Two words, two problemsStrong evidence
Each vertebra in the lower back has a solid block at the front and a ring of bone at the back. The pars interarticularis is a thin bridge in that ring, and the American Academy of Orthopaedic Surgeons calls it "the weakest portion of the vertebra". A crack or stress fracture there is spondylolysis. It is most often in the lowest lumbar vertebra (L5), on one side or both.
Spondylolisthesis is a vertebra that has slipped forward on the one below. It isn't the same as a slipped disc, and it doesn't always cause symptoms. Slips are graded by how far the vertebra has moved: up to a quarter of its width is grade I, the commonest, and more than half is a high-grade slip, which is more likely to press on nerves.
Sources 1, 2, 3
Three different groups
Young athletes: the crack is a stress injury from repeated arching and twisting, in sports such as gymnastics, football and weightlifting. When both sides crack, the vertebra can slip, most often during a growth spurt, though an NHS children's team calls a slip in children very rare.
Adults with an isthmic slip: the crack usually dates from childhood and went unnoticed. Symptoms often start in middle age, and the slip very rarely gets much worse.
Older adults with a degenerative slip: the whole vertebra slips forward as the discs and the small joints at the back of the spine wear, mostly after 50 and more often in women. Narrowing of the spaces the nerves run through (spinal stenosis) is common with it.
Sources 1, 4, 5
Common, and often painlessStrong evidence
In 500 US schoolchildren followed from the age of 6, 4.4% had a crack at 6, rising to 6% by adulthood. Slips reached at most 28% of the vertebra's width and rarely progressed after the teenage years. Forty-five years on, people with a crack on one side only had never slipped, slips had slowed with each decade and none had passed 40%, and their quality-of-life scores matched the general population's.
In a US community study using CT (computed tomography) scans, 11.5% of 188 adults aged 40 to 80 had a crack, men about 3 times as often as women, and it wasn't linked with back pain. Degenerative slips were about 3 times as common in women and became commoner with age. In 4,151 Danish adults, degenerative slips were found in 8.4% of women and 2.7% of men, linked with age but not with lifting at work.
Sources 6, 7, 8, 9
In young athletes, a common cause of back pain that lastsEmerging evidence
In a US sports clinic, spondylolysis explained the back pain of 47 of 100 teenage athletes, against 5 of 100 adults. In 1,025 non-elite teenage athletes with back pain, 30% had it. A review put the highest rates in diving (35% of athletes), cricket (32%), baseball and softball (27%), rugby (22%) and weightlifting (19%). In 40 English academy fast bowlers aged 14 to 17, 20.5% had a bone stress injury in the lower back on screening MRI (magnetic resonance imaging), and a national screening programme found 197 such injuries in 142 young fast bowlers over 15 years.
Back pain in general is common at this age: about 12% of children and teenagers have it at any one time, and 34% over a year. An NHS paediatric orthopaedic team says spondylolysis is a recognised cause, but "by far the most common cause of low back pain is simple muscle pain".
Sources 10, 11, 12, 13, 14, 15, 5
How it's recognised
The NHS lists lower back pain that is often worse standing or walking and eased by sitting or bending forward, pain spreading to the bottom or thighs, tight hamstrings, and pain, numbness or tingling down one leg (sciatica). In young athletes the pain typically builds with activity, eases when they stop the sport, and hurts on arching back.
A GP may arrange an X-ray, and an MRI scan if there is leg pain, numbness or weakness. For teenagers, an NHS children's team uses MRI first, because X-rays are not very good at picking up an early crack, and CT or X-rays to follow healing. A specialist review found MRI picked up about 83% of the cracks CT showed, and says the one-legged arching test is of little use.
Sources 2, 5, 16
What usually happensEmerging evidence
Young athletes: most do well. In the 2026 trial described below, both groups had low pain and disability scores by 3 and 12 months. Whether the crack knits depends on how early it is caught: in 60 Japanese children treated with rest, no sport and a brace, very early cracks all healed, in about 2.5 months, and more advanced ones healed in 80%, in about 3.6 months, but 26% came back. Across 10 studies, 68% of fresh cracks healed, 28% of more advanced ones and none of the long-standing ones, and one-sided cracks healed far more often than two-sided ones (71% against 18%).
A crack that doesn't knit can still stop hurting: in pooled studies, 84% of young people did well although only 28% of cracks healed. In a Finnish study of 57 children, those whose crack healed had better quality of life at 2 years, and 5 developed a small slip. Back pain often comes back: in one cohort, 45% had significant symptoms again within about 3 years.
Adults: in people followed for 45 years, slips slowed with each decade. In 145 people with a degenerative slip managed without surgery for 10 years or more, the slip got worse in 34%, with no link to their symptoms. Of those without nerve problems at the start, 76% stayed free of them; of those with nerve symptoms who declined surgery, 83% got worse.
Sources 17, 18, 3, 19, 20, 7, 21
