not responded to other treatment, a pars injection may be offered. A mixture of local anaesthetic and steroid is placed directly into the fracture. The anaesthetic numbs the area for a few hours, and the steroid calms inflammation over the following days or weeks. If your pain eases afterwards, that strongly suggests the pars defect is the culprit. For some people the relief is long-lasting.

What happens

The injection is a day-case procedure, done either under local anaesthetic or with sedation through a drip. You lie on your front, the skin is cleaned, and live X-ray guidance is used to steer a fine needle into the defect. The injection itself takes only a few minutes, and most people are discharged a couple of hours later.

Once the numbness wears off, the pain often returns for a while, occasionally a little worse, until the steroid starts working. Keep taking your usual painkillers until then, and never stop strong painkillers or nerve-pain medicines suddenly. If you had sedation, BASS advises not driving for 48 hours and having an adult stay with you overnight.

How long the benefit lasts varies from weeks to years. If an injection helped, BASS notes that most surgeons prefer to leave a gap of at least six months before giving another. If there is no improvement by six weeks, or only short-lived relief, surgery may be the next step for some people.

Risks of pars injections

BASS describes pars injections as having very few risks, and gives no specific figures. Uncommon problems include bleeding (tell your team if you take blood thinners), infection (rare, but more likely if the skin of the back has eczema or psoriasis), temporary facial flushing or changes to periods, raised blood sugar for a few days in people with diabetes and, rarely, a needle puncture of the dura causing a headache on standing for a few days.

Surgery

Surgery is considered when symptoms remain disabling despite good non-surgical care. The type of operation depends on whether the vertebra has slipped:

• Direct pars repair: for little or no slip, a screw is passed across the fracture with bone graft so the crack heals.
• Decompression: bone and inflamed tissue around the fracture are removed to free pinched nerves.
• Fusion: screws and rods hold the slipped segment still while bone graft joins the vertebrae; BASS describes this as taking about 3 to 6 months. The disc may also be replaced with a cage from the back (PLIF or TLIF) or the front (ALIF). See our guide to lumbar spinal fusion.

BASS patient information reports good relief of back and leg symptoms in about 70% of people (up to 7 in 10), often building over several months. Long-standing numbness or weakness may not fully recover. Most people go home 1 to 2 days after surgery.

Risks of surgery

These figures come from BASS patient information on surgery for spondylolysis and spondylolytic spondylolisthesis.

Risk	Approximate frequency (BASS)
Nerve root injury	Under 1 in 100 for a first operation; up to 10 in 100 for repeat surgery
Tear in the dura (nerve lining)	Fewer than 5 in 100
Further operation for a persistent fluid leak	Less than 0.05%
Superficial / deep wound infection	Up to 4 in 100 / fewer than 1 in 100
Blood clot in the leg or lung	Fewer than 1 in 700
Bones fail to fuse	Up to 5 in 100
Cage or implant moves	Up to 2 in 100; about 1 in 100 need another operation
Death from injury to major vessels or organs	About 1 in 10,000
Fatal anaesthetic complication	About 1 in 250,000
ALIF only: injury to major blood vessels	Up to 15 in 100
ALIF only: retrograde ejaculation (men)	Under 1 in 100
ALIF only: temporary bowel standstill (ileus)	About 11 in 100

Other risks include bleeding, recurring leg pain from scarring, screw-related nerve injury or breakage, positioning injuries and, rarely, worse symptoms. Very rarely, bleeding into the canal can cause paralysis. Smoking is the biggest threat to fusion; BASS advises stopping ideally 2 to 3 months before surgery.

Further reading

• BASS patient booklet: Lumbar Spondylolysis and Spondylitic Spondylolisthesis
• BASS patient booklet: Pars Injection for Lumbar Spondylolysis
• NHS: Back pain
• Spinemathema: Lumbar spinal fusion (TLIF and ALIF)

This guide is general information and is not a substitute for advice about your own situation. Please speak to your GP or specialist.